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RICH2019:

A proposal for the LHCb-RICH upgrade

LHCb Public Note

Reference : LHCb-PUB-2013-011

Created : 08-July-2013

Last modified : 08-July-2013

Prepared by1 : Carmelo D’Ambrosioa , Sajan Easob,

Christoph Freia, Alessandro Petrolinic

a: CERN, Geneva, Switzerland

b: STFC, Rutherford Appleton Laboratory,Didcot,UK

c: INFN , Sezione di Genova, Italy

1 This work is the conclusion of several upgrade meetings, where most of the RICH community took part. It is therefore almost impossible to name all the people, who directly or indirectly have contributed to it. Allow us therefore to thank first the whole RICH team and then the LHCb collaboration for their help and interest.

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RICH2019: A proposal for the LHCb-RICH upgrade

In the Wake of RICH2008

Proponents1: C. D'Ambrosio, S.Easo, C. Frei, A. Petrolini

Abstract

A proposal for a two-RICH system upgrade for 2019 is presented. The proposal relies on the experience gained in running the current RICHes. We apply small changes to the layout of RICH1, in order to reduce the expected occupancies to acceptable values and improve performance at high luminosities, while retaining the current RICH2 layout. As such, the proposed upgrade appears to be a well-known, safe and robust option. Regardless of the chosen option and in addition to the change of the layout, the current HPDs will have to be replaced with a new photon detector. At present the baseline choice is the Multi anode Photon Multiplier Tube. The proposed layout, while still requiring optimization and fine-tuning, already shows satisfactory physics performances, which we expect to further improve as the optics performance is demonstrated to be superior to the current layout. It is shown that the proposed new RICH1 can fit into the current infrastructure with minimal engineering work.

1 This work is the conclusion of several upgrade meetings, where most of the RICH community took part. It is therefore almost impossible to name all the people, who directly or indirectly have contributed to it. Allow us therefore to thank first the whole RICH team and then the LHCb collaboration for their help and interest.

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1 Main Considerations for the LHCb RICH system upgrade

LHCb [1] is one of the four major experiments at the LHC, and is dedicated to the study ofCP violation and the rare decay of heavy flavors. It is a forward spectrometer designed toaccept forward-going b-and c-hadrons produced in proton-proton collisions.

The RICH system of the LHCb experiment [2] provides charged particle identification (PID) over a wide momentum range, 2÷100 GeV/c, by using two separate RICH detectors: RICH1,close to the collision point, covering large acceptances (from 25 mrad to 300(H)/250(V)mrad) and 2÷50 GeV/c momentum range; and RICH2, placed just before the calorimetry system, covering from 15 mrad to 120 mrad acceptance and 15÷100 GeV/c momentum range.

The LHC accelerator started at the end of 2009 and ran at a centre-of-mass energy of 7 TeV until the end of 2011, followed by 8 TeV in 2012. The instantaneous luminosity rapidly increased, reaching the LHCb nominal operating value of 2 × 1032 cm 2 s 1 at the end of 2010, and double (4 × 1032 cm 2 s 1 ) for the 2012 year, running at 50 ns bunch spacing and 1.6 visible interactions per bunch crossing.

For its upgrade, LHCb has chosen to implement a flexible software trigger with the capability of processing up to 40 MHz rates and to consequently increase the collision volumeluminosity up to 20 × 1032 cm 2 s 1 (Lumi20) [3]. It is assumed that the upgrade should be ready for running in the second half of 2019 [4].

In this scenario, the present RICH system will have to undergo a series of important upgrades.

The first being the change of the whole opto-electronic chain, as the present photon detector (Hybrid Photon Detector) was designed to output up to 1.1 MHz signal rates, in line with the present LHCb specifications. Our baseline choice is to use Multi anode Photo Multipliers (MaPMTs) with a specific front-end electronics (see Appendix 1 and 2). As in the present RICH system, the price of photon detectors is the biggest contribution to the overall cost.

Figure 1: Estimated occupancies (left-end side and blue color) and average numbers of hits per event (right-end side and green color) in B-triggered events versus luminosities for the present RICH1, instrumented with MaPMTs [5].

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Changing only the photon detector with the present optical layout in place, implies a certaintythat the Particle Identification (PID) performance is unscathed at the upgrade LHC running conditions, which are essentially summarized as follows: up to 20 × 1032 cm 2 s 1 luminosity at 25 ns bunch crossing and a center of mass energy of 14 TeV. This is not at all the case. Fig.1 shows estimated occupancies (left-end side) and average numbers of hits per event(right-end side) in B-triggered events versus luminosities for the present RICH1, instrumented with MaPMTs ([5]). The dependence of PID efficiency upon the previous quantities becomes clear when looking at a typical event produced in both RICH detectors. Fig.2 shows a real triggered event as seen by the present RICH system for the current LHC running conditions (4× 1032 cm 2 s 1 at 50 ns bunch crossing and 8 TeV).

Figure 2: An example of a typical LHCb event as seen by the RICH detectors. The upper/lower HPD panels in RICH1 and the left/right panels in RICH2 are shown separately [2].

size of dots, not to scale

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Average recorded hits per event is in excess of 2000 hit signals in each detector, with tails up to more than 4000 hits per event. In spite of the apparent randomness of the patterns displayed, this event can be perfectly reconstructed by the RICH pattern recognition together with the Tracking system information. However, even if we consider peak occupancies up to ~20% manageable by our pattern recognition, the transition between 10 × 1032 cm 2 s 1

(Lumi10) and Lumi20 seems to result in the necessity of reducing them by modifying the optical design of RICH1. RICH2 highest peak occupancies never reach 7% [5].

2 RICH2019 and RICH2008 upgrade options

Over the previously discussed scenario, we present an upgrade option (RICH2019), which tries to maintain (and improve) the present system performance at the foreseen increased luminosities and energy. As a matter of fact, the present RICH system has been a great success, thus we build our proposal on its proven performance and reliability and on our own experience.

The optical system of RICH1 is modified, but not its position inside the LHCb detector (see Sect.3). Its spherical mirrors radius of curvature increases from 2.7 m up to 3.8 m, therefore halving occupancies. RICH2 is left unchanged. Both are fully instrumented with MaPMTs(RICH2019-F, where F is for Full). Together with RICH2019, we also describe the present RICH1 and RICH2 systems performance at the upgrade conditions (RICH2008F). The detailed optical design and specifications of RICH1 v2019 will be shown in Sect.3 and the fully simulated Detector and PID performances for both RICH2019 and RICH2008 in their F configuration will be shown in Sect.4 (a 3D image of RICH1 v2019 is shown in Fig.3).

Figure 3: 3D image of RICH1 v2019.

Both systems could use lenses in low occupancy areas, in order to reduce MaPMTs numbers and decrease overall cost. This can be done only if the physics performance is not affected,which we do not yet know and based on a trade-off including complexity and costs. Here, we

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will perform the exercise and look at these configurations (we shall call them Lite, L) for both systems only when costing and schedules are discussed (Sect.5). Nevertheless we do not expect important variations in performances for these with respect to the full configurations. Full simulations will start soon to study the L option.

Before closing this section and go to the details of the new configurations, it is worth clarifying the reasons of the choice to maintain two separated RICHes.

RICH1 sits as close as possible to the collision point in order to cover large acceptances (25 mrad ÷ 300(H)/250(V) mrad) (and 2÷50 GeV/c momentum range) with limited detector volume, optical and photon detector surfaces. Apart from the cost (increasing fast with distance from the beam collision point), material budget and technical constraints (ex.: gas parameters, environmental conditions, optical and mechanical stability, etc.) can be controlled well and minimized inside a well-known scenario.

The present (and the future) RICH1 features only a 4.7% of X0 (C4F10 gas, 2.6%, sph. mirror, 1.5% and exit window, 0.6%, no aerogel) and uses the VELO exit window as its own entrance window. The spherical mirror surface is (and will remain) only 2 m2 and the photon detector plane will go from 1.5 m2 to only 1.6 m2 2. Finally, it delivers a very uniform response in photon yield (see Fig.9).

RICH-1 RICH-1 RICH-2present 2019 2008/2019

Acceptance [mrad] 25-300 25-300 15-120% X0 4.7 4.7 15Sph. Mirr. Surf. [m2] 2.1 1.5 8.2

Flat. Mirr Surf. [m2] 2.1 2.5 6.2

Ph. Det. Surf. [m2] 1.5 1.6 2.1

Cherenkov Gas C4F10 C4F10 CF4

Ch. Sig. Gas Vol. [m3] 2.5 2.5 10

Avr. Ph.electron Yield 25 (30)* 35 22

*Value from data (expected) [2].Table 1: Material budget, optical surfaces and photoelectron yields.

RICH2, placed just before the Calorimeters, covers from 15 mrad to 120 mrad acceptance and 15 ÷ 100 GeV/c momentum range. It presents a material budget of ~15% X0, for a spherical mirror surface of 4.1 m2 and a photon detector plane of 2.1 m2. Although its vessel is large, the mirror surfaces are small. This allows the engineering of a stable, totally elastic and light mechanics (honeycomb flat panels). The gas volume involved in the Cherenkov light signal

2 These figures could be compared (with due care) to the TRIDENT option [6]. For example, the spherical mirror surface is ~48 m2, to be made of highly precise and stable mirrors in order to maintain the expected resolution. This is needed to cover the 300 mrad (250 mrad) horizontal (vertical) acceptance requirement. The material budget of TRIDENT will also be important, possibly around 30% in the wide acceptance area.

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generation is only ~10 m3, which minimizes fluctuations due to pressure and temperature variations of the gas radiator. These parameters are presented in Table 1.

Last but not least, an important factor comes into play about this upgrade. In order to be ready for the integration in the pit, the upgraded system will have to be ready for the end of year 2017. The conception, design, engineering, fabrication and procurement of highly specialized manufacts, the quality controls and tests, and finally its construction, all these will have to be achieved during three years of LHC running and one of maintenance and consolidation (LS1), in which our resources will be all stretched to deliver the best physics for LHCb. Therefore, we have decided to optimize our resources, going for a concept, a technique and a technology, which have demonstrated high reliability and excellent performance in these four running LHC years. And that we know extremely well in all their facets. Finally, it should improve and ease a schedule, which is already very heavy, due to the challenging development of the new opto-electronic chain.

3 Optical layout of RICH2019

Configuration: modified RICH1 optical system and current RICH2.

In principle, minimal changes are required to the current RICH1 to reduce the high-occupancy ( 30%) attained with the current optics. This is accomplished by increasing the focal length of the spherical mirror by a factor 2, which in turn halves the occupancy (for a discussion on the need of keeping it low, see Sect.4 . A realistic option was developed, which fits into the currently perceived constraints, like for example the space presently occupied by RICH1 and therefore available.

As a consequence of the increased focal length, the Photon Detector (PD) Plane is moved further away from the beam-line, letting it fit into the available space with minimal engineering changes. This is a safe and well-known scenario, easily extrapolated from construction and years of experience with the current RICH1. We refer, for illustration purposes, to Figure 4: Layout of the current RICH1, from the RICH1 EDR [7] together with the newly proposed RICH1 v2019.

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Figure 4: Layout of the current RICH1, from the RICH1 EDR [7] together with the newly proposed RICH1 v2019.

3.1 Constraints (from the RICH1 EDR, LHCb-2004-121)The following constraints have been tentatively, and conservatively, assumed [7]:

The allowed region in z is: zMIN 1118.5 mm z zMAX 2005.1 mm.The allowed region in y is assumed to be: 0 mm y 2000 mm.The allowed region in x is assumed to be: < ±1000 mmHorizontal acceptance: 300 mrad.Vertical acceptance: 250 mrad.

3.2 Assumptions

The following assumptions are used in the design of the new optical layout:1. Radius of the spherical mirror R=3800mm. Minimum possible tilt of the spherical mirror

required to drive all photons to the plane mirror, in order to have smaller aberrations and longer track path length in the gas radiator. Point 2 is located at zMAX.

2. The flat mirror is located just outside the geometrical acceptance, to avoid shadowing while minimizing the tilting of the spherical mirror. Point 4 is located at zMIN. The plane mirror is tilted to have the focal surface roughly centered in the z coordinate, in order to find space for the installation of the PD-assembly without exceeding the limits in z.

3. The PD plane is tilted to have zero average photon incidence angle projected on the y-zplane in order to improve the efficiency of the MaPMT and of the possible light collector system, in order to reduce the shadowing of the magnetic shield of the MaPMT and to reduce the size of the PD plane. This is made possible by the large depth of focus, thus allowing defocusing on lengths of a few hundreds of mm with little changes to the resulting resolutions.

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3.3 Optical layout geometry specs

The optimization of the optical layout was carried out using a professional optical CAD software3. Its geometry was determined as follows:1. Straight tracks were generated inside the acceptance; for each track saturated Cherenkov

photons were generated along the track at fixed azimuth; the best-focus point of this set of photons was determined. A fit to a plane (the PD-plane) was carried out on the set of best-focus points obtained by many tracks.

2. The PD plane was tilted to have zero average incidence angle projected on the y-z plane.3. The PD-plane was moved closer to the beam-line, defocusing, as long as the spot size

stays less than the finite pixel size uncertainty, in order to reduce the size needed in the vertical direction, and to stay inside the z limits of RICH1.

The optical layout (upper half only), in the vertical plane and its 3D view are shown in Figure 5: Optical layout, y-z projection and 3-D view of RICH1 v.2019. The key geometrical data (refer to Figure 4: Layout of the current RICH1, from the RICH1 EDR [7] together with the newly proposed RICH1 v2019.) are shown in Table 2.

Coordinatesz (mm) y (mm)

IP 0 0Center of Curvature of the spherical mirror -1740.59 1193.25Point 1 1867.20 0Point 2 2005.10 553.07Point 3 1352.79 345.42Point 4 1118.50 1162.50Point 8 1663.72 1737.62AnglesTilting of the axis of the spherical mirror with respect to the beam axis 14.00°Tilting of plane mirror with respect to the beam axis 16.00°Tilting of PD-plane with respect to the beam axis 68.28°

Table 2

3 Optica 3, http://www.opticasoftware.com and Wolfram Mathematica 9, http://www.wolfram.com/mathematica.

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Figure 5: Optical layout, y-z projection and 3-D view of RICH1 v.2019.

3.4 Optical performance

The first performance figure is the quality of the focusing of the optics. A good enough focusing is a necessary but not a sufficient condition for a good RICH performance.The optics was optimized first stand-alone in order not to mix spurious effects and to differentiate the various effects and contributions to the total Cherenkov angle uncertainty.

The following performance figures are obtained.1. RMS spot size on the PD-plane: 1.11 mm +/- 0.03 mm; maximum spot size diameter: 3.3

mm (to be compared with the pixel size of the MaPMT: 2.9 mm).2. PD-plane size for full acceptance: 580 mm × 1410 mm (about 1940 MaPMT @ 29 mm

pitch).3. Average photon incidence angle on the PD-plane: 2.4°.

Together with these purely optical figures, more errors add up when the actual photon detector, the gas radiator and Cherenkov radiation specific emission are included in the design. These are, typically and respectively, the pixel size, the chromatic and the emission point errors.

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3.5 Evaluation of the optical performance

In order to evaluate the purely optical performance with the physical elements included in the design, a stand-alone simulation/reconstruction was setup based on the same framework used to optimize the optics. Moreover the same simulation was used to cross-check the estimates of the chromatic and pixel size errors and to test their independence, allowing to sum all them in quadrature. x and

y, and saturated Cherenkov photons were emitted randomly along the track accounting for the chromatic error and the finite pixel size. When the starting and arrival point of a photon are known the trajectory is unambiguously determined by Fermat’s principle. Therefore, after assuming that all photons were emitted at the middle of the track, the emission Cherenkov angles were reconstructed and compared with the true Cherenkov angles. The resulting single-photon errors are summarized in the Tab.3 for the RICH2008 configuration (present configuration instrumented with MaPMTs) and for the RICH2019 configuration, RICH2 configuration being the same for both (RICH2 v.2008). The different contributions were determined by switching-off in turn the various error sources in all possible combinations.

RICH1 v2008 RICH1 v2019 RICH2 Notes

Emission Point Error 0.57 mrad 0.43 mrad 0.24 mrad Dependent on track direction

Chromatic Error 0.59 mrad 0.59 mrad 0.24 mrad

Pixel Size Error 0.62 mrad 0.44 mrad 0.19 mrad Assuming 2.9 mm pixel size.

Combined Error 1.02 mrad 0.85 mrad 0.39 mrad Single photon Ch. angle res.

Table 3: Optical resolutions

Tab.3 shows that the newly proposed optics performs better than the current RICH1 optics: the reduced aberrations coming from the larger focal length overcome the adverse effect of the about 2° larger tilt of the optical axis of the mirror. Moreover the combined overall error corresponds to summing the three contributions in quadrature, showing that the three error sources are uncorrelated. It should be noted that the emission point error strongly depends on

x y of the track inside the acceptance. The values in the table are the median of distributions of the emission point error for tracks at different angles.

Clearly, in addition to the above errors sources, which are the main drivers of the detector design, the total single-photon resolution is affected by many other factors in the real LHCb environment which require a full-simulation but are independent of any optics design. These include: errors on the track, effect of curved low-momentum tracks, particle conversion, photon background, etc., which tend to enlarge the Cherenkov angle distributions and to generate tails. The overall resolution in a realistic situation is discussed in the coming section4, where the results of a full simulation will be shown.

4 Physics Performance Evaluation

The physics performance criteria are based on the following parameters, all extracted by running a full simulation:

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1. Single photon Cherenkov angle resolutions. This is the evolution of the number given in Table 3, adding the realistic conditions of the LHCb experiment.

2. Photoelectron yields.3. Hit occupancy.4. PID performance.

The resolutions and yields are estimated with particle gun events with saturated tracks, while the occupancy and the PID performance are determined with Bs events.

The PID performance depends on the other three parameters in this list, on the trackingsystems performance and on the pattern and reconstruction algorithms, specifically applied.The first three parameters will be discussed in Sexct.4.1, while PID performances in Sect.4.2. It is worth noting that the performance is evaluated by implementing the whole simulated present LHCb geometry, which uses GEANT4. The simulations do not include spill over and, apart from assuming binary readout for the data, no attempt is yet made to simulate a real front-end readout chain. Finally, the events generated from the simulations are subjected to the standard LHCb reconstruction and particle identification algorithms to evaluate the PID performance.

4.1 Detector Performance Evaluation

The modelling of the R11265 Hamamatsu MaPMTs for the simulation is described here, while details on the hardware can be found in Appendix 1. They have an active area of 23 mm × 23 mm, divided into an array of 8 × 8 pixels. The collection efficiency at the first dynode is assumed to be 0.9. The Quantum Efficiency curves used for the bi-alkali photocathodes for these types of MaPMTs are shown in Fig.A-2. For the simulations, the Super Bialkali (SBA) version with a borosilicate window is used.

Figure 6: Panoramix picture of Modules and MaPMTs

The MaPMTs are arranged in modules. In the simulations, each module contains a set of 16 MaPMTs configured as a 4 × 4 array with a pitch of 28 mm and the size of each module is assumed to be 113.5 mm × 113.5 mm. Fig.6 shows a panoramix picture of one module.

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We are showing the RICH2008F and RICH2019F options. They are listed in Tab.4,specifically for RICH14. Panoramix pictures of the RICH1 v2008 geometry is shown in Fig.7and RICH1 v2019 geometry is shown in Fig.8.

Optics Version Description ROCRICH1 V2008 Current RICH1 Optics 2710 mmRICH1 V2019 Improve RICH1, reduce occupancies, keep the same space 3800 mm

Table 4: RICH1 optics versions. ROC= Radius of curvature of the spherical mirror

Figure 7: RICH1 v2008 panoramix picture

4 In order to understand the performance limits of RICH1 v2019, we have tested a few more options. In particular, an "horizontal" and an “ideal” RICH1 have been run through the full simulation process. This lastlayout was intended to minimize mirrors aberrations and to maximize photoelectron yields. Although impossible to build, as it occupies some VELO space, it provides us with important clues on how to improve the present version (see [8] and [9]).

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Figure 8: RICH1 v2019 panoramix picture

4.1.1 Resolutions and photon yields for RICH 2008 and RICH2019

The overall error and its components for the RICH1 v2019 geometry are listed in Tab.5. In Fig.8, the corresponding distributions are shown. For a discussion on how these were obtained see [8]. In addition to these is an uncertainty coming from the measurement of the direction of the charged tracks, which is estimated to be 0.4 mrad for all cases.

Resolutions RICH1 v2008 RICH1 v2019 RICH2

Emission Point [mrad] 0.63 0.47 0.37

Chromatic [mrad] 0.58 0.58 0.32

Pixel [mrad] 0.61 0.42 0.19

Overall [mrad] 1.05 0.85 0.52

Photoelec. Yield 32 (rms=7.6) 35 (rms=7.6) 22 (rms=5.)

Table 5: Single photon resolutions (rms) for optical configurations after full simulation. All units are in mrad. Last row shows the expected photoelectron yields with their rms.

Photoelectron yield is defined as the number of photons generated by a track and detected (Hits). The average yields obtained for the different optical configurations are listed in Tab.5, last row. The plots are shown in Fig.9. It is worth noting the nicely gaussian shaped distributions.

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Figure 9: Photoelectron yields for a) RICH1 v2019, b) RICH1 v2008 and c) RICH2

c)

b)

a)

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4.1.2 Hit Occupancies

The hit occupancy and the PID performance are evaluated at different run configurations.These are listed in Table 3. All the events used are for Bs events with no spill over.

Table 6: Run Conditions. Here LB is the luminosity per bunch crossing.

The Hit Occupancy is defined as the number of hits per pixel in percent, averaged over one MaPMT. For each MaPMT, it is shown in Fig.10 for a) Lumi2, b) Lumi10 and c) Lumi20 run conditions of RICH1 v2008. Fig.11 shows the occupancies of RICH1 v2019 for Lumi20 run condition. The X-axis in the figures is the MaPMT-id number and there is one entry per MaPMT. The high occupancy in RICH1 v2008 seen at Lumi20 is well over a comfortable value. Although this is only in a small region of the PD plane (about 5% of the total), with its close surroundings it represents most of the event. By increasing by a factor 2, the mirror focal length, we expect to halve occupancies at Lumi20. This is shown in Fig.11 for the RICH2019 configuration.

Luminositycm-2 s-1

# bunches LB cm-2 s-1 Beam Energy(TeV)

Lumi2-S 3.2 x 10 32 1300 0.247x1030 3.5 2

Lumi10-S 10 X 10 32 2400 0.433x1030 7 3.9

Lumi20-S 20 x 10 32 2670 0.749x1030 7 6.8

a) b)

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Figure 10: Occupancy in percent for RICH1 v2008, a) Lumi2, b) Lumi10 and c) Lumi20.

Figure 11: Occupancy in percent for RICH1 v2019 at Lumi20. The factor 2 decrease is almost exactly respected, as the difference is due to the higher photoelectron yield of RICH1 v2019.

c)

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At this point, we are quite comfortable to go into PID performances and compare the different options. The Detector performance, through its resolutions and occupancies at high luminosity, seem to be adapted to the following challenge.

4.2 PID performance with full simulation

The PID performance is obtained after running through the whole reconstruction usingstandard LHCb software. This software runs daily on real data and provides LHCb with excellent hadronic particle identification. PID performance is measured in terms of the probability for misidentifying a kaon as a pion versus the efficiency for identifying a kaonover a wide range of momenta. In the ideal case, those quantities should be 0 and 1 respectively. It is usually plotted for different cuts of the delta-log-likelihood difference between kaons and pions [10]. The basic algorithm used for particle identification is described elsewhere [11] . The PID performance is calculated for tracks above 1.5 GeV/c momentum and 0.5 GeV/c transverse momentum. Tracks in the full acceptance up to 300 mrad are used for this. For all the RICH1 configurations, the data from current RICH2 is used in the PID algorithm. Hence the performance obtained is that of the whole RICH system.

In Figs 12 and 13, the PID performances of RICH2008F and of RICH2019F are respectivelyshown for the three run conditions. In Fig.14, the PID performance is compared between different options. In this Figure the plots from Fig.12 are superposed with Lumi20 performances from RICH2019. We would like to stress the fact that RICH2019F performances are still in a preliminary phase of assessment, and therefore to be taken with "a pinch of salt".

Figure 12: PID performance of the RICH2008F for the three run conditions

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Figure 13: PID performance of the RICH2019F for the three run conditions

Figure 14: PID performance of RICH2008F for the three run conditions, with superimposed the RICH2019F (in green).

For both configurations, increasing the luminosity (and therefore the occupancy) degrades the PID performance. However, Fig.14 shows the improvement achieved with RICH2019F. In particular, RICH2008F at Lumi10 coincides quite well with RICH2019F at Lumi 20. Knowing that these two configurations feature roughly the same occupancies at the indicated luminosities (see Figs 10b and 11), the result seems to be as expected. However we are

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convinced that we can improve its performance with further optimization, to get as close as possible to the present RICH system performance at Lumi2. Moreover, we must always keep in mind that the occupancies have been reduced by a factor 2 and this represents an important safety margin, which does not necessarily appear in the Figure.

5 Evaluation of Costs and Lens Option

For the costing of the RICH2019 option, we start from the tables presented in [12] to arrive to Tab.7. It shows the RICH2008 and 2019 costing with Full (F) configuration and with Lite (L) configuration, in which we use lenses to decrease the number of MaPMTs where possible.

Columns 1 and 3 show the cost for the RICH2008F and RICH2019F versions respectively. We tend to cost the detector in terms of number of MaPMTs. Column 5 (RICH2019 zero) shows that this accounts for ~70% of the total. Of this, ~15% is the cost of the front-end board.

It is also worth noting that the number of MaPMTs used for the Full configuration covers exactly the present PD plane. However, our experience with real data and performance shows that it is possible to reduce this surface to at least ~75% of the present value5). This is a very important reduction in number of MaPMTs to be acquired. From the 2560 MaPMTs used for RICH2, it could be reduced to around 2000 (a cost saving of 25%). However, following to the letter our present simulation conditions, we do not yet apply this important factor. Shortlystudies will be focussed on the optimization of the PD plane surfaces. An example is shown in Fig.15 with its associated table.

Figure 15: 2D image of the occupancy in RICH1 v2019 at Lumi20, when cut at different levels (0, 1%, 3%). The reduction in equivalent number of tubes is impressive, signalling that most of the PD surface is at low very occupancy.[13] 5 Indeed, this is the present running configuration for RICH2, where essentially 4 edge rows are filled with anodes, with no loss of physics.

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Table 7: Cost estimation for various options (see text)

Lite: partially covered with lensesFull: no lensesRICH2019: 1960 data links [12]RICH2008: 1570 data links [12]1)Price per MaPMT ~10% lower (>5000 total MaPMTs)

Column 2 and 4 show the equivalent costing in case of using lenses in regions of very low occupancies (<1.0% at Lumi20) in order to decrease the number of MaPMTs. Although all along this note, the detector and PID performances are evaluated without using lenses, we feel it is important to point out this option.

RICH2 has demonstrated a large safety margin in terms of performance and it is overdesigned in its optical resolution. By applying a lens with a ~1.4 demagnification in front of a MaPMT,its effective detection surface becomes double. The loss of pixel resolution is also a factor 1.4, but it does not affect the total resolution, dominated by other factors (see Sects 3 and 4). Therefore, by optimizing the outer regions, leaving the RICH2 central region with bare MaPMTs (a bit less than 1/2 of the total surface) and covering the rest with the MaPMT +lens system, we find a number of ~1500 MaPMTs.

For RICH1 v.2019, we follow the same reasoning [14], and we find a number of 1400 MaPMTs, using strictly the same lens system as for RICH2. In this case, the surface covered with lenses is also ~1/2 of the total. The lens system can be applied on the photon detector plane of RICH1 v.2019, due to its better resolution. This is not the case for RICH1 v.2008,where we have not envisaged the use of lenses (the number of MaPMTs remains 1152 also in the Lite option). The optoelectronic chain (from MaPMT to GBT) is the same everywhere and only two types of mechanics will be needed for the whole system, by using the same lens.

Finally, it is essential to realize that the intensity of data flow will not change between different options, when the photon yields are similar and for the same performance. For example, a lower number of MaPMTs with lenses will reduce the resolution, but not the event

Syst. Version RICH2008 RICH2019Full Lite Full1) Lite Zero

No of MaPMTs 3712 2652 4480 2900 0Cost in kCHFMech.&optics 365 512 1268 1363 1201Services 250 250 250 250 250MaPMTs 4950 3536 5374 3867 0Electr&DAQ 2771 2310 3096 2647 1523

Total 8336 6608 9988 8127 2974

Spare MaPMT (11%) 507 361 576 396 0

Tot. w. cont (15%) 10169 8014 12149 9801 3420

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data size (zero suppressed) or reducing occupancies by "spreading" them on a larger surface, will not reduce it either. Therefore, all what is related to DAQ in the Table is kept untouched for reason of consistency (although numbers have to be verified between options).

We are careful not to speculate further. All these numbers can and should be further optimized, however only with the constant validation of the full PID performance simulation.

6 Possible Timeline for RICH2019

6.1 Consideration on the mechanical design and timeline

Changing the optics to adopt the R=3.8 m radius of curvature, will require to replace most of the mechanical components of the current RICH1 detector (Figs 16, 17 and 18). However, the design and the production of all the new systems will clearly benefit from the experience that has been acquired from the construction of the present RICH1. This advantage will be notable in terms of manpower, cost and timescale. The construction will be essentially similar to the existing design. As the current RICH1 has given an excellent performance, we can be sure onthe validity of this proposed new version.

Some of the crucial components are the optics and their supports. From our experience with composite mirrors carried out for the current RICH1, we can safely envisage the same optionfor the new RICH1. We think that some efforts to find alternative manufacturers for such components would be valuable. Similarly, new flat mirrors made of glass and quartz windows will be required. In our experience, all these procurements will demand a close follow-up on the production, test and quality control.

Figure 16: View from the top of present mechanical envelope with v2008 (blue) and v2019 (red) optical surfaces (projected on the x-z plane).

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Following the timescale that was needed to design, produce, install and commission the present RICH1 detector, the period 2014-2019 is tight but still suitable for accomplish theupgraded detector. Having said that, it should also not be forgotten that part of the resourcesin the RICH team will still be devoted to the operation and maintenance of the current RICH system

A general timeline for the mechanical systems is envisaged as follow:

- Complete the simulation for physics performance and finalizing the optics layout;- Update the optics design of the detector (main dimensions defined, envelope);- Perform a first magnetic simulation with the new layout of the shielding (this shield

the MaPMTs AND injects B field just after VeLo);- Conceptual (mechanic) design of the detectors.

10/2013 Technical Design Review:

In 2014 it follows the mechanical design of the different systems (including finite element analysis, verification and tests for new components, etc.).List of main systems (WPs)

- Magnetic shielding,o Upper and lower shieldo Supportso Interface with UT detector

- Gas enclosure:o Structure/Shelf.o Side doors.o Seal to VeLo.6

o Exit window7

o Seal to beampipe (downstream)6

o Interface with optical elements (mirror supports, quartz, photon detector assemblies).

o Interfacing with the gas distribution module. - Optical Elements:

o Spherical mirrors + kinematic supports.o Flat mirrors + kinematic supports.o Quartz windows.o Calibration system for measuring the magnetic distortion

- Photon detector assemblies:o Mechanical support.o Enclosure: light+gas tight box.o Module of the photon detectors.o Cooling components for the electronics.o Electric/optics connections.

6 As this stage, one can envisage to re-use the existing one. (Velo team might profit of this opportunity to improvement supports to reduce slightly the radiation length in the experiment, this will be done anyway..) 7 Supposing that the new optics layout stays in the present RICH1 space, the existing window and the seal to the beampipe might be re-used.

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First half 2015 Engineering Design Review

At this stage, all the final design will be frozen, manufacturing drawings prepared, the different production processes established, including final cost and delivery time of the components listed above. Furthermore to the follow up of the fabrication, procurement, test and preparation of the detector, a set of equipment and tooling for installation and maintenance is required:

- Equipment for the new magnetic shields installation.- Equipment for the gas new enclosure installation.- Tooling for the optics installation (mirrors, quartz).- Envisage a permanent access platform or appropriate access and systems for a short

maintenance time of the photon detectors and its electronics- Equipment for dismounting the gas enclosure.8

- Equipment for dismounting the magnetic shielding.7

First half 2016 Production Design Review

6.2 Integration of RICH v2019 in the LHCb experiment

By changing the radius of curvature of the spherical mirror, the focal plane is moved far away from the LHC beam axis. As the room available is limited, the mirror radius has been also tuned in respect to this constrain. The proposed optics fits in the allowed space without requiring any modifications on the concrete of the LHCb cavern. This also requires replacing the two magnetic shielding and the gas enclosure. As shown in Fig.17, the new shielding would have to be extended and will reach the concrete floor. Similarly, the upper shield will be moved up.

Figure 17: Old and proposed new RICH1 mechanical envelopes (not a technical drawing)

8 It might be needed to adapt the existing equipment for an appropriate dismounting procedure.

Comment [CF1]: Added for comparison : current RICH1 and with extendend shield

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When starting to investigate this new optics, the obvious option was to turn horizontally the optics, similar to the ones in RICH2. This configuration has an advantage for the maintenance aspect, where the photon detectors are mounted on each side of the structure, and, moreover, all the Photo-Detector design would be a re-scaled version of the one for RICH2, thus providing considerable savings in terms of manpower for development and design. But the space constrains required for the access to the VeLo and TT detectors do not seem to allow such an option. In any case, from our experience of the RICH1 assembling and maintenance aspect with photon detectors seated above and under, it is important that a detailed study be carried out to insure smooth maintenance.

One important aspect is the removing of the existing magnetic shielding. This operation requires removing services surrounding the RICH1 detector, including the mechanical support of the TT detector and services for the VeLo and TT detectors. However, it appears that many of these services will anyway need to be replaced for the upgraded versions of these neighboring detectors.

It will not be an easy task to integrate RICH1 v2019 in a pit already occupied with other hardware. We have only described a few aspects in order to prepare the interested reader. A preliminary presentation has been given by the LHCb technical coordination [15] and from there it is not clear whether the proposed upgrade can be integrated into LHCb during LS2 (Long Shutdown 2, its time span has not yet been decided, at present ~18 months). From the RICH project side, we still think that we should propose the best possible option inside the present space and time.

In case there would be a demonstrated impossibility to integrate and ready RICH1 v2019 for the year 2019, a compromise solution could be found, where keeping integrally the present mechanics, only the optical system is changed to support a spherical mirror with a radius of curvature around 3.3 m, thanks to the fact that the new opto-electronic chain should stay in anenvelope of ~300 mm (see Fig.18, identical to Fig.4).

Figure 18: RICH1 v2008 and RICH1 v2019 optics represented in the same presentmechanical envelope.

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Conclusions

A proposal for a two-RICH system upgrade for 2019 has been presented called simply RICH2019. It is really an evolution and adaptation of the present RICH system to the future running conditions. The basic idea is to retain the experience and the excellent reliability of the present system and adapt a modified optics layout to all the existing constraints. We have shown that this is not a minimalistic solution in terms of performance, as the last improvesconsiderably together with safety margin for unexpected conditions. However, it is minimalistic in terms of cost, resource and time. In fact it is a realistic proposal, well adapted to the tight schedule which both LHCb operation and general LHC upgrade are imposing. It is challenging too, would be only that the whole opto-electronic chain has to be changedtogether with the optical system. As an example, we are very much focused in achieving acomplete system before the end of year 2014, which can parasitically run in RICH2 during the 2015 - 2017 years. And, would this proposal be accepted, we would all commit to ready a new RICH system for the year 2019. Not a small achievement!

We apply "smooth" changes to the layout of RICH1, in order to reduce the expected occupancies to acceptable values and improve performance at high luminosities, while retaining the current RICH2 layout. As such, the proposed upgrade appears to be a well-known, safe and robust option. However, it is also flexible and improved. It offers better tuning of the optics, smarter use of available space, possible employment of optical adapters to reduce costs and, last but not least, leaves RICH2 optics, mechanics, gas system, etc., which have performed exceptionally well, untouched, to offer both a powerful and reliable system from the very beginning of data acquisition and the chance to be able to focus our main resources on just RICH1 v2019.

With a preliminary PID performance study, the proposed layout already shows satisfactory physics performances. It performs at Lumi20 as RICH2008 does at Lumi10 and we are confident to be able to push this further (the aim being to reach the same performance as RICH2008 at Lumi2).

We have also offered a realistic (although general) timeline for the realization of RICH2019. We are confident that the magnetic shielding (and B injection on VeLo exit) can smoothly be adapted to the new configuration and aware that the momentum resolution between the future VeLo and UT is not suffering from the tiny material budget due to RICH1.

Integration at the pit of RICH1 v2019 appears to be challenging too, due to time and space constraints. This is out of our hands. However, should this be proven a showstopper, we have proposed a compromise, which foresees a modification of the optical system to strictly remain into the present mechanical envelope, though still lowering occupancies and improving performance.

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Appendix 1 ThePhoto-Sensor

The baseline photo-sensor is the R11265 MaPMT from Hamamatsu, readout by dedicated front-end electronics. The main characteristics relevant to the RICH upgrade design are presented here. We note that also TRIDENT would use the same Photo-Sensor as a baseline.

Specifications of the R11265

It is a one-inch square 8×8 multi-anode, fast time-response, 0.8 mm thick UV/borosilicate input window, with SBA/UBA photocathode, 12-stage metal channel dynodes, head-on PMT.One MaPMT is shown in Figure A-1, installed on the custom socket (for 4 MaPMTs) developed for the LHCb RICH upgrade.

Figure A-1: Photo of one R11265 MaPMT installed on the custom socket (for 4 MaPMTs)developed for the LHCb RICH upgrade.

The minimum geometrical sensitive area is 23mm × 23 mm, giving a geometrical acceptance of the bare MaPMT of 0.82. Typical channel to channel uniformity is 1:3. Dark current is 0.4 nA per anode pixel. The typical gain with a standard voltage divider optimized for single-photon detection (that is non tapered, except for the first and last stages) is G=106 at 1 kV supply voltage. The gain increase per 0.1 kV voltage increase is 2.5. The QE curves for bi-alkali photo-cathodes are shown in Figure A-2.

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Figure A-2: QE curves for bialkali photo-cathodes (From Hamamatsu)

UBA features higher QE, higher cost and does not seem at the moment available for mass production yet, so the SBA appears to be a good compromise between photon yield and cost. Transmittance curves for different input windows are shown in Figure A-3

Figure A-3: Transmittance of glasses for PMTs (from: PhotoMultipliers tubes – Basics and Applications – Hamamatsu Corporation).

Choice of the input window

In the choice of the input window, between borosilicate glass and UV glass, a trade-off is required among light transmission (maximized by the UV glass), chromatic error (increasing with the UV glass), ageing due to radiation damage (better for UV glass) and cost. As a

29

baseline for all our simulation, borosilicate glass is being used. Further investigation is being carried out in order to reach a final decision.

Power consumption

67With the standard voltage divider ratio suggested by the manufacturer and 3 MOhm total resistance the power consumption is 333 mW at V=1kV and a gain non-linearity of G/G 0.12 at occupancy 10% is expected. Various solutions are available to reduce the power consumption (if needed) and/or improve the gain non-linearity, by employing more elaborate power supply schemes. For instance, using two more independent voltages at the last and last-but-one dynodes one can obtain a gain non-linearity of G/G 0.04 at occupancy 30% (at the same power consumption of 333 mW at V=1kV). Due to the largely different occupancies in different parts of the PD-plane, different powering schemes might be used, in a trade-off between power-consumption and gain linearity.

Photo-Detection efficiency

In addition to the Quantum Efficiency the photo-electron collection efficiency at the first dynode affects the total photo-detection efficiency. For this type of MaPMT a figure of about 0.9 is usually assumed.

Pixel size

The pixel size of the bare MaPMT is 2.9 mm. The pitch between different MaPMT, forengineering purposes, is estimated to be about 28 mm. Assuming to use an optical adapter to recover just this clearance one would get an effective pixel size of 3.5 mm. Assuming to use the optical adapter to reduce the number of MaPMT by demagnifing a factor 1.4 the effective pixel size would be 4 mm. A reference value of 2.9 mm is assumed in the simulations. In case of use of an optical adapter, the pixel size error can be re-scaled accordingly.

Magnetic shielding

In addition to the overall magnetic shield, every MaPMT has to be protected by individual magnetic shields in order to avoid changes of gain and efficiency losses. Measurements for characterization are on-going. As the global magnetic shielding provides also a local B field just after the VELO for triggering purposes, a detailed study and design will have to be carried out soon for RICH2019.

Optical adapter

The use of a possible optical adapter (a lens system) in front of the MaPMT is being considered. It can be used to recover both the missing geometrical acceptance of the single MaPMT and the dead regions due to the packing of the different MaPMT as well as to reduce the number of required MaPMT in low-occupancy regions. Its use requires a trade-off accounting for losses at the optical surfaces, image degradation, more complex engineering and cost. Different solutions can be considered and have been studied, including: solid tapered light guides made of silica; hollow tapered light guides (Winston cone); one single thin lens near the focal plane (actually an eye-piece to the focusing mirrors); one thick hemi-spherical lens

30

(solid immersion lens), as proposed by Roger Forty; two-lens system (one field lens on the focal surface plus eye-piece), as used both in COMPASS and HERA-B. Its detailed design is strongly dependent on the precise design of the focusing optics and, therefore, it is not attempted at this stage. A study was carried out in the last decade during the development of LHCb [16,17].In any case, on the basis of previous experience, given the long depth of focus of the optics, the almost perpendicular incidence of the photons and the small angular spread of the photons from a single track it is assumed that a simple optical system with demagnification 1.5÷2.0 is realistic.

Appendix 2 The readout electronics, housing of the MaPMT and PD Assembly

While electronics and mechanics will be the subject of separate reviews, including the engineering aspects, we summarize here their most important aspects in order to provide a complete overview of the proposed upgrade.

Front-end electronics readout chain

The MaPMT readout must conform to the upgraded 40 MHz LHCb electronics architecture.Binary readout has been chosen as baseline, as it is the simplest option and minimizes the off-detector data throughput. The binary readout requires the ability to adjust channel-to-channel gain variations of the MaPMTs prior to discrimination in the front-end ASIC, as well as capability to adjust the discriminator thresholds. The development and prototyping of a custom front-end electronics readout chip, named CLARO and tailored to the LHCb RICH application, is underway [18]. It features a response pulse returning to the baseline value before 25 ns, thus eliminating possible spill-over and dead-time effects for LHC operation and very low power-consumption.As an alternative solution, the currently-available MAROC3 chip [19] is being evaluated for its suitability. Simulations will be made to investigate whether the MAROC3 shaping time is compliant with the expected maximum occupancy and whether spill-over and dead-time effects are tolerable. A new version of MAROC is also awaited.The decision on the final readout chip to be adopted will be taken after test-beam operation and radiation testing, and is a major milestone scheduled for 2013.The front-end data flow is shown in Figure A-4.

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Figure A-4: Front-end data flow diagram

Photo-Detector Assembly

The PD-assembly is being designed as a totally modular system, making it adaptable to any option with only minor changes. Moreover, the wide ranges of occupancies require flexibility, in addition to modularity. Fully autonomous functional units, the Elementary Cells (EC), are the atomic fully functional assembly, housing and operating four MaPMTs. A suitable number of EC is then assembled into suitable Photo-Detector Modules (PDM). PDMs are assembled into a super-structure providing overall mechanical support, global thermal active cooling, global magnetic screening and allowing ease of access for maintenance.

MaPMT housing (EC and PDM)

The Elementary Cell (EC) and Photo-Detector Module (PDM) house the MaPMT and all its ancillary systems required for the operation of the MaPMT. The development is based on the experience and ideas developed for building the current RICH as well as similar experiments [16,20,21]. The PDM, already prototyped and tested until 2005, is made of an assembly of ECs, each one integrating four MaPMTs. The base-board of the Elementary Cell is a thick PCB, with custom sockets to house four MaPMTs and integrating different functions together, including: mechanical supporting structure for the MaPMT and ancillary systems; electrical connections from/to the MaPMT (readout board, HV, DCS, etc.); voltage divider passive components; passive thermal dissipation components; magnetic shielding; optical adapter, ifneeded. The backside of the PCB hosts the interface towards the front-end readout electronics chain. A few EC are secured to aluminum plates to form a PDM which also hosts the front-end electronics readout boards. The PDM are finally secured to the support super-structure.The magnetic shield of the MaPMT will be also integrated into the Elementary Cell, as well as, if needed, the optical adapter; the latter is strictly connected to the magnetic shielding, and therefore an integrated design is foreseen.The current design for the PDM according to the CLARO option is shown in Figure.A-5.The current design for the PDM according to the MAROC3 option is shown in Figure A-6.

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Figure A-5: PDM for the CLARO option.

33

Figure A-6: PDM for the MAROC3 option

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Super-structure of the photo-detector assembly

The super-structure of the photo-detector assembly interfaces the PDM with the rest of the RICH/LHCb infrastructure, which includes: the overall mechanics, the global thermal and active cooling, the global magnetic screening, and electro-optical cabling.Moreover, the accessibility for RICH1 has to be improved, allowing ease of access for removal of PDM for easy maintenance.

Global magnetic shield

A general mechanical issue is represented by the magnetic shielding of RICH-1 that, in the present configuration, has a special design to convey the fringe field from the dipole magnet towards the beam axis to allow a rough measurement of track momenta using VELO and Trigger Tracker to be used at trigger level. As the baseline is to redesign the layout of the RICH-1 detector, also this special magnetic screen has to be redesigned. In case RICH-1 will be completely removed, a magnetic structure to shape the field in the proximity of the beam axis will be in any case needed.

Appendix 3 Full simulation resolution distributions

Figure A-7: Various distributions from the full simulation. a) Reconstructed Cherenkov angle, showing the chromatic error (Gaussian fit); b) Pixel error contribution; c) Emission Point Error contribution and d) Reconstructed Cherenkov angle showing the overall resolution

Width=0.58 mrad

Width=0.42 mrad

Width=0.47 mrad

Width=0.85 mrad

(a) (b)

(c)

(d)

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References

[1] LHCb collaboration, A.A. Alves Jr. et al., JINST 3 (2008) S08005[2] Performance of the LHCb RICH detector at the LHC, The LHCb RICH Group, CERN-LHCb-DP2012-003, Nov. 27, 2012[3] Letter of Intent for the LHCb Upgrade, The LHCb coll., CERN-LHCC-2011-001[4] Framework TDR for the LHCb Upgrade, The LHCb coll., CERN-LHCC-2012-007[5] Rich upgrade meeting, Sajan Easo, Jun-18-2012[6] TRIDENT, Roger Forty, May-8-2013[7] Figure 4: Layout of the current RICH1, from the RICH1 EDR [7] together with the newly proposed RICH1 v2019.[8] Rich-upgrade-review, Sajan Easo, April-4-2013[9] Rich-upgrade-review, Sajan Easo, May-8-2013[10] Ring Imaging Cherenkov detectors for LHCb, Roger Forty, LHCb 96-005[11] RICH Pattern Recognition for LHCb, Roger Forty, NIMA433 (1999) 257[12] Rich upgrade review, Neville Harnew, April-4-2013[13] LHCb week Upgrade Summary Talks, Carmelo D'Ambrosio, June-22-2012[14] Rich upgrade meeting, Sajan Easo, November-26-2012[15] Rich upgrade review, Rolf Lindner, May-8-2013[16] LHCb 2000-005 RICH - 19/02/2000;[17] Nucl. Instr. and Meth. A 497 (2003) 314 330;[18] P. Carniti, M. De Matteis, A. Giachero, C. Gotti, M. Maino, G. Pessina, “CLARO-CMOS, a very low power ASIC for fast photon counting with pixellated photodetectors”, JINST 7 P11026 (2012), http://iopscience.iop.org/1748-0221/7/11/P11026[19] S. Blin, P. Barrillon, C. De la Taille, “MAROC, a generic photomultiplier readout chip”, Nuclear Science Symposium Conference Record (NSS/MIC), 2010 IEEE, 10.1109/NSSMIC.2010.5874062 .[20] Nucl. Instr. and Meth A 550 (2005) 559 566;[21] Nucl. Instr. and Meth A 518 (2004) 210 212;

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Addendum to the note (C. D'Ambrosio, S. Easo, C. Frei, A. Petrolini)

This addendum to the note is intended to describe the progress of the RICH2019 option, where the magnetic shielding boxes are not modified and the whole optical system is contained into them.

The need of such an approach stems from the fact that installation and integration of RICH2019 into the LHCb detector in 18 months or less during LS2, creates important critical issues and it would need a complicated schedule, which is likely longer than the allowed period. In the note, this new approach was already pointed out as a possible step. Now it has become our main option.

The basic idea is to enlarge as much as possible the gas enclosure only along the Z coordinate (along the beam line) and move the optical system in such a way that it offers the appropriate optical path between the spherical mirror and the photon detector plane, while keeping it inside the present space constraint.

A first optimization towards this goal is discussed in Section 1, the corresponding PID performance results from full simulations are quoted in Section 2 and finally in Section 3 a preliminary schedule and an estimate of cost are given for RICH2019.

Details and discussions on this topic can be also found in the presentations by C. Ambrosio and S.Easo on 17-6-2013 at https://indico.cern.ch/conferenceDisplay.py?confId=257144

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Section 1 : Optimization of Optics

In an effort to minimize the impact on the current infrastructure and on the LS2 schedule of the proposed new RICH1 v.2019, an updated optimized layout is presented, which fits into the currently perceived constraints. This layout features a better optical focusing with respect to all previous options and produces about 10 % more Cherenkov photons.

A careful investigation of the engineering of the current RICH1 led to the constraints shown in Figure AD-1. This figure shows that the spherical mirror can be moved downstream and the plane mirror moved upstream (gaining in total about 18 cm along z) in order to increase the path of the tracks inside the gas radiator, increasing the number of Cherenkov photons, and to allow a reduced tilt of the spherical mirror, reducing optical aberrations.

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Figure AD-5: The mechanical envelopes of the current RICH1

The layout of the re-optimized optics, with radius of curvature of the spherical mirror R=3800mm, is shown in Figure AD-2. The photo-detector plane is placed at the optimal position, to minimize the emission point error, with the constraint of being perpendicular to the direction of the incident photons projected on the vertical plane (for low-angle tracks).

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Figure AD-6: Optical layout of the RICH 2019 optics.

The key geometrical data are shown in Table AD-1 (which is an update of Table 2 in the note and hence supersedes it).

The emission point error, which uniquely characterizes the quality of the focusing of the optics, is improved both with respect to the current RICH1 and to the first RICH 2019 version (thanks to the reduced tilt of the spherical mirror even if the path length of the tracks is increased): the median value is 0.34 mrad, 75% of the tracks have emission point error less than 0.5 mrad, while the maximum value, for tracks at the extreme edges of the acceptance, is

0.8 mrad. These figures makes the emission point error smaller than chromatic error, pixel error and track error, thus rendering little value for any further attempt to optimize the optics.

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Coordinatesz (mm) y (mm)

IP 0 0Center of Curvature of the spherical mirror -1639.00 977.51Point 1 2032.13 0Point 2 2145.00 640.12Point 3 1320.33 337.14Point 4 1100.00 1189.17Point 8 1694.71 1492.33AnglesTilting of the axis of the spherical mirror with respect to the beam axis 10.00°Tilting of the plane mirror with respect to the beam axis 14.50°Tilting of PD-plane with respect to the beam axis 58.80°PD-plane area (requiring full geometrical acceptance)x-size 1386in the y-z plane 637

Table AD-1

The average angular-to-linear magnification (size on the PD-plane divide by Cherenkov angle) is 1.9 mm/mrad (depends on the track angle and on the direction on the PD-plane). The average incidence angle of the photons on the PD-plane is 3°.

The space behind the focal plane (the blue box in the figure) is about 27 cm deep. In the framework of the currently perceived constraints this appears to be a workable starting point for further engineering studies.

It should be noted that this space could be increased, if necessary, by moving the PD-plane closer to the beam line, either by accepting a slightly worse focusing of the optics and/or by slightly reducing the radius of curvature of the mirror.

Section 2 : Results from full simulations

Compared to the geometry described in the main note, the shift of the spherical mirror downstream has improved the photoelectron yield and the reduction of the spherical mirror tilt has improved the emission point error. This version of the optics is labeled as ‘v7G’ and it indicates current status of the optimization performed for RICH-2019. Figure AD-3 shows the

41

photoelectron yield for this version. Table AD-2 shows a comparison of the photoelectron yields and single photon resolutions for different versions including those for v7G, using full simulations. The corresponding plots for the single photon resolutions for v7G can be found in the appendix of this addendum.

Figure AD-3: RICH1 Photoelectron yield for v7G optics

C4F

10

42

C4F10 RICH1-2008

v0-HPD

RICH1-v2008

v0-PMT

RICH1-v2019

v7G-PMT

Emission point [mrad]

0.61, QW:0.12 0.63 0.37

Chromatic [mrad] 0.84 0.58 0.58

Pixel [mrad] 0.6, PSF=0.79 0.61 0.44

Overall[mrad]

Overall+Track[mrad]

1.45

1.50

1.05

1.12

0.78

0.88

Photoelectron yield 34 32 40

Table AD-2: Single photon resolutions and yields for RICH1. Here a track resolution of 0.4 mrad used to get the combined resolution in the row labeled “Overall+Track”. In column 2 the QW refer to the resolution component from curved HPD quartz window and PSF refer to the resolution component from HPD point spread function.

To determine the PID performance, 20000 events of Bs were generated and processed through the full chain of LHCb software for simulation and reconstruction. This used standard LHCb software, available for several years as described in the paper [22] and was used for the results in the RICH detector paper [23] . The results from this, for v7G optics are shown in Figures AD-4 a,b and c. The excellent performance from this last version can be seen from Figure AD-5. The performance at Lumi20-S for the v7G version of RICH2019 is compared to that of the RICH2008 in Figure AD-6.

43

(a)

(b)

mis-id

(c)

mis-id

efficiency

efficiency

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Figure AD-4: Mis-id versus efficiency plots for a) Lumi2-S, b) Lumi10-S and c) Lumi20-Srespectively. The different curves correspond to different minimum PT cuts. All curves are for the v7G version of optics. The mis-id and efficiency are plotted in percent.

Figure AD-5: PID performance curves for RICH2019 (V7G version) at different luminosity for the same minimum PT cut (PT>0.5 GeV/c ). The black curve is for lumi2-S, blue curve is for lumi10-S and the red curve is for Lumi20-S.

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Figure AD-6: Comparison between RICH2008 (V0 Optics) and the newly optimized RICH2019 (V7G Optics). All plots are with PT>0.5 GeV/c and all use PMTs. The black (Lumi2-S), blue(Lumi10-S) and red (Lumi20-S) curves are for RICH-2008. The yellow (Lumi20-S) curve is for v7G optics.

Section 3 : Schedule and cost estimates

The previous estimated schedule, on top of the 18 months LS2 foreseen schedule (here called FW-TDR schedule), is shown in Figure AD-7 a), while the lightened schedule, without magnetic shielding boxes modifications is shown in Figure AD-7 b). This last is simply stemming from AD-7a, without the work on the magnetic shielding. Further optimizations and parallelisms with the present FW-TDR will be searched for, to insure a comfortable installation, integration and commissioning of RICH2019 inside the LS2 time span.

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Figure AD-7 a

The estimated cost of the RICH2019 Full MaPMT and Lite MaPMT options is shown in Table AD-3.

These costs have to be taken with a pinch of salt and represent in our view the highest price (Full) and the lowest (Lite). Optimizing the detector will likely bring more cost reduction. The photo detector plane is not yet optimized in physics performance versus number for MaPMTs. RICH2 v2019 has not gone through any cost optimization process yet and this will carry further cost reductions in our opinion.

47

Figure AD-7 b

48

Syst. Version RICH2019

Full1) Lite

No of MaPMTs 4480 2900

Cost in kCHF

Mech.&optics 1085 1180

Services 50 50

MaPMTs 5374 3867

Electr&DAQ 3003 2151

Total 9512 7248

Spare MaPMT (11%) 576 396

Tot. w. cont (15%) 11571 8821

Lite: partially covered with lenses

Full: no lenses

1)Price per MaPMT ~10% lower (>5000 total MaPMTs)

Table AD-3: Estimated costs of RICH2019, Full and Lite versions.

Conclusions

In short, we have shown that RICH2019 can be built without modification of the magnetic shielding and provide excellent particle identification for the LHCb upgrade experiment. It can be produced before the start of LS2 and installed during LS2. The overall cost seems to be under control and further optimization will quite likely reduce the cost . This would finalize the design on firmer basis.

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Appendix:

Plots for the single photon resolutions for ‘v7G’.

Figure AD-8: Resolutions for the v7G optics. Cherenkov angle reconstructed in different ways showing (a) Chromatic error (Gaussian fit) (b) Pixel error (c) Emission Point error (d) Overall resolution (Gaussian fit ).

References

[22] Simulation of LHCb RICH Detectors using GEANT4. S.Easo et.al., IEEE.Trans. Nucl. Sci 52 (2005) 1665

[23] Performance of the LHCb RICH detector at the LHC, The LHCb RICH Collaboration,EPJ C (2013) 73:2431

(a) (b)

(c) (d)

Width=0.58 mrad

Width=0.44 mrad

Width=0.37 mrad

Width=0.78 mrad