LHC Luminosity Workshop December 9, 2002 Gregory Snow (University of Nebraska) 1 Toward a...

LHC Luminosity Workshop December 9, 2002 Gregory Snow (University of Nebraska)

1

Toward a Measurement and Monitoring

of pp Luminosity in the CMS Experiment

Gregory R. SnowUniversity of Nebraska, USA(for the CMS Collaboration)

CMS Luminosity Goals and ConstraintsComplementary Techniques ConsideredOccupancy in Very Forward CalorimeterCounting Fast-Reconstructed Vertices

Counting Z’s


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The CMS Detector

Totem T1here

Totem T1here


3

Luminosity Goals and Constraints

GOALS • Aim for luminosity determination to 5% (absolute and relative)• Bunch-by-bunch measurement desired• Best to have redundant, cross-checkable measurements• Provide real-time luminosity information to CMS and LHC

CONSTRAINTS• No new detectors; use calibrated rates in existing CMS subdetectors ORGANIZATION• CMS/TOTEM luminosity working group formed after 2-day Luminosity Workshop in November 1999• Lumi Working Group under umbrella of Forward/Diffractive Physics Working Group


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TOTEM Total Cross Section Measurement

• Measurement of total cross-section (tot) at 14 TeV• Early in LHC running• Low luminosity ( 1028 cm-2 s-1)• Special high insertion, zero crossing angle• Fewer bunches (36 x 36)• Luminosity independent method• Elastic cross section to smallest possible

• During TOTEM running• Calibrate monitors for high-luminosity measurements

t


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Luminosity-Independent Method for tot

inelel

teltot NN

dtdN

0

2

)(

1

16

• Optical theorem relates tot to elastic scattering extrapolated to t = 0

• Must count elastic (Nel) and inelastic (Ninel) interactions

• Elastic to as low-|t| as possible, to minimize extrapolation error

• Correction from (1 + 2) term is small at LHC

• Correction less than 2%, known to about 0.5%

• Aim: few % uncertainty on tot

005.0017.0

02.013.0)0(Im

)0(Re

2

f

f


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Importance of precisemeas. of tot and itscomponents(D0 experience with various tot measurements: CDF, E710, E811)

Determined by MonteCarlo and, for CMS, cross-calibration duringTOTEM tot measurement

Used successfully inD0 employing small-anglescintillator arrays on eachside of I.P.


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CMS Very Forward Calorimeters (HF)

20o HF wedges


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HF

HAD (143 cm)

EM (165 cm)

5mmTo cope with high radiation levels (>1 Grad accumulated in 10 years) the active part is Quartz fibers: the energy measured through the Cerenkov light generated by shower particles.

This is the cause of two of the peculiar features of this calorimeter:

The visible energy is carried by relativistic particles, i.e. electrons: the calorimeter is sensitive to the EM component (0) of the hadronic shower. Shower size depends on Moliere radius not i

The light is generated preferentially at 45 degrees: light propagation is far from ‘usual’ meridian one.

Iron calorimeter Covers 5 > > 3 Total of 1728 towers, i.e.2 x 432 towers for EM and HAD x segmentation (0.175 x 0.175)

HF is fast and transverse shower size is small


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HF Segmentation

• 3 < || < 5• = 0.175 10o = 0.175 0.175• 432 towers per side

“Zeros” based onoccupancy in HF towersabove a fixed ET threshold

Toy Monte Carlo, 2000 zero-bias eventsfrom DPMJET

Outer and inner phi ringseliminated from M.C. study

()


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Event Characteristics

0 Phi distribution 2

0 Theta distribution

distribution

HF HF


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Particle types in zero-bias events

Neutrinos and muonseliminated from

countingLogscale

p

e

K

KL

KS


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Number of interactions/Xing vs. Luminosity

DifferentLuminosity values

set by numberof interactionsper crossing

103

4

10331032

Avera

ge n

o.

of

inte

ract

ion

s/cr

oss

ing

Luminosity (cm-2 s-1)


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Poisson distributed number of interactions

DifferentLuminosity values

set by numberof interactionsper crossing

Most probablevalue shown

in each histogram

Events overlaidby choosing

randomly fromDPMJET sample

5 8

12 18


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Low Luminosity – 1 interaction per Xing

20% of events have zero HF towerswith ET > 10 GeV

These histograms willbe accumulated in the Global Cal Trigger boardsand read out on demand for online monitoring

+

-

HF occupancydistributions

0

0


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Medium Luminosity – 5 ints. Per Xing

HF threshold sensitivitynot severe

HF occupancydistributions vs.threshold inrestricted region of HF

0

0

0


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Medium Luminosity – 6 ints. Per Xing

Threshold sensitivitysmall compared tovariation of zeroswith luminosity

HF occupancydistributions vs.threshold inrestricted region of HF

0

0

0


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Low Luminosity

103410331032

Use all HF towers


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Medium Luminosity

103410331032

Use 4 symmetric slicesof HF towers


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High Luminosity

103410331032

Use 4 symmetric boxesof HF towers


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1%

Boxes

Slices

• Nominally require >1% of bunch crossings to report a “zero”

• Calibrate boxes vs. slices for luminosities where both are above 1%

• Switch to boxes when slices fall below 1%

Transferring Calibration to High L


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Global Calorimeter Trigger Electronics

• The L1 Global Calorimeter Trigger (GCT) receives “trigger primitives” from the Regional Calorimeter Trigger

• Among the trigger primitives are the HF trigger tower (2 towers in azimuth, 3 towers in pseudorapidity) energies – each bunch crossing, no dead-time

• Histograms will be accumulated using the same FPGA-based Trigger Processing Modules as the rest of the GCT

• For each bunch crossing, 64 histogram bins (16 bits/bin) can be arranged to store required occupancy information


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• Linear Impact Parameter in XY based on 2 pixel hits

• IP < 2 mm

• Propagate track candidate to third layer in RZ and find Zexp

Z3=|Zexp-Zrec|<1.5 mm

4 cm

7 cm

0 cmXY

4 cm

7 cm

23 cm

RZ

• Reconstruct PT based on 3 stereo hits and select PT > 0.9GeV/c

A. Giassi & A. Starodumov / INFN-Pisa /

Fast track reconstruction method

Barrel pixel detector


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40% events withPT > 1 GeV, ||<1.6

Fast track reconstruction methodQCD events by PythiaMomentum: PT > 50MeV/cPrimary vertex: xy=15m, z=53mmAcceptance: ||<1.6 (barrel only)Samples: 1, 2, 5, 10 pile-up events


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Preliminary Results

Increasingluminosity

Verticesfound

Interactions/crossing

Fast vertexreco. eff.

Npv Ntot <Nv> r(%) (%)

1

2

5

10

40.01.4

39.71.5

40.00.9

41.61.3

2000

1000

2000

1000

1.0

1.17

2.0

4.16

40

68

97

100


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L (cm-2 s-1) 1026 1027 1028 1034

Rm 2/s 20/s 200/s 2108/s

Time(2000

events)16min 2min 10s < 1s

Scale factor

1 1 1Rm/

RL1RL1/Rtape

Fast vertices reconstructed

per second

Preliminary Results


26

• PixelLines (Danek) for PV reconstruction• Poisson distribution of pile-up events• Highest luminosity values need study• Fake rate study• Systematic error study

Future work


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Counting Z’s

• Improved theoretical cross section calculations for Z l+l- are approaching the precision needed to use inclusive Z production as an absolute luminosity monitor

• V. Khoze et al. [Eur. Phys. J. C19, 313-322 (2001)] predict BR(Z e+e- or +-) = 1.85 nb at 14 TeV with an accuracy of 4% (NNLO, input parton distributions, s, splitting functions, …)

• Other theorists predict uncertainty to shrink to 1-2% by LHC turn-on

• Experimental advantage to monitor both Z e+e- and Z +-

(Different systematic uncertainties involved)

• Given 30 hour luminosity lifetime and nominal 20-hour run, how well can we do, statistically, in a day’s run at LHC design luminosity?


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Counting Z’s

• Inclusive rate of Z +- at L = 1 1034 cm-2s-1 is 18.5 Hz

• Assume 50% recovered (acceptance ||< 2.0, trigger efficiency, …) for monitoring

• 20 hour run (L = 5 1033 by the end) yields 540,000 Z +- • Negligible statistical uncertainty for full run• Every 2.5 minutes, 1000 Z’s yield 3% statistical uncertainty• Statistics not sufficient for bunch-by-bunch monitoring over full run

(Only 200 Z’s per bunch crossing over full run)

• BUT, “monitoring” bandwidth may be restricted to 1-2 Hz out of 100 Hz• Full run yields 144,000 Z’s divided into +- and e+e- • 15 minutes to accumulate 1000 Z’s in each channel for 3% uncertainty

• Ongoing CMS study of evolution of trigger and reconstruction efficiencies over a wide range of pile-up events/luminosity values


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Summary

The CMS Luminosity measurement is conceptualized and needs work for implementation

Firmware for FPGA occupancy histograms on Global Calorimeter Trigger boards

Online monitoring of luminosity for CMS and LHC

Luminosity database for physics analysis

Special run requests

Luminosity Conceptual Design Report in preparation, to be followed

by Technical Design Report

LHC Luminosity Workshop December 9, 2002 Gregory Snow (University of Nebraska) 1 Toward a...

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