Detection methods for long-lived particles at the LHC

S. Viganò, A. De Min

Università di Milano Bicocca

IFAE, 2006-04-19 S. Viganò 2

Outline

• Introduction

• Detection of heavy stable charged particles:

• Detection of non-pointing photons:

• Conclusions

• TOF measurement in muon chambers • dE/dx in the tracker and in ECAL

• From shower shape in the e.m. Calorimeter• From late showers in the muon system• TOF measurement in the e.m. Calorimeter• From ECAL + HCAL

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IntroductionMassive long-lived particles are expected in several models

beyond SM, for examples:

• Models with a weakly broken symmetry, where the particle would be stable if the symmetry were exact

→ for ex. SUSY models with tiny RPV

• Models with an exact symmetry which forbids the decay of heavy exotics into ordinary particles, where the decay into neutral particle is suppressed by small coupling or by phase space

→ for ex. SUSY models with exact RP where the gravitino is the LSP or some other ‘hidden sector’ particle

→ or pseudo-Higgs models with an absolutely stable neutral pseudo-Higgs and a possible very small mass gap to the lightest charged one

In this talk we focus on the second class of models and, in particular, on SUSY model with gauge-mediated SUSY breaking (GMSB)

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GMSB scenarioSUSY breakingHidden sector

N gauge singlets of SU(5)Messenger sector

MSSMVisible sector

Λ: effective scale of the breaking

M: messenger mass scale

Mass spectrum depends on 5 parameters: N, M, Λ, tanβ, sign(μ)

Gravitino is the Lightest Supersymmetric Particle and has couplings inversely proportional to its mass. For masses > 1 eV/c2 decay into LSP has long lifetime.

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NLSPGravitino: light and non-

interacting GMSB phenomenology

defined by Next to Lightest Supersymmetric Particle

NLSP is not univocally determined but depends on GMSB parameters

2 candidates:

The lightest slepton -> Stau

The lightest Neutralino

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Experimental possibilitiesNLSP decays into its SM partner and gravitino with:

From the experimental point of view there are 3 possibilities:

• clarge compared to detector dimensions

• cof the order of detector dimensions

• csmall w.r.t. detector dimensions

45

NLSP TeV1000m

GeV100)m3.1(

Fc

1. Stau NLSP looks like a heavy muon2. Neutralino will escape detection

Both types of NLSP can decay inside the detector and lifetime can be directly measured

All NLSP decay inside the detector

P

GM

Fm

'3

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Stable stau signatureLong-lived stau differs from muons by considerably

lower

main observables useful to distinguish between the 2 cases:

• Time of flight

• Specific ionization

Muon chambers

Tracker and/or e.m. calorimeter

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CMS muon system: 4 muon stations in between the iron yoke slabs

Geometric coverage: up to ||<2.1

Each barrel muon station (MS) equipped with Drift Tubes (DT)

12 layers grouped in 3 superlayers per station:

• 2 for RΦ measurement

• 1 for z() measurement (0 in the outermost chamber)

Precise tracking with spatial resolution of the order of

75÷150 m per tracking point

CMS muon chambers: layout

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TOF measurement=1 <1

Position of a particle determined from the drift time of electrons to anode wires

A starting time t0 (time of flight from the production point to the measuring station) is needed

For the stau: t0 is a free parameter function of

is measured in order to minimize χ2 of the reconstructed track

If =1 is assumed in the case of stable

stau, points would not align!

Time resolution: 1ns

(both ATLAS and CMS)

CMS

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Mass and lifetime determinationSquared mass of the particle determined from the 1/ measurement

Unquestionable signal significance even after 500 pb-1 (1 week @ 1033 cm-2s-1)

In each event a couple of NLSP is produced

⇒ lifetime can be inferred by the ratio N1/N2, since it holds:

pcmLeLP /)(

CMS ATLAS

Precision after 30 fb-1:

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HSCP from dE/dx (CMS)The measurement of specific ionization is a complementary approach• Precise dE/dx only from tracker could be difficult because:

• Information from the ECAL can be useful because:

1. High densityof low momentum tracks (min bias)

2. Limited resolution (7-10%)

3. Limited dynamic range in the readout

4. Saturation effects

1. Cleaner environment

2. More gaussian distribution of dE/dx

3. Sensitive to particles that don’t reach the muon chambers for which no TOF is available

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Kinematics

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HSCP in the CMS ECAL• ECAL can provide non-negligible information on

dE/dx

• Its performance on particles which do not shower has been studied in the context of calibration with cosmic rays → very promising results

• MIPs deposit 11 MeV/cm in PbWO4 → 250 MeV if they pass through a single crystal

• If the tracker is used to determined which crystal is traversed the resolution (considering 40 MeV RMS noise per channel) is:

1. 16% at 250 MeV (1 m.i.p. equivalent)

2. 4% at 1 GeV (4 m.i.p. equivalent)

3. 2% at 5 GeV (20 m.i.p. equivalent)

GeV#

En

trie

s

Pedestalmip

Cosmics

Pions

1 ADC count = 8.4 MeV

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Neutralino decaysWhen the neutralino c is of the order of the detector dimensions (which correspond to gravitino mass in the range 1-100 eV) its decay occurs in the tracker volume and the experimental signature is missing energy (carried by the 2 gravitinos of the event) and 2 high energy photons which don’t point to the interaction vertex

With the electromagnetic calorimeter either in ATLAS or in CMS it is possible to determinethe photon impact direction and hence to give information on neutralino lifetime

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Impact direction reconstruction• The ATLAS e.m. calorimeter is segmented longitudinally so it can provide

directional information

• In his first compartment it has narrow strips that give good resolution in • In the barrel:

• Efficiency to detect an isolated photon as non-pointing as a function of the significance:

)GeV(

mrad60

E

i.e. Requiring to be non-zero by 5 gives an efficiency of 82%

ATLAS

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Shower shape of non pointing photonsCMS ECAL not longitudinally segmented

The impact direction of the photon can be inferred from the asymmetry in the shape of the energy deposition among the crystals

The difference between the length of the two axes A1 and A2 is a good variable to quantify the degree of tilt of non pointing particles

is the degree of asymmetry of the shadow (relative difference between the size of the shadow along and the direction perpendicular to it)

Rejection of pointing photons can be performed at a level of 99% keeping an acceptance for the signal photons bigger than 0.8 (for =0.4 rad = 23°)

for different tilts

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Photon direction from ECAL+HCALAlternatively, use ECAL-HCAL lever

arm to

determine photon direction. Three difficulties:

1) Limited space granularity

2) Need photon leakage in HCAL, so sensitive for high-energy showers (and high-energy neutralinos)

3) But photon energy ( Neutralino energy) negatively correlated with photon angle (high-energy Neutralinos produce collinear photons!)

c=0 cm

c=500 cm

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TOF measurementIf the mass and the momentum distributions of

the neutralinos are determined from other measurements it is possible to convert the rate into a lifetime measurement

Photons from neutralino decaying into gravitino are delayed w.r.t. prompt photons due to the geometry of their path and to the velocity of the neutralino

Mean delay of non-pointing photons of the order of 2 ns

ATLAS e.m. calorimeter has a time resolution of about 100 ps

CMS ECAL better than 1 ns for signal amplitudes greater than 2 GeV (ultimate precision determined by constant term ~ 0.1 ns)

⇒ Independent way to detect photons from neutralino decays

CMS

ATLAS

psGeVE

nst 19

)(

62.1)(

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Late shower developmentWhen neutralinoc is in the range 1÷100 m a large fraction of decays take place inside the muon detector.Photon inside the iron yoke develops an e.m. showers Showers can leak to the muon station ⇒ Muon chambers should register from a few tens to few hundreds of hits

Measured flight path (distance between interaction point and the entrance point to the muon station which is supposed to see the accumulation of hits) depends on neutralino lifetime

This method starts to be sensitive at c~20 cm Maximal acceptance at c~10 m

CMS

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Conclusions

• We have shown that both ATLAS and CMS have sensitivity to massive long-lived particles (such as staus or neutralinos in GMSB models) through direct detection or through their decay products

• Most techniques require some “improper” use of detectors:– Timing information from calorimeters and muon detectors– Showers in muon stations– Asymmetries in shower shapes

• In some cases also the decay lifetime can be measured with a good accuracy (which in GMSB would allow the computation of the SUSY breaking scale F1/2 )

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References• Measurement of the time of flight of stable stau:

– CMS CR 1999-019, M. Kazana, G. Wrochna, and P. Zalewski – Hep-ph/0012192, S. Ambrosiano, B. Mele et al.

• HSCP in the CMS ECAL:– CMS NOTE 2005-023, M. Bonesini, T. Camporesi et al.– C. Marchica, talk @ ECAL TB meeting July 13th 2005

• Neutralino decays into non-pointing photons:– ATLAS Physics TDR (chapter 20)– G. Franzoni PhD Thesis (The CMS electromagnetic calorimeter and its

sensitivity to non pointing photons)– CMS CR 1999-019, M. Kazana, G. Wrochna, and P. Zalewski– F. Tartarelli, talk @ HEP 2005 (Final test beam results from ATLAS

electromagnetic calorimeter series modules)– CMS NOTE 2006-037, R. Bruneliere and A. Zabi