Detection methods for long-lived particles at the LHC

1
Detection methods for long-lived particles at the
LHC

• S. Viganò, A. De Min
• Università di Milano Bicocca

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

3
Introduction

• Massive 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)

4
GMSB scenario
Mass spectrum depends on 5 parameters N, M, ?,
tanß, sign(µ) Gravitino is the Lightest
Supersymmetric Particle and has couplings
inversely proportional to its mass. For masses gt
1 eV/c2 decay into LSP has long lifetime.
5
NLSP

• Gravitino 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 -gt Stau
• The lightest Neutralino

6
Experimental possibilities

• NLSP decays into its SM partner and gravitino
  with
• From the experimental point of view there are 3
  possibilities
• ct large compared to detector dimensions
• ct of the order of detector dimensions
• ct small w.r.t. detector dimensions

1. Stau NLSP looks like a heavy muon
2. 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
7
Stable stau signature

• Long-lived stau differs from muons by
  considerably lower b
• 2 main observables useful to distinguish between
  the 2 cases
• Time of flight
• Specific ionization

Muon chambers
Tracker and/or e.m. calorimeter
8
CMS muon chambers layout

• CMS muon system 4 muon stations in between the
  iron yoke slabs
• Geometric coverage up to hlt2.1
• Each barrel muon station (MS) equipped with Drift
  Tubes (DT)
• 12 layers grouped in 3 superlayers per station
• 2 for RF measurement
• 1 for z(q) measurement (0 in the outermost
  chamber)
• Precise tracking with spatial resolution of the
  order of
• 75150 mm per tracking point

9
TOF measurement
CMS
If b1 is assumed in the case of stable stau,
points would not align!

• 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
  b
• is measured in order to minimize ?2 of the
  reconstructed track

Time resolution 1ns (both ATLAS and CMS)
10
Mass and lifetime determination
In each event a couple of NLSP is produced ?
lifetime can be inferred by the ratio N1/N2,
since it holds
Squared mass of the particle determined from the
1/b measurement Unquestionable signal
significance even after 500 pb-1 (1 week _at_ 1033
cm-2s-1)
11
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 density of 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 dont reach the muon
   chambers for which no TOF is available

12
Kinematics
13
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 ADC count 8.4 MeV

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)

Pions
14
Neutralino decays
When the neutralino ct 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 dont point to the interaction
vertex
With the electromagnetic calorimeter either in
ATLAS or in CMS it is possible to determine the
photon impact direction and hence to give
information on neutralino lifetime
15
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 q
• In the barrel
• Efficiency to detect an isolated photon as
  non-pointing as a function of the significance

i.e. Requiring Dq to be non-zero by 5s gives an
efficiency of 82
ATLAS
16
Shower shape of non pointing photons

• CMS 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 a 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 b0.4 rad
  23)

D for different tilts
17
Photon direction from ECALHCAL

• Alternatively, use ECAL-HCAL lever arm to
• determine photon direction. Three difficulties
• Limited space granularity
• Need photon leakage in HCAL, so sensitive for
  high-energy showers (and high-energy neutralinos)
• But photon energy ( ? Neutralino energy)
  negatively correlated with photon angle
  (high-energy Neutralinos produce collinear
  photons!)

ct0 cm
ct500 cm
18
TOF measurement

• If 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

ATLAS
CMS
19
Late shower development
When neutralino ct is in the range 1100 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 ct20 cm Maximal
acceptance at ct10 m
CMS
20
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 )

21
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 _at_ 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 _at_ HEP 2005 (Final test beam
  results from ATLAS electromagnetic calorimeter
  series modules)
• CMS NOTE 2006-037, R. Bruneliere and A. Zabi