Detection methods for long-lived particles at the LHC

1. Detection methods forlong-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 S. Viganò

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) S. Viganò

4. Λ: effective scale of the breaking Hidden sector SUSY breaking N gauge singlets of SU(5) M: messenger mass scale Messenger sector MSSM Visible sector GMSB scenario 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. S. Viganò

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 -> Stau • The lightest Neutralino S. Viganò

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 • Stau NLSP looks like a heavy muon • 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 S. Viganò

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 S. Viganò

8. CMS muon chambers: layout • CMS muon system: 4 muon stations in between the iron yoke slabs • Geometric coverage: up to |h|<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(q) measurement (0 in the outermost chamber) • Precise tracking with spatial resolution of the order of • 75÷150 mm per tracking point S. Viganò

9. b=1 b<1 TOF measurement CMS If b=1 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 χ2 of the reconstructed track Time resolution: 1ns (both ATLAS and CMS) S. Viganò

10. Precision after 30 fb-1: CMS ATLAS Mass and lifetime determination In each event a couple of NLSP is produced ⇒ lifetime can be inferred by the ratioN1/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 @ 1033 cm-2s-1) S. Viganò

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: • High densityof low momentum tracks (min bias) • Limited resolution (7-10%) • Limited dynamic range in the readout • Saturation effects • Cleaner environment • More gaussian distribution of dE/dx • Sensitive to particles that don’t reach the muon chambers for which no TOF is available S. Viganò

12. Kinematics S. Viganò

13. # Entries Pedestal mip Cosmics GeV 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 • 16% at 250 MeV (1 m.i.p. equivalent) • 4% at 1 GeV (4 m.i.p. equivalent) • 2% at 5 GeV (20 m.i.p. equivalent) Pions S. Viganò

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 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 S. Viganò

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 S. Viganò

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 b=0.4 rad = 23°) D for different tilts S. Viganò

17. Photon direction from ECAL+HCAL • 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!) ct=0 cm ct=500 cm S. Viganò

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 S. Viganò

19. Late shower development When neutralinoct is in the range 1÷100m 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 ct~20 cm Maximal acceptance at ct~10 m CMS S. Viganò

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 ) S. Viganò

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 @ 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 S. Viganò