Reconstruction and Identification of Hadronic Decays of Taus using the CMS Detector

1. Reconstruction and Identification of Hadronic Decays of Taus using the CMS Detector Michele Pioppi – CERN On behalf of the CMS collaboration TAU 08 Novosibirsk – Russian Federation 25 September 2008 michele.pioppi@cern.ch

2. Outline • The CMS detector • Physics with hadronic t at CMS • Hadronic t reconstruction • Hadronic t identification • t trigger • Conclusions michele.pioppi@cern.ch

3. The CMS detector Resolution Coverage Tracker |h|<2.4 ECAL |h|<3.0 HCAL |h|<3.0 barrel |h|< 5.0 forward Muon |h|<2.4 michele.pioppi@cern.ch

4. Physics with tSM Higgs • qqqqH(tt) VBF Forward jet tagging Central Higgs decay products to trigger michele.pioppi@cern.ch

5. Physics with tMSSM Higgs Neutral Higgs (A.H,h) Charged Higgs (H±) Production mechanism 5 s discovery potential michele.pioppi@cern.ch

6. Physics with tSUSY In mSUGRA models, light mass SUSY can be discovered soon in di-t final states through the decay chain 5 s discovery potential michele.pioppi@cern.ch

7. t reconstruction strategy • Particle Flow reconstruction High efficiency, low fake rate and optimal resolution for each kind of particle • Common t selection used as a basis for all the final states Robustness wrt unexpected detector effects, high t reconstruction efficiency and sufficient QCD background rejection • Sophisticated t identification Suitable and tunable t reco and id algorithms for each individual analysis michele.pioppi@cern.ch

8. The particle flow algorithm Particle Flow consists in identifying and reconstructing each particle in an event followed by the best possible determination of the energy and direction, by including the information of all CMS subdetectors. Jet, tau and missing transverse energy reconstruction is then made from these reconstructed and calibrated particles directly. michele.pioppi@cern.ch

9. t Pre-selection Sample Entries Signal Ztt 250K Background QCD(22) 750K Jet PT >15 GeV/c Lead track PT >5 GeV/c Lead track coneDR<0.1 An iterative tracking approach (allows to have good tracks with only 3hits) is significantly improving the leading track finding michele.pioppi@cern.ch

10. Isolation algorithm All the t decay products are expected to be in a narrow signal cone around the leading track. If the t is isolated an isolation annulus, expected to contain little activity, is defined. t candidates with charged particles (Pt>1GeV/c) and neutrals (Pt>1.5GeV/c) in the isolation cone are rejected michele.pioppi@cern.ch

11. Shrinking vs fixed cone CDF implements a 3D signal cone that shrinks as a function of jet E-1,while historically CMS uses a fixed signal cone (DR=0.07) The shrinking cone is defined in h-f plane and scales as E-1 to be extended in the forward region. In both the cases the Isolation cone = 0.5 michele.pioppi@cern.ch

12. Common selection performance The shrinking cone algorithm improves signal efficiency at the cost of increasing QCD fake rate. The range affected by the cone algorithm is between 20 and 60 GeV/c michele.pioppi@cern.ch

13. Secondary background sources • The common selection is aimed to fight the main source of background (QCD jets) • Secondary sources are in order of importance: • Electrons Due to the high material budget in the tracker, several electrons often emit a significant fraction of energy by radiation. A special treatment is needed to reduce electron contamination • Photons Photons convert frequently, and the isolation is much more difficult for such photons (under study). • Muons Very high identification efficiency michele.pioppi@cern.ch

14. Electron rejection • A veto is applied to all the tracks pre-identified as electrons in the particle-flow • The electron pre-identification is aimed to identify electrons (isolated and within jets) in a wide range of transverse momentum, pseudo-rapidity and physics case • The algorithm uses a multi-variate analysis of information from calorimeters and tracker(more efficient for electrons emitting high-energy Bremmstrahlung photons) • Eff(e)>95% • Eff(p)=5% michele.pioppi@cern.ch

15. Electron rejection The main electron source of background for isolated t are isolated electrons. For such electrons the rejection can be improved by using inclusive calorimetric information. EEcal= sum of the cluster energy in a window (around the extrapolated impact point of the leading track) |Dh|<0.04 and Df<0.5 in the direction of the expected brem photon deposition EHcal= sum of the cluster energy in a window (around the extrapolated impact point of the leading track) in a window DR <0.184 michele.pioppi@cern.ch

16. t trigger The hadronic t trigger is crucial for final states with a single t (e.g H±tn) Level1 trigger relies on pure calorimetric information Three different paths dedicated to t have been designed michele.pioppi@cern.ch

17. t trigger • HLT is composed by 3 steps(Lvl2, Lvl2.5,Lvl3) of increasing complexity • Lvl2 is based on jet reconstruction and isolation • Lvl 2.5/3 is based on tracking seeded by the jet direction Efficiency for SingleTau path michele.pioppi@cern.ch

18. Summary • t physics program in CMS is ambitious • A common and robust selection for hadronic t has been developed • t (Pt>40 GeV/c) eff > 50% • QCD (Pt>40 GeV/c) eff<3% • Sophisticated identification to reduce secondary source of background • Ztt eff 92% • Zee eff 1% • Dedicated trigger for hadronic t in place to achieve the physics goal michele.pioppi@cern.ch