1. 1 Lepton Identification at Hadron Colliders c. mills

2. 2December 13, 2005 Introduction: Leptons in Physics At hadron colliders, QCD processes prevail  Higher cross-section than electroweak Leptons only produced by electroweak processes  Flag for these rarer processes  Used in triggers and “offline” selection  Look for W, Z, top (strong production, weak decay), and … ? Start with general idea, then move to actual implementation

3. 3December 13, 2005 Leptons in a Generic Detector Nature: 3 leptons  e (stable)   (2.2 x 10 -6 s)  Even a 10 GeV muon has a 99.99% chance of escaping the detector (5 m radius) without decaying   (2.9 x 10 -13 s)  Even a 1 TeV tau has an immeasurably small (1 part in 10 45 ) chance to escape the detector Jargon: “lepton” = e or  Decays inside detector, usually hadronically, into a “jet” of particles

4. 4December 13, 2005 ` A Generic Detector Electrons  Track  Stop (shower) in EM calorimeter Muons  Track  Passes through calorimeter  track in muon detector EM cal tracking Hadronic cal. Muon detectors muon electron

5. 5December 13, 2005 Electron Backgrounds Jet: Catch-all term for fakes of hadronic origin  Tracks + energy in calorimeter  Nasty case:  +  0 gives one track + EM energy Photon  Need to pick up a track  Conversion:   e e Muon  Yes, really: Energetic muons can emit bremstrahlung: photon in EM cal + track from muon (rare) Heavy-flavor decay  Real electrons but treated as background: tricky

6. 6December 13, 2005 Muon Backgrounds Less background than electrons in general Jet: Catch-all term for fakes of hadronic origin  Tracks + energy in calorimeter  Nasty case: “punch-throughs”, K decay-in-flight Cosmic rays  Real muons Heavy-flavor decay  Real muons but treated as background: tricky

7. 7December 13, 2005 CDF: A Real Detector Forward-backward and azimuthally symmetric From the beamline outward:  Silicon vertex detector  Drift chamber tracker  Solenoid  Electromagnetic calorimeter (with shower maximum)  Hadronic calorimeter  Shielding  Muon chambers and scintillator Cutaway view of the CDF II detector Protons go in here Interaction point

8. 8December 13, 2005 CDF Tracking Drift chamber tracking  Metal wires in closed chamber full of gas  Charged particle ionizes gas  Alternating R-phi and stereo layers (4 of each) Algorithms reconstruct tracks from hits  Group wires/strips with signal above threshold into clusters = “hits”  Momentum from curvature in 1.4 T field  Use track quality, number of tracks Silicon strip tracking (Solid state)  Charged particle creates electron- hole pairs, apply HV to “collect charge”  Good resolution, radiation tolerance (close to IP)  R-phi, stereo, and Z type layers (7- 8 layers, some double-sided)

9. 9December 13, 2005 CDF Tracking Apparently this is also a “CDF tracker”… The Grumman S-2T Turbine Tracker

10. 10December 13, 2005 CDF Calorimetry High-mass: particle interacts with matter, stops (= transfers all its momentum)  CDF: alternating layers of scintillator, heavy material  Shower develops in heavy material  Collect photons from scintillator Electromagnetic calorimeter stops electrons/photons first (ideally)  Lead-scintillator Hadronic calorimeter stops hadrons  Iron-scintillator Designed to measure particle energy  Very coarse granularity in eta, phi Projective geometry  “Towers” point back at interaction point interaction point forward central one “tower” scintillator iron lead shower maximum detector

11. 11December 13, 2005 CDF “Small Tracking” Shower maximum detectors: electrons  Small, shallow tracking at depth where EM shower peaks  Wire chamber in central, scintillator strips in plug  Better spatial resolution than calorimetry  Run clustering algorithms, like central tracker  ,  location of shower centroid  Shower profile (collimated/ spread out?) Muon chambers  Shallow wire tracker outside of calorimetry, shielding  Short tracks, called stubs, indicate muons

12. 12December 13, 2005 Kinematic vs. ID selection Kinematic = what’s usable  E T or p T cuts  Fiducial (in volume where detector can measure reliably)  Fraction of signal events passing these cuts determined by physics process (Acceptance) Identification (“ID”) cuts assume you have the above, aim is to reject backgrounds  Probability for real lepton to pass is Efficiency  Probability for something else to pass is the Fake Rate

13. 13December 13, 2005 Electron Identification Jet rejection  Calorimeter Isolation: Ratio of energy in a cone around the electron to the electron energy. Jets are wider objects  Track Isolation: Require electron track to be much higher p T than any other track around it  Had/Em: Ratio of energy in the hadronic calorimeter to energy in EM calorimeter. Jets typically deposit most of their energy in the hadronic calorimeter

14. 14December 13, 2005 Electron Identification Jet rejection (continued)  Shower profile: should be narrow (related to isolation)  Track-shower max matching: track should point at cluster centroid (particularly good for rejecting sneaky  +  0 s Most of these (especially isolation-type variables, track-centroid matching) are also very good at rejecting real electrons from heavy-flavor decay, but not as powerful against that…

15. 15December 13, 2005 Electron Identification Photons  Correct EM signature  Requiring a track gets rid of prompt photons  Conversions: Algorithm looks for opposite-sign tracks originating from the same, displaced point Muons  Rare, but it happens  Reject some with track- centroid matching  Get rid of the rest by requiring that the electron not be pointing right at missing energy An exaggerated conversion  e+ e-    radiated photon showers in EM detector, just like an electron muon track points right at the cluster

16. 16December 13, 2005 Muon Identification Jet rejection similar to electrons  Calorimeter, Track Isolation  MIP signature: Require there to be almost nothing (few GeV) in the calorimeters  Muon stub: Very few hadronic particles make it out of the calorimetry  Impact parameter, track quality:  Kaon decays-in-flight have two low-p T tracks strung together to make one lousy high-p T track Smaller fake rates, still worry about real muons from heavy flavor decays

17. 17December 13, 2005 Muon Identification Cosmic rays  Impact parameter: unlikely to have crossed detector at exactly the interaction point  Cosmic tagging algorithm looks at track timing information: consistent with beam crossing?

18. 18December 13, 2005 Use in Analysis Ideally, apply all selection criteria to a Monte Carlo of the physics process of interest In practice, detector modeling is rarely perfect Trust MC for your acceptance, but not efficiency Quantify data/MC discrepancy by measuring the efficiency in both  Pure sample of leptons? At CDF, use Z bosons (mass window + opposite charge), background 2% or less)  Compare to Z MC Take scale factor = ratio of  (data)/  (MC) eff, multiply MC A*  by this correction factor

19. 19December 13, 2005 Moving to CMS @ the LHC

20. 20December 13, 2005 Moving to CMS @ the LHC  A physicist’s-eye view

21. 21December 13, 2005 CMS Tracking Pixels – lower occupancy close to interaction point Strips are faster to readout and easier to track with (less combinations) Endcap structures as well as radial Stronger field (4 T) will provide better momentum resolution for higher p T particles All silicon, all the time Almost 10 M readout channels

22. 22December 13, 2005 CMS EM Calorimetry Instead of alternating dense material and scintillator, a very dense scintillator  Crystals of lead tungstate (PbWO 4, 98% metal by mass but completely transparent) Finer  -  resolution  Crystals are 1 Moliere radius (= typical width of EM shower = 22 mm) wide No shower max detector Instead, pre-radiator:  Two layers of lead (to start shower) followed by silicon layers (to measure position) one crystal

23. 23December 13, 2005 CMS Hadron Calorimetry Sampling calorimeters, like CDF  Central: copper-scintillator sandwich  Forward: steel-quartz sandwich  Robust for higher radiation evironment: uses Cerenkov light instead of scintillation. Spatial resolution (central): 0.87 x 0.87 in  -  (compare to CDF at 0.11 x 0.26) All the calorimetry is inside the magnet  Less material in front of calorimetry (except the tracker…) Additional scintillator outside of magnet to get up to 11 absorption lengths

24. 24December 13, 2005 CMS muon detectors 4 “muon” stations interleaved with iron absorber/ flux return  Each “station” is layers of wire chambers Right outside the solenoid Enough lever arm for independent tracking

25. 25December 13, 2005 Signal, Background at 14 TeV From pp at 2 TeV to pp at 14 TeV More energetic leptons  More bremstrahlung  Adds tracks, confuses calorimeter information  A use for the better tracking More “noise” in the event from underlying, softer interactions  Need to re-think isolation variables?

26. 26December 13, 2005 Electron ID at CMS Much finer segmentation in calorimetry  More detailed isolation and shower shape variables  Instead of just an isolation ratio, look at shape of energy distrubution (electrons should be confined to ~ one crystal)  Important as events are very busy and occupancy is high With preradiator, may be able to discriminate against  +  0  look for indications of two particles, better resolution for track/cluster mismatch More material in tracker  Conversions will be more of a problem, but perhaps it will be easier to catch them?

27. 27December 13, 2005 Muon ID at CMS All silicon tracking  More stringent track quality requirements  Forward muons more practical (coverage)  Pointing at vertex in Z as well as   d0 resolution?  Must understand tracking to do muon ID well Matching silicon track to muon chamber tracks More material, more energetic muons  Challenge: muons may radiate  Too much acceptance loss from requiring MIP signature in ECAL?  Use ECAL, preradiatior, accept muons that appear to be paired with a photon  Still require MIP in HCAL

28. 28December 13, 2005 Summary Electrons and muons can be identified with good efficiency/ high purity  Use to identify interesting physics Use all parts of detector to discriminate against backgrounds CMS brings new challenges but new tools to use as well