Lepton Identification at Hadron Colliders

1. Lepton Identification at Hadron Colliders c. mills

2. 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. Leptons in a Generic Detector • Nature: 3 leptons • e (stable) • m (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 • t (2.9 x 10-13 s) • Even a 1 TeV tau has an immeasurably small (1 part in 1045) chance to escape the detector • Jargon: “lepton” = e or m Decays inside detector, usually hadronically, into a “jet” of particles

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

5. Electron Backgrounds • Jet: Catch-all term for fakes of hadronic origin • Tracks + energy in calorimeter • Nasty case: p+p0 gives one track + EM energy • Photon • Need to pick up a track • Conversion: g 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. 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. CDF: A Real Detector Cutaway view of the CDF II 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 Protons go in here Interaction point

8. CDF Tracking • 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) • 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

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

10. 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 scintillator iron scintillator lead shower maximum detector one “tower” central forward interaction point

11. 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 • h, j 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. Kinematic vs. ID selection • Kinematic = what’s usable • ET or pT 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. 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 pT 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. 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 p+p0 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. Electron Identification e+ • 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 e- g An exaggerated conversion m radiated photon showers in EM detector, just like an electron g m muon track points right at the cluster

16. 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-pT tracks strung together to make one lousy high-pT track • Smaller fake rates, still worry about real muons from heavy flavor decays

17. 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. 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 e (data)/ e (MC) eff, multiply MC A*e by this correction factor

19. Moving to CMS @ the LHC

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

21. 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 pT particles All silicon, all the time Almost 10 M readout channels

22. CMS EM Calorimetry • Instead of alternating dense material and scintillator, a very dense scintillator • Crystals of lead tungstate (PbWO4, 98% metal by mass but completely transparent) • Finer h-j 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. 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 h-j (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. 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. 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. 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 p+p0 • 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. 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 j • 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. 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