Transcript Lepton Identification at Hadron Colliders
Lepton Identification at Hadron Colliders
c. mills
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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
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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 t escaping the detector (5 m radius) without decaying (2.9 x 10 -13 s)
Decays inside detector, usually hadronically, into a “jet” of particles
Even a 1 TeV tau has an immeasurably small (1 part in 10 45 ) chance to escape the detector • Jargon: “lepton” = e or m
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A Generic Detector
• Electrons Track Stop (shower) in EM calorimeter • Muons Track Passes through calorimeter track in muon detector
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Muon detectors Hadronic cal.
EM cal tracking `
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Electron Backgrounds
• Jet: Catch-all term for fakes of hadronic origin Tracks + energy in calorimeter Nasty case: p + p 0 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
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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
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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 scintillator and
Cutaway view of the CDF II detector Interaction point
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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
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CDF Tracking
Apparently this is also a “CDF tracker”… The
Grumman S-2T Turbine Tracker 9
one “tower”
CDF Calorimetry
scintillator iron scintillator central lead shower maximum detector • 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 electrons/photons first (ideally) Lead-scintillator • Hadronic calorimeter stops 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
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CDF “Small Tracking”
• Shower maximum detectors: electrons Small, shallow tracking at depth where EM shower peaks Wire chamber in central, scintillator strips in plug Run clustering algorithms, like central tracker h , j location of shower centroid Better spatial resolution than calorimetry Shower profile (collimated/ spread out?) • Muon chambers Shallow wire tracker outside of calorimetry, shielding Short tracks, called stubs, indicate muons
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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
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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
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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 + p 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…
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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 m
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g
An exaggerated conversion
g m
e radiated photon showers in EM detector, just like an electron muon track points right at the cluster
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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 high-p T T tracks strung together to make one lousy track • Smaller fake rates, still worry about real muons from heavy flavor decays
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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?
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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 (MC) eff, multiply MC A* e (data)/ by this e correction factor
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Moving to CMS @ the LHC
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Moving to CMS @ the LHC
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A physicist’s-eye view
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CMS Tracking All silicon, all the time
Almost 10 M readout channels
• 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
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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 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
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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
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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
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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?
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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 + p 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?
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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
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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
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