Lepton Identification at Hadron Colliders

1

Lepton Identification at Hadron Colliders

c. mills

December 13, 2005 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

December 13, 2005 3

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 1045) chance to escape the detector

• Jargon: “lepton” = e or

Decays inside detector, usually hadronically, into a

“jet” of particles

December 13, 2005 4

`

A Generic Detector• Electrons

Track Stop (shower)

in EM calorimeter

• Muons Track Passes

through calorimeter

track in muon detector

EM caltracking

Hadronic cal.

Muon detectors muo

n

electr

on

December 13, 2005 5

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

December 13, 2005 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

December 13, 2005 7

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 chambersMuon chambers and and

scintillatorscintillator

Cutaway view of the CDF II detector

Protons go in here

Interaction point

December 13, 2005 8

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)

December 13, 2005 9

CDF Tracking

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

December 13, 2005 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

interaction point

forw

ard

centralone “tower”

scintillator

scintillator

iron

leadshower

maximum detector


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


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


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


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…


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


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


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?


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


Moving to CMS @ the LHC


Moving to CMS @ the LHC

A physicist’s-eye view


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


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 - 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


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


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


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?


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?


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


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

Lepton Identification at Hadron Colliders

Documents

Transcript of Lepton Identification at Hadron Colliders