Physics with W and Z Bosons at Hadron Colliders

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Physics with W and Z Bosons at Hadron Colliders

• Suyong Choi
• SKKU

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Outline

• Introduction
• Part I
• W and Z
• Production at Hadron Colliders
• Part II
• QCD
• Electroweak
• As backgrounds to exotic phenomena
• Conclusion

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Introduction

• As a standard candle, seeing W and Z boson will
  be an important first step at LHC
• Relevance to experiment
• Masses energy and momentum calibration
• Luminosity measurement
• Efficiency measurements
• Relevance to physics
• Electroweak and QCD
• Backgrounds to exotic phenomena

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Why W and Z?

• It is easier to identify leptons (e,?,?) at
  hadron colliders
• Good resolution and efficiency
• jets are messy
• Leptonic final states of new phenomena are
  easiest to discover
• Cascade decay New particle ? ? W or Z ?
  leptons
• Direct decay New particle ? ? leptons
• Directly or indirectly W and Z are major
  backgrounds to events with leptons

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The W Boson

• Spin 1
• Charge ?1
• MW 80.398 ? 0.025 GeV
• ?W2.106 ? 0.050 GeV
• Couples to- Particles of different flavor-
  left-handed fermion orright-handed antifermions
• Leptonic decay

time
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Review of the W Boson

• Hadronic decays
• W- - particle ? antiparticle

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Decays of a real W

• Cannot decay into t-quark
• per lepton species
• Ignoring mass

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Review of the W Boson

• Beta decay via a virtual W boson
• md , mu a few MeV W is virtual

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Decay of Top Quark

• mt 174 GeVmb 5 GeV
• In this case, W is a real particle

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Production of W at Hadron Colliders

• Flip the direction of outgoing quark
• quark-antiquark annihilation
• Real W boson can be produced if the incoming
  quarks have enough energy

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Production of W at Hadron Colliders

• Traditionally, W bosons were produced at
  colliders
• anti quark exists as a valence quark in
  anti-protons
• For LHC, W is produced by q and q-bar from
  protons
• antiquark exists as sea quark in protons

W
antiproton
proton
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Production Cross Section of W Boson at Hadron
Colliders
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How to Identify W Bosons

• Signature charged lepton neutrino
• charged lepton is identified easily
• neutrino escapes the detector
• Kinematics for W rest
• ? and ? have p40 GeV and back-to-back
• peak in pT distribution at 40 GeV
• Missing ET signature of neutrino
• Total momentum from detector in transverse plane
  ? 0 since neutrino escapes the detector
• Vector sum of energies calorimeter cells

Jacobian peak
pT
MW/2
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Discovery of W Boson from UA1 _at_ CERN
Electron
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W Event from DØ Experiment in Run I
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W Event Side View
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More About W Events

• Calculating the cross section

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Parton Densities inside Proton

• Consider probability for partons(quarks and
  gluons) with momentum fraction x inside proton
• proton is a dynamic object

probability density of partons
scattering of quarks
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Electron ET Distribution

• Smooth shape
• Finite width of W
• W not at rest
• Energy resolution

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Neutrino Transverse Momenta
Calculated after correcting missing ET
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Transverse Mass

• How to calculate mass of W?
• We cannot measure the pz of neutrino
• Form a scalar quantity under boost in z-direction
• Transverse vectors are not affected
• Edge of transverse mass distribution is around MW

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Theoretical Transverse Mass Distribution
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Transverse Mass Distribution from CDF Data
background
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Wjets Events

• W is not produced at rest
• Gluon emission from quarks
• Wn jets

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Review of Z Boson

• Spin 1
• Charge 0
• MZ 91.1876?0.0021 GeV
• ?Z 2.4952 0.0023 GeV
• Couples to particles of same flavor
• Leptonic decay

time
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Review of the Z Boson

• Hadronic Decays
• Since Z couples to a combination of electric and
  weak charge, the branching fraction pattern is
  complicated
• charged lepton 3.3 per family
• neutrinos 20
• Quarks 67

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Production of Z Bosons at Hadron Colliders

• Production through qq-bar annihilation
• Identification through 2 leptons

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Z Boson in UA1 Detector in April 30, 1983
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D0 Experiment in RunI
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Experimental Importance of W/Z at LHC
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Energy and Momentum Scale Calibration

• Calorimeter energy calibration
• conversion factor for scintillation photons in
  crystals ? energy
• Peak position must be at known MZ
• Muon momentum calibration
• Material effects
• Systematic offset in detector alignment and
  simulation

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Resolution

• Variation in detector response
• calorimeter cell
• misalignment of tracking detectors

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Efficiency Measurement from Data using Z Events

• Tag-and-probe method
• If M(? track)MZ in an event, then the track
  must be another muon

tag
Z
probe track
How often is the track identified as a muon?
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W Bosons

• Missing ET is calculated using calorimeter cells
  with some minimum energies
• Width and distribution of missing ET tells us the
  inherent noise of the calorimeter

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Integrated Luminosity Measurement and W/Z

• Important for integrated luminosity measurement
• Number of events of a process we expect
• But how do we get ?
• Systematic uncertainties of A and ? difficult to
  estimate
• Using W or Z events could have smaller error

cross section
Integrated luminosity
Acceptance
Measured by luminosity counter
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Physics with W/Z Bosons

• QCD
• Electroweak
• Higgs
• SUSY

Tevatron - proton antiproton collider at 1.96
TeV Experiments at Tevatron CDF and D0
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Quantum Chromodynamics (QCD)
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W/Z as Standard Candles

• Proton structure
• Parton distribution function
• W/Z Rapidity
• perturbative QCD
• Well-defined scale for perturbative QCD
• W/Z n jets
• pT of W and Z bosons

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Parton Density Function (PDF) of Proton
fraction of pz of proton carried by a quark
or gluon
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W Boson Rapidity at Tevatron

• Quarks inside proton
• Quarks inside antiproton

p
p-bar
u
d-bar
W
d
u-bar
W-
pzgt0
pzlt0
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W/Z Boson Rapidity at LHC

• Quarks inside proton have different average
  momenta
• Boost in z-direction of bosons are different
• Boson rapidity distribution sensitive to parton
  densities

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LO, NLO, NNLO Predictions

• lt5 theoretical uncertainty on total cross
  sections

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Zjets

• From Zjets distributions
• Probe gluons inside proton
• Test reliability of QCD calculations
• Zn jets fixed order perturbative QCD
• pT of Z boson resummation in QCD

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W/Zn jets Prediction from Theory

• Open points are NLO calculations LO can be off
  by more than 100 !

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Zn jets at D0
1st jet
2nd jet
3rd jet
MCFM Next-to-Leading order (NLO) calculation
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Transverse Momentum Distribution of Z

• Tests the resummation calculation at low pT

Leading order calculation
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Wjets at Tevatron
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Jets in Wjets
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Jet ET Distribution from Wjets Events
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Jet Opening Angle Distribution
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Slide from Michelangelo Manganos Talk at CTEQ07
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Rediscovering Standard Model at LHC
Assumed selection efficiency W? l?, Z? ll
20 tt ? l?X 1.5 (no b-tag, inside mass bin)
lots of minimum-bias and jets (107 events in 2
weeks of data taking if 20 of trigger
bandwidth allocated)
10 pb-1 ? 1 month at 1030 and lt 2 weeks at 1031,
?50
1 fb-1 Similar statistics to D0/CDF
100 pb-1 ? few days at 1032 , ?50
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W, Z n-jets at CMS
CMS AN-2006/102 CMS AN-2006/108

• Background to many searches
• Lepton multijets
• Cross sections are high, can be measured with
  1fb-1, already dominated by
• detector effects,
• theo. uncertainties,
• ttnjet bkg subtraction (for Ws)
• W/Z heavy flavor jets

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W/Z jets Backgrounds to Higgs
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Electroweak Physics with W/Z
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W Boson Mass

• Copious W production - strong point of hadron
  colliders
• MW is one of the most important parameters in the
  SM

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Connection with the Higgs Sector

• Weak mixing angle
• Higgs loop effect on W/Z
• Logarithmic dependence

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At CMS

• By using kinematics of leptons from Z decays,
  total error in systematics to 30 MeV is
  possible.
• energy scale and linearity 14 MeV
• resolution 5 MeV

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Diboson Production

• Probe of trilinear gauge coupling

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Diboson at CMS

• Multilepton signature major background to
  search for new physics

significance 12.8
significance 13
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Summary

• Standard Candle Inclusive W/Z are the well
  understood part of the Standard Model
• Calibration of detector and physics
• One of the most important channels
• W/Zjets
• NLO calculation available up to 2 jets
• Need 4 and 5 jets calculation, currently only in
  LO 100 error