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Electroweak Physics at the LHC Precision Measurements and New Physics. PY898: Special Topics in LHC Physics By Keith Otis 4/13/2009. Outline. Electroweak Parameters W-Mass Top-Mass Electroweak Mixing Angle Drell-Yan Forward-Backward Asymmerty in Z Decays (A FB )

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electroweak physics at the lhc precision measurements and new physics

Electroweak Physics at the LHCPrecision Measurements and New Physics

PY898: Special Topics in LHC Physics

By Keith Otis


  • Electroweak Parameters
    • W-Mass
    • Top-Mass
    • Electroweak Mixing Angle
  • Drell-Yan
  • Forward-Backward Asymmerty in Z Decays (AFB)
  • Triple Gauge Boson Couplings
    • Charged TGCs
    • Neutral TGCs
    • Anomalous Quartic Couplings
  • Heavy Leptons
electroweak parameters
Electroweak Parameters
  • The main parameters of EW theory are measured to very high precision.
    • The mass of the W (MW) is known with an uncertainty of 0.03%
      • MW= 80.428 ± 0.039 GeV UA2/CDF/D0, 80.376 ± 0.033 GeV LEP 2
      • SM predicts 80.375 ± 0.015 GeV
    • The uncertainty in the mass of the Top (mt) is 0.7%
      • mt= 170.9 ± 1.8 GeV
      • SM predicts 171.1 ± 1.9 GeV
    • The uncertainty on the electroweak-mixing angle (θw) is 0.07%
      • cos(θw)= Mw/Mz
  • The LHC will be able to improve on these in a relatively short period of time.
Tevatron: 2 TeV
  • LHC: 10-14TeV
  • LHC @ 1033 Luminosity
    • 150 Hz W
    • 50 Hz z
    • 1 Hz tT
  • 10 pb-1 of Luminocity
    • 150k W→eν
    • 15k Z→ee
    • 10k tT
w mass
  • W→lv signature
    • Isolated charged lepton pT > 25 GeV || < 2.4
    • Missing transverse energy ETMiss > 25 GeV
    • No jets with pT > 30 GeV
    • Recoil < 20GeV
  • For an integrated luminosity of 1 fb−1, 4 million events with W→lv(ℓ = e or μ) decays are expected.

Summer 2005 result

68% CL

68% CL





w mass extraction
W mass extraction
  • The W mass is extracted from the measured plT distribution or from the Jacobian peak observed in the transverse mass of the lepton-neutrino system, MWT.
  • The W mass is obtained by comparing the measured distributions with template distributions generated from data (Z events) or MC.

W-mass: ATLAS


Based on 10fb-1 of data corresponding to ~10M Wln

Fit MT(W) or pT (e) to Z0 tuned MC

MC Template method

Z-Samples play a crucial role in reducing systematics and theoretical uncertainties

Requires further study


W-mass: CMS


Scaled observables

Systematic uncertainties (MeV) on W-mass for 10fb-1

  • Use Z events as templates
    • Scaled observable using weighting fn
    • Morphing using kinematic transformation
    • Limited by Z-statistics
  • Detector and theoretical effects cancel at least partially
  • Pt(l) better than MT as it is not sensitive to ET systematics
    • But needs PT(W) to be understood

PT(W) needs to reduced


new physics
New Physics?

Mar 06

  • MW is a fundamental SM parameter linked to the top, Higgs masses and sinqW.

LEP2+Tevatron DMW=30MeV

Tevatron run2 (2fb-1) 30MeV combined

  • For equal contribution to MH uncertainty:

DMt< 2 GeV DMW < 15 MeV

Can get dMH/MH~30%

Important cross-check with direct measurements


W-mass summary

  • A number of methods have been studied
    • Direct measurement of MT pT(l)
      • Z-events used to tune W MC
    • scaled observable pt(l), ‘morphing’
      • Z-events used as a template
    • Systematics greatly improved using Z-samples
    • All methods are giving DMw in range of 20MeV per channel per variable, so combined <15MeV per experiment seems to be achievable for 10fb-1
      • Need to understand correlations
      • Main issues at ET for MT and PT(W) for pt(l)
      • DMW ~10MeV looks possible
      • Requires DMt<1GeV for EW fits
top mass
Top Mass
  • Top quark pairs, mainly produced via gluon fusion, yields a production cross-section of 833 pb, at next to leading order, 100 times higher than at Tevatron.
  • The "golden" channel is the semi-leptonic channel:
    • tT→Wb+WB→ (lv)b+(jj)B
top mass1
Top Mass
  • Golden Channel event selection:
    • Isolated high pT lepton, EmissT and at least 4 jets, two of which are b-tagged.
    • This gives a signal efficiency of ~5% with a signal to background ratio of the order 10.
  • Primary backgrounds
    • The main backgrounds are single top events, mainly reduced by the4 jet cut, fully hadronic t T events, reduced by the lepton requirements, W+jet and Z+jet events
finding the top mass
Finding the Top Mass
  • Reconstruction of the hadronic side of the decay is done by minimization procedure.
    • This minimization constrains the light jet pair mass to Mw, via corrections to the light jet energies.
    • After trying all possible jet combinations the one minimizing the χ2 is kept.
    • The b-jet closest to the hadronic W is associated to the chosen pair.
    • The three jet invariant mass is then fitted with a Gaussian plus a polynomial
    • The Result: Mt= 175.0±0.2(stat.)±1.0(syst.) GeV, for an input mass of 175 GeV and 1 fb−1.
drell yan
  • As discussed in previous weeks this is where a heavier neutral gauge boson (Z’) would show up
  • AFB of the leptons from Zs

Drell Yan

  • Important benchmark process:
  • Measure cross-section
  • parton-parton luminosity functions
  • constrain PDFs
  • measure sin2qW
  • Deviations from SM

Determination of sin2θefflept(MZ2)

AFB = b { a - sin2θefflept( MZ2) }

Measure Afb with leptons in Z0 DY events

Can fit with Mt to constrain MH

a, b calculated to NLO QED and QCD.

Need to define direction

At the Tevatron -- well defined



  • At LHC no asymmetry wrt beam
  • Assume that there is a
  • Q-qbar collision
  • quark direction from y(ll)
  • Requires measurement at high y(ll)





Determination of sin2θefflept(MZ2)

Current error on world average 1.6x10-4


Can be further improved by combining Z decay channels

Systematics: PDF, lepton acc. (~0.1%), radiative correction calculations


Triple gauge boson couplings

  • SM gauge group SU(2)LxU(1)Y
  • WWg and WWZ couplings

(charged TGCs)

Couplings described by

5 independent parameters

All are zero in SM

Any deviations is a signal of new physics


Anomalous couplings in WWg




~3000 evts

Most sensitive measurement is looking for high pT Zs or gs


Charged TGC predictions

Results expected to be ~x10 better than LEP/Tevatron

Results are statistics limited

(except for g1Z)


Neutral TGCs


No tree level neutral couplings in SM

All are zero in SM

Leads to 3-5 order of magnitude improvement compared to LEP


Anomalous Quartic couplings

Look for Wgg, low production threshold at Mw


ATLAS 30fb-1 e-ngg ~14 events(~x4 for l+/-ngg)

heavy leptons
Heavy Leptons
  • “Evidence grows for charged heavy lepton at 1.8-2.0 GeV”- Physics Today (1977)
  • Current limits: mL(±) >100.8GeV
  • Neutral Heavy Lepton Mass Limits
    • Mass m> 45.0 GeV, 95% CL (Dirac)
    • Mass m> 39.5 GeV, 95% CL (Majorana)
heavy leptons1
Heavy Leptons
  • Relic abundance of the leptons must not “over-close” the universe.
    • Can’t provide more than the critical energy density (10-5GeV cm-3)
  • A stable, charged lepton must have a low enough relic abundance for it not to have been detected in searches for heavy isotopes in ordinary matter
  • The mass and lifetime of the new leptons musts not be such that they would have been detected in a previous collider experiment.
  • There are no theoretical constraints found for lifetimes less that ~106 s even for masses up to the TeV scale.
  • Only limits are the experimental ones
heavy leptons2
Heavy Leptons
  • Where do we look for heavy Leptons?
    • Drell-Yan
    • Other mechanisms
      • pp→γγ→L+L-
      • pp→Zγ→L+L-
  • Mechanisms for introducing new leptons
    • New fermionic degrees of freedom
      • Vector Singlet Model (VSM)
      • Vector Doublet Model (VDM)
      • Fermion-mirror-fermion Model (FMFM)
heavy leptons3
Heavy Leptons
  • In these new models:
    • Exotic leptons mix with the standard leptons through the standard weak vector bosons and according to the Lagrangians
l detection
L± Detection
  • Time-of-Flight
    • Heavy particles
    • Detectable in both the central tracker and muon chambers
    • Use measured momentum and time delay to reconstruct the mass
l detection1
L± Detection
  • Imperfections in the time and momentum resolutions will cause a spread in the mass peak
  • Bunch crossing identification
    • Muons from D-Y and heavy quark decays
      • For a background signal to look like a heavy lepton neutral current two opposite charge muons would have to be mis-identified at the same time.
      • Make pT cut at 50 GeV to eliminate heavy quark decays
l detection2
L± Detection
  • Detection at the LHC is entirely cross section limited.
l detection3
L± Detection
  • Detection of up to 1TeV should be possible a the LHC
leptons vs sleptons
Leptons vs. Sleptons
  • Study the angular distribution
heavy lepton summary
Heavy Lepton Summary
  • Assuming standard model couplings and long lifetime:
    • We can detect heavy charged leptons in intermediate scale models up to 950 GeV with 100 fb-1
    • Above 580 GeV it’s hard to distinguish them from scalar leptons
  • The Electroweak sector, while one of the better understood sectors of the SM, still holds important information and even some exciting new physics at the LHC


Constraining gluon PDF with Ws

  • Many W+Z measurements have pdf uncertainties
  • at LHC Q2~MZ2 corresponds to sea-sea collisions  depends on gluon from gqq
  • Need to improve understanding of gluon

GOAL: syst. exp. error ~3-5%

W Rapidity Distributions for different PDFs

~ ±3.6% @ y=0

~ ±5.2% @y=0

~ ±8.7% @y=0




ZEUS to MRST01 central value difference ~5%

ZEUS to CTEQ6.1 central value difference ~3.5%

(From LHAPDF eigenvectors)



e+ rapidity







Generator Level

Error boxes

are the

Full PDF Uncertainties


Detector Level

with sel. cuts

Electron distributions

  • At Detector level reflects generator level distributions
  • ~8% PDF uncertainty at y=0 remains
first measurements of w and z


First measurements of W and Z

W and Z cross-sections

For ~1fb-1 data, systematics dominate

Main theoretical contribution

PT(W/Z) LO-NLO ~2%


tracker efficiency

Initial luminosity uncertainty ~10%, reduced to 5%

m-reconstruction efficiency

from 20pb-1 Zmm

Barrel and endcap

To 0.5% in 0.2h bins



LHC prediction


Minimum bias and Underlying Event


PYTHIA6.214 - tuned


● CDF 1.8 TeV

● CDF 1.8 TeV



PYTHIA6.214 - tuned

UE includes radiation and small impact parameter bias

MB only


First measurements at the LHC ?Charged particle density at  = 0

(Only need central inner tracker and a few thousand pp events)


  • Min bias events are also crucial for intercalibration
  •  CMS require 18M events to intercalibrate ECAL in h at 2%
  • ATLAS studies of use of MB events to study L1 trigger rates

Measuring the minimum bias events at ATLAS

Black = Generated (Pythia6.2)

Blue = TrkTrack: iPatRec

Red = TrkTrack: xKalman


Only a fraction of tracks reconstructed,:

  • limited rapidity coverage

Measure central plateau

  • can only reconstruct track pT with good efficiency down to ~500MeV, but most particles in min-bias events have pT < 500MeV

 Hard extrapolation.



Reconstruct tracks with:

1) pT>500MeV

2) |d0| < 1mm

3) # B-layer hits >= 1

4) # precision hits >= 8

pT (MeV)

ue uncertainties
UE uncertainties


PYTHIA6.214 - tuned


Transverse < Nchg >

x 3


CDF definition of UE

  • Extrapolation of UE to LHC is unknown
  • Depends on
  • Multiple interactions
  • Radiation
  • PDFs

ATLAS DC2 Simulated data

Reconstructing the underlying event

Njets > 1,

|ηjet| < 2.5,

ETjet >10 GeV,

|ηtrack| < 2.5,

pTtrack > 1.0 GeV/c

Analyse with first data

(a la CDF)

Need to ensure overlap between MB and jet trigger

Require ~20M MB events to get ptjet~30GeV

Ratio <NTrackReco>/<NTrackMC>

Leading jet ET (GeV)