Electroweak physics at the lhc precision measurements and new physics
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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 LHCPrecision 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 (AFB)

  • Triple Gauge Boson Couplings

    • Charged TGCs

    • Neutral TGCs

    • Anomalous Quartic Couplings

  • Heavy Leptons


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

direct

indirect

)

(


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

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

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

ET-systematics


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 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 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

  • 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

q

q

  • 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)

Q

q


[%]

Determination of sin2θefflept(MZ2)

Current error on world average 1.6x10-4

sin2θeff=0.23153±0.00016

Can be further improved by combining Z decay channels

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


Associated Production of Gauge Bosons


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

CMS

ATLAS

30fb-1

~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

CMS

No tree level neutral couplings in SM

All are zero in SM

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


Quartic Couplings


Anomalous Quartic couplings

Look for Wgg, low production threshold at Mw

S/B~1

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


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 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 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 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

  • 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± 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± Detection

  • Detection at the LHC is entirely cross section limited.


L± Detection

  • Detection of up to 1TeV should be possible a the LHC


Leptons vs. Sleptons

  • Study the angular distribution


Leptons vs. Sleptons


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


Summary

  • 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


Backup slides


|h|<2.5

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

CTEQ6.1M

MRST02

ZEUS-S

ZEUS to MRST01 central value difference ~5%

ZEUS to CTEQ6.1 central value difference ~3.5%

(From LHAPDF eigenvectors)


e-rapidity

e+ rapidity

CTEQ61

CTEQ61

MRST01

MRST01

ZEUS-S

ZEUS-S

Generator Level

Error boxes

are the

Full PDF Uncertainties

ATLAS

Detector Level

with sel. cuts

Electron distributions

  • At Detector level reflects generator level distributions

  • ~8% PDF uncertainty at y=0 remains


ET

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%

(CMS)

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

ATLAS


LHC prediction

Tevatron

Minimum bias and Underlying Event

LHC

PYTHIA6.214 - tuned

Tevatron

● CDF 1.8 TeV

● CDF 1.8 TeV

~80%

~200%

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)

LHC?

  • 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

dNch/d

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.

h

dNch/dpT

Reconstruct tracks with:

1) pT>500MeV

2) |d0| < 1mm

3) # B-layer hits >= 1

4) # precision hits >= 8

pT (MeV)


UE uncertainties

LHC

PYTHIA6.214 - tuned

PHOJET1.12

Transverse < Nchg >

x 3

x1.5

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)


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