ElectroWeak Physics at LHC

Kajari Mazumdar Tata Institute of Fundamental Research, Mumbai, India

BSM@LHC, IACS, Kolkata January, 2009

LHC is a discovery machine, why should we do ElectroWeak (precision) physics @ such a facility?

Which measurements are important?

• W,Z physics

-- production cross-sections

-- mass measurement

-- asymmetries

• Drell-Yan events

• Di-boson production at LHC

LHC opens up a vast an new kinematic range. Various measurements will be performed at kinematic regions different than earlier experiments.

Proton-proton collision at LHC expected in 2009 2nd half.Initial beam energy 450 GeV.

Physics run @ 10 TeV during September to November, integrated luminosity ~ few pb-1.

LHC in 2010: 14 TeV operation, expected integrated luminosity ~ 2.5 fb-1.

LHC Start-up schedule and energy

Very Early phase of LHC data-taking

• Properties of underlying and minimum bias events will be the first physics.• Crucial for understanding the detector and its performance.• Consistency check for Standard Model processes.• Validate and tune MC: preparation for New Physics

Theoretical situation at LHC startup: W,Z cross-section known to ~ 3% ttbar cross-section to ~ 10% Min. bias charged multiplicity to ~ 50% !

Very large event samples! in 10 /pb:150K W e, 15K Z ee, 10K tt

Statistical accuracy will be reached fast. allows detailed systematic studies.

• Energy and momentum scale calibration from Zℓℓ (ℓ = e/μ )• ET miss calibration from Wμν and Zττ• Understand W+jets and Z+jets: – important background for searches• Measure the W τν cross section: – validation of τ-ID needed for searches

Utility of W, Z events at LHC start up

• W,Z physics will be done from early phase of LHC, from few pb-1 onwards.• Early discovery is challenging have to rediscover Standard Model first.• W, Z events will enjoy the phase of both signal and calibration physics Large cross-sections Clean leptonic signatures. Robust selections (loose mass cuts, etc.) possible. Events samples can be selected with high efficiency and purity. Precision EW physics will be the main stream physics at LHCCaveats: Precision EW does not mean only Wmass! Precision studies are slower than searches!

Electroweak physics at LHC : Measurements with Standard Candles

Why precision measurement will take longer?- LHC detectors are very complex: 10e7 electronic channels.- Detector material interferes with measurements: kinematics of W decay products need to be known at the point of decay, not far away. Material modelling is tested/tuned based on E/p of electron.- Detectors are thicker larger correction, as well as better relative corrections needed time consuming.

The Standard Model (SM) is remarkable: at tree level, only three parameters are needed to predict everything. (eg., GF, and sin2w)

Precision electroweak measurements • make it possible to constrain MH• attempt to look for deviations from the simple 3-parameter theory i.e. evidence that three parameters are not enough

•Loop effects: Correction @few %, depends on M2(t) and log(MH)• Couplings that differ from SM values. consistency check of SM may give hints of New Physics or provide constraints, since new particles contribute to loop corrections.

Precision ElectroWeak Physics

No valence antiquarks at LHC sea density of proton and higher order effects need to be known accurately.

Sea-sea parton interaction dominate at low-x and high Q**2 ~ 25% of W at LHC are produced by heavier flavours .Strong correlation between induced variations of W, Z distrns.

Statistically with 10 fb-1 data: electron, muon channel Mw ~ 2 MeV Precision calculations mandatory for precision measurements.QCD corrections are important needs continuous dedicated effort. Already at Tevatron QCD effects are ¼ of systematic uncertainty.

NLO contributions are large at LHCMore energy available for extra jet radiation.

Distrtibutions, relevant for extracting prescision EW parameters , are predicted accurately by perturbative QCD resummation calculationFortunately NNLO calculations are available.

Theoretical issues for W, Z physics at LHC

Examples of various effects of accounting for higher order:• Significant electroweak corrections to gauge boson hadroproduction (taking into account photon radiations): K_EW (NLO) > K_QCD (NNLO)

• NLO EW corrections to W-rapidity ~ NNLO QCD and PDF uncertaintyRelevant for precision lumi and PDF constraints.

• Lepton identification requirements (isolation) and detector effects strongly affect final state radiation.

• Large uncertanties (due to soft gluon emission) affect the transverse momentum preditions for W and Z .

Problem for experiments: No MC generator , which takes care of both EW and QCD corrections, is available till now !

• To derive precise measurements of the electroweak parameters MW, W, sin2θ Relevant observables: leptons’ transverse momentum, W transverse mass, ratio of W/Z distributions, forward-backward asymmetry.

• To monitor the collider luminosity and constrain the parton distribution functions (PDFs) Relevant observables: total cross section, W rapidity and charge asymmetry , lepton pseudorapidity .

• To search for New Physics Relevant observables: Z invariant mass distribution and W transverse mass MW in the high tail.

Note: production models of W and Z are similar (same QCD considerations) Use Z to predict pT spectrum of W.• Precision MC needed to correct for different phase-space (MW vs. MZ)• Different EWK couplings

Accurate experimental measurements with W and Z

Consistency check of SM (constraint on gauge sym. breaking)

Indirect measurement of Higgs and SUSY

For equal contributions of ∆Mt and Mw to ∆MH, need

∆Mw ~ 0.007 ∆mt

requires all contributions to W-mass uncertainty below ~ 10 MeV

Precision measurement of MW

Same QCD effects for both!

Use Z to predict W pT spectrumPrecision MC needed to correct for• Different phase-space (MW MZ)• Different EWK couplings

Two approaches:1. Model pT(W) with the measured pT(Z) :

promising, estimated precision MW 2 MeV

2. Fit W with measured transverse distributions in Z events (one lepton killed) corrected for the cross section ratio soft gluonemission cancels in the ratio exactly calculable in

(actually most of the uncertainties cancel in the ratio)

Production of W and Z

Early Measurements of W,Z in electron channel

Systematic uncertainties dominate for L > 1 fb-1Effect on measurement of MW

Main experimental issues: Imperfect estimation of absolute energy scale Uncertainties in kinematic distributions of W (rapidity, transeverse mom. ) (due to uncertainties in PDF, higher order corrections, )

W,Z in muon channel: 10 /pb

Uncertainties in Z cross-section at 100 /pb• systematics at the 1% level (efficiencies,background,. . . )• theoretical error at 2%, rel. acceptance determination and PDFs• luminosity uncertainty: 10% (at start up, 5% in long term).

Systematic uncertainties in Mz measurement with muons

Experimental: Misalignment, knowledge of magnetic field,collision point uncertainty,Pile up effects,Underlying eventsTheoretical: PDF , initial state radiation Pt effects (LO to NLO)

Measurement of W-massTraditional template method suffers from uncertainties due to lepton energy,momentum scales, resolutions, recoil model of W, PDF, backg.,..

• Startup uncertainties not competitive, but serves to establish analysis method.• Long term/high lumi measurements: i) use known Z-mass to generate templates for different W-mass values from real Z-data, ii) compare with realW-events to extract W-mass. RRatios of distributions corr. Z and W are accurately calculated in pQCD. scaled observables method, only the difference between W and Z matters

Method works, rescaled distribution of electron Pt in CMS for 1 /fb

Traditionally, use transverse mass and transverse momentum of lepton

MT distribution: • independent of PtW to 1st order• detector effects dominated by resolution pf pT() ie, EtmissMT distribution sensitive to hadronic recoil and its modelling and multiple interactions

Lepton momentum distribution: sensitive to that of W (ie, to higher order QCD corrections), independent of Etmiss resolution

Summer 08: MW = 25 MeV, should finally go down to about 10 MeV at LHCVery tough job! Can’t be obtained quickly.

ATLAS expectation: ~ 7 MeV per lepton channel with 10 /fbCMS: ~ 10 MeV ….

Determination of W-mass

• Large rate: measure cross-section and asymmetries.• Important benchmark process from initial phase of LHC. • Deviations from SM cross section indicate new physics.• Possibility of early discovery with very clean signature.• With 100 pb-1 @ 14 TeV, range probed > 800 GeV.

Drell Yan process:

DY background negligible, <1.5 events at 1 /fb for M_ll > 1.5 TeV!

Determination of sin2θW(MZ2

): Forward-Backward Asymmetry

Stat. Error for L = 100 fb-1 (ATLAS, |Yz|>1) ∆AFB ∆sin2θW

|y(l1,l2)| < 2.5 3.0*10-4 4.0*10-4

|y(l1)| < 2.5 + 2.3*10-4 1.4*10-4|y(l2)| < 4.9

Current error on world average 1.6x10-4

need small systematic error: PDF uncertainty, precise knowledge of lepton acceptance and efficiency effects of higher order QCD

[%]

ATLAS

At LHC no asymmetry wrt beam, assume qqbar collision

qbar direction from y(ll) measurement at high y(ll)

•quark direction is the same as the boost of the Zquark direction is the same as the boost of the Z• Measurement possible only in electron channelMeasurement possible only in electron channel

Double Gauge Boson signal in detectors

Cross-section range from 20 pb for W to 0.2 pb ZZ production.Signals with in final state has important background due to fake photons

High significance in first /fb!Almost background-free!5observation with ~ 150 /pb!

Consequence of non-Abelian structure of Standard Model. self-coupling of gauge bosons : measure triple gauge coupling (TGC)Charged TGC: WWZ, WWg allowed in SM study WW and WZ Neutral TGC: ZZZ, ZZg, Zgg forbidden in SM study ZZ processes

Probing TGC is at the core of testing SM: values are O(0.001) deviations from SM New PhysicsImportant and irreducible background for Higgs search (H--> WW) and New Physics searches

Double Gauge Boson Production at LHC

Wproduction: 3 interfering diagrams produce finite cross-section

CP conserving effective Lagrangian for WWcouplings, SM

Unitarity may be violated for non-SM needs form factor

• Radiation Amplitude Zero (absence of radiation or events) arises from factorization of amplitude probe of gauge symmetry.

In general factorization possible in gauge theories for tree amplitude of 4 particles involving one or two gauge bosons.

Zero at =-1/3 for W+

Problem: knowledge of quark direction not accurate!May choose charge-signed rapidity difference .Get a dip at -0.3 in SM.

But, final state radiation or non-SM couplings may wash out the dip!

Tevatron data indicates existence of RAZ, to be studied at LHC.

Note, photon measurement is difficult!

Over all reconstruction eff. of (trigger + offline): 97-93% for mass range 0.2 to 5 TeV. Mass resolution ~ 1.8 to 6% for mass range 0.2 to 5 TeV. Effect of initial misalignment: 2.3% at Mz, 25% at 3 TeV………..long term … 1.1% 5%

Backgrounds: ZZ, WZ,WW, tt mostly negligible.DY production of bb-pair and their semileptonic decays: also negligible after isolation.

Forward-backward asymmetry:Systematic uncertainties dominate for L > 100 fb-1 & M > 500 GeVStatistical uncertainty dominate for M > 1 TeV

Drell-Yan events

Conclusion

W and Z events provide important early measurements at LHC.

Help to understand detectors and physics performance .Large samples for systematic studies.

Precision measurements with data of 1 fb-1 get limited by thoeretical uncertainties reduce theoretical errors, mainly by constraining PDFs.

Rates of diboson production provide order of magnitude improvement on current limits.Possibility to probe anomalous gauge couplings already with a few fb-1

EW physics will continue to play crucial role beyond early phase.

Even after finding a Higgs signal, precise value of W mass is important:• A Higgs is not necessarily a SM Higgs!• There may be more than one Higgs!• Indirect constraints will help interpretation of gauge symmetry breaking.

Backup

p-p detectors at LHC: ATLAS & CMS

W e v rapidity ditribution

CTEQ61

MRST02 ZEUS02

CTEQ61

MRST02 ZEUS02

e- rapidity e+ rapidity

GeneratedGenerated

y

d(W

e

)/dy

y

d(W

e

)/dyReconstructed Reconstructed

u d W e ν

d u W e ν

W production at LHC over |y|< 2.5 => 10e-4<x1,2< 0.1

Low x region dominated by g qq: sea present PDFs have large uncertainties at low x (4-8%)

In global fits, LHC data will constrain PDFs

Early measuremnets of e+,e- angular distr.At LHC will discriminate among PDFsIf experimental precision < 5%

x1,2 = 1/√s * M e±y

Semiclassical W

Interaction of W with electromagnetic field : completely determined by 3 numbers1. el. Charge of W: effect on el. Field ~ 1/r2 2. magnetic dipole moment: effect on B-filed goes as 1/r33. Electric quadrupole momnet: ~1/r4. Measuremnet of TGC is equivalent to measuring the 3rd and 4th numbers. Due to higher powers of r, they are measured better at lower r(ie, higher energy)