1. ElectroWeak Physics at LHC Kajari Mazumdar Tata Institute of Fundamental Research, Mumbai, India BSM@LHC, IACS, Kolkata January, 2009

2. 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.

3. Proton-proton collision at LHC expected in 2009 2 nd 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

4. 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 Z  ℓℓ (ℓ = e/μ ) ET miss calibration from W  μν and Z  ττ 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

5. 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 LHC Caveats: 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.

6. The Standard Model (SM) is remarkable: at tree level, only three parameters are needed to predict everything. (eg., GF,  and sin2 w) 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

7. 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 LHC More energy available for extra jet radiation. Distrtibutions, relevant for extracting prescision EW parameters, are predicted accurately by perturbative QCD resummation calculation Fortunately NNLO calculations are available. Theoretical issues for W, Z physics at LHC

8. 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 uncertainty Relevant 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 !

9. 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

10. Consistency check of SM (constraint on gauge sym. breaking) Indirect measurement of Higgs and SUSY For equal contributions of ∆M t and  M w  to ∆M H, need ∆M w ~ 0.007 ∆m t  requires all contributions to W-mass uncertainty below ~ 10 MeV Precision measurement of M W

11. Same QCD effects for both! Use Z to predict W p T spectrum  Precision MC needed to correct for Different phase-space (M W  M Z ) Different EWK couplings Two approaches: 1.Model p T (W) with the measured p T (Z) :  promising, estimated precision   M W  2 MeV 2. Fit W with measured transverse distributions in Z events (one lepton killed) corrected for the cross section ratio soft gluon emission cancels in the ratio  exactly calculable in (actually most of the uncertainties cancel in the ratio) Production of W and Z

12. Early Measurements of W,Z in electron channel Systematic uncertainties dominate for L > 1 fb-1 Effect 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, )

13. 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 events Theoretical: PDF, initial state radiation Pt effects (LO to NLO)

14. Measurement of W-mass Traditional 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 real W-events to extract W-mass. R Ratios 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

15. Traditionally, use transverse mass and transverse momentum of lepton MT distribution: independent of PtW to 1 st order detector effects dominated by resolution pf pT( ) ie, Etmiss  MT 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 LHC Very tough job! Can’t be obtained quickly. ATLAS expectation: ~ 7 MeV per lepton channel with 10 /fb CMS: ~ 10 MeV …. Determination of W-mass

16. 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 TeV!

17. Determination of sin 2 θ W (M Z 2 ): Forward-Backward Asymmetry Stat. Error for L = 100 fb -1 (ATLAS, |Yz|>1) ∆A FB ∆sin 2 θ W |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 Z quark direction is the same as the boost of the Z  Measurement possible only in electron channel  Measurement possible only in electron channel

18. 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! 5  observation with ~ 150 /pb!

19. 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 Physics Important and irreducible background for Higgs search (H--> WW) and New Physics searches Double Gauge Boson Production at LHC

20. W  production: 3 interfering diagrams produce finite cross-section CP conserving effective Lagrangian for WW  couplings, SM  Unitarity may be violated for non-SM  needs form factor 

21. 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!

22. 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 GeV Statistical uncertainty dominate for M  > 1 TeV Drell-Yan events

23. 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.

24. Backup

25. p-p detectors at LHC: ATLAS & CMS

26. W  e v rapidity ditribution CTEQ61 MRST02 ZEUS02 CTEQ61 MRST02 ZEUS02 e - rapidity e + rapidity Generated y d  (W  e )/d y y Reconstructed W production at LHC over |y| 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 PDFs If experimental precision < 5% x1,2 = 1/√s * M e±y

27. Semiclassical W Interaction of W with electromagnetic field : completely determined by 3 numbers 1.el. Charge of W: effect on el. Field ~ 1/r2 2. magnetic dipole moment: effect on B-filed goes as 1/r3 3. Electric quadrupole momnet: ~1/r4.  Measuremnet of TGC is equivalent to measuring the 3 rd and 4 th numbers.  Due to higher powers of r, they are measured better at lower r(ie, higher energy)