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ElectroWeak Physics at LHC

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

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ElectroWeak Physics at LHC

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

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

  3. Very Early phase of LHC data-taking LHC Start-up schedule and energy Proton-proton collision at LHC expected in 2009 2nd half. Initial beam energy 450 GeV. Physics run @10 TeVduring September to November, integrated luminosity ~ few pb-1. LHC in 2010: 14 TeV operation, expected integrated luminosity ~ 2.5 fb-1. • 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. Very large event samples! •  in 10 /pb: • 150K W e n, 15K Z ee, 10K tt • Statistical accuracy will be reached fast.  allows detailed systematic studies. Theoretical situation at LHC startup: W,Z cross-section known to ~ 3% ttbar cross-section to ~ 10% Min. bias charged multiplicity to ~ 50% ! Utility of W, Z events at LHC start up • 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

  5. Electroweak physics at LHC : Measurements with Standard Candles • 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! 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. Precision ElectroWeak Physics • The Standard Model (SM) is remarkable: at tree level, only three parameters are needed to predict everything. (eg., GF, a 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.

  7. Theoretical issues for W, Z physics at LHC • 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  DMw ~ 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.

  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. Accurate experimental measurements with W and Z • To deriveprecise 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

  10. Precision measurement of MW 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

  11. Production of W and Z Same QCD effects for both! Use Z to predict W pT spectrum • Precision MC needed to correct for • Different phase-space (MW MZ) • Different EWK couplings Two approaches: • Model pT(W) with the measured pT(Z) :  promising,estimated precision MW 2 MeV 2. Fit W with measuredtransverse 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)

  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 Systematic uncertainties in Mz measurement with muons 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). 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 Determination of W-mass • MT distribution: • independent of PtW to 1st order • detector effects dominated by • resolution pf pT(n) 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: DMW = 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 ….

  16. Drell Yan process: • 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. DY background negligible, <1.5 events at 1 /fb for M_ll > 1.5 TeV!

  17. Determination of sin2θW(MZ2): Forward-Backward Asymmetry [%] 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 •  Measurement possible only in electron channel 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

  18. Double Gauge Boson signal in detectors Cross-section range from 20 pb for Wg to 0.2 pb ZZ production. Signals with g in final state has important background due to fake photons Almost background-free! 5s observation with ~ 150 /pb! High significance in first /fb!

  19. Double Gauge Boson Production at LHC 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

  20. Wg production: 3 interfering diagrams produce finite cross-section CP conserving effective Lagrangian for WWg 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. Drell-Yan events 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

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

  27. Semiclassical W • Interaction of W with electromagnetic field : completely determined by 3 numbers • 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 3rd and 4th numbers. • Due to higher powers of r, they are measured better at lower r(ie, higher energy)

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