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ElectroWeak Physics at LHC

This talk discusses the plan for studying ElectroWeak physics at the LHC, including the production cross-sections, mass measurement, asymmetries, and di-boson production. It also covers the LHC machine, detectors, and the early phase of data-taking. Theoretical issues and challenges for W, Z physics are also addressed.

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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) On behalf of CMS & ATLAS Collaboration HCP 2008, Galena May 27, 2008

  2. Plan of talk • LHC machine and p-p Detectors • W,Z physics -- production cross-sections -- mass measurement -- asymmetries • Drell-Yan events • Di-boson production at LHC Won’t cover EW physics related to Top quark, bottom quark, Higgs boson and Beyond Standard Model Will cover topics which can be done with upto few fb-1

  3. LHC Machine schematics

  4. LHC Start up schedule and energy LHC in 2008: CM energy 10 TeV Machine cool down: end-June Experimental caverns closed: mid-July Commissioning starts: end July Expect collision at 10 TeV during September to November 40 Physics days @ 5·1031 Expected integrated luminosity ~ 20 pb-1 Machine shut down: end- November LHC in 2009: Hardware commissioning to beam energy of 7 TeV. Machine checkout ~ 1month Commissioning with beam @ 75 ns ~ April 25 ns operation follows. 150 days Physics run during May to Nov. @ 1033 Expected integrated luminosity ~ 2.5 fb-1

  5. ATLAS and CMS p-p detectors at LHC: ATLAS & CMS |η|<2.5 coverage |η|<2.6 coverage • Tracker • EM Calorimeter • HAD Calorimeter • Muon Spectrometer |η|<3 coverage |η|<3 coverage |η|<4.9 coverage |η|<4.9 coverage |η|<2.6 coverage: |η|<2.7 coverage: (1TeV muons) (1TeV muons)

  6. CMS mostly built on surface and then lowered into cavern: ATLAS built inside cavern. CMS Endcap detector being lowered in the cavern.

  7. Central Beam Pipe Installation CMS Detector Status Trial of Forward Pixel installation Endcap Beam pipe, installed on April 30 Cosmic Run at Zero Tesla: May 5-9. Collected 30 Million cosmic triggers. Data reconstructed at Tier0, latency < 1 hr. Data shifted to CMS Analysis Facility, Tier1, Tier2 centres for prompt analysis. Computing Challenge on-going. June : Cosmic Run at 4 Tesla. CMS closed by mid-July.

  8. Early phase of LHC data-taking • Crucial for understanding the detector and its performance. • Consistency check for Standard Model processes. • Conditions envisaged during early phase: • Low luminosity detector ready • tracking system not fully aligned (CMS : 100 mm) • Calorimeter not fully calibrated (CMS: ECAL ~ 1.5%, HCAL ~ 5%) • Suitable trigger menu for startup, pilot-run, low –luminosity: • eg., f-asymmetry, p0 for ECAL calibration, minimum bias events, soft-jets, sum-Et, e, m w/o tracking isolation, … • Earliest data to understand beam background, pile up, tracking eff. • Extraction of offline lepton identification efficiencies, etc. • From underlying events main observables: charge, energy densities • Validate and tune MC: preparation for New Physics

  9. Electroweak physics at LHC : Measurements with Standard Candles • will be done from early phase, from few pb-1 onwards . • Early discovery is challenging have to rediscover Standard Model first. • W,Z measurements are some of the first analysis at LHC •  Large cross-sections • Clean leptonic signatures • Robust selections (loose mass cuts, etc.) possible • Events samples can be selected with high efficiency and purity • Data driven methods are essential. • Theoretical situation at LHC startup: • W,Z cross-section known to ~ 3% • ttbar cross-section to ~ 10% • Min. bias charged multiplicity to ~ 50% !!! • Typical rate at 1032 cm2 s-1 • Inelastic: 107 Hz • bb production: 104 Hz • W  ln : 1 Hz, Z ll : 0.1 Hz • tt production: 0.1 Hz • SM Higgs (mH= 115 GeV): 0.001 Hz

  10. Foreseen ElectroWeak measurements at LHC: Measurement of single W, Z processes Drell-Yan Events (beyond Mz) Di-boson productions Various measurements will be performed at a kinematic region different than earlier experiments. • LHC is W/Z factory  statistics is not at all an issue • (Wl) ~ 19 nb  107 events in L dt = 1 fb-1 • (Zll) ~ 1.9 nb  106 events in L dt = 1 fb-1 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

  11. Theoretical issues for W, Z physics at LHC • Sea-sea parton interaction dominate at low-x and high Q2 • ~ 25% of W at LHC are produced by heavier flavours . • Strong correlation between induced variations of W, Z distrns. • Precision calculations mandatory for precision measurements. • Large uncertanties (due to soft gluon emission) affect the transverse momentaum, rapidity,.. preditions for W and Z . • Distrtibutions, relevant for extracting prescision EW parameters , are • predicted accurately by perturbative QCD resummation calculation • fortunately NNLO calculations are available. • Also significant electroweak corrections to gauge boson hadroproduction (taking into account photon radiations): • K_EW (NLO) > K_QCD (NNLO) • No MC generator , which takes care of both , is available till now !

  12. Necessity for accurate measurements with W and Z • To derive precise measurements of the electroweak parameters • MW, G W, sin2 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... • 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 g radiation. • Pole approximation agrees well with full calculation around MW.

  13. Precision measurement of MW • Consistency check of SM (constraint on gauge sym. breaking) • Indirect measurement of Higgs and SUSY • For equal ∆Mt and DMw get ∆MH indirectly •  ∆Mw ~ 0.007 ∆mt • Statistically with 10 fb-1 data: electron, muon channel  DMW ~ 2 MeV • Aimed precision of ∆MW~ 15 MeV per expt., ∆mt ~ 2 GeV •  MH estimate within 30% • requires all contributions to W-mass uncertainty below ~ 10 MeV • Main experimental issues: • Imperfect estimation of absolute energy scale due to uncertainties in kinematic distr. of W  Alternative methods to estimate MW at LHC essential.

  14. Production model 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 effects cancel in the ratio, • also exactly calculable in pQCD

  15. Early Measurements of W,Z in electron channel CMS: simple and robust electron selection suitable for imperfect calibration, alignment during initial phase. Tag and Probe method for determination of selection eff. from data. Estimation of QCD background from data (relaxing electron isolation, missing Et spectrum does not change)

  16. CMS results expected at 10 pb-1 in electron channel Wen Z/g* e+e- Nsel - Nbkg. 67954 ± 674 3914 ± 63 (Nbkg=0) Eff. for tag & probe (%) 65.1 ± 0.5 68.1 ± 0.6 Acceptance (%) 52.3 ± 0.2 32.39 ± 0.18 Int. lumi (pb-1) 10 10 s * Br (nb) 19.97 ± 0.25 1.775 ± .034 Cross-section used (nb) 19.78 1.787 Systematic uncertainties dominate for L > 1 fb-1 Potential sources: uncertainties in background estimate uncertainties in background subtraction/correction unaccounted correlations Other sources of inefficiency

  17. W,Z in muon channel: CMS, 10 pb-1 Efficiencies to be determined by tag-and-probe method from data. Background for Wmn: Determination of QCD rate by matrix method  simultaneous analysis of 2 almost uncorrelated background discriminating variables, e.g., SPT of tracks in m- isolation cone vs. MT(W). NQCD = NA*NB/NC, assuming small correlations among cuts NA: events with SPT <3 GeV, MT< X, NB: events with SPT > Y, MT> 50 GeV, NC: events with SPT > Y, MT< X Shape of W mn events from Z mm events Calorimetric missing Et = missing Et distribution in Z mm events

  18. CMS performance at 10 pb-1 Reconstruction efficiency Misaligned detector, distorted mag. field Pt scale ~2.7% Tracker misal.~.9% m system misal. ~ .3% Mag. Field err ~.5%

  19. Fractional uncertainties on Z  mmcross-section, 100 pb-1 • ATLAS : 0.004(stat) ± 0.008(sys) ± 0.02(th) ± 0.1(lumi) • CMS : 0.004(stat) ± 0.011(sys) ± 0.02(th) ± 0.1(lumi) • systematics at the 1% level (efficiencies,background,. . . ) • theoretical error at 2%, rel. acceptance determination and PDFs • limited by luminosity uncertainty: 10%. • Comparison with W measurements: • though higher production rate, lower trigger eff. • moderate value to missing transverse energy • no precise mass constraint • reasonable dependence on tracker and muon detectors’ performance • Higher systematics for W, more background  use knowledge acquired from Z

  20. Cross section measurement accuracies for muon channel CMS results for L =1fb-1 s(Zmm + X) = 1160 ± 1.5 (stat) ± 27 (syst)  116 (lumi) pb Exp. error due to uncertainty in tracker effeciency: 1% + ... Theo. error due to Pt effects LO/NLO : 1.83% + PDF choice : 0.7% +... • s(Wmn + X) = 14700 ± 6 (stat) ± 485 (syst)  1470 (lumi) pb • Theo. error due to Pt effects LO/NLO : 2.29% + PDF choice : 0.9% +... • Exp. error due to uncertainties in Missing Et: 1.33% + • Trigger efficiency: 1% + Muon efficiency: 1% + .... Momentum, energy scales of leptons to be done via accurate measurement of Z-mass, during commissioning of physics tools. Additionally various systematic studies for the detector with Z ll

  21. Systematic uncertainties in MZ measurement with muons Experimental: Detector misalignment Knowledge of magnetic field Collision point uncertainty Pile up effects Underlying events Overall systematic uncertainty of <0.35% for both detectors Theoretical: PDF choice ~ 0.9% Initial state radiation ~ 0.2% pT effects (LO to NLO) ~ 1.8% Uncertainties in ATLAS: Mu reco eff ~ 0.6% Kinematic cuts ~ 0.3% Trigger ~0.2% Isolation ~0.2% Uncertainties in CMS: Tracking ~ 1% Trigger ~0.2% Isolation ~0.2%

  22. Determination of MW • Traditional template method:simple, suffers from uncertainties due to • lepton energy,momentum scales, resolutions, recoil model of W, PDF, • GW, bkg. etc. • 2. Prediction of W-distributions, e.g. pTl, MT(W), sensitive to MW, to be • determined from corresponding distributions of Z • scaled observables method, only the difference between W and Z matters Precision expected: DMW ~2 MeV MT,pTe in Wen decay

  23. Effect of PDF on W/Z production model Correlation of W and Z rapidity MW  20 MeV CMS-NOTE 2006/061 M.Boonekamp, 2007 • Expected DMW 1 MeV after PDF uncetainty reduction • Achievable at the LHC from dZ/dy measurement at 10 fb-1

  24. Potential of constraining PDF ATLAS W-data: Simulate real experimental conditions: 1M “data” sample with CTEQ6.1 PDF through detector simulation, with imposed 4% systematic error Redo global ZEUS PDF fit (with Det.Gen. level correction). Shift in central value. ZEUS-PDF BEFORE including W data e+CTEQ6.1 pseudo-data ZEUS-PDFAFTER including W data low-x gluon shape parameter λ, xg(x) ~ x –λ: BEFORE λ = -0.199 ± 0.046 , AFTER λ = -0.186 ± 0.027  41% reduction in error

  25. 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 With 100 pb-1 @ 14 TeV, range probed < 800 GeV Di-lepton mass spectrum CMS

  26. Double Gauge Boson Production at LHC • Consequence of non-Abelian structure of Standard Model • self-coupling of gauge bosons : triple gauge coupling (TGC) • Charged TGC: WWZ, WWg  allowed in SM  study WW and WZ processes • 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 • Anomalous coupling • enhancement of cross-section at high pT (g- and l-type couplings) •  changes in h and angular distributions (k-type coupling) • Also di-boson production: important and irreducible background for New Physics searches .

  27. ZZ signal: keep only ZZ events On-shell Z:70 GeV < MZ < 110 GeV ZZ (72%), Zg* (26%), gg(2%) sNLO(ZZ4e)=18.7 fb 10 fb-1 Nearly background free!

  28. Ideal simulation miscalibration Systematic effects WZ 3l 4 channels combined (3e,2e1m,21e,3m) Backgrounds: Zbb, ZZ High significance in the first fb-1 ! Expected signal and background yields for 1 fb-1 5 s observation : ZZ : ~1 fb-1 WZ : ~150 pb-1

  29. Conclusion • W and Z events provide important early measurements at LHC. • Crucial to understand detectors and physics performance . • Large samples possible for systematic studies. • Precision measurements of EW variables with data > 1 fb-1 get limited by theoretical uncertainties. 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.

  30. Backup

  31. Most general description of the TGC vertex by an Lorentz invariant effective Lagrangian V=Z,γ Δκ= κ -1 gZ 1Δ = g Z -1 WWγ : Dkλγ Δκ= κ -1 , ΔgZ 1 = gZ 1 -1Δ = g Z -1 ( gg= 1 : EM gauge invariance) Atlas-Phys-Pub-2006-011

  32. TAG Jacobian peak PROBE Measurement of lepton efficiency: ATLAS: Boonekamp, Besson CMS Knowledge of Z-decay distribution and involved errors needed in the differential method Tag and Probe method : use one lepton to search for the other, needs detector redundancy Critical for electrons, MW  10 MeV, Muons ~ 2 MeV

  33. LHC commissioning The commissioning of and unprecedently complex LHC machine and the detectors is a big challenge, matched only by the anticipation of Discovery. Only with well commissioned experiments and proved performance we will be able to open the door to the new physics world! Some examples of performance

  34. Backgrounds for W  e nchannel @ 1fb-1 : • Main sources: Zll (1%) , W (1%), Z (0.2%) •  predictable shape and rate • QCD bkg. expected small (0.1%) after tight lepton selections • Shape and rate uncertain  can be checked with data [CMS-NOTE 2006/061] MW = 5 MeV for B/B= 5% Projected uncertainties for 10 fb-1: PDFs : 2 MeV, QCD corrections: 2 MeV, lepton eficiency: <10 MeV Energy& scale resolution: < 10 MeV, backgrounds < 5 MeV

  35. e-rapidity e+ rapidity CTEQ61 CTEQ61 MRST02 MRST02 ZEUS02 ZEUS02 ds(We)/dy Generated Generated y ds(We)/dy 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%

  36. In situ determination of detector performance with Z events: • Efficiency determination in data • ‘Tag and Probe’ method • Limitations: ‘tag’ and ‘probe’ correlations, background processes, Ф-symmetric inefficiencies • Determination of detector resolutions • Folding the Monte Carlo predicted resolution by a smearing function to reproduce the measured Z boson resonance curve Expected precision Tracking error : 0.2 -0.5% Trigger : 0.2% Momentum scale to few per mille

  37. Dilepton resonance : possibility for early discovery with clean signature! DY background : negligible, <1.5 events at 1 fb-1 for M_ll > 1.5 TeV

  38. 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 & Mmm > 500 GeV Statistical uncertainty dominate for Mmm > 1 TeV

  39. Systematics for diboson measurements in CMS WZ Systematic uncertainties on cross section ZZ Systematic uncertainties on significance

  40. Atlas study: λZ, Δκγ, λγ: maximum likelihood fit to 1-d PT (V) distr. Δκz, Δg1z : fit to 2-d distr. of PT(Z) vs. PT(l_W) TGC limits for 30 fb-1:95% CL incl. syst. λZ: (-0.0073, 0.0073) λγ : (-0.0035, 0.0035) ΔκZ : (-0.11, 0.12) Δκγ : (-0.075, 0.076) Δg1Z : (-0.0086, 0.011)

  41. WW→lνlν @ Atlas ee μ μ eμ Tot./fb-1 NS 36.7 37.6 284.4 358.7 NB 188.6 112.1 59.4 360.1 SL 2.6 3.4 25.3 16.6 Reject jet contribution NS/√NB 2.7 3.6 36.9 18.9 WZ→lllν @ Atlas tt 3e 2e1 μ 1e2μ3μTot./fb-1 NS 16.9 17.1 21.9 19.8 75.7 NB 1.7 0.9 1.7 2.0 6.3 SL 7.4 8.6 8.9 8.0 16.4 Reject low Pt Fake events ZZ→llll @ Atlas WW Zg 13.4 candidate events are expected at 1 fb-1. Almost background free ! Zj ZZ

  42. Morphing method for MW measurement pT(ll) MW =0.3pT m*(ll) Kinematic transformation, measured from Z events, parametrised as a function of boson masses and widths phenomenological approach. • Fit W with measuredtransverse mass distributions in Z events • (one lepton killed) corrected for the cross section ratio . • Scaled pTl method  DMW ~ 30 MeV for 1 fb-1, less systematics. ATLAS

  43. Effect of lepton scale on M_W Expect ~2% at start up, ultimately 0.02% for el. Energy ~1% at start up and 0.02% for muon momentum scale  “Data” - - - Best Fit ____ MC Scale: 1.0038 ± 0.0002 • MW  3 MeV • Z peak sets the scale and resolution at MZ • 10-5 on the scale • 10-4 on the resolution • Differential calibration needed to compensate for non-linearities • Calibrate peak and resolution in energy bins of electron pairs • Propagate uncertainty to pT spectrum and fit • MW < 10 MeV • Caveats: rapidity dependence, QED FSR, efficiency effects ATLAS-PHYS-PUB-2006-07

  44. Determination of sin2θW(MZ2): Forward-Backward Asymmetry [%] • At LHC no asymmetry wrt beam, assume qqbar collision • q direction from boost of Z • 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

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