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Electroweak physics at LHC

Electroweak physics at LHC. Marina Cobal, University of Udine ‘IFAE’, Torino Apr 14 - Apr 16, 2004. Outline. Introduction W mass measurement Top mass measurement EW single top quark production: measurement of V tb . A FB asymmetry in dilepton production: sin 2 θ eff lept (M Z 2 ) .

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Electroweak physics at LHC

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  1. Electroweak physics at LHC Marina Cobal, University of Udine ‘IFAE’, Torino Apr 14 - Apr 16, 2004

  2. Outline • Introduction • W mass measurement • Top mass measurement • EW single top quark production: measurement of Vtb. • AFB asymmetry in dilepton production: sin2θefflept(MZ2). • Conclusion.

  3. pp (mainly) at s = 14 TeV Startup in April 2007 Initial/low lumiL~1033 cm-2 s-1  2 minimum bias/x-ing  “Tevatron-like” environment 10 fb-1 /year Design/high lumiL=1034 cm-2 s-1 after ~ 3 years ~ 20 minimum bias/x-ing  fast ( 50 ns) radhard detect 100 fb-1 /year LHC and experiments TOTEM 27 km ring 1232 dipoles B=8.3 T ATLAS and CMS : pp, general purpose ALICE : heavy ions, p-ions LHCb : pp, B-physics

  4. ATLAS cavern CMS yoke Huge activities in construction of both CMS and ATLAS

  5. from n, m decays from meson decays GF (1) VCKM (4) mfermions (9) from direct measurements SM (17 parameters) predictions mbosons (2) (down to 0.1% level) mH not discovered yet H  ln(mH/mW) t  mt2 What we know No observable directly related to mH. Dependence through radiative corrections. Example: mW=mW(mt2,log(mH)) By making precision measurements (already interesting per se): • one can get information on the missing parameter mH • one can test the validity of the Standard Model

  6. Where we are Uncertainties on mt, mW are the dominating ones in the ew fit Each “observable” (Aq, Al, Rq, Rl, , m, sinW,…) can be calculated as a function of: had, s(mZ), mZ, mt,mH At the Tevatron run II: mt  2.5 GeV ; mW  30 MeV  mH/mH  35% What can we expect/should be our goal at LHC?

  7. √s [TeV] Luminosity [cm-2s-1] ∫L [fb-1/y] TeVatron 2 <1032 0.3 LHC (low lum) 14 1033 10 LHC LHC (high lum) 14 1034 100 TeVatron Predictions - -

  8. n e W beam line (missing pT) MW measurement at the LHC year 2004: MW 30 MeV , year 2007: MW< 30 MeV (Lep2+Tevatron) • mtop and MW: equal weight in the EW fitifMW 0.007 mtop at LHC: mtop 2 GeV gives the precision MW :  15 MeV • Methods: same as at the Tevatron, with Wl decays butLHC statistics is higher Wl 60 M reconstructed @ lowL(x10 RunII) • PTl spectrum • R=MTW / MTZ spectrum • MTW Pile-up , theo. know. of PTW Small syst, sample /10 Best choice for low lum phase

  9. Mw measurement: Transverse Mass • Use Transverse mass to cope with unmeasured PL • W mass: fit exp. shape to MC sample with different Values of MW • stat. error negligible • syst. error: MC modelling of the physics • and the detector responses physics detector

  10. Advantages of LHC Profit from Tevatron experience: Many potential hidden sources of systematics from the detector description in the simulations (+pile-up, + radiation)  use data samples for in-situ calibrations ! Advantages of the LHC: Huge statistics of signal & control samples like Z leptonic decays:  Larger production cross-section for the control process Z(e/)(e/) (6 millions/y/exp)  granularity + acceptance + resolution are better Error from control samples at the LHC < than at the Tevatron

  11. Detector systematics 0.02% on lepton E/p scale precision needed  Zee, Z mass reconstruction  leptonic decays of , J/ ~15 MeV/y-e-l can be reached … and ~1-2% uncertainty in E+p resolution  Z width reconstruction  beam tests data ~5 MeV/y-e-l can be reached Recoil modelling (UE + detector)  determine response/resolution of the recoil (+UE) from Z data ~5 MeV/y-e-l can be reached low extrapolation error ZW

  12. Implicit SM assumption to determine W From LEP From theory Physics systematics pT(W)  use pT(Z) from Z, use pT(W)/pT(Z) to model MC. ~5 MeV/y-e-l can be reached Uncertainties on the parton density functions  Compare different models, constrain pdfs through Z+W data ~10 MeV/y-e-l can be reached W width  use measurement of <10 MeV/y-e-l can be reached Radiative decays  constrained through W? <10 MeV/y-e-l, theory work needed Background  checks on Z and W (composition similar to Tevatron) <5 MeV/y-e-l should be reached

  13. MW , Wl one lepton species low lum, per exp., per year Much better! Internal calibration from Z data mainly. Need excellent control of energy flow+ p scale • Combining channels and exp., should reach MW  15 MeV

  14. Cross section determined to NLO precision Total NLO(tt) = 834 ± 100 pb Largest uncertainty from scale variation Compare to other production processes: Top production cross section approximately 100x Tevatron Mtop measurement: production ~90% gg~10% qq Low lumi Opposite @ FNAL LHC is a top factory!

  15. Br(ttbbjjl)=30%for electron + muon Golden channel Clean trigger from isolated lepton The reconstruction starts with the W mass: different ways to pair the right jets to form the W jet energies calibrated using mW Important to tag the b-jets: enormously reduces background (physics and combinatorial) clean up the reconstruction Lepton side Hadron side MTop from lepton+jet Typical selection efficiency: ~5-10%: • Isolated lepton PT>20 GeV • ETmiss>20 GeV • 4 jets with ET>40 GeV • >1 b-jet (b40%, uds10-3, c10-2) Background: <2% W/Z+jets, WW/ZZ/WZ

  16. Hadronic side W from jet pair with closest invariant mass to MW Require |MW-Mjj|<20 GeV Assign a b-jet to the W to reconstruct Mtop Kinematic fit Using remaining l+b-jet, the leptonic part is reconstructed |mlb -<mjjb>| < 35 GeV Kinematic fit to the tt hypothesis, using MW constraints j2 j1 b-jet t Lepton + jet: reconstruct top Borjanovic et al., SN-ATLAS-2004-040 W-mass • Selection efficiency 5-10%

  17. Method works: Linear with input Mtop Largely independent on Top PT Biggest uncertainties: Jet energy calibration FSR: ‘out of cone’ give large variations in mass B-fragmentation Verified with detailed detector simulation and realistic calibration Mtop systematics Challenge: determine the mass of the top around 1 GeV accuracy in one year of LHC

  18. Mtop Alternative mass determination • Select high PT back-to-back top events: • Hemisphere separation (bckgnd reduction, much less combinatorial) • Higher probability for jet overlapping • Use the events where both W’s decay leptonically (Br~5%) • Much cleaner environment • Less information available from two ’s • Use events where both W’s decay hadronically (Br~45%) • Difficult ‘jet’ environment • Select PT>200 GeV Various methods all have different systematics

  19. Use exclusive b-decays with high mass products (J/) Higher correlation with Mtop Clean reconstruction (background free) BR(ttqqb+J/)  5 10-5  ~ 30%  103 ev./100 fb-1(need high lumi) Mtop from J/ MlJ/ Different systematics (almost no sensitivity to FSR) Uncertainty on the b-quark fragmentation function becomes the dominant error M(J/+l) M(J/+l) Mtop

  20. Determination MTop in initial phase Use ‘Golden plated’ lepton+jet Selection: Isolated lepton with PT>20 GeV Exactly 4 jets (R=0.4) with PT>40 GeV Reconstruction: Select 3 jets with maximal resulting PT Signal can be improved by kinematic constrained fit Assuming MW1=MW2 and MT1=MT2 Commissioning the detectors • Calibrating detector in comissioning phase • Assume pessimistic scenario: • -) No b-tagging • -) No jet calibration • -) But: Good lepton identification No background included

  21. Signal plus background at initial phase of LHC Most important background for top: W+4 jets Leptonic decay of W, with 4 extra ‘light’ jets Alpgen, Monte Carlo has ‘hard’ matrix element for 4 extra jets(not available in Pythia/Herwig) Commissioning the detectors ALPGEN: W+4 extra light jets Jet: PT>10, ||<2.5, R>0.4 No lepton cuts Effective : ~2400 pb L = 150 pb-1 (2/3 days low lumi) With extreme simple selection and reconstruction the top-peak should be visible at LHC measure top mass (to 5-7 GeV) give feedback on detector performance

  22. Direct determination of the tWb vertex (=Vtb) Discriminants: - Jet multiplicity (higher for Wt) More than one b-jet (increase W* signal over W- gluon fusion) 2-jets mass distribution (mjj ~ mW for the Wt signal only) Three production mechanisms: Main Background [xBR(W→ℓ), ℓ=e,μ]: ttσ=833 pb [ 246 pb] Wbbσ=300 pb [ 66.7 pb] Wjjσ=18·103 pb [4·103 pb] Single top production 1) Determination of Vtb 2) Independent mass measurement +16.6 Wg fusion: 245±27 pb S.Willenbrock et al., Phys.Rev.D56, 5919 Wt: 62.2 pb A.Belyaev, E.Boos, Phys.Rev.D63, 034012 W* 10.2±0.7 pb M.Smith et al., Phys.Rev.D54, 6696 -3. 7 Wg [54.2 pb] Wt [17.8 pb] W* [2.2 pb]

  23. Single top results • Detector performance critical to observe signal • Fake lepton rate • b and fake rate id  • Reconstruction and vetoing of low energy jets • Identification of forward jets • Each of the processes have different systematic errors for Vtb and are sensitive to different new physics • heavy W’  increase in the s-channel W* • FCNC gu  t  increase in the W-gluon fusion channel • Signal unambiguous, after 30 fb-1: • Complementary methods to extract Vtb • With 30 fb-1 of data, Vtb can be determined to %-level or better(experimentally)

  24. Drell-Yan production at the LHC Goals: • Measure pdf • Determine parton lumi • Measure sin2W

  25. sin2leff measurement • sin2θefflept is one of the fundamental parameters of the SM! • precise determination will constrain the Higgs mass and check consistency of the SM. • From AFB in p-pZ, l+ l- • at the Z pole, AFB comes from interference • of the V and A components of the couplings • [hep/ph9707301] • At pp collider, forward direction not naturally definedBUT • -Always from sea  softer: direction given by the sign of y(ll) • - Asymmetry increase with y(ll) AFB = b { a - sin2θefflept( MZ2) } a and b calculated to NLO in QED and QCD.

  26. L= 100 fb-1 sin2leff measurement • To reach World Average precision, need to • use full calorimeter coverage (e channel only) • - Z + - ||<2. • - Z e+ e- ||<2.5 jet rejection factor 1000 • - Z e+ e- 2.5<||<5 jet rejection factor 10 - 100 [%] σ ( Z → l +l - ) ~ 1.5 nb (for either e or μ) ( statistical ) ( statistical ) • Main sys. effect: uncertainty on pdf, lepton acceptance (~0.1%), radiative correction Can be further improved: combine channels/experiments.

  27. Conclusions: • LHC will allow precision measurements: unexplored kinematic regions, high-statistics (W, Z, b, t factory); • MWcan be measured with a precision of 15 MeV (combinig e/μ and ATLAS + CMS); • Mtop: ~ 1 GeV(combined with ∆mW~15 MeV, constrains MH to ~ 25%); • EW single top production: direct measurement of Vtb; measurement of top polarization (Wg with statistical precision of ~ 1.6%); • sin2θefflept( MZ2) can be determined with statistical precision of 1.4x10-4(competitive to lepton collider measurements!)

  28. challenge • lepton E,p scale • goal ~0.02% mandatory for MW • knowledge of ID material to 1% • precise alignment (<1m) • precise magnetic fields maps (<0.1%) • differents sub-detectors concerned •   jet energy scale (light jets / b jets) • goal ~1%mandatory for Mtop • coverage • ||<2.5  • ||<5 e, , jets ATLAS challenge CMS

  29. Backup Stan Bentvelsen Moriond-QCD 2004

  30. MW & Mtop measurement- From D. Glenzinski (CDF), ICHEP2002 10 fb-1 of LHC (per experiment) 2 fb-1 of RunII (per experiment)

  31. tt Cross Section at 14TeV total NNLO:uncertainty from scale (mt/2, 2mt)< 3%!!! (N.Kidonakis, hep-ph/0401147)

  32. Assuming total uncertainty on W-mass of 15 MeV Combined LHC prospect Very challenging measurement! Repeat the Electro-Weak fit changing the uncertainties: Mtop=1 GeV MW =15 MeV Same central values SM constraints on MHiggs: Summer 2003 values: Chances to rule out SM! Consequences from Mtop Thanks to M.Grunewald direct EXCLUDED (mH/mH  32%) (mH/mH  53%)

  33. j2 j1 b-jet Mtop t High Pt sample • The high pT selected sample deserves independent analysis: • Hemisphere separation (bckgnd reduction, much less combinatorial) • Higher probability for jet overlapping • Use all clusters in a large cone R=[0.8-1.2] around the reconstructed top- direction • Less prone to QCD, FSR, calibration • UE can be subtracted Mtop Statistics seems OK and syst. under control R

  34. Use events where both W’s decay hadronically (Br~45%) Difficult ‘jet’ environment (QCD, Pt>100) ~ 1.73 mb (signal) ~ 370 pb Perform kinematic fit on whole event b-jet to W assignment for combination that minimize top mass difference Increase S/B: Require pT(tops)>200 GeV Top mass from hadronic decay • Selection • 6 jets (R=0.4), Pt>40 GeV • 2 b-tagged jets • Note: Event shape variables like HT, A, S, C, etc not effective at LHC (contrast to Tevatron) 3300 events selected: (tt) = 0.63 % (QCD)= 2·10-5 % S/B = 18

  35. Continuous jet algorithm Reduce dependence on MC Reduce jet scale uncertainty Repeat analysis for many cone sizes R Sum all determined top mass:robust estimator top-mass Determining Mtop from (tt)? huge statistics, totally different systematics But: Theory uncertainty on the pdfs kills the idea 10% th. uncertainty  mt  4 GeV Constraining the pdf would be very precious… (up to a few % might not be a dream !!!) Alternative methods • Luminosity uncertainty then plays the game (5%?) Luminosity uncertainty then plays the game (5%?)

  36. W TGC vertex γ/Z W L= 30 fb-1 L= 30 fb-1 ▓ SM ▓ SM ∆g1Z = 0.05 λ1 = 0.01 Triple gauge boson couplings • TGC of the type WWγor WWZ provides a direct test of the non-Abelian structure of the SM (EW symmetry breaking). • It may also indicate hints of new physics: new processes are expected to give anomalous contributions to the TGC. • This sector of the SM is often described by 5 params: g1Z, κγ, κZ, λγ and λγ (SM values are equal to g1Z = κγ = κZ = 1 and λγ = λγ = 0, tree level). • New physics could show up as deviations of these parameters from their SM values. • Anomalous contribution to TGC is enhanced at high √s (increase of production cross-section).

  37. sensitive to high-energy behaviour: mWV, pTV • Variables: Wγ: (mWγ, |ηγ*| ) and (pTγ, θ* ) WZ: (mWZ , |ηZ*| ) and (pTZ, θ* ) sensitive to angular information: |ηV*|, θ* • SM: vanishing helicity at low |η| Non-standard TGC: partially eliminates `zero radiation’ Systematic uncertainties: • At the LHC, sensitivity to TGC is a combination of the very high energy and high luminosity. L = 30 fb-1 Using max-Likelihood fit to mWV |ηV*| • Uncertainties arising from low pT background will be quite small: anomalous TGC signature will be found at high pT. • Theoretical uncertainties: p.d.f.’s & higher order corrections

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