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Electroweak Physics at the LHC Precision Measurements and New Physics

Electroweak Physics at the LHC Precision Measurements and New Physics. PY898: Special Topics in LHC Physics By Keith Otis 4/13/2009. Outline. Electroweak Parameters W-Mass Top-Mass Electroweak Mixing Angle Drell-Yan Forward-Backward Asymmerty in Z Decays (A FB )

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Electroweak Physics at the LHC Precision Measurements and New Physics

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  1. Electroweak Physics at the LHCPrecision Measurements and New Physics PY898: Special Topics in LHC Physics By Keith Otis 4/13/2009

  2. Outline • Electroweak Parameters • W-Mass • Top-Mass • Electroweak Mixing Angle • Drell-Yan • Forward-Backward Asymmerty in Z Decays (AFB) • Triple Gauge Boson Couplings • Charged TGCs • Neutral TGCs • Anomalous Quartic Couplings • Heavy Leptons

  3. Electroweak Parameters • The main parameters of EW theory are measured to very high precision. • The mass of the W (MW) is known with an uncertainty of 0.03% • MW= 80.428 ± 0.039 GeV UA2/CDF/D0, 80.376 ± 0.033 GeV LEP 2 • SM predicts 80.375 ± 0.015 GeV • The uncertainty in the mass of the Top (mt) is 0.7% • mt= 170.9 ± 1.8 GeV • SM predicts 171.1 ± 1.9 GeV • The uncertainty on the electroweak-mixing angle (θw) is 0.07% • cos(θw)= Mw/Mz • The LHC will be able to improve on these in a relatively short period of time.

  4. Tevatron: 2 TeV • LHC: 10-14TeV • LHC @ 1033 Luminosity • 150 Hz W • 50 Hz z • 1 Hz tT • 10 pb-1 of Luminocity • 150k W→eν • 15k Z→ee • 10k tT

  5. W-Mass • W→lv signature • Isolated charged lepton pT > 25 GeV || < 2.4 • Missing transverse energy ETMiss > 25 GeV • No jets with pT > 30 GeV • Recoil < 20GeV • For an integrated luminosity of 1 fb−1, 4 million events with W→lv(ℓ = e or μ) decays are expected. Summer 2005 result 68% CL 68% CL direct indirect ) (

  6. W mass extraction • The W mass is extracted from the measured plT distribution or from the Jacobian peak observed in the transverse mass of the lepton-neutrino system, MWT. • The W mass is obtained by comparing the measured distributions with template distributions generated from data (Z events) or MC.

  7. W-mass: ATLAS ATLAS Based on 10fb-1 of data corresponding to ~10M Wln Fit MT(W) or pT (e) to Z0 tuned MC MC Template method Z-Samples play a crucial role in reducing systematics and theoretical uncertainties Requires further study

  8. W-mass: CMS CMS Scaled observables Systematic uncertainties (MeV) on W-mass for 10fb-1 • Use Z events as templates • Scaled observable using weighting fn • Morphing using kinematic transformation • Limited by Z-statistics • Detector and theoretical effects cancel at least partially • Pt(l) better than MT as it is not sensitive to ET systematics • But needs PT(W) to be understood PT(W) needs to reduced ET-systematics

  9. New Physics? Mar 06 • MW is a fundamental SM parameter linked to the top, Higgs masses and sinqW. LEP2+Tevatron DMW=30MeV Tevatron run2 (2fb-1) 30MeV combined • For equal contribution to MH uncertainty: DMt< 2 GeV DMW < 15 MeV Can get dMH/MH~30% Important cross-check with direct measurements

  10. W-mass summary • A number of methods have been studied • Direct measurement of MT pT(l) • Z-events used to tune W MC • scaled observable pt(l), ‘morphing’ • Z-events used as a template • Systematics greatly improved using Z-samples • All methods are giving DMw in range of 20MeV per channel per variable, so combined <15MeV per experiment seems to be achievable for 10fb-1 • Need to understand correlations • Main issues at ET for MT and PT(W) for pt(l) • DMW ~10MeV looks possible • Requires DMt<1GeV for EW fits

  11. Top Mass • Top quark pairs, mainly produced via gluon fusion, yields a production cross-section of 833 pb, at next to leading order, 100 times higher than at Tevatron. • The "golden" channel is the semi-leptonic channel: • tT→Wb+WB→ (lv)b+(jj)B

  12. Top Mass • Golden Channel event selection: • Isolated high pT lepton, EmissT and at least 4 jets, two of which are b-tagged. • This gives a signal efficiency of ~5% with a signal to background ratio of the order 10. • Primary backgrounds • The main backgrounds are single top events, mainly reduced by the4 jet cut, fully hadronic t T events, reduced by the lepton requirements, W+jet and Z+jet events

  13. Finding the Top Mass • Reconstruction of the hadronic side of the decay is done by minimization procedure. • This minimization constrains the light jet pair mass to Mw, via corrections to the light jet energies. • After trying all possible jet combinations the one minimizing the χ2 is kept. • The b-jet closest to the hadronic W is associated to the chosen pair. • The three jet invariant mass is then fitted with a Gaussian plus a polynomial • The Result: Mt= 175.0±0.2(stat.)±1.0(syst.) GeV, for an input mass of 175 GeV and 1 fb−1.

  14. Drell-Yan • As discussed in previous weeks this is where a heavier neutral gauge boson (Z’) would show up • AFB of the leptons from Zs

  15. Drell Yan • Important benchmark process: • Measure cross-section • parton-parton luminosity functions • constrain PDFs • measure sin2qW • Deviations from SM

  16. Determination of sin2θefflept(MZ2) AFB = b { a - sin2θefflept( MZ2) } Measure Afb with leptons in Z0 DY events Can fit with Mt to constrain MH a, b calculated to NLO QED and QCD. Need to define direction At the Tevatron -- well defined q q • At LHC no asymmetry wrt beam • Assume that there is a • Q-qbar collision • quark direction from y(ll) • Requires measurement at high y(ll) Q q

  17. [%] Determination of sin2θefflept(MZ2) Current error on world average 1.6x10-4 sin2θeff=0.23153±0.00016 Can be further improved by combining Z decay channels Systematics: PDF, lepton acc. (~0.1%), radiative correction calculations

  18. Associated Production of Gauge Bosons

  19. Triple gauge boson couplings • SM gauge group SU(2)LxU(1)Y • WWg and WWZ couplings (charged TGCs) Couplings described by 5 independent parameters All are zero in SM Any deviations is a signal of new physics

  20. Anomalous couplings in WWg CMS ATLAS 30fb-1 ~3000 evts Most sensitive measurement is looking for high pT Zs or gs

  21. Charged TGC predictions Results expected to be ~x10 better than LEP/Tevatron Results are statistics limited (except for g1Z)

  22. Neutral TGCs CMS No tree level neutral couplings in SM All are zero in SM Leads to 3-5 order of magnitude improvement compared to LEP

  23. Quartic Couplings

  24. Anomalous Quartic couplings Look for Wgg, low production threshold at Mw S/B~1 ATLAS 30fb-1 e-ngg ~14 events(~x4 for l+/-ngg)

  25. Heavy Leptons • “Evidence grows for charged heavy lepton at 1.8-2.0 GeV”- Physics Today (1977) • Current limits: mL(±) >100.8GeV • Neutral Heavy Lepton Mass Limits • Mass m> 45.0 GeV, 95% CL (Dirac) • Mass m> 39.5 GeV, 95% CL (Majorana)

  26. Heavy Leptons • Relic abundance of the leptons must not “over-close” the universe. • Can’t provide more than the critical energy density (10-5GeV cm-3) • A stable, charged lepton must have a low enough relic abundance for it not to have been detected in searches for heavy isotopes in ordinary matter • The mass and lifetime of the new leptons musts not be such that they would have been detected in a previous collider experiment. • There are no theoretical constraints found for lifetimes less that ~106 s even for masses up to the TeV scale. • Only limits are the experimental ones

  27. Heavy Leptons • Where do we look for heavy Leptons? • Drell-Yan • Other mechanisms • pp→γγ→L+L- • pp→Zγ→L+L- • Mechanisms for introducing new leptons • New fermionic degrees of freedom • Vector Singlet Model (VSM) • Vector Doublet Model (VDM) • Fermion-mirror-fermion Model (FMFM)

  28. Heavy Leptons • In these new models: • Exotic leptons mix with the standard leptons through the standard weak vector bosons and according to the Lagrangians

  29. L± Detection • Time-of-Flight • Heavy particles • Detectable in both the central tracker and muon chambers • Use measured momentum and time delay to reconstruct the mass

  30. L± Detection • Imperfections in the time and momentum resolutions will cause a spread in the mass peak • Bunch crossing identification • Muons from D-Y and heavy quark decays • For a background signal to look like a heavy lepton neutral current two opposite charge muons would have to be mis-identified at the same time. • Make pT cut at 50 GeV to eliminate heavy quark decays

  31. L± Detection • Detection at the LHC is entirely cross section limited.

  32. L± Detection • Detection of up to 1TeV should be possible a the LHC

  33. Leptons vs. Sleptons • Study the angular distribution

  34. Leptons vs. Sleptons

  35. Heavy Lepton Summary • Assuming standard model couplings and long lifetime: • We can detect heavy charged leptons in intermediate scale models up to 950 GeV with 100 fb-1 • Above 580 GeV it’s hard to distinguish them from scalar leptons

  36. Summary • The Electroweak sector, while one of the better understood sectors of the SM, still holds important information and even some exciting new physics at the LHC

  37. Backup slides

  38. |h|<2.5 Constraining gluon PDF with Ws • Many W+Z measurements have pdf uncertainties • at LHC Q2~MZ2 corresponds to sea-sea collisions  depends on gluon from gqq • Need to improve understanding of gluon

  39. GOAL: syst. exp. error ~3-5% W Rapidity Distributions for different PDFs ~ ±3.6% @ y=0 ~ ±5.2% @y=0 ~ ±8.7% @y=0 CTEQ6.1M MRST02 ZEUS-S ZEUS to MRST01 central value difference ~5% ZEUS to CTEQ6.1 central value difference ~3.5% (From LHAPDF eigenvectors)

  40. e-rapidity e+ rapidity CTEQ61 CTEQ61 MRST01 MRST01 ZEUS-S ZEUS-S Generator Level Error boxes are the Full PDF Uncertainties ATLAS Detector Level with sel. cuts Electron distributions • At Detector level reflects generator level distributions • ~8% PDF uncertainty at y=0 remains

  41. ET First measurements of W and Z W and Z cross-sections For ~1fb-1 data, systematics dominate Main theoretical contribution PT(W/Z) LO-NLO ~2% (CMS) tracker efficiency Initial luminosity uncertainty ~10%, reduced to 5% m-reconstruction efficiency from 20pb-1 Zmm Barrel and endcap To 0.5% in 0.2h bins ATLAS

  42. LHC prediction Tevatron Minimum bias and Underlying Event LHC PYTHIA6.214 - tuned Tevatron ● CDF 1.8 TeV ● CDF 1.8 TeV ~80% ~200% PYTHIA6.214 - tuned UE includes radiation and small impact parameter bias MB only

  43. First measurements at the LHC ?Charged particle density at  = 0 (Only need central inner tracker and a few thousand pp events) LHC? • Min bias events are also crucial for intercalibration •  CMS require 18M events to intercalibrate ECAL in h at 2% • ATLAS studies of use of MB events to study L1 trigger rates

  44. Measuring the minimum bias events at ATLAS Black = Generated (Pythia6.2) Blue = TrkTrack: iPatRec Red = TrkTrack: xKalman dNch/d Only a fraction of tracks reconstructed,: • limited rapidity coverage Measure central plateau • can only reconstruct track pT with good efficiency down to ~500MeV, but most particles in min-bias events have pT < 500MeV  Hard extrapolation. h dNch/dpT Reconstruct tracks with: 1) pT>500MeV 2) |d0| < 1mm 3) # B-layer hits >= 1 4) # precision hits >= 8 pT (MeV)

  45. UE uncertainties LHC PYTHIA6.214 - tuned PHOJET1.12 Transverse < Nchg > x 3 x1.5 CDF definition of UE • Extrapolation of UE to LHC is unknown • Depends on • Multiple interactions • Radiation • PDFs

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