Electroweak physics at the LHC with ATLAS APS April 2003 Meeting – Philadelphia, PA Arthur M. Moraes University of Sheffield, UK (on behalf of the ATLAS collaboration)

APS April 8, 2003 EW physics at ATLASA. M. Moraes Outline LHC and ATLAS. W mass measurement Improvements in the measurements of the mass of the top quark ( m t ). A FB asymmetry in dilepton production: sin 2 θ eff lept (M Z 2 ). EW single top quark production: direct measurement of V tb. Triple gauge boson couplings (TGC). Conclusion.

APS April 8, 2003 EW physics at ATLASA. M. Moraes LHC (Large Hadron Collider): p-p collisions at √s = 14 TeV bunch crossing every 25 ns (40 MHz) Huge discovery potential for new physics: Higgs and Supersymmetry. Large statistics → small statistical error! Processσ (nb)Events/year ( L = 10 fb -1 ) W → e ν 15~ 10 8 Z → e + e ― 1.5~ 10 7 tt 0.8~ 10 7 Inclusive jets p T > 200 GeV 100~ 10 9  low-luminosity: L ≈ 2 x cm -2 s -1 ( L ≈ 20 fb -1 /year)  high-luminosity: L ≈ cm -2 s -1 ( L ≈ 100 fb -1 /year) W, Z, top, b, … factory ! -

APS April 8, 2003 EW physics at ATLASA. M. Moraes ATLAS: A Toroidal LHC AparatuS ~44m ~22m 7,000 tons A. M. Moraes Multi-purpose detector coverage up to | η |=5 ; design to operate at L= cm -2 s -1 Inner Detector (tracker) Si pixel & strip detectors + TRT; 2 T magnetic field; coverage up to | η |< 2.5. Calorimetry highly granular LAr EM calorimeter (| η | < 3.2); hadron calorimeter – scintillator tile (| η | < 4.9). Muon Spectrometer air-core toroid system (| η | < 2.7). Lepton energy scale: precision of 0.02% ( Z → ll ) Jet energy scale: precision of 1% ( W → jj; Z ( ll) + jets) Absolute luminosity: precision ≤ 5% ( machine, optical theorem, rate of known processes)

APS April 8, 2003 EW physics at ATLASA. M. Moraes W mass measurement W mass is one of the fundamental parameters of the SM ( α QED, G F, sin θ W ) Precise measurements will constrain the mass of the SM Higgs or the h boson of the MSSM; Equal weights in a χ 2 test: At the LHC ∆m t ~ 2 GeV At the time of the LHC start-up the W mass will be known with a precision of about 30 MeV (LEP2 + Tevatron) M H (GeV) M W (GeV) experimental lower bound on M H current LHC M W should be known with a precision of about 15 MeV (combining e/ μ and CMS data). (achievable during the low-luminosity phase at ATLAS) constrains M H to ~25%. M W = ± GeV (LEP2 – PDG)

APS April 8, 2003 EW physics at ATLASA. M. Moraes W mass measurement Lepton energy and momentum scale: ~0.1% at Tevatron ~0.02% at LHC – ATLAS (tuned to, l =e, μ) Sources of uncertainty: p.d.f.’s & radiative corrections: improve theoretical calculations!  Detector resolution + pile-up will smear significantly the transverse mass distribution. (method limited to the low-luminosity phase!) Statistical uncertainty : < 2 MeV for L ≈ 10 fb -1 σ = 30 nb ( l =e,μ) 3x10 8 events ( L ≈ 10 fb -1 ) Systematic error a) physics: W p t spectrum, structure functions, W width, radiative decays and background. b) detector performance: lepton scale, energy/ momentum resolution and response to recoil.

APS April 8, 2003 EW physics at ATLASA. M. Moraes Top mass tt production is expected to be the main background to new physics processes: production and decay of Higgs bosons and SUSY particles. = 833 pb at LHC > 8x10 6 events at ( L ≈ 10 fb -1 ) Together with M W, m t helps to constrain the SM Higgs mass. - Precision measurements in the top sector are important to get more clues on the origin of the fermion mass hierarchy. Top events will be used to calibrate the calorimeter jet scale ( W→jj from t→bW ). tt leptonic decays (t→bW) single lepton: W →lν, W → jj 29.6% (2.5 x 10 6 events) di-lepton: W →lν, W →lν 4.9% (400,000 events) L ≈ 10 fb -1 - Jet energy scale: ~3% at Tevatron ~1% at LHC – ATLAS (including W→jj from t→bW) ∆ m t at LHC will be dominated by systematic errors! Final state gluon radiation (~1%) (~7 pb at Tevatron) Best channel for m t measurement will be tt→WWbb→( l ν )(jj)bb --- ∆ m t ~ 2 GeV (m t = m jjb ) SM dominant decay gg → tt (~ 90%) qq → tt (~ 10%) m t = ± 4.4 GeV (global fit – PDG)

APS April 8, 2003 EW physics at ATLASA. M. Moraes Determination of sin 2 θ eff lept (M Z 2 ) Main systematic effect: uncertainty on the p.d.f.’s, lepton acceptance (~0.1%), radiative correction calculations. sin 2 θ eff lept is one of the fundamental parameters of the SM! precise determination will constrain the Higgs mass and check consistency of the SM. sin 2 θ eff lept will be determined at the LHC by measuring A FB in dilepton production near the Z pole. σ ( Z → l + l - ) ~ 1.5 nb (for either e or μ) y cuts – e + e - ( | y (Z) | > 1 ) ∆A FB ∆sin 2 θ eff lept | y ( l 1,2 ) | < x x | y ( l 1 ) | < 2.5; | y ( l 2 ) | < x x sin 2 θ eff lept ( M Z 2 ) = ± 1.7 x (global fit PDG) A FB = b { a - sin 2 θ eff lept ( M Z 2 ) } L = 100 fb -1 a and b calculated to NLO in QED and QCD. ( statistical ) Can be further improved: combine channels/experiments. [%]

APS April 8, 2003 EW physics at ATLASA. M. Moraes EW single top quark production W q q’ b t W q t b b g g W t b b b q t W W-gluon fusion Wt process W* process σ Wg ~ 250 pb σ Wt ~ 60 pb σ W* ~ 10 pb Probe the t-W-b vertex Background: tt, Wbb, Wjj -- ( not yet observed! ) Directly measurement (only) of the CKM matrix element V tb at ATLAS (assumes CKM unitarity ) ( lower theoretical uncertainties! ) for each process: σ ∝ |V tb | 2 ProcessS/BS/√B∆V tb / V tb – statistical ∆V tb / V tb – theory W-gluon %7.5% Wt %9.5% W* %3.8% L = 30 fb -1 Systematic errors : b-jet tagging, luminosity ( ∆ L ~ 5 – 10%), theoretical (dominate V tb measurements!). New physics : heavy vector boson W’ Source of high polarized tops!

APS April 8, 2003 EW physics at ATLASA. M. Moraes Triple gauge boson couplings TGC of the type WW γ or WWZ provides a direct test of the non-Abelian structure of the SM (EW symmetry breaking). This sector of the SM is often described by 5 parameters: g 1 Z, κ γ, κ Z, λ γ and λ γ, (SM values are equal to g 1 Z = κ γ = κ Z = 1 and λ γ = λ γ = 0, at the tree level). Anomalous contribution to TGC is enhanced at high √s ( increase of production cross-section ). W W γ/Z TGC vertex It may also indicate hints of new physics : new processes are expected to give anomalous contributions to the TGC. New physics could show up as deviations of these parameters from their SM values. L = 30 fb -1 ∆g 1 Z = 0.05 λ 1 = 0.01 Gauge, C and P invariance ▓ SM

APS April 8, 2003 EW physics at ATLASA. M. Moraes ParametersStatistical (at 95% C.L.) Systematic (at 95% C.L.) ∆g 1 Z ± ∆κZ∆κZ ±0.024 λZλZ ∆κγ∆κγ λγλγ ±0.0033± Systematic uncertainties: At the LHC, sensitivity to TGC is a combination of the very high energy and high luminosity. Uncertainties arising from low p T background will be quite small: anomalous TGC signature will be found at high p T. Theoretical uncertainties: p.d.f.’s & higher order corrections L = 30 fb -1 Variables: W γ : (m W γ, | η γ * | ) and (p T γ, θ * ) WZ : (m WZ, | η Z * | ) and (p T Z, θ * ) sensitive to high-energy behaviour: m WV, p T V sensitive to angular information: | η V * |, θ * SM: vanishing helicity at low | η | Non-standard TGC: partially eliminates `zero radiation’ Using max-Likelihood fit to m WV  | η V * |

APS April 8, 2003 EW physics at ATLASA. M. Moraes Conclusions: LHC will allow precision measurements: unexplored kinematic regions, high-statistics (W, Z, b, t factory); ATLAS: valuable precision measurements of SM parameters; W mass can be measured with a precision of 15 MeV (combinig e/ μ and ATLAS + CMS); Top mass: ~ 2 GeV (combined with ∆m W ~15 MeV, constrains M H to ~ 25%); sin 2 θ eff lept ( M Z 2 ) can be determined with statistical precision of 1.4x10 -4 (competitive to lepton collider measurements!) EW single top production: direct measurement of V tb ; measurement of top polarization (Wg with statistical precision of ~ 1.6%); Sensitivity to anomalous TGC’s: indicative of new physics!