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Forward-Backward Charge Asymmetry in Z production at the LHC. Introduction ATLAS & CMS Detectors FB asymmetry measurement study Conclusions. Mohamed Aharrouche (LAPP-Annecy ) (for the ATLAS and CMS Collaborations). Motivation. Forward-Backward asymmetry
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Forward-Backward Charge Asymmetry in Z production at the LHC • Introduction • ATLAS & CMS Detectors • FB asymmetry measurement study • Conclusions Mohamed Aharrouche(LAPP-Annecy ) (for the ATLAS and CMS Collaborations) IVIIthRencontres de Moriond-QCD
Motivation • Forward-Backward asymmetry • Provides a test of the Standard Model (i.e. universality) • At Z-pole • Determination of the Weinberg effective angle and the precision on this value AFB = b(a – sin²(eff)) • At high mass • Sensitive to many scenarios of physics beyond the Standard Model J. L. Rosner, Phys. Rev. D 35 (1987) U. Baur et al. Phys.Rev. D57 (1998) K. Sliwa et al.ATL-PHYS-2000-018 IVIIthRencontres de Moriond-QCD
LHC Proton-Proton collisions at ECM = 14 TeV ATLAS and CMS: general purpose detectors Z factory Production ~1.5x107 events/year at low luminosity (1033cm-2s-1) From qq annihilation xqxqbar ~ 4x10-5 Longitudinal momentum PL = 0.5*√s*(xq -xqbar) Decay To two energetic fermions with opposite charge 70% to quarks pair: dominated by the background leptonic decay channels: cleaner Z Properties at LHC IVIIthRencontres de Moriond-QCD
e- q q e+ Charge Asymmetry • Parity violation in the neutral current • Consequence: Asymmetry in the angular distribution of leptons from Z decay • Theprobabilities to produce a lepton with a polar angle and with π- are different • Theta dependence of the cross section IVIIthRencontres de Moriond-QCD
Detectors ATLAS CMS ●4T Solenoid (for muons & inner tracker) ● Pure silicon tracker (pixel + strips) (||<2.6) ● Homogeneous EM calorimeter + hadronic sampling Calo. ( ||<4.9) ●2T Solenoid for inner tracker ● Tracker: silicon (pixel + strips) and TRT (||<2.5) ● Sampling calorimetry (||<4.9) ● Toroid system for muons IVIIthRencontres de Moriond-QCD
Detector requirements • Z→μ+μ- • Muons detection limited • Acceptance (|| < 2.7) • Z→e+e- • electrons can be detected in very forward regions (||<4.9) • No tracker in ||>2.5 • At least one electron should be in the central region • Opposite charge assigned to the forward electron =2.5 • All events • |e1|<2.5 |e2|<4.9 • |e1| & |e2| < 2.5 IVIIthRencontres de Moriond-QCD
e- * p p cos(*) • Defined in the Collins-Soper frame to take into account the non zero transverse momentum of the in incoming quark • In pp collisions we suppose that the quark direction is the same as the Z boost Dielectron around the Z mass (Mz±6 GeV) IVIIthRencontres de Moriond-QCD
Quark direction all events Events with quark direction correctly estimated Less than 60% of events with correct quark direction at |Y| < 1 IVIIthRencontres de Moriond-QCD
Counting method AFB= (F-B)/(F+B) F= number of events with cos*>0B= number of events with cos*<0 ee invariant mass(GeV) Calculating AFB Forward events Backward events IVIIthRencontres de Moriond-QCD
GeV Case 1 Case 2 38% Analysis method - Z→e+e- • Distinguish the detector regions: • Case-1 • electrons in central region: (||<2.5) • Case-2 • One fiducial electron (||<2.5) andother electron (||<4.9) • Selection: • Two high PT electrons (PT > 20 GeV) • Mass window MZ±6 GeV • Missing Et cut < 20 GeV • Fast simulation • Signal • Drell-Yan events Ze+e- • Background • QCD dijets (dominant) • ttbar • 1 year at high luminosity (100 /fb) • All events • Case 1 • Case 1 & |y(Z)|>1 • All events • Case 2 • Case 2 & |y(Z)|>1 IVIIthRencontres de Moriond-QCD
Case 2 Case 1 Z→e+e- results • AFB vs YZ rapidity • sensitivity increases with forward electrons • (statistical) δAFB vs forward electron/jet rejection • Forward electron efficiency fixed at 50% IVIIthRencontres de Moriond-QCD
Z→e+e- results cont. LEP: δsin2efflep = 1.6x10-4 (stat and syst) IVIIthRencontres de Moriond-QCD
Can we measure electrons with such requirements in the forward region? IVIIthRencontres de Moriond-QCD
1- BkgEff Forward electrons • Full simulation • electrons from Ze+e- • QCD jets • Discriminant analysis • Multivariate • 5 selected input variables • Energy fraction of the most energetic cell • Longitudinal moment • ... 2.5<||<3.2 3.2<||<4.9 signal backg Neural network output IVIIthRencontres de Moriond-QCD
Systematics! • Ongoing study to answer to: • Can we control the systematics? • Can we use AFB to constrain PDFs at LHC? preliminary IVIIthRencontres de Moriond-QCD
Conclusions • Very large statistics expected in Z production at LHC • The determination of the weak mixing angle with a high statistical precision (10-4) seems reachable • Requirements: • At least one electron in the central region • Electron efficiency more than 50% in the forward region • e/jet rejection > 100 in the forward region IVIIthRencontres de Moriond-QCD
Extra Slides IVIIthRencontres de Moriond-QCD
AFB – Z’→μ+μ - CMS IVIIthRencontres de Moriond-QCD