Forward-Backward Charge Asymmetry in Z production at the LHC

M. Aharrouche ([email protected])

page 1IVIIth Rencontres de Moriond-QCD

Forward-Backward Charge Asymmetry in Z production at the LHC

Mohamed Aharrouche (LAPP-Annecy )

(for the ATLAS and CMS Collaborations)

•Introduction

•ATLAS & CMS Detectors

•FB asymmetry measurement study •Conclusions



Motivation

Forward-Backward asymmetryProvides a test of the Standard Model (i.e. universality)

At Z-poleDetermination of the Weinberg effective angle and

the precision on this value AFB = b(a – sin²(eff))

At high massSensitive 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



Z Properties at LHC 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



Charge Asymmetry Parity violation in the neutral current

Consequence: Asymmetry in the angular distribution of leptons from Z decay

The probabilities to produce a lepton with a polar angle and with π- are different

Theta dependence of the cross section

q q

e-

e+



Detectors

● 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

ATLASCMS



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

•All events•|e1|<2.5 |e2|<4.9•|e1| & |e2| < 2.5•

=2.5



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

pp

*

e-

Dielectron around the Z mass (Mz±6 GeV)



Quark direction

Events withquark direction

correctly estimated

Less than 60% of events with correct quark direction at |Y| < 1

all events



Calculating AFB

ee invariant mass(GeV)

Counting method AFB= (F-B)/(F+B)

F= number of events with cos*>0B= number of events with cos*<0

Forward events

Backward events



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)

GeV

Case 1

Case 2

38%

•All events•Case 1•Case 1 & |y(Z)|>1

•All events•Case 2•Case 2 & |y(Z)|>1



Z→e+e- results

Case 1

Case 2

AFB vs YZ rapidity sensitivity increases with

forward electrons

(statistical) δAFB vs forward electron/jet rejection Forward electron efficiency

fixed at 50%



Z→e+e- results cont.

LEP: δsin2efflep = 1.6x10-4 (stat and syst)



Can we measure electrons with such requirements in the

forward region?



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 ...

Neural network output

signalbackg

1-

Bkg

Eff

2.5<||<3.2 3.2<||<4.9

method Eff(% )atRej=100

Eff(% )atRej=10

FisherLikelihoodANN (MLP)

587372

939496



Systematics!

Ongoing study to answer to: Can we control the systematics? Can we use AFB to constrain PDFs at LHC?

preliminary



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



Extra Slides



AFB – Z’→μ+μ -

CMS

Forward-Backward Charge Asymmetry in Z production at the LHC

Documents

Transcript of Forward-Backward Charge Asymmetry in Z production at the LHC