Z Cross Section etc

Paul Telford (and Emily Nurse)University of Manchester

Manchester Group MeetingJanuary 5th 2005

Introduction

• Main body of the talk will focus on the updated Z cross section result which I worked upon with Emily

• Will briefly discuss other work that I have been involved in

• Includes W cross section measurement, which we hope to update soon

Z Cross section Motivation

• In combination with W cross section allows an indirect measurement of the W width

• Experimentally useful as allows calibration of detector, triggers.

• Use to gain understanding of effects such as Underlying event, tracker alignment in well understood process

• Determine using following formula

• BR = Ncand.(1-fbkd)/.Luminosity

• Count number of candidate events Ncand

• Determine the luminosity of sample

• Estimate the Background fraction of Ncand

using data (cosmics, QCD) and MC (Z)

• Evaluate acceptance using fast MC and efficiencies from data

• Won’t talk about event selection in detail… leave this to Nasim

• Backgrounds small (~1%), won’t discuss

• Evaluate with fast MC simulation, PMCS

• Parameterise the effect of detector resolution

• Tune parameters to data

• Measure track, muon-ID and trigger effs in data

• Introduce into PMCS as function of geometry

• Co-ordinates chosen for unbiased efficiencies

Total Efficiency

Result • Result = 291.3 ± 3.0 (stat) ± 6.9 (syst) ± 18.9

(lumi) pb cf theoretical prediction ≈ 250 pb

• Largest systematic due to luminosity (6.5%)

• Second largest due to Choice of PDF (1.7%)

• Largest `experimental’ uncertainty due to modelling of the muon system efficiencies (1.0%)

• Others from Uncertainty in beamspot distribution (0.6%), backgrounds in Efficiency(0.5%)….

• Statistical Uncertainty on candidates (0.9%)

• Statistical Uncertainty on the Efficiency (0.6%)

PDF Uncertainty

• PDFs parameterise the probability to get parton of a given type with given energy

• CTEQ6 comes with uncertainties on parameters

• Uncertainty on observable calculated from sum of uncertainties due to those on parameters

• Requires to evaluate observable 41 times…need large samples to control stat uncertainty

• Wrote toy MC to model these fluctuations … need 100M events to get uncertainty to < 0.1%

Muon System Uncertainty

• Muon chambers divided into octants in phi

• Efficiency lower in boundary between octants

• No hard definition of these regions

• Try to distinguish models of these regions, but limited by statistics

Cross Checks

• Main body of work this year has been cross checks

• Use GEANT simulation to check no biases present in tag and probe

• Check acceptance using HERWIG

• Check trigger efficiency using independent trigger

• And lots more ….

W Cross Section

• Also involved with a measurement of the W cross section

• Similar to Zs, but have tighter cuts due to higher backgrounds

• My particular concern is the efficiencies, in particular the correlations between the W and Z analyses

Other Work

• Am responsible for making public the aspects of the fast MC utilised in these analyses

• Have assumed responsibility for interfacing the new PDF interface with D0 software

• Done `Consultancy’ work for MC aspects of equivalent measurements in Electron Channel

• Given talk at DPF conference on Doubly Charged Higgs production in four muon decay channel

Conclusions and Future Plans

• Described an overview of the measurement of the Z cross section

• Immediate future involves updating W cross section and the ratio between W and Z cross sections

• Also continue to release (and document) the fast MC side of things for the benefit of all the collaboration