2011 QCD and High Energy Interactions - Rencontres de Moriond ...

W/Z + Jets results from CMS
V. CIULLI on behalf of CMS Collaboration
Dipartimento di Fisica, Università di Firenze, via G. Sansone 1,
50025 Pontedera (PI) Italy
We present an overview of the studies of W and Z boson production with jets in pp collisions
at a centre-of-mass energy of 7 TeV using 36 pb −1 of data collected in the CMS experiment.
We report the measurement of the jet rates as well as the first measurement of the polarization
of W bosons with large transverse momentum, and the observation of Z bosons in association
with a b-quark jet.
1 Introduction
The study of the production of W and Z vector boson with jets provides a stringent and important
test of perturbative QCD calculations. Furthermore, events with vector bosons and
jets are the main background to searches for physics beyond the Standard Model. A precise
measurement of the cross section and of the jet and boson kinematics is therefore essential. This
paper presents the measurement of the jet rates 1 , the first observation of W polarization 2 , and
of a Z boson and a b-quark jet 3 , obtained with the full data sample recorded with the CMS
experiment 4 during 2010, which amounts to an integrated luminosity of 36 ± 4 pb −1 .
2 Event selection and jet rates
The selection of W boson candidates requires one electron (muon), with pT > 20 GeV in |η| <
2.4 (2.1). Misidentified leptons, as well as non prompt leptons arising from decays of heavyflavor
hadrons or decays of light mesons within jets, are suppressed by requiring the lepton to be
isolated, i.e. by limiting the additional hadronic activity surrounding the lepton candidate. The
transverse mass MT of the W candidate is calculated from the missing transverse energy (MET)
in the event, measured using the particle flow algorithm, and W candidates with MT < 20 GeV
are rejected. If a second electron (muon) with pT > 10 GeV in |η| < 2.4 is found, then the
event is considered for a Z candidate. Only Z candidates with a di-lepton mass between 60 and
120 GeV are retained for the following analysis.
Jets are reconstructed from the particle collection created with the particle flow algorithm
and are formed with the anti-kT clustering algorithm 5 with a size parameter of R = 0.5.
Jet energy corrections are applied to improve the accuracy of the jet transverse energy (ET)
measurement and to flatten the jet energy response as a function of η and ET 7 . We require
|η| < 2.4, so that the jets fall within the tracker acceptance, and ET > 30 GeV. The observed,
uncorrected distribution of the number of reconstructed jets in the W → µν is shown in Figs. 1.
The distribution from simulation, including background processes, is also shown. Overall, a
good agreement is found up to n = 6 jets. The agreement between data and simulation is