Measurement of Inclusive W and Z Boson Production Cross Sections in pp Collisions at ffiffi s p 1⁄4 8 TeV

A measurement of total and fiducial inclusiveW and Z boson production cross sections in pp collisions at ffiffi s p 1⁄4 8 TeV is presented. Electron and muon final states are analyzed in a data sample collected with the CMS detector corresponding to an integrated luminosity of 18.2 0.5 pb−1. The measured total inclusive cross sections times branching fractions are σðpp→WXÞ×BðW→lνÞ1⁄412.21 0.03ðstatÞ 0.24ðsystÞ 0.32ðlumÞnb and σðpp→ZXÞ×BðZ→lþl−Þ1⁄41.15 0.01ðstatÞ 0.02ðsystÞ 0.03ðlumÞ nb for the dilepton mass in the range of 60—120 GeV. The measured values agree with next-to-next-to-leadingorder QCD cross section calculations. Ratios of cross sections are reported with a precision of 2%. This is the first measurement of inclusive W and Z boson production in proton-proton collisions at ffiffi s p 1⁄4 8 TeV.

The production of W and Z bosons is one of the most prominent examples of hard scattering processes at hadron colliders [1].Theoretical predictions are available at nextto-next-to-leading order (NNLO) [2][3][4][5][6] in perturbative quantum chromodynamics (QCD).The calculations are limited by uncertainties in parton distribution functions (PDFs), missing higher-order QCD effects, and electroweak (EW) radiative corrections, which are available at next-to-leading order (NLO) [7][8][9][10].Precise measurements of inclusive cross sections provide tests of perturbative QCD and validate the theoretical predictions of higherorder corrections.Additionally, accurate measurements can be used to constrain PDFs.
Inclusive W and Z boson production cross sections and their ratios were previously measured by the ATLAS and CMS Collaborations at the Large Hadron Collider (LHC) in proton-proton collisions at ffiffi ffi s p ¼ 7 TeV [11][12][13].
This Letter describes the inclusive measurement at ffiffi ffi s p ¼ 8 TeV, performed in the electron and muon decay channels, with the CMS detector.A data sample collected in 2012 corresponding to an integrated luminosity of 18.2 AE 0.5 pb −1 is used.The levels of instantaneous luminosity reached by the LHC in 2012 present challenges for the precise measurement of the W boson cross section because of the degraded missing transverse momentum resolution resulting from the large number of pp interactions per bunch crossing (pileup).A data sample with low pileup was collected in May 2012 by adjusting the beam separation during data taking.An average of 4 interactions per bunch crossing was achieved, compared with the average of 21 during the rest of 2012.The measurements of the W and Z boson production cross sections are performed using this data sample.
The central feature of the CMS apparatus is a superconducting solenoid, of 6 m internal diameter, providing a field of 3.8 T. Within the field volume are a silicon pixel and strip tracker, a crystal electromagnetic calorimeter (ECAL), and a brass or scintillator hadron calorimeter.Muons are measured in gas-ionization detectors embedded in the steel flux-return yoke of the magnet.CMS uses a right-handed coordinate system, with the origin at the nominal interaction point, the x axis pointing to the center of the LHC, the y axis pointing upwards, perpendicular to the plane of the LHC ring, and the z axis along the counterclockwise-beam direction.The polar angle θ is measured from the positive z axis, and the azimuthal angle ϕ is measured in the x-y plane.The pseudorapidity η is defined by η ¼ − ln½tanðθ=2Þ.Details of the CMS detector and its performance can be found elsewhere [14].
Leptonic W boson decays are characterized by a prompt, energetic, and isolated charged lepton and a neutrino giving rise to significant missing transverse energy E miss T .Events used in the cross section measurement are not required to have a minimum reconstructed E miss T , but the E miss T distribution is used as a discriminant against background from multijet events.Z boson decays to leptons are selected by requiring two energetic and isolated leptons of the same flavor and opposite charge.The Z boson candidates are required to have a reconstructed dilepton mass of between 60 and 120 GeV.Samples of Z boson candidates satisfying looser lepton requirements are used to estimate efficiencies.
Because of the high rate of collisions and the limited bandwidth for data processing, the data acquisition system must be selective in deciding which events are sufficiently interesting to be kept for analysis.Triggers make rapid decisions by executing simplified muon and electron reconstruction algorithms.For this analysis, the events are collected when triggered by the presence of a muon with large transverse momentum p T > 15 GeV and jηj < 2.1 or an electron with large transverse energy E T > 22 GeV and jηj < 2.5, with loose isolation and identification requirements.
Electrons are identified as clusters of energy deposits in the ECAL matched to tracks measured with the silicon tracker [15][16][17][18][19].The ECAL fiducial region is defined by jηj < 1.44 (barrel) or 1.57 < jηj < 2.5 (end cap), where η is the pseudorapidity of the energy cluster.The barrel-end cap transition region and the first ring of end cap trigger towers are excluded because they are partially obscured by cables and services exiting between the barrel and end caps.A cluster is considered to be within the acceptance of the ECAL if it is within the ECAL fiducial region and has transverse energy E T > 25 GeV.Electrons are required to be isolated from other reconstructed particles in a cone of . Particle candidates are identified using a particle-flow algorithm [20,21] that provides a complete description of the event in terms of electrons, muons, photons, charged hadrons, and neutral hadrons.An electron candidate is selected if the sum of transverse momenta of particles in the cone is less than 15% of the candidate's transverse energy.
Muons are reconstructed from seed tracks in the muon detector combined with silicon strip and pixel information using a global fit [22,23].In the p T range of interest, the momentum resolution is driven by the inner tracking system.Muons with p T > 25 GeV and jηj < 2.1 are selected, which is consistent with the acceptance of the single muon trigger.A relative isolation variable is computed as discussed for electrons, but in a cone of radius ΔR ¼ 0.4 and with an isolation selection requirement of less than 12%.
The acceptance for W or Z boson events is the fraction of generated events for which the leptons satisfy the restrictions on η and p T .The event selection criteria will select a subset of the accepted events, and the efficiency specifies the fraction of events selected.This accounts, for example, for the region of the ECAL from 1.44 < jηj < 1.55.Other effects, such as crystal boundaries, are accounted for in the efficiency to reconstruct leptons.Using this acceptance definition, we are able to separate experimental from theoretical uncertainties in the measurement.The detector response is simulated using a detailed description of the CMS detector, based on the GEANT4 package [24].Additional proton-proton interactions are taken into account using an admixture of simulated minimum bias events, and the same reconstruction code is applied for data and simulated events.Data to simulation ratios of efficiencies are used as scale factors.No single event generator gives a reliable description of both EW and QCD effects.The acceptance is estimated using Monte Carlo simulation based on POWHEG [25][26][27][28].The effects of nonperturbative QCD, higher-order QCD, and electroweak corrections on the estimated acceptance are investigated using specific simulation tools, from which uncertainties are derived [7][8][9][10]29,30].The uncertainty related to the PDFs is estimated following closely the prescription of the PDF4LHC working group [31] to combine uncertainties related to the choice of the NLO PDF and the strong coupling constant α s .
The W boson candidate events are required to have an identified electron or muon.The W boson signal and background yields are obtained from the E miss T distributions using a binned maximum-likelihood fit.The missing transverse energy is calculated with the particle-flow algorithm by adding the transverse energy vectors of all identified particles.An accurate E miss T measurement is essential to distinguish the W signal from QCD multijet backgrounds.To account for shortcomings of the simulation in modeling the recoil against the W boson, a correction is derived from a Z boson sample.The recoil in these events is studied, in data as well as in simulation, and the differences are propagated to the W boson simulation as a function of the p T of the generated W or Z boson.Other background processes from W → τν, Drell-Yan, diboson, and top-pair production also become significant at high E miss T , contributing about 6% of the total selected yield.The background contribution from cosmic rays in the W → μν channel is negligible.The E miss T model is fitted to the observed distribution as the sum of three contributions: the W signal, the QCD background, and other backgrounds.The QCD background is modeled by an analytic function, while the signal and EW backgrounds are modeled with simulation-based fitting functions [11].The EW contributions are normalized to the W signal yield in the fit through the ratios of the theoretical cross sections.Figure 1 shows the E miss T distributions of the inclusive W boson samples and the results of the fit.
To extract the Z boson yield, the events in the dilepton mass window are counted.The yields contain a contribution of 3% from γ Ã -mediated processes, including interference effects, as estimated with MCFM [32].Background contamination is estimated from simulation to be about 0.4%.Figure 2 shows the dilepton mass distributions of the inclusive Z samples.The signal yields, the acceptances, and the efficiencies are summarized in the Supplemental Material [33].
The systematic uncertainties are summarized in The luminosity of the data sample is measured with an uncertainty of 2.6% by counting the number of clusters per event in the silicon pixel detector.The highly granular detector, consisting of ∼60 million channels, guarantees an excellent linearity of the pixel detector response versus pileup.The method is calibrated by means of a procedure pioneered by van der Meer [40], consisting of beam scans along the vertical and horizontal directions.This van der Meer technique determines the luminosity at the percent level from a measurement of the beam parameters [41].The dominant contribution to the luminosity uncertainty originates from the assumptions on the functional form of the beam shapes.
The theoretical predictions of cross sections and cross section ratios are computed at NNLO with the program FEWZ [42] and the MSTW2008 [43] set of PDFs.The uncertainties in these predictions, at the 68% confidence level (CL ), include contributions from the uncertainty of the strong coupling constant α s [44,45], the choice of heavy-quark masses (charm and bottom quarks) [46], as well as neglected higher-order corrections beyond NNLO, Events / 2.0 GeV , where N obs is the number of observed events and N exp is the total of the fitted signal and background yields., where N obs is the number of observed events and N exp is the total of the signal and background yields.
which are estimated by allowing the renormalization and factorization scales to vary.The NNLO predictions for the total cross sections times branching fractions are 7.12 AE 0.20 nb for W þ , 5.06 AE 0.13 nb for W − , and 1.13 AE 0.04 nb for Z boson production.The Z boson cross section requires an invariant mass within the range 60-120 GeV, and it includes the effects of virtual photons.
The results in the electron and muon decay channels are compatible with a p value of 0.42.Assuming universality of lepton couplings to W and Z bosons, the channels are combined by calculating an average cross section value weighted by their statistical and systematic uncertainties, taking into account the correlated uncertainties.The two leptonic decay channels are combined by assuming fully correlated uncertainties for the acceptance and luminosity, but with other uncertainties assumed to be uncorrelated.
In measurements of the ratios of cross sections some systematic uncertainties cancel, most importantly the uncertainty in the luminosity.The uncertainties in the lepton reconstruction and identification are treated as uncorrelated, and the resulting experimental uncertainty in the ratio measurements can, therefore, be larger than that for individual cross section measurements.A summary of the measurements is given in Table II, including the results obtained within the fiducial regions in p T and η.See Supplemental Material [33] for the total cross sections times branching fractions and ratios for the electron and muon decay channels.
The upper two plots in Fig. 3 show the measured and predicted W versus Z and W þ versus W − cross sections for different PDF sets.The uncertainties in the theoretical predictions correspond to the PDF uncertainties only.This approach eliminates the need to propagate acceptance uncertainties originating from the PDFs and higher-order corrections into the measurement.The final measurement is compared with the predicted cross sections in the acceptance region for three different PDFs with their respective uncertainty bands propagated through the prediction.
In summary, we have performed the first measurements of total and fiducial inclusive W and Z production cross sections times branching fractions in pp collisions at ffiffi ffi s p ¼ 8 TeV using 18.2 AE 0.5 pb −1 of low-pileup data recorded with the CMS detector.The W and Z bosons are observed via their decays to electrons and muons.The measured total inclusive production cross sections times branching fractions are σðpp→WXÞ×BðW →lνÞ¼12.21AE0.03ðstatÞAE0.24ðsystÞAE0.32ðlumÞnb and, for the dilepton mass in the range of 60-120 GeV, σðpp 15 AE 0.01ðstatÞ AE 0.02ðsystÞ AE 0.03ðlumÞ nb.In addition  to the inclusive cross sections, we present ratios of cross sections measured with a precision of 2%.The measurements in the electron and muon channels are consistent and in agreement with NNLO calculations.Additional figures summarizing our measurements are available in the Supplemental Material [33].
We congratulate our colleagues in the CERN accelerator departments for the excellent performance of the LHC and thank the technical and administrative staffs at CERN and at other CMS institutes for their contributions to the success of the CMS effort.In addition, we gratefully acknowledge the computing centers and personnel of the Worldwide LHC Computing Grid for delivering so effectively the computing infrastructure essential to our analyses.Finally, we acknowledge the enduring support for the construction and operation of the LHC and the CMS detector provided by the following funding agencies: BMWF and FWF (Austria); FNRS and FWO (Belgium); CNPq, CAPES,   [43], NNPDF 2.3 [47], and CT10 [48].PRL 112, 191802 (2014) P ggg Also at Argonne National Laboratory, Argonne, USA.hhh Also at Erzincan University, Erzincan, Turkey.
iii Also at Yildiz Technical University, Istanbul, Turkey.jjj Also at Texas A&M University at Qatar, Doha, Qatar.kkk Also at Kyungpook National University, Daegu, Korea.

FIG. 1 (
FIG.1(color online).The missing transverse energy distributions for W boson candidate events in the electron (top) and muon (bottom) final states.The variable χ shown in the lower plot is defined asðN obs − N exp Þ= ffiffiffiffiffiffiffiffiffi N obs p, where N obs is the number of observed events and N exp is the total of the fitted signal and background yields.

FIG. 2 (
FIG.2(color online).The dilepton mass distributions for Z boson candidate events in the electron (top) and muon (bottom) final states.The variable χ shown in the lower plot is defined asðN obs − N exp Þ= ffiffiffiffiffiffiffiffiffi N obs p, where N obs is the number of observed events and N exp is the total of the signal and background yields.

FIG. 3 (
FIG. 3 (color online).Measured and predicted W versus Z boson (left column) and W þ versus W − boson (right column) production cross sections times branching fractions.The ellipses illustrate the 68% CL coverage for total uncertainties (open) and excluding the luminosity uncertainty (filled).The top row shows the inclusive cross sections times branching fractions and the bottom row shows the results within the fiducial regions.The uncertainties in the theoretical predictions correspond to the PDF uncertainty components only and are evaluated for MSTW 2008 NLO[43],NNPDF 2.3 [47], and CT10[48].

TABLE I .
Systematic uncertainties in percent for the electron and muon channels; "…" means that the source either does not apply or is negligible.

TABLE II .
Summary of total and fiducial W þ , W − , W, and Z production cross sections times branching fractions, W to Z and W þ to W − ratios, and their theoretical predictions.
Also at Vienna University of Technology, Vienna, Austria.c Also at CERN, European Organization for Nuclear Research, Geneva, Switzerland.d Also at Institut Pluridisciplinaire Hubert Curien, Université de Strasbourg, Université de Haute Alsace Mulhouse, CNRS/IN2P3, Strasbourg, France.e Also at National Institute of Chemical Physics and Biophysics, Tallinn, Estonia.f Also at Skobeltsyn Institute of Nuclear Physics, Lomonosov Moscow State University, Moscow, Russia.g Also at Universidade Estadual de Campinas, Campinas, Brazil.h Also at California Institute of Technology, Pasadena, USA.i Also at Laboratoire Leprince-Ringuet, Ecole Polytechnique, IN2P3-CNRS, Palaiseau, France.j Also at Zewail City of Science and Technology, Zewail, Egypt.k Also at Suez Canal University, Suez, Egypt.l Also at Cairo University, Cairo, Egypt.m Also at Fayoum University, El-Fayoum, Egypt.n Also at British University in Egypt, Cairo, Egypt.o Present address: Ain Shams University, Cairo, Egypt.p Also at National Centre for Nuclear Research, Swierk, Poland.q Also at Université de Haute Alsace, Mulhouse, France.r Also at Joint Institute for Nuclear Research, Dubna, Russia.s Also at Brandenburg University of Technology, Cottbus, Germany.t Also at The University of Kansas, Lawrence, USA.u Also at Institute of Nuclear Research ATOMKI, Debrecen, Hungary.v Also at Eötvös Loránd University, Budapest, Hungary.w Also at Tata Institute of Fundamental Research-EHEP, Mumbai, India.x Also at Tata Institute of Fundamental Research-HECR, Mumbai, India.y Present address: King Abdulaziz University, Jeddah, Saudi Arabia.z Also at University of Visva-Bharati, Santiniketan, India.aa Also at University of Ruhuna, Matara, Sri Lanka.bb Also at Sharif University of Technology, Tehran, Iran.cc Also at Isfahan University of Technology, Isfahan, Iran.dd Also at Plasma Physics Research Center, Science and Research Branch, Islamic Azad University, Tehran, Iran.ee Also at Università degli Studi di Siena, Siena, Italy.ff Also at Universidad Michoacana de San Nicolas de Hidalgo, Morelia, Mexico.gg Also at St. Petersburg State Polytechnical University, St. Petersburg, Russia.hh Also at Faculty of Physics, University of Belgrade, Belgrade, Serbia.Also at Rutherford Appleton Laboratory, Didcot, United Kingdom.nn Also at Paul Scherrer Institut, Villigen, Switzerland.oo Also at Institute for Theoretical and Experimental Physics, Moscow, Russia.pp Also at Albert Einstein Center for Fundamental Physics, Bern, Switzerland.qq Also at Gaziosmanpasa University, Tokat, Turkey.rr Also at Adiyaman University, Adiyaman, Turkey.ss Also at Cag University, Mersin, Turkey.tt Also at Mersin University, Mersin, Turkey.uu Also at Izmir Institute of Technology, Izmir, Turkey.vv Also at Ozyegin University, Istanbul, Turkey.ww Also at Kafkas University, Kars, Turkey.xx Also at Suleyman Demirel University, Isparta, Turkey.yy Also at Ege University, Izmir, Turkey.zz Also at Mimar Sinan University, Istanbul, Istanbul, Turkey.aaa Also at Kahramanmaras Sütcü Imam University, Kahramanmaras, Turkey.bbb Also at School of Physics and Astronomy, University of Southampton, Southampton, United Kingdom.Also at University of Belgrade, Faculty of Physics and Vinca Institute of Nuclear Sciences, Belgrade, Serbia.
a Deceased.bii Also at Facoltà Ingegneria, Università di Roma, Roma, Italy.jjAlso at Scuola Normale e Sezione dell'INFN, Pisa, Italy.kkAlso at INFN Sezione di Roma, Roma, Italy.llAlso at University of Athens, Athens, Greece.mm ccc Also at INFN Sezione di Perugia, Università di Perugia, Perugia, Italy.ddd Also at Utah Valley University, Orem, USA.eee Also at Institute for Nuclear Research, Moscow, Russia.fff