Measurement of forward W and Z boson production in pp collisions at \( \sqrt{s}=8 \) TeV

Published for SISSA by Springer Received: November 26, 2015 Accepted: January 6, 2016 Published: January 26, 2016 The LHCb collaboration E-mail: Abstract: Measurements are presented of electroweak boson production using data from √ pp collisions at a centre-of-mass energy of s = 8 TeV. The analysis is based on an integrated luminosity of 2.0 fb−1 recorded with the LHCb detector. The bosons are identified in the W → µν and Z → µ+ µ− decay channels. The cross-sections are measured for muons in the pseudorapidity range 2.0 < η < 4.5, with transverse momenta pT > 20 GeV/c and, in the case of the Z boson, a dimuon mass within 60 < Mµ+ µ− < 120 GeV/c2 . The results are σW + →µ+ ν = 1093.6 ± 2.1 ± 7.2 ± 10.9 ± 12.7 pb , σW − →µ− ν = 818.4 ± 1.9 ± 5.0 ± 7.0 ± 9.5 pb , σZ→µ+ µ− = 95.0 ± 0.3 ± 0.7 ± 1.1 ± 1.1 pb , where the first uncertainties are statistical, the second are systematic, the third are due to the knowledge of the LHC beam energy and the fourth are due to the luminosity determination. The evolution of the W and Z boson cross-sections with centre-of-mass energy is studied using previously reported measurements with 1.0 fb−1 of data at 7 TeV. Differential distributions are also presented. Results are in good agreement with theoretical predictions at next-to-next-to-leading order in perturbative quantum chromodynamics. Keywords: Electroweak interaction, Hadron-Hadron scattering, Forward physics ArXiv ePrint: 1511.08039 Open Access, Copyright CERN, for the benefit of the LHCb Collaboration. Article funded by SCOAP3 . doi:10.1007/JHEP01(2016)155 JHEP01(2016)155 Measurement of forward W and Z boson production √ in pp collisions at s = 8 TeV Contents 1 2 Detector and data set 2 3 Event yield 3 4 Cross-section measurement 4.1 Muon reconstruction efficiencies 4.2 GEC efficiency 4.3 Final-state radiation 4.4 Selection efficiencies 4.5 Acceptance 4.6 Unfolding the detector response 4.7 Systematic uncertainties 5 6 6 7 7 7 7 8 5 Results √ 5.1 Cross-sections at s = 8 TeV √ 5.2 Ratios of cross-sections at s = 8 TeV 5.3 Ratios of cross-sections at different centre-of-mass energies 9 9 12 14 6 Conclusions 20 A Differential measurements 22 B Correlation matrices 30 The LHCb collaboration 40 1 Introduction Measurements of W and Z boson production cross-sections at hadron colliders constitute important tests of the Standard Model (SM).1 Theoretical predictions for these crosssections are available at next-to-next-to-leading order (NNLO) in perturbative quantum chromodynamics [1–5]. The dominant uncertainty on these predictions reflects the uncertainties on the parton density functions (PDFs). The forward acceptance of the LHCb detector allows the PDFs to be constrained at Bjorken-x values down to 10−4 [6]. Ratios of the W and Z cross-sections provide precise tests of the SM as the sensitivity to the PDFs in the theoretical calculations is reduced and many of the experimental uncertainties cancel. 1 Throughout this article Z is used to denote the Z/γ ∗ contributions. –1– JHEP01(2016)155 1 Introduction φ∗η ≡ tan (φacop /2) , cosh (∆η/2) (1.1) where the angle φacop = π − |∆φ| depends on the difference ∆φ in azimuthal angle between the two muon momenta, while the difference between their pseudorapidities is denoted by ∆η. Differential cross-section ratios and the muon charge asymmetry, arising from the W production charge asymmetry, are also determined as a function of the muon pseudorapidity. This article is organised as follows: section 2 describes the LHCb detector; section 3 details the selection of W and Z boson candidate samples; section 4 defines the W and Z boson cross-sections and summarises the relevant sources of systematic uncertainty, as well as their estimation; section 5 presents the results and section 6 concludes the article. Appendices A and B provide tables of differential cross-sections and correlations between these measurements. 2 Detector and data set The LHCb detector [18, 19] is a single-arm forward spectrometer covering the pseudorapidity range 2 < η < 5, designed for the study of particles containing b or c quarks. The detector includes a high-precision tracking system consisting of a silicon-strip vertex detector surrounding the pp interaction region, a large-area silicon-strip detector located upstream of a dipole magnet with a bending power of about 4 Tm, and three stations of silicon-strip detectors and straw drift tubes placed downstream of the magnet. The tracking system provides a measurement of momentum, p, of charged particles with 2 The φ∗η definition in this article is equivalent to the definitions in refs. [9, 10, 12]. –2– JHEP01(2016)155 √ During LHC Run 1, data were collected at centre-of-mass energies, s, of 7 TeV and 8 TeV, providing two distinct samples for measurements of the electroweak boson production cross-sections. The evolution of the cross-sections, and cross-section ratios, may be used to infer the existence of physics beyond the Standard Model (BSM) [7]. √ LHCb has measured the W boson production cross-section at s = 7 TeV using the muon channel [8], and that of Z bosons decaying to muon [9], electron [10] and tau lepton [11] pairs, using a data set of 1.0 fb−1 . The Z boson production cross-section at √ s = 8 TeV has also been measured using decays to electron pairs [12]. Similar measurements have also been performed by the ATLAS [13] and CMS [14–16] collaborations, although in different kinematic regions. √ The measurements of inclusive W and Z boson cross-sections at s = 8 TeV described here are performed following the same procedure as detailed in refs. [8, 9]. The crosssections are defined for muons with transverse momentum pT > 20 GeV/c and pseudorapidity in the range 2.0 < η < 4.5. In the case of the Z boson measurements, the invariant mass of the two muons is required to be in the range 60 < Mµµ < 120 GeV/c2 . These kinematic requirements define the fiducial region of the measurement and are referred to as the fiducial requirements in this article. Total cross-sections are presented, as well as differential cross-sections as functions of η of the muons, and of the Z boson rapidity, yZ , transverse momentum, pT,Z , and φ∗η [17]. Here φ∗η is defined as2 3 Event yield Events for this analysis must satisfy the selection criteria detailed in refs. [8, 9]. The trigger requires a single muon with pT > 1.5 GeV/c at the hardware stage, and includes an upper threshold of 600 hits in the SPD to prevent high-particle multiplicity events from –3– JHEP01(2016)155 a relative uncertainty that varies from 0.5% at low momentum to 1.0% at 200 GeV/c. The minimum distance of a track to a primary vertex, the impact parameter (IP), is measured with a resolution of (15 + 29/pT ) µm, where pT is the component of the momentum transverse to the beam, in GeV/c. Different types of charged hadrons are distinguished using information from two ring-imaging Cherenkov detectors. Photons, electrons and hadr (...truncated)