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

Journal of High Energy Physics, Jan 2016

Measurements are presented of electroweak boson production using data from pp collisions at a centre-of-mass energy of \( \sqrt{s}=8 \) 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 p T > 20 GeV/c and, in the case of the Z boson, a dimuon mass within \( 60 < {M}_{\mu }{{{}_{{}^{+}}}_{\mu}}_{{}^{-}}<120 \) GeV/c 2. The results are $$ \begin{array}{l}{\sigma}_W{{}_{{}^{+}}}_{\to \mu }{{}_{{}^{+}}}_{\nu }=1093.6\pm 2.1\pm 7.2\pm 10.9\pm 12.7\;\mathrm{p}\mathrm{b},\hfill \\ {}{\sigma}_W{{}_{{}^{-}}}_{\to \mu }{{}_{{}^{-}}}_{\overline{\nu}}=818.4\pm 1.9\pm 5.0\pm 7.0\pm 9.5\;\mathrm{p}\mathrm{b},\hfill \\ {}{\sigma}_{Z\to \mu }{{{}_{{}^{+}}}_{\mu}}_{{}^{-}}=95.0\pm 0.3\pm 0.7\pm 1.1\pm 1.1\;\mathrm{p}\mathrm{b},\hfill \end{array} $$ 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.

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

JHE in pp collisions at s Measurements are presented of electroweak boson production using data from pp collisions at a centre-of-mass energy of p s = 8 TeV. The analysis is based on an integrated luminosity of 2:0 fb 1 recorded with the LHCb detector. The bosons are identi ed Electroweak interaction; Hadron-Hadron scattering; Forward physics - and Z boson production The LHCb collaboration in the W ! and Z ! in the pseudorapidity range 2:0 < + decay channels. The cross-sections are measured for muons < 4:5, with transverse momenta pT > 20 GeV=c and, in the case of the Z boson, a dimuon mass within 60 < M + < 120 GeV=c2. The results are W +! + = 1093:6 W ! Z! + = 818:4 = 95:0 2:1 1:9 0:3 7:2 5:0 0:7 10:9 7:0 1:1 12:7 pb ; 9:5 pb ; 1:1 pb ; where the rst 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. Di erential distributions are also presented. Results are in good agreement with theoretical predictions at next-to-next-to-leading order in perturbative quantum chromodynamics. 1 Introduction 2 3 4 Ratios of cross-sections at di erent centre-of-mass energies 6 Conclusions A Di erential measurements B Correlation matrices The LHCb collaboration 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 re ects 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 theoretical calculations is reduced and many of the experimental uncertainties cancel. 1Throughout this article Z is used to denote the Z= contributions. { 1 { During LHC Run 1, data were collected at centre-of-mass energies, ps, of 7 TeV and p 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 p 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 di erent kinematic regions. The measurements of inclusive W and Z boson cross-sections at p s = 8 TeV described here are performed following the same procedure as detailed in refs. [8, 9]. The crosssections are de ned for muons with transverse momentum pT > 20 GeV=c and pseudorapidity in the range 2:0 < < 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 < M < 120 GeV=c2. These kinematic requirements de ne the ducial region of the measurement and are referred to as the ducial requirements in this article. Total cross-sections are presented, as well as di erential cross-sections as functions of of the muons, and of the Z boson rapidity, yZ , transverse momentum, pT;Z , and [17]. Here is de ned as2 tan ( acop=2) cosh ( =2) ; (1.1) where the angle acop = j j depends on the di erence in azimuthal angle between the two muon momenta, while the di erence between their pseudorapidities is denoted by . Di erential 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 de nes 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 di erential 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 < < 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 (...truncated)


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R. Aaij, C. Abellán Beteta, B. Adeva, M. Adinolfi. Measurement of forward W and Z boson production in pp collisions at \( \sqrt{s}=8 \) TeV, Journal of High Energy Physics, 2016, pp. 155, Volume 2016, Issue 1, DOI: 10.1007/JHEP01(2016)155