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)