Observation of $Z$ production in proton-lead collisions at LHCb

TL;DR

This paper reports the first observation of Z boson production in proton-lead collisions at the LHCb, measuring the production cross-sections in different beam directions using muon pairs.

Contribution

It provides the first measurement of Z boson production in proton-lead collisions at the LHC, expanding understanding of electroweak processes in nuclear environments.

Findings

Measured Z production cross-section in proton beam direction: 13.5 nb

Measured Z production cross-section in lead beam direction: 10.7 nb

First observation of Z bosons in proton-lead collisions at this energy

Abstract

The first observation of $Z$ boson production in proton-lead collisions at a centre-of-mass energy per proton-nucleon pair of $\sqrt{s_{NN}}=5~\text{TeV}$ is presented. The data sample corresponds to an integrated luminosity of $1.6~\text{nb}^{-1}$ collected with the LHCb detector. The $Z$ candidates are reconstructed from pairs of oppositely charged muons with pseudorapidities between 2.0 and 4.5 and transverse momenta above $20~\text{GeV}/c$. The invariant dimuon mass is restricted to the range $60-120~\text{GeV}/c^2$. The $Z$ production cross-section is measured to be \begin{eqnarray*} \sigma_{Z\to\mu^+\mu^-}(\text{fwd})&=&13.5^{+5.4}_{-4.0}\text{(stat.)}\pm1.2\text{(syst.)}~\text{nb} \end{eqnarray*} in the direction of the proton beam and \begin{eqnarray*} \sigma_{Z\to\mu^+\mu^-}(\text{bwd}) & =&10.7^{+8.4}_{-5.1}\text{(stat.)}\pm1.0\text{(syst.)}~\text{nb} \end{eqnarray*} in the direction of the lead beam, where the first uncertainty is statistical and the second systematic.