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

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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.

Figures and captions

Invariant dimuon mass distribution of selected $ Z $ candidates in (a) the backward and (b) the forward sample are shown by the black data points with error bars. The red line shows the distribution obtained from simulation using Pythia8 with the MSTW08 PDF set normalised to the number of observed candidates.

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Rapidity distribution of selected $ Z $ candidates in (a) the backward and in (b) the forward sample are shown by the black data points. The red line shows the distribution obtained from simulation using Pythia8 with the MSTW08 PDF set normalised to the number of observed candidates. The top $x$-axis shows the rapidity, $y_\text{Lab}$, in the laboratory frame, the bottom one the rapidity, $y$, in the centre-of-mass frame.

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Experimental results and the theoretical predictions for the $ Z \!\rightarrow \mu ^+\mu ^- $ production cross-section. The inner error bars of the experimental results show the systematic uncertainties. The uncertainties on the theoretical predictions are negligible compared to those on the experimental results.

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Tables and captions

Systematic uncertainties in the cross-section calculation for $\sigma_{ Z \!\rightarrow \mu ^+\mu ^- }$. The uncertainties on $\rho$ and $\varepsilon^\text{cand}$ are assumed to be fully correlated between the forward and the backward sample.

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$ Z \!\rightarrow \mu ^+\mu ^- $ cross-section predictions based on NNLO calculations using Fewz with the MSTW08 PDF set. The nuclear modifications are parametrised by the EPS09 nPDF set. The first uncertainty is the scale uncertainty evaluated by changing the renormalisation and factorisation scales by a factor two up and down. The second one comes from the uncertainties on the PDF and the third in case of the predictions with nuclear effects from the nPDF.

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Supplementary Material [file]

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Supplementary material for LHCb-PAPER-2014-022 ============================================== This ZIP file contains supplemetary material for the publication LHCb-PAPER-2012-011. The files are: supplementary.pdf : An overview of the extra figures *.pdf, *.png, *.eps, *.C : figures in the different formats (Fig1-S not in only in pdf- and png-format)

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Created on 09 December 2018.Citation count from INSPIRE on 18 December 2018.