# Observation of associated production of a $Z$ boson with a $D$ meson in the forward region

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## Abstract

A search for associated production of a $Z$ boson with an open charm meson is presented using a data sample, corresponding to an integrated luminosity of $1.0\,\mathrm{fb}^{-`}$ of proton--proton collisions at a centre-of-mass energy of 7\,TeV, collected by the LHCb experiment. %% Seven candidate events for associated production of a $Z$ boson with a $D^0$ meson and four candidate events for a $Z$ boson with a $D^+$ meson are observed with a combined significance of 5.1standard deviations. The production cross-sections in the forward region are measured to be $$\sigma_{Z\rightarrow\mu^+\mu^-\!,D^0} = 2.50\pm1.12\pm0.22pb$$ $$\sigma_{Z\rightarrow\mu^+\mu^-\!,D^+} = 0.44\pm0.23\pm0.03pb,$$ where the first uncertainty is statistical and the second systematic.

## Figures and captions

 Invariant mass distribution for $Z$ (left) and $D$ (right) candidates for $Z + D ^0$ (top) and $Z + D ^+$ (bottom) events. The superimposed curves represent the projection of the fit described in Sect. 4. Z-ZD0_base.pdf [14 KiB] HiDef png [122 KiB] Thumbnail [36 KiB] *.C file D-ZD0_base.pdf [14 KiB] HiDef png [110 KiB] Thumbnail [33 KiB] *.C file Z-ZDp_base.pdf [14 KiB] HiDef png [122 KiB] Thumbnail [36 KiB] *.C file D-ZDp_base.pdf [14 KiB] HiDef png [113 KiB] Thumbnail [34 KiB] *.C file Invariant mass of the $Z$ and $D ^0$ (left) and $Z$ and $D ^+$ (right) candidates (shown as black dots) compared to the fit (see text) that was used to extract the combinatorial background. The fit shown includes the signal and the background components. The colour scale shows the PDF value at any given point. ZD0_base.pdf [126 KiB] HiDef png [1 MiB] Thumbnail [217 KiB] *.C file ZDp_base.pdf [128 KiB] HiDef png [1 MiB] Thumbnail [217 KiB] *.C file Animated gif made out of all figures. PAPER-2013-062.gif Thumbnail

## Tables and captions

 Relative systematic uncertainties for the production cross-section of a $Z$ boson with an open charm meson $\left[\%\right]$. Table_1.pdf [45 KiB] HiDef png [55 KiB] Thumbnail [9 KiB] tex code Comparison of the measured cross-sections $\left[\rm \,pb \right]$ and the theoretical predictions for the associated production of a $Z$ boson with an open charm meson. For the measured cross-section the first uncertainty is statistical and the second systematic. For {\sc{MCFM}} the first uncertainty is related to the uncertainties of the parton distribution functions, the second is the scale uncertainty and the third due to uncertainties associated with $c$ -quark hadronisation as discussed in the text. Table_2.pdf [59 KiB] HiDef png [45 KiB] Thumbnail [7 KiB] tex code

## Supplementary Material [file]

 Supplementary material full pdf supp.pdf [78 KiB] xsec-br.pdf [15 KiB] HiDef png [202 KiB] Thumbnail [80 KiB] *C file

Created on 20 April 2019.Citation count from INSPIRE on 20 April 2019.