Inclusive Low-Mass Drell-Yan Cross-Section at LHCb at $\sqrt{s}$ = 8 TeV

The LHCb experiment, one of the four main experiments at the LHC, is optimized for decays of particles containing a $b$- or $c$-quark. The LHCb detector is a single-arm forward spectrometer with an acceptance from approximately 30 to 250 mrad, with respect to the incoming proton beams. In addition to its main goal, its unique geometry makes it also a very interesting detector to probe general physics in the forward region. This includes electroweak boson production, which can provide important insights into the parton distribution functions (PDFs) of the proton. As part of this electroweak program, a measurement of the differential and double-differential inclusive Drell-Yan cross-sections with subsequent decay to muon-pairs \begin{equation*} \frac{\mathrm{d} \sigma(pp \to Z/\gamma* \to \mu^+\mu^-)}{\mathrm{d} M_{\mu\mu}}\quad\text{and}\quad \frac{\mathrm{d}^2 \sigma(pp \to Z/\gamma* \to \mu^+\mu^-)}{\mathrm{d} y\, \mathrm{d} M_{\mu\mu}} \end{equation*} is performed in the range $10.5 < M_{\mu\mu} < 110$ GeV/$c^2$ and $2 < y < 4.5$. The cross-section measurement benefits from the high-precision calibration of the absolute luminosity at LHCb. For this measurement, data corresponding to 2.0\invfb collected with the LHCb detector at a centre-of-mass energy of $\sqrt{s}$ = 8 TeV are analysed. The cross-sections are compared to theoretical next-to-next-to leading order perturbative QCD predictions using four different PDF sets, none of which included measurements from this hitherto unexplored region in phase space. The differential cross-section as a function of invariant mass of the dilepton pair is found to be in general agreement with the different predictions, however some systematic discrepancies are found when studying the double-differential cross-section as a function of rapidity of the dilepton pair. Overall the measurement presented here is systematically limited in the region which would be of most interest as input for future PDF sets, the low mass region. Nevertheless, it is hoped that the data presented here will help place new constraints on the quark and anti-quark content of the proton PDFs down to very low values of the momentum fraction of the partons, $x$.