Luminosity measurements at LHCb using dimuon pairs produced via elastic two photon fusion.

This note outlines the feasibility of using the elastic two photon process pp$\rightarrow$ p+$\mu^{+}\mu^{-}+p$ to make luminosity measurements at LHCb. The overall efficiency at LHCb for recording and selecting pp$\rightarrow$ p+$\mu^{+}\mu^{-}+p$ events produced within 1.6<$\eta$<5 has been determined using Monte-Carlo to be 0.0587 $\pm$ 0.0008, yielding 5210$\pm$71(stat.) events for an integrated luminosity of 1fb$^{-1}$. The main background processes where dimuons are produced via inelastic two-photon fusion and double Pomeron exchange have been studied using the full LHCb detector simulation while the other background sources, including backgrounds caused by K/$\pi$ mis-identification, have been studied at four vector level. The background is estimated to be (4.1 $\pm$ 0.5(stat.) $\pm$ 0.6(syst.))% of the signal level. Most of this background comes from K/$\pi$ mis-identification, although the largest source of uncertainty in the estimation is due to knowledge of the number of events produced via double Pomeron exchange. Systematic uncertainties on a luminosity measurement at LHCb using this channel are estimated to be ~1% the largest of which are the uncertainty on the predicted cross-section for events containing dimuons produced via double Pomeron exchange, and the knowledge of the trigger and reconstruction efficiency for muons in LHCb.