Testing the electroweak sector and determining the absolute luminosity at LHCb using dimuon final states

The LHCb detector, which commenced data-taking at the Large Hadron Collider in September 2008, is a foward single arm spectrometer optimised for measurements of CP-violating and rare decays in the b quark sector. This thesis describes preparatory Monte-Carlo based studies of two proposed physics measurements that will be made at LHCb. Firstly, the trigger, reconstruction and selection efficiencies for recording $Zightarrowmu^{+}mu^{-}$ events at the LHCb experiment are presented. From simulation we conclude that LHCb will be capable of reconstructing Z bosons via this channel with rapidities in the range 1.7$<$y$<$4.9 with an overall efficiency of 0.358$pm$0.002(stat.), yielding $sim$172,500 events for an integrated luminosity of 1$fb^{-1}$. The background has been studied at the four-vector level and is estimated to be (3.0$pm$2.9)% of the signal level with the dominant contribution coming from events where two hadrons are misidentified as muons, a background source that can best be estimated from the data themselves. Systematic uncertainties in the efficiency and purity are discussed and expected to be $<$0.5% for a cross-section measurement in the forward region (1.7$<$y$<$4.9). Assuming the luminosity can be determined at a similar level, LHCb will rapidly be able to make a unique measurement of $sigma_{Z}cdot Br(Zightarrowmu^{+}mu^{-})$ at high rapidities with a precision of about 1%. This measurement wi ll provide an important cross-check for the corresponding measurements at CMS and ATLAS, and, in conjunction with a measurement of the W cross section, precisely test the electroweak sector at LHC energies. In addition measurements of the differential distributions, d$sigma$/dy and d$sigma$/d$P_{T}$, will constrain the proton parton distribution functions and provide a stringent test of QCD at high-$Q^{2}$. Secondly, the feasibility of using the elastic two photon process $pp ightarrow p+mu^{+}mu^{-}+p$ to make luminosity measurements at LHCb is investigated. The overall efficiency at LHCb for recording and selecting $pp ightarrow p+mu^{+}mu^{-}+p$ events produced within the pseudorapidity range $1.6<eta<5$ has been determined using Monte-Carlo to be 0.059$pm$0.001(stat.), yielding $sim$5200 events for an integrated luminosity of 1$fb^{-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 the four-vector level. The background is estimated to be (4.1$pm$0.5(stat.)$pm$1.0(syst.))% of the signal level with the dominant contribution due to K/$pi$ mis-identification. Systematic uncertainties on a luminosity measurement at LHCb using this channel are estimated to be $sim$1.3% and are dom inated by the uncertainty on the predicted cross-section for events containing dimuons produced via double pomeron exchange, an uncertainty that is expected to be reduced in the near future.