The $Z \to \mu^{+}\mu^{-}$ decay channel in the CMS experiment at LHC: from cross-section measurement with the 2010 7 TeV collision dataset to offline machine luminosity monitor

This thesis has been possible thank to a bourse from Italian Minister of Education, MIUR. During the phd program I had the opportunity to take part actively to the CMS experiments at CERN: many aspects of the chain to take and analyze the data have been studied. Indeed, in the first year of the phd program a tool to store and retrieve the CMS condition data in the CMS databases has been developed. A web monitor tool has been also deployed to trace and check the correctness of the transactions. Instead, in the second and third year of the phd program the decay of the Z boson in two muons has been analyzed. The CMS detector, which took 7 TeV collision data for the entire 2010 at CERN, is designed to provide precise measurements of TeV proton-proton collisions at the Large Hadron Collider. The measurements of the cross section of the Z boson production at LHC provides a first test of the Standard Model in a new energy domain and may reveal exotic physics processes. Moreover, the properties of the Z boson resonance and its decay into two muons are known to very high precision from LEP experiments and hence can be used as a physics process for calibration and alignment. The Z boson production is also a common background process for many other physics analyses and must therefore be well understood. This thesis describes the measurement strategy of the cross section for the process $pp \to Z + X \to \mu^{+}\mu^{-} + X$ at the CMS experiment during i ts start-up phase with low integrated luminosity available. The originality of the method relies on a fully data driven approach: the $Z \to \mu^{+}\mu^{-}$ inclusive cross-section is determined with a simultaneous fit of the yield of $Z \to \mu^{+}\mu^{-}$ events, the average reconstruction muon efficiencies in the tracker and in the muon detector, the trigger efficiency, as well as the efficiency of the cut applied to select isolated muons. The extracted Z yield has to be just corrected for the geometrical acceptance and for the integrated luminosity in order to measure the cross section. The measurements obtained with the first 2.9$pb^{-1}$ is $\sigma(pp \to ZX) x BF(Z\to\mu^{+}\mu^{-})=0.924\pm0.031(stat)\pm0.022(syst.)\pm0.101(lumi.)nb$,(2) referred to the invariant mass range $60 < m_{\mu^{+}\mu^{-}}<120GeV/c^{2}$. The value measured is in agreement with the Standard Model prediction. That measurement has been recently published in the paper [1]. The entire 2010 collision dataset has been also analyzed a first preliminary results for the cross-section measurement with 35 $pb^{-1}$ of data is also presented. I followed constantly the data taking in 2010, thank to frequent travel to CERN, from the first $Z \to \mu^{+}\mu^{-}$ candidate hunting in the first month, to the very first MC driven cross-section measurement showed at ICHEP conference in Paris, in July 2010, using 198 $pb^{-1}$ of data. The data-driven technique has been applied with the dataset of 2.9 $pb^{-1}$. The update of the measurement with the entire 2010 dataset corresponding to about 35 $pb^{-1}$ is also shown, together with the prospects to use the $Z \to \mu^{+}\mu^{-}$ and $W \to \mu\upsilon$ selected events to perform an offline evaluation of the integrated luminosity furnished to the experiment.