Measurement of single top t-channelproduction cross section through thet → W b → μνb decay in 7 TeV proton-protoncollisions with the Compact Muon Solenoiddetector at the Large Hadron Collider

Abstract

In my PhD I actively participated the Compact Muon Solenoid (CMS) experiment at the Large Hadron Collider (LHC) of CERN. My work over the three years of the PhD encompassed several aspects of the experiment, including participation to data taking activities, performance studies of the detector and data analysis finalized to the measurement of single top quark production. During the first year of my Ph.D. program I had the chance to participate in the commissioning effort of the CMS Muon System, studying the performances of the Resistive Plate Chambers detectors using cosmic ray data collected in 2008-2009 and early collision data taken in 2010. The second and third years of my Ph.D. program were devoted to the study of the production of single-top events in proton-proton collisions at LHC. The LHC has been successfully running since the end of 2009 and is providing 7 TeV center of mass proton-proton collision data since March 2010, collecting data corresponding to an integrated luminosity of 36.1 inverse pb . With the data taking of 2011, the instantaneous luminosity of the machine was constantly upgraded, allowing to collect 5.73 inverse fb of integrated luminosity. The CMS detector has successfully collected collision data through 2010 and 2011, collecting a total integrated luminosity of 5.22 inverse fb . The LHC is often considered to be a “top quark factory”, allowing for a production of top quarks through several mechanisms: tt pair production, mainly occurring through processes which involve the strong interaction, and single top quark production, which occurs through electroweak interactions and has been object of study of my thesis. There are three mechanisms of production of single top, known as t,s, and tW channels. The study of single top processes provides the unique opportunity to explore the electroweak sector of top quark physics, allowing for example a direct measurement of Vtb as well as the study of Flavour Changing Neutral Currents. On the other hand single top channels offer a window for searches for physics beyond standard model, being for instance sensitive to W resonances. Amongst the three channels, I focused my work on the t channel, which yields a production cross section of 64.6 pb, the highest amongst the single top processes. I developed and deployed an analysis which focused on the top quark decay channel t → W b → μνb aiming at early observation of single top events with the 2010 dataset. I was successful in achieving the first experimental evidence of single top at the LHC, which resulted in a paper published on PRL . With 2011 data incoming, I developed and deployed a new analysis, adapting the strategy to a different goal: the new analysis should have minimized the systematic uncertainty and at the same time be model-independent to allow for further studies of single top polarization and beyond standard model physics searches. The results have been updated with a luminosity of 1299 inverse pb , and a combination with the electron channel results has been performed to increase the precision of the measurement.