Quality of Jet Measurements and Impact on a Search for New Physics at CMS

The search for new physics beyond the Standard Model is one of the primary enterprises inparticle physics today. Many models predict the production of new, coloured particles atthe LHC. These particles would eventually decay into quarks and gluons, which manifestas particle jets in the detector. Hence, a precise understanding of the properties of jets isessential to study the underlying hard interaction process of hadronic final-states in orderto probe the Standard Model and enter the territory beyond.In this thesis, a measurement of the jet transverse-momentum (pT ) response function atthe CMS experiment at the CERN LHC is presented, and its impact on a QCD-backgroundestimation method in a search for new physics is described.The jet-pT response function is determined in two steps from dijet data collected inproton-proton collisions at a centre-of-mass energy of 7 TeV and corresponding to an integrated luminosity of up to 4.90 fb−1 . The jets are clustered with the anti-kT algorithm fromindividually measured particles that are reconstructed with the Particle-Flow algorithmcombining the information from all relevant subdetectors. Firstly, the relative jet-pT resolution σ/pT is measured using an unbinned maximum-likelihood approach that is based onthe pT balance in QCD-dijet events and incorporates the jet-pT spectrum. In the centraldetector region, the relative resolution in data is approximately 10% at pT = 100 GeV andimproves to 5.5% at 600 GeV. It is approximately 5% larger on average than in the simulation. Secondly, the tails of the jet-pT response function are measured studying the dijetpT -asymmetry. The tails are caused by semileptonically decaying heavy-flavour quarks aswell as instrumental effects and contribute at the percent level to the response function.Their relative size in data is larger than in the simulation by 20 – 60% in the centraldetector region at medium to high pT .Furthermore, a search for new physics in a multijet final-state with no leptons and alarge pT imbalance is reviewed, which has been conducted by CMS with 4.98 fb−1 of data.The QCD-multijet background is estimated from data in a control region by modelling thejet-pT mismeasurements using the response function determined from dijet events. Takinginto account the uncertainties on the response-function measurement, a prediction of theQCD background in the search region with a typical uncertainty of 60 – 70% is achieved.No excess in the number of events has been observed. Derived exclusion limits on theparameters of the Constrained Minimal Supersymmetric Standard Model correspond to alower limit on equal squark and gluino masses of approximately 1300 GeV.