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Measurement of the Jet Energy Scale in the CMS exepriment with the first LHC Proton Collisions

Wednesday, 26 October, 2011 - 10:00
Seminar room of the Physics Department
Isfahan University of Technology, Teheran, Iran
Maryam Zeinali
phd defence

Calibration of jets are important in the studies of new physics, since when jets are better
calibrated, the new physics events can be better reconstructed and differentiated from the
background. In e+jets ttbar events, the reconstructed jets originate from the decay of the
W boson and the top quark. As the masses of both the W boson and the top quark have
been already measured at LEP and Tevatron with great precision, therefore, the mass
values can be used to constrain the system. Mass constraints are imposed in an event-byevent
least square technique by means of Lagrange Multipliers which is referred to as a
kinematic fit. The kinematic fit return a probability which reflects how likely the
hypothesis of originating three reconstructed jets from the decay of the top quark is
correct. The jet energy scale calibration factors are then adopted so that the probability
returned by the kinematic fit is maximized. The method is also appliedon the 2010
collision data recorded by the CMS experiment and the results have been reported. Good
agreement between simulation and collision data is found. The results are statistically
limited which can be fixed when running the method on larger amount of statistics
collected in the 2011 runs.