Kinematic fit in CMS & the use of top quarks for calibration

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As a first topic, a kinematic fit implementation is described as it is used in CMS at the LHC collider. The importance of a kinematic fit for the top quark mass measurement is illustrated. Not only is the resolution on the final mass estimator reduced significantly, the probability of the kinematic fit can also be translated in a strong separation power between correct and wrong jet pairings. The two other topics covered in this contribution, the measurement of the b-tagging efficiency and the measurement of the inclusive jet energy scale of light quark jets, are both examples of how the abundantly produced top quark pairs can be used for calibration tasks. In the first study a highly b-enriched jet sample is constructed from the fully leptonic as well as the semileptonic t t decays, using a likelihood ratio method. The possible systematic uncertainties on the estimated b-purity were evaluated, leading to an optimized estimate of the expected preci- sion on the measurement of the b-jet identification performance. In the second study the absolute jet energy scale of the two reconstructed light quark jets from the semileptonic t t decay (t t → W bW b → l νbqqb) was determined via the precisely measured W- boson mass. Even for an integrated luminosity of 0.5 fb-1 the systematic effects, mainly pile-up, on the optimal inclusive jet energy shift dominate the statistical uncertainty. A precise knowledge of both the b-tag efficiency and the jet energy scale will be of major im- portance for precision measurements (e.g. top quark mass) and a whole spectrum of new physics studies (e.g. Higgs-boson search).

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