Offline b-Tagging Calibration

The CMS detector will be equiped with a full silicon tracking device, allowing for performant b-jet identification algorithms. The identification of b-jets is essential for a large part of the physics program at the LHC. Searches for the Higgs boson and supersymmetry often need good b-tagging tools to differentiate the signal from the background. But also in Standard Model physics, for precise measurements in the top quark sector for example, b identification is crucial.

Various b-tagging algorithms have been implemented for CMS, and their performance has already been studied with dedicated Monte-Carlo simulations. However, the efficiencies and purities of these algorithms will have to be calibrated. Due to the high energy and luminosity of the LHC accelerator, a completely new method to calibrate b-tagging algorithms can be exploited. Indeed, during the first years of operation, the LHC will produce more than 8 million top anti-top pairs each year. As the decay t->Wb has a branching fraction very near to 100%, these ttbar events can be used to isolate a jet sample with a highly enriched b-jet content. This sample can then be exploited to calibrate the b-jet identification algorithms.

At the IIHE we develop two independent methods to extract a b-jet enriched jet sample in ttbar events.

• The first one exploits the semi-leptonic decay with final state bbqq'lnu. Backgrounds can be suppressed by requiring one b-tagged jet, and the combinatorial ambiguities can be handled with the many kinematical constraints in the event.
• A second method uses the fully leptonic bbl1nu1l2nu2 final state, which is cleaner but suffers from lower statistics and important contamination from WW and Drell-Yan backgrounds.

On both b-jet enriched data samples the b-tagging performance can then be measured similarly. A likelihood ratio based method has been developed to perform this measurement along with an estimation of the most important contributions to the systematic uncertainties. The research on this subject is currently being finalized and will be published in the Physics TDR Volume 1 in december 2005. By then an estimate will be obtained for the expected precision on the measurement of the b-jet identification performance, which serves as direct input for each physics analysis that relies on the considered b-tagging algorithms.