Fitting of Event Topologies with External Kinematic Constraints in CMS

The resolution of the measured objects in the final state of proton collisions at the Large Hadron Collider can be improved by forcing well-defined kinematic hypotheses on the event. The kinematic constraints proposed by the hypotheses can be applied by means of Lagrange multipliers in a general event-by-event least square fitting technique. The resulting chi-square of the fit can be interpreted as the probability of the proposed kinematic hypotheses to be true for the observed event. This note describes both the general mathematical concept of kinematic fitting and its implementation within the analysis framework of the Compact Muon Solenoid experiment. The performance of the method is demonstrated in a few typical case-studies where relative improvements are emphasized.