Improvement of the Higgs mass resolution in the ZH -> L+L- b b channel using a Kinematic Fit with constraints

The measurement of the Higgs mass in the H → bb¯ decay suffers from low accuracy of b-jet momentum reconstruction. In order to improve the resolution in mbb¯ a least squares kinematic fit was applied in the ZH → l+l−bb¯ channel. In the fit the b-jet momentum vectors are changed such that they are closer to those of the true b-quarks. Momentum conservation in the transverse plane was used as a constraint. Initial state radiation carries away momentum of the ZH system. To account for the loss in transverse momentum an additional momentum vector is constructed. An improvement of 8 % was achieved in the Higgs mass resolution. With a cut on the chi-square probability of the kinematic fit the expected significance could be improved by 17 %. A cross check on the ZZ → l+l−bb¯ decay channel showed an improved of 23 % in the expected significance.