Topical seminar: Measuring the Higgs boson mass using event-by-event uncertainties

by Mr Antonio Castelli (Nikhef)

Tuesday 1 Sept 2015, 14:00 → 15:00 Europe/Brussels

Universe

abstract: After the Higgs boson discovery in 2012, the measurement of the quantum numbers of this particle has become of the utmost importance to test the validity of the Standard Model. This talk presents the observation, by the ATLAS experiment, of the decay of a Standard Model Higgs boson into four charged leptons. A sharp peak over the expected background (corresponding to a local significance of 8 sigma) is observed around a value of the four lepton invariant mass of 125 GeV. The mass, the signal strength and the width of the Higgs boson are extracted by applying an unbinned maximum likelihood fit to the selected dataset. Different models can be used in the fit. The seminar focuses on the description of a signal model based on analytic functions. The signal probability density function (PDF) is given by a sum of convolutions of an Exponential to a Gaussian. Using such a PDF it is possible to exploit in the fitting procedure the information provided by the uncertainty measured on the four lepton invariant mass in each event. The inclusion of this observable allows to reduce by 6% the uncertainty measured on the Higgs boson mass and signal strength presented in the last ATLAS publications. The signal model can be easily extended to be also sensitive to the Higgs boson decay width. Because of the lack of statistics only an upper limit can be set on this quantity. The limit is in agreement with the most recent publications.