Description:

Abstract: An apparent feature of the Standard Model is that all electroweak bosons couple equally to all three lepton generations, a property known as lepton flavor universality (LFU).  Experimental evidence of a significant deviation from LFU would be a sign of new physics.  The current, most precise determination of the leptonic W boson branching fractions, based on a combination of measurements by the four LEP experiments, shows a 2.6 standard deviation discrepancy in the decay ratio of W to tau leptons compared with decays to the lighter flavor leptons.  I will present a measurement of the leptonic and inclusive hadronic decay branching fractions of the W boson based on 35.9/fb of proton-proton collision data collected by the CMS experiment at sqrt(s) = 13 TeV.  The analysis largely relies on identifying W boson decays in events originating from top pair production, and the W boson branching fractions are determined based on a binned, maximum likelihood estimate that is performed simultaneously in each event category.  Based on the ratio of leptonic to hadronic branching fractions, three additional standard model quantities can subsequently be derived: the sum of squared elements in the first two rows of the Cabibbo--Kobayashi--Maskawa (CKM) matrix, the CKM element |V_cs|, and the strong coupling constant at the W boson mass scale.

Spekaer: Nate Odell, Postdoctoral Fellow, Northwestern University 

Host: Michael Schmitt