Measurement of the W boson mass with the LHCb experiment

Precision measurements of electroweak parameters permit fundamental consistency tests of the Standard Model of Particle Physics. The resulting constraints on physics beyond the Standard Model are primarily limited by the precision of direct measurements of the mass of the W boson (mW ). Over the past thirty years, many experiments have performed measurements of mW , but an improvement in its precision is still needed. This thesis presents the first measurement of mW by the LHCb experiment at the facilities of the European Organization for Nuclear Research (CERN) in Geneva. This measurement is based on data collected by LHCb during the year 2016 at centre-of-mass-energy √s = 13 TeV. A simultaneous fit is performed on the q/pT distribution of a sample of W → μν decays and the φ∗ distribution of a sample of Z → μμ decays. The value of mW is determined to be mW = 80354 ± 23stat ± 10exp ± 17theory ± 9PDF MeV, based on an arithmetic average of results with three different Parton Distribution Function sets. This is considered a pathfinder measurement aimed at starting discussions with the theory and experimental community to reduce the still large systematic uncertainties on mW . The new methodologies and strategies adopted for this work lay the foundation of what will be the legacy measurement with the full dataset collected between 2015 and 2018. While this thesis describes the full measurement, greater detail will be given to the treatment of two of the most important sources of systematic uncertainty on the theoretical and experimental side, namely the Parton Distribution Functions and the measurement of the muon momenta. New methods, published in peer-reviewed journals, and ad hoc strategies have been developed to reduce their associated systematic uncertainties. Some of these studies have been applied to the proof-of-principle measurement described in this thesis. The others will be extremely important in the attempt of reducing the total uncertainty in future measurements of mW.