Measurement of the W boson mass with LHCb

Lead Research Organisation: University of Oxford
Department Name: Oxford Physics


This project aims to make the first ever measurement of the W boson mass using the LHCb experiment at the CERN Large Hadron Collider. It is universally accepted that there must be new matter species and forces beyond those of the Standard Model. One of the most powerful discovery approaches is to confront the precisely calculable relations between the fundamental parameters of the SM. A global fit of the electroweak parameters is highly sensitive to possible effects of heavy new particles in quantum loops. The sensitivity of this test is currently limited by the precision with which the W boson mass has been measured. It is therefore of paramount importance that the W mass is measured more precisely. The most precise measurements of the W mass to date come from the CDF and D0 experiments at the Fermilab Tevatron proton antiproton collider. Measurements are underway with the ATLAS and CMS experiments, with ATLAS having released their first analysis in December 2016. This measurement has an uncertainty of 19 MeV, of which 14 MeV is attributed to the modelling of W boson production. LHCb is yet to be exploited in the measurement of the W mass, but could play a very important role in the ultimate precision in the W mass, due to its unique angular coverage. Unlike the other detectors which cover wide angle particle production, LHCb focusses on small angles with respect to the colliding proton beams. This means that many systematic uncertainties, especially those related to W boson production modelling, will be orthogonal to, or in some cases anti-correlated with, those of ATLAS and CMS. LHCb already has a vast dataset with several million leptonic W boson decays, and its full Run-II dataset will have been recorded by midway through the studentship. This dataset will allow a measurement with a statistical uncertainty of around 10 MeV. LHCb is well suited to the study of muonic W decays, thanks to its excellent charged particle tracking, and muon identification capabilities. However, the unique design of LHCb means that different challenges will face this measurement, compared to those of ATLAS and CMS. This will require novel new approaches, for example to control the level of background, and to model W boson production in the small-angle region covered by LHCb. One of the first objectives will be to precisely study Z boson production in the LHCb region, and to compare measured distributions with state-of-the-art theoretical models. The accuracy of these models is central to the goal of measuring the W mass. That is because the kinematic distributions of the charged lepton from leptonic W decays must be compared with templates from these tools, in order to extract the W mass. The Z data will be critical in this regard.This promises to be a very exciting research project, with potential for a high impact on the sensitivity of precision null tests of the Standard Model.


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Studentship Projects

Project Reference Relationship Related To Start End Student Name
ST/R505006/1 01/10/2017 30/09/2021
1937404 Studentship ST/R505006/1 01/10/2017 31/03/2021 Martina Pili
Description Guide at CERN Open Days 
Form Of Engagement Activity Participation in an open day or visit at my research institution
Part Of Official Scheme? No
Geographic Reach International
Primary Audience Public/other audiences
Results and Impact The Open Days are the largest outreach events that CERN hosts once every five years. Visitors were guided across CERN's main experiment sites, including the undeground areas where the LHC experiments are installed. More than two thousand people visited the LHCb site in two days.
Year(s) Of Engagement Activity 2019
Description School Visit 
Form Of Engagement Activity A talk or presentation
Part Of Official Scheme? No
Geographic Reach Regional
Primary Audience Schools
Results and Impact Students from the Abingdon School (Oxfordshire) visited CERN. As Oxford student based at CERN I gave a brief talk describing the physics programme of my experiment and my own project goals.
Year(s) Of Engagement Activity 2019