The Standard Model of particle physics provides an elegant description of the fundamental constituents of matter and their interactions. Two of the four basic forces of nature, namely the electromagnetic and weak forces, are combined into a unified electroweak theory. In the electroweak theory the mass of the W boson is predicted to a precision of one part in ten-thousand. If experiments measure a different value this would be a strong hint of new particles and interactions. Unfortunately, the W mass is extremely difficult to measure. The most precise measurements are expected to come from the study of leptonic W decays at the LHC, but a key challenge is to understand the structure of the colliding protons well enough. Measurements of the W mass have long been part of the program of the ATLAS and CMS experiments at the LHC, but they could eventually hit an irreducible limit due to the proton structure uncertainties. It has recently been suggested that a new measurement of the W mass by the LHCb experiment, which uniquely covers particle production at small angles with respect to the colliding beams, would help to alleviate this problem. Researchers at Warwick, and collaborators from Oxford and Liverpool, looked into the role of the proton structure uncertainties in detail, and realised how a re-optimisation of the analysis method could help to reduce the uncertainties by roughly a factor of two. This should ultimately lead to an enhanced sensitivity to physics beyond the Standard Model through these precision tests. Work is now ongoing to apply the proposed method in the first ever measurement of the W mass with data from LHCb.

• Caption to figure: The expected proton structure uncertainty in 1000 simulated measurements of the W boson mass with the LHCb experiment. The red histogram (which is scaled down by a factor of ten) corresponds to the traditional approach of performing a fit to the charged lepton transverse momentum distribution. The blue histogram corresponds to the inclusion of weights that consider the compatibility of the data with the proton structure assumptions. The green histogram corresponds to the new method with a fit to the two dimensional distribution of the transverse momentum and angle of the charged lepton.
• Publication: https://arxiv.org/abs/1902.04323