Understanding and constraining the PDF uncertainties in a W boson mass measurement with forward muons at the LHC

Precision electroweak tests are a powerful probe of physics beyond the Standard Model, but the sensitivity is limited by the precision with which the W boson mass ( M_W ) has been measured. The Parton Distribution Function (PDF) uncertainties are a potential limitation for measurements of M_W with LHC data. It has recently been pointed out that, thanks to LHCb’s unique forward rapidity acceptance, a new measurement of M_W by LHCb can improve this situation. Here we report on a detailed study on the mechanism driving the PDF uncertainty in the LHCb measurement of M_W , and propose an approach which should reduce this uncertainty by roughly a factor of two using LHCb Run 2 data.