List of current students

LHCb has an active electroweak (EW) physics programme with measurements of inclusive processes such as Z and W production in leptonic final states already published. The EW working group is also branching into jet physics with a completed Z+jet analysis and Z/W+b-jet analyses in internal review. More detailed and precise measurements of jet physics and the extraction of SM parameters are expected.

These measurements rely on simulation in modelling efficiencies, controlling backgrounds and in fitting for signal event yields. However, the standard simulation used at LHCb is not ideal as generators are generally not tuned for forward physics and leading-order (LO) matrix elements of Drell-Yan production do not account for the number, and hardness, of jets accurately.

During a three-month internship with the Lund HEP theory group, we have investigated two advanced Monte Carlo (MC) techniques to improve the modelling of electroweak physics in the forward region. Firstly, we used matched and merged (M&M) MC schemes, whereby matrix elements of W/Z+n jets for finite n are matched to a parton shower (PS) to approximately model additional parton branchings beyond n. Secondly, we looked at weak showers where vector bosons are allowed to be emitted within the parton shower. Weak showers give a more realistic description of jet substructure, complement the standard Drell-Yan production of W/Z bosons and enable more accurate parton shower histories to be created, which are used in M&M schemes.

We found that M&M schemes generally give a good description of Z+jet in the forward region, particularly in regions of phase space where a LO Drell-Yan plus PS description suffers. We also found that M&M MC behaves in a similar way in the forward region as it does centrally. This can be considered as a validation of the universality of these techniques. Finally, we found that current measurements of EW physics at LHCb do not yet fully probe the effect of weak showers. Weak showers become a significant, or even dominant, production mechanism of W/Z for event topologies with two, or more, jets and scales beyond 100 GeV in the pT of the jets.