02 Dec 2020
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LHCb - Beauty mesons as a probe of new physics

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Co-supervisors: PPD - S. Ricciardi, University of Edinburgh - Prof F. Muheim

For this studentship special conditions are available. International students are eligible for this studentship, which will cover all fees, stipend, and other related costs of the PhD.

The LHCb experiment recently (Oct 2020) announced the fi rst observation of time-dependent CP violation in the strange-beauty (Bs0 ) meson, through decays into a pair of kaons, Bs0 -> K+K. This result achieved a further milestone in the field of CP violation. The asymmetry between matter and antimatter requires that CP violation exists between particles and antiparticles, but the Standard Model (SM) prediction for this effect falls short by nine orders of magnitude. New physics beyond the SM must exist and can be probed with beauty mesons. This opens up exciting prospects of additional studies in B0 and Bs0 mesons, both in the Run 1-2 data sample already collected by the LHCb experiment, and in the upcoming Run 3 period in which the LHCb experiment
will use an almost entirely new detector.

In this project the student will study the B0 and Bs0 meson systems in decays which are sensitive to the same underlying CP-violating processes, and provide key measurements to test the standard model of particle physics. Building on the well-established expertise and tools developed by the Edinburgh group, the student will reconstruct and study the time-dependence of B -> J/Psi X decays. This will open up, for example, the possibility to measure the lifetime difference between B0 meson mass eigenstates (d), an important outstanding measurement from LHCb which is sensitive to new physics in B0 decays. The student will primarily focus on analysing Run 2 data, and in particular will help to develop new favour tagging tools which can signi cantly boost the sensitivity of a wide range of CP violation measurements. Improving the tagging power by a factor of 2, which is feasible through a total event tagger and the use of multivariate machinery, would be the equivalent of doubling the data sample, giving a large gain in precision. The student will also work on preparing for the Run 3 data collection period, and use their experience from the Run 2 data analysis and tagging algorithms to validate the early Run 3 data.

The student will also work on detector hardware for future LHCb upgrades. Excellent discrimination among pion, kaons and protons, referred to as particle identi cation (PID), is essential for flavour tagging. The PID provided by the Ring Imaging Cherenkov (RICH) system has been central to the success of LHCb. The student will contribute first to the commissioning of the newly installed RICH photon detectors and then to RICH operation during the upcoming Run 3 data. To operate at higher luminosities, however, the RICH system needs to be redesigned. A signi cant R&D programme is needed to produce a photon detector with sufficient timing capabilities, able to operate with low noise in the harsh radiation environment and with sufficiently fine granularity to cope with the expected occupancy. Silicon PhotoMultipliers (SiPMs) are the baseline choice for the future detector upgrade. Recent progress in SiPMs technology motivates a vigorous R&D programme, starting in 2021, to develop efficient and fast single-photon capable devices. The student will contribute at the characterization of SiPMs at RAL, where a dedicated lab is being set-up to lead the UK R&D on the use of SiPMs in LHCb.

This project offers a combination of crucial detector development activities, and an analysis programme which could lead to several high impact publications. Both aspects are ideally suited to the experience and interests of the RAL and Edinburgh LHCb groups.

Contact: Ricciardi, Stefania (STFC,RAL,PPD)