Ph.D. studentships on ATLAS starting in 2025.

A Ph.D. on ATLAS involves two (or more) separate areas of work. One is the main thesis activity: new research which is the primary focus of the project. Example projects follow below, but these are indicative only - details will be agreed with the supervisor. The other is taking on a responsibility for some aspect of detector operations or upgrade. The Warwick group is involved in building the ITk tracker to replace the current ATLAS tracking system in HL-LHC, and the student could join in this work. Other recent responsibilities have included monitoring the existing tracker, trigger optimisation and operations, and calibration of algorithms to identify b-quark jets.

1. Higgs boson searches. The standard model is known to be incomplete in several ways. One of those is the absence of sufficient CP violation to explain the matter-antimatter asymmetry in the Universe. A very exciting possibility is electro-weak baryogenesis, which would require modifications to the Higgs potential, and probably more Higgs bosons. If there are others, their behaviours are much less predictable than the first, and therefore harder to find. This project is to search for a charged Higgs boson decaying into a b quark and a t quark. The enormous, and growing, dataset recorded by ATLAS allows rare searches with ever increasing sensitivity.
2. CP violation could show up in studies of the Higgs boson discovered in 2012. The original measurements of the Higgs boson often assumed its kinematic properties to be consistent with the Standard Model prediction, and tried to measure the rates. This analysis is to work on differential cross-section measurements to test the Standard Model more carefully. The combined Run 2 and Run 3 datasets, allied with ever improving tool performance, will allow studies in unprecedented detail of the vector-boson fusion production mode in the decay of the Higgs boson to two tau leptons. Interesting distributions are the transverse momentum of the Higgs boson, testing the Higgs boson coupling or the angle between the two leading jets testing the CP invariance of the Higgs boson.
3. Probing the Higgs potential through di-Higgs production: The main way that the LHC can constrain the form of the Higgs potential is through observing the simultaneous production of two Higgs bosons and measuring the properties of such events. So far we have not discovered this process, but the search for it is heating up as our limits on the production process approach the Standard Model expectation! A large number of interesting new techniques for improved triggers, object identification, signal vs background discrimination, and modelling of the main backgrounds have been developed over the last few years, but more advancements are still needed for us to observe this important process at the LHC.
4. Physics of and with the Top quark. The ATLAS experiment has produced roughly 100 million top/anti-top quark pairs and, as the most recently discovered of the six quarks, these decays are a fascinating and world-best place to search for new and unexpected phenomena in top decays. We have an active program to measure the decay of the top quark to flavour-tagged final states, and in particular, we are close to completing a measurement of the branching fraction t -> b bbar c, which is sensitive to the Vcb element of the CKM matrix. The unique selling-point of this measurement, is that the decays to the (bbar c) pair come direct from a real charged W gauge boson, rather than from the highly virtual ones which are normally involved in measuring this fundamental parameter of the Standard Model. This current measurement is based on the ATLAS Run 2 data, and there is a PhD project to apply similar methods to the new Run 3 data sample, together with improvements in the data analysis. The same data sample will also be used to test the up-charm unitarity of the CKM quark mixing matrix, and to test for unexpected sources of CP violation in top decays.