My research is based on the LHCb experiment at CERN's Large Hadron Collider. The experiment is an international collaboration of ~1000 physicists from ~50 institutes that is designed to study the decays of B mesons. I lead a group of three academics, two postdoctoral staff and a number of PhD students who, together with engineering and technical staff, are contributing to a wide range of activities at LHCb.
I am particularly interested in the use of rare decay modes as probe for physics beyond the current "Standard Model" of particle physics. My group have played a leading role in several measurements where we have seen some hints of tension with the Standard Model predictions. These so called "B anomalies" have been among the highest profile measurements in particle physics in recent years:
- We have led the measurements of the lepton flavour universality ratio, RK, between the branching fractions of B →K mu mu- and B →K e e- decays. Our most recent measurement is 3.1 sigma from the Standard Model prediction, giving evidence for lepton flavour universality violation in these decays (see our paper, here, and associated media coverage here, here, here, here and here). Our previous measurement of this ratio was published in Physics Review Letters (here) and was the 2nd most cited experimental particle physics paper in the field in 2019. These measurements agree with similar tensions we observe in B0→K*0l l- decays (see this paper) and the techniques have been used to develop related measurements, see e.g. here.
- We have made the world's best measurements of the rare decay B0→K*0mu mu- . I led the group that made the first LHCb measurements of this decay and we have played a leading role in every subsequent generation of these measurements (see e.g. our papers here, here, here and here). The techniques developed have enabled an entire suite of measurements of related decays (e.g. here and here). Our most recent measurements of the B0→K*0mu mu- angular observable P5' are in tension with Standard Model predictions and were published in Physics Review Letters (here). We are leading work to update these measurements with the full dataset collected at LHCb, have developed the measurement technique and are collaborating with theorists to develop the phenomenology, see e.g. here and here.
- We developed the LHCb measurement of the lepton universality ratio RD* (see here and here) which is a ratio between semileptonic decays involving taus and muons. Combining this measurement with other analogous measurements again gives a further 3 sigma tension with Standard Model predictions. We are developing the first such measurement using suppressed b→u quark transitions and have made the first observation of the decay B→pp mu nu (see paper here) and are leading a search for B→pp tau nu.
These anomalies can be interpreted coherently in new physics models with a new vector or axial-vector particle such as Leptoquark or Z' and have caused a great deal of excitement in the particle physics community. However, questions remain about whether the Standard Model predictions are correct and further measurements are critical to clarify the situation. I have used an associateship at IPPP to start a major workshop series in this area, “Beyond the Flavour Anomalies”, see here and here.
My research group have also performed a wide range of other measurements:
- We pioneered the B0→K*0mu mu- branching fraction measurements (here) which, together with other b→smu mu- branching fractions, is in tension with Standard Model predictions.
- We made the world's first observation of the decay B →pi mu mu-. This was the first b→dmu mu- transition ever observed and allowed us to constrain the parameters |Vts/Vtd|2 which are sensitive to new heavy particles and allow us to test the minimal flavour violation hypothesis in such decays for the first time. Our paper was published in JHEP and subsequent update here. We also made the first observation of the analogous baryonic decay ΛB→p pi mu mu- (paper).
- The observation of a significant isospin asymmetry in B→Kmu mu- decays. This analysis involved the first rare decay studied at LHCb with a long-lived particle. Our paper show some tension with the SM predictions and was published in JHEP.
- The first measurement of the CKM parameter Vub using ΛB→pmunu decays, published in nature physics (paper).
- Searches for exotic new physics - see, for example, here, here and here.
My group have also played an active role in designing and building one of LHCb's Ring Imaging CHerenkov (RICH) detectors for the first generation LHCb detector and for the upgrade that we will start taking data with from 2022. See papers on our test-beam programme here and the full detector description here. For a future, further upgrade to the LHCb experiment, we are interested in developing a variety of different subsystems, including the RICH but also the tracking system and data processing. In addition to these activities, I have served on various committees for the research council STFC and am a former member of the heavy flavour averaging group (HFLAV) - see the latest combination of rare decay properties here.
et al., 2021, Measurement of J/psi production cross-sections in pp collisions at root s=5 TeV, Journal of High Energy Physics, ISSN:1029-8479
et al., 2021, Search for CP violation in Ξb−→pK−K− decays, Physical Review D, Vol:104, ISSN:2470-0010
et al., 2021, Search for CP violation in Xi(-)(b) -> pK(-)K(-) decays, Physical Review D, Vol:104, ISSN:2470-0010
et al., 2021, Observation of the decay Lambda b0 -> chi(c1)p pi(-), Journal of High Energy Physics, ISSN:1029-8479
et al., 2021, Observation of the decay Λb0 → χ<inf>c1</inf>pπ<sup>−</sup>, Journal of High Energy Physics, Vol:2021