Edinburgh takes on the flavour of beauty

The magnificent Playfair Library in the historic centre of Edinburgh provided a spectacular setting for the scientific presentations of the 15th International Conference on B-Physics at Frontier Machines (Beauty 2014). The purpose of this conference series is to review the state of the art in the field of heavy-flavour physics, and to address the physics potential of existing and future B-physics experiments. This line of research aims to explore the Standard Model at the high-precision frontier, the goal being to reveal footprints of “new physics” originating from physics beyond the Standard Model in observables that can be predicted reliably. Hosted by the University of Edinburgh on 14–18 July, Beauty 2014 attracted around 90 physicists, including leading experts on flavour physics from across the world, to present and discuss the latest results in the field.

The key topics in flavour physics are strongly suppressed rare decays and decay-rate asymmetries that probe the phenomenon of CP violation. The non-invariance of weak interactions under combined charge-conjugation (C) and parity (P) transformations was discovered 50 years ago through the observation of K_L → π⁺π^– decays (CERN Courier July/August 2014 p21). The Cabibbo–Kobayashi–Maskawa (CKM) mechanism, postulated 10 years later, allows CP violation to arise in the Standard Model, in particular in the decays of B mesons (CERN Courier December 2012 p15). These particles are hadronic bound states of a b antiquark and a u, d, s or c quark. In the case of the neutral B⁰_d and B⁰_s mesons, quantum-mechanical particle–antiparticle oscillations give rise to interference effects, which can induce manifestations of CP violation. Flavour-changing neutral currents are forbidden at the tree level in the Standard Model, and are therefore sensitive to new particles that might reveal themselves indirectly through their contributions to loop processes. These features are at the basis of the search for new physics at the high-precision frontier.

The exploration of B physics is dominated currently by the dedicated LHCb experiment, as well as the general-purpose ATLAS and CMS experiments at the LHC. The completion of the upgrade of the KEKB collider and the Belle detector in Japan in the coming years will see KEK re-join the B-physics programme, when the Belle II experiment starts up at SuperKEKB (CERN Courier January/February 2012 p21).

At Beauty 2014, the programme of 13 topical sessions included 61 invited talks. The majority covered a variety of new analyses and experimental results, complemented by a series of review talks on theoretical aspects. In addition, seven early-career researchers (PhD students and postdocs) presented posters in a dedicated session.

Highlights of the conference included a measurement of CP violation in the decay B⁰_s → φφ, new results on the determination of the angle γ of the unitarity triangle from B → DK and B⁰_s → D^±_sK^± decays – the former of which receives contributions from “tree” topologies only – and B⁰_s → K⁺K^– and B⁰_d → π⁺π^– decays, which also receive “penguin” contributions where new particles might enter in the loops. The results for γ are consistent among one another within the uncertainties and the information on the unitarity triangle coming from global fits of various observables. The error on direct γ measurements is now approximately 9°, with significant contributions from the latest results from LHCb, which will continue to improve this precision. Impressive new measurements of the weak phase φ_s and decay-width difference ΔΓ_s were presented by CMS and LHCb in B⁰_s → J/ψφ and B⁰_s → J/ψππ decays. The latter is now the most precise φ_s result, with an uncertainty of 68 mrad, and the results are in agreement with the predictions of the Standard Model.

In the field of rare B-meson decays, there were reports on impressive theoretical progress for B⁰_s → μ⁺μ^– decays. This is one of the rarest decays that nature has to offer, and is therefore a very sensitive probe of new physics. Theoretical improvements relate to the calculation of higher-order electroweak and QCD corrections, which resulted in a higher precision on the predicted theoretical Standard Model branching ratio for this channel. The experimental evidence for this decay was reported by the CMS and LHCb collaborations in the summer of 2013, and is one of the highlights of Run 1 of the LHC. New combined results have recently been made public by the two collaborations.

Measurements of the angular distribution of the rare B⁰_d → K*⁰μ⁺μ^– decay and comparison with respect to calculations within the Standard Model was another hot topic. A discrepancy is observed in a single bin in the distribution of the so-called P₅´ observable. The key question is whether strong-interaction processes or new physics effects are causing this discrepancy. The possibilities led to interesting discussions during the session, which continued during the coffee breaks. Improved statistics on this and related channels from Run 2 at the LHC are awaited eagerly.

The opening talk of the conference was given by John Ellis of King’s College London and CERN, who presented his perspective and vision for the search for new physics

In the ratio of the rates of B⁺ → K⁺μ⁺μ^– and B⁺ → K⁺e⁺e^– decays, which test lepton-flavour universality, LHCb reported a new 2.6σ deviation from the Standard Model, which has to be explored in more detail. Moreover, first results on measurements of the photon polarization in b → sγ by the B factories and LHCb were presented, and this will be studied in a more powerful way by Belle II and the upgraded LHCb.

Many other interesting measurements and developments were discussed at the conference. One of these concerned the first observation of a heavy-flavoured spin-3 particle, the D*_s(2860)^– meson, observed by LHCb in the decay of a B⁰_s meson (CERN Courier September 2014 p8). Another was the confirmation of an exotic resonance Z(4430) composed of four quarks, also by LHCb (CERN Courier June 2014 p12). In addition, many more results were presented on heavy-flavour production and spectroscopy at the B factories, at Fermilab’s Tevatron and at the ALICE, ATLAS, CMS and LHCb experiments.

On the theory frontier, there was an excellent review of the spectroscopy of B hadrons and bottomonium. Impressive progress reported in the calculation of non-perturbative parameters with lattice QCD has already had an important impact on various analyses. Other topics included the status of lepton-flavour violation and models of physics beyond the Standard Model, searches for exotic new physics such as Majorana neutrinos, charm physics and rare kaon decays.

The opening talk of the conference was given by John Ellis of King’s College London and CERN, who presented his perspective and vision for the search for new physics – in particular supersymmetry – at the LHC and beyond. A whole session was devoted to prospects for the future B-physics programme, addressing the upgrades of LHCb, ATLAS, CMS and Belle II. An exciting summary and outlook talk by Hassan Jawahery of the University of Maryland concluded the conference.

The University of Edinburgh provided an impressive social programme. No visit to Scotland is complete without whisky tasting, and participants were treated to the option of 25 different samples. A walking tour of the historic Edinburgh Castle was complemented by a bus tour and a boat ride under the famous Forth Bridge. The conference dinner, held at the Dynamic Earth museum, included another Scottish speciality – haggis.

In conclusion, the 15th Beauty conference was a great success, with presentations of exciting new results. Now it is time to look forward to the next edition, to be held in the spring of 2016.