Warwick hosts a feast of flavour

In September 2010, the University of Warwick played host to CKM2010, the 6th International Workshop on the CKM Unitarity Triangle. The CKM workshops, named after the Cabibbo-Kobayashi-Maskawa matrix that describes quark mixing in the Standard Model, date from 2002 when the first meeting took place at CERN. The workshop has since established itself as one of the most important meetings in the field.

With a two-year gap since the previous meeting, there was much at CKM2010 for theorists and experimentalists alike to discuss. This was the first time since the inauguration of the series that the workshop occurred with neither of the B-factory experiments – BaBar and Belle – being operational. A generation of experiments in charm and kaon physics have also completed data-taking. While much is being done to archive the knowledge that has been accumulated from this era, the organizers of CKM2010 chose instead to look to the future.

Uncharted territory

Only by looking forwards is it possible to address the many open questions in flavour physics, which Paride Paradisi of the Technische Universität München presented in the first of the opening plenary sessions. The biggest issue, perhaps, concerns the fact that there is still no real understanding of the underlying reason for the flavour structure of the Standard Model. More pressing, however, is the so-called “new-physics flavour puzzle”: how is the need for physics beyond the Standard Model at the tera-electron-volt scale – to resolve the hierarchy problem – to be reconciled with the absence of such new physics in precision flavour measurements? The most popular solution is the “minimal flavour violation” hypothesis, which can be tested by observables that are either highly suppressed or precisely predicted in the Standard Model.

Two sectors where the experimental measurements do not yet reach the desired sensitivity are those of the D⁰ and B_s mesons. Guy Wilkinson of the University of Oxford described the progress made at Fermilab’s Tevatron over the past few years, emphasizing the potential of the LHC experiments at CERN – particularly LHCb – to explore uncharted territory. It will be interesting to see if the datasets with larger statistics confirm the hints of contributions from new physics to B_s mixing that have been seen by the CDF and DØ experiments at the Tevatron. The large yields of D, J/ψ, B and U mesons already observed by the LHC experiments augur well for exciting results in the near future.

However, the LHC will not be the only player in flavour physics in the next decade. Yangheng Zheng of the Graduate University of Chinese Academy of Sciences and Marco Sozzi of the Università di Pisa and INFN described the new facilities and experiments that are coming online in the charm and kaon sectors, respectively. The BEPCII collider in Beijing has achieved an instantaneous luminosity above 3×10³² cm^–2 s^–1, and the BES III collaboration has already published the first results from its world’s largest datasets of electron–positron collisions in the charmonium resonance region. The kaon experiments NA62 at CERN and K0T0 at J-PARC are well on the way towards studies of the ultra-rare decays K⁺ → π⁺νν and K_L → π⁰νν.

Meanwhile, there are plans for a new generation of B factories, which Peter Križan of the University of Ljubljana and J Stefan Institute described. The clean environment of electron–positron colliders provides a unique capability for various measurements, such as B⁺→τν_τ. The upgrade of the KEKB facility and the Belle detector to allow operation with a peak luminosity of 8×10³⁵ cm^–2 s^–1 (40 times higher than achieved to date) has been approved and construction is now ongoing, with commissioning due to start in 2014. The design shares many common features – most notably the “crab-waist” collision scheme – with the SuperB project, recently approved by the Italian government (Italian government approves SuperB).

Maximizing the impact of these new experiments will require progress in lattice QCD calculations. Junko Shigemitsu of Ohio State University described recent developments in this field, showing that accuracy below a per cent has been reached for several parameters in the kaon sector, with calculations using different lattice actions giving consistent results. In the charm sector, determinations of constants are approaching the per cent level of precision; this advance, when combined with new measurements, appears to have resolved the apparent discrepancy in the value of the D_s decay constant. Further work is needed to reach the desired level of precision in B physics but excellent progress is being made by several groups around the world.

The main body of the workshop consisted of parallel meetings of six working groups, which provided opportunities for detailed discussions between experts. The summaries from these working groups were presented in two plenary sessions on the final day.

Working group I, convened by Federico Mescia of the Universitat de Barcelona, Albert Young of the University of North Carolina and Tommaso Spadaro of INFN Frascati, focused on the precise determination of |V_ud| and |V_us|. A measurement of the muon lifetime at a precision of one part per million by the MuLAN collaboration determines the reference value of the Fermi coupling. Improved measurements of |V_ud| and |V_us|, mainly from nuclear β-decay and (semi-)leptonic kaon decay, respectively, set constraints on the unitarity of the first row of the CKM matrix at better than 1 permille. Interesting discrepancies in the measurements of the neutron lifetime and of |V_us| demand further studies.

Hint of new physics?

Working group II, convened by Jack Laiho of the University of Glasgow, Ben Pecjak of the University of Mainz and Christoph Schwanda of the Institute of High Energy Physics in Vienna, had as its subject the determination of |V_ub|, |V_cb|, |V_cs| and |V_cd|. This is an area where dialogue between theorists and experimentalists has been extremely fruitful in driving down the uncertainties. Lively discussions continue, stimulated in part by the apparent discrepancies between inclusive and exclusive determinations of both |V_cb| and |V_ub|. The latest data on the leptonic decay B⁺→τν_τ, which is sensitive to contributions from charged Higgs bosons, show an interesting discrepancy that may prove to be a first hint of new physics.

Working group III, convened by Martin Gorbahn of the Technische Universität München, Mitesh Patel of Imperial College London and Steven Robertson of the Canadian Institute for Particle Physics, at McGill University and SLAC, tackled rare B, D and K decays. One particularly interesting decay is B→K^*l⁺l^–, where first measurements of the forward-backward asymmetry by BaBar, Belle and CDF hint at non-standard contributions. This is exciting for LHCb, where additional kinematic variables will be studied. Inclusive rare decays, such as b→sγ, and those with missing energy in the final state are better studied in electron–positron collisions and help to motivate the next generation of B factories. Among other golden modes, improved results on B_s→μ⁺μ^– and K→πνν remain eagerly anticipated by theorists, who continue to refine the expectations for these decays in various models.

The fourth working group, convened by Alexander Lenz of the Technische Universität Dortmund and Universität Regensburg, Olivier Leroy of the Centre de Physique des Particules de Marseille and Michal Kreps of the University of Warwick, was concerned with the determination of the magnitudes and relative phases of V_td, V_ts and V_tb. While the Tevatron experiments have started to set constraints on these quantities from direct top production, with further improvement anticipated at the LHC, the strongest tests at present come from studies of the oscillations of charm and beauty mesons. Hints for new physics contributions in the B_s sector provided the main talking point, but the potential for and the importance of improved searches for CP violation in charm oscillations was also noted.

Measurements of the angles of the unitarity triangle were the subject of the remaining two working groups. Working group V, convened by Robert Fleischer of NIKHEF and Stefania Ricciardi of the Rutherford Appleton Laboratory, focused on determinations of the angle γ using B→DK decays, while working group VI, convened by Matt Graham of SLAC, Diego Tonelli of Fermilab and Jure Zupan of the University of Ljubljana and the J Stefan Institute, covered measurements using charmless B decays. The angle γ plays a special role because it is has negligible theoretical uncertainty. The precision of the measurements is not yet below 10°, leaving room for results from LHCb – combined with measurements from charm decays – to have a big impact on the unitarity triangle fits. The measurements based on charmless decays, which are dominated by loop (“penguin”) amplitudes, tend to have significant theoretical uncertainties that must be tamed to isolate any new physics contribution. The main issue concerns developing methods to understand whether existing anomalous results (such as the pattern of CP asymmetries in B→Kπ decays) are caused by QCD corrections or by something more exotic.

A common feature of all working groups was the strong emphasis on the sensitivity to new physics and the utility of flavour observables to distinguish different extensions of the Standard Model. Less than two years after the award of the Nobel prize to Kobayashi and Maskawa “for the discovery of the origin of the broken symmetry which predicts the existence of at least three families of quarks in nature”, their greatest legacy – and that of Nicola Cabibbo (see box) – will perhaps be a discovery that finally goes beyond the paradigm of the Standard Model.

• CKM2010 was generously supported by the University of Warwick, the Science and Technology Facilities Council, the Institute for Particle Physics Phenomenology and the Institute of Physics.