Le τ, un laboratoire d'idées

Ces dernières années, le τ est devenu un outil de recherche pour la physique au-delà du Modèle standard, et ses perspectives sont prometteuses, comme l'a révélé le 11e atelier international sur la physique du lepton tau. Un atelier Tau 2010 a eu lieu à l'Université de Manchester en septembre. Quelque 80 délégués issus de 20 pays ont participé à cette réunion animée, où de nombreux sujets ont été abordés : violation de saveur du lepton, chromodynamique quantique, physique du neutrino du tau, physique du tau au Tevatron et au LHC, et perspectives de la discipline.

In the 20 years since the first International Tau Workshop there has been remarkable progress, through work by experiments such as CLEO, BaBar, Belle, those at the Large Electron-Positron collider and the Tevatron, as well as several neutrino experiments, not to mention the work by theorists. The early efforts saw the acceptance of the τ as a standard lepton, a heavy copy of the electron and the muon. But the τ is massive enough to decay into hadrons as well as leptons, so its decays provide a rich laboratory for studies of a large range of physics topics. More recently, the τ has come into use as a tool to search for physics beyond the Standard Model, for example through lepton-flavour violation, charge-parity violation or the production of the τ in decays of possible new particles produced at the Tevatron and the LHC.

Tau 2010, the 11th International Workshop on Tau Lepton Physics, took place at the University of Manchester on 13–19 September. Some 80 delegates from 20 countries participated in a lively meeting that covered many topics in τ physics, including lepton-flavour violation, QCD, the muon-anomalous magnetic moment, τ neutrino physics, τ physics at the Tevatron and the LHC, as well as the outlook for the field. The workshop had a programme of more than 60 talks.

New physics signatures

The τ lepton is important as a potential way to observe the violation of lepton flavour (LFV). Apostolos Pilaftsis of Manchester stressed in his introduction to the LFV session that any observation of it would be an unambiguous signature for physics beyond the Standard Model. While the τ cannot be produced in such copious quantities as the muon, a significant advantage comes from its relatively large mass. The session contained a number of theory contributions and updates on searches for LFV from BaBar and Belle. In his summary of work by the Heavy Flavour Averaging Group, Swagato Banerjee of Victoria showed the recent enormous progress that these B-factory experiments have made in this area (figure 1). Huge improvements on the limits for LFV in τ decays to many possible final states have been made in comparison with those from the CLEO experiment, which had previously been the leader in this area. Further progress is to be expected at Belle II and at the proposed SuperB facility. For LFV in muon decays, the MEG collaboration – searching for muon decays to an electron plus a photon – reported an analysis of its first data, with tantalizing evidence for some possible candidates. Plans for the proposed Mu2e, COMET and PRISM/Prime experiments were also outlined at the workshop.

New physics might also be found in the τ sector through unexpected violations of charge-parity (CPV). This well known phenomenon in K and B physics reflects subtle differences in nature between the behaviour of matter and antimatter. Both the BaBar and Belle experiments have new, complementary results from the decay τ → πK0ν. A small amount of CPV is expected in this process from the properties of the K0 system, but neither experiment has found any excess and each has set limits on the strength of any possible CP-violating contributions.

Measurements of the decays of B→ τν were presented from both BaBar and Belle, each reporting an excess above the rate expected in the Standard Model. Such an excess could come from mediation of the decay by a virtual charged Higgs particle. However, the excess is small, and more data are needed to confirm its existence.

The mass of the τ is a fundamental parameter in the Standard Model of particle physics, and therefore important to measure in its own right. Also, a precise value for the mass is needed for testing lepton universality, by relating the lepton electroweak couplings and the muon and τ lifetimes. The most precise measurement to date, from the KEDR experiment at the VEPP-4M electron–positron collider in Novosibirsk, was reported at the workshop. The measurement comes from a threshold scan of the τ-pair cross-section using the technique of resonant depolarization to obtain a precise measurement of the beam energy. The new result for the τ mass is 1776.69 GeV with a precision of 0.013%. Plans are in place to improve this further at the BES III experiment in Beijing.

In his introductory talk for the QCD session, Antonio Pich of Valencia stressed the great value of the hadronic decays of the τ as a laboratory for studying QCD. With naive counting of possible fermionic final states, the τ would decay about 60% of the time via qqν. The hadronic decays make up about 65% of the total – the small difference from 60% arising mainly from QCD effects. It turns out that the non-perturbative contribution to the QCD corrections is small despite the low mass of the τ.

The workshop saw some lively discussion of the various approaches to the calculation of the perturbative terms, with recent developments in contour-improved perturbation theory challenging the approaches based on fixed-order perturbation theory. Despite the theoretical uncertainties, the value of the strong coupling constant, αs, obtained from the τ decay data remains the most precise experimental measurement and provides a low-energy measurement with a small uncertainty that helps to confirm the running of αs expected in QCD.

Also in the quark sector, τ decays to strange final states allow for determination of the Cabibbo-Kobayashi-Maskawa matrix element Vus. The measurement reported from BaBar, based on the ratio of the rate for τ → Kν to that for τ → πν, agrees well with results from other methods, while a more inclusive method based on the use of all strange decay modes gives a lower result. This may be a result of missing decay modes and/or problems with the underlying theory. More progress is expected.

Muons and neutrinos

It has been known for some years that the measured value of the muon's anomalous magnetic moment, g-2, deviates from theory by a few standard deviations. The measurement, made by the E821 experiment at Brookhaven, remains one of the few hints for new physics. While electroweak theory and perturbative QCD allow for precise calculations of the principal contributions to the muon g-2, the non-perturbative QCD part from vacuum polarization effects (when a virtual photon fluctuates to a hadronic system) has to be based on experiment.

Andreas Hoeker of CERN introduced the session on this topic, showing how data on τ decays can be used to help with the calculations of the non-perturbative part via the use of conserved vector current to relate the isovector, spin-1 component of the τ decays to the hadronic systems produced in low-energy electron–positron annihilation. A great deal of recent experimental and theoretical progress was reported and while some discrepancies remain to be resolved, the difference between theory and experiment in the value of the muon's anomalous magnetic moment remains at over 3σ. The workshop heard about plans for improved measurements of g-2 at both Fermilab and the Japan Proton Accelerator Research Complex.

William Marciano from Brookhaven introduced the session on neutrino oscillations, noting that there is great potential for major discoveries and surprises in the present and future neutrino experiments. There were reports from the OPERA experiment, including strong evidence for ντ appearance, searches for atmospheric ντ in SuperKamiokande, the status of the T2K experiment, searches for astrophysical ντ in the IceCube detector and latest results from the MINOS experiment.

The session on τ physics at the Tevatron and the LHC produced a particularly lively discussion. Among the highlights were limits on Higgs production from the DØ experiment at the Tevatron, a reconstructed candidate for the decay W → τν in ATLAS and a signal of some 20 events in CMS for the decay Z → ττ (figure 2). These were seen as encouraging indications of the thorough work done to develop suitable triggers and algorithms for τ selection at the hadron colliders. Clearly a rich harvest of τ-related physics is yet to come from the Tevatron and, in particular, from the LHC. The last session at the workshop pointed to exciting future potential for much new τ physics from Belle II and the proposed SuperB facility.

Michel Davier of the Laboratoire de l'Accélérateur Linéaire, Orsay, gallantly gave up a visit to Chatsworth House and the Derbyshire Peak District, as well as dinner in the Manchester Museum of Science and Industry, to prepare what was an excellent summary talk of the workshop. The series of Tau Workshops, which Davier in fact initiated in 1990, will now continue into its third decade, with Tau 2012 scheduled to take place in Nagoya, late in 2012.