Final days, brief summary

Tuesday was the days of the quarks -- almost every talk was about QCD or flavour phyics.

Unfortunately, the lattice QCD talk did perhaps not make the best of the opportunity to convince a larger HEP audience of the importance of lattice results, since Yoshinobu Kuramashi chose to pass by the many important contributions that lattice QCD can make and has made towards flavour physics, and concentrated on the derivation of nuclear properties from lattice QCD instead. This is clearly a very important topic and replacing phenomenological models with true first-principles predictions from QCD will have an enormous impact on nuclear physics; the present audience of experimental high-energy particle physicists and Beyond-the-Standard-Model theorists might have been more excited to hear about lattice determinations of the decay constants and semileptonic form factors of heavy mesons, however.

The decays of B and B_s mesons are also the field in which the most exciting discrepancies between experimental results and Standard Model predictions keep appearing. While the D_s discrepancy of last year has disappeared in the meantime, there is now some tension in B meson decays, the most significant of which (at 3.2σ) is the like-sign dimuon charge asymmetry measured at the Tevatron. However, combining all results from D0 and CDF gives a discrepancy of less than 2σ when compared with the Standard Model, while all individual results are mutually compatible within errors. So perhaps this is again a fluctuation, or else something really weird is going on here. Phenomenologists may have a clearer picture of what that could be, see the post by Jester.

Today (i.e. Wednesday) started out with a session on neutrinos. Neutrinos have their own version of flavour physics -- neutrino oscillations. The MNS matrix, which is the leptonic analogue of the CKM matrix, is a lot less well-known than its hadronic cousin, however. In particular, it is not clear whether the mixing angle θ₁₃ describing the mixing between the first and third generations is non-zero, although recent results indicate that it likely is. The flavour structure of the neutrino sector might be much richer than the quark one, though, since the existence of sterile neutrinos (i.e. neutrinos not partnered with a charged lepton via the weak interactions) cannot be ruled out at present.

The neutrinos detected at great effort by huge detectors can come from a variety of sources, some of which got their own talks: Alain Bellerive talked about solar neutrinos (i.e. the ones created by nuclear fusion in the Sun), where the Solar Neutrino Observatory (SNO) is now beginning to achieve precision measurements, which favour a scenario of large neutrino mixing angles. Tsuyoshi Nakaya presented long-baseline accelerator neutrino experiments (where neutrinos beams are created at an accelerator from the leptonic decays of a beam of charged particles), among whom OPERA has recently reported the first candidate for a ν_τ oscillation. Fabrice Piquemol spoke about reactor-based experiments (where the neutrinos come from the reactions in nuclear reactors) and single and double β decays; the tail of the energy distribution of the electrons produced in β decays can impose an upper limit on the mass of the electron neutrino (the limit of 2.3 eV does not seem terribly competitive with other limits, however), and the observation of neutrinoless (or "0ν") double β decay would establish that neutrinos are Majorana particles, indentical to their antiparticles.

Today's second session was about the links between particle physics and cosmology: Dark matter is one of the big "known unkowns" in our present understanding of the universe. What is known is that it cannot be normal (hadronic) matter because of limits imposed by Big Bang nucleosynthesis, and that it cannot be neutrinos, either. The best candidate would be a WIMP (i.e. a Weakly Interacting Massive Particle), such as the lightest stable superpartner of a Standard Model particle if SUSY exists in nature. The discovery of such a particle at the LHC would thus have significant impact also on cosmology.