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.

Highlights of my Day One at ICHEP

With great excitements I arrived at Palais des Congres this morning for Day One at ICHEP. Most of the talks today were outside of my area of research. This is a good thing in a sense as it means that I may get to learn new things from the talks. And yes, it was indeed the case. On the experimental side, I have enjoyed the well attended Tevatron Higgs session this afternoon on updated limits in both the high and low mass regions from CDF and D0. On the theory side, the highlight was the talk on Emergent Gravity by Erik Verlinde based on the paper (arXiv:1001.0785) he wrote earlier this year claiming that

"Gravity is not a fundamental force"

but it emerges as an entropic force caused by changed in the information associated with the positions of material bodies. Given that this claim changes fundamentally how we think about gravity, there were many very critical questions from the audience as you can imagine, such as the scale at which the entropic interpretation breaks down, the role of graviton in this scenario, the equivalence of the entropic scenario and other known theories in which gravity is a fundamental force.

As pointed out by my fellow bloggers as well that the most important thing being at conferences probably is to interact with other physicists face to face. Besides the coffee breaks, poster sessions also provide such an opportunity to informally discuss and learn new results and ideas from one another.

So what did I learn during the poster session today??

"Is there LSND anomaly?"

According to Poster # 588 from Harp - CDP, which presents the new results they just released a few weeks ago on the low energy pion backgrounds, the answer is NO. What they found is that there is a significant asymmetry in producing pi+ and pi-. The effects of this was not taken into account by LSND, according to the poster. More specifically, LSND underestimated the backgrounds by a factor of 2 and it underestimated the systematic errors by at least by a factor of 2, according to Poster #588. If the analysis stands, the still unanswered question then is, could the excess seen by MiniBoone in both the neutrino and antineutrino modes similarly due to the low energy pion backgrounds?

Tomorrow will be an exciting day as well. It will start out with presentations on first physics results from the LHC. Very much looking forward to it!

Hello World from Athens, Greece!

Greetings from Athens where I have been for the Neutrino 2010 Conference! As this is my first post, let me just briefly introduce myself. I am a particle theorist working on beyond the standard model physics. Particularly I have been interested in the origin of fermion masses and mixing (including the neutrinos, of course!) and have been trying to gain some insights into this problem by constructing models utilizing a variety kinds of new physics (grand unification, extra dimensions, Z'...).

Getting back to the Neutrino 2010 Conference. Some of the highlights of the conference were the new results from MINOS and MiniBoone. The MINOS Collaboration updated their results on the neutrino mode with more than double the data from their previous update in 2008. Updated results on the antineutrino mode were also presented. What is puzzling is the fact that the best fit value for the mass square difference extracted from the antineutrino mode is 3.36 x 10(-3) eV^2, which is far higher than the value determined using the neutrino mode data, 2.35 x 10^(-3) eV^2. One should note that the result for the neutrino mode was based on 7 x 10^(20) POT while the antineutrino mode result was based on significantly low statistics of 1.71 x 10^(20) POT. If the results turn out to be correct, it could mean CPT violation or the existence of new neutrino interaction. Clearly these data are still very preliminary and we should wait and see how this discrepancy disolve when more data becomes available.

MiniBoone also presented the results for both the neutrino and antineutrino modes. For both modes, an excess in the electron-like events was found in the low energy region. For the neutrino mode, there is an unexplained 3 sigma electron-like excess in the region of energy > 475 MeV, while in the region below 475 MeV the fit does not agree with LSND. For the anti-neutrino mode, there is a small excess above 475 MeV and the fit below 475 MeV agree with LSND result. It would be interesting to see if the low energy excess is still present when more statistics have been accumulated and when we have a better understanding of the background in the low energy region.

For the Neutrino Conference series, it has been the case that there are plenary sessions only. One thing that has been started since Neutrino 2008 in Christ Church, New Zealand, is the poster session. This is not uncommon in physics conferences, but what is special is the 3 min talks that enable the poster presenters to advertise their posters. Given that there are more than one hundred of them, the execution of the "3 min talk sessions" was quite an operation! Fortunately, the sessions went very smoothly, and since the talks were only for 3 min duration each, the speakers really had to get to the point right away making it an efficient way to learn numerous new results.

In addition to the exciting physics, life in Athens has been quite an experience. As you all know, there have been several protests and riots in Athens due to the economic crisis. Today is the third day since the Metro has gone on strike. This has posted a big problem for conference participants as most of us are staying quite far from the Megaron conference center and the Metro is the best way to get there from the hotels especially given that it is extrememly hard to get a taxis in central Athens. Hopefully the strike will end soon, or at the very least that it will not be extended to the airport in the next few days when everyone will be flying out of Athens.

This is it for this post. In the next one, I will report on the theoretical developments and additional experimental updates that I have learned at the conference as well as my overall impressions. So stay tuned.