Double Dare

(There's nothing like a little rant on a holiday morning)

Last Wednesday I noticed this press release from Double Chooz which announced "the observation of the disappearance of (anti-)neutrinos in the expected flux observed from the nuclear reactor" which implies "complementary and important evidence of oscillation also involving the third angle". Wow, I thought, they've nailed down theta13! But it turned out to be much more exciting than just another fundamental parameter. A more careful reading reveals that, based on the first 100 days of data, Double Chooz found sin^2(2 theta13) = 0.085 ± 0.051. Clearly something interesting is going on. To an untrained eye, the Double Chooz result is... consistent with zero; moreover it is similar, even if slightly less precise, to the null result from MINOS: sin^2(2 theta13) = 0.04 ± 0.04. However now in the 21st century one needs a more inspired approach to statistics...

To better understand what's going on, go back a few months. In June this year the T2K experiment also issued a press release about theta13, announcing "an indication that muon neutrinos are able to transform into electron neutrino". T2K is an experiment in Japan where a beam of muon neutrinos with GeV energies is produced in J-PARC and sent over a 300km tunnel ;-) to the SuperKamiokande detector. It is established that muon neutrinos can oscillate into tau neutrinos, the process being governed by the theta23 angle in the MNS neutrino mixing matrix whose value is close ot 45 degrees. If the angle theta13 in that same matrix is non-zero then the process ν_μ → ν_e is also allowed. For this reason, T2K was searching for an appearance of electron neutrino in the muon neutrino beam. The T2K announcement was based on the detection of 6 electron neutrino events, versus about 2 expected from background. At the time some of us wondered why they put such a spin on a merely 2.5 sigma excess, given that neutrino experiments had already produced many confusing results with similar or larger significance (LSND, MiniBoone, later OPERA). After all, neutrino beam experiments are plagued by difficult systematic uncertainties which are due to our incomplete understanding of the dirty hadronic physics involved in the beam production. Indeed, the subsequent results from MINOS turned out to disfavor the T2K central value of sin^2(2 theta13) of about 0.11.

In hindsight, the T2K press release was a pioneering step in data interpretation and the gauntlet was recently picked up Double Chooz. The latter experiment is targeting the transformation of anti-electron neutrinos into other type which, at short distances, is also controlled by the theta13 angle. More precisely, Double Chooz is looking for disappearance of MeV anti-electron neutrinos at a distance of 1 km away from the French nuclear reactor Chooz B where the antineutrinos are produced. They observe a small deficit of events in the energy range 2-5 MeV compared to the no-oscillation hypothesis, see the picture. While T2K was spinning a less-than-3-sigma excess, the Double Chooz press release made a further bold step and presented a less-than-2-sigma one as an evidence. There is still a long way to adapt the standards used in psychology and behavioral sciences. But, little by little, this approach could be applied to wider areas of physics, especially to high energy physics which suffers from dearth of discoveries. Just think of it: if we could call a 2 sigma excess an indication then every week the LHC could deliver an indication of new physics!

But, seriously... I also expect that the value of theta13 is non-zero and the experiments may be seeing the first hint of it. One argument is that global fits to the neutrino oscillation data point to sin^2(2 theta13) = 0.05 and 3 sigma away from zero. Besides, there is no compelling theoretical reason why theta13 should be zero (and if you believe in anarchy there is a reason to the contrary). The smoke should clear up in the next few years thanks to Double Chooz, Daya Bay, NOvA, and others. However the current experimental situation is far from being conclusive and the latest Double Chooz results did not change much in this respect, as can be seen in the fit to the right. I guess this could have been said without diminishing the importance of Double Chooz, and without treating the public as retarded...

See the web page of Double Chooz and this post on Quantum Diaries for more details.