The Double Chooz Experiment
Masahiro Kuze (right) discussed the status of Double Chooz to open today’s session at Neutel. He started by quoting the present knowledge of the neutrino mixing matrix parameters. Speaking of which, I would like to incidentally mention that a good mnemonic trick to remember the angles of the PMNS matrix is the following:
theta_12 = 34 degrees (simple numerical succession)
theta_23 = 45 degrees (simple numerical succession)
theta_13 = 9 degrees (quadratic series)
I hope this is not taken as a numerology argument – it’s just a good trick to remember these values, which may have not stuck in physicists’ minds yet (most determinations are quite recent!).
Anyway, back to the talk. The speaker went on to explain the baseline needed for oscillation maxima for the three modes, which can be derived from the values of delta_m^2 between pairs of states. For reactor neutrino experiments, one looks at disappearance of neutrinos from the flux, as the energy is too low for appearance. Double Chooz, located at the border between Belgium and France, benefits from two reactors of 4.27 GWatts, and has a near detector at 400 meters and a far detector at 1050 meters from the source.
The target of Double Chooz is a Gd-loaded liquid scintillator, 10 cubic meters, with layers of photon catchers and an inner veto of 90 cubic meters of scintillator. Charged current reactions of antineutrinos produces a prompt signal from positrons, whose energy can be measured, and a delayed neutron-capture energy release, yielding 8 MeV of photons after 30 microseconds. The delayed coincidence drastically reduces backgrounds.
Double Chooz started its scientific results output using the far detector, by reporting the first indication of theta_13 value. From 2015 they started taking both near and far detector data, with first results shown last year in Moriond, with theta_13=0.111+-0.018.
Since in DC the ratio of the flux from the two reactors is the same in the two detectors, this reduces systematics due to the operation history of the reactors. During a short period of 7 days with both reactors closed for maintenance, they could strongly constrain backgrounds.
Recently DC started to use the gamma-catcher volume (30 tons) where the capture of the neutron can also occur in hydrogen. This yields a much larger statistics for neutrino events. They use an artificial neural network to reject backgrounds, based on three features of the observed events; this is maybe overkill, but the speaker showed nice rejection plots and there is nothing wrong in principle in using a cannonball to kill a fly!
The results on theta_13 from last september seminar indicate a small tension with the most precise determination of that quantity (by Daya Bay). In the future they aim at reducing detection systematics and bringing down their uncertainty bars. The speaker mentioned a workshop of experts from Daya Bay, Double Chooz, and RENO that took place last October in Seoul. A new event will happen soon in Paris. This is bringing better understanding in the techniques used by each experiment, from which all collaborations are gaining knowledge.