Kamis, 08 Maret 2012

Daya Bay observes theta13 at 5 sigma!

Too many news these days, so just a brief note on something that deserves a long article. The
Daya Bay experiment just announced the measurement of one of the last unknown fundamental parameters in the Standard Model (understood as the old Standard Model extended by the neutrino mass operators). The parameter is called the theta13 mixing angle and, roughly speaking, controls the oscillation probability of electron neutrinos. One way it could manifest itself is via appearance of electron neutrinos in a beam of muon neutrinos sent over several hundred kilometers. Another possible manifestation is via oscillation of electron neutrinos into the other flavors over a distance of a few hundred meters. More precisely, the survival probability of an electron neutrino with the energy E at the distance L from the source is given bywhere Δm31 is approximately equal to the "atmospheric mass difference" known to be of order 0.05 eV.

There is no theoretical reason for theta13 to be zero, however it is known to be a bit smaller than the other two neutrino mixing angles (who are known quite precisely). Several experiments have been racing to measure it: T2K in Japan, Minos in the US, Double Chooz in France, RENO in South Korea, and Daya Bay in China. Recently, there has been a few experimental hints that the value is about 10 degrees, although none of the experiments could by itself present a 3 sigma evidence.

Now it seems the first prize has been snatched by the Chinese. Daya Bay looks for disappearance of electron antineutrinos produced in nuclear reactors (if an electron neutrino transforms into other flavors it cannot be detected by this experiment, so effectively it "disappears"). Comparing the observed flux in near (~500 m) detectors and a far (~1500m) detector they conclude that about 6% of the electron neutrinos disappear in between. Based on that they quote the value of the mixing angle
or theta13 ≈ 9 degrees in more familiar units. This result suggests that neutrinos are anarchists. Unlike the quark mixing angles that display a highly hierarchical structure, the neutrino mixing angles are of similar magnitude and apparently random. The deeper reason for either of these 2 facts is currently a mystery...

So the last thing we don't know about the Standard Model is the absolute scale of the neutrino masses, and the CP violating phase in the neutrino mixing matrix. We'll probably learn those too before the end of the century.

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