T2K: >2 sigma preference for neutrino CP violation

[mfb]

Presentation at ICHEP
Report at university website

The main plot is on slide 18: $\delta_{CP}$ seems to be somewhere around -2, with 0 appearing unlikely. In other words, muon neutrinos oscillate more likely to electron neutrinos than anti-muon neutrinos oscillate to anti-electron neutrinos.

The measurement is not very precise yet, but unlike for most LHC measurements, there is no reason to expect 0 - the CP phase is a free parameter, it can be anything. T2K plans to collect 10 times more data within the next 10 years, so we will get more precise measurements in the not so distant future. Other experiments will contribute as well.

 

[Orodruin]

Well, let us not get over-excited with this. This result does not add a lot to what we already had from T2K and CP conservation is still essentially within the $2\sigma$ region. If there are only standard oscillations, the early T2K results in the neutrino channel would be a statistical fluctuation regardless of the parameters, just one that would be more likely in the case of maximal CP-violation.

This being said, T2K will continue to take data for some time still and NOvA as well. However, to me it appears unrealistic for the current generation of experiments to exclude CP-conservation at high confidence ($5\sigma +$). However, the next generation of experiments will stand a good chance of doing so and probing the mass ordering.

 

[mfb]

Well, there is no prediction of δ_CP = 0, it can have any value. That makes the 2 sigma more relevant than "we saw a 2 sigma deviation from the standard model prediction".

The expected significance was a bit below 2 sigma, a naive scaling with the protons on target would lead to an expected significance close to 5 sigma in 2027 from T2K alone.

 

[Lord Crc]

So is the point here that it's a free parameter in the standard model, but not in certain BSM approaches?

 

[mfb]

Neutrino mixing is beyond the standard model, and it is a free parameter in all models I am aware of.