New T2K Measurements Of Neutrino Oscillation Parameters

The T2K experiment has used to brute force of running lots of collisions and new clever analysis techniques to come up with its latest measurement of neutrino oscillation parameters. 

Jester retweets physicist Peter Denton who notes in commenting on this paper that:

Their weak evidence for CP violation has decreased somewhat. Was just under 3sigma, now just under 2sigma (depending on the choice of prior).

The bottom line is as follows:

Frequentist and Bayesian analyses are presented, including results on sin2θ13 and the impact of priors on the δCP measurement. Both analyses prefer the normal mass ordering and upper octant of sin2θ23 with a nearly maximally CP-violating phase. 

Assuming the normal ordering and using the constraint on sin2θ13 from reactors, sin2θ23=0.561+0.021−0.032 using Feldman--Cousins corrected intervals, and Δm232=2.494+0.041−0.058×10−3 eV2 using constant Δχ2 intervals. The CP-violating phase is constrained to δCP=−1.97+0.97−0.70 using Feldman--Cousins corrected intervals, and δCP=0,π is excluded at more than 90% confidence level. A Jarlskog invariant of zero is excluded at more than 2σ credible level using a flat prior in δCP, and just below 2σ using a flat prior in sinδCP. 

When the external constraint on sin2θ13 is removed, sin2θ13=28.0+2.8−6.5×10−3, in agreement with measurements from reactor experiments. These results are consistent with previous T2K analyses.

From the body text:

The analysis with (without) the reactor constraint sees a Bayes factor (BF) of 3.35 (1.43) for the upper over the lower θ23 octant; 4.21 (1.83) for the normal over inverted mass ordering; and a combined factor of 1.58 (0.63) for upper θ23 octant and normal ordering. 

When calculating the BFs, the alternate hypothesis is any other combination of octant and mass ordering. Interpreting the largest BFs with the Jeffreys’ scale, there is substantial evidence for the normal ordering when marginalising over the octant, and substantial evidence for the upper octant when marginalising over the mass ordering. In the more recent interpretation of BFs by Kass and Raftery, these both correspond to positive evidence. Importantly, the Jeffreys and Kass–Raftery definitions of “evidence” do not equate to the criteria often used in particle physics. For instance, a probability of 95.4% (“2σ”) is equivalent to a BF of 20.7, which is deemed as “decisive” on the Jeffreys’ scale, and as “strong” on the Kass–Raftery scale.