Thursday, March 31, 2016

Progress Made In Determining CP Violation Angle Of Standard Model

You have to run to establish the pass.

In particle physics, that means that you have to have solid measurements of the fundamental constants of the Standard Model to support all other more glamorous predictions of the Standard Model and to constrain any possible beyond the Standard Model physics (the Hail Mary pass of the physics world).

One of the least precisely know physical constants, which is measured mostly in the decays of B mesons through intermediate D meson states, is one of the three angles of the "unitary triangle" implied by the CKM matrix that governs quark flavor changes in weak force interactions called gamma.  It is known only to about +/- 10% accuracy and plays a large role in CP violation (i.e. discrepancies in the rate at which interactions happen going "forward" and "backward" in time). Gamma is also insensitive to beyond the Standard Model physics and top quark physics.

While the unitary triangle partial parameterization of the CKM matrix doesn't do so, it is possible to explain all CP violation in the Standard Model with just one of the four CKM matrix parameters chosen appropriately, unifying myriad observations of CP violation in many possible kinds of hadron decays that are not obviously related to each other until you understand the Standard Model.

A new paper summarizes the data from 20 million B meson decays in the roughly 600,000 cases involving intermediate D meson states, allowing for precision measurements of rare four body decays and minimizing statistical error.  This is the most accurate measurement ever of twenty-one observables related to these decays.

CP violation is established at the five sigma discovery level in some decays where it was expected but not previously demonstrated experimentally.  Weaker, but still strong, evidence of CP violation is seen where expected in some other decays with very small branching fractions and/or in which CP violation is suppressed at the "tree level" for some (predicted and well understood) reason, reducing the statistical power associated even with the observation of 20 million decays.

Most of these results are either a good fit to the existing world average measurements of gamma and related derived constants, or are in only very modest tension with it.  But, there is only one out of 19 results are not consistent with the world average at the two-sigma level (the chart in the paper shows one sigma intervals). This is about par for the course for two-sigma confidence intervals, which on average, produce one in twenty results outside the confidence intervals. 

Perhaps unsurprisingly, the greatest tension appears to be in the observed CP-violation in four body decays that produce three pions and one kaon, which constitute about 678 of the 600,000 or so relevant decays (the smallest number of events of any of the possibilities measured), which could simply be due to underestimates of the error bars that use a Gaussian distribution when some other distribution with somewhat fatter or asymmetric tails (perhaps a Poisson distribution) would be a more accurate description of the true expected probability distribution.  Or, it could just be that small samples are more prone to be quirky, all other things being equal, than bigger ones.

The new paper does not derive a new world averages for gamma or any more fundamental Standard Model constant from its new data in this paper, as opposed to comparing its results to the old world average, although all the data required to do so is available in the paper. But, within a year or two, the precision with which we know the value of gamma should be improved somewhat and the world average bets fit value should change modestly. It would probably take me a couple of weeks of research and calculations to work this out as I'm rusty when it comes to the relationships between the various observables and the fundamental constants that they are derived from in principle.

If the Standard Model is correct, the sum of the three angles of the "unitary triangle" calculated using the true value of the relevant fundamental constants in the CKM matrix should sum to 180 degrees. Prior to this study, the data showed a sum of the three angles (alpha, beta and gamma) was 175 +/- 9 degrees, which is consistent at the one-half sigma level, with the 7 degree uncertainty in gamma dominating the uncertainty in the total.  A global fit of the three angles suggests that the true value of gamma should be at the high end of its +/- 7 degree range of experimentally measured values.  

I look forward to seeing the revised estimate based on this data soon.  If the best fit value and margin of error for gamma both go down, this hints that the Standard Model CKM matrix may be insufficient to explain the observed phenomena and that beyond the Standard Model theories may be needed.  If the best fit value of gamma rises and the margin of error decreases, but not so much that the sum is inconsistent with 180 degrees, then it is likely that the four parameters of the Standard Model CKM matrix completely describe quark flavor transitions via the weak force, and that any beyond the Standard Model particles don't interact with Standard Model particles via the weak force.

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