CP Violation Has Been Seen In The Decays Of Heavy Charged Mesons

* The earliest evidence of CP violation was in light, neutral mesons.  But, in 2012, the LHCb experiment provided experimental evidence of CP violation in the decays of charged B mesons, where the effects are less obvious without precision experimental measurements.

This confirms the Standard Model, while ruling out theories predicated on the limited circumstances in which CP violation had actually been observed, despite its presents in every W boson interaction as a result of the CP violating phase of the CKM matrix.

Results from 2012 are old news, but it seemed new to me when I discovered this result this spring (although I now known that I've mentioned this in at least one previous post), and I feel compelled to call attention to this discovery, because I've called attention to its non-discovery to date in earlier posts (some of which were accurate at the time or made the appropriate qualifications, but some of which may have been sloppy or inaccurate).

* In other useful background basic physics, Tommaso Dorigo, explains parton distributions functions.

This sheds interesting insight on the nature of the proton (which is more complex than a naive three quarks linked by gluons model would lead you to believe), and also illustrates have particle physicists of circumvented an inability to do some key QCD calculations from first principles by fitting curves (parton distribution functions) to a very rich data set.

The reality is that it is not currently possible to predict the outcome of LHC class proton-proton collisions doing calculations from first principles using only the fundamental constants of the Standard Model, although in principle, it should be possible to do so with enough computing power and the right techniques. Where the data run out, at about 3 TeV, the uncertainty in calculating cross-sections of interactions (i.e. likelihoods of particular decays) rises quickly to above 10%, which greatly impairs the ability of scientists at the LHC to distinguish background phenomena from signals of new physics.

Dorigo's concluding remarks on this point bear mentioning:

What this means is that we will have a hard time discovering new particles at that high-energy end, given our insufficient knowledge of the proton PDF: if we see an excess in the data we might be tempted to call it a new resonance, but we will always have to reckon with the fact that we cannot be sure of the real behaviour of the PDF at the very high end.

Something similar happened to CDF in 1996, when an excess at high energy of the cross section of inclusive jet production got many excited, as it could be the first signal of small constituents within quarks. Later it was discovered how the PDF model used was insufficiently precise, and the data returned in agreement with the new predictions.

But let's not be discouraged - what everybody hopes in fact is that we'll hit some pretty unmistakable new physics signal in Run 2, one which no PDF uncertainty could explain away. I remain sceptical, but I would be among the first to celebrate...

Of course, an inability to detect subtle new physics at high energies also implies in inability to rule out subtle new physics at high energies.