Back in November, I described a surprising result from the LHCb experiment at the Large Hadron Collider [LHC] concerning “CP violation” in the decays of particles called “D mesons” (which are hadrons that contain an unpaired charm quark or an unpaired charm antiquark) at a level much larger than expected by theorists. Rather than rehashing the explanation for what that was all about, I’m going to point you to what I wrote in November.
There’s news this week that the CDF experiment at the now-closed Tevatron has updated its result for a measurement of the same quantity, using the full CDF data set. And they now find a very similar result to what LHCb found. This is indicated on the slide shown below, taken from the talk given by Angelo Di Canto at the La Thuile conference (but edited by me to fix a big typo; I hope he does not mind.) You see that while LHCb found a CP-violating asymmetry of -0.82%, with statistical and systematic uncertainties of 0.21% and 0.11%, CDF finds -0.62%, with almost identical uncertainties — a little closer to zero, but still well away from it.
This lends support to LHCb’s result, and putting the two results (and a couple of other weaker ones) together makes their combination discrepant from zero by about 3.8 standard deviations. That’s great, but not as great as it would have been if what theorists thought a few years ago was still considered reliable. Back then, the relevant experts (and I should emphasize I am not one of them) would have told you that they were pretty darn certain that the Standard Model [the equations we use to describe the known particles and forces] could not produce CP violation of this type, and any observation of a non-zero signal would imply the existence of previously unknown particles. But the experts have been backing away from this point of view recently, worrying that maybe they know less about how to calculate this in the Standard Model than they used to think. If we’re to be sure this is really a sign of new particles in nature, and not just a sign that theorists have trouble predicting this quantity, we’re going to need additional evidence from another quarter. And so far, we haven’t got any.