A Stunning New Neutral D Meson Oscillation Anomaly

Shorter Summary 

For the first time ever, an experiment at the Large Hadron Collider (LHC) has seen a highly statistically significant (albeit slight) difference in mass between a particle and its antiparticle, contrary to the Standard Model. This has suddenly risen to the most statistically significant anomaly in all of high energy physics.

Analysis

It isn't clear why this is the case, or whether there is any good reason to suspect underestimated systemic error (other than the fact that it contradicts the Standard Model and wasn't strongly predicted by any of the front runner beyond the Standard Model physics theories currently in circulation as viable proposals in light of other HEP experimental data).

If this results is independently replicated by another experiment (since the LHCb operates at lower energies than the ATLAS and CMS experiments, there are other colliders in the world that can do so), this will be a very big deal, probably implying "new physics" of some undetermined nature. 

But, despite the high statistical significance of the error, because the discrepancy is so small in both absolute and relative terms, it is easy to imagine that an overlooked source of systemic error in the measurement that could resolve the anomaly on highly technical grounds (although I have no specific ones in mind).

Is This A Discovery Yet?

While "5 sigma" statistical significance is the standard for making a new discovery in high energy physics, this isn't the only requirement. 

The preprint results still need to be peer reviewed, the 5 sigma observation needs to be independently replicated, and the proponents of new physics based upon the observation need to propose some theory to explain that result that is consistent with the rest of the laws of physics supported by empirical observations in contexts where anomalies aren't seen. 

This long road is a bit like the Roman Catholic church's arduous process for declaring someone to be a "saint." But this is enough to get the ball rollings towards a widely recognized beyond the Standard Model physics result that all credible high energy physicists would have to accept and reckon with somehow.

The New Results In Context

A Dº meson has two valence quarks, a charm quark and an anti-up quark, bound by gluons. The Dº mass is about 1865 MeV. Its antiparticle, anti-Dº meson also has two valence quarks, an anti-charm quark and an up quark. Since it is neutral, the electromagnetic charge of the particle and the antiparticle is the same. Both the particle and its antiparticle are pseudoscalar (i.e. spin-0 odd parity) mesons.

The particle and its antiparticle can't oscillate in "tree level" (i.e. single step Feynman diagram) processes. But, they can oscillate via a number of two step processes.

For example, the charm quark can decay to a strange quark which can decay to an up quark, while the anti-up quark can become an anti-strange quark which can become an anti-charm quark, in each case, via two rounds of simultaneous W boson transitions, one W- and one W+ each, and those four weak force bosons can be virtual ones that cancel each other out.

Because it can happen, the Standard Model predicts that it will happen, so Dº-anti-Dº meson oscillation while nice to observe to confirm that hypothesis, is no big deal.

The probability of going from a Dº to an anti-Dº meson and the probability of going from an anti-Dº meson to a Dº meson are not identical due to the CP violating phase in the CKM matrix. So, the observed oscillating Dº-anti-Dº pair, isn't exactly a 50-50 mix of each of them, although the difference between what was observed and a 50-50 mix wasn't statistically significant (as predicted in the Standard Model given the precision of the measurements done).

But, as a PDG review paper explains, in the Standard Model, the mass and decay width of the Dº meson and the mass and decay width of the anti-Dº meson should be identical.

A preprint of a June 7, 2021 letter from the LHCb experiment, however, concludes that there is a small, but still 7.3 sigma mass difference (without considering look elsewhere effects) between a Dº meson and an anti-Dº meson, the first time ever that a particle and its antiparticle of any kind have been observed to have statistically significantly different masses, and a statistically significant 3.1 sigma difference in decay widths.

Normalized by the decay width, the mass difference is 0.397% of the decay width, and the decay width difference is 0.459% of the average decay width.  The average decay width is less than 2.1 MeV. So, the observed discrepancy between the masses of the Dº and anti-Dº meson is on the order of 0.008 MeV or less, and the observed discrepancy between the decay widths of the Dº and anti-Dº meson is on the order of 0.009 MeV or less. 

In both absolute terms (less than 0.2% of the electron mass), and relative to the mass of the Dº meson (on the order of two or three parts per million), these differences are very small. 

But, as noted above, the differences are reported to be statistically significant despite the fact that in the Standard Model they shouldn't occur at all. 

Look elsewhere effects probably reduce the differences in decay widths to a non-statistically significant level of 2 sigma or less because over the years probably something like a thousand or more HEP matter-antimatter mass comparisons have been done, all producing null results. But, even with look elsewhere effects, the locally 7.3 sigma mass difference should be at least 4-5 sigma, and it is also worth noting that the mean percentage mass difference in reported in this preprint does replicate the prior world average for this measurement (which previously was "marginally compatible with" no mass difference due to a much larger experimental uncertainty in previous measurements).

The Letter contains no meaningful or insightful commentary regarding what could be leading to this bombshell conclusion.