A Baryon CP Violation Observed at CERN

[PeterDonis]

TL;DR Summary

CERN announces observation of CP violation in the decay of the beauty baryon.

The LHCb collaboration has announced that they have observed CP violation in the decay of the beauty baryon. The Nature paper is here:

https://www.nature.com/articles/s41586-025-09119-3

And the arxiv preprint is here:

https://arxiv.org/pdf/2503.16954

According to the paper, CP violation has previously been observed in mesons, but this is the first observation of it in baryons.

 

[Dale]

Was this a prediction of the standard model or something new? If it was already a prediction, does the measurement put new constraints on any of the standard model parameters?

 

[pines-demon]

Was this a prediction of the standard model or something new? If it was already a prediction, does the measurement put new constraints on any of the standard model parameters?

According to the introduction, the usual CP asymmetry by the Cabibbo mechanism is not enough to explain the matter-antimatter asymmetry in the universe. In the paper they detect a different mechanism (still explainable by Cabibbo theory but it has a larger amplitude than usual) with 5.2 standard deviations of confidence.

 

[Dale]

So is that still considered within the SM or is that outside of the SM?

 

[pines-demon]

So is that still considered within the SM or is that outside of the SM?

It says that is hard to tell, because calculating the terms that they need is not easy (it is a challenging chromodynamics calculation).

 

[Dale]

Thanks, I will be interested to see how that settles out.

 

[PeterDonis]

Was this a prediction of the standard model

The SM predicts CP violation in certain quark processes, which includes both mesons and baryons. As far as I can tell from the paper, these new results don't rule out the possibility of other physics being involved beyond the SM, but they don't show any definite signs of such physics either.

 

[SergejMaterov]

why is CP violation in baryons so small compared to mesons? paradox?