Charm CP-violation [2019]

This piece of news appeared in The Ph-Word when it was a newsletter and refers to events from March’19; the actual issue can still be read here.

It was time for Les Rencontres de Moriond again, the particle physics conference-cum-excuse-for-skiing. (Seriously, that’s where inspiration is supposed to hit).
The main takeaway from the conference was, once more, that the Standard Model of elementary particles keeps getting confirmed. But also like last year, the only new news came from the experiment which is turning out to be the sliest one in the Large Hadron Collider: the LHCb.
The LHCb crowd was done analyzing the data from all these years’ proton smashing and announced that they observed for the first time CP-violating decays involving the charm quark. Now, hold on. It’s bullet time:

– Take an object and make a copy of it, only with everything reversed like in a mirror; you’d expect it to work and play in the same way as the original. This sounds so intuitive (I guess) that even elementary particles comply.
– Now take a particle and its anti-particle, that is change its electric charge and a few other such numbers that we’d better not get into right now. It will still indulge us by behaving in the same way as the original except for the opposite charges.
– Given the above, what would one expect from an anti-particle that is also reversed? Why, to behave like the particle that we started with. (Where behaving usually refers to emitting away smaller particles after a while, i.e. its “decays”.)
– All this symmetric behaviour is known as the CP-symmetry – for charge and parity.
– Now, since the swinging ’60s it is known that some decays sometimes break this simple rule… These decays involve strange quarks and bottom quarks (these are their given names, okay?).
– This is as baffling as it might sound. Actually it’s one of the most mysterious facts involving particles. And it’s known as CP-violation.
– The LHCb experiment saw CP-violation now also involving the charm quarks.
– What they actually measured was the difference in decays of composite particles that are made up of charm quarks among others. And the small difference was clearly there.

The violation is not just a riddle but possibly also related to why the universe is made out of matter instead of antimatter. In any case any new piece in the CP-puzzle is always welcome.

Leave a Reply

Your email address will not be published. Required fields are marked *