,
23 tweets,
9 min read
Read on Twitter
Exciting news! @LHCbExperiment is announcing the observation of CP violation in charm decays. indico.cern.ch/event/807176/ What does that mean? A somewhat long thread.
The laws of physics that affect us (gravity and electrodynamics) are left-right symmetric. One of the pictures below is mirror reversed. Physics doesn't let you tell which. (Social constructs may help: numbers, letters, watches, is the man left-handed?...) 
Until 1956 that had not been tested for the weak nuclear interaction, responsible for radioactive decays. Lee and Yang proposed an experiment, that Wu (known as "Madame Wu") conducted.
Result: Weak interactions are not left-right symmetric. We say they violate the "P" (for parity) symmetry. Lee and Yang got the @NobelPrize for it. Wu did not. Another mystery of science.
The "C" symmetry stands for charge-reversal. Conceptually you could imagine doing the above experiment with antimatter: positive electrons and negative antiprotons. The result would be similar: We expect P to be violated in antimatter.
But at the time it was assumed that physics would be symmetric under P and C applied simultaneously: Antimatter seen in a mirror looks like matter.
That was until Cronin and Fitch found in 1964 that CP violation did not hold in decays of kaons. Kaons are particles containing an anti-s quark and a d quark. Another Nobel Prize.
They found decays of the K-long particle two two pi mesons. Neutrally charged kaons come in two species: the short-lived K-short that decays to two pi mesons, and the longer-lived K-long that decays to three. That's what CP symmetry predicts. The opposite is violating CP symmetry
That is a good thing. (Peace Nobel Prize, but also a physicist) Sakharov explains that CP violation is needed to create a Universe made of matter, like ours. Without CP violation we would not be there.
In 1973 Kobayashi and Maskawa (Nobel, again) explained how CP violation was realised in Nature. But they needed 6 quarks to exist for the model to work. Quite a hypothesis, as at the time only three quarks were known.
One year later, quark number 4 was found : the charm quark. And in 1979 the beauty quark. (Nobel and Nobel). (Quark number 6, top, discovered in 1995, is not really needed in this story).
According to the Standard Model of particle physics CP violation only occurs in interactions of quarks and the W boson at the bottom right of the picture (and probably in neutrinos too). (@PHDcomics and @DanielWhiteson ).
So with all ingredients in place, one could check if the mechanism of Kobayashi and Maskawa is really explaining CP violation. Two experiments were built in the USA and Japan to check. I was working on the latter, Belle. It's the predecessor of @belle2collab .
In 2001, Belle and its US counterpart BaBar announced the observation of CP violation in neutral beauty mesons. In the plot below the black and white points do not match. If CP was conserved they would.
Later @LHCbExperiment reported the observation of CP violation in Bs mesons, containing a beauty and a strange quark: the dark bump on the left (Bs mesons) is bigger than that on the right (anti-Bs mesons).
Why is that important? Because we need CP violation for the universe to exist, but we need more than the amount we observe. Much more. We thus need to look for other sources of CP violation, not explained by Kobayashi and Maskawa
And CP violation in decays of charm? So far it was never seen. And for a good reason: Kobayashi and Maskawa predict it to be a tiny effect. If we see something large, it must be New Physics. Maybe the New Physics needed by Sakharov.
And today is the day: we announce CP violation in charm at @_moriond and @CERN! And indeed it's a tiny effect: 1.5 per-mille. We needed 70 million charmed mesons to measure it.
So it is New Physics? Well, we don't know. The predicted amount of CP violation predicted is smaller, but the calculations are uncertain. More work for @petrov and his colleagues.
The CERN press release is here
Nice summary by @MarcoGersabeck theconversation.com/amp/cern-study…
This thread made it to the @thewire_in thewire.in/the-sciences/s…
