Do a physics experiment. Now take that experiment, change all the particles to antiparticles, and reflect the entire apparatus around some fixed plane. If you get an equivalent result, we say that the experiment preserves charge/parity symmetry, or CP for short. Most mid-century physicists originally assumed that CP would be a good symmetry of nature — switching matter with antimatter also requires switching left with right, but why should left-handed particles behave any differently than right-handed antiparticles? But in the 1960’s Cronin and Fitch showed that it was violated by the decays of neutral kaons, for which they picked up a Nobel Prize in 1980.
Since then, studying CP violation has been a fruitful pursuit for particle physicists. The decay of various quarks into each other generically violates CP (as shown by Kobayashi and Maskawa, Nobel 2008), so searching for CP violation gives us a lot of leverage when we try to map out the dynamics of particles in the Standard Model. Which is why it was big news today when CERN announced that the LHCb experiment has observed CP violation in a brand-new system, decays of the Bs meson. (Here’s the paper.) It’s only the fourth known particle to have CP-violating decays, joining the kaon, the D meson, and the regular B meson. (The subscript s means there is a strange quark involved.) A brand-new way to study a mysterious subatomic process, learn more about the Standard Model, and launch an ambitious search for new physics! Should be enough to get anyone excited.
But it’s not, of course — there are people out there who stubbornly resist the charms of precision electroweak particle physics. So it’s traditional to make an appeal to something nominally more sexy: the matter/antimatter asymmetry of the universe.
I’ve complained about this before, to little avail. The logic is as irresistible as it is faulty: the process of baryogenesis, by which matter came to dominate over antimatter, requires that there be CP violation in the early universe; we are studying CP violation here in the late universe; obviously, what we’re doing helps us understand the matter/antimatter asymmetry. But that’s only true if the kind of CP violation we are studying is actually somehow related to baryogenesis. Which, most experts believe, it is not.
Here’s a piece in Symmetry Breaking which makes the case against itself quite clearly. It starts with:
When the universe was less than a minute old, a tiny difference in the behavior of matter and antimatter led to the matter-dominated existence we experience today. Today, particle physicists on CERN’s LHCb collaboration announced that, for the first time, they have observed particles called strange beauty mesons, or B0s, contributing to this imbalance.
That seems pretty unambiguous: they are saying that physicists have observed a process that contributed to the matter/antimatter asymmetry. It’s only at the end of the article that they admit you’ve been duped:
However, the Standard Model predicts only a tiny portion of the amount of CP violation needed to explain the huge deficit of antimatter in the universe. While these results help scientists understand the mechanics of CP violation, the case of the missing antimatter remains unsolved. “We expected a certain amount of CP violation to be found in the strange beauty system,” says Pierluigi Campana, the LHCb spokesperson. “But finding the missing fraction of CP violation in the early universe will be new physics, which the Standard Model can’t predict.”
That’s the point: baryogenesis requires CP violation, and the Standard Model has CP violation, but almost everyone agrees that the Standard Model by itself can’t possibly explain baryogenesis. But it can explain the new results from LHCb. Chances are extremely high that the CP violation observed at CERN has nothing at all to do with the asymmetry of matter and antimatter. But who wants an inconvenient fact to get in the way of a good story?
What’s going on here is exactly the same bait-and-switch syndrome that’s responsible for the “God Particle” name, or selling a cosmology book by pretending it’s about why there is something rather than nothing, or mixing up time-reversal violation with the arrow of time. I got in trouble for complaining about that last one, too, with folks who thought I was denigrating a good piece of experimental science. But it’s quite the opposite: I’m saying that the truth is interesting enough, there’s no need to try to sell it via dubious connections with something that supposedly is more marketable!
The Higgs boson, modern cosmology, time-reversal invariance, CP violation — these are really interesting topics. It’s our duty to sell them and explain them at the same time; not do the former at the cost of the latter. It doesn’t do any good if people think that what we do is interesting, but only because we’ve misled them about what that actually is. The good folks at LHCb have every reason to be extremely proud that they’ve discovered a new system that violates CP, and launched a new way to study Standard Model physics and hopefully look for phenomena that stretch beyond that. They don’t need to hitch their wagon to the baryogenesis star.