The Sciences

Marketing CP Violation

Cosmic VarianceBy Sean CarrollJun 4, 2010 1:49 PM


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A couple of weeks ago we heard news that the Tevatron at Fermilab, soon to be superseded by the LHC at CERN as the world's cutting-edge high-energy particle accelerator, might not be completely out of surprises just yet. The D0 experiment released results that seemed to indicate an asymmetry between the properties of matter and antimatter, at a level just a smidgen above what you need to claim a statistically significant result. Blogs started chattering right away, of course, but this was big enough news to be splashed across the front page of the New York Times. The measurement concerns the decay of B mesons -- particles consisting of one bottom (b) quark and one lighter antiquark, or vice-versa. If the other quark is a down, the corresponding meson


is electrically neutral, as is its antiparticle. They can therefore practically indistinguishable, and can oscillate back and forth between each other. The one difference is that the meson and anti-meson decay a little bit differently; this has been studied in great detail at B-factories, with results that have been very useful in determining values of parameters in the Standard Model of Particle Physics. The new D0 results use a different kind of particle -- the


meson, in which a strange quark rather than a down quark is stuck to the bottom quark. They measured the relative rate of decay of the


and its antiparticle, and found a discrepancy that appears inconsistent -- barely -- with the Standard Model. In particular, they looked at decays that produced muons or anti-muons.

You would expect that a single collision would produce one


and one anti-


, and that one would decay into a muon and the other into an anti-muon. But because the neutral B mesons can oscillate into their own antiparticles, sometimes you will get decays into the same kind of particle -- both muons, or both anti-muons. If matter and antimatter were completely symmetric, each possibility should happen equally often; 50% of the time you'd get two muons, and 50% of the time you'd get two anti-muons. But you don't; D0 reports that they see muons more often than anti-muons. That breaks the symmetry between matter and antimatter, and in a way that doesn't seem compatible with the Standard Model. If the only thing going on was ordinary Standard Model interactions, the discrepancy should be too small to be observed by the experiment. That's what all the excitement is about. Like most just-barely-significant results, this one is very likely to ultimately go away once more data are obtained. Indeed, the competing CDF experiment at Fermilab has already indicated that they don't see the effect. But you never know. And after that lengthy introduction, what I actually wanted to say is: I find the way that exciting results about matter/antimatter asymmetry are marketed to be somewhat annoying. (I know you are fascinated to hear about my pet peeves.) In technical jargon, what's actually being measured is CP violation. Built into the framework of quantum field theory, which is the basis for all of modern particle physics, are three different "reflection" symmetries -- transformations with the property that, if you do them twice, you come back to where you started. One is time reversal, labeled T; one is parity or mirror symmetry, labeled P; and one is "charge conjugation", or matter-antimatter exchange, labeled C. Every one of them was originally believed to be a symmetry, i.e. that the behavior of matter stayed the same under these transformations; in every case, we were wrong and Nature chooses to violate them. We still believe that the combination of all three, labeled CPT, is a good symmetry, but by now we're a bit more open-minded. Charge conjugation C is violated pretty blatantly in the standard model. Fermions -- "matter" particles like quarks and leptons, in contrast to bosons that are "force" particles like photons and gluons -- come in right-handed and left-handed varieties. These are related by parity; if you have a right-handed particle and you do a P transformation, you get a left-handed particle. The weak interactions of particle physics, as it turns out, only involve left-handed fermions and right-handed antifermions; the right-handed fermions and left-handed antifermions simply don't feel the weak interactions at all. Charge conjugation would change a left-handed electron, which does feel the weak interactions, into a left-handed positron, which does not. That's a pretty easy difference to detect, so C is dramatically violated in the Standard Model. But the combination CP changes a left-handed electron into a right-handed positron, both of which do feel the weak interactions. So this is a good symmetry -- almost. It turns out that much more subtle effects do violate CP (including the decays of B mesons). Nobel Prizes were handed out for the experimental discovery in 1980, and for the theoretical background in 2008. So CP violation is interesting -- it's a deep feature of particle physics, representing a breakdown of a fundamental symmetry, for which Nobel Prizes are handed out on multiple occasions. But that's doesn't seem juicy enough to some people. Whenever a new result concerning CP violation is announced, it's never enough to give the kind of explanation I just did. It's always couched in terms of "Why are we here?" The point is that CP violation plays a crucial role in baryogenesis, the mysterious process that accounts for the excess of matter over antimatter in our actual universe. Long ago Andrei Sakharov showed that you couldn't generate such an imbalance unless you violated CP. And baryogenesis is very important -- we wouldn't be here, blogging, if there were equal numbers of particles and antiparticles in the universe. So in some general terms, the subject of CP violation and the subject of "Why are we here?" are intertwined. But not that much. The logic seems to be something like this:

  1. CP violation has something to do with baryogenesis.

  2. This experiment has something to do with CP violation.

  3. Therefore, this experiment has something to do with baryogenesis.

I'll leave it to the trained philosophers in the audience to find the logical flaw in that argument. Try substituting "George Washington" and "cherry trees" for "CP violation" and "baryogenesis." The point is that the conclusion doesn't hold -- not everything about CP violation is necessarily related to baryogenesis. We don't know how baryogenesis actually happened -- there are many theories on the market, and any of them or none of them may be right. Therefore, there's no way of knowing whether any particular manifestation of CP violation is in any way related to baryogenesis. There could be lots of different ways in which CP is violated. In particular, there's no compelling theoretical reason why the CP violation being studied in the decays of B mesons has anything at all to do with baryogenesis. It's possible -- lots of things are possible. But what's being studied isn't baryogenesis; it's CP violation. So why isn't that enough? The answer is obvious -- explaining why we are here seems to be something that a wider audience can get excited by more directly than studying the details of a slightly-broken symmetry. The only problem is that it's not true; these experiments aren't really studying why we are here. We can't blame journalists for this one; here is a case where they are just reporting what the scientists tell them, and the scientists are quite willing to be shameless. I understand the motivation for being shameless -- it's hard to explain the details, and the results are legitimately interesting. But ultimately I don't think it's right to say untrue things in the name of getting people excited about true things. I would therefore like to see particle physicists take a slightly more honest tack about the importance of CP violation. It's perfectly okay to say that it gives us insight into the difference between matter and antimatter -- that's true. And that should be enough! It's not okay to say that it gives us insight into the imbalance between matter and antimatter in our observable universe; it's completely possible (even likely) that such a statement is simply false. If we get people excited about what we're doing by causing them to misunderstand what that actually is, we're ultimately not winning.

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