Marketing CP Violation

By Sean Carroll | June 4, 2010 9:49 am

A couple of weeks ago we heard news that the Tevatron at Fermilab, soon to be superseded by the LHC at CERN as the world’s cutting-edge high-energy particle accelerator, might not be completely out of surprises just yet.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 Bd 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 Bs 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 Bs 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 Bs and one anti-Bs, 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.

CATEGORIZED UNDER: Science, Science and the Media
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  • http://Facebook Vianna Biehl

    I had to read this to find out what CP Violation meant; I am not a physicist. The added value for me was finding out what isn’t meant by the term.

    Facts are best without embellishment.

  • lee

    Ha! At least it is a (brief) break from stories about “God Particles”…

  • Pieter Kok

    I’m not sure I agree with this. The question is not whether this particular instance of CP violation has anything to do with the matter-antimatter balance, but whether it gets front line physics on the front page of a newspaper. Somewhere, there will be a kid reading this and getting excited about physics. He or she will figure out the subtleties about baryogenesis when it becomes important.

    And when was a press article about physics ever completely accurate?

    • Sean

      I would prefer to get kids excited about physics by telling them the truth.

  • Aaron Sheldon

    You think its bad in HEP, trying being a level headed statistician in the health sciences.

    Personally I have become so overly inundated with barely significant exploratory analysis being blown up to overly important inferential conclusions that I have stopped reading my peers research. Their mathematics is generally 200 years behind the state of the art, and their conclusions so badly over stated that it is pointless to even try to argue with them. So I just stopped reading health science statistical literature… its completely pointless.

  • Giotis

    This is because they are not really marketing CP violation or “Physics”, they are marketing themselves.

  • King Cynic

    Sorry, I’m going to take the experimentalists’ side here. These sorts of CP violation studies very well could tell us something important about baryogenesis. Just because the theorists can’t immediately relate the observations to the observed matter/antimatter asymmetry doesn’t mean that these measurements don’t have consequences for, or that they aren’t motivated by, baryogenesis. We’re doing these experiments exactly because we hope they will have a bearing on the question of why we’re here, and I cannot take anything that D0 has said or done about this to be overselling on this grounds. Sean’s implication in comment #5 that the experimentalists just haven’t been telling the truth is wrong-headed and just a little mean.

  • Pieter Kok

    “I would prefer to get kids excited about physics by telling them the truth.”

    Sure, so would I. But a new CP violation on its own will not reach the front page of the New York Times. I think some pragmatism is appropriate.


  • Ahmed Hamza

    >at a level just a smidgen above what you need to claim a statistically significant result

    If I may ask, how exactly is this defined for a modern physicist? In biology, machine learning, and other statistically dependent programs, there may be orders of difference between the accepted ‘confidence’ value, from one study to the next.

  • Brett

    I find the regular reiteration that CP violation experiments are really about why we are here less annoying than the “obligatory” (and people often describe it that way) slide in people’s talks about the mass/energy budget of the universe. This slide shows up even when people are talking about the small slivers corresponding to lepton or photon energies, even in noncosmological contexts.

    Also, the matter-antimatter asymmetry doesn’t really require CP violation. It’s the T violation that matters, really. If CPT is a good symmetry, the two are equivalent, but as Sean says, we can be more flexible. Sakharov’s necessary conditions for baryon asymmetry were B (baryon number) nonconservation, C violation, CP violation, and non-equilibrium behavior. This is correct, assuming CPT. More generally, B nonconservation, C violation, and T violation are really necessary; you also need either CPT or non-equilibrium evolution.

  • Kaleberg

    I find it hard to take any news story about the BS particle very seriously. Was this from The Onion?

  • locke

    I have to go with King Cynic here: let’s take a quote from what Sean said:
    “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.” and change it to a logically equivalent and equally true: “to say that it gives us insight into the imbalance between matter and antimatter in our observable universe; it’s completely possible (though unlikey) that such a statement is simply true”. Seems the most the people involved can be accused of is not emphasizing the “though unlikely” part of the results. Probably ALL of the conceivable reasons behind the matter-antimatter imbalance are unlikely, but one (perhaps more than one?) is going to turn out to be correct. Unless you’re a telepath (or unless they tell you), you cannot KNOW what motivated this research. I suspect that grant proposals in this field (written to be read by experts) also enphasize the possible connection to matter-antimatter imbalance (not my field, so I emphasize the “suspect”).

  • Baal

    In neither the Dzero paper nor the NY Times article that was linked, can I see any quote that I can interpret as a scientist misleading the public. We have for example from Borissov:

    “This result may provide an important input for explaining the matter dominance in our universe.”

    That’s a completely true statement. If anybody chooses to misread or mishear the words “may provide” as “definitely does provide”, then that’s their own problem. Since some non-Standard-Model CP violation is necessary to explain baryogenesis, it would be foolish to not consider any previously unknown or unobserved source of CP violation as possibly relevant to the baryogenesis issue.

    I think a more fair representation of the logic being used is:

    1. Baryogenesis requires a new, previously unknown source of CP violation.

    2. We may be seeing (or looking for) a new, previously unknown source of CP violation.

    3. Therefore, what we are seeing (or looking for) may be related to baryogenesis.

    If you catch somebody substituting the word “is” for “may be” in the last part, then that’s not fair.
    But otherwise this logic is not only fallacy-free, but good science. As far as I can tell, the NYTimes article and the Dzero paper seem to be saying things the right way.

    Maybe Sean had some other particular marketing offense in mind. But in any case, more generally, it is worth noting that what journalists claim a physicist told them is sometimes a barely recognizable distortion of what the physicist actually said. Journalists also have motive to make the story more interesting. When confronted with what appears to be hype or unfair marketing or false scientific statements in the media, it is a very good idea to entertain the hypothesis that the journalism is at fault, not the scientist.

  • Per

    When you guys (hey Sean, that’s you :) stay out of the sceptic, religious, wacky American right wing, whatever nonsense you have an opinion about, and stick to the facts, then you really kick ass. This post was awsome.


  • Claire C Smith

    “I’ll leave it to the trained philosophers in the audience to find the logical flaw in that argument.” Sean.

    Full circle for me here, as starting commenting on this blog before it went to Discover Magazine – which cf course is a super magazine. Am now dissapointed in quoted comment. Has been made – unsure if it’s means ruling out contributary laymans and amateur input.

  • Jason

    You wrote this whole post just because a journalist tried to implicate that the experiment shed light on our origins? Journalists are supposed to do crap like that, because titles need to catch the eyes of the reader who may not care about science or honesty in science journalism. His job is to sell his article, to get you to click on the link and keep reading. The NY Times’ job is to sell papers. It’s about making money. The NY Times doesn’t care about science or honesty in science journalism. It uses science stories and other kinds of stories to sell a product to us. It cares about selling newspapers and making money, not about being accurate in the title of their article about PHYSICS (a subject which the general population hates anyway). So they have to jazz up the title a bit and say something grand like “our origins” or “why we are here”.

    No amount of blogging will make science journalists change their ways because their number one priority is to sell articles and newspapers and to keep their jobs.

  • Guennadi

    I just reread the scientific paper itself, and the presentation of the result at the seminar at Fermilab, and did not find a single place where it is said, according to your words: “It gives us insight into the imbalance between matter and antimatter in our observable universe”. The only claim in the presentation is: “This result may provide an important input for explaining the matter dominance in our Universe”. What do you see wrong with this statement? What is the source of your claim of non-honesty of physicists, which is quite strong conclusion? Could you give at least one example? If you refer to some journal paper, may be you address your blame to that particular journal and to the particular journalists, instead of blaming the team of physicists, who are, I am sure, as honest, as only possible.

  • Tona

    There is nothing wrong with saying that it MAY explain the dominance of matter in the universe. That is correct, it may and it may not.

    I think it is naïve to think that you can hope to draw young minds to the field without giving them approachable content that does not require they already have a physics degree. Capturing their imagination about how physics affects the world around them is what will make them want to study physics in the first place.

    The New York Times does not write the way it does solely to sell newspapers. The job of the general audience, mainstream science media is also to engage the public that does NOT have a deep science background. That is good for the field and for democracy. You can not expect the general public to support research funding or make intelligent votes on policy that involves science if you do not explain the science to them in a way that will interest them and be understandable. Expecting them to be interested in an article about CP violation without first telling them why they personally should care about CP violation in a way that does not require a physics degree — ie. how it could have affected the development of the universe – is unrealistic and exactly the attitude that has made people shy away from learning about physics and given physicists a reputation as elitists.

    Writing blogs and newspaper articles to “preach to the choir” of the already science literate population will not expand science literacy and funding any further than their current states.

  • Tona

    Sean says: “Indeed, the competing CDF experiment at Fermilab has already indicated that they don’t see the effect. But you never know.”

    CDF and DZero have differently designed detectors. CDF is not capable at the moment of testing the validity of the DZero result. CDF collaborators are attempting to find a way to test the result indirectly, but because of detector design they can not test it directly. The LHC, however, will be able to validate or invalidate the result in the future.

  • Roto

    I think you are going too far, Sean. Had we not seen CP violation, we’d be at much more at a loss as to how the baryon asymmetry arose. CP violation is much more peculiar than cherry trees or people named George Washington. Sure, doesn’t mean Standard Model CP violation is precisely the origin of the baryon asymmetry, but, the experimental proof of CP violation validates the hunting license to imagine some kind of CP violation is likely the source of the baryon asymmetry; other mechanisms are less likely by one tooth fairy, one very significant tooth fairy.

    Three other important points: 1) CP violation is oddly absent from the strong interaction. Now that is weird, and were the absence of CP violation in QCD our only experience with CP violation, we’d never postulate the Sakharov mechanism for the matter/antimatter asymmetry. 2)What D0 did was look for new large types of CP violation… if they are right (and I think they are wrong) then their CP violation would be much more likely tied to the baryon asymmetry than the Standard Model types. 3)Ramsey and Purcell used the baryon asymmetry back in the early 1950’s to seek the neutron electron dipole moment, which would have been a hint of CP violation (really T violation). So the connection is way older and more distinguished than you might know, Sean.


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Cosmic Variance

Random samplings from a universe of ideas.

About Sean Carroll

Sean Carroll is a Senior Research Associate in the Department of Physics at the California Institute of Technology. His research interests include theoretical aspects of cosmology, field theory, and gravitation. His most recent book is The Particle at the End of the Universe, about the Large Hadron Collider and the search for the Higgs boson. Here are some of his favorite blog posts, home page, and email: carroll [at] .


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