LHC Research May Help Explain the Universe's Matter/Antimatter Imbalance

suraj.sun sends this excerpt from the BBC: "Particles called D-mesons seem to decay slightly differently from their antiparticles, LHCb physicist Matthew Charles told the HCP 2011 meeting on Monday. The result may help explain why we see so much more matter than antimatter. The team stresses that further analysis will be needed to shore up the result. At the moment, they are claiming a statistical certainty of '3.5 sigma' — suggesting that there is less than a 0.05% chance that the result they see is down to chance. The team has nearly double the amount of data that they have analyzed so far, so time will tell whether the result reaches the 'five-sigma' level that qualifies it for a formal discovery."

Kaon decay

[tylersoze]

CP violation in weak interactions has been known for some time, specifically in neutral Kaon decay. If I'm understanding this results correctly, the surprise here seems to be the magnitude of the CP violation in this case.

Re:Kaon decay

[torako]

CP violation in Kaon decays can be explained by the Standard Model, but if the magnitude of CP violation they have claimed exists in the D system can not. It would be the first actual hint of physics beyond the Standard Model at the LHC. That would be some very exciting news (especially because everybody expected the big "discovery" detectors ATLAS and CMS to actually find something new first, i.e. the Higgs or Supersymmetry).

Re:How do we know matter is more common?

[bsane]

The assumption is- if the universe had a fair amount of both, we'd see the gamma radiation leftovers from collisions, and we don't...

Re:sigma?

[Anonymous Coward]

No, this is not MBA-speak, sigma is standard statistical terminology for "standard deviation":

In some fields, for example nuclear and particle physics, it is common to express statistical significance in units of the standard deviation of a normal distribution. [wikipedia.org]

Careful: QCD hard!

[Roger W Moore]

CP violation in Kaon decays can be explained by the Standard Model, but if the magnitude of CP violation they have claimed exists in the D system can not.

The calculations required to predict the amount of CP violation in meson systems are extremely hard to do. When I worked on the NA48 experiment, which measured direct CPV in the kaon system, the theorists were initially adamant that there was no way the parameter we measured (espilon-prime over epsilon) could be above 0.001 in the Standard Model. Several year later after both NA48 and KTeV had published results putting the parameter at well above that I saw a theory talk saying that these results were in perfect agreement with the Standard Model!

Now the discrepancy seems a lot larger here but, nevertheless, even if the result holds I'd give the theorists time to think about this and see whether they find problems in the calculations. I have a huge amount of respect for my theory colleagues but QCD calculations like this are fantastically hard so it is not at all uncommon for the results to change.

Re:We don't "see" much more matter

[Surt]

We know what annihilation looks like. If there were anti-stars in our galaxy, we'd see some substantial annihilation signatures in the mixing in nebulae for example. Even if whole galaxies were anit-matter, we'd see some signature where the galaxies mix. The smallest unit of mass that could be anti matter unnoticeably is probably the supercluster. Even then, doubtful that we couldn't see annihilation signatures along the great walls, for example.

Significance

[kievit]

Being a physicist myself I am very happy that this topic makes it into the news. But it is important to keep cool and skeptical. The statement that a statistical fluke has a probability of 0.05% implies that it is bound to happen if you let 2000 students do data analyses on independent data sets. There are indeed literally thousands of PhD students doing such analyses LHC data, trying to address hundreds of specific research questions that each require different data selections. So it is very likely that some of them will find a result several standard deviations away from the expectation. Actually 3.5 sigma deviations happen very often, because of all sorts of mistakes and inaccuracies in the analyses, but most of the time these mistakes are scrutinzed away before loud public announcements are made. After all scrutiny a few genuine statistical flukes should still remain, and recognized as such.

(For the xkcd inclined: green jellybeans linked to acne [xkcd.com].)

More caveats:

• On slide 14 and 15 you see a summary of the estimated systematic errors and the final result: the deviation of the observed value from the expected value is 0.82 ± 0.21(stat.) ± 0.11(sys.) %. Estimating and combining systematic errors is almost by definition dark magic. It looks like the "3.5 sigma" was obtained by adding the statistical and systematic error in quadrature, which yields a total error of 0.237, and 0.82/0.237=3.5.
• The statement that the probability of this 3.5 sigma deviation is 0.05% is based on the assumption that if you repeat this analysis several times on more data with exactly the same experimental setup, the deviations from expectation are distributed like a Gaussian (bell curve) with a sigma equal to the total error mentioned in the previous bullet point. That is a major idealization, it could be distributed in many other ways, and then the relation between the deviation (in units of sigma, which is also defined for non-Gaussian distributions) and "the fraction of events with such a deviations or larger" can be quite different. Furthermore, when repeating the identical experiment the systematical errors do *not* fluctuate (that is one of the aspects in which they differ from statistical errors), so aforementioned idealized Gaussian would have an arbitrary offset with a magnitude of the order of the estimated systematic error (0.11), in either direction, and a width of the actual statistical error, 0.21. Depending on what this systematic error really is, the true statistical significance is much larger or much smaller than the quoted 3.5 sigma.

So this is a very interesting result, but more study is needed and in my experience such flukes almost always evaporate in the light of more data and scrutiny. Still, it's not completely excluded that this was indeed the first hint of a real discovery (otherwise no researcher would ever do all that work).

OK, enough for now. Sorry for misinterpretations and other errors I might have made.

Re:How do we know matter is more common?

[HiThere]

Black hole? Something's wrong with your physics model.

OTOH, as stars emit gas, and so do galaxies, you'd expect to detect a lot of matter-antimatter annihilation going on. This is a particular energy range of gamma radiation that we just aren't seeing, so we believe that there's no sizable amount of antimatter in the universe. This isn't entirely certain, as glaxy clusters are much more separated than individual galaxies, but if there is a significant amount of antimatter it's at the galactic cluster range of size, and it's really hard to explain why it would chunk in that way. (OTOH, I'm not a cosmologist. I could be wrong. But in this case I'd make a reasonable bet that I wasn't.)