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Tuesday, 5 July 2011

Tevatron finds CP-violation at 3.9 sigma: could this be mechanism for matter-antimatter asymmetry?

Matter-antimatter asymmetry, as I wrote in this blog post and in this introductory article, is one of the great unsolved mysteries of the Universe. The problem in simple terms is that there is an excess of matter over antimatter in the Universe, and scientists and astronomers are baffled as to why. One would naively expect the Universe to behave symmetrically, treating matter and antimatter exactly the same rather than preferring one to the other. After all, symmetry and conservation laws play a huge part in physics.

However, physicists have increasingly found that this is not the case: some processes in particle physics produce more matter than antimatter, an effect known as baryogenesis. These processes violate CP symmetry: a combination of charge conjugation symmetry and parity symmetry.

Click here for an introduction to CP violation as a mechanism for baryogenesis.

Following the discovery of interactions that violate CP symmetry in 1964, the Standard Model of particle physics was modified to take account of CP violation, by adding a complex phase into the CKM matrix describing quark mixing. However, the maximum amount of CP violation that can be included in the Standard Model in this way is still much much too small to account for the observed preponderance of matter over antimatter in the Universe. To put it in perspective, even if the parameters of the Standard Model were adjusted so as to give the maximum possible amount of CP violation, it would still only account for an excess of matter roughly equal in size to one galaxy - and there are millions of galaxies in the Universe!

Recent experiments provide evidence that the amount of CP violation observed in nature is greater than the amount that is allowed by the Standard Model. Researchers at the Tevatron* have measured the dimuon charge asymmetry – the number of muons compared to antimuons that are produced in a particular reaction – and found that more muons are produced than antimuons. The amount by which muon production exceeds antimuon production is larger than that predicted by the standard model.

The researchers have quoted the disagreement between their results and the standard model as 3.9 sigma – this means that if the Standard Model prediction is correct, there is a 0.005% probability of obtaining the result they did. When an experiment gives a result that the standard theory says should only occur 0.005% of the time, it becomes sensible to ask whether the standard theory might be wrong. However, with thousands of particle physics experiments currently taking place around the world, one would expect to see a few anomalous results occurring, even if there was nothing wrong with the underlying theory. For this reason, the convention in particle physics is to disbelieve the current theory only if the level of disagreement between the theory and the results is 5 sigma – i.e. if the theory predicts that the observed result will occur 0.00003% of the time.

In conclusion, this experiment has provided strong evidence that there may be more CP-violation occurring in the Universe than current Standard Model particle physics can explain. This CP-violation could be the mechanism by which the early Universe produced more matter than antimatter. If these results can be confirmed at the desired 5 sigma level of accuracy, they provide further motivation to develop a theory of particle physics that goes beyond the Standard Model.


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