Tuesday, 15 November 2011

CP violation at the LHC

Even if you have only a passing interest in particle physics, you have probably heard by now that the LHC has just produced a welcome surprise, following months of results which have merely improved the bounds on hypothetical new physics.

So has the Higgs boson finally been hunted down? Have gluinos or squarks finally shown up, providing long-awaited evidence for supersymmetry? Has a monstrous black hole been created, which will destroy us all over the course of the coming days? Not quite (and if you thought that last one was a realistic option, you have been badly misled!). Direct searches for the Higgs, or superpartners, or other new particles are largely the domain of two different experiments at the LHC — the CMS and ATLAS detectors. Instead, this new result comes from LHC-b, an experiment which has so far seen far less of the limelight. Mat Charles, from right here at Oxford, made the announcement in a talk at the HCP 2011 conference yesterday, and was kind enough to give us the same talk today in a local seminar.

What LHC-b has done is found the first evidence for 'CP violation' which cannot be explained by the standard model. If you know what that means, you can skip the rest of this paragraph. In a recent post, I tried to explain that there is really no fundamental difference between matter and anti-matter. While this is true, they are allowed to have slightly different interactions, and indeed we know that this is true in nature; the 1980 Nobel Prize for physics was awarded for this discovery. An example is that certain particles called kaons decay more commonly to positrons (which are anti-electrons, remember!) than electrons. Mathematically, a particle and its anti-particle are related by changing the sign of all Charges, combined with a Parity transformation, which reverses the spin (I skipped over this point in my last post). So for example, if we apply this combined 'CP' transformation to an electron which is spinning clockwise around its direction of travel, we get a positron which is spinning anticlockwise around its direction of travel. Any difference in their interactions indicates that CP is not an exact symmetry of nature; in physics parlance, CP invariance is 'violated'.

The LHC-b results concern D-mesons, which are particles made up of a charmed quark with an up or a down anti-quark, or a charmed anti-quark with an up or a down quark. Their measurements indicate that CP is violated in the interactions of these D-mesons by much more than would be predicted by the standard model. In particular, their measured value of a particular CP-violating parameter is -0.82 +- 0.21(stat.) +- 0.11 (sys.), where the two "plus or minus" values are respectively the statistical and systematic errors. The calculation of what this value should be in the standard model is complicated, and people are being somewhat coy about it, but supposedly its magnitude is expected to be around 0.1. Obviously the central value of the measurement is much bigger than this; if the results holds up under more scrutiny and with the collection of more data, it points to new physics, e.g. quantum effects of new heavy particles which have not yet been seen directly.

A couple of comments are in order. First, the statistical significance of the result is currently about 3.5 sigma, meaning it has much less than a one percent chance of being a statistical fluctuation. Of course, this is still possible, which is why the standard for a discovery in particle physics is significantly higher, at 5 sigma. But as you can see above, the uncertainty is currently dominated by the statistical uncertainty, which should decrease as the inverse of the square root of the number of events analysed. At present, they have only analysed half of their data, and of course they will collect a lot more data next year. Also, partly because the statistics are currently the limiting factor, they have been very conservative about estimating their systematic errors. With work, these are expected to come down as well.

The other point to be made is that the standard model expectation for this measurement needs to be pinned down with certainty, and I'm sure the relevant experts are already on the case. Finally, let me say that even if it turns out after more careful consideration that the measurement agrees with the standard model after all, it is still a nice piece of work from the LHC-b team, so congratulations to them!

(I am far from first with this news. In particular, my fellow ex-Melbourne student Anna Phan, who is a member of LHC-b, reported the news here, while Jester and Tommaso Dorigo each give more technical discussions, here and here respectively.)

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