CP violation at the LHC

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Yesterday’s big news in the physics world: the LHCb experiment has observed a \(3.5\sigma\) asymmetry between the decays of \(D^0\) and \(\bar{D}^0\) mesons. This has already been described in detail elsewhere on the web: Sean Carroll has a nice explanation accessible to non-experts, or you can look at the presentation of the results from the HCP conference (which itself is reasonably clear and informative, if you have some experience looking at particle physics presentations).

For those who are not inclined to click on links, here’s a quick summary of the story. CP symmetry violation is a difference between the behaviors of a particle and the mirror image of its antiparticle. The probability of a CP-violating process to occur is controlled by a complex phase parameter in the quark mixing matrix. There are two kinds of CP-violating processes that we can detect:

  • Some particles (kaons, D and B mesons) transform into their antiparticles and back as they propagate. CP violation means that the oscillation probability for going from the particle to the antiparticle is different from the probability to go from the antiparticle to the particle. Intuitively, you could imagine that the meson spends more time as a particle than as an antiparticle, or vice versa (in the sense of a time-averaged expectation value of the state). This is called indirect CP violation.
  • Most particles decay, and each reaction by which a particle can decay has a characteristic time constant that gives the average lifetime of the particles that decay via that reaction. (whew!) If the time constant of a particular decay reaction that starts with the particle differs from the time constant of the opposite reaction that starts with the antiparticle, that is called direct CP violation.

So far we have observed both direct and indirect CP violation in kaons and B mesons, which contain strange quarks and bottom quarks respectively. The new results from LHCb, if they pan out, will add direct CP violation in D mesons (which contain charm quarks) to that list. Finding and measuring the properties of CP violation with a new quark type will go a long way toward increasing our understanding of the mechanism behind this violation. That in turn will help explain why the universe contains matter and not antimatter — which is, in a sense, the ultimate example of CP violation.