Physicists (like us) are, with good reason, eagerly anticipating results from the Large Hadron Collider at CERN, scheduled to turn on next year. The LHC will collide protons at much higher energies than ever before, giving us direct access to a regime that has been hidden from us up to now. But until then, a whole host of smaller experiments are interrogating particle physics from a variety of different angles, using clever techniques to get indirect information about new physics. Just a quick rundown of some recent results:
- Yesterday the MINOS experiment at Fermilab (Main Injector Neutrino Oscillation Search) released their first results. (More from Andrew Jaffe.) This is one of those fun experiments that shoots neutrinos from a particle accelerator onto an underground journey, to be detected in a facility hundreds of miles away — in this case, the Soudan mine in Minnesota. They confirm the existence of neutrino oscillations, with a difference in mass between the two neutrino states of Δm2 = 0.0031 eV2. The neutrinos left Fermilab as muon neutrinos, and oscillated into either electron or tau neutrinos, or something more exotic. MINOS can be thought of as a follow-up to the similar K2K experiment in Japan, with a longer baseline and more neutrinos.
- The previous week, the D0 experiment at Fermilab’s Tevatron (the main proton-antiproton collider) released new results on the oscillations of a different kind of particle, the Bs meson (a composite of a strange quark and a bottom antiquark), as reported in this paper. For better or for worse, the results are splendidly consistent with the predictions of the minimal Standard Model. These B-mixing experiments are very sensitive to higher-order contributions from new physics at high energies, such as supersymmetry. D0 is telling us something we have heard elsewhere: that susy could already have easily been detected if it is there at the electroweak scale, but it hasn’t been seen yet. Either it’s cleverly hiding, or there is no susy at the weak scale — which would come as a surprise (a disappointing one) to many people.
- Finally, a little-noticed experiment in Italy has been looking for axion-like particles — and claims to have seen evidence for them! (See also Doug Natelson and Chad Orzel.) The usual (although still hypothetical) axion is a light spin-0 particle that helps explain why CP violation is not observed in the strong interactions. (There is a free parameter governing strong CP violation, that should be of order unity, and is experimentally constrained to be less than 10-10.) The axion is a “pseudoscalar” (changes sign under parity), and couples to electromagnetism in a particular way, so that photons can convert into axions in a strong magnetic field. (Another mixing experiment!) The axion relevant to the strong CP problem has certain definite properties, but other similar spin-0 particles may exist that couple to photons in similar ways, and these are generically referred to as axion-like. Zavattini et al. have fired a laser through a magnetic field and noticed that the polarization has rotated, which can be explained by an axion-like particle with a mass around 10-3eV, and a coupling of around (4×105eV)-1. My expert friends tell me that the experimentalists are very good, and the result deserves to be taken seriously. Trouble is, the particle you need to invoke is in strong conflict with bounds from astrophysics — these particles can be produced in stars, leading to various sorts of unusual behavior that aren’t observed. Now maybe the astrophysical bound can somehow be avoided; in fact, I’m sure some clever theorists are working on it already. But it would also be nice to get independent confirmation of the experimental effect.