Musings

They have measured oscillation between the 2^nd and 3^rd families of neutrinos and inferred a value of $\Delta m_{2 3}^2= (3.1\pm 0.6 (\text{statistical}) \pm 0.1 (\text{systematic})) \times 10^{-3} \text{eV}^2$. The mixing is also close to maximal.

MINOS results for $\Delta m_{2 3}^2$ v.s. $\sin^2(2\theta_{2 3})$, compared to those of Super-Kamiokande and K2K. (MINOS Press Release)

These are really impressive first results from an experiment which should produce even better results down the road.

PVLAS

For reasons that escape me, both Sean and Luboš have recent posts about the PVLAS experiment which made a splash last summer, purporting to detect vacuum birefringence in a strong (5 Tesla) magnetic field (see also a more recent conference proceedings). This, obviously, suggests an axion-like field, $a$, with a coupling $\mathcal{L}=\dots -\frac{g}{4} a F_{\mu\nu}\tilde{F}^{\mu\nu}$ to photons. But it’s pretty much impossible to reconcile the apparent strength of the coupling, $1.7\times 10^{-6} \text{GeV}^{-1} \lesssim g \lesssim 1.0\times 10^{-5} \text{GeV}^{-1}$, suggested by PVLAS with astrophysical bounds from energy transport in stars and from the CAST experiment (see, e.g., the review by George Raffelt).

That’s not to say that people haven’t tried. Proposing a model in which the aforementioned “axion” is composite, with compositeness scale of a few KeV, built out of preons with electric charges $\sim 10^{-9}e$, and possibly also requiring the photon to have a small mixing with another unbroken $U(1)$ gauge boson, and hoping that the whole Rube Goldbergesque contraption is not already ruled out by a host of existing experiments, is, even the authors admit, a bit of a stretch.

Andreas Ringwald has a review of the status of these proposals. The smart money’s on the experimental result going away.