W Boson Helicity

In the Standard Model, W and Z bosons, the weak force carrier bosons, interact only with left-parity fermions.

It is perhaps unsurprising then, that the helicity of W bosons (and analogous trait to parity in photons and some other bosons) is likewise asymmetric. About 30% +/- 4% were measured to have left helicity, about 72% +/- 5% were measured to have neither left nor right helicity (aka longitudinal helicity), and the negative 2% +/- 2% with right helicity was statistically indistinguishable from zero, in a recent measurement at the large hadron collider (measured for the first time in this LHC study).

This is a good fit to the Standard Model prediction. According to the study:

The SM predictions for the W boson helicity fractions at next-to-next-to-leading-order (NNLO) in the strong coupling constant, including the finite b quark mass and electroweak effects, are FL = 0.311 ± 0.005, F0 = 0.687 ± 0.005, and FR = 0.0017 ± 0.0001 for a bottom quark mass mb = 4.8 GeV and a top quark mass mt = 172.8 ± 1.3 GeV. The current experimental results for the W boson helicity fractions, all extracted using tt events, are in good agreement with the SM predictions.

Once again, the Standard Model, formulated four decades ago (albeit with updated top and bottom quark mass constants), nails an experimental result, for an experiment that has never been conducted before, with flying colors.  Any alternative to the Standard Model must consistently produce such accurate results.

In other news, there is still no evidence for Standard Model prohibited neutron-antineutron oscillation (which would violate baryon number), despite the extreme precision of the measurement.