How it started How it's going

Today @LHCbExperiment released the most precise photon polarisation measurement in b→sγ transitions. These are very rare transitions that occur via "penguin" diagrams involving some of the heaviest known particles.

They are a precise test of the weak interaction, which is purely left-handed. The photon should be totally polarised, so measuring the polarisation of the photon tests if there are other unknown forces that contribute.

But how do you measure the polarisation of the photon? Our particle detectors can't do it and putting a polariser filter is not an option. https://twitter.com/PKoppenburg/status/1299234895856840704?s=20

One trick is to look for virtual photons. Heisenberg allows to produce a photon with a mass for a very short time. Then they will transform to something stable, here an electron and a positron.

Once you catch those, you can measure their angular distribution. We do that with the same procedure as for the similar process B→K*μ⁺μ⁻ https://twitter.com/PKoppenburg/status/1237067266287710208?s=20.

The result is that Standard Model wins. We can set constraints on the right-handed contribution to these decays, called C₇' and compare it to the expected left-handed part C₇. And as it's quantum mechanics we get a complex number out, with a real and an imaginary part.

The paper: https://twitter.com/LHCbPhysics/status/1316286149778968577?s=20