Polarisation confirmed

The polarisation of photons emitted in the decay of a bottom quark into a strange quark, as predicted by the Standard Model, has just been observed for the first time by the LHCb collaboration. More detailed research is still required to determine the value of this polarisation with precision.


In this LHCb event, K, π and γ are emitted from a B+ → K+π-π+γ decay. This was investigated by the LHCb collaboration in order to study the photon (γ) polarisation.

If we imagine that photons are like little spinning tops which spin around an axis aligned with their direction of propagation, we can identify two types of photons. Those that are “right-handed” turn in the same direction as a corkscrew, and those that are “left-handed” turn in the opposite direction. If for a large number of decays of a given type we can observe an imbalance between the production of right-handed photons and the production of left-handed protons, we can say that there is a polarisation.

At CERN, the LHCb collaboration has been looking at precisely this phenomenon. In particular it has been studying the polarisation of the photon (γ) emitted in the decay of a bottom quark (b) into a strange quark (s): b → sγ. According to the predictions of the Standard Model of particle physics, the photons emitted in this decay should almost always be left-handed. But until now, this polarisation had not been demonstrated in an experiment. “Thanks to the data gathered by LHCb in 2011 and 2012, we have been able to study around 14,000 b → sγ decays," explains Olivier Schneider, a physicist at EPFL and a member of the LHCb collaboration. “By counting the number of photons emitted in different directions, we have successfully demonstrated polarisation (see box). Further research is needed to determine if this is polarisation with an excess of left-handed photons, as predicted by the Standard Model, or an excess of right-handed photons, and in what proportions."

If the polarisation turns out to be different from the Standard Model prediction, where almost 100% left-handed photons are expected, it could mean a U-turn for particle physics: “If our research eventually shows a right-handed polarisation, or even just a left-handed polarisation different to that predicted by the Standard Model, it would open the door to new physics,” enthuses Olivier Schneider. “In fact, various theories beyond the Standard Model predict other polarisation values for the b → sγ transition. If these predictions were confirmed, it would open up a whole new front for particle physics.” Something which would be music to the ears of many physicists.

Want to know more?

Figure 1the light blue plane is defined by the momenta of the K+, π- and π+ particles. By comparing the number of photons detected above (up) and below (down) this plane, physicists can calculate the Aud asymmetry, which is proportional to the photon polarisation.

To be more precise, the LHCb collaboration investigated the B+ → K+π-π+γ decay, in which the b → sγ transition takes place. The imbalance between right-handed photons and left-handed photons can be revealed by the “up-down asymmetry (Aud)”, which was measured by comparing the number of photons detected above (up) and below (down) the plane defined by the K+, π- and π+ momenta in the rest frame of these three particles (see Figure 1).

The Aud asymmetry was calculated for four mass intervals of the Kππ system: between 1100 and 1300 MeV/c2; between 1300 and 1400 MeV/c2; between 1400 and 1600 MeV/c2; and between 1600 and 1900 MeV/c2. These four Aud measurements are globally incompatible with the zero value, with a statistical significance of 5.2 sigma (see Figure 2), which indicates that the photons are indeed polarised.

Figure 2measurements of the Aud asymmetry for four mass intervals of the Kππ system.
Note that the Aud asymmetry does not directly provide the λ polarisation value, but is proportional to it according to the relationship: Aud = k * λ, where k is a constant that is in principle different for each mass interval of the Kππ system. A more detailed study could allow the value of k to be determined for each mass of the Kππ system. This would also allow the polarisation to be calculated.


To find out more, read the article published by the LHCb collaboration in arXiv and in Physical Review Letters.

by Anaïs Schaeffer