In a paper published today in the journal Physical Review Letters, the COMPASS experiment at CERN reports a key measurement on the strong interaction. The strong interaction not only binds quarks into protons and neutrons, it also binds protons and neutrons into nuclei. Inside those nuclei, pions – made up of a quark and an antiquark – mediate the interaction. Strong interaction theory makes a precise prediction on the polarisability of pions – the degree to which their oppositely-charged constituents can be separated in an electromagnetic field. This has baffled scientists since the 1980s, when the first measurements appeared to be at odds with the theory. Today's COMPASS result is in close agreement with theory.

To measure the polarisability of the pion, COMPASS shot a beam of pions at a target of nickel. As the pions approached the nickel at distances only twice the radius of the particles themselves, they experienced the very strong electric field of the nickel nucleus, which caused them to deform and change trajectory, emitting a photon in the process. It is by measuring this photon and the deflection of the pion for a large sample of 63,000 pions that the polarisability could be measured. The result shows the pion to be significantly small polarisability than previous measurements, as expected from strong interaction theory.

Read more: "Measurement of the Charged-Pion Polarizability" – Physical Review Letters