New light on the strong interaction

A collision recorded by CDF with two high-energy photons produced and no other particles.

When a proton and an antiproton collide at very high energy in Fermilab’s Tevatron, about 25 percent of the time they just scatter away from the point of collision at a very small angle without losing any energy. We call that elastic scattering. This can happen in two ways: via the electromagnetic force (a photon is exchanged) or by the strong force if the protons overlap. In the other 75 percent of cases some of the energy of the collision goes into creating new strongly interacting particles or hadrons, such as π mesons.

The CDF collaboration has found (see Exclusive Diphoton Production) that there is a third class of collisions. In this class, the particles lose a small fraction (less than about 1 percent) of their energy, but no hadrons are created. The collision only creates a pair of energetic photons, or gamma rays, out of the lost energy. This is the first observation of this process (see figure).

The new photon pair creation process involves both the strong and the electromagnetic interactions. Scientists predicted this process, but it is hard to calculate.

Gluons are the quanta of the strong force binding quarks inside protons. When two gluons collide they can create a quark-antiquark pair that immediately converts into a pair of photons. These nearly always appear together with many other particles. It rarely happens that another gluon is exchanged in such a way that no hadrons are created. The measured very small rate at which such isolated photon pairs are created agrees with the Standard Model expectation. This rate is about one in 25 billion collisions. This result was presented at a recent Fermilab seminar.

edited by Andy Beretvas

These physicists were responsible for this analysis: From left: Mike Albrow, Fermilab; Erik Brücken, Helsinki, Finland; and Risto Orava, Helsinki, Finland.