Spotlight studies the proton’s glue

Photons, the particles of light, make excellent probes of the strong-force interactions that hold the proton together.

The strong force is difficult to study because gluons, the carriers of the strong force, interact with each other. This makes the inside of a proton a very busy place, since the gluons that the constituent quarks (two up and one down) exchange to bind together interact in complicated ways. On the other hand, photons, which are the particles of light and carry the electromagnetic force, do not interact with each other or with gluons. This makes the photons that are produced directly out of proton collisions excellent probes of the strong force and helps physicists test models of the complex interactions that hold a proton together. A recent analysis at DZero sheds new light on proton structure by looking at events with a photon and a quark or gluon.

Finding the “right” photons was the major challenge in this analysis. When quarks and gluons are made in a collision, strong-force interactions make them almost immediately develop into a spray of particles called a jet. To get the most information about the proton’s structure, analyzers focused on photons that were made directly during the collision and not as part of the formation of a jet. A special discriminant was built based on detector information that could help select direct photons and reject the background from jets. The shape of the discriminant output allowed the analyzers to determine how pure their sample of photons was from background contamination in each region studied.

In most of the regions tested, the analyzers found good agreement between the observed data and the predictions made with various models. However, in some regions the data showed trends that did not match any prediction. The new observations, which have precision on par with many of the predictions, will be useful for improving models of the proton and gaining an even deeper understanding of the strong force.

Mike Cooke

These physicists made major contributions to this analysis.
Internships at Fermilab give undergraduates the opportunity to learn about high-energy physics while contributing to ongoing research. These undergraduate students spent their summer at DZero, where they made significant contributions to ongoing studies of the Higgs boson.