Photon pairs probe proton’s parts

Pairs of photons can probe the structure of a proton and help improve the modeling of strong force interactions.

A lot of activity is packed into a proton, and accurately modeling that particle maelstrom is important to measuring properties of the Higgs boson and searching for new physics. Most of a proton’s momentum is carried by its three “valence quarks,” which are two up quarks and one down quark for a proton. However, those valence quarks exchange many gluons, the particles that carry the strong force, and those gluons continually split into quark-antiquark pairs that exist briefly before annihilating back into a gluon. The complex interactions between valence quarks and the sea of other quarks and gluons within the proton are difficult to calculate directly in the Standard Model and are instead constrained by experiment.

Photons make an excellent probe for studying proton structure and the complex interactions of the strong force because they do not directly interact with gluons, which allows them to escape the maelstrom. A recent analysis from DZero examines events where pairs of photons were produced by a collision in the Tevatron, a process of particular importance because it is a direct background to Higgs boson decays to photons. An important part of this analysis was to accurately model background events containing a neutral meson (a quark-antiquark pair) that could mimic a photon signal. Discriminating between different neutral particles requires a careful study of the energy deposit distribution a particle leaves in DZero’s finely segmented electromagnetic calorimeter.

The analyzers compared the observed behavior of photon pair events to the predictions from a number of models, and found that, while a broad agreement between simulation and data was observed, no single model described all aspects of the data well. These measurements will help improve future models of the proton’s structure and the strong force interactions related to photon pair production, which will be used during future studies of the Higgs boson’s properties and searches for new physics.

Mike Cooke

These physicists made major contributions to this analysis.
As building manager for the DZero complex, Pete Simon (Fermilab) coordinates numerous efforts critical to the success of the collaboration, from maintaining essential detector infrastructure to sustaining a healthy and productive office environment.