Constraining the proton sea

The measured differential cross sections of events with a photon and a heavy b quark as a function of photon energy. The data are compared to four theoretical predictions.

A simple picture of the proton is that it is made out of three quarks: one down quark and two up quarks. The quarks are “glued” together by gluons. However, this picture is incomplete. The so-called “sea” in the proton plays an essential role. The sea is produced by the gluons in the proton—gluons that split continuously into quark and anti-quark pairs or radiate other gluons. The sea quarks can also be quarks from the heavier quark families, for example charm quarks and bottom quarks. The heavy quarks within the proton sea are extremely scarce, so their properties are not well understood. In this experiment CDF physicists have studied these properties using the entire Run II data set.

CDF scientists studied proton-antiproton collisions that produced a photon and a jet from a heavy quark. Photons are measured in the central part of the detector and in the energy range from 30 to 300 GeV. When the photon energy is relatively low, the photon production rate is sensitive to the low-energy heavy quarks in the proton sea and can be used to improve our understanding of those components. When the photon energy is higher, the associated high-energy heavy quarks come from a different source: the gluons produced in the collision that split into quark and antiquark pairs. The events with high-energy photons can provide useful information on the gluon production and its splitting rate into heavy quarks.

The rate of production of a photon and a heavy jet is shown in the top figure. The results are compared with four theoretical calculations. Most of the calculations describe the lower-energy photon production rate well, which indicates a good understanding of the heavy sea quarks. Two of the calculations, kT factorization and SHERPA, are in reasonable agreement with the measured cross section over the full range of the data. However, some calculations underestimate the rate of higher-energy photons, which indicates it is important to improve the modeling of gluon splitting rate into heavy quarks in these calculations. These results are useful for quantum chromodynamics calculations and searches for new physics.

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edited by Andy Beretvas

Physicists Ray Culbertson, Costas Vellidis and Tingjun Yang, all from Fermilab, were responsible for this analysis.