Gluon jets contain more color than quark jets

The plot shows the number of events as a function of the dijet invariant mass for the dilepton plus dijet selection. There are three channels, and the channel with the most quark-like jets is shown (quark-tagged events). Fit results are overlaid for the ZW and ZZ processes.

Quarks, the particles that make up nuclear matter, and gluons, the particles that mediate the strong force that holds nuclei together, share an important property: they both have “color.” Color, in particle physics, is like the strong force’s version of electric charge, but it can be a bit stranger. For instance, particles with color can’t live very long on their own. So when quarks and gluons are produced in particle colliders like Fermilab’s Tevatron, they form a stream of more stable, color-neutral particles that particle physicists call jets. These jets are then detected by calorimeters, which measure their energy and other properties.

While they look similar at first glance, quark jets and gluon jets are actually different in appearance. Because gluons have a little more color than quarks, gluon jets tend to have more particles in them and thus tend to be wider than quark jets. These properties can make studying jets more difficult: The algorithm for finding jets can capture more of the particles in the narrower quark jets and so lead to better energy measurements of quarks over gluons. But physicists can use these same characteristics to try to tell the difference between quark and gluon jets, which is important when searching for new processes that should create either quarks or gluons, but not both.

That is exactly what physicists searching for the rare production of boson pairs at CDF did. They wanted to find proton-antiproton collisions where a W or Z boson was produced and decayed into a pair of quarks, producing two quark jets alongside another Z boson that decayed into more easily identifiable electrons or muons. In order to better pick out events with two jets originating from quarks, they developed a new variable that separates quark jets from gluon jets by looking at things such as the spatial spread of the jet. By reducing more of their gluon jet background, they increased their sensitivity to the W and Z bosons they were looking for. Also, to better model their backgrounds, they found ways to calibrate their measurements of the jets’ energies based on the type of particles — quarks or gluons — that created them.

By looking at the full Tevatron Run II data set (8.9 inverse femtobarns), CDF physicists have determined the production cross section for ZW and ZZ boson pairs. They measured a cross section σ{ZW +ZZ} = 2.5 +2.0 / -1.0 picobarns, a result that is consistent with the Standard Model prediction of 5.1 picobarns.

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edited by Wesley Ketchum and Andy Beretvas

These physicists were responsible for this analysis: Top row: Young-Kee Kim, University of Chicago. Second row, from left: Vadim Rusu, Fermilab, and Wesley Ketchum, University of Chicago.