Double parton interactions – four times the charm!

A new publication by the DZero collaboration measures the production of pairs of J/ψ mesons. There are two possible mechanisms for this process, depending on the number of parton interactions in the proton-antiproton collision. The contributions from single- and double-parton interactions are separated by examining the angular separation of the two mesons, which is quite different for the two cases.

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Most of the time, a proton-antiproton “collision” is actually the interaction of one parton (quark or gluon) from the proton and one from the antiproton. The rest of the (anti)proton remnants generally continue their paths, undetected, down the beam pipe. Every so often, however, a second parton-parton interaction can occur in the same collision. The fraction of these “double-parton” (DP) interactions is an interesting quantity because it provides unique information about the distribution of the partons inside the proton: Are they spread out uniformly throughout the proton, or are they more clustered together?

The DZero experiment this week releases a new measurement of the prevalence of both single-parton and double-parton interactions by investigating the production of J/ψ (“jay psi”) mesons, either singly or in pairs. These mesons themselves consist of a charm quark-antiquark pair. At the Tevatron, J/ψ mesons were usually produced from gluons, which split into the charm quark-antiquark pair, and so this measurement is especially sensitive to the distribution and properties of gluons within the colliding (anti)protons.

As well as gluon splitting, there is a second significant source of J/ψ mesons that must be accounted for in the analysis: the decay of hadrons containing a beauty quark. Such decays must be identified and subtracted from the total sample since they are not sensitive to the same underlying process. Fortunately, these beauty hadrons tend to travel some small but measurable distance in the detector, whereas direct J/ψ production happens at the initial collision. This allows the two components to be differentiated and the fractions of each computed with good precision.

For the single J/ψ events, scientists measured the production cross section and found it to be in excellent agreement with the Standard Model predictions. For the J/ψ pair events, there are two possible mechanisms: a single-parton interaction producing both mesons or a double-parton interaction, with each producing a single J/ψ meson. Again, the two possible sources are differentiated by examining the properties of the events, here the angular separation of the two J/ψ mesons. For the single-parton case, the mesons tend to be produced close together, while for DP interactions they are more uniformly distributed in space (see the figure above).

Using this technique, scientists measured the cross sections for both single- and double-parton interactions. The data indicate that both types of J/ψ pair production occur more often than theory would predict, with the DP cross section in particular being three times higher than predictions. This indicates exactly how important it is to produce such measurements, providing new constraints to improve the theoretical modeling.

Finally, taking the ratio of DP to single J/ψ production, we obtain the quantity σeff, which is a measure of the average distance between colliding partons in the (anti)proton. The measured value is 5.0 millibarns (or 5×10-27 square centimeters!), somewhat smaller than similar measurements sensitive to quark (rather than gluon) distributions. This is suggestive of gluons being more closely clustered within the proton than are quarks. The fact that we begin to be sensitive to such differences is a remarkable feat, testimony to the many years of dedicated study of the unique Tevatron data set.

Mark Williams

These DZero members all made significant contributions to this publication.
Erich Varnes, from the University of Arizona, was recently re-elected as the co-chair of the DZero Institutional Board for the next three years. The Institutional Board has an important governance role in the collaboration, working with the spokespersons to ensure that all aspects of the experiment are coordinated in an efficient and productive manner. We congratulate Erich, thank him for all his contributions to the experiment and wish him the best of luck in his continued position.