Seeing double in proton-antiproton collisions

Protons and antiprotons are composite objects, formed from a constantly changing mixture of quarks and gluons (partons). In a small fraction of collisions there can be more than one parton-parton interaction. Measuring the rate of such double interactions provides important information about the structure of the proton.

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What happens when protons and antiprotons collide? To answer this question, we first need to consider what we mean by a particle collision. The proton is not fundamental: It contains an evolving mixture of quarks, antiquarks and gluons. These quarks and gluons are collectively called partons and are held together by the QCD interaction, like marbles in a bag. While the marble metaphor is flawed in many ways, like most real-world imaginings of quantum behavior, it provides a helpful analogy to consider the various possible outcomes of particle collisions, so let’s go with it.

The most common outcome of a proton-antiproton collision is that the two hadrons simply break apart — the two bags of marbles break — weakly scattering the internal quarks and gluons. This is called a “soft” interaction. The collisions that we are usually interested in are those in which a parton from the proton interacts directly with a parton from the antiproton in a “hard” scattering process that can produce new particles such as Higgs, W and Z bosons and other quarks. In our analogy, this would be two marbles, one from each bag, hitting each other and breaking apart.

Occasionally, however, there can be more than one hard interaction per collision: In other words, two separate pairs of marbles smash together or two pairs of partons interact. These so-called double-parton interactions are much rarer than the usual single parton case, and their prevalence provides important information on the spatial distribution and transverse momentum of the quarks and gluons inside the proton. This week the DZero collaboration will release a new publication, measuring the rate of double-parton interactions using events with three quarks and a photon in the final state.

The measurement uses a clever feature of the data to improve the precision: Sometimes two single-parton interactions can occur in separate proton-antiproton collisions. In our analogy, two bags of marbles collide at point A, and another two collide at point B. By comparing the number of selected events with one versus two separate collision points, the ratio of single- to double-parton interactions can be extracted with minimal reliance on the details of the detector efficiency. Furthermore, particular characteristics of the double-parton events allow them to be identified by their experimental signature and discriminated from the main backgrounds.

The final results are expressed in terms of an effective cross section (the transverse area in the (anti)proton occupied by the interacting partons) and are the most precise ever made. In addition, for the first time the measurement is also performed separately for events where one of the partons was a heavy quark (beauty or charm). Interestingly, the measurement indicates that the probability of a double-parton interaction is the same regardless of the flavor of the initial parton: Unlike most of us, parton interactions don’t seem to be influenced by charm and beauty!

Mark Williams

These DZero members all made significant contributions to this publication.
The DZero collaboration continues to thrive more than two years after the final Tevatron collisions took place. This week sees the coming together of scientists from around the world at Fermilab for the winter DZero collaboration meeting. The aim is to push forward a range of measurements toward publication, with more than 20 different analyses being presented for discussion.