Two b or not two b?

A new publication by the DZero collaboration measures the production of both single- and double-b quarks in conjunction with a photon, probing the gluon and b quark content of the proton. Among the findings is the first observation that pairs of b quarks are more likely to be produced at higher energies, as predicted by the theory of quantum chromodynamics.

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We traditionally consider the proton as comprising three quarks, two up and one down. In fact, we have long known that this is a simplified picture, representing only the average quark content; in reality the proton is a maelstrom of gluons and quark-antiquark pairs in constant dynamic interaction. This means that at any given time, a proton in fact contains quarks and antiquarks of all types, even the heavy bottom (also known as beauty, or b) quarks.

Measuring the fraction of the proton energy carried by these rare heavier quarks is an important goal, allowing the complex calculations of quantum chromodynamics (QCD) to be tuned and improved to better represent reality. In turn this leads to more accurate predictions for other processes necessary, for example, for future measurements of the W boson mass and searches for new signals beyond the Standard Model.

The DZero experiment this week released a new measurement that is highly sensitive to both the b quark content of the proton and to the gluon content. The analyzers search for events containing a high-energy photon in association with one or two b quark jets, all produced from the initial proton-antiproton collision. Some of these events come from the colliding (anti)proton’s gluons, which later split into a b quark-antiquark pair, and some come directly from a b quark in the proton itself. The blend of the two processes is one prediction of QCD that can be tested by this measurement.

The final aim is to measure the normalized production probability (or cross section) for both photon+b and photon+bb events as a function of the photon momentum and compare to predictions from QCD calculations. We also report the ratio of double- to single-b quark cross sections, where most of the experimental uncertainties cancel, giving an even more precise test of the predictions.

The main challenge is in measuring and subtracting the contributions from non-photon+b(or bb) processes, which can mimic the signal. Photons can be produced from the decays of common particles inside the detector (such as neutral pions), and so the photon purity must be determined. Similarly, the tools to identify b quark jets are susceptible to contamination from similar charm (c) quark jets. For both photons and b jets, the final signal yields are measured by examining distributions that can discriminate between signal and background components.

After performing the full analysis to extract the signal yields versus photon momentum, the two cross sections and their ratio are reported and compared to different predictions from theory and simulation. One interesting finding is that the ratio of double-b to single-b cross sections increases with photon momentum, as shown in the figure above, which is consistent with the predictions that the gluon splitting process should dominate at higher energies. Two b or not two b? Well, that depends on the momentum!

Mark Williams

These DZero members all made significant contributions to this publication.
The DZero Speakers’ Bureau (which also includes the spokespersons Dmitri Denisov and Gregorio Bernardi) is tasked with the challenging job of selecting worthy candidates to present DZero results at conferences all over the world. Speakers are chosen based on their contributions to the experiment, including analysis, leadership, and hardware and software service work. The bureau also keeps track of all talks ever given by DZero collaborators in an online database.