Heavy light and heavy quarks

The top diagram shows one of several ways to create a Z along with a bottom quark-antiquark pair. The initial quark and antiquark on the left are partons in the proton-antiproton collisions of the Tevatron.The bottom diagram shows a similar process without the Z. Because the Z is so massive, the gluon, shown in bright blue, will have different energy and momenta in the two cases.

The force that holds a proton together also makes it more difficult to figure out what happens when protons and antiprotons collide. When a parton — a particle inside the proton — is produced in a collision, it feels the strong nuclear forces that hold the proton together. Those forces will rapidly deflect or decelerate the parton as it leaves the collision, and so the parton you actually can measure is not quite the same as the parton originally produced.

One way around this problem is to look at collisions that produce particles that do not feel the strong force. Such particles will not be deflected or decelerated before being measured. One such particle is the Z boson. The Z is much like the photon, the particle of light, but it does not travel at the speed of light, since the Z (unlike the photon) has mass. So you might think of a Z as heavy light.

There is a second reason to look at proton collisions that produce Zs. The Z is about 100 times more massive than the proton and the energy needed to produce that mass is not available for creating other particles. That is different from the case where a massive particle is not created, and a theoretical calculation that works in one case might not work in the other.

Now about the heavy quarks. Protons don’t contain bottom quarks, per se. They do contain gluons that can turn into bottom quark-antiquark pairs. (You may remember the phrase "sea quarks" from the December 18, 2014 DZero column; b quarks in protons are always sea quarks.) So measuring the production of bottom quark-antiquark pairs tells us about the gluon content of the proton. Measuring the production of bottom quark-antiquark pairs produced together with a Z tells us about the gluon part when a lot of the collision energy goes into making a heavy particle.

DZero has recently measured the production of a Z with a bottom quark-antiquark pair. More precisely, what we measured was the ratio of the production of a Z with two jets produced from bottom quarks over the production of a Z with two jets of any sort. In this way, many possible causes of measurement uncertainty are removed. The result, a ratio of 2.36 ± 0.47 percent, matches the existing theoretical calculations well, strengthening our confidence in our understanding of the gluon content of the proton. It also provides an important constraint on the backgrounds for such rare processes as Higgs boson production. This is the first measurement of this important ratio at hadron colliders, shedding heavy light on heavy quarks.

Leo Bellantoni

The following scientists contributed to this result: top row, from left: Manbir Kaur and Suman Bala, Panjab University; Ashish Kumar, SUNY. Second row, from left:Thomas Hebbeker, RWTH Aachen University; Karl Jacobs, Freiburg University. Third row, from left: Arnulf Quadt, Georg-August University, Gottingen; Volker Buescher, University of Mainz; Dorothee Schaile, Ludwig-Maximilians University, Munchen. Fourth row, from left: Peter Maettig, University of Wuppertal; Norbert Wermes, University of Bonn.