One jet, two jets, three…

When a proton collision creates a W or Z boson and jets, it is about five times less likely to make two jets than one, and another five times less likely to make three jets than two, and so on…

About a hundred million pairs of protons collide in the LHC each second. The rate of collisions must be so high for two reasons: to improve the chances of seeing something rare and to allow patterns to emerge out of the noise.

Each collision results in a unique splatter of debris, and no one can predict what will result from any individual impact. Some collisions produce photons – single-particle flashes of light. Some make heavy relatives of photons known as W and Z bosons. Many collisions create narrow streams of particles known as jets.

If we group collision events by type, they begin to form patterns. For example, consider the collisions that result in a W or Z boson and one or more jets. Collisions producing one jet are more common than those that make two jets, which are more common than those that make three and so on. Just as in bird-watching, we cannot predict what the next collision will bring, but Canadian Geese are more common than Cackling Geese.

With enough data, the pattern becomes sharp. A recent CMS paper presents an analysis of the rates of these collisions. The scientists showed, with unprecedented precision, that each additional jet makes the collision type more rare by the same factor, approximately a factor of five. That is, for every 25,000 cases with one jet, there were about 5,000 cases with two jets, about 1,000 cases with three jets and about 200 with four jets.

Patterns like this reveal the inner world of quarks and gluons. The standard theory of the force that binds them dictates that an energetic quark or gluon is about as likely to appear as any other energetic quark or gluon. These particles then split up into jets like the ones observed in CMS. By counting jets, the CMS scientists showed that energetic quarks and gluons do seem to be produced independently.

This measurement is also important for searches for new phenomena. Many theoretical particles are expected to disintegrate into W bosons, Z bosons and jets, so they would look just like the cases studied here. In fact, the CDF collaboration observed an unexpected excess of cases with a W boson and two jets. Without a precise understanding of how many jets to expect at the LHC, a similar excess would be inconclusive. This measurement is a substantial improvement in our understanding and puts us in a much better position as we push our searches into the unknown realm of discovery.

—Jim Pivarski

The physicists pictured above made important contributions to this high-precision analysis.
The physicists shown here are managers of the US-CMS Detector Operations Program.