Quarks, Gluons and Jets

An updated version of this post is now on The Guardian. See also the glossary section after Chapter 1.5 of Smashing Physics.

Yesterday we (ATLAS) released our first (preliminary) jet cross section measurement.

“Cross section” in this context is basically a probability. If you fire two footballs at each other, they have a bigger cross-sectional area than two snooker balls, so they are more likely to hit each other. A “jet cross section” is a measure of how likely we are to see jets when we fire two protons at each other.

An ATLAS event with two jets.

Jets are what quarks and gluons do when they try to escape. The proton is made up of quarks stuck together by gluons (the name gluon is itself, I guess, an early geekjoke). Most of the fundamental forces get weaker with distance – the Earth’s gravitational pull gets weaker the the further out into space you go, for example. But the strong nuclear force is the other way round.

The force between two quarks actually gets stronger as you pull them apart, more like an elastic band. When two quarks in LHC protons bounce off each other they head away really quickly, feeling almost no force at first (physics buzzwords: asymptotic freedom. See this Nobel Prize citation). But at some point that has to end, because as they get further and further from the protons they were knocked out of, the force pulling them back gets stronger and stronger.

You can think of the quarks as being the ends of the elastic band. They fly away from each other until at some point the band snaps and two new ends (new quarks) are produced. Eventually, we see a spray of hadrons (particles, like the proton, which contain quarks and generate amusing typos). Because the initial quarks get kicked so hard, this spray is collimated into a jet, and despite all the splitting and production of new quarks, the direction of the jet reflects pretty well the initial direction of the quark.


So, what you see in the graph reflects the distribution of quarks and gluons scattered in collisions at the LHC.

The jet cross section shown as a function of the jet momentum transverse to the LHC beams. The lower inset shows the ratio of the data to the theory. The orange band is the uncertainty on the theory, the blue shows the systematic uncertainty on the data.

When we collide protons, we really care most about about the collisions between the proton’s constituents – quarks or gluons. Unfortunately the quarks and gluons only carry a fraction of the energy of the proton, and we have no way of choosing how much. If the fraction was a half, for example, then we would have jets with 1750 GeV of energy (half of 3.5 TeV). But most of the quarks and gluons carry much smaller fractions.

At the moment, even though these are the most energetic proton collisions ever created in the lab, they are not quite the most energetic quark collisions. You’ll see the horizontal axis only goes up to 600 Giga-electron Volts. This is an energy where the Tevatron has already seen some jets.

As we get more data it is already going higher, and we are in really new territory, colliding quarks at higher energies than ever before. This is the zone where we might see new particles or forces come into play. There are already searches from ATLAS looking for these.

To have a real measurement of this, and show that the theory prediction  (Quantum Chromodynamics, labelled QCD on the plot) agrees with the data, is a real achievement. This result, like the minimum bias results, is part of finding our feet in the new energy regime of the LHC – but these collisions are much closer to where we want to be. And we already have ten times more data to play with than is shown here – thank you LHC!

These and lots of other great results from ATLAS, as well as other experiments at the LHC and elsewhere, will be shown in ICHEP next week. I’ll also be writing for the Guardian Science blog there, as well as filming the final Colliding Particles episode with Mike, Adam and Gavin. Don’t miss…



About Jon Butterworth

UCL Physics prof, works on LHC, writes (books, Guardian Science and elsewhere). Citizen of England, UK, Europe & Nowhere, apparently.
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6 Responses to Quarks, Gluons and Jets

  1. lily asquith says:

    On the plots it says NLO “Next to leading order” and I was wondering, if LO “leading order” is like a drawing of a stick man and “all orders” is like a photograph of a real person, would NLO have arms coming out if its face? I understand this is a barmy question, but I’m becoming less and less confident with this stufff to NLO and I thought you might know what I mean.

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