Multiparton interactions at the Large Hadron Collider
Away from the high-profile Higgs hunting, a new paper sheds some light on the complex inner life of the proton, and how it affects results from CERN’s LHC
For most of the past two years, before it stopped last week for a while, the LHC was making protons collide head on with each other. The protons are made of of quarks and gluons, and most of the physics results we publish discussing each proton-proton collision as though it were a collision between a pair of these constituents. The rest of the proton – all those other quarks and gluons – is essentially just a nuisance.
However, a recent paper does something different. It measures what seem to be multiple quark and gluon collisions in the same proton-proton collisions.
This phenomena is known as “multi-parton interaction”. A “parton” is a part of the proton, as the name suggests. The term was introduced by Richard Feynman in 1969 to describe small pointlike bits (ok, parts) of very fast moving protons. Gell-Mann and Zweig had already proposed that hadrons were made of objects which Gell-Mann called quarks; but whether these were just mathematical tools to describe the symmetries of hadrons, or whether they had any more substantial reality was not clear.
“Substantial reality” is probably a trigger phrase for some philosophical types, so I should say immediately that what I mean by it is; could you see quarks by smashing things into them. This is the particle physics way. Indeed, Feynman’s partons were inspired by an experiment, at the Stanford Linear Accelerator Center, which did just that. Scattering electrons off protons showed strong evidence that there were actual partons inside. Subsequent work by Bjorken and Paschos showed that these partons shared many properties with quarks. Eventually, as people worked out the theory of the strong nuclear interaction – which binds protons, neutrons and atomic nuclei together – the conclusion that Feynman’s partons were nothing other than the quarks of Gell-Mann and Zweig plus the gluons which stick them together became inescapable*.
What this means that the distribution of partons inside the proton has to be understood at some level if you want to do physics at the LHC. Mostly we do this by using things called “Parton Density Functions”, which we know from data elsewhere (from the Stanford experiment and others, but most precisely from the more recent HERA collider). These functions describe the probability of finding a parton with a given momentum inside a proton. Most calculations for LHC processes would take one parton with some probability like this, and ignore the rest of them.
In this picture you can think of the LHC as a parton collider. All the correlations arising from the fact that the partons are bound up inside protons are ignored. But the proton is complex and important. In fact, most of the mass of the proton, and thus most of the visible mass of the universe, comes not from the Higgs but from the binding between partons, which is completely neglected in this picture.
This paper takes a step back from that approximation, and as I said, explicitly tries to measure some of the correlations between pairs of partons – multiparton interactions.
This is done by looking for events in which two things happen; the production of a W boson, and the production of a pair of hadronic jets. The idea is that, sometimes at least, the W is produced by one pair of partons, and the jets are the result of a collision between a different pair – but both happen in the same proton-proton collision.
One of the key distributions is this:
The vertical axis is the number of measured proton-proton events. Along the horizontal axis, the quantity “Delta jets” is the difference in the magnitude of the momenta of the two jets. If the jets have equal and opposite momenta, “Delta jets” is zero. This is because in general, a jet pair which comes from a parton-parton collision which did not involve a W boson would be balanced (since the incoming pair of partons have no momentum transverse to the beams). That’s why the blue histogram, which is a sample of events with only a pair of jets in them, is concentrated down at low values. In events which also produced a W boson, then the pair of jets can recoil against the W, they do not have to balance each other, and the distribution is much broader.
Anyhow, the fact that the resulting fit agrees better with the data than does the dotted histogram implies that their are “extra” pairs of jets in these events. The extra pairs are concentrated at low values of the “Delta jet” variable, and so most likely come from an additional parton-parton scatter taking place in the proton-proton scatter in which the W boson was produced.
There has already been plenty of evidence that multiparton interactions exist but this is the first time they have been measured at the LHC and one of the clearest observations of them anywhere to date. While it does not have the “eureka” feel of discovering a new boson, this paper may turn out to be one of the more influential ones from the LHC, eventually.
A rather mundane reason for this is that if two pairs of partons collide in a single proton-proton collision, this can lead to some very peculiar-looking events. Two “standard” collisions can gang up and look very non-standard, possibly leading you to thing you have found some physics beyond the Standard Model. (Probably supersymmetry.) So if you want to claim this, you need some understanding of the probability of multiparton interactions occurring in a single proton-proton collision.
But on top of that, the issue of how a strongly interacting quantum field theory gives rise to the proton, an ultra-stable bound state with a lifetime longer than the age of the universe, present in the heart of every atom of matter, is an exciting open question. It’s a problem being tackled from several experimental, theoretical and computational directions. I think understanding correlations between partons in high energy collisions will make an important contribution to this.
* There’s a nice account of this by two of the people who won the Nobel prize for it in this recent CERN courier.