The European Physical Society High Energy Physics conference is taking place now in Vienna. This is the first big chance for the experiments at the Large Hadron Collider (LHC) to show off what they have managed to extract from the new data they have recorded since 3 June, when the LHC restarted particle collisions after a two-year break.
The new collisions are at a higher energy – 13 TeV¹ compared to the previous record of 8 TeV. Since we are bumping up against the speed-of-light barrier, this means the speed of the protons increases from 299 792 449 metres per second to 299 792 454 m/s (the speed of light is 299 792 458 m/s). An increase of only 5 m/s, which doesn’t sound terribly important. But speed is the wrong way to judge the significance of the increase. The main point of high energies in particle colliders is that they allow us to see into the heart of atoms and study the structure of matter at tiny distance scales; in a way the LHC is like a giant microscope. Turning up the energy is like turning up the power of the microscope, and we are eager to see what that might reveal.
The ‘reveal’ doesn’t all happen at once of course. As well as the energy, the amount of data is also important, and so far we only have a tiny fraction of what the LHC will deliver.
One of the first things to do is simply count the particles produced when protons collide at these new energies. These so-called “minimum bias” measurements set the stage on which all other measurements are made. They tell us what a typical collision looks like, before we start trying to select rarer and more interesting varieties.
Both ATLAS and CMS have produced results on the number of particles produced in 13 TeV collisions. The ATLAS result follows the template of our earlier results at lower energies, and shows how well (or how badly) various theoretical models predict the energy dependence of the particle mulitiplicity. The CMS result is a bit different. It takes advantage of a data-taking period during which their solenoidal magnet (the “S” in CMS) was off.
Having no magnet is a bad thing in general, because charged particles bend in the magnetic field, and from that bending, their momentum can be measured. No magnetic field, no momentum measurement. However, with the magnet on, very low momentum particles get bent so much that their paths curl up inside the LHC beam-pipe, and they never make it to the detector, and so never get detected at all.
With the magnet off, those tracks can be detected. We don’t know their momentum, but CMS have measured how many of them there are and what direction they are going in. All useful information for constraining the theory models, and understanding the environment in which the rest of LHC physics will be done.
Another ‘counting particles’ type of measurement is the so-called ‘ridge’, first measured by CMS very early in 7 TeV data-taking, back 2010. This involves saving a lot of events which are not ‘average’, but in which an unusually large number of particles have been produced. Then you look at the correlations between pairs of those particles.
The CMS measurement first showed a surprising ‘ridge’, indicating an increased probability of emitting particles at a similar azimuthal angle (perpendicual to the beams) even when the angle along the beam (the rapidity) was very different. This kind of correlation has been seen as evidence that a sort of plasma, or liquid, of quarks and gluons has been formed, and are undgoing some kind of “collective flow”… essentially following each other around. Such an effect is expected (and seen) in heavy ion collisions, but very unexpected in proton-proton collisions. ATLAS confirms the CMS result (and now at higher energy), and in the meantime a wide variety of alternate theoretical explanations have arisen.
Jet of hadrons are produced when quarks and gluons smash into each other at short distances. The shorter the distance, the higher the energy of the jet. In fact measurements of jets give us or first glimpse of this really short distance physics, the reason we went for higher energies in the first place.
The first ATLAS jet measurement at 13 TeV was released for this conference; so far it only covers the range of jet energies that we’ve covered before, but it shows that, as expected, these jets are produced much more frequently in 13 TeV collisions that at 7 or 8 TeV, and illustrates the potential of these measurements as we get more data.
There are more results coming, and things are happening so quickly that I have probably missed some good ones here already. ATLAS is in a frenzy of reviewing and approving preliminary results at the moment and I would assume that CMS, LHCb and ALICE are in a similar state. Apologies to colleagues, especially on those other experiments, if I have missed your favourite highlight – it wasn’t deliberate and it may well feature on these pages soon anyway.
In short, there’s a lot going on at the moment. And the best is definitely still to come, with updates on the top quark, W, Z and Higgs boson production and (my current favourite) the question as to whether this bump will be seen again in the new data or not?