The picture at the top shows what happened in the CMS particle detector when xenon nuclei were circulated in the LHC and brought into head-on collision. The yellow is made up of tracks of electrically-charged particles, produced in such numbers that the whole of the centre of the picture is a yellow blur, with individual tracks only visible near the edges. The blue and green blocks indicate energy deposited by both charged and neutral particles in the CMS calorimeter.
Collisions between protons look significantly less busy than this, with fewer particles produced. But both xenon and lead nuclei are packed with protons and neutrons, and though lead has more of them, by eye I don’t think anyone could tell the difference between a xenon-xenon collision and a lead-lead one.
There are however expected to be differences in detail, on average, in the shape and properties of the exotic ball of material produced in the heart of the collisions.
Before it flies apart, this material is a plasma of quarks and gluons, the basic constituents of all nuclear material. Measuring the differences between lead collisions and xenon collisions may teach us more about this strange stuff. If nothing else, it should allow us to make some “control” measurements, a good way of reducing systematic uncertainties. And measuring something new could always throw up a surprise. Time will tell, as the recorded data are carefully analysed.
Xenon itself was a “target of opportunity” for the LHC. The noble gas is being injected into the pre-accelerators of the LHC for the benefit of the NA61 experiment, also known as SHINE, the SPS Heavy Ion and Neutrino Experiment (a decent enough acronym by particle physics standards). It was decided to retune the LHC so that, for one day only, xenon could make it all the way into the ALICE, ATLAS, CMS and LHCb experiments too¹.
One of the things SHINE is doing is measuring different nuclear collisions in an attempt to scope out the threshold for actually producing a quark-gluon plasma. To make a plasma, you need a lot of protons and neutrons with a lot of energy each. NA61 aims to find out exactly how many and how much.
In a coincidental aside, a xenon nucleus contains 54 protons, and about 77 neutrons on average, which makes its total mass quite close to the mass of the Higgs boson. This coincidence has caused confusion in some circles and I would not be surprised if some theories related to this were to show up in comments below the line.
There should be no confusion really. According to the Standard Model, the Higgs boson is infinitely small. We can’t measure “infinitely small”, of course, but from the way it behaves, we can set some kind of upper limit on the physical size of a Higgs. This tells us that if the xenon nucleus were the size of a beachball, the Higgs boson would smaller than the finest grain of sand.
This would go some way toward explaining why the Higgs boson was not discovered until 2012, whereas xenon was discovered in 1898. Another reason would be that the Higgs boson rapidly and spontaneously decays into other particles, whilst xenon is stable.
Unless, of course, you treat it like we did on Friday in the LHC.
¹ Add 16/10/17, I’d missed LHCb from this list initially, apologies to colleagues there, they also got rather nice data. Also I’d loosely said “retune the magnets” when in fact it is only the radio-frequency cavities which need to change, not the bending magnets, otherwise it would be a much bigger operation. See this piece Symmetry Magazine for more details.
Jon Butterworth’s latest book A Map of the Invisible: Journeys into Particle Physics was published on 5 Oct 2017 by Penguin.
