A day of xenon collisions at CERN

On Friday, the Large Hadron Collider at CERN had a day of smashing xenon nuclei together, a departure from its usual diet of protons or lead

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.


Jon Butterworth

The GuardianTramp

Related Content

Article image
Anomalous bottoms at Cern and the case for a new collider
Particles known as “bottom mesons” are not decaying in the way the Standard Model of particle physics says they should, and it’s causing some excitement

Ben Allanach and Tevong You

01, Nov, 2017 @4:36 PM

Article image
And we're off! CERN declares start of 2016 LHC physics season
Weasels be damned, we’re running again! And first thing on the list is to find out whether ‘those bumps’ are new subatomic particles, or just statistical noise

Jon Butterworth

09, May, 2016 @4:34 PM

Article image
Europe chooses its 'roadmap' for science facilities
Stop what you’re doing! ESFRI has spoken. Including probably the best list of major science acronyms on the planet

Jon Butterworth

08, May, 2016 @10:08 AM

Article image
Two quarks for Muster Higgs
Since the big discovery of 2012, the Large Hadron Collider at CERN has been accumulating data and making steady progress. Two recent results establish the origins of the mass of the two heaviest quarks

Jon Butterworth

23, Jul, 2018 @6:30 AM

Article image
Life, Physics and Everything
When the Guardian’s science blog network closes, Life & Physics will have been here for eight years. Physics has come a long way in that time, but there is (as always) more to be done...

Jon Butterworth

13, Aug, 2018 @6:34 AM

Article image
Newly discovered particles, and what's in them
Quarks, basically. But more charming than usual

Jon Butterworth

20, Aug, 2017 @7:30 AM

Article image
Experiment reveals evidence for a previously unseen behaviour of light
Beams of light do not, generally speaking, bounce off each other like snooker balls. But at the high energies in the Large Hadron Collider at CERN they have just been observed doing exactly that

Jon Butterworth

03, Sep, 2017 @12:56 PM

Article image
From gravity to the Higgs we're still waiting for new physics
Annual physics jamboree Rencontres de Moriond has a history of revealing exciting results from colliders, and this year new theories and evidence abound

Ben Allanach

24, Mar, 2017 @1:11 PM

Article image
Is the Standard Model isolated?
The Large Hadron Collider at CERN revealed the Higgs boson in 2012, but has led to no comparable discovery since. It is worth asking what we hope to learn from the new data coming soon – and indeed from any particle physics experiment in the near future

Jon Butterworth

12, Mar, 2017 @10:56 AM

Article image
How to hide a 'fifth force' – and how to find one
Several big problems with physics at the moment involve gravity. But because Einstein’s theory works so well, it’s very difficult to change it. Some recent ideas show a possible way forward

Jon Butterworth

04, Dec, 2016 @11:20 AM