As CERN Director General Rolf Heuer puts it
It is a tradition to collide ions over one month every year as part of our diverse research programme at the LHC. This year however is special as we reach a new energy and will explore matter at an even earlier stage of our universe.
Traditions form quickly it seems, as, compared to say eating chocolate or storing money for mysterious strangers, the LHC is a comparative newcomer on the Swiss scene. But indeed, on 25 November - last Wednesday - “stable beams” were declared in the Large Hadron Collider, and heavy ion collisions began again.
As I wrote when we first did this, bringing lead nuclei into collision, rather than the protons which the LHC usually smashes together, gives us access to a complementary kind of physics. The energy density is not as high, and we won’t be making any Higgs bosons, but the volume of hot dense stuff created is bigger, and it makes sense to think of it as a new form of quark-gluon matter, with a temperature and a density. Studying its “equation of state” - meaning how this matter develops as temperature, pressure and volume change - is important. As with the proton collisions previously, the energy has been increased, and this increases the volume and the temperature of the quark-gluon matter that we can study.
To be brutally honest, it is a little personally frustrating to pause the proton-proton collisions just when they were going so well, and spend time recommissioning the beams for heavy ions. But that’s just being selfish, it is important physics and deserves its turn. And anyway if it wasn’t for the neutrons in the lead, we would just be the Large Proton Collider, or LPC instead of LHC, which would bring an annoying confusion with the Legal Practice Course (although we would still not have as serious a naming issue as the neutron source at Harwell).
The strong interaction is the force carried by gluons, which binds quarks inside hadrons and dominates the interactions within the quark-gluon soup being created by the LHC right now. It is one of the three fundamental forces at play in the Standard Model of particle physics (with the fourth, gravity, sitting sulkily off to one side muttering something about geometry).
A couple of weeks ago I spent four days discussing the strange, wonderful and useful consequences of electromagnetism, another of those three Standard Model forces. Arguably almost the whole of everyday life - chemistry, biology, philosophy, technology - is a consequence of electromagnetism. Everything except mathematics and nuclear power stations, perhaps¹. On the face of it, the strong force is even richer than electromagnetism in its potential for new phenomena. Because it is so strong and operates at such short distances, studying it is, unfortunately, very challenging both for theory and experiment. But it seems a bit presumptious to think that it can’t possibly have any more surprises to reveal.
Pentaquarks were the most recent addition to the strong interaction scene. Even more intriguingly, there are proposals that odd, stable combinations of strongly-interacting matter may even be good candidates for “Dark Matter”. Dark Matter is the stuff that, from astrophysical observations, we know constitutes about 80% of the mass of the universe. The observations pin it down via its gravitational influence, but Dark Matter itself is invisible to conventional astronomy. Most physicists think that it must be a new kind of particle, not present in the Standard Model. But perhaps the Standard Model has more surprises in store for us, and can generate a Dark Matter candidate from the known quarks and gluons?
It is speculative stuff, and we’re unlikely to find Dark Matter before Christmas. But heavy ion collisions offer another window into the world of quarks and gluons and we will be peering through it intently.
Jon Butterworth’s book Smashing Physics is available as “Most Wanted Particle” in Canada & the US and was shortlisted for the Royal Society Winton Prize for Science Books.