What next at the Large Hadron Collider?
It is rare for an academic discipline to double the observable landscape that it’s able to study, but that’s what’s currently happening in particle physics. Over the past two years, around 400 engineers and technicians have together put in about a million hours to prepare the Large Hadron Collider (LHC) to produce collisions between protons at almost twice the energy previously attained. In parallel, physicists have been upgrading their detectors, electronics and software frameworks to handle the increased future demands. The LHC was scheduled to fully circulate beams about two weeks ago, but an intermittent short circuit in one of the machine’s magnet circuits caused a postponement. Apart from being mildly frustrating for all involved, this served to illustrate the required level of care in safely recommissioning the world’s largest machine and the sheer volume of components which have to work simultaneously. The good news is that the required delay was only a short one and the long-awaited ‘Run 2’ has now commenced.
So far the LHC has delivered about 1% of its total planned collisions. By the end of Run 2 (around 2018), we’ll still only have about 3%, but crucially, the new data will be very close to the design energy. Physicists often talk about the ‘Energy Frontier’, by which we mean the maximum amount of energy that we can cram into a single collision between sub-atomic particles. Following Einstein’s famous equation E=mc2, that corresponds more-or-less directly to the maximum mass of a previously unknown particle that could be produced in the collisions. The increased beam energies mean that, for the second time in five years, the LHC is giving us sensitivity to possible new particles in a large, previously unexplored mass range. It would be entirely wrong to think that this sort of expansion into virgin territory is routine. Since I started work in particle physics 23 years ago, it has happened twice, one of which was the start of the first LHC run. Barring a complete revolution in accelerator technology, it will happen at most once more in the next 23 years and almost certainly not at CERN, since the LHC programme stretches to around 2035 in current schedules.
In the Higgs boson, the LHC has already led to the discovery of a new particle so fundamentally different from anything we previously knew about that we need to introduce a third basic category of ‘stuff’, quite distinct from matter particles and from the forces that act between them. That discovery has quickly given way to characterisation and we’re now deep into asking questions about the Higgs boson using the samples already delivered by the LHC: is it produced at the rate we predict? Does it decay in the ways we predict? Is there more than one? To give just one example of the rate of progress, we now know the Higgs mass to a precision of about 1 part in 400. A major task for the LHC Run 2 and beyond will be to continue asking such questions about the Higgs boson, through studies at ever increasing precision.
Despite being a triumph for experiment, theory and engineering, not to mention large-scale multi-national collaboration, the arrival of the Higgs boson was widely anticipated. According to my rather unscientific straw polls at the time, around half of the physicists involved expected to find the Higgs (I confess I was not among them). This time, it’s different; there is no consensus view on what LHC Run 2 will reveal, but rather a wide range of ideas, many of which are far stranger even than the Higgs mechanism. One thing we do know for sure is that our so-called Standard Model of particle physics is not a complete description of the Universe. There are more problems remaining in fundamental physics than there is room to include here, but perhaps the most glaring is our complete failure to understand about 95% of the content of the Universe. For several good reasons, astronomers and cosmologists are sure that there's around five times more ‘dark matter’ out there than there is ordinary matter of types that can be explained by the particles of the Standard Model, not to mention the even larger and more mysterious ‘dark energy’ component. Producing new heavy dark matter particles in the LHC would be an astonishing linking together of problems and solutions in diverse areas of fundamental science. This topic was nicely tackled in a recent BBC Horizon programme, Dancing in the Dark – The End of Physics? narrated by comedian David Mitchell. In that programme, ‘Dave the atom smasher from Birmingham’ (aka ATLAS spokesperson, Professor Dave Charlton) explained the possibility of discovering dark matter by not seeing it – ie, by inferring its presence in the debris of LHC collisions from an imbalance among the outgoing particles, implying that something has slipped away unseen.
Exhilarating though they may be, such thoughts of fresh discoveries are premature. A lot more hard work will be required before we even get to analyse the collisions that might contain the first hints of new massive particles. Along with our UK and international colleagues, Birmingham physicists will be scrutinising every detail of the new high energy data, as well as developing the ultra-radiation hard detectors that will later be required to collect the other 97% of the LHC collisions. The simple fact this time is that we don’t know what to expect, but we do know we’re privileged to be looking in places that nobody has looked before and we do know we have to explore every possibility... because we can.
Professor Paul Newman
Head of Particle Physics, School of Physics and Astronomy, University of Birmingham