If we want to continue to probe the structure of matter, to understand what the smallest constituents of nature are and how they interact, we have to think big and plan for the long term. Possibilities include machines that would dwarf the Large Hadron Collider, and neutrino beams crossing half a continent

Just over a year ago I was up a mountain, in fog and hail, at the South-Western tip of Sicily. Along with about fifty other delegates, I was discussing the future of particle physics. This was the Erice meeting where we drafted the update of the European Strategy for particle physics. Although the meeting was convened by the council of CERN, it concerned much more than the future of the laboratory in Geneva that currently runs the Large Hadron Collider - the 27km circumference accelerator where the Higgs boson was recently discovered.



The year 2012 saw not only the Higgs boson discovery, but also the measurement of a key parameter, θ₁₃, describing the way that neutrinos behave, and numerous other significant results. These results have a big impact of what we might do next in our exploration of fundamental physics. From days of argument in the cold, stone-floored rooms of Erice, four large high-priority projects emerged. There has been news recently on all of them, and here is an update.

The Large Hadron Collider



The obvious and unanimous top priority was to get all the great science we can out of the LHC. The machine is currently shut down for maintenance and enhancements so that it can, starting in 2015, safely collide beams at close to its design energy of 14 TeV. The fact that we already managed to discover the Higgs boson with beams colliding at 7 and 8 TeV is a great success, and also a bit lucky. With increased energy, we will learn more about this boson, and will be able to explore further into the unknown regions of physics above the “electroweak symmetry breaking” scale, which is where the Brout-Englert-Higgs mechanism generates the masses of the fundamental particles. Above this energy, the weak and electromagnetic forces are in a sense unified, and physics is rather different. And it may still be that surprises will show up. Supersymmetry is still a possibility, though if it doesn’t appear in the next run of the LHC, even some of its biggest fans will start to lose interest. Whatever it holds, the new data are eagerly awaited.



The current shut down is called “Long Shutdown One” (LS1), and we already wishing we hadn’t called it a “long” shutdown, as it seems far too short. The problem is not so much the work on the machine - this is very challenging and complex, as you can see from the “Long Shutdown 1 Dashboard”, a live chart showing progress. But you can also see from this that things are going well. The problem seems to be preparing the experiments for the new data while simultaneously publishing all the important results from the unique dataset we have already recorded from the first run. You might think 3000 physicists is plenty, but somehow we seem to be short of people almost everywhere. Good papers are still coming out, but we need to be faster.

Plans are already well-advanced for upgrades to the accelerator which will increase the data rates, and to the detectors to allow them to continue to cope with this. With these upgrades, the LHC can continue delivering amazing physics from the energy frontier for two more decades. The indicative schedule was given in Frédérick Bordry’s slides at last week’s meeting on Future Circular Colliders.

The proposed LHC running schedule beyond the first "Long Shutdown". Photograph: /from Frédérick Bordry's presentation at the Future Circular Colliders workshop, Feb 2014.

CERN after the LHC



That workshop last week at the University of Geneva directly addressed another of the main outcomes of the European Strategy. CERN should start thinking about its options after the LHC. This may seem a bit premature if the LHC could keep delivering physics until 2035, but the time scales are long. After a meeting at the Royal Academy of Engineers last week, Steve Myers passed me some of the first documents discussing what became the LHC. A nice quote

It should be clear from this requirement of “Ten Tesla Magnets” alone that such a project is not for the near future and that it should not be attempted before the technology is ready.



In short, they didn’t know how to build it, but they had to start doing research and development, or the technology might never exist. To build such machines, you don’t just have to exploit cutting-edge technology and engineering, you have to develop new technologies. These documents led to a meeting in 1984 much like the one that just took place. Twenty-six years later, the LHC began high-energy operation. These things take time.



The picture at the top of this article shows a possible route for 100km tunnel, encircling the town of Geneva and the Salève mountain, which, if filled with suitably powerful magnets, could collide protons at energies of 100 TeV or so.

A Linear Electron-Positron Collider



Now that we know where (in mass) the Higgs boson is, we can go right after it with leptons. Colliding electrons and positrons together at an energy just above 216 GeV will produce Higgs (mass 125 GeV) and Z (mass 91 GeV) bosons together at a reasonable rate. The fact that (unlike protons), electrons and positrons are fundamental means some things can be done much more precisely this way than at the LHC. For example, by measuring the Z boson alone and knowing the incoming energy, we can determine how many Higgs bosons are being produced, even if some of them decay invisibly, say to neutrinos or some exotic Dark Matter.



There is a serious proposal to build an electron positron collider in Japan which could make Higgs and Z bosons this way, and also go to higher energies if required. For example there are beautiful measurements one could make with 350 GeV of energy, where pairs of top quarks can be produced (mass 175 GeV each). Lots of physicists in Europe and the US would like to work on such an experiment, but the ball is really in the Japanese government’s park. Since the Erice meeting, they have selected a site for the experiment, and the project has been official adopted by the government, which has allocated a modest budget line to it. More will be required but these are all positive steps.



Neutrinos



Last week it was reported that the UK has decided to participate in the next Long-Baseline Neutrino Experiment (imaginatively titled LBNE) in the US. LBNE will use a neutrino beam provided by Fermilab in Chicago, and a detector in a place called Lead, in Sanford, South Dakota. The UK interest is a significant step. The world of particle physics is largely agreed that we need a new long-baseline experiment to explore the strange behaviour of neutrinos. There are three such projects currently under discussion: LBNE in the US, some ideas at CERN, and a project in Japan. The world of particle physics (and beyond, probably) is also largely of the opinion that we really can only afford one of these. Colleagues from UCL, as well as Cambridge, Lancaster, Liverpool, Manchester, Oxford, Sheffield, Sussex and Warwick and the Rutherford Appleton Laboratory, have signed up for LBNE. While things may be a little more complicated than that, with some UK universities preferring the Japanese project*, I agree with John Womersley (CEO of the STFC, the research council responsible for funding particle physics in the UK) as quoted in the BBC article:

The UK has shown its interest in the Fermilab initiative. What I hope is that other European participants will get involved. If it can go ahead, it will be an important step for the US and an important step for Europe for a global physics programme.

Fermilab already has a long-baseline neutrino beam, providing science for the NOvA and MINOS+ experiments, and this can be used for R&D in preparation for the next steps, including LBNE. One proposal also moving forward in the UK and US is called “CHIPS” (a rather contorted acronym for Cherenkov Detectors in Mine Pits) which could make important contributions toward the eventual precision of LBNE, using some of the neutrinos which Fermilab is currently “spewing into space”, as Jenny Thomas (a UCL colleague and one of the CHIPS leaders) puts it. CHIPS could start doing physics rather sooner than LBNE too, and as Jenny says, “Starting small can still yield important results”.



And more



Particle physics touches on astrophysics and nuclear physics, and requires precision theory and high-performance computing amongst many other things. Dark matter searches, experiments searching for rare particle decays, and relatively small-scale precision measurements also form an essential part of this exploration. However, the four projects above are of a size requiring global coordination. How much of this actually happens of course also depends upon the governments and taxpayers of the world being willing and able to continue to support and invest in the adventure. But with these recent developments, it seems we might be at least be approaching a rational and credibly-distributed global plan for continued exploration of the smallest structures and highest energies.

* Also (added 23/2/2014) it is worth clarifying several of those universities, and RAL, have signed up for the Japanese project as well as the US one.



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Jon Butterworth’s book, Smashing Physics, is out on 22 May. Order it now!





