Physicists Plan to Build a Bigger LHC 263
ananyo writes "When Europe's Large Hadron Collider (LHC) started up in 2008, particle physicists would not have dreamt of asking for something bigger until they got their US$5-billion machine to work. But with the 2012 discovery of the Higgs boson, the LHC has fulfilled its original promise — and physicists are beginning to get excited about designing a machine that might one day succeed it: the Very Large Hadron Collider. The giant machine would dwarf all of its predecessors (see 'Lord of the rings'). It would collide protons at energies around 100 TeV, compared with the planned 14TeV of the LHC at CERN, Europe's particle-physics lab near Geneva in Switzerland. And it would require a tunnel 80-100 kilometres around, compared with the LHC's 27-km circumference. For the past decade or so, there has been little research money available worldwide to develop the concept. But this summer, at the Snowmass meeting in Minneapolis, Minnesota — where hundreds of particle physicists assembled to dream up machines for their field's long-term future — the VLHC concept stood out as a favorite."
Re:Dallas? (Score:5, Informative)
This is true, but no so simple: in a straight line, you gain energy with the distance. When going round, you lose energy to stay in the loop as a function of the radius (the infinite radius case brings you back to the straight line). Thus, each time you want more energy, your collider ring needs to have considerably larger radius (following a third power law). At some point (basically the point after this proposal) you have to loop around the solar system :)
Re:You need another one? So soon? (Score:4, Informative)
Re:Dallas? (Score:5, Informative)
There's a tradeoff in circular/linear accelerators. Linear accelerators let you collide leptons (usually electrons) efficiently and leptons provide a MUCH cleaner signal. A comparable energy circular accelerator can be shorter, but due to bremsstrahlung losses, you have to collide hadrons (like protons), which provides a much messier signal.
After you do some rough calculations of what particles you can collide, their energies and the number of interactions per second, you then take those numbers and plug them into a model of a hypothetical detector along with a number of theories you'd like to explore to see which configuration gives you the biggest "bang for your buck"
The issue is that different people are more interested in probing different kinds of physics and it's impossible to make a detector/accelerator that's sensitive enough to fully probe everything, so big arguments happen at places like Snowmass. We know that we basically can only ask for one multi-billion dollar accelerator, so everyone's fighting to keep their pet research alive.
Re:WHY NOT IN THE FIRST PLACE !! (Score:5, Informative)
Well, many of these tunnels, including the one the LHC uses, have been refurbished multiple times already. Cern's main ring was built to be somewhat future-proof, but that was a long time ago. A google search came up with The history of CERN [web.cern.ch], which dates the groundbreaking to 1954.
In accelerators you have two basic designs: linear and circular(ish). In linear accelerators each boosting element (RF cavity or whatnot) gets one chance to give the beam particles a kick, so the energy is limited to how hard you kick (limited by technology) and how many elements / how long (limited by budget).
In circular accelerators you are limited by synchrotron radiation. At some point the energy pumped into the beam matches the energy lost via synchrotron radiation. To move in a circle you have to accelerate inwardly, and an accelerating charged particle radiates light. At particle accelerator energies, this radiation is in the x-ray spectrum. You can reduce the loss by using a larger ring -- a smaller curvature requires less centripetal acceleration and hence less radiation loss. You can also of course build stronger boosting elements, but the radiation also heats the beamline and surrounding superconducting magnets, so it's not "that simple."
The other thing to vary is the kind of particle accelerated. Electrons have a very small mass and lose a larger fraction of their momentum to synchrotron radiation. SLAC and KEK are linear accelerators that use electrons. (Cornell's CESR is a ring that accelerates electrons too, but at lower energies compared to these others.) Protons are the other obvious choice, which is what Fermilab and CERN's LHC (after the upgrade) are accelerating. Being much more massive, the protons slough off less of their momentum to synchrotron radiation and can be accelerated to higher energies given the same size ring. The disadvantage of protons is that the energy of the proton is shared among its three quarks (and gluons I think) whereas the electron is truly singular as far as can be told.
I've been out of touch lately but as of at least 8 years ago three proposals were being discussed: VLHC -- big ring accelerating protons. Next Linear Collider (NLC) -- long linear accelerator for electrons. Muon collider -- a smaller ring (actually with straight sections like a track&field track) that produces and accelerates muons. Muons are just like electrons only 200 times more massive and is unstable with a half-life of 2 microseconds. The muon collider was thought to be an ideal Higgs factory, but with a lot of design challenges. One of the main challenges is to not only accelerate the muons before they decay, but also collimate, or "cool", the beam very fast as well so that you can create as many head-on collisions as possible.
So the news that the VLHC design is currently in favor is interesting, but this is hardly the first time the issue has been discussed and I doubt it will be the last. Several years ago the NLC design seemed most favorable, but this would, by its length, be limited to a specific design energy and probably be built to produce Higgs, Higgs, and more Higgs. It seems to me like a VLHC would have more discovery potential for more massive Higgs particles, signs of supersymmetry, or whatever else might exist.
Re:helium? (Score:2, Informative)
The helium at LHC is liquid helium used to cool the superconducting magnets, not to fill the tunnel.
But maybe by the time this is built we'll have room-temperature (or nearly so) superconductors that can sustain that kind of magnetic field. (AFAIK the LN2-cooled ceramic superconductors can't.)
Re:Question... (Score:4, Informative)
You hit numerical problems if you calculate it that way. Wikipedia [wikipedia.org] gives a series expansion that works well for large values of gamma:
v (in units of c) = 1 - 1/2 \gamma^(-2)
v = c (1 - 1.8e-10), or 0.99999999982 c
Re:Dallas? (Score:5, Informative)
Muon colliders are a great concept - but they are difficult REALLY difficult. There is a significant ongoing effort to work on the technologies but they are far from ready now.
Personally I love the idea of high gradient RF cavities fabricated from Beryllium, filled with high pressure hydrogen, with megawatt high energy muon beams. There are however some possible....failure modes. Then there are the problems with neutrino radiation (I'm not kidding - it can exceed allowable dose limits).
A potentially more serious issue is that while the muon collisions themselves are very clean, the decaying muons create a huge amount of background noise in the detectors.
I think its a great project and work should continue - but like laser acceleration we can't build a machine like this yet.