Future Particle Accelerators

Tomorrow’s plenary sessions at the ICHEP conference are keenly awaited by particle physicists because of the rumoured possibility of some new physics results, but there is another reason for them to get excited. There will also be a lot of discussion about the future of particle accelerators.

The day will open with a talk by Steve Meyers about the progress so far with the commissioning of the LHC. He will also talk about the long-term future in which we could see the LHC upgraded from its current design limit of 14 TeV energy to 31 TeV. There will also be possible improvements in luminosity that could see it producing 200 inverse femtobarns of data each year. Although that talk is tomorrow the slides are already up so you can get an idea of what he is going to say already.  The slide below gives an overview of the plans taken from yesterdays talk “LHC Machine Upgrades” by Roger Bailey.

After the talk by Meyers there will be a some reports of highlights so far from the LHC experiments. That will be followed by an address from Monsieur le Président de la République AKA Nicolaus Sarkozy and then there will be a press conference over the lunchtime break. Hopefully all this will be included in the webcast. It all seems like a lot of fuss to make over a physics conference so we hope this means that there will be some political announcement concerning the future of particle accelerators.

After some more interesting physics talks in the afternoon that will include the much-anticipated “Higgs Searches at the Tevatron” by Ben Kilminster, the day will end with a couple of reports that includes one from the International Committee for Future Accelerators. Sadly they have not yet posted their slides so we can only guess what they have in store for us. 

Luckily there were some talks yesterday about future colliders so we can look at some of those:

The ILC (International Linear Collider) is the most talked about possibility for the next generation of collider. It will be an electron-positron collider like LEP, but unlike LEP it will be a linear design rather than a ring.

The advantage of lepton colliders over hadron colliders is that leptons have no known structure so they give much cleaner results. A high energy electron positron collider would allow detailed measurements to be made of particles discovered at the LHC. The disadvantage is that it is hard to accelerate electrons up to the same energies as protons. because they are much lighter they need to be pushed to higher speeds so that the relativistic gamma factor generates more energy, but at higher velocity they radiate away energy much faster when they are circulated in a circle. For this reason a linear design is mor effective.

In fact the design of a linear collider can incorporate circular and linear features in combination. One possible design for the ILC is like this,

The nominal design now under consideration will provide 500 GeV of centre-of-mass energy. That may seem a bit feeble compared to the energy of the Large Hadron Collider which is 14 times as much, but because leptons are not seen as composite at these energies, more of the energy will go into forming new particles than is normally possible with composite hadrons. Nevertheless, if they pick the ILC design now and then the LHC finds a new 600 GeV particle later this year, it will be a little frustrating.

The alternative is to adopt a more ambitious design for a linear collider known as CLIC (Compact LInear Collider)  which would use normal conducting cavities rather than superconducting cavities. This could allow it to reach energies of 3 TeV. The main issue with CLIC is that its feasibility has not yet been demonstrated. If they are going to make a definitive decision now, it probably wont be CLIC.

There are other alternatives such as the LHeC project which is a compromise that fires electrons at protons. This has some advantages of its own but it does not appear to be a leading contender as yet.

An even more ambitious project that has been studied is a muon collider. Muons are leptons so they have all the advantages of electrons over protons in collision, while also being heavy. The trouble is that muons are unstable with lifetimes of  two microseconds. At relativistic speeds this would be dilated but only to seconds at best. Given that it takes an hour or so to ramp up the energy of protons in the LHC this option does not seem very good. At best a very small number of collisions could be achieved. We will have to hope for the discovery of a more stable charged heavy particle if we want such a radical new approach (and who knows?)

The conclusion is straight forward. The only presently viable option for the next generation of colliders is the ILC. Tomorrow physicists may know more details about what their government sponsors have planned for them.


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