LHC@home

As well as the regular monthly edition, there is a Physics special edition of the Scientific American currently on sale in England, & I guess in other countries too.

It gives a lot of info about frontiers in Physics, and the anticipated switch on of the LHC in 2007 is mentioned more than once with explanations of the physics that might be found by this machine. (Lots of other experiments are mentioned too, of course).

Higgs boson, what is beyond the Standard Model, the Supersymettric Standard Model, possible relativity violations, etc etc - a good write up of the whole area all in English rather than maths and all pretty much as accurate as you can get without the maths.

River~~

PS - I hope this doesn't get moderated as commercial spam - I am not connected with the magazine except as a long term customer!

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You forgot to mention in your list of breakthrough experiments in the LHC one of the most exciting tests the LHC is supposed to make, namely the experimental prove of hidden dimensions.

Lisa Randall in her book "Warped Passages" (highly recommended reading also for laypeople without serious math training) she is just dying to see the LHC fired up to test her ideas whether hidden dimensions are responsible for the weakness of gravity in our universe.

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You forgot to mention in your list of breakthrough experiments in the LHC one of the most exciting tests the LHC is supposed to make, namely the experimental prove of hidden dimensions...

Hidden dimensions are described in the SciAm special edition, but (unless I missed that point) are not tied specifically to the LHC in the magazine. You are quite right, the LHC is hoping to be the first to demonstrate how many extra dimensions there are - hoping to beat the cosmologists to the experimental proof of their own theory...

River~~

I happen to read this article as well and thought that it was interesting that Linear accelerators have the potential to be more powerful than cyclical ones because the particles going around a curve above a certain speed give off synchronic radiation slowing them down. Why would CERN build a cyclical one then? Isn't power what they are after?

You forgot to mention in your list of breakthrough experiments in the LHC one of the most exciting tests the LHC is supposed to make, namely the experimental prove of hidden dimensions...

Hidden dimensions are described in the SciAm special edition, but (unless I missed that point) are not tied specifically to the LHC in the magazine. You are quite right, the LHC is hoping to be the first to demonstrate how many extra dimensions there are - hoping to beat the cosmologists to the experimental proof of their own theory...

River~~

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I happen to read this article as well and thought that it was interesting that Linear accelerators have the potential to be more powerful than cyclical ones because the particles going around a curve above a certain speed give off synchronic radiation slowing them down. Why would CERN build a cyclical one then? Isn't power what they are after?

Power is what they are after.

Both the round accelrators and the liner ones accelerate the particles once they are in the machine, by making the particles 'surf' on an electromagnetic wave that pushes them always a tad faster then they are already going.

A round machine has the disadvantage you mention - but has the advantage that the particles can take more than one pass through the machine to build up to their max energy. I don't know what the parameters are for the LHC, but if the particles go round 100 times between injection and collision, that is 100 x pi x diameter. The acceleration is achieved by making the fields in the machine move round faster as the particles accelerate. I'd guess that some of what we have been crunching is simulating the acceleration fields, and some the (more difficult job) of simulating the bending fields that keep the particles in the ring, and some simulating the coordination between the two sets of fields.

A linac (liner accelrator) is more simple. The containment fields are very simple - they only need to keep the particles moving in a straight line, and apart from the interactions between the particles they tend to do that anyway.

The particles are always moving with the same energy at each specific place in the machine - it is injected at one end with (say) 1 Gev, at this point just here it has reached 2GeV, and up there it has reached 3GeV, etc. Each magnet is quite simple, as it only has to be designed for that one energy.

Despite all those advantages, the big disadvantage of the linac is that each magnet or electrode is only passed once. There is only just so much of a push that the particle can be given as it flies through at close to c, so you need an awful lot of magnets / electrodes and you need an awful lot of real estate to fit them all in a straight line.

Your infrastructure is cheaper per unit length than the round accelarator, and you don't need to make the particles go so far because they don't have the sycnhotron losses. You don't need 100 x pi x D to compete with a round accelerator of diameter D, but you certainly need more than pi x D.

If you fire into a stationary target, you lose more than half the energy in recoil. It is more efficient to fire at moving particles coming the other way. For that, you need to build yourself a second linac at almost (but not quite) 180 deg to the first. Hey, the cost just doubled!

For a round machine, if you run postive particles in one direction and negative particles in the other, the two sets of particles need only one set of major bending magnets most of the way round the beam line, though there will need to be some independent tuning so there will some magnets that are designed to affect the two lines differently.

By playing well thought out games with the positions of the injector and the distances to the crossing points, you can use geometry rather then electronics to synchronise the pulse arrivals -- at least to first order, there will no doubt again be some fine tuning issues.

At the end of the day, as with any engineering job, there is more than one viable solution. Which solution is best in all the circumstances depends on which way the trade-offs go. The ring size grows with energy due to the limits set by the sycnotron radiation. There is only so much energy you can add with each electrode, so the number of electrodes in a linac, and hence total machine length increase with the designed energy. So both geometries scale at about the same rate with design energy, and the same trade offs have occurred time and time again at each scale.

In general, if you want to run a collider, the ring has usually been the cheaper option. If you want to run into a stationary target (which has the advantage that there can be kilogrammes of target to hit) then a linac tends to have the edge. Either way it is ultimately an economic decision - with unlimited budget either geometry could be made to do either job.

And finally labs tend to do what they are already good at. CERN is good at rings, it has years of expertise on site, more than once in the past CERN has had the best ring in the world. They can continue to bulid on their existing learning curve. Other labs have made themselves centres of excellence at building linacs. It makes sense for both sets of labs to continue developing what they are already good at.

River~~