Tuesday, 11 September 2012


Way back at the beginning of August, I spotted a paper proposing a new experiment: LEP 3.  This struck me as somewhat amusing; you see, LEP—the Large Electron Positron collider—was the predecessor to the LHC.  When LEP finished its second run, it was dismantled and the LHC built in its tunnel.1  But the idea is serious, from the people at CERN no less.

To understand why, we need to consider the fundamental difference of the LHC and LEP.  As the name states, LEP collided electrons and positrons (anti-electrons).  The LHC collides protons.  There are two relevant differences: protons are much heavier, but are composite objects made up of quarks and gluons, while we believe electrons to be fundamental.  These have the consequence that the LHC (and similar proton machines) can reach higher energies, but with less precision.

The energy limit is a little non-trivial.  The LHC, like most particle accelerators for several decades, is a ring.  This lets the particles in the machine pass through the accelerating elements many times over.  The energy limit is then given by how much energy is lost going around the loop.  Electrons and protons are electrically charged; that's how we accelerate them in the first place.  But it is a fundamental consequence of Maxwell's laws that accelerating charges emit electromagnetic radiation; and to a physicist, changing direction is a type of acceleration.  The rate of energy loss is proportional to the acceleration, and in turn to the speed.  So the same machine can accelerate electrons and protons to (roughly) the same speed.  The energy depends both on the speed and the mass, however, so the the heavier proton will have more energy.

The sub-components of the proton are problematic, because it means that we do not know the exact initial state of each collision.  In particular, high energy collisions are overwhelmingly between quarks and/or gluons, rather than the proton as a whole.  We know the total energy and momentum of the proton, but not how that is divided between its components.  It is made worse by the quantum nature of the system, where particles are created from the vacuum all the time.  That's not to say we have no idea about what's going on.  One very useful point is that the components of the proton are essentially all going in the direction of the proton; we know that the momentum perpendicular to the beam axis is zero.  We also know the energy of the proton's components on a statistical basis.  But there always remains a residual uncertainty in any measurement from this lack of knowledge.

This leads to the basic point.  Proton colliders like the LHC are sometimes called discovery machines, because they are built to discover things; electron colliders are built to study things we've already found with precision.  This has happened before; LEP itself was designed to study the W and Z bosons after their discovery at SpS.  And so a LEP 3 run would be devoted to studying the LHC's recent discovery (the Higgs, for those of you at the back).

Now, you might very well ask: if it's common to follow up a proton discovery machine with a precision electron experiment, why haven't people being preparing for this?  The answer is that they have.  There are three candidates that have been discussed for up to twenty years in some cases, two different designs for electron-positron machines and a more speculative muon collider.  However, all three of these have been considered under the common pre-LHC assumption that we'd find supersymmetry (or something like it) almost immediately, but it would take a while to find the Higgs.  That the reverse has turned out to be true means we might want to consider new options, and that's where LEP 3 fills in.

Compared to the alternatives, LEP 3 would run at a much lower energy; unsuited for studies of supersymmetry, but ideal for the Higgs.  The idea is that if we don't find anything by, say, 2017, then instead of planned upgrades to the LHC, convert the apparatus to an electron-positron machine.  The costs would be relatively low; we again get to reuse the tunnel, and the plan is to leave the existing detectors in situ and reuse them also.  It would presumably be relatively quick to set this up, too.

Further, a LEP 3 run might ultimately be better suited to finding new physics than the LHC.  If we continue to not find anything through 2017, then a few extra years of running is unlikely to help.  Precision measurements of the W, Z and Higgs masses and couplings could reveal indirect evidence, thanks to the highly predictive nature of the Standard Model Higgs sector.

I'm not really best placed to judge the merits of different experimental ideas.  But as noted, I thought the whole concept was amusing.  In the larger sense, though, I hope we don't have to go this way; I along with most theorists am still hoping we find something juicy at the LHC itself.

1. This was a major cost saver; one of the most expensive parts of a particle accelerator is digging the tunnel to put it in.  It's not feasible to build them on the surface, due to the background radiation.

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