What the LHC is really looking for

AS DAMP squibs go, it was quite a spectacular one. Amid great pomp and ceremony - not to mention dark offstage rumblings that the end of the world was nigh - the Large Hadron Collider (LHC), the world's mightiest particle smasher, fired up in September last year. Nine days later a short circuit and a catastrophic leak of liquid helium ignominiously shut the machine down.

Now for take two. Any day now, if all goes to plan, proton beams will start racing all the way round the ring deep beneath CERN, the LHC's home on the outskirts of Geneva, Switzerland.

Nobel laureate Steven Weinberg is worried. It's not that he thinks the LHC will create a black hole that will engulf the planet, or even that the restart will end in a technical debacle like last year's. No: he's actually worried that the LHC will find what some call the "God particle", the popular and embarrassingly grandiose moniker for the hitherto undetected Higgs boson.

"I'm terrified," he says. "Discovering just the Higgs would really be a crisis."

Why so? Evidence for the Higgs would be the capstone of an edifice that particle physicists have been building for half a century - the phenomenally successful theory known simply as the standard model. It describes all known particles, as well as three of the four forces that act on them: electromagnetism and the weak and strong nuclear forces.

It is also manifestly incomplete. We know from what the theory doesn't explain that it must be just part of something much bigger. So if the LHC finds the Higgs and nothing but the Higgs, the standard model will be sewn up. But then particle physics will be at a dead end, with no clues where to turn next.

Hence Weinberg's fears. However, if the theorists are right, before it ever finds the Higgs, the LHC will see the first outline of something far bigger: the grand, overarching theory known as supersymmetry. SUSY, as it is endearingly called, is a daring theory that doubles the number of particles needed to explain the world. And it could be just what particle physicists need to set them on the path to fresh enlightenment.

So what's so wrong with the standard model? First off, there are some obvious sins of omission. It has nothing whatsoever to say about the fourth fundamental force of nature, gravity, and it is also silent on the nature of dark matter. Dark matter is no trivial matter: if our interpretation of certain astronomical observations is correct, the stuff outweighs conventional matter in the cosmos by more than 4 to 1.

Ironically enough, though, the real trouble begins with the Higgs. The Higgs came about to solve a truly massive problem: the fact that the basic building blocks of ordinary matter (things such as electrons and quarks, collectively known as fermions) and the particles that carry forces (collectively called bosons) all have a property we call mass. Theories could see no rhyme or reason in particles' masses and could not predict them; they had to be measured in experiments and added into the theory by hand.

These "free parameters" were embarrassing loose threads in the theories that were being woven together to form what eventually became the standard model. In 1964,Peter Higgs of the University of Edinburgh, UK, and François Englert and Robert Brout of the Free University of Brussels (ULB) in Belgium independently hit upon a way to tie them up.

That mechanism was an unseen quantum field that suffuses the entire cosmos. Later dubbed the Higgs field, it imparts mass to all particles. The mass an elementary particle such as an electron or quark acquires depends on the strength of its interactions with the Higgs field, whose "quanta" are Higgs bosons.

Fields like this are key to the standard model as they describe how the electromagnetic and the weak and strong nuclear forces act on particles through the exchange of various bosons - the W and Z particles, gluons and photons. But the Higgs theory, though elegant, comes with a nasty sting in its tail: what is the mass of the Higgs itself? It should consist of a core mass plus contributions from its interactions with all the other elementary particles. When you tot up those contributions, the Higgs mass balloons out of control.

The experimental clues we already have suggest that the Higgs's mass should lie somewhere between 114 and 180 gigaelectronvolts - between 120 and 190 times the mass of a proton or neutron, and easily the sort of energy the LHC can reach. Theory, however, comes up with values 17 or 18 orders of magnitude greater - a catastrophic discrepancy dubbed "the hierarchy problem". The only way to get rid of it in the standard model is to fine-tune certain parameters with an accuracy of 1 part in 10³⁴, something that physicists find unnatural and abhorrent.