The LHC and the search for Susy

Anil Ananthaswam

• Last Updated: November 12. 2009 10:18PM UAE / November 12. 2009 6:18PM GMT

Scientists at Cern are preparing to fire up the Large Hadron Collider again to search for the Higgs Boson Fabrice Coffrini / AFP

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 the European Organisation for Nuclear Research (Cern), the LHC’s home on the outskirts of Geneva.

The Nobel laureate Steven Weinberg is worried. It is not that he thinks the LHC will create a black hole that will engulf the planet. No: he is actually worried that the LHC will find what some call the “God particle”, the popular and embarrassingly grandiose moniker for the hitherto undetected particle, the 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 does not 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 Dr 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, doubles the number of particles needed to explain the world.

So what’s so wrong with the standard model? First off, it has nothing to say about the fourth fundamental force of nature, gravity, and it is also silent on the nature of dark matter, which outweighs conventional matter in the cosmos by more than four to one.
Ironically, 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. Then, in 1964, Peter Higgs of the University of Edinburgh in Scotland and François Englert and Robert Brout of the Free University of Brussels in Belgium independently hit upon a way to tie them up. But the Higgs theory, though elegant, comes with a nasty sting in its tail: what is the mass of the Higgs itself?

The experimental clues we already have suggest that the Higgs’s mass should lie somewhere 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.

This is not the only defect in the standard model. A new way of tackling these and other problems appeared in the early 1970s with supersymmetry. According to this theory, each fermion is paired with a more massive supersymmetric boson, and each boson with a fermionic supersibling. For example, the electron has the selectron (a boson) as its supersymmetric partner, while the photon is partnered with the photino (a fermion). In essence, the particles we know now are merely the runts of a litter double the size.

The key to the theory is that in the high-energy soup of the early universe, particles and their superpartners were indistinguishable. Each pair co-existed as single massless entities. As the universe expanded and cooled, though, this supersymmetry broke down. Partners and superpartners went their separate ways, becoming individual particles with a distinctive mass.

Supersymmetry was a bold idea, one which dealt with the problem of the mass of the Higgs. That was not all. When they began studying some of the questions raised by the new theory, things became really interesting.

One pressing question concerned the present-day whereabouts of supersymmetric particles. If such particles exist, they must be extremely massive indeed, requiring huge amounts of energy to fabricate.

Such huge particles would long since have decayed into a residue of the lightest, stable supersymmetric particles, dubbed neutralinos. When physicists calculated exactly how much of the neutralino residue there should be, they were taken aback. Neutralinos fulfilled all the requirements for the dark matter that astronomical observations persuade us must dominate the cosmos.

Supersymmetry’s scope does not end there. As Nathan Seiberg and his Princeton University colleague Edward Witten have shown, the theory can also explain why quarks are never seen on their own, but are always corralled together by the strong force into larger particles such as protons and neutrons. All this seems to point to some fundamental truth locked up within the theory.

The reason physicists are so excited about the LHC, though, is that the kind of supersymmetry that best solves the problem with the Higgs will become visible at the higher energies the LHC will explore. Similarly, if neutralinos have the right mass to make up dark matter, they should be produced in great numbers at the LHC.

Any sighting of something that looks like a neutralino would be very big news indeed. Even better, it would tell us that nature is fundamentally supersymmetric.

There is a palpable sense of excitement about what the LHC might find in the coming years. “I’ll be delighted if it is supersymmetry,” says Dr Seiberg.

“But I’ll also be delighted if it is something else. We need more clues from nature. The LHC will give us these clues.”

New Scientist