The physics world was abuzz with some tantalizing news a couple of weeks ago. At a meeting of the European Physical Society in Grenoble, France, physicists announced that the latest data from the Large Hadron Collider (LHC) might hint at the existence of the ever-elusive Higgs boson.

According to the Standard Model, the remarkably successful theory of how all the fundamental particles interact, the Higgs boson is responsible for endowing every other particle with mass. And as the last remaining particle pr edicted by the Standard Model yet to be detected, its discovery is one of the chief goals of the LHC, the most powerful particle accelerator on Earth and perhaps the most complex scientific endeavor ever attempted.

Sitting underground near Geneva, Switzerland, the LHC accelerates protons around a ring almost five miles wide to nearly the speed of light, producing two proton beams that careen toward each other. Most of the protons just keep on going past each other, but a small fraction of them collide, creating other particles in the process. But these particles are fleeting, decaying into lighter particles before they can be detected. The challenge for physicists is to pick out hints of new, exotic physics from the flurry of newly minted particles. By sifting through the data, they hope to tease out signs that some of these particles are Higgs bosons.

The LHC is equipped with several detectors, but the ones that are the largest and are going after the Higgs are called ATLAS (A Toroidal LHC Apparatus) and the Compact Muon Solenoid (CMS); Caltech plays a prominent role in the latter. Both experiments recently reported what physicists are calling "excess events." That is, the LHC appears to have created slightly more events than would be expected if the Higgs does not exist. The bump occurred in the region between 130 and 150 gigaelectron volts (GeV—a unit of energy that is also a unit of mass, via E = mc², where the speed of light, c, is set to a value of one), which is the expected mass range of the Higgs. But the data are not yet statistically significant enough to be called a definite signal, let alone a discovery of the Higgs particle, says Harvey Newman, professor of physics.

There are two possible explanations for these results, he says. The bump in the data could just be background events due to some unknown source or it could be the first signs of the Higgs. "One could speculate that it's an unusual statistical fluctuation," he says. "But I don't think so."

The LHC is now operating with 7 teraelectron volts (TeV, a thousand times higher than a GeV) of energy at the center of mass between the two proton beams, and may increase to 8 TeV next year (the maximum energy is 14 TeV, which will be reached by 2014).

Physicists will continue to ramp up the LHC, boosting it to higher energies and increasing the number of collisions to improve the chances of producing Higgs bosons. With several times more particle interactions, the physicists are continuing to close in on the Higgs, as well as other new particles and interactions. There's a chance that by the end of next year, they may determine, once and for all, whether the Higgs exists.

Searching for SUSY
If it turns out that the Higgs does not exist, then physicists will have to do some serious rethinking about the Standard Model. "But even if the Higgs exists, the Standard Model still has fundamental problems," Newman says. For example, the theory is not self-consistent. "The most natural way to solve these problems, is with supersymmetry." he says

Evidence for supersymmetry, abbreviated SUSY ("soosie"), is also something that physicists had anticipated at the LHC. The theory proposes that each fundamental particle has a supersymmetric partner - for example, a quark's partner is called a "squark." There are many versions of the theory, from simple toy models to subtler ones. So far, however, the LHC hasn't detected any signs of supersymmetry. "Many of the models we're excluding are toy models," says Maria Spiropulu, an associate professor of physics. So even though people might be disappointed, it's way too early to rule out the theory. "Some people get depressed that SUSY is being excluded. But it's quite the opposite - we're confirming that nature is much more subtle than what the obvious thing would be."

Source: Caltech