The Higgs boson is a wily, elusive little particle, but scientists at both Fermilab and CERN are hot on its heels, and if recent experimental results are any indication, the Higgs is fast running out of places to hide — at least the version of the Higgs predicted by the Standard Model of particle physics.
Let's check in with Fermilab's Tevatron first, shall we? As regular readers know, after decades of world-class research and pivotal discoveries, the Tevatron's days are numbered. Ongoing budget cuts mean the massive (four miles in circumference) accelerator in Illinois will shut down this fall. So scientists on the two main experiments there, CDF and DZero, are working overtime to pick up hints of this last missing piece to the particle physics puzzle.
Earlier this month, scientists with each of those collaborations presented new results that exclude key regions of the range of possible masses for the Higgs, based on additional data collected and more sophisticated techniques for analyzing that data. See, scientists aren't entirely sure where to look for the Higgs; the more they can narrow the target range, the better their chances of finally detecting its telltale signature.
There are two primary scenarios: one that involves a high-mass Higgs boson (heavier than 130 GeV, or giga-electron volts, up to around 600 GeV), and one that predicts a low-mass Higgs (between 114 GeV and 129 GeV). The latest results focused on the high-mass scenario, and based on those findings, Fermilab scientists say they now can exclude a Higgs with a mass between 158 and 173 GeV with about 95 percent certainty.
There's still a 5 percent chance the Higgs is hiding there, but the general consensus is that this means the low-mass Higgs is emerging as the more likely scenario. The Tevatron teams will keep taking data from collisions in hopes of reducing the statistical fluctuations over the next few months. According to DZero co-spokesperson Dmitri Denisov of Fermilab, who told Symmetry Breaking, "If the Higgs boson exists, hints of its presence will emerge from the Tevatron data. If it does not exist, the CDF and DZero collaborations expect to rule out the remainder of the allowed mass range and scientists would have to wonder again: how do fundamental particles acquire mass?"
Exciting times! Scientists with the Large Hadron Collider's CMS and ATLAS experiments noted the Tevatron results with great interest, and predict that they, in turn, will be able to further reduce the mass range where the Higgs might be lurking, ruling out masses between 120 and 530 GeV. I think we can all see where this is going: in that case, the Higgs would have to have a low mass of between 114 and 119 GeV, or an uber-heavy mass of 531 to 600 GeV — or our current models are wrong.
Of course, this prediction comes with a few caveats, namely, it is based on the assumption that the LHC will remain on track to hit its data collection goals in 2011 and beyond. As with the Tevatron, the more data the LHC experiments collect, the more they can narrow target ranges with an ever-increasing certainty, and the better chance they will have at discovering — or excluding! — the Higgs predicted by the Standard Model.
But the Higgs isn't the only game in town for the LHC and Tevatron physicists. A paper recently appeared on the arXiv announcing mounting evidence for a potential new particle that nobody was really looking for in the first place. The smoking gun is in the directions that top quarks and their counterparts, antiquarks, travel after a collision; theory predicts that the particles should show a preference for one direction in particular 5 percent of the time.
However, both the CDF and DZero collaborations at Fermilab found an unexpected asymmetry: 15 percent of the particles showed a preferred direction, with top quarks showing a preference for moving forward and antitops showing a preference for moving backward. Furthermore, another study showed that above certain energies (greater than 450 GeV), the particles exhibited this weird asymmetry almost half (48 percent) of the time.
It's not the Higgs, according to CDF physicist Fabrizio Margaroli, because the Higgs wouldn't exhibit that kind of behavior. The most likely explanation is that an as-yet-undiscovered particle — just heavy enough to escape detection by the Tevatron — is interfering in some way and causing the asymmetry. But the LHC, with its higher energies, could find definitive evidence of this proposed particle — possibly in the next few weeks or months.
So if the Higgs keeps playing hard to get, there could be a nice little consolation prize in the mix for the LHC: signs of new physics. Things are definitely getting interesting.