While much of North America was roiling in the summer heat over the weekend, the search for the elusive Higgs boson was also heating up, as particle physicists gathered in Grenoble, France, for the International Europhysics Conference on High Energy Physics, to hear about the latest results in their field.
Most eagerly awaited were new results from two experiments currently underway at the Large Hadron Collider: ATLAS and CMS. The good news is that there are tantalizing hints of the Higgs in that data.
The less good news is that those hoping for the announcement of a bona fide discovery will be disappointed. The purported signals indicating the possible presence of the Higgs remain too faint to make such a claim: less than 3 sigma in both cases, although CMS is getting close to 3 sigma. However, the two experiments show the same excesses in a specific mass range: 130-150 GeV. This is a bit surprising, since they had expected to be able to exclude the mass range from 130 to 200 GeV.
Why are narrowing the range of masses, and achieving at least a 3-sigma result, so important? Well, scientists aren’t entirely sure where to look for the Higgs.
As Aidan Randle-Conde said at Quantum Diaries, “Like anyone else, physicists have issues with confidence,” by which he means “the extent to which they trust a trust a measurement…. Our data are statistically limited, so we can never be 100 percent certain in any of our measurements.”
That’s why limits are so important in particle physics. The more physicists can narrow the target mass range for the Higgs boson, the better their chances of finally detecting its tell-tale 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).
Earlier this year, scientists with Fermilab’s DZero and CDF collaborations presented results that limited the possible range of masses to between 158 and 173 GeV with about 95 percent certainty. ATLAS has ruled out 155-190 GeV and 295-450 GeV; CMS, in turn, has excluded 149-206 GeV and 300-440 GeV. (Symmetry Breaking has a nice explanation of the different approaches employed by Fermilab and the LHC in their hunt for the Higgs.) This significantly narrows the range of masses in which the Higgs might be hiding.
What does this dizzying array of numbers really mean in terms of finding the Higgs? Based on the latest results from both the LHC and Fermilab, the low-mass Higgs is starting to emerge as the more likely scenario — or possibly a very heavy Higgs. It’s also possible that our current models are wrong, which would spark a flurry of new theoretical work to incorporate the new data.
Physicists are cautious about claiming anything more than that, and with good reason. All four experiments will continue collecting data, although Fermilab’s Tevatron is slated to shut down operations in September. Still, the additional data should enable Fermilab scientists to make further refinements to their analyses within the 114-180 GeV mass range.
Meanwhile, ATLAS and CMS will likely double the amount of data by the end of the year. And it’s very possible that when that new data is figured into the mix, those tantalizing excesses will disappear. Statistical “bumps” come and go all the time in particle physics, which is why the field requires a 5-sigma signal in order to claim “discovery.” We should know within a few months whether the same will be true here.