[credit provider=”CERN” url=”http://cdsweb.cern.ch/record/1459500″]
AS DISCOVERIES go, that of the Higgs boson is as big as they get. Much of modern physics hinges on the particle, first predicted 48 years ago by Peter Higgs, a Briton, and independently by two other teams of theorists, and finally observed earlier this year at CERN, Europe’s main particle-physics laboratory, outside Geneva (earning the seven leading experimenters a $3m prize earlier this week from a Russian internet mogul). Paradoxically, however, many boffins would rather Dr Higgs’s prediction not be borne out to the letter. The latest results from ATLAS, one of the two experiments spearheading the Higgs hunt at CERN, offers the contrarians a glint of hope.The reason the Higgs is imperative is that without it, or something like it, to give mass to other particles, the entire elaborate mathematical edifice erected over the past four decades to describe the most fundamental constituents of reality would topple. The reason many physicists would prefer it to differ from what Dr Higgs and the others postulated is that this would give them an inkling of where the successor to the Standard Model, as that venerable theory is known, might be hiding.
For all its explanatory power, the Standard Model leaves some big questions, like why the universe is made of matter, unanswered. Theorists have plenty of ideas which explain everything the model does, and more. But if observations conform precisely to its predictions, they have no way of telling which of these ideas reflects reality. For that, they need data at odds with the reigning theory.
On December 13th ATLAS obliged—sort of. An unstable beast, the Higgs cannot be observed directly. Instead, physicists look for telltale patterns left in the detector by longer-lived particles it decays into. Theory allows for a number of distinct decay modes, as the patterns are called. For example, the Higgs can break up into two photons or into two heavier particles called Z bosons, among other possibilities. By measuring the energy of these daughter particles, scientists can, by dint of Albert Einstein’s famous equation E=mc2, work backwards to determine the mass of the parent. When ATLAS researchers did this with their latest batch of data, instead of both modes pointing to the same mass of around 125 giga-electron-volts (GeV), the esoteric unit used to weigh subatomic particles, they yielded two, slightly different masses: 123.5GeV for the Zs and 126.6Gev for the photons.
The difference is about three times the ATLAS detector’s resolution of 1GeV or so. The odds that the result is down to chance are around one in 100, well below the exacting one-in-3.5m standard particle physicists have set themselves to claim a discovery, but enough to stoke speculation.
ATLAS scientists are doing their utmost to cool any excitement. For one thing, although some theoretical proposals imply the existence of more than one Higgs, it would be quite a coincidence for their masses to be so close together. Odder still would be the two Higgses’ preference for distinct daughters; theorists expect any Higgs-like particle to display either both decay modes, or neither.
Crucially, CERN’s other big Higgs-hunting experiment, called CMS, is not seeing double. Its data indicate a single Higgs mass of 126GeV. Indeed, for all decay modes taken together, so do ATLAS’s. In all likelihood, then, the discrepant result is an artefact, caused by the limited precision of the apparatus or a fluke of statistics.
Still, physicists will draw subversive comfort from another piece of information which emerged from both experiments this week. Besides enumerating the possible decay modes, the Standard Model makes firm predictions about how often the different patterns ought to occur. For break-up into Z bosons, as well as for some other analysed patterns, the data dovetail neatly with theory. But the rate for the decay into two photons is consistently higher than expected. If this were due to a statistical fluctuation, you would expect the effect to diminish with more data. Instead, it refuses to budge. This, too, might be mere statistical noise. But many boffins are keeping their fingers crossed that it is the first note of a whole new symphony.
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