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Tuesday, December 18, 2012

Discrepancies in the Mass of the Higgs Boson

The Economist notes that some recent tests to measure the mass of the Higgs Boson have revealed discrepancies. The article explains:
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. 
... 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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