Friday, July 22, 2011

Surprise LHC blip hints at Higgs - again

Particle watchers could be forgiven for feeling a little weary. An unexpected blip in the data glimpsed at the Large Hadron Collider is once again being attributed to the Higgs boson - the hypothetical particle thought to endow all others with mass.

The news raises hopes that the long-anticipated particle might finally be within reach - and is on somewhat firmer ground than a tantalising report earlier this year that turned out to be a false alarm. However, the blip - an excess of particles of a certain energy - is not yet big enough to rule out a statistical fluke that might vanish when more data is gathered.

In the wreckage of colliding protons, the ATLAS detector at the LHC, located near Geneva, Switzerland, has found an unexpected abundance of pairs of W bosons, which carry the weak nuclear force, with energies between about 120 to 140 gigaelectronvolts (GeV).

That could be due to a Higgs particle with a mass in that range decaying into pairs of W bosons (particle masses and energy are treated interchangeably because mass is readily converted into energy in particle collisions and decays).

The excess was reported on Friday by Kyle Cranmer of New York University at the Europhysics Conference in Grenoble, France.

The team also saw smaller excesses in pairs of photons and Z bosons, which carry the weak nuclear force and could be due to Higgs' decays too.

The combined statistical significance, taking all three types of excess reported by ATLAS into account, is 2.8 sigma, slightly below the 3 sigma threshold (equivalent to a 1-in-370 chance of being due to a fluke) that a measurement must pass to count as "evidence" for something new: only 5 sigma data, equivalent to a 1-in-1.7 million chance of being due to a fluke, gains "discovery" status.

The other main detector at the LHC, called CMS, has found an excess in a similar range, between 130 and 150 GeV, reports Nature. The size of that excess is roughly 2 sigma, writes physicist Adam Falkowski on the Resonaances blog.

If all this sounds a tad familiar, rewind back to April, when four physicists claimed to have found hints of the Higgs in ATLAS data in a study abstract leaked online. A subsequent official analysis by the collaboration of 700 physicists who run ATLAS concluded (pdf) that result was an error. Unlike that claim, the new excesses have been vetted by the ATLAS and CMS collaborations respectively.

And what of the Tevatron accelerator in Batavia, Illinois, which has been locked in a race with the LHC to be the first to spot the Higgs? The Tevatron has not yet found evidence for the Higgs and is set to shut down at the end of September, but there is still time for it to weigh in on this with the data it is still collecting. Exciting times ahead, but expect more fits and starts.


Post a Comment