Higgs boson hints multiply in US Tevatron facility data

Tevatron accelerator aerial view (Fermilab) The Tevatron led the high-energy physics world for two decades before its shutoff in 2011

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Scientists from the US Tevatron accelerator say they have spotted possible hints of the Higgs boson at a mass similar to that seen at the LHC.

The findings add weight to the idea that the Higgs - purported to give all other particles their mass - exists near a mass of 125 gigaelectronvolts.

However, the new data are not themselves sufficiently statistically assured to rate the find a "discovery".

The results were presented at the Moriond physics conference in Italy.

The Tevatron, based at the US national laboratory Fermilab, was for two decades the world's premier particle accelerator, but was shut down in 2011 after negotiations to extend funding failed.

Statistics of a 'discovery'

Quarter
  • Particle physics has an accepted definition for a discovery: a "five-sigma" (or five standard-deviation) level of certainty
  • The number of sigmas measures how unlikely it is to get a certain experimental result as a matter of chance rather than due to a real effect
  • Similarly, tossing a coin and getting a number of heads in a row may just be chance, rather than a sign of a "loaded" coin
  • A "three-sigma" level represents about the same likelihood as tossing about eight heads in a row
  • Five sigma, on the other hand, would correspond to tossing more than 20 in a row
  • Independent confirmation by other experiments turns five-sigma findings into accepted discoveries

However, like all particle-smashers, the Tevatron created a tremendous amount of data that remained to be analysed.

The latest data hint at the existence of a particle between 115 and 135 gigaelectronvolts (GeV; this is between about 120 and 140 times as heavy as the protons found in every atom) with a certainty of about 2.2 sigma.

That means that there is about a one in 36 chance that the anomaly they see is the result of happenstance - far less assured than the "five sigma" threshold that physicists use to demarcate a formal discovery.

However, what makes the find more compelling is that the Large Hadron Collider has found a suggestive "bump" in its data at about the same mass, despite being a radically different experiment.

The LHC collides protons together, while the Tevatron used protons and their antimatter counterpart, antiprotons.

Both experiments hunt for the Higgs by looking at what those high-energy particles decay into.

At the Tevatron, the data are from the production of bottom quarks and their counterparts bottom antiquarks, whereas at the LHC the primary search is for the production of the light particles known as photons.

"It's a different accelerator, different detectors and a different decay channel," said Rob Roser, spokesman for CDF, one of the two main Tevatron detectors.

What is an electronvolt?

Particle interaction simulation (SPL)
  • Charged particles tend to speed up in an electric field, defined as an electric potential - or voltage - spread over a distance
  • One electronvolt (eV) is the energy gained by a single electron as it accelerates through a potential of one volt
  • It is a convenient unit of measure for particle accelerators, which speed particles up through much higher electric potentials
  • But because of the equivalence of mass and energy laid out in E=MC2, physicists also speak of the mass of particles in electronvolts

"It adds to the picture, and it's starting to make a compelling case," he told BBC News. "But we can't make quite as bold a statement as we would like.

"I just wish either one of us just had more data right now. It's frustrating."

The two main detectors at the LHC, CMS and Atlas, also presented results at the meeting on Wednesday, but the experiments have precious little further data relative to those presented late last year.

That will radically change later this year as the facility will produce three times the amount of data this year as in 2011.

However, recent analysis of Atlas data has "excluded" the mass range up to 122.5 GeV. The Tevatron data, meanwhile, exclude its presence at the heavier masses of 147-179 GeV, also completely consistent with what the LHC has found.

As has been said before, if it indeed exists, there are few places left for the Higgs boson to hide.

Tony Weidberg, a University of Oxford physicist who works at the LHC's Atlas detector, said that the Tevatron results were consistent with the idea of a comparatively "light" Higgs boson.

"It's interesting because it's another little hint," Dr Weidberg told BBC News. "It makes it a little bit more likely that we're going to end the year with a discovery rather than an exclusion.

"The proof of the pudding will be in the LHC data that we'll get this year; by the end of the year we'll have moved away from hints to either discovery or exclusion - and either of those results is exciting to me."

Statistics of a 'discovery'

Quarter
  • Particle physics has an accepted definition for a "discovery": a five-sigma level of certainty
  • The number of standard deviations, or sigmas, is a measure of how unlikely it is that an experimental result is simply down to chance, in the absence of a real effect
  • Similarly, tossing a coin and getting a number of heads in a row may just be chance, rather than a sign of a "loaded" coin
  • The "three sigma" level represents about the same likelihood of tossing nine heads in a row
  • Five sigma, on the other hand, would correspond to tossing more than 21 in a row
  • Unlikely results are more probable when several experiments are carried out at once - equivalent to several people flipping coins at the same time
  • With independent confirmation by other experiments, five-sigma findings become accepted discoveries

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