LHC: Higgs boson 'may have been glimpsed'

Control room at Cern Two teams at the LHC have seen hints of what may well prove to be the Higgs

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The most coveted prize in particle physics - the Higgs boson - may have been glimpsed, say researchers reporting at the Large Hadron Collider (LHC) in Geneva.

The particle is purported to be the means by which things in the Universe obtain their mass.

Scientists say that two experiments at the LHC see hints of the Higgs at the same mass, fuelling huge excitement.

But the LHC does not yet have enough data to claim a discovery.

Finding the Higgs would be one of the biggest scientific advances of the last 60 years. It is crucial for allowing us to make sense of the Universe, but has never been observed by experiments.

The Higgs boson

  • The Higgs is a sub-atomic particle that is predicted to exist, but has not yet been seen
  • It was proposed as a mechanism to explain mass by six physicists, including Peter Higgs, in 1964
  • It imparts mass to other fundamental particles via the associated Higgs field
  • It is the last missing member of the Standard Model, which explains how particles interact

This basic building block of the Universe is a significant missing component of the Standard Model - the "instruction booklet" that describes how particles and forces interact.

Two separate experiments at the LHC - Atlas and CMS - have been conducting independent searches for the Higgs. Because the Standard Model does not predict an exact mass for the Higgs, physicists have to use particle accelerators like the LHC to systematically look for it across a broad search area.

At a seminar at Cern (the organisation that operates the LHC) on Tuesday, the heads of Atlas and CMS said they see "spikes" in their data at roughly the same mass: 124-125 gigaelectronvolts (GeV; this is about 130 times as heavy as the protons found in atomic nuclei).

"The excess may be due to a fluctuation, but it could also be something more interesting. We cannot exclude anything at this stage," said Fabiola Gianotti, spokesperson for the Atlas experiment.

Professor Rolf-Dieter Heuer, director-general of Cern: ''We have made extremely good progress''

Guido Tonelli, spokesperson for the CMS experiment, said: "The excess is most compatible with a Standard Model Higgs in the vicinity of 124 GeV and below, but the statistical significance is not large enough to say anything conclusive.

"As of today, what we see is consistent either with a background fluctuation or with the presence of the boson."

'Exciting'

Prof Rolf-Dieter Heuer, director-general of Cern, told BBC News: "Such signals can come and go… Although there is correspondence between the two experiments, we need more solid numbers."

None of the spikes seen by the experiments is at much more than the "two sigma" level of certainty.

Statistics of a 'discovery'

Five Swiss francs
  • 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

A level of "five sigma" is required to claim a discovery, meaning there is less than a one in a million chance the data spike is down to a statistical fluke.

Another complicating factor is that these tantalising hints consist only of a handful of events among the billions of particle collisions analysed at the LHC.

Prof Heuer said: "We can be misled by small numbers, so we need more statistics," but added: "It is exciting."

If it exists, the Higgs is very short-lived, quickly decaying - or transforming - into more stable particles. There are several different ways this can happen, which provides scientists with different routes to search for the boson.

They looked at particular decay routes for the Higgs that produce only a handful of events, but have the advantage of having less background noise in the data. This background noise consists of random combinations of events, some of which can look like Higgs decays.

Other decay modes produce more events - which are better for statistical certainty - but also more background noise. Prof Heuer said physicists were "squeezed" between these two options.

Prof Stefan Soldner-Rembold, from the University of Manchester, called the quality of the LHC's results "exceptional", adding: "Within one year we will probably know whether the Higgs particle exists, but it is likely not going to be a Christmas present."

The simple fact that both Atlas and CMS seem to be seeing a data spike at the same mass has been enough to cause enormous excitement in the particle physics community.

Statistics of a 'discovery'

Five Swiss francs
  • 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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