Is the Large Hadron Collider closing in on Higgs particle?

Atlas calorimeter (Cern) If the Higgs exists, the Large Hadron Collider could find it by the end of next year

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Are the Higgs hunters closing in on their quarry?

Listening to the buzz at the Europhysics conference in Grenoble, one might be forgiven for thinking scientists are on the verge of something historic.

They have been analysing an impressive amount of data amassed by the Large Hadron Collider (LHC) over a year-and-a-half of operations.

The vast machine, housed in a circular tunnel below the French-Swiss border, was designed to uncover the Higgs boson - the sub-atomic particle that has so far avoided detection by any previous particle accelerator.

Improvements to the LHC's US counterpart - the Tevatron - have enabled it to stay in the Higgs race. At the weekend, it emerged that scientists working on the 20-year-old accelerator have also caught possible hints of the particle.

Both machines work by accelerating two beams of particles to high energies and then smashing them together. This is done in order to generate new particles in the collisions - perhaps even the Higgs.

Predicted to exist by Edinburgh University physicist Peter Higgs and others in the 1960s, the boson particle is the last missing piece in the Standard Model - the framework built to describe the interactions of sub-atomic particles, and the most widely accepted theory of particle physics.

The Higgs explains why other particles have mass, making it crucial to our understanding of the cosmos.

The LHC's predecessor - the Lep 2 - lacked the power to produce the Higgs. Its replacement is the highest energy accelerator ever built, capable of probing an unexplored domain in particle physics.

But hints of the Higgs boson have come and gone before.

Intriguing surplus

The much anticipated results presented here in France show what physicists describe as "excess events" across the search area - or mass region - where the Higgs is predicted to be found.

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 electron volt (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
  • The first accelerators only created bunches of particles with an energy of about a million eV
  • The LHC can reach particle energies a million times higher: up to several teraelectronvolts (TeV)
  • This is still only the energy in the motion of a flying mosquito
  • But LHC beams include hundreds of trillions of these particles, each travelling at 99.99999999% of the speed of light
  • Together, an LHC beam carries the same energy as a TGV high-speed train travelling at 150 km/h

The most significant of these is a surplus of unusual particle events at a mass of 140-145 gigaelectronvolts (GeV). A Higgs particle with this mass would be at the low end of the range allowed for the boson, right where physicists predict it is most likely to be found.

Professor Stefan Soldner-Rembold, spokesman for the DZero experiment at the Tevatron, said the fact that both the LHC and its US counterpart were seeing similar things at this mass was intriguing.

"There might be some picture emerging from the fog," he told me.

As physicists hunt for the Higgs across the mass range where it could be, they exclude or eliminate more and more of this territory until the Higgs either shows up, or doesn't.

Rolf-Dieter Heuer, the director-general of Cern - the organisation that operates the LHC - told BBC News: "Either we find the Higgs boson in the next 12 months or we can exclude it in its (allowed) mass region."

"Both would be a tremendous discovery."

While the region below 150 GeV is still "wide open", Professor Heuer said the mass range from 150-200 GeV has now been eliminated as a possible "hiding place" for the Higgs.

But he said it was interesting that a region between 200 and 300 GeV had not yet been excluded.

Combined results

Indeed, members of the Atlas collaboration - which, along with the CMS collaboration, is one of two teams searching for the Higgs at the LHC - presented results in Grenoble showing a surplus of interesting events at around 250 GeV.

Professor Dave Charlton, deputy spokesperson for Atlas, told BBC News this was "intriguing" because the ability of physicists to resolve the type of interesting events which were seen in this mass region was particularly good.

Physicists working at the Tevatron will present the combined results of their Higgs search this week at the Europhysics conference.

The Atlas and CMS teams at the LHC gather their results separately. They are set to combine them in time for a scientific conference in Mumbai, India, next month.

But what if, after the entire "search area" has been excluded, the Higgs has still not shown up? It's a scenario that some researchers consider a more exciting one than discovering the simplest Standard Model Higgs boson.

As one audience member at the conference summed it up: "God forbid that all we find at the LHC is the Standard Model Higgs and no new physics."

If the hadron collider search turns up nothing, physicists must find some other way to explain how matter particles get their mass.

The Standard Model is the best model at present to explain the world around us. But it is imperfect, which has led physicists to propose the existence of new physics phenomena beyond this framework.

There were no signs of such new physics - such as the model known as supersymmetry - in the latest searches at the LHC.

As Rolf Heuer explains, the Standard Model is holding up pretty well, which means - for now at least - the race for the Higgs is still on.

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