Large Hadron Collider results excite scientists

Atlas experiment (Cern) The Atlas experiment is one of two multi-purpose experiments at the LHC

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The Large Hadron Collider (LHC) has picked up tantalising fluctuations which might - or might not - be hints of the sought-after Higgs boson particle.

But scientists stress caution over these "excess events", because similar wrinkles have been detected before only to disappear after further analysis.

Either way, if the sub-atomic particle exists it is running out of places to hide, says the head of the European Organization for Nuclear Research (Cern), which runs the LHC.

He told BBC News the collider had now ruled out more of the "mass range" where the Higgs might be.

The new results are based on analyses of data, gathered as the vast machine smashes beams of protons together at close to light speeds.

Primary goals

Scientists from two different experiments (Atlas and CMS) based at the LHC are scouring the wreckage of these collisions.

One of their primary goals is to search for hints of the Higgs, which is the last missing piece in the Standard Model - the most widely accepted theory of particle physics.

What is an inverse femtobarn?

Red barn (Eyewire)
  • The "barn" is a unit of area used in particle collider physics
  • It derives from the measure of a uranium atom's nucleus - comparatively large among atoms, or as physicists joked "as big as a barn"
  • A femtobarn is a millionth of a billionth of a barn
  • That's just 0.000000000000000000000000000000000000001 square centimetres
  • The inverse femtobarn is a measure of how many particles have smashed into one another in an area equal to one femtobarn
  • The LHC passed 10 inverse femtobarns of collisions in 2010: roughly equivalent to the number of sand grains on a stretch of beach 1.5km long, 100m wide and 1m deep

Without the Higgs, physicists cannot explain why particles have mass. But despite the best efforts of scientists working on both sides of the Atlantic to detect it experimentally, the boson remains a theoretical sub-atomic particle.

The Standard Model is a framework that explains how the known sub-atomic particles interact with each other. If the Higgs boson is not found, physicists would have to find some other mechanism to explain where particles get their mass from. It would also require researchers to change the Standard Model.

Rolf-Dieter Heuer, director-general of Cern, said the amount of data gathered was a factor of 20 greater than had been amassed at the same time last year.

"With one inverse femtobarn, you cannot cover the entire mass region which is allowed for the Higgs boson," Professor Heuer told me.

"However, the experiments can now - unfortunately - exclude quite a large part of this allowed mass region."

Physicists think the Higgs will most probably be found in the low-mass region - between 114 GeV (gigaelectronvolts) and 140 GeV. While the gigaelectronvolt is a unit of energy, in particle physics, mass and energy can be interchanged because of Einstein's equivalence idea (E=MC2).

Fluctuations

Professor Heuer said that searches at low masses had picked up small fluctuations "here and there", but that this was expected because physicists were analysing small numbers across a number of different "channels".

"The whole thing becomes more interesting the more data we collect," he explained.

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

News of the surplus of interesting events - seen by both the Atlas and CMS teams - were outlined at the European Physical Society's HEP 2011 conference here in Grenoble, France.

The most significant excess is seen at a mass of 145 GeV and is above the two-sigma level of certainty. Another fluctuation is seen by the Atlas experiment at the higher mass of 250 GeV, with a two-sigma level of certainty.

A three-sigma result means there is roughly a one in 1,000 chance that the result is attributable to some statistical quirk in the data.

Five sigma means there is about a one in 1,000,000 chance that the "bump" is just a fluke and is the level generally required for a formal discovery.

Dave Charlton, who works on the Atlas experiment at the LHC, called the excess of events "intriguing".

But the particle physicist from the University of Birmingham, UK, told BBC News these "could go up to three sigma, or they could disappear".

The Large Hadron Collider is a vast machine built in an underground tunnel that runs in a circle for 27km under the French-Swiss border.

It accelerates two beams of proton particles at near light-speed around the circular tunnel and smashes them together at selected collision points around the underground ring. By looking at what is produced in these particle collisions, physicists should be able to shed further light on the nature of the cosmos

HEP 2011 runs until 29 July in Grenoble.

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