Large Hadron Collider results excite scientists
By Paul Rincon
Science reporter, BBC News, Grenoble
22 July 2011
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 one inverse femtobarn of data, gathered as the vast machine smashes beams of protons together at close to light-speed.
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.
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.
Statistics of a 'discovery'
Particle physics has an accepted definition for a "discovery": a five-sigma level of certainty
The number of sigmas (or standard deviations) is a measure of how unlikely it is that an experimental result is simply down to chance rather than 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 more than eight heads in a row
Five sigma, on the other hand, would correspond to tossing more than 20 in a row
A five-sigma result is highly unlikely to happen by chance, and thus an experimental result becomes an accepted discovery
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).
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.
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.
One candidate noted by the Atlas team occurs at the higher mass of 250 GeV and has reached the 2.8 sigma level of certainty. A three-sigma result means there is roughly a 1 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-one-million chance that the "bump" is just a fluke and is the level generally required for a formal discovery.
Another Atlas fluctuation occurs between 130 GeV and 150 GeV and is at the 2.5-sigma level.
Professor 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".
HEP 2011 runs until 29 July in Grenoble.
Source; http://www.bbc.co.uk/news/science-environment-14258601
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