'Moment of truth' approaching in Higgs boson hunt

The giant Atlas experiment The Higgs search is approaching its endgame at Cern

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In recent months, news headlines have been dominated by one story from the world of particle physics - those befuddling faster-than-light neutrinos.

Such is the interest in those speedy sub-atomic particles that developments in the search for the elusive Higgs boson - usually covered at every twist and turn by journalists - have been all-but eclipsed.

Earlier this month, physicists announced results of a combined search for the Higgs by the Atlas and CMS experiments at the Large Hadron Collider (LHC).

Their analysis, presented at a meeting in Paris, shows that physicists have now covered a large chunk of the search area in detail, ruling out a broad part of the mass range where the boson could be lurking.

An even more important milestone in the Higgs hunt beckons in December.

The Higgs explains why other particles have mass, making it crucial to our understanding of the Universe. But it has never been observed by experiments.

Small window

Researchers have now excluded the possibility that the Higgs (in its conventional form) will be found between the masses of 141 gigaelectronvolts (GeV) and 476 GeV.

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

Finding the Higgs boson at a mass of 476 GeV or more is considered highly unlikely.

This means that physicists are now focussing their hunt on the remaining "low mass" range - a small window between 114 GeV and 141 GeV.

Within that window, there is an intriguing "excess" in observations - a Higgs hint, perhaps - that stands out at 120 GeV.

But as fluctuations go, this one is relatively weak - at around the two-sigma level of certainty.

This roughly equates to a one in 22 chance that the observation is down to chance. A five sigma level is needed for a formal discovery.

There is also a broader "excess" above that mass. And it must be stressed that such hints may come and go.

But there is an even more intriguing possibility: that the boson may not exist at all, at least in its simplest form.

This is the version of the Higgs that conforms to the Standard Model, the framework drawn up to explain how the known particles - from the quarks to the W and Z bosons to the neutrinos - interact.

In this "zoo" of particles, the Higgs remains hidden in the long grass of its enclosure, invisible to the prying eyes of visitors.

Beginning of the end

The search by the LHC has already moved on from the data presented earlier this month.

Start Quote

It is the first yes or no... we are entering a phase where it will be very interesting - this I know”

End Quote Dr Guido Tonelli CMS spokesperson

Teams of scientists at the facility on the Franco-Swiss border have been busy analysing a whopping five inverse femtobarns of data collected by the LHC's experiments up to October this year.

The Atlas and CMS collaborations will present independent analyses of this data set at a seminar in Geneva on 13 December. The respective teams have not had the time to combine their results, as they did for the Paris seminar.

They might see completely different things.

Or, more promisingly, they could both see a fluctuation at around the same mass - as they did when researchers presented findings at the Europhysics meeting in Grenoble, France, in July.

"If you look at the data, it's about five times as much as was presented at the summer conferences," said Dr James Gillies, director of communications at Cern (the Geneva-based organisation that operates the LHC).

"It's possible to exclude much more of the available range for the Higgs.

"It's possible - but I think extraordinarily unlikely - to exclude the Higgs definitively. It's possible that there will be signs something is there.

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

"But what's not possible is to give a definitive discovery announcement, on the status of the analysis, given the time they've had."

Either way, scientists are waiting with bated breath for the December seminar, which will - at the very least - mark the beginning of the end for the Higgs race.

"We are pushing very hard to present preliminary results on the entire statistics," said Dr Guido Tonelli, spokesperson for the CMS collaboration.

He told BBC News that with five inverse femtobarns of data, the researchers will have sufficient sensitivity that "if there is something, we should see first hints. If there is nothing we should see no excess".

"It is the first yes or no. It will very likely not be conclusive - to be really sure at the highest confidence level, we might need to combine the data [from Atlas and CMS] again and collect additional data next year.

"But we are entering a phase where it will be very interesting - this I know."

'Uneasy rumour'

The rumour mill is already churning vigorously, and is likely to enter overdrive as the December seminar approaches.

The blogger known as Jester recently proffered a Soviet-inspired analogy: "An uneasy rumour is starting among the working class and the lower-ranked party officials.

"Is the first secretary dead? Or on life support? Or, if he's all right, why he's not showing in public?"

A definitive statement about the Higgs is likely to come next year.

The suggestion that particle physicists have been chasing a chimera for decades is one that some will not want to contemplate. But others regard as a more exciting possibility.

A no-show would open up a new era of activity in particle physics - one focussed on finding an alternative theory to patch up the hole in the Standard Model left by the excision of the Higgs.

Indeed, there is already a substantial body of work on alternatives to the Standard Model Higgs.

As Prof Rolf-Dieter Heuer, director-general of Cern, says, either scenario would represent "a tremendous discovery".

And one particle physicist speaking at the Europhysics conference this year summed it up thus: "God forbid that all we find at the LHC is the Standard Model Higgs and no new physics."

Paul.Rincon-INTERNET@bbc.co.uk

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