Higgs boson: LHC scientists to release best evidence

Higgs boson decay simulation The Higgs, so far, exists definitively only in simulations

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Anticipation is building in the run-up to presentations of the best-yet evidence for - or against - the existence of the Higgs boson.

The famed particle is a missing link in current theories of physics, used to explain how things gains their mass.

Rumours have been swirling about the findings for weeks, ahead of the announcement on Tuesday afternoon.

It is likely to yield only tantalising hints, as the teams do not have enough data to claim a formal discovery.

However, most physicists concede that not finding the Higgs boson is as exciting a prospect as finding it in the place where existing theory predicts it should be.

"If we wouldn't find it it would be even - in a way - more exciting, but you know, both ways, it's a win-win situation," said Prof Stefan Soldner-Rembold, a particle physicist from the University of Manchester.

"[If] we find it, we know this theory's complete, but there's still more things to look for. If we don't find it, we know there must be something else which we haven't understood yet."

Field day

Finding the Higgs was a key goal for the $10bn (£6bn) Large Hadron Collider (LHC) - a 27km (17-mile) circumference accelerator ring of superconducting magnets, designed to re-create the conditions just after the Big Bang in an attempt to answer fundamental questions of science and the Universe itself.

The collider hosts two experiments - Atlas and CMS - that are searching for the particle independently.

There is intense excitement among physicists working at Cern, the Geneva-based organisation which operates the collider, over hints that the hunters have cornered their quarry.

"It is a fantastic time at the moment, you can feel people are enthusiastic," Dr Christoph Rembser, a senior scientist on the Atlas experiment, told BBC News. "It is really very lively."

Start Quote

If the Universe really is like that, I find it really quite breathtaking and humbling that we can understand it”

End Quote Dr Tara Shears University of Liverpool, UK

Prof Soldner-Rembold 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."

He told me: "The Higgs particle would, of course, be a great discovery, but it would be an even greater discovery if it didn't exist where theory predicts it to be."

The Higgs boson is a "fundamental" particle; one of the basic building blocks of the Universe. It is also the last missing piece in the leading theory of particle physics - known as the Standard Model - which describes how particles and forces interact.

The Higgs explains why other particles have mass. As the Universe cooled after the Big Bang, an invisible force known as the Higgs field formed together with its associated boson particle.

It is this field (and not the boson) that imparts mass to the fundamental particles that make up atoms. Without it, these particles would zip through the cosmos at the speed of light.

Mass mechanism

The way the Higgs field works has been likened to the way photographers and reporters congregate around a celebrity. The cluster of people are strongly attracted to the celebrity and create resistance to his or her movement across a room. In other words, they give the celebrity "mass".

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

"The thing about the Higgs is that we always say we need it to explain mass. But the real importance of it is that we need it to make sense of the Universe," said Dr Tara Shears, a particle physicist at Liverpool University.

She told BBC News: "Discovering the Higgs confirms that the approach we have been taking to understand the Universe is correct."

Such deeper motivations underlie the current effort at Cern to dislodge the Higgs and other phenomena. Housed in a 27km-long circular tunnel beneath the French-Swiss border, the LHC smashes particle beams together at close to the speed of light, with the aim of detecting new particles in the debris.

Physicists do not know the mass of the Higgs itself, which has made hunting for it more difficult. They have to look for the particle by systematically searching a range of masses where it is predicted to be.

From 1989-2000, Cern's LEP particle smasher ruled the Higgs out up to a mass of 114 gigaelectronvolts (GeV). To search for the Higgs beyond that mass, physicists needed a much more powerful machine - the LHC.

Statistics of a 'discovery'

Two-pence piece
  • 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

The two detectors Atlas and CMS are looking for signs of it among the billions of collisions that are occurring in each experiment. Hints of the Higgs would look like a little spike or "bump" in physicists' graphs.

For more than a week, rumours have been circulating on physics blogs that Atlas and CMS see a Higgs signal at 125 GeV, between the 2.5 and 3.5 sigma level of certainty.

These numbers represent a measure of the likelihood that any bump the scientists see is down to chance, rather than caused by a real physical phenomenon.

If those are the numbers quoted on Tuesday, it would not be enough for Cern to make a definitive statement. Three sigma counts as an "observation", while five sigma is regarded as the threshold for claiming a discovery.

Indeed, Cern's director-general Rolf-Dieter Heuer has told staff by email that the announcement would not be conclusive.

Any such spike could diminish as more data are gathered. But if Atlas and CMS both see a signal in about the same place, there would be an irresistible temptation to pop champagne corks - though behind closed doors.

In public, however, physicists would be obligated to say that a definitive "yes" or "no" would need to wait until 2012.

Asked where a Higgs discovery would rank among scientific milestones of the last 100 years, Dr Shears said: "I don't think that I could compare it to any other scientific advance... it is quite different.

"This is a prediction that stems from a very mathematical approach to understanding the Universe, which is guided by the idea that it is simple at heart.

"If the Universe really is like that, I find it really quite breathtaking and humbling that we can understand it."

Paul.Rincon-INTERNET@bbc.co.uk

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