15 Dec 2011

Mary (whether "virgin" or merely "young"!) Higgs...mas, Bosons!

"Our Christian tradition of 2,000 years is that Mary remains a virgin and that Jesus is the son of God, not Joseph", says Lyndsay Freer, spokeswoman for the Catholic Diocese of Auckland where this billboard was defaced within hours of its unveiling...

Yet, all this "traditional" indoctrinating gibberish (which is hardly about the translation of the word "almah") is about to perish at the scientists' hands rather than of the scholars... Though it may not be the best Christmas pressie, it is humanity's best chance ever to evade the mysticist narratives prevalent in the Age of Ghost-Modernism...

By Paul Rincon,
Science editor, BBC News website, Geneva

The most coveted prize in particle physics - the Higgs boson - may have been glimpsed, say researchers reporting at the Large Hadron Collider (LHC) in Geneva.

Two teams at the LHC have seen hints of what may well prove to be the Higgs

The particle is purported to be the means by which everything in the Universe obtains its mass.

Scientists say that two experiments at the LHC see hints of the Higgs at the same mass, fuelling huge excitement.

But the LHC does not yet have enough data to claim a discovery.

Finding the Higgs would be one of the biggest scientific advances of the last 60 years. It is crucial for allowing us to make sense of the Universe, but has never been observed by experiments.

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

Six theoreticians, including the English physicist Peter Higgs, first proposed the Higgs mechanism in 1964

What is the Higgs boson?

The Higgs so far definitively exists only in the minds of theoretical physicists. There is a nearly complete theory for how the Universe works - all of the particles that make up atoms and molecules and all the matter we see, along with more exotic particles. This is called the Standard Model. However, there is a glaring hole in the theory: it does not explain how it is that all those particles have mass. The Higgs mechanism was proposed in 1964 by six physicists, including the Edinburgh-based theoretician Peter Higgs, as an explanation to fill this hole.

What is so important about mass?

Mass is, quite simply, a measure of how much stuff an object - a particle, a molecule, or a Yorkshire terrier - contains. If not for mass, all of the fundamental particles that make up atoms and terriers would whiz around at light speed, and the Universe as we know it could not have clumped up into matter. The Higgs mechanism proposes that there is a field permeating the Universe - the Higgs field - that allows particles to obtain their mass. Interactions with the field - with the Higgs bosons that come from it - are purported to give particles mass. This is not unlike a field of snow, in which trudging through impedes progress; your shoes interacting with snow particles slows you down.

How do scientists search for the Higgs boson?

Ironically, the Standard Model does not predict an exact mass for the Higgs itself. Particle accelerators such as the LHC are used to systematically search for the particle over a range of masses where it could plausibly be. The LHC works by smashing together two beams of the sub-atomic particles called protons at close to light speed. This generates a vast shower of particles that are only created at high energies. The Higgs will probably never be observed directly, but scientists at the LHC hope that a Higgs will momentarily exist in this soup of particles. If it behaves as researchers think it will, it should decay further into yet more particles, leaving a trail that could prove its existence.

It is not the first machine to hunt for the particle. The LEP machine, which ran at Cern from 1989-2000, ruled out the Higgs up to a certain mass, and the US Tevatron accelerator searched for the particle above this range before it was switched off this year. These data are still being analysed, and could yet be important in helping confirm or rule out the particle. The LHC, as the most powerful particle accelerator ever built, is just the most high-profile of the experiments that could shed light on the Higgs hunt.

When will we know if we have found it?

As with all particle physics, this is a tricky point. The Higgs should show up in a particular range of masses, and signals that indicate it is there in the mess of particles will show up as a kind of "bump" in the data. Making sure that bump is really due to a Higgs is a different matter. If you flip a coin ten times and get eight heads, you might think the coin is somehow "loaded". But only after hundreds of flips can you say so with the kind of certainty that physics requires for a formal "discovery". What is clear about the LHC results so far is that the two teams working to find it do not have enough data - enough "flips" - to say that the Higgs has been found or excluded beyond doubt. More experiments will be needed for that.

How do we know the Higgs exists?

Strictly speaking, we do not, and that is what is so exciting about the status of the hunt at the LHC - the giant experiment that was built in part to hunt for the Higgs. In its simplest form, the theory predicts a "Standard Model Higgs", which is the focus of the current hunt. But history has shown that predictions from theory can be wrong, and the absence of the simplest Higgs particle may suggest that it exists at different energies, decays into different particles, or perhaps doesn't exist at all.

What if we don't find it?

Most professional physicists would say that finding the Higgs in precisely the form that theory predicts would actually be a disappointment. Large-scale projects such as the LHC are built with the aim of expanding knowledge, and confirming the existence of the Higgs right where we expect it - while it would be a triumph for our understanding of physics - would be far less exciting than not finding it. If future studies definitively confirm that the Higgs does not exist, much if not all of the Standard Model would have to be rewritten. That in turn would launch new lines of enquiry that would almost certainly revolutionise our understanding of the Universe, in much the same way as something missing in physics a century ago led to the development of the revolutionary ideas of quantum mechanics.

This basic building block of the Universe is a significant missing component of the Standard Model - the "instruction booklet" that describes how particles and forces interact.
Two separate experiments at the LHC - Atlas and CMS - have been conducting independent searches for the Higgs. Because the Standard Model does not predict an exact mass for the Higgs, physicists have to use particle accelerators like the LHC to systematically look for it across a broad search area.

At a seminar at Cern (the organisation that operates the LHC) on Tuesday, the heads of Atlas and CMS said they see "spikes" in their data at roughly the same mass: 124-125 gigaelectronvolts (GeV; this is about 130 times as heavy as the protons found in atomic nuclei).

"The excess may be due to a fluctuation, but it could also be something more interesting. We cannot exclude anything at this stage," said Fabiola Gianotti, spokesperson for the Atlas experiment.

Guido Tonelli, spokesperson for the CMS experiment, said: "The excess is most compatible with a Standard Model Higgs in the vicinity of 124 GeV and below, but the statistical significance is not large enough to say anything conclusive.

"As of today, what we see is consistent either with a background fluctuation or with the presence of the boson."


Prof Rolf-Dieter Heuer, director-general of Cern, told BBC News: "Such signals can come and go… Although there is correspondence between the two experiments, we need more solid numbers."

None of the spikes seen by the experiments is at much more than the "two sigma" level of certainty.

A level of "five sigma" is required to claim a discovery, meaning there is less than a one in a million chance the data spike is down to a statistical fluke.

A level of "five sigma" is required to claim a discovery, meaning there is less than a one in a million chance the data spike is down to a statistical fluke.

Statistics of a 'discovery'

  • 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 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
  • Unlikely results can occur if several experiments are being 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

Another complicating factor is that these tantalising hints consist only of a handful of events among the billions of particle collisions analysed at the LHC.

Professor Rolf-Dieter Heuer, director-general of Cern told BBC News: "We can be misled by small numbers, so we need more statistics," but added: "It is exciting."

If it exists, the Higgs is very short-lived, quickly decaying - or transforming - into more stable particles. There are several different ways this can happen, which provides scientists with different routes to search for the boson.

They looked at particular decay routes for the Higgs that produce only a handful of events, but have the advantage of having less background noise in the data. This background noise consists of random combinations of events, some of which can look like Higgs decays.

Other decay modes produce more events - which are better for statistical certainty - but also more background noise. Prof Heuer said physicists were "squeezed" between these two options.

Prof Stefan Soldner-Rembold, from the University of Manchester, 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."

The simple fact that both Atlas and CMS seem to be seeing a data spike at the same mass has been enough to cause enormous excitement in the particle physics community.

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