Higgs boson results from LHC 'get even stronger'

Higgs announcement, 4 July The announcement on 4 July was greeted with enthusiasm around the world

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The Higgs boson-like particle whose discovery was announced on 4 July looks significantly more certain to exist.

The particle has been the subject of a decades-long hunt as the last missing piece of physics' Standard Model, explaining why matter has mass.

Now one Higgs-hunting team at the Large Hadron Collider report a "5.9 sigma" levels of certainty it exists.

That equates to a one-in-550 million chance if the Higgs did not exist, the team would see these same results.

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 formal threshold for claiming the discovery of a particle is a 5-sigma level - equivalent to a one-in-3.5 million chance.

That is the level that was claimed by the team behind Atlas, one of the LHC's Higgs-hunting experiments, during the 4 July announcement. The other, known as CMS, claimed results between 4.9 and 5 sigma.

The range reported by CMS at the time reflects the fact that there are a number of ways to look for the Higgs boson, none of which can observe it directly.

Accelerators like the LHC smash together particles at extraordinary energies in a bid to create a Higgs, which should exist only for a fleeting fraction of a second before decaying into other particles or flashes of light that can be caught and counted.

Now the Atlas team has submitted fuller analyses of these "decay channels", including one in which the Higgs ends up as two lighter particles known as W bosons.

As they report online in a paper submitted to Physics Letters B, their data now reach a significance of 5.9 sigma.

The CMS team reiterated its 5-sigma certainty in a paper submitted to the same journal, after incorporating more data from collisions at the heightened particle energies at which the LHC is running this year.

The findings only shore up a result that, as far as physicists were concerned, had already passed muster for declaring the existence of a new particle.

However, many questions remain as to whether the particle is indeed the long-sought Higgs boson; the announcement was carefully phrased to describe a "Higgs-like" particle.

More analyses will be needed to ensure it fits neatly into the Standard Model - the most complete theory we have for particles and forces - as it currently exists.

The Standard Model and the Higgs boson

Standard model

The Standard Model is the simplest set of ingredients - elementary particles - needed to make up the world we see in the heavens and in the laboratory

Quarks combine together to make, for example, the proton and neutron - which make up the nuclei of atoms today - though more exotic combinations were around in the Universe's early days

Leptons come in charged and uncharged versions; electrons - the most familiar charged lepton - together with quarks make up all the matter we can see; the uncharged leptons are neutrinos, which rarely interact with matter

The "force carriers" are particles whose movements are observed as familiar forces such as those behind electricity and light (electromagnetism) and radioactive decay (the weak nuclear force)

The Higgs boson came about because although the Standard Model holds together neatly, nothing requires the particles to have mass; for a fuller theory, the Higgs - or something else - must fill in that gap

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