Particle physics: Experiments give shape to Higgs

PARIS (AFP) - Physicists on Sunday said they had learned more about the identity of the Higgs Boson, the elusive particle whose ground-breaking discovery was announced nearly two years ago.

Work at the Large Hadron Collider (LHC) - the particle smasher on the French-Swiss border where the breakthrough was made - has answered long-standing questions about how the Higgs behaves, they said.

The Higgs was theorised in the 1960s as being the sub-atomic particle that gives other particles mass. Without it, matter would not exist.

Decades of work followed to explore the idea until on July 4, 2012, rival teams at the LHC announced they had independently found a particle consistent with the Higgs.

But further work was needed to flesh out this discovery and to see how it fits with the Standard Model, the conceptual framework for explaining visible matter in the Universe.

In a study published in the journal Nature Physics, one of the LHC teams said the boson behaves as predicted, and is not an "imposter that looks like it."

Analysis of the mountain of data from collisions at the LHC shows the boson decays neatly to a group of sub-particles called fermions, in line with Standard Model theory, the paper said.

"This is an enormous breakthrough," said Markus Klute of the Massachusetts Institute of Technology (MIT) who led the research at the LHC's Compact Muon Solenoid (CMS).

"Now we know that particles like electrons get their mass by coupling to the Higgs field, which is really exciting." Finding the Higgs was only possible through the building of the LHC, the world's biggest laboratory, made up of a 27km ring-shaped tunnel.

An army of physicists from around the world sifted through the rubble left from billions of proton smashups, hunting for a telltale signature from a fleeting particle.

The initial discovery put the Higgs' mass at between 125 to 126 gigaelectronvolts, a standard unit of measurement at sub-atomic level.

Later analysis of the data from these experiments also found that the boson has no "spin," and rapidly decays into pairs of photons (particles of light) and so-called W or Z bosons.

"We have now established the main characteristics of this new particle," said Klute in a press release issued by MIT.

"All of these things are consistent with the Standard Model." Experiments at the LHC are currently on hold while the collider goes through an upgrade, although scientists are still trawling through reams of data generated from smashups before the shutdown.

Operations are due to resume in 2015, with a three-year programme that will see scientists use more powerful collisions to explore theorised phenomena such as "super-symmetry" which may explain dark matter, the substance that makes up most of the Universe.

The boson is named after Peter Higgs, a British physicist who co-won the Nobel prize last year with Francois Englert of Belgium.

Other physicists who made big contributions were Robert Brout, also a Belgian, who died in 2011, and a US-British team of Dick Hagen, Gerald Guralnik and Tom Kibble.

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