Wednesday, 15 January 2014

CERN discovers Higgs-like Boson

Physicists working at the Large Hadron Collider (LHC) have announced the discovery of the Higgs boson – or at least a particle that resembles the Higgs. In two special seminars this morning at the CERN particle-physics lab in Geneva, spokespeople for the LHC's two main experiments – ATLAS and CMS – both reported measurements of the Higgs' mass at confidence levels of 5σ. Any finding that passes this statistical threshold is generally, but not always, considered a "discovery" among the particle-physics community.
However, today's announcement of a discovery of a particle that looks like the Higgs is by no means the end of the story, as physicists have yet to understand its complete nature.
Physicists have had the Higgs boson in their sights for nearly 50 years because its discovery would complete the Standard Model of particle physics. The particle and its associated field explain how electroweak symmetry broke just after the Big Bang, which gave certain elementary particles the property of mass. The Standard Model does not, however, predict the mass of the Higgs, and successive experimental programmes at CERN's Large Electron–Positron Collider (LEP), Fermilab's Tevatron and now the LHC have tried to measure the particle's mass.
Presenting the latest results from the CMS experiment, spokesperson Joe Incandela announced that his experiment has discovered the Higgs boson at a mass of 125 GeV/c2 and a statistical significance of 5σ.
Incandela described the result as "A phenomenal effort considering that we stopped taking data two weeks ago."
Incandela was followed by ATLAS spokesperson Fabiola Gianotti, who says that ATLAS has measured the mass of the Higgs as 126 GeV/c2, which agrees with preliminary results released by CERN in December 2011. The statistical significance of the measurement is 5σ.
"The search is more advanced today than we imagined possible," says Gianotti. However, she cautioned that "a little more time is needed to finalize these results, and more data and more study will be needed to determine the new particle's properties".
Measurements with 5σ from both detectors – combined with previous searches by Tevatron and LEP – leave no doubt that a "Higgs-like" particle has been discovered by the LHC.
"We have reached a milestone in our understanding of nature," says CERN director general Rolf-Dieter Heuer, who described the new particle as being "consistent with the Higgs boson".
Speaking in the CERN auditorium immediately after the results had been presented, Edinburgh University particle theorist Peter Higgs congratulated researchers on their finding. "For me, it's a really incredible thing that it's happened in my lifetime," he said.
A typical Higgs candidate event captured by the CMS detector at CERN

However, mass appeal is not why we chose the discovery as our breakthrough of the year. The July announcement was much anticipated because physicists have had the Higgs boson in their sights for nearly 50 years. Its discovery completes the Standard Model of particle physics – making it the most important physics breakthrough so far in the 21st century.
The Higgs boson and its associated field explain how electroweak symmetry was broken just after the Big Bang to give certain elementary particles the property of mass. The Standard Model does not, however, predict the mass of the Higgs, which had remained a mystery until July. That was when both CERN experiments announced that they had independently discovered a particle with a mass of about 125 GeV/c2. Crucially, both experiments were able to claim this figure with confidence levels of 5σ. Any finding that passes this statistical threshold is generally considered a "discovery" in the particle-physics community.
And if that were not enough, the CMS and ATLAS collaborations stand out because of the sheer scale of what has been achieved by their thousands of members over the past two decades. Starting in the early 1990s, when plans were first hatched for the Large Hadron Collider (LHC), physicists began thinking about how they could build detectors the size of small office blocks to capture and measure the multitude of particles produced when two protons collide at TeV energies. Some focused on how vast quantities of collision data could be stored and distributed to physicists around the world. Yet more began developing methods for analysing this vast and bewildering amount of information.
If both the ATLAS and CMS experiments had simply functioned as expected, that alone would have been worthy of a Physics Worldaward. However, both have overachieved since they first started taking data in 2010. Indeed, current ATLAS spokesperson Fabiola Gianotti told us that the accelerator has produced 10 times more data than would have been expected by this time. "The experiments, the computing grid and the LHC accelerator are performing well beyond our expectations," she says.
These are just a few reasons why both experiments have been able to home in on the Higgs after just a shade over two years of data-taking. In fact, the precise nature of the new particle is revealed by how it decays into other particles, which are then detected by the ATLAS and CMS collaborations. And while Physics World has been careful to call the discovery a "Higgs-like particle" – just as the collaborations themselves have done – evidence is now growing that the particle is a Higgs boson as described by the Standard Model of particle physics.

shailesh shukla

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