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Physicists Detect New Heavy Particle

Wed, 2012-07-04 15:26 -- cnicolini
A proton-proton collision event in the CMS experiment producing two high-energy photons (red towers). This is what would be expected from the decay of a Higgs boson but it is also consistent with other, more common physics processes.
Image credit: 
CERN
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© CERN 2012
Signature resembles long-sought 'Higgs boson.'
Originally published: 
Jul 4 2012 - 3:30pm
By: 
Virat Markandeya, ISNS Contributor

(ISNS) -- Scientists from two experiments at the Large Hadron Collider in Europe confirmed the existence of a new heavy particle, likely to be the long-sought Higgs boson, thanks to troves of particle-collision data that yielded discovery-level certainty upon analysis. The results, announced at a major particle physics conference in Melbourne, Australia, mark the culmination of a search for a heavy particle believed to give mass to elementary particles such as electrons and quarks.


RELATED: Higgs Boson Could Help Explain How We Exist [Video]


The announcements represent the current high-water mark for the $7-billion LHC particle accelerator at the Franco-Swiss border, which smashes together subatomic particles known as protons at super-high energies to recreate conditions thought to exist fractions of a second after the Big Bang.

"To me it's really an incredible thing that it's happened in my lifetime," said Peter Higgs, the British theorist after whom the particle is named, who was present at the conference and at times appeared choked with emotion. The ideas were in the air when Higgs had written a brief 2-page paper in 1964 that led to the moniker "Higgs boson." The Higgs has been the last undiscovered particle predicted by the Standard Model, the bedrock theory of known subatomic particles and the forces that govern them, and it also holds the promise of revealing new physics phenomena.

It was vindication for a generation of scientists. "Tears came in my eyes when the five-sigma number came up," said Brookhaven National Laboratory's Howard Gordon, who is the U.S. deputy operations program manager for the ATLAS experiment at the LHC. The U.S. contingent of ATLAS, hosted by Brookhaven and consisting of over 700 people from 44 institutions, helped to build many of its key detectors and handles about 20 percent of the worldwide computing effort involved in simulation and analysis of its data.

The two experiments, CMS and ATLAS, analyzed particle decay data from approximately 500 trillion particle collisions.  Joe Incandela, spokesperson for the CMS experiment, said that if you imagine each collision as a grain of sand, you'd have enough sand to fill an Olympic swimming pool. However, Higgs-related collisions are so rare that the grains of sand would only cover the tip of a finger, he said.

At the ICHEP2012 conference earlier this morning, both groups reported "bumps" in their collision data indicating the presence of a particle with mass measured at around 125 -126 billion electron volts, abbreviated as GeV. This is over a hundred times heavier than the proton, which is the core of the basic hydrogen atom, and is only about 1 GeV.

Each LHC experiment confirmed these results to about five sigmas of certainty, indicating that there is less than a one in a million probability that these were chance results resulting from something other than the presence of a new particle.

"We have a new particle consistent with a Higgs boson," said Rolf-Dieter Heuer, Director-General of CERN.

The new particle was detected by the sprays of particles into which it decays. The Standard Model predicts that the Higgs can decay into pairs of about half a dozen types of particles, but other, more ordinary matter can also make similar decays. So experimental physicists must  tally up the number of events in each of the ways a Higgs particle can decay and look for unexpected excesses in these decay events.  At a very basic level, it is like rolling a die again and again to figure out if it is loaded.

The CMS experiment studied five decay channels of which two, where the decay is into a pair of photons or particles known as Z bosons, are the most important because they allow for more precise measurements of the Higgs' mass. The combined significance of the signal for all five was 4.9 sigma.

The ATLAS experiment studied two main channels where the Higgs-like particle decays, into two photons or four leptons, a category of particles such as electrons and muons. It found a signature at around 126 GeV at a 5 sigma significance, combining the data from the two kinds of events.

"It is very, very nice for the Standard Model Higgs boson to be at that mass,"  said ATLAS spokesperson Fabiola Gianotti, because LHC is well suited to studying particles at that mass. "So thanks, nature," she added.

These results follow earlier LHC data and the Monday announcement from the Fermi National Accelerator Laboratory in Illinois of evidence, extracted from the less powerful and now-retired Tevatron accelerator, for the particle at a level of three sigma.

The Higgs mechanism answers the fundamental question of why most of elementary particles have masses. Without the Higgs, everything from stars to atoms would not exist. It was the last remaining piece of the Standard Model. Elementary particles, such as electrons and quarks, interact with the Higgs field and their interaction creates their mass.  

Today's results represent a "marvelous achievement, the cornerstone of 400 years of efforts to explain what we observe in the universe," said Gordon Kane, a University of Michigan theoretical physicist who did not work on the experiment. The data, he said, suggest a particle that's remarkably close to the Higgs predicted by the Standard Model. Time will tell, he said, as to whether the small discrepancies that currently exist between the data and the predictions of the Standard Model are experimental errors or if they point to new physics beyond the Standard Model.  If confirmed, even small deviations from the Standard Model could lead the way to new physics, and could be consistent with the predictions of major extensions to the Standard Model such as string theory.

The latest tranche represents merely a third of the data that ATLAS is expected to obtain in 2012.  The current results are expected to be published by the end of July. "There's more to come…Please, in particular theorists, be patient. There is a long path to go," ATLAS spokesperson Gianotti jestfully chided the scientific audience.        

The results came in the last runs of the experiments which ended about two weeks ago. The announcements were met by standing ovations and a surprisingly low number of questions at the end of the scientific presentations.

More questions flowed from layperson reporters at a post-seminar news conference. One reporter asked whether the results have any relevance to him, being that he's made up of elementary particles.

"I think it has a lot of relevance to you," said Heuer, the CERN Director-General of CERN, "because if that [Higgs field] would not exist, then you would not exist."


Virat Markandeya is a contributing writer to Inside Science News Service.


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