Closing in on the ‘God particle’

Image from the CMS. Copyright CERN

Physicists are closing in on the Higgs boson, the missing piece of the Standard Model of particle physics — but they aren’t quite there yet.

UC Davis physics professor Mani Tripathi said that although not yet conclusive, the results so far were a “shot in the arm” for the thousands of young scientists working on the project.

“The students and postdocs have been working very hard and such positive developments keep the enthusiasm level high,” he said.

Two international teams of thousands of scientists working at the Large Hadron Collider in CERN, Switzerland presented their latest results today. The ATLAS and CMS experiments have made progress in narrowing the energy range at which the Higgs could exist, but have yet to pin it down with enough statistical precision, or “five sigma” in mathematical jargon.

“The fact that the results from two different experiments, CMS and ATLAS, have an excess at basically the same mass, around 125 GeV, is very exciting,” said UC Davis physicist John Conway, who is a collaborator on the CMS experiment.

“When the results are combined in the coming weeks, the significance of this excess will be quite strong.  It won’t be the golden “five sigma” for discovery, but I think that there is every reason to believe this is the start of the exploration of the Higgs boson at the LHC.  There is much to do, and in the next year we’ll learn a lot more about it!”

“Of course it is exciting that both ATLAS and CMS see excesses consistent with the expectations for a SM (Standard Model) Higgs boson (or one that is close to SM-like) at 125 GeV (Giga electron-volts).  That they see such excesses consistent with the same mass persuades me that this is very unlikely to be a statistical fluctuation,” said UC Davis Professor Jack Gunion, who literally wrote the book — first published in 1983 — on how to hunt down the Higgs.

“It is quite striking that two independent experiments both find excess of events in the same region. This overlap is over and above the fact that various decay channels of the Higgs also find excesses in the same region.  If this is all a coincidence, nature is too cruel on us physicists!” Tripathi said. 

The Higgs particle is the missing piece of the Standard Model of particle physics, which effectively explains the behavior of the particles that make up the visible universe. The existence of the Higgs boson was proposed in the 1960s by British physicist Peter Higgs to explain why particles have mass. If Higgs bosons exist, according to theory, they can only appear for a fleeting time from extremely high energy collisions between other particles. The Large Hadron Collider, the most powerful particle accelerator ever built, was designed to produce these high energy collisions and identify the particles that spill out of them.

However, in recent years physicists and astronomers have discovered that most of the universe is made up of ‘dark matter’ and ‘dark energy’ that does not appear to be made of the same matter and energy as our visible universe. There is also a long-standing problem of reconciling the Standard Model, which deals with extremely small particles, with General Relativity, which describes the behavior of gravity on an extremely large scale.

Conclusively identifying — or ruling out — the Higgs boson would help physicists confirm or reject the various theories that aim to unify the Standard Model and Relativity or attempt to address dark matter and dark energy.

“Theoretically, if you accept a SM-like Higgs at 125 GeV, then this discards about 80 percent of the theories out there and strongly constrains many others.  If other types of new physics are seen (nothing so far) or strongly limited, the theoretical possibilities will become even more constrained,” Gunion said.

A number of UC Davis physicists are working on the Large Hadron Collider, especially at the CMS experiment which is run primarily by U.S. scientists.

Read my 2008 feature about the convergence of cosmology and particle physics for UC Davis magazine.

More information:

CERN press release

John Conway blogs on “Making the Higgs sausage”



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