Definitely, maybe: evidence grows for positive ID of Higgs boson

It’s now safe to drop the ‘like’ from Higgs-like particle.

Physicists say they are now almost certain that the curious “Higgs-like particle” revealed last year at CERN is a Higgs boson.

Data presented this week at the Rencontres de Moriond meeting in Italy – billed as a place for physicists “to discuss recent findings and new ideas in physics in a pleasant, relaxed and convivial atmosphere” – showed that the Higgs-like particle behaves in ways that suggest it is actually a Higgs boson.

The Higgs boson is the missing piece of the puzzle in the Standard Model of physics, and is crucial to understanding why particles have the masses they do.

A statement on the CERN website said the new scientists speaking at Moriond announced last week “that the new particle discovered at CERN last year is looking more and more like a Higgs boson. However, more analysis is still required before a definitive statement can be made.”

“Finding out what kind of Higgs it is will rely on carefully measuring the particle’s interactions with other particles, and that may take several years to resolve,” the statement said.

The new findings, presented by CERN scientist Adam Falkowski and colleagues, show that the Higgs-like particle decays into W and Z bosons – crucial criteria that must be met before the particle could be confirmed as a real Higgs boson.

“Previous measurements of the ‘Higgs-like’ boson determined that it is probably a boson, which means it has a spin quantum number of zero and that it had positive parity. These are qualities we would expect of the Higgs boson, but also of just a Higgs boson (there are models with more than one, and of differing types) or another particle entirely,” said Jonathan Carroll, Post Doctoral Research Associate at the University of Adelaide’s Centre for the Subatomic Structure of Matter.

“What remains is to sort out a small excess in the two-photon decay channel; to observe it actually decay into fermions; rule out spin-2 to make sure it really is a spin-0 boson (though observing the fermion decays will serve this purpose just fine); and to make sure that it doesn’t have a twin,” said Dr Carroll.

“These will quite likely occur in the next few years, but at the moment ‘Higgs’ seems to be the best label to put on this particle.”

Dr Nitesh Soni, ARC Research Associate at the Centre of Excellence for Particle Physics at Tera Scale) at the University of Adelaide, said the fresh evidence was “compelling enough to drop the "like” from Higgs-like particle and say it was a Higgs boson.“

"But the remaining question is if it is the ultimate boson or do we have more similar, unobserved bosons?”

“To come up with a significant answer on fermions, we will have to wait for the restart of Large Hadron Collider machine,” he said.

The University of Adelaide’s Professor Anthony Thomas, director of the Adelaide node of the ARC Centre of Excellence in Particle Physics at the Terascale, said it is crucial that the Higgs boson’s existence and properties be established beyond doubt.

“This work adds some confidence concerning one feature of the Higgs, namely its couplings to bosons. However, the essential missing element is the coupling to fermions and there we still need more information and this may not come until after the shutdown of the Large Hadron Collider – not until 2015,” he said.

“The whole subatomic physics world is waiting for that information with great anticipation.”