Mystery boson earns Higgs status thanks to W particle

Say hello to Higgs. New data confirms that the unsatisfyingly named "Higgs-like particle" announced at CERN last year really is a Higgs boson.

Back then, the particle's discoverers were sure it was a boson, one of two types of elementary particle, and that its mass was about 126 GeV. But their data couldn't reveal all its properties.

Theory dictates that a Higgs boson must have a value of zero for a quantum mechanical property called spin, and positive "parity", which can be thought of as looking the same when reflected in a mirror. Data reported at the Moriond meeting in La Thuile, Italy, last week all but nailed these two qualities.

Most importantly, though, a Higgs boson needs to have the role it was dreamed up to perform. Peter Higgs suggested that a mass-giving Higgs field pervades all space, in order to explain why the W and Z bosons have mass but the photon, another boson, doesn't. Photons zip through the field, others are slowed to different degrees corresponding to mass and the Higgs boson is a ripple in the field.

W decay

So the real clincher came when data presented at Moriond showed the first strong sign that the new particle decays into W bosons. A Higgs boson must decay into particles its field gives mass to. There was plenty of evidence that the particle decayed into Zs, but until Moriond, no good evidence that it decayed into Ws. The Moriond result prompted Adam Falkowski of CERN and colleagues to post the analysis "A Higgs at Last" online .

There's still an important distinction, though. "It is legitimate to call this beastie 'a' Higgs boson," says Raymond Volkas, of the University of Melbourne in Australia, but not "the Higgs".

While "a" Higgs must at least give mass to the W and Z bosons, the leading standard model of particle physics assumes the boson gives mass to other elementary particles too, called fermions. This "standard model Higgs" is "the" Higgs: testing for this involves measuring its rate of decay into fermions.

Though measurements so far indicate the new particle explains some of the mass of fermions, huge numbers of decays would need to be gathered to confirm it explains all of it ? something the Large Hadron Collider at CERN may never be able to do.

Time to drop "Higgs-like boson"? Falkowski thinks so: "At this point, 'like' sounds silly."

Reference: arxiv.org/abs/1303.1812

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