Have we really discovered the Higgs boson?

7 Jul 2012, 5:42 pm   -   Source: Celeste Biever, CERN, The New Scientist
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Subatomic particles collide. (AAP)

Subatomic particles collide. (AAP)

What could the new subatomic particle be if it isn't the Higgs boson? Do scientists prefer a standard-model Higgs boson or an alien? A CERN scientist takes a look. 

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"As a layman, I would say, I think we have it," said Rolf-Dieter Heuer, director general of CERN at Wednesday's seminar announcing the results of the search for the Higgs boson. But when pressed by journalists afterwards on what exactly "it" was, things got more complicated. "We have discovered a boson – now we have to find out what boson it is," he said cryptically. Eh? What kind of particle could it be if it isn't the Higgs boson? And why would it show up right where scientists were looking for the Higgs? We asked scientists at CERN to explain.

If we don't know the new particle is a Higgs, what do we know about it?
We know it is some kind of boson, says Vivek Sharma of CMS, one of the two Large Hadron Collider experiments that presented results on Wednesday. There are only two types of elementary particle in the standard model: fermions, which include electrons, quarks and neutrinos, and bosons, which include photons and the W and Z bosons. The Higgs is a boson – and we know the new particle is too because one of the things it decays into is a pair of high-energy photons, or gamma rays. According to the rules of mathematical symmetry, only a boson could decay into exactly two other photons.

Anything else?
Another thing we can say about the new particle is that nothing yet suggests it isn't a Higgs. The standard model, our leading explanation for the known particles and the forces that act on them, predicts the rate at which a Higgs of a given mass should decay into various particles. The rates of decay reported for the new particle yesterday are not exactly what would be predicted for its mass of about 125 gigaelectronvolts (GeV) – leaving the door open to more exotic stuff. "If there is such a thing as a 125 GeV Higgs, we know what its rate of decay should be," says Sharma. But the decay rates are close enough for the differences to be statistical anomalies that will disappear once more data is taken. "There are no serious inconsistencies," says Joe Incandela, head of CMS, who reported the results on Wednesday.

In that case, are the CERN scientists just being too cautious? What would be enough evidence to call it a Higgs boson?
As there could be many different kinds of Higgs bosons, there's no straight answer. An easier question to answer is: what would make the new particle neatly fulfil the Higgs boson's duty in the standard model? Number one is to give other particles mass via the Higgs field – an omnipresent entity that "slows" some particles down more than others, resulting in mass. Any particle that makes up this field must be "scalar". The opposite of a vector, this means that, unlike a magnetic field, or gravity, it doesn't have any directionality. "Only a scalar boson fixes the problem," says Oliver Buchmueller, also of CMS.

When will we know whether it's a scalar boson?
By the end of the year, reckons Buchmueller, when at least one outstanding property of the new particle – its spin – should be determined. Scalars' lack of directionality means they have spin 0. As the particle is a boson, we already know its spin is a whole number and as it decays into two photons, mathematical symmetry again dictates that the spin can't be 1. Buchmueller says LHC researchers will able to determine whether it has a spin of 0 or 2 by examining whether the Higgs' decay particles shoot into the detector in all directions or with a preferred direction – the former would suggest spin 0. "Most people think it is a scalar, but it still needs to be proven," says Buchmueller. Sharma is pretty sure it's a scalar boson – that's because it is more difficult to make a boson with spin 2. He adds that, although it is expected, confirmation that this is a scalar boson is still very exciting: "The beautiful thing is, if this turns out to be a scalar particle, we are seeing a new kind of particle. We have never seen a fundamental particle that is a scalar."

If the Higgs is predicted by the standard model, how could a Higgs not be a standard-model Higgs boson? What other kinds are there?
Although the Higgs is part of the standard model, some extensions to the standard model also contain Higgs particles, such as supersymmetryMovie Camera. In this theory, which is unproven but potentially more comprehensive because it contains dark matter, there is not one Higgs but several. Right now, it's impossible to tell whether the boson reported at CERN on Wednesday is a lone actor, as specified by the standard model, or one of such a supersymmetric army. As Buchmueller says, this particle could still be an "alien or stranger".

Which do scientists prefer – a standard-model Higgs boson or an alien?
"I would be excited if it is a Higgs boson but perhaps not the standard-model Higgs boson," says Fabiola Gianotti, head of ATLAS, CERN's other large LHC experiment, who presented the team's Higgs results at the seminar. "Something that is a bit uncomfortable for the standard model." That would be a sign of physics beyond the standard model. Physicists currently know we need this, but don't know where to look. A non-standard-model Higgs would give them a clue. Incandela is more equivocal. "As experimentalists, we really want to observe nature. We have no pre-judgement," he says. "I am just excited that we have a discovery."

Higgs Boson Particle

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