Happy birthday, Higgs boson! A year ago today, the discovery of the particle credited with giving others mass was announced to a packed and jubilant auditorium at CERN near Geneva, Switzerland. The moment marked the end of a 50-year hunt. But although the boson has been found, there is still plenty we do not know about the celebrated particle. Here are the most interesting unknowns that surround the Higgs boson.
What kind of Higgs boson is it?
When the particle's discovery was announced, researchers working with the Large Hadron Collider (LHC) at CERN resisted calling their quarry the Higgs boson outright, preferring the vaguer "Higgs-like boson", or "particle resembling the Higgs". They knew the particle they had glimpsed was a brand new boson, one of two types of elementary particle. But it was not clear if its properties corresponded exactly to those laid out for the Higgs in the standard model of particle physics, which describes all known particles and the forces acting on them.
In fact, many physicists were hoping the boson would prove to be something more exotic, because this would suggest ways to extend the standard model, which currently cannot explain dark matter or gravity, for example.
A year on, key properties known as spin and parity, as well as the exact particles the boson decays into, are gradually being pinned down, and the boson seems to be behaving as expected, leading to the award of official Higgs status in March. "We have made enough property measurements to start to convince ourselves that what we are looking at is a Higgs boson," says Oliver Buchmueller of Imperial College London, a member of the LHC's CMS collaboration, one of the two teams that announced evidence for the Higgs.
But even though it fulfils the minimum requirements of a Higgs boson, that doesn't mean it is vanilla, says Buchmueller. "The precision with which we are measuring these properties today is not sufficient to say whether this is the minimal realisation of the Higgs mechanism – or something more." One mystery that remains is why the Higgs boson decays into more photons than expected. This excess was initially reported last July. In the latest analyses, ATLAS still sees the Higgs decaying into too many photons, while in data collected by CMS there is no excess.
Why is the Higgs so light?
When the particle was found, its mass was also pinned down and, coming in at about 125 gigaelectronvolts (GeV), the Higgs is surprisingly light. If this mass is plugged into the standard model along with the masses of all the other known particles, it leads to a prediction that the universe is unstable. And that points to new physics, says Buchmueller. Obviously the universe is here now, so a light Higgs raises the prospect of further particles to stabilise things. "If the Higgs boson had a mass of 135 to 140 GeV, the universe would have been stable. Then we really would have problems for what to do next," says Buchmueller.
How much more can the LHC tell us about the Higgs?
Whatever physicists do next, they'll be doing it without a major particle collider for a few years. The LHC is currently in sleep mode, undergoing two years' worth of upgrades that will push its colliding proton beams to 7 teraelectronvolts (TeV) each, its maximum design energy. The supercharged LHC might throw up evidence for dark matter or extra dimensions, which could influence our understanding of the Higgs.
The upgrade will also boost the rate of Higgs production to twice as fast as before, giving us plenty of statistics for studying its properties further. Buchmueller is particularly excited about this, because the Higgs and its associated field are the first such entities known to have a spin of 0, which means that – unlike gravity, for instance – they lack directionality. Further Higgs production may teach us about "spin-zero fields" in general, which might offer clues to the nature of dark energy, the mysterious force causing the universe's expansion to accelerate, and which may be a spin-zero field.
How many Higgses are out there?
A more powerful LHC could also reveal whether the Higgs has siblings. "No one will be surprised if there is a second type of Higgs particle in nature," wrote Matt Strassler, a physicist at Rutgers University in Piscataway, New Jersey, in a blog entry on 2 July exploring the question of multiple Higgses. The entry was sparked by recent rumours of evidence for a second Higgs-like particle, which have since been dismissed by Strassler and other physicists contacted by New Scientist.
But as Strassler says, there is nothing ruling out the existence of further Higgses. For example, one popular way to extend the standard model is a theory called supersymmetry, or SUSY, which specifies a minimum of five types of Higgs boson. Though evidence of SUSY has so far failed to show up at the LHC, physicists will be watching for it keenly when the collider switches on again in 2015.
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