What the latest LHC revelations say about the Higgs

Hint of the Higgs? (Image: Thomas McCauley, Lucas Taylor/CERN)

In December 2011, the elusive Higgs boson was back in the limelight when hints of the particle emerged in the wreckage of proton collisions at the world's most powerful particle smasher – the Large Hadron Collider (LHC) near Geneva, Switzerland.

There have been no new collisions since, but researchers from the LHC's two main detectors have given the existing data a more careful look, culminating in two fresh Higgs analyses released on Tuesday. Here New Scientist disentangles what we can and can't conclude about the particle that is swiftly becoming the people's favourite

Remind me: what was the scoop in December?
The Higgs boson is the missing piece of the standard model of physics, the leading theory for how particles and forces interact. The Higgs is thought to endow others with mass but has yet to be positively observed, so its mass – and existence – are still unconfirmed.

In December, the LHC's two main particle detectors, CMS and ATLAS, each reported excesses of events, such as the appearance of a pair of photons in the shrapnel from particle collisions. These excess events could be due to a Higgs with a mass of around 125 gigaelectron volts (GeV; particle masses and energy can be treated interchangeably).

Great. End of story?
Far from it. Since more mundane reactions can produce such events too, they do not provide definitive evidence for the Higgs.

By convention, researchers only declare a discovery when an anomaly reaches a statistical significance known as 5 sigma, which means there is less than a 1-in-a-million chance it is just a fluke. The size of the anomalies reported by the two detectors in seminar in December was 1.9 sigma for CMS and 2.5 sigma for ATLAS, which indicate a probability of a fluke of roughly 1 per cent.

So what's the latest?
After taking a closer look at the data, ATLAS finds little need for revision, reporting the same 2.5 sigma significance as before. But the CMS analysis reports some extra events that were not analysed in time for LHC's December announcement. These events could be due to a Higgs boson of around 125 GeV decaying into a pair of photons – after being produced from the collision of two particles called W and Z bosons, that transmit the weak force.

Adding these events to the December analysis gives a small boost to the overall statistical significance of CMS's Higgs hint, which now stands at 2.1 sigma. "It's not a radically different picture," says Greg Landsberg of Brown University in Providence, Rhode Island, who works on CMS.

What's this about 4.3 sigma, then?
A blog post on the Nature website on Tuesday states that an unofficial synthesis of the two results gives a significance of 4.3 sigma. But Pauline Gagnon of ATLAS says it would be premature to reach that conclusion before the Moriond conference in La Thuile, Italy, in March. "Combining the two experiments may happen after the first week of March but not before," she says. "Until then, it is anyone's guess."

Will combining the CMS and ATLAS results boost the significance?
Not necessarily, Landsberg says. The two detectors do not see an excess at exactly the same mass: CMS sees one at 124 GeV, ATLAS at 126 GeV. That might be due to errors in mass measurements by at least one of the detectors, in which case further analysis could bring the two masses into line.

The other possibility is that they are both seeing flukes that happened to show up at nearly the same mass. "You have to carefully sift through different sources of uncertainties," warns Landsberg, to figure out which answer is more likely.

When will we know whether the Higgs is out there?
More data will be needed. The LHC will start up again in late March or early April after its winter shutdown. CERN officials are still trying to decide whether to boost the energy of this year's collisions from 7000 GeV to 8000 GeV. A decision could be announced as early as Friday, says Laurent Serin of the French National Institute of Nuclear and Particle Physics (CNRS/IN2P3).

If the Higgs exists, collisions at the higher energy would produce more Higgs particles, making it easier to spot. Either way, the LHC will confirm or rule out the Higgs by the end of 2012, says Landsberg. "It's a very exciting year, and hopefully a year from now I can point to discovery papers."

References: CMS report, arxiv.org/abs/arXiv:1202.1488; ATLAS report, arxiv.org/abs/arXiv:1202.1408

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