Higgsino or hallucination?

Out of the formless mist of 400 trillion particle collisions, physicists last December saw a blip in their data. At least, it might be a blip.

In 2011 and 2012, such a blip gradually turned into the “God particle” Higgs boson, the biggest discovery in particle physics in decades. If this new one is real, it would be much bigger news than the Higgs – an unexpected new particle, or even a totally new force for nature.

Then again, it could be nothing.

Actually, it’s probably nothing.

The blip: what is it?

The blip appears as a few more pairs of photons than expected at a particular energy (about 750 GeV).  This could indicate the existence of a new particle 750 times the mass of a proton (or about six times as massive as the Higgs boson).

Scientists at the Large Hadron Collider at CERN, the European Organisation for Nuclear Research in Switzerland, first reported seeing the excess in December 2015. The statistical significance of the result was low, meaning there was a good chance the blip was just a fluke, destined to be washed out in the rising tide of more data.

But what made particle physicists do a double take in December was that two detectors – the Compact Muon Solenoid (CMS) and A Toroidal LHC Apparatus (ATLAS) – saw a photon excess in that same region.

Last week at a conference in Italy, the two CERN collaborations presented a reanalysis of the same evidence. The results are a mix.

The CMS collaboration was slightly more confident about that signal in their data was not a fluke. The ATLAS data was slightly less so.

If it’s real, what could it mean?

There is no shortage of theories. Since the preliminary result three months ago, some 200 theoretical papers have proposed explanations.

Probably the most exciting possibility is that the new particle could be a graviton, a particle that transmits gravity. First proposed in the 1930s, their discovery would unite quantum theory with gravity, thereby solving perhaps the greatest mystery in all physics.

Others believe the new particle could be a relative of the Higgs boson – which some theorists believe comes in multiple flavours.

Then there’s supersymmetry, a proposed extension to the Standard Model of particle physics whereby every particle has an exotic twin. Evidence of supersymmetry would open up a world of squarks, sleptons and even a Higgsino – making particle physic textbooks read a little like a Dr Seuss story. The possible new particle could be the first in a new wave of particle discoveries.

Another idea is that this particle made up of particles that we know about, but bound together by a new, super-strong force. This would add a fifth force of nature, thus rewriting the physics textbooks.

But really, what is it?                                              

Though many physicists are entertaining the bump as intriguing, most agree that the signal will probably fade away like a mirage in the desert.

The physicists’ excitement is tempered by the statistical confidence in the data.

The thing is, human brains tend to pick out patterns in random noise, such as seeing a face in a gnarled oak tree. To really judge whether a given bump is meaningful, scientists calculate its statistical significance. In physics, this is measured in “sigma”, a way of measuring deviations from expectations.

The CMS data is a 1.6 sigma, meaning the chances of it being a fluke are the same as flipping a few heads in a row. The ATLAS data is about 2 sigma, the equivalent of tossing five heads in a row, or about 3% chance of being a fluke.

Such signals come and go all the time. The search for the Higgs itself was marked by many false alarms. As University of Pisa physicist Guido Tonelli, one of the leaders in the discovery of the Higgs boson told The New York Times in 2013: “We’ve made many discoveries, most of them false.”

Three sigma is conventionally described as “evidence” – if this were a courtroom, the exhibits would barely be admissible.

To be judged a “discovery” requires a five sigma effect, or 99.99994% confidence. (Roughly a one in three million chance that it’s a fluke.) Crossing this hurdle was what marked the announcement of the Higgs boson discovery in March 2013.

So will this bump herald a discovery? Only more data will tell.

With maintenance work at CERN just completed, the Large Hadron Collider is expected back online in the next two weeks. By July, both CMS and ATLAS teams expect to have enough data to decide whether the 750 GeV bump is real, or a mirage.

Until then, the jury is out.

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