Tantalising LHC 'blip' not a particle after all


Hints of an exotic new particle disappear in a deluge of new data from the Large Hadron Collider. Cathal O'Connell reports. 


New particle ... or nothing at all? Unfortunately, a bump in a graph from the Large Hadron Collider turned out to be the latter.
David Parker / Science Photo Library

It was good while it lasted. For the past eight months particle physicists debated the meaning of a mysterious ‘blip’ which appeared in data from the European Organisation for Nuclear Research (CERN) Large Hadron Collider in December. Could it be a totally unexpected, exotic new particle – one that would overturn the order of physics as we know it?

Alas, the researchers announced on Friday at a conference in Chicago, the signal faded like a mirage in the desert.

We’ve all had that experience of a weird coincidence, such as thinking of somebody just before they call. On its own, the event seems remarkable, but among the myriad chance events that constitute our lives, these coincidences are inevitable, and so, meaningless.

Such an anomaly piqued the interest of CERN physicists in December last year when two detectors – A Toroidal LHC Apparatus (ATLAS) and Compact Muon Solenoid (CMS) – seemed to discern similar evidence for an exotic new particle.

These blips can come and go, but the fact that two detectors noticed the same excess of gamma photons at an energy level of 750 GeV (750 billion electronvolts) got physicists excited.

A similar blip at 125 GeV in 2012 eventually manifested as the discovery of the “god particle”, the Higgs Boson.

In March, after reanalyses of the same data, the auspicious signs refused to disappear. More than 500 papers have been written since December purporting to explain the signal.

Some physicists hoped these were the first inklings of a groundbreaking discovery – something bigger than the Higgs boson in 2012, or than the gravitational waves caught earlier this year.

Possible explanations included a heavier cousin of the Higgs particle, or even a graviton – the long sought-after gravity particle that could connect general relativity and quantum mechanics.

Those hopes have now been dashed.

Between April and July, the Large Hadron Collider collected four times more data than it had previously at a collision energy of 13 TeV or (99.999999% the speed of light). Now armed with a huge dataset totalling 1,000 trillion proton-proton collisions, the blips at 750 GeV have disappeared.

This is a recurring story in the particle physics world. The eventual discovery of the Higgs, for example, was preceded by several false alarms.

The December data (and the March reanalysis) both carried a confidence level of about 97% (meaning a 3% chance of fluke) – which is deemed significant in some fields of science.

Not the pedantic world of particle physics, though, where a suspect signal needs 99.99994% confidence to be heralded as a "discovery". The fleeting appearance of the 750 GeV signal justifies this level of rigour. Better safe than sorry.

In the meantime, the LHC has been breaking all records in terms of its luminosity (related to how many particle collisions it can orchestrate per second). Physicists are collecting data at a far higher rate than ever before, and are still on the hunt for its grandest quarry – dark matter.

Cathal 2016.png?ixlib=rails 2.1
Cathal O'Connell is a science writer based in Melbourne.
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