In December 2015, physicists at the Large Hadron Collider experiment at CERN, Geneva, unveiled a plot of results obtained from the giant particle collider. It showed a curious bump in the curve – indicative of a particle weighing about 750 GeV/c2. The physicists were excited because such a particle’s existence had not been predicted by existing theories, and it signified the possible start of a new chapter in physics.
But it isn’t to be. Since December, the LHC has been smashing more particles and collecting more data – at least four-times more than what was available when the Higgs boson was discovered in January 2013. Strong rumours in the particle physics circuit, as well as blog posts by physicists involved in the experiment, have it that the bump is no longer visible on the plot. In fact, there might even be a dip there instead.
Particle physicists are keen for the ‘existing theories’ that explain the behaviour of elementary particles, collected under an umbrella framework called the Standard Model, to be disproved. Over the last few decades, the model has predicted the existence of a family of particles all of which have been discovered. However, it remains unable to answer some lingering questions. The LHC is aiding the quest, to find some new answers, by running experiments to find particles doing things that the model can’t explain. In other words, physicists are trying to break the model – and failing.
It’s a nightmarish place to be: to have some knowledge, know that it is faulty in some way, but not know which way.
The rumours of the 750-GeV particle’s disappearance started to trickle in in mid-July. They are intensifying now as the 38th International Conference on High-Energy Physics (ICHEP), the world’s largest particle physics conference, kicked off in Chicago on August 3. Various experimental collaborations from the LHC will be presenting hundreds of results obtained from the latest experiments. The definitive word, insofar as there is one, on the 750 GeV bump will be presented on August 5.
The December 2015 announcement, which revealed the bump for the first time, came from the ATLAS collaboration at the LHC, which works with the detector it is named for. Even at that time, a problem with the plot was widely discussed: that it was composed of too few results. The ATLAS detector records what happens when trillions of protons are collided at almost-light-speed at its centre by the LHC. One output, obtained after intense filtering and analysis, is a plot showing what heavier particles might’ve formed and how they decayed to lighter particles.
A bump on this plot is called a resonance. It signifies an unusual accumulation of energy at a point, construed as a particle. Such a particle is not directly observed because it decays too quickly into a signature group of lighter particles. Instead, physicists use the lighter particles and their properties to predict what could’ve decayed into them. The 750 GeV particle was thought to have decayed into two photons – referred to as the diphoton signal.
However, because of the number of collisions being conducted, it’s quite likely that the detector picked up on a ‘fluke’ event. Physicists try to eliminate such events by analysing larger and larger datasets in which flukes are evened out. And this is what happened since December 2015.
Matt Strassler, a theoretical physicist, wrote on his blog, “… it seems that what we saw in those December plots was a fluke. It happens. I’m certainly disappointed, but hardly surprised. Funny things happen with small amounts of data.”
Another theoretical physicist, Adam Falkowski, wrote on his: “In the recent years, physics beyond the Standard Model has seen two other flops of comparable impact: the faster-than-light neutrinos in OPERA, and the CMB tensor fluctuations in BICEP. Much as the diphoton signal, both of the above triggered a binge of theoretical explanations, followed by a massive hangover. There was one big difference, however: the OPERA and BICEP signals were due to embarrassing errors on the experiments’ side. This doesn’t seem to be the case for the diphoton bump at the LHC.”
That’s right. This is a trying time for theoretical physicists, not the experimentalists. Between December 2015 and August 2016, more than 500 papers explaining the 750-GeV bump in various ways uploaded to the arXiv preprint server – and all of which now stand dashed. As Falkowski goes on to discuss, the biggest takeaway from this episode is that even though the ATLAS bump was pretty statistically significant (apart from the fact that it involved a smaller dataset), and was somewhat corroborated by the CMS detector at the LHC earlier this year, it still turned out to be a fluke.
“Given that for 30 years we have been looking for a clue about the fundamental theory beyond the Standard Model, our reaction was not disproportionate once a seemingly reliable one had arrived. Excitement is an inherent part of physics research. And so is disappointment, apparently.”
No other anomalous results are expected to be presented at ICHEP this year. Nonetheless, thousands of physicists will be flocking to the conference – and catching updates via new papers and Facebook videos – to hear the LHC’s other results. After all, the results have been gleaned from the largest deposit of data ever obtained of the world of high-energy physics. There might yet be something.