The possible discovery of a new particle in Hungary, and its subsequent interpretation as the force behind dark matter, has kicked up some dust. However, something’s off about the Hungarian results…
It’s called the Atomki anomaly. ‘Atomki’ is the nuclear physics research centre at the Hungarian Academy of Sciences in Debrecen, Hungary, and the site of a certain experiment that first spotted the anomaly about two years ago. Though there are some doubts about what really has been found, the news of something being anomalous at all – a new particle? – has stoked excitement in a community desperately looking for something new. In fact, one interpretation would have us believe that, if other tests around the world are able to hold up the Atomki results, it could be a phenomenal new discovery: of a fifth fundamental force in nature, possibly related to dark matter.
In the experiment, scientists fire protons at a lithium atom. A lithium atom contains four neutrons and three protons. When it captures an extra proton, it transmutates from a lithium-7 atom into a beryllium-8 atom, 8 being the new sum of protons and neutrons: four and four. However, the stable beryllium atom needs five neutrons and three protons, so it starts to lose the extra proton’s worth of energy through radioactive decay. In this process, the beryllium-8 atom emits a photon that then decays into one electron and one positron (the electron’s antimatter counterpart).
Particle X
So far so good – until the Atomki team saw something weird happening. The beryllium-8 atoms would emit photons at different energies each time they decayed. But once every few million times they decayed by emitting a photon of higher energy, the electron and positron the photon decayed into flew off at a larger angle than usual. Specifically, a particle emitted at around 17 MeV would decay into an electron and positron flying off at a high 140º. Is this particle still a photon? Or, at a simpler level, is this something we’re measuring now because we’ve better detectors or are performing more advanced experiments?
Apparently not. The Atomki team performed multiple tests and found that the anomalous decay angle kept manifesting itself. They also tried various computer simulations to see if it was simply a previously unobserved but perfectly explicable phenomenon. Their conclusion appeared in a pre-print paper they submitted in April 2015 analysing the results: “To the best of our knowledge, the observed anomaly can not have a nuclear physics related origin.”
This was both not-so significant and very significant at once. It was not-so significant because the field of nuclear physics is more well-explored than the field of particle physics, where the particle’s inexplicable origins found greater relevance. So a nuclear physicist being stumped about a particle-physics finding was okay. On the other hand, the result was also significant because their paper on the experiment was published in the prestigious journal Physical Review Letters in January 2016…
… at the same time the particle physics community was grappling with the possibility of a new particle – this one much heavier – being spotted at the Large Hadron Collider in CERN, Geneva. And both announcements fed into each other’s excitement because of what the community as such was hoping for: new physics. New physics is the branch of physics thought to be able to explain facts about the universe that our current level of understanding struggles to. After many years of trying with the LHC and other experiments, physicists hadn’t found any signs of new physics – but suddenly, they had two leads on their hands. Was it too good to be true?
As it turned out: yes. The LHC particle, which some physicists hoped would be an elusive carrier of the gravitational force, turned out to be a glitch in the data. The official declaration was made on August 5 at the 38th International Conference on High-energy Physics, Chicago. Fortunately, the Atomki anomaly particle still survives, and because of the climate it survives in – of a desperation to find signs of new physics – it continues to amass significance.
In fact, the particle shot to the limelight almost a year after its presence was announced when a team of American physicists figured one way to explain its properties would be to think of it as a new boson, a force-carrying particle, called a dark photon. Their interpretation implies that the dark photon acted through a fifth fundamental force, one possibly dictating how the mystery substance known as dark matter interacts with other particles. We currently know only four forces: the strong nuclear, the weak nuclear, the electromagnetic and gravitational forces. A fifth force would be a tremendous claim. And if confirmed, it will be a momentous occasion in the history of the study of the natural universe – the same way the discovery of the Higgs boson by January 2013 was.
Fluke happens
But will it be confirmed?
There is some apprehension on this count for two reasons. The first is that scientists with the Atomki experiment have claimed to have discovered the same particle twice before 2015 – in 2008 and 2012 – and each time at a different energy level from the previous time. Oscar Naviliat-Cuncic, a nuclear physicist at Michigan State University, flagged in a report in Quanta the multiple problems with the Atomki team’s claims:
- On what grounds were the previous claims of having found the same particle retracted?
- If the same claim was retracted twice, does it mean the team is not recording the uncertainties in its measurements? And if so, does this mean the current claim will be retracted in four years?
- Why has the Atomki team not reported any results in the last few years where it hasn’t found a boson?
… and so forth.
Another issue with the result concerns why the particle hasn’t shown up in previous high-energy physics experiments. This is particularly so because the mass of the supposed particle, about 17 MeV (about 34-times as heavy as an electron), is low enough to have been found by the LHC, which probes energy levels up to little more than a dozen thousand MeV.
On a separate note, a quick Google Scholar search for the work of Attila Krasznahorkay, the head of Atomki and the leader of the team that claims to have found the new particle, involving beryllium-8 atoms throws up at least five papers, from 2005, 2008, 2012, 2015 and 2016. Each claims a discovery involving a new kind of boson. Moreover, between 2001 and 2005, Naviliat-Cuncic had found that a previous leader of the Atomki group had found ‘evidence’ pointing to the existence of multiple bosons, but had never directly found any of them.
Then again, working in Atomki’s favour is the fact that many physicists have acknowledged the Hungarians’ paper in Physical Review Letters to be theoretically sound and free of obvious mistakes.
Double-checking strong results
The second reason for apprehension has to do with the misfortune that has befallen many other claims in particle physics. One particularly infamous claim in the recent past was of the discovery of signs of primordial gravitational waves by the BICEP2 observatory operated at the South Pole. Scientists from the Harvard-Smithsonian Centre for Astrophysics, who analysed the results, reported in March 2014 that the observatory had found the signs imprinted in a certain form of radiation known to be spread throughout the universe. The odds of it being a fluke were 1-in-588 million. However, the scientists had made a mistake in assuming that what they were looking for had only one cause – primordial gravitational waves – and had failed to eliminate other possibilities. This oversight, a form of bias, led to their claims being disproved by the ESA Planck telescope within a year despite the strength of observations.
As it happens, the Atomki particle was found with the fluke-odds at an immensely minuscule 1-in-200 billion. Yet, it can’t be taken for granted until other experiments are able to replicate the results to the same effect – especially when there is reason to suspect that the Hungarians might be cherry-picking from their data to show only bosons. These checks are also necessary because the Atomki particle, if indeed it is something exotic, is at the outer edge of what physics knowledge we have, in the midst of uncertainties we aren’t fully aware of.
It also pays to note that the Americans’ interpretation of the Atomki particle is just one interpretation. Other interpretations may not yet have been publicised but theoretical particle physics has been known to play host to a great variety of explanations, to explain the same thing in different ways. For example, physicists had written over 500 papers by June of the particle spotted at the LHC in December 2015, each outlining a distinct way to understand how the new particle might have been created. Over time, the Atomki anomaly will also surely feed a similar number of papers if its mystery persists.
But curiously enough, the Americans – physicists from the University of California, Irvine, and the University of Kentucky, Lexington – were able to bolster their idea by claiming it would be able to solve another persistent problem in physics as well. This is an added bonus because one theory being able to resolve multiple issues is a sign of the theory’s validity and robustness. The second problem has to do with a particle called a muon, and how it behaves inside a magnetic field. There is a difference between how our existing theories of physics predict the muon will behave and how the particle has been observed to behave. The Americans argue that their dark photon interferes with the muon’s interactions with the field. And because the second problem has to do with a different particle, it becomes an additional way to test the ‘dark photon theory’.
But for all its supporters and detractors, the Atomki anomaly out of Debrecen has got one thing straight: show up at a crucial time for particle physicists, who in their desperation are following up the slightest deviations from normal in their search for any signs of new physics. If the anomaly was in fact a cherry-picked result by Krasznoharkay & Co., now is the time – under the blinding glare of experimental collaborations from around the world – when its reincarnations will also be laid to rest. After all, one does not simply walk into new physics territory.