Many think the sterile neutrino could be behind the reactor anti-neutrino anomaly – as well as a candidate particle of dark matter.
New results from an experiment in China are hinting at the existence of a hitherto unknown kind of matter, and piling on suspicions about it since the early 1990s. The ‘matter’ comes in the form of a neutrino – a ghostly fundamental particle three forms, or flavours, of which are already known: electron neutrino, muon neutrino and tau neutrino.
The existence of neutrinos was postulated by the Swiss-American physicist Wolfgang Pauli in 1930 to account for some missing energy in particle physics experiments that the theory wasn’t able to account for. These particles are incredibly hard to detect because they interact with others only via the weak nuclear force and gravity. At any given moment, there are hundreds of trillions of neutrinos passing through you with your body noticing nothing at all. So the detectors built to detect them are made of extremely pure materials and maintained at a heightened sensitivity.
And it was at such an experiment – the Liquid Scintillator Neutrino Detector (LSND), New Mexico – that physicists first started to conjecture a fourth kind of neutrino between 1993 and 1998. Then, a beam of antiparticles called anti-muons were made to strike a target and release anti-neutrinos. LSND found that more electron anti-neutrinos were being produced than expected. The cause has since been attributed, among other alternatives, to a mysterious (yet hypothetical) fourth kind of neutrino. Physicists think it doesn’t even interact via the weak nuclear force and makes do with gravity, earning it the name of a ‘sterile’ neutrino.
Its presence was sensed by detectors of the International Daya Bay Collaboration, which receive neutrinos emitted at a nuclear reactor located within two kilometres away. Daya Bay in particular is noted for its study of neutrino oscillations, a tendency of the particles to change from one flavour to another as they travel. So if a beam of X electron neutrinos starts off from the source, detectors at the destination will be able to detect fewer than X electron neutrinos as well as few tau or muon neutrinos, which some electron neutrinos will have oscillated into.
In results published on February 12, in the journal Physical Review Letters, physicists at Daya Bay noted that 6% fewer electron antineutrinos (the particles’ anti-matter counterparts) were being detected than what the current paradigms predicted. This is called the ‘reactor antineutrino anomaly’ (RAA). The results also indicated that there were more electron antineutrinos weighing 5 MeV than there should be.
Many think the sterile neutrino could be behind both mysteries but in different ways. The more straightforward case for their existence is from the RAA, where the 6% of electron antineutrinos could’ve oscillated into sterile antineutrinos and so become invisible to the detectors. However, the odds of this detection being a fluke are about 1 in 350 – not enough to claim a discovery (for that, the odds of a fluke will have to be 1 in 3.5 million). To close this link, physicists will require more sensitive detectors that can observe more antineutrinos and so amass more proof that the deficit really exists.
The RAA as a concept entered into use following a French investigation in 2011. At the time, physicists waiting for the Double Chooz experiment in Chooz, France, to come online were double-checking an estimate of neutrino and antineutrino detection rates first obtained in the late 1980s. Then, they realised that their modern methods were detecting 3% more antineutrinos than older experiments had. The find further legitimised the LSND experiment’s results in 1993-1998 as well as the findings of a Russian-American experiment in 2005.
On the other hand, the abundance of electron antineutrinos at 5 MeV has been observed such that the odds of a fluke are at most about 1 in 100,000. This is still not enough to claim a discovery but does make for some strong evidence. “This unexpected disagreement between our observation and predictions strongly suggested that the current calculations would need some refinement,” said Kam-Biu Luk of the University of California at Berkeley and co-spokesperson of the Daya Bay Collaboration in a statement.
If that refinement is achieved by using the sterile neutrino, then it will bolster the case for this new kind of fundamental particle. But in the process, it will also destabilise the Standard Model, a set of theories used to describe the behaviour of all known fundamental particles. The model’s last predicted particle – the Higgs boson – was found in 2012*, and it has no room for a sterile neutrino. But if the sterile neutrino is confirmed to exist, then theoretical physicists will have to rework the model and, effectively, over six decades of work that went into putting it together.
The refinement itself concerns one of the central problems of neutrino physics, even particle physics in general: the neutrino mass hierarchy – determining which flavour is the lightest and which is the heaviest. The relationship between their masses is determined by a number called theta-13, measured in degrees of an angle, and experiments typically log data to inform physicists’ calculations of it. It was the Daya Bay Collaboration that measured theta-13 most accurately in 2012, arriving at 8-9º. Another upcoming Chinese neutrino experiment, called the Jiangmen Underground Neutrino Observatory, will continue to try to resolve the hierarchy.
The value of this angle and the neutrinos’ tiny masses has implications for many aspects of physics – going all the way to determining the number of galactic clusters in the universe. Before the first stars formed, and the universe was a hot and dense blob of plasma, the masses of the fundamental particles stood to determine how much the matter made of them would clump together once the universe started to cool down. Two physicists calculated in 2014 that if the three known flavours of neutrinos altogether weighed about five times more than they’re thought to, the resulting effect on clumping would explain the distribution of galaxies predicted by other observations.
Moreover, because the sterile neutrino interacts only via the force of gravity, it is also being considered as a candidate for dark matter.
*The Higgs boson was actually found sometime in 2011, the discovery announced in 2012, and confirmed to be so in 2013.