Frontiers Physics World  October 2021

Do high-energy neutrinos lurk in SN1987A?

The SN1987A supernova event might be the source of four particles detected in Japan and the US – and so possibly explain the origin of the most energetic cosmic rays, as Edwin Cartlidge reports

Cosmic conundrum Did the Kamiokande-II or IMB experiments intercept high-energy neutrinos from the SN1987A event? (Courtesy: ESO)

Data collected more than 30 years ago could contain evidence of high-energy neutrinos generated by a supernova. That is the claim of Yuichi Oyama, a physicist at the KEK research institute in Japan, who says two experiments appear to have intercepted such particles from the SN1987A event on 23 February 1987 (arXiv:2108.05347). The finding could help explain the origin of the most energetic cosmic rays – although other experts say there is not enough evidence to back up the claims. 

Supernovae are enormous explosions occurring when heavy stars run out of nuclear fuel and implode – in the process creating shock waves that eject the star’s exterior. SN1987A was the closest supernova to be seen for well over 300 years, taking place about 170,000 light-years from Earth in the Large Magellanic Cloud. Its light arrived in February 1987 and reached peak brightness three months later. It was also the first supernova from which physicists detected neutrinos, with 25 of the elusive subatomic particles being registered by two underground experiments – Kamiokande-II in Japan and the Irvine-Michigan-Brookhaven detector (IMB) in the US.

Those neutrinos were all detected within the space of just 13 seconds, confirming existing models of supernovae and earning the then Kamiokande-II spokesperson, Masatoshi Koshiba, a share of the 2002 Nobel Prize for Physics. However, the neutrinos all had relatively low energies – a few tens of millions of electronvolts. Theorists have predicted, though, that such supernovae should also give off neutrinos with billions of electronvolts within about a year of the explosion. They would arise from the decay of pions produced when protons accelerated by the remnant star collide with the ejected material.

Observation of these high-energy neutrinos would confirm that at least some of the extremely energetic cosmic-ray protons reaching Earth are accelerated within supernovae. But identifying such particles is a formidable task. To detect neutrinos, scientists fill huge tanks with water or other liquids and measure the Cherenkov light given off when muons created by the interaction of neutrinos with nuclei in surrounding rock pass through the liquid. But to do so researchers must screen out detections of muons produced by cosmic-ray protons in the atmosphere (the cosmic rays themselves being susceptible to magnetic deflection on their journey to Earth and therefore unable to reveal their own points of origin).

The trick in the case of high-energy neutrinos from supernovae is to look only for those neutrinos reaching detectors from below, having passed through the Earth to get there. Any muon generated by cosmic-ray protons in the atmosphere on the far side of the planet will penetrate a few kilometres at most into the Earth’s crust, and therefore cannot be confused with the muons of interest.

Oyama has analysed the “upward-going muons” recorded by Kamiokande-II and IMB. To work out how many of these events can be tied to high-energy neutrinos from SN1987A, he whittled them down using two criteria: that they occurred between 11 August and 20 October 1987; and that they were no more than 10° away from the direction of SN1987A. In doing so, he found four events that fit the bill – two in each of the experiments.

As Oyama points out, these events might still be noise – specifically, neutrinos generated by cosmic rays on the far side of the Earth. But such background neutrinos are themselves rare. Indeed, he says the Kamiokande-II and IMB data show that not even one such neutrino would be expected in the spatial and temporal window he selected. Combining the individual probabilities that each of the four events is background, he calculates the odds of them not originating in SN1987A to be 0.27%.

Oyama says that neither the Kamiokande-II or IMB collaborations considered their respective detections to constitute statistically significant evidence, and they only learned of their counterparts’ data when some of the IMB members went to work in Japan in 2004. He released his paper only after having tried, and failed, to persuade the remaining members of the two collaborations to issue a joint publication. Indeed, one anonymous former colleague from Kamiokande-II does not agree with how the analysis was done, arguing that it relies on an a posteriori statistical calculation.

Those doubts are shared by Francis Halzen of the University of Wisconsin-Madison, US, who is principal investigator of the IceCube neutrino detector in the South Pole. He points out that Oyama has not employed a “blind analysis” in which the time period and angular window would be chosen before the data are known. Oyama points out it was impossible to do a blind analysis on data that are more than 30 years old but says that he tried to guard against making the choice of spatial and temporal windows “too intentional”, adding that very slightly smaller windows would have boosted the statistical significance considerably. The important thing, Oyama maintains, is to release the data and let others decide for themselves.