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Are ‘sterile neutrinos’ dark-matter particles after all?

Graph showing recent constrains on sterile neutrino production

Could sterile neutrinos constitute dark matter? (Courtesy: Bubul et al./ arXiv:1402.2301)


By Tushna Commissariat

It is always interesting to us at Physics World when a particular topic suddenly attracts the attentions of the physics community, especially when it’s a rather hotly debated subject. The past couple of days, for example, have seen a lot of talk about “sterile neutrinos”, based on two papers – published in quick succession on the arXiv preprint server – that suggest the tentative detection of these hypothetical paricles.

Both papers are based on an unidentified emission line seen in the X-ray spectrum of some galaxy clusters obtained by the European Space Agency’s XMM-Newton observatory. Intriguingly, sterile neutrinos are also considered to be possible dark-matter candidates, meaning that – if discovered – they would be the first fundamental particles to lie beyond the bounds of the Standard Model of particle physics.

For those of you who haven’t heard about the sterile neutrino before, it’s a proposed and much-debated fourth type of neutrino that would have a mass but would not interact with the weak force at all, in contrast to the three types of neutrinos we do know exist – the electron, muon and tau neutrinos. Indeed, sterile neutrinos (if they exist) are likely to be even trickier to detect than the others as they would interact only with these “active” neutrinos.

In the new research, Bulbul et al. and Boyarsky et al. both identified an extremely weak, monochromatic, 3.5 keV line in the X-ray spectrum that could be interpreted as a signal emerging from a decaying 7 keV sterile neutrino. The graph above, taken from Bulbul’s paper, shows all the recent constraints placed on sterile neutrino production models — the measurement obtained by Bulbul and colleagues is marked with the star in red and is consistent with previous upper limits and is in a region of space that has not yet been ruled out. The researchers involved in both papers clearly state that the observation itself is currently very uncertain. That’s because the signal is weak and is located within several well-known faint lines – plus there are “significant modelling uncertainties”.

All of which means that the detection might be just one more false alarm. In fact, experiments have long been trying to detect sterile neutrinos and, so far, placing limits on their mass or lifetime is the best that has been achieved.

Adam Falkowski, who writes the Resonaances blog, has discussed the papers in some depth and what he has to say is worth reading. As he explains, if the signal itself is found to be accurate and indicative of “new physics”, then one possible explanation may point towards the sterile neutrino. While Falkowski is cautiously optimistic about this finding, others such as Matt Strassler, who writes the Of Particular Significance blog, are more sceptical. In fact, Strassler suggests that we don’t lose any sleep over it until the signal is confirmed.

On the other hand, I’ve been surprised that no neutrino researchers have been commenting the papers, so I dropped Maury Goodman, the leader of the Argonne high-energy-physics neutrino group, an e-mail asking for his view. While Goodman has not had a chance to read though the new research, he’s clear where he stands on the subject of sterile neutrinos: he doesn’t believe they exist.

Goodman explains that measurements made on neutrino mass and mixing angles over the last 15 years have been mostly consistent, with the exception of a few measurements that some researchers interpret as evidence of more neutrinos. “It is my understanding that it is impossible to fit any of those anomalies together in a consistent way, and I hear a lot of statistical arguments that are, in my opinion, misguided,” he insists. Goodman goes on to say that while “a sizable minority of neutrino physicists are considering sterile-neutrino searches, I think a majority of us discount the idea”.

The bottom line is that the new research has got many physicists talking – both Sean Carroll and Jim Al-Khalili tweeted about it yesterday and there is lots more chatter about the particle on Twitter. (Only yesterday, we published another news story that revolved around the sterile neutrino, but I will leave you to take a look at that for yourselves, as it is based on an unrelated observation.)

Whether anything truly groundbreaking will emerge from this anomalous signal or if it too will be banished to the vaults of near-miss signals remains to be seen.

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  1. If it’s true we only observe 4% of the whole universe the other 96% being covered by the unknown energy fields and matter famously known as Dark Energy and Dark Matter…
    The impacts of the discovery of these particles, if truelly they are part of the dark matter, then they will utterly change the world as we know it and our perception of reality as we perceive it..
    Our belief systems and our overall understanding of the universe… Nice work, hope to see the results

  2. John Duffield

    It’s thin gruel, it’s clutching at straws, and it’s damn statistics. And moreover it repeats the myth that dark matter must consist of particles. It’s like relativity never happened. Take a look at the Foundation of the General Theory Relativity, Doc 30 page 185: “the energy of the gravitational field shall act gravitatively in the same way as any other kind of energy”. That energy is spatial energy, it doesn’t consist of particles, and yet it has a mass equivalence and a gravitational effect. A gravitational field is inhomogeneous space, see But “the FLRW metric starts with the assumption of homogeneity and isotropy of space”. And so relativity remains the Cinderella of contemporary physics.

  3. Robert L. Oldershaw

    OK. But if this latest gambit for particle dark matter turns out to be yet one more in a decades-long series of false hints, false positives, false leads and much wasted time/money, will our heroic particle dark matter theorists admit that they might have been leading us astray for all those decades?

    Will other physicists, the scientific community and science journalists start demanding equal time for astrophysical dark matter candidates.

    Hypothetical WIMPs, axions and sterile (indeed!) neutrinos are not the only game in dark matter town. Why did we ever assume they were?

    • John Duffield

      “…will our heroic particle dark matter theorists admit that they might have been leading us astray for all those decades?”

      No. They don’t mention the alternative relativity solutions, and they even delete comments that refer to them. They aren’t honest. They are promoting “their” hypotheses and denying their competitors any airtime.

      • Robert L. Oldershaw

        As the Talking Heads song goes: “Same as it ever was”.

        Only new empirical can sort it out, as was the case for the Ptolemaic/Galilean controversy, atomic theory and Relativity.

        If the whole “WIMP” hypothesis turns out to be wrong and fundamentally misguided, imagine the loss of time, money and careers that could have been spent more productively.

        It never should have been “the only game in town”, but that is the way it has been treated for decades.

  4. M. Asghar

    Scientific work is hard, even longwinded and iterative in any domain and nothing comes down as a revelation save for those who do not pass days and nights sleepless.

  5. Gary Conner

    We live in exciting times. This could prove to be another red herring, or, the most important discovery of the 21st century. I am content to await further research for now.

  6. Trackback: Sterile Neutrinos als Dunkle Materie? – Hier wohnen Drachen

  7. Trackback: Dying Flash from DM - maybe

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