Tag Archives: neutrinos

The oldest problem in astronomy – solved?

It’s probably the oldest problem in astronomy: what’s the origin of high-energy cosmic rays? Finally, the question might have been solved.

Victor Hess discovered cosmic rays back in 1912, but it proved incredibly difficult to identify the astrophysical source of these bullets. The obstacle to progress was the fact that charged cosmic rays – whether protons or atomic nuclei – don’t follow a straight-line path from source to Earth. Instead, the paths get bent and twisted by magnetic fields in space. Just because a cosmic ray appears to come from a particular direction of sky doesn’t mean it really did come from that direction. It seems to be an insurmountable problem.

But we are now in the era of multi-messenger astronomy! And that allows astronomers to answer questions that once seemed impossible.

The key to unlocking the cosmic ray mystery is that the violent events that generate high-energy charged particles will also generate neutrinos. And neutrinos do follow a straight-line path from source to Earth: because they interact solely via the weak force their paths aren’t bent by magnetic fields, and they don’t get absorbed or scattered by intervening matter. Neutrinos can act as tracers of high-energy cosmic rays. Of course, the same properties that make them useful tracers also make them incredibly difficult to detect: indeed until recently, apart from a diffuse neutrino background,  astronomers had managed to confirm only two astrophysical sources of neutrinos: the Sun and SN1987A (the latter being a relatively close supernova). The IceCube observatory, however, now has good evidence for a third source: TXS 0506+056. And this might have solved the mystery of high-energy cosmic rays.

In September 2017, IceCube – a neutrino telescope consisting of detectors buried in a cubic kilometer of South Pole ice – spotted a neutrino with an energy of 290TeV. (That’s 40 times more energetic than the particles accelerated by the LHC.) Astronomers could trace it back to a source in the direction of Orion. IceCube sent out an alert to observatories around the world, and several of them – Fermi, MAGIC, HAWC and others – detected an increase in gamma-ray activity from the same patch of sky. The culprit was TXS 0506+056 – a blazar that’s about four billion light years away.

Artist's depiction of a blazar

A blazar is an active galactic nucleus in which one of the jets points directly at Earth. Charged particles are deflected by magnetic fields, but neutrinos and EM radiation can head straight towards Earth. Needless to say, this artist’s depiction is not to scale! (Credit: IceCube/NASA)

A blazar is an active galactic nucleus – the compact central region of a galaxy where a supermassive black hole sucks material onto an accretion disk and spews out radiation in two opposing relativistic jets. When we see a blazar, we just happen to be looking directly down one of the jets. It’s quite a thought: four billion years ago the central black hole of a galaxy hurled neutrinos and charged particles and gamma radiation towards Earth. Magnetic fields steered the charged particles away from us. But the neutrinos and gamma rays made it to Earth. And, in September 2017, IceCube detected one of those neutrinos.

The “Case of the High-Energy Cosmic Rays” isn’t entirely closed. Astronomers would want to see more examples before they can be sure that active galactic nuclei are the source. But the observation is very, very suggestive.

And, as with all else in science, the answer to one question raises others: Can other objects besides active galactic nuclei produce high-energy cosmic rays? What is the exact mechanism whereby these particles are produced? And what is the source of the most powerful cosmic rays – are blazars responsible for them too? Now that we are in the age of multi-messenger astronomy, an age in which we can observe astrophysical events not only across the entire electromagnetic spectrum but also with gravitational wave telescopes and neutrino telescopes … well, the answers might start to come more quickly.

Bert and Ernie and a new type of astronomy?

In May 2013, scientists presented a preliminary analysis of 28 high-energy events captured by the IceCube Neutrino Observatory, a strange telescope entombed deep in Antarctic ice. Two of these events – dubbed Bert and Ernie – had an energy above 1 PeV. (I wrote about these events in an earlier post.) The other 26 events had an energy in excess of 30 TeV. The initial analysis suggested that these 28 events were likely to be from extraterrestrial sources. A more detailed analysis, published today in the journal Science, suggests that only about 11 of the 28 events are likely to have been caused by atmospheric muons or neutrinos. This means that, at a 4? level of certainty, IceCube has detected high-energy neutrinos from outside the Solar System. A 4? result is not quite at the 5? level that is usually said to constitute a discovery, but it is highly suggestive: there is only one chance in 15000 that all those detections were of purely atmospheric events.

IceCube building

The IceCube Neutrino Observatory consists of dozens of photomultiplier tubes attached to 86 cables, each of which are up to 2.5 km long and buried deep in Antarctic ice. The photomultipliers detect the Cerenkov radiation from fast-moving secondary particles created when neutrinos strike nuclei in the ice. The structure here is just the tip of the observatory! (Credit: IceCube Collaboration)

The exciting thing, I believe, is that the IceCube team now know how and where to look for high-energy neutrinos. They’ll find more astrophysical neutrinos, for sure, and the neutrino sky suddenly looks much more interesting. For many years, the only extraterrestrial neutrinos that astronomers had detected were those from the Sun and a few from SN1987A. IceCube has thus broken new ground.

The IceCube discovery has caused many commentators to hail a new type of astronomy: neutrino astronomy. Well, I don’t think we are quite there yet. The problem is that we don’t know where Bert, Ernie or the other neutrinos originated. To do neutrino astronomy one needs to be able to correlate neutrinos with specific astrophysical objects; the IceCube measurements lacked the angular resolution to do this. But that, too, will come. And new neutrino telescopes, such as the KM3NeT facility that is being constructed in the Mediterranean, will help.

We can’t do neutrino astronomy just yet, but it won’t be long before we’re studying the universe from an entirely new vantage point. And then, for the first time, astronomers will be able to study the distant universe using something other than electromagnetic radiation. IceCube is opening its eyes.

Bert and Ernie – dark matter candidates?

At the time New Eyes on the Universe was published, the only confirmed sources extraterrestrial neutrinos were the Sun and SN1987A. The view of the sky afforded by neutrino telescopes was rather dull.

That view of the neutrino sky is beginning to change. The IceCube SouthPole Neutrino Observatory – a “telescope” consisting of particle detectors buried in one cubic kilometre of Antarctic ice – has detected 28 neutrinos with an energy in excess of 30 TeV (a teraelectronvolt is 1012 eV). Two of these neutrinos, dubbed Bert and Ernie, had energies in excess of 1 PeV (that’s 1015 eV) – far in excess of energies available at the Large Hadron Collider.

Artist's impression

An artist’s impression of the array of optical sensors, buried in Antarctic ice, that form the IceCube telescope. If a high-energy neutrino interacts with an oxygen atom in the ice, a charged particle can be produced that will be moving through the ice faster than light itself can travel through the ice. A cone of Cerenkov radiation, with its characteristic blue hue, will be produced – and it’s this radiation that the sensors detect. (Credit: IceCube Collaboration/NSF)

It’s possible that Bert and Ernie were produced by high-energy cosmic rays smashing into Earth’s atmosphere, but an extraterrestrial origin for these neutrinos does seem more likely than not. And If IceCube has indeed detected high-energy neutrinos from the depths of space the question becomes: what was their source? That’s where things get interesting. If they came from some violent astrophysical source then astronomers have a telescope that lets us study them. Or perhaps they came from the decay of dark matter particles – a suggestion made in a recent preprint by Arman Esmaili and Pasquale Serpico (Are IceCube neutrinos unveiling PeV-scale decaying dark matter?). Whatever the source of Bert and Ernie turns out to be, it seems certain that IceCube truly is giving us some new eyes through which to view the universe.

The cosmic ray gun

It’s one of the most long-lasting questions in astrophysics: what’s the source of those really high-energy cosmic rays that sometimes hit Earth? What cosmic gun could possibly shoot such high-energy bullets towards us?

There are two obvious candidates: active galactic nuclei and gamma ray bursts.

There are seem to be two types of progenitors for gamma ray bursts, but the most luminous events probably come from the collapse of very massive, rapidly rotating stars. Models of such collapse events suggest that the fireball should, alongside the generation of extremely energetic gamma rays, generate high-energy cosmic rays and neutrinos. And scientists now have a detector that can hunt for neutrinos from gamma ray bursts: IceCube.

Artist's impression of the IceCube observatory

An artist's depiction of the IceCube observatory: 86 strings containing 5160 Digital Optical Modules are used to detect neutrino events.
(Danielle Vevea/NSF & Jamie Yang/NSF)

I don’t propose to discuss IceCube in detail in this post. I’ll surely do that in later posts, and you could always read the relevant chapter in New Eyes on the Universe. The exciting news yesterday is that IceCube has been used to look for neutrinos from 300 gamma ray bursts detected by the Swift and Fermi space telescopes. Neutrinos are of course notoriously difficult to spot, but IceCube should have seen several neutrinos from these exceptionally luminous events. It saw nothing.

This negative result suggests that our models of particle production in the fireball of a gamma ray burst might need some major tweaking – and perhaps that high-energy cosmic rays don’t come from burst after all. Perhaps active galactic nuclei are the guns that fire those cosmic ray bullets at us.

Nattering with neutrinos

Solution 16 in If the Universe is Teeming with Aliens…Where is Everybody? is entitled “They are signaling, but we do not know how to listen”. In that section I discuss a solution to the Fermi paradox that some scientists have occasionally proposed: extraterrestrial civilizations are out there, and sending signals, but we don’t hear them because we’re listening in the wrong way. Perhaps, the argument goes, we’re listening for electromagnetic signals when we should be listening for modulated neutrino beams, gravitational waves, or cosmic rays. In particular, the possibility of using neutrinos for interstellar communication was proposed by Mieczyslaw Subotowicz as long ago as 1979 (in the paper “Interstellar communication by neutrino beam”, which appeared in volume 6 of Acta Astronoautica; see pages 213-220).

When I wrote Where is Everybody?, neutrino communication was not possible and it seemed to me unlikely that it would ever be possible in my lifetime. How quickly technology progresses! A recent paper by Daniel Stancil and colleagues (Demonstration of communication using neutrinos) reported on how a communications link was established between the NuMI beam line and the MINERvA detector (both at Fermilab). OK, so the distance involved here are not on an interstellar scale (in fact, the separation was only 1035m); and the transmission figures aren’t stunning (they achieved a decoded data rate of 0.1 bits per second, with a bit error rate of 1%). But it’s a start! Ten years ago this couldn’t be done; in ten years time this will be routine.

The work by Stancil and his colleagues will eventually have applications, particularly in scenarios where communication using electromagnetic waves is difficult or impossible. (The 1035m over which the neutrino beam allowed communication to take place included 240m of solid earth: this was direct communication, rather than boring a tunnel that could house a optical fiber that could then carry an electromagnetic. Or consider the case of communication with a submerged nuclear submarine: seawater is opaque to short-wavelength electromagnetic radiation, which is why submarines must come close to the surface and float a communications line, but neutrinos go straight through water – as they do with everything else.) So neutrinos may have a role in future communications technology. But will they play a role in communicating over interstellar distances?

I remain unconvinced that any extraterrestrial civilizations would choose to broadcast a signal using neutrinos. First, electromagnetic radiation is a faster signal carrier than a neutrino beam. (Despite the recent story about those OPERA neutrinos, we know that neutrinos don’t travel faster than light.) Second, its vastly easier to generate modulated electromagnetic radiation than it is to generate a modulated neutrino beam. Third, its vastly easier to detect modulated electromagnetic radiation than it is to detect a modulated neutrino beam. Fourth, because we share the same universe and are subject to the same laws of physics, we can be reasonably sure than any extraterrestrial civilization will know all of the above – they’ll know that we know that they know all this.

If there are any extraterrestrial civilizations out there trying to contact others in the universe, surely they’ll be using electromagnetic radiation. Won’t they?

So has the fat lady sang at OPERA?

Well, it had to happen. The OPERA team has identified two potential problems in their measurement of neutrino velocities. (You remember that story, right? The one where it seemed as if the neutrinos were superluminal…)

It turns out that there’s a problem with an atomic clock that they used to get start/stop times for the measurement. (The error here would tend to increase the measured time-of-flight, and thus reduce the measured speed.) There was also a problem with the optical fibre connection between the main clock and the GPS system. (Surprisingly, the error here would tend to increase the measured speed.)

The identification of these two systematic errors means that the OPERA team can no longer claim to have seen superluminal neutrinos. Further experiments later this year, both at OPERA and elsewhere, will surely put the story to bed once and for all.

What has been fascinating here, though, has been the reaction of the scientific community to the claim. I think we all knew that this result was never going to stand. But that doesn’t mean the OPERA team were wrong to publish. Their initial result caught the public imagination, and their identification of systematic errors in the experiment showed the public how science progresses in the real world.

They showed that science is sometimes messy, sometimes confusing. But they also showed that science is transparent, and eventually it gives us knowledge we can rely on. Well done OPERA.

When will the fat lady sing at OPERA?

Some of the world’s finest physicists and cosmologists have in recent weeks been pouring scorn on the now infamous OPERA result. (If you’ve just been released from one of those Mars simulation missions, such as Mars500, then I guess it’s possible that you might have missed what has the potential to be the biggest physics result in a century: the report by the OPERA collaboration that muon neutrinos produced by CERN travelled ever-so-slightly faster than light while on their way to detectors at Gran Sasso.) I’m sure that those scientists, many of whom I admire tremendously, are right: those neutrinos are surely not travelling faster than light. It wasn’t as if the neutrinos acted like resublimated thiotimoline, somehow arriving at the OPERA detectors before they were produced. The OPERA team were making tremendously difficult measurements, and at this point it’s safer to assume that their finding is the result of some unknown source of error in the experiment. But there’s one point on which I think those eminent critics of OPERA have it wrong.

The criticism is that the OPERA team contacted the media and called a press conference before they published their results in a peer-reviewed paper: irresponsible behaviour, clearly, particularly where such a controversial result is involved. Thing is, the OPERA researchers didn’t announce their results at a press conference: they announced them at a CERN seminar. And they didn’t draft a press release: they submitted a technical preprint to arXiv. Surely they did everything that responsible scientists should do?

Once, not many years ago, you could put a preprint on arXiv and you knew you’d be reaching an audience of physicists. We now live in a world of blogs (well, you’re reading this one aren’t you?) and Twitter. Put a preprint on arXiv that says in effect “Einstein was wrong” and you may as well shout it out loud while standing naked at Speaker’s Corner. Perhaps unfortunately for OPERA, in the modern world of social media there’s no way that the original seminar could go unnoticed; the press conferences that followed were inevitable – and then so was the criticism that the collaboration hadn’t followed proper processes.

Neutrino beam going from CERN to Gran Sasso

CERN sends neutrinos directly through the Earth to the Gran Sasso Laboratory, some 730km away Credit: CERN

If there’s a criticism to be made of OPERA it is, I believe, that they hadn’t ruled out all sources of systematic error before giving that initial CERN seminar. Indeed, that’s probably why ten senior members of the collaboration decided not to sign the arXiv submission. One obvious concern with the experiment, which many physicists voiced immediately, is that CERN was sending long neutrino pulses (about 10 microseconds long) to Gran Sasso; the effect they were observing, though, involved a shift that was a tiny fraction of that pulse length (the shift was about 60 nanoseconds). For their analysis to work, the collaboration needed to know the shape of the neutrino pulse quite precisely; but they were only able to infer the neutrino pulse shape. (The neutrinos come from protons smashing into a target; OPERA infer the neutrino pulse shape from the initial proton pulse shape.) Get that inference just a little bit wrong and they would end up seeing things that just aren’t there.

Fortunately, there’s a really simple way to get round this difficulty: repeat the experiment, but send a series of short neutrino pulses separated by large gaps. That way you don’t need to know the neutrino pulse shape: each pulse from CERN is unambiguously linked to the OPERA detector.

The OPERA collaboration has now run precisely this experiment. They asked CERN to generate proton pulses lasting just 3 nanoseconds, and recorded 20 neutrino events. And the result? Well, again the neutrinos reached Gran Sasso about 60 nanoseconds before light itself could have reached there. The anomaly remains.

So when will the fat lady sing at OPERA? When will we know what systematic error is to blame for this bizarre result? (And for what it’s worth I think it will turn out to be a systematic, probably to do the use of GPS in the experiment.) Well, it’s clear that independent checks are required. The first project to be in a position to do those checks is likely to be MINOS at Fermilab. We might get results from MINOS some time in 2012. If MINOS replicates the OPERA result… well, then we’ll be living in interesting times.