Tag Archives: neutrinos

“Seeing” Double: Neutrinos and Photons Observed from the Same Cosmic Source

There has long been a question as to what types of events and processes are responsible for the highest-energy neutrinos coming from space and observed by scientists.  Another question, probably related, is what creates the majority of high-energy cosmic rays — the particles, mostly protons, that are constantly raining down upon the Earth.

As scientists’ ability to detect high-energy neutrinos (particles that are hugely abundant, electrically neutral, very light-weight, and very difficult to observe) and high-energy photons (particles of light, though not necessarily of visible light) have become more powerful and precise, there’s been considerable hope of getting an answer to these question.  One of the things we’ve been awaiting (and been disappointed a couple of times) is a violent explosion out in the universe that produces both high-energy photons and neutrinos at the same time, at a high enough rate that both types of particles can be observed at the same time coming from the same direction.

In recent years, there has been some indirect evidence that blazars — narrow jets of particles, pointed in our general direction like the barrel of a gun, and created as material swirls near and almost into giant black holes in the centers of very distant galaxies — may be responsible for the high-energy neutrinos.  Strong direct evidence in favor of this hypothesis has just been presented today.   Last year, one of these blazars flared brightly, and the flare created both high-energy neutrinos and high-energy photons that were observed within the same period, coming from the same place in the sky.

I have written about the IceCube neutrino observatory before; it’s a cubic kilometer of ice under the South Pole, instrumented with light detectors, and it’s ideal for observing neutrinos whose motion-energy far exceeds that of the protons in the Large Hadron Collider, where the Higgs particle was discovered.  These neutrinos mostly pass through Ice Cube undetected, but one in 100,000 hits something, and debris from the collision produces visible light that Ice Cube’s detectors can record.   IceCube has already made important discoveries, detecting a new class of high-energy neutrinos.

On Sept 22 of last year, one of these very high-energy neutrinos was observed at IceCube. More precisely, a muon created underground by the collision of this neutrino with an atomic nucleus was observed in IceCube.  To create the observed muon, the neutrino must have had a motion-energy tens of thousand times larger than than the motion-energy of each proton at the Large Hadron Collider (LHC).  And the direction of the neutrino’s motion is known too; it’s essentially the same as that of the observed muon.  So IceCube’s scientists knew where, on the sky, this neutrino had come from.

(This doesn’t work for typical cosmic rays; protons, for instance, travel in curved paths because they are deflected by cosmic magnetic fields, so even if you measure their travel direction at their arrival to Earth, you don’t then know where they came from. Neutrinos, beng electrically neutral, aren’t affected by magnetic fields and travel in a straight line, just as photons do.)

Very close to that direction is a well-known blazar (TXS-0506), four billion light years away (a good fraction of the distance across the visible universe).

The IceCube scientists immediately reported their neutrino observation to scientists with high-energy photon detectors.  (I’ve also written about some of the detectors used to study the very high-energy photons that we find in the sky: in particular, the Fermi/LAT satellite played a role in this latest discovery.) Fermi/LAT, which continuously monitors the sky, was already detecting high-energy photons coming from the same direction.   Within a few days the Fermi scientists had confirmed that TXS-0506 was indeed flaring at the time — already starting in April 2017 in fact, six times as bright as normal.  With this news from IceCube and Fermi/LAT, many other telescopes (including the MAGIC cosmic ray detector telescopes among others) then followed suit and studied the blazar, learning more about the properties of its flare.

Now, just a single neutrino on its own isn’t entirely convincing; is it possible that this was all just a coincidence?  So the IceCube folks went back to their older data to snoop around.  There they discovered, in their 2014-2015 data, a dramatic flare in neutrinos — more than a dozen neutrinos, seen over 150 days, had come from the same direction in the sky where TXS-0506 is sitting.  (More precisely, nearly 20 from this direction were seen, in a time period where normally there’d just be 6 or 7 by random chance.)  This confirms that this blazar is indeed a source of neutrinos.  And from the energies of the neutrinos in this flare, yet more can be learned about this blazar, and how it makes  high-energy photons and neutrinos at the same time.  Interestingly, so far at least, there’s no strong evidence for this 2014 flare in photons, except perhaps an increase in the number of the highest-energy photons… but not in the total brightness of the source.

The full picture, still emerging, tends to support the idea that the blazar arises from a supermassive black hole, acting as a natural particle accelerator, making a narrow spray of particles, including protons, at extremely high energy.  These protons, millions of times more energetic than those at the Large Hadron Collider, then collide with more ordinary particles that are just wandering around, such as visible-light photons from starlight or infrared photons from the ambient heat of the universe.  The collisions produce particles called pions, made from quarks and anti-quarks and gluons (just as protons are), which in turn decay either to photons or to (among other things) neutrinos.  And its those resulting photons and neutrinos which have now been jointly observed.

Since cosmic rays, the mysterious high energy particles from outer space that are constantly raining down on our planet, are mostly protons, this is evidence that many, perhaps most, of the highest energy cosmic rays are created in the natural particle accelerators associated with blazars. Many scientists have suspected that the most extreme cosmic rays are associated with the most active black holes at the centers of galaxies, and now we have evidence and more details in favor of this idea.  It now appears likely that that this question will be answerable over time, as more blazar flares are observed and studied.

The announcement of this important discovery was made at the National Science Foundation by Francis Halzen, the IceCube principal investigator, Olga Botner, former IceCube spokesperson, Regina Caputo, the Fermi-LAT analysis coordinator, and Razmik Mirzoyan, MAGIC spokesperson.

The fact that both photons and neutrinos have been observed from the same source is an example of what people are now calling “multi-messenger astronomy”; a previous example was the observation in gravitational waves, and in photons of many different energies, of two merging neutron stars.  Of course, something like this already happened in 1987, when a supernova was seen by eye, and also observed in neutrinos.  But in this case, the neutrinos and photons have energies millions and billions of times larger!


More on Dark Matter and the Large Hadron Collider

As promised in my last post, I’ve now written the answer to the second of the three questions I posed about how the Large Hadron Collider [LHC] can search for dark matter.  You can read the answers to the first two questions here. The first question was about how scientists can possibly look for something that passes through a detector without leaving any trace!  The second question is how scientists can tell the difference between ordinary production of neutrinos — which also leave no trace — and production of something else. [The answer to the third question — how one could determine this “something else” really is what makes up dark matter — will be added to the article later this week.]

In the meantime, after Monday’s post, I got a number of interesting questions about dark matter, why most experts are confident it exists, etc.  There are many reasons to be confident; it’s not just one argument, but a set of interlocking arguments.  One of the most powerful comes from simulations of the universe’s history.  These simulations

  • start with what we think we know about the early universe from the cosmic microwave background [CMB], including the amount of ordinary and dark matter inferred from the CMB (assuming Einstein’s gravity theory is right), and also including the degree of non-uniformity of the local temperature and density;
  • and use equations for known physics, including Einstein’s gravity, the behavior of gas and dust when compressed and heated, the effects of various forms of electromagnetic radiation on matter, etc.

The output of the these simulations is a prediction for the universe today — and indeed, it roughly has the properties of the one we inhabit.

Here’s a video from the Illustris collaboration, which has done the most detailed simulation of the universe so far.  Note the age of the universe listed at the bottom as the video proceeds.  On the left side of the video you see dark matter.  It quickly clumps under the force of gravity, forming a wispy, filamentary structure with dense knots, which then becomes rather stable; moderately dense regions are blue, highly dense regions are pink.  On the right side is shown gas.  You see that after the dark matter structure begins to form, that structure attracts gas, also through gravity, which then forms galaxies (blue knots) around the dense knots of dark matter.  The galaxies then form black holes with energetic disks and jets, and stars, many of which explode.   These much more complicated astrophysical effects blow clouds of heated gas (red) into intergalactic space.

Meanwhile, the distribution of galaxies in the real universe, as measured by astronomers, is illustrated in this video from the Sloan Digital Sky Survey.   You can see by eye that the galaxies in our universe show a filamentary structure, with big nearly-empty spaces, and loose strings of galaxies ending in big clusters.  That’s consistent with what is seen in the Illustris simulation.

Now if you’d like to drop the dark matter idea, the question you have to ask is this: could the simulations still give a universe similar to ours if you took dark matter out and instead modified Einstein’s gravity somehow?  [Usually this type of change goes under the name of MOND.]

In the simulation, gravity causes the dark matter, which is “cold” (cosmo-speak for “made from objects traveling much slower than light speed”), to form filamentary structures that then serve as the seeds for gas to clump and form galaxies.  So if you want to take the dark matter out, and instead change gravity to explain other features that are normally explained by dark matter, you have a challenge.   You are in danger of not creating the filamentary structure seen in our universe.  Somehow your change in the equations for gravity has to cause the gas to form galaxies along filaments, and do so in the time allotted.  Otherwise it won’t lead to the type of universe that we actually live in.

Challenging, yes.  Challenging is not the same as impossible. But everyone one should understand that the arguments in favor of dark matter are by no means limited to the questions of how stars move in galaxies and how galaxies move in galaxy clusters.  Any implementation of MOND has to explain a lot of other things that, in most experts’ eyes, are efficiently taken care of by cold dark matter.

X-Rays From Dark Matter? A Little Hint For You To Enjoy

Well it’s not much to write home about, and I’m not going to write about it in detail right now, but the Resonaances blog has done so (and he’s asking for your traffic, so please click):

A team of six astronomers reports that when they examine the light (more specifically, the X-rays) coming from clusters of galaxies around the sky, and account for all the X-ray emission lines [light emitted in extremely narrow bands by atoms or their nuclei] they know about, there’s an excess of photons [particles of light] with energy E=(3.55-3.57)+/-0.03 keV, a “weak unidentified emission line”, that can’t easily be explained.  What could it be?

[A keV is 1000 eV; an eV is an electron-volt, an amount of energy typical of chemical reactions.  Note that physicists and astronomers commonly use the word “light” to refer not just to “visible light” — the light you can see — but to all electromagnetic waves, no matter what their frequency. ]

Well first: is this emission line really there?  The astronomers claim to detect it in several ways, but “the detection is at the limit of the current instrument capabilities and subject to significant modeling uncertainties” — in other words, it requires some squinting — so they are cautious in their statements.

Second: if it’s really there, what’s it due to?  Well, the most exciting and least likely possibility is that it’s from dark matter particles decaying to a photon with the above-mentioned energy plus a second, unobserved, particle — perhaps a neutrino, perhaps something else.   I’ll let Resonaances explain the sterile neutrino hypothesis, in which the dark matter particles are kind of like neutrinos — they’re fermions, like neutrinos, and they are connected to neutrinos in some way, though they aren’t as directly affected by the weak nuclear force.

But before you get excited, note that the authors state: “However, based on the cluster masses and distances, the line in Perseus is much brighter than expected in this model, significantly deviating from other subsamples.”  In other words: don’t get excited, because something very funny is going on in the Perseus cluster, and until that’s understood, the data can’t be said to be particularly consistent with a dark matter hypothesis.

One more anomaly — one more hint of dark matter — to put on the pile of weak and largely unrelated hints that we’ve already got!  I don’t suggest losing sleep over it… at least not until it’s confirmed by other groups and the Perseus cluster’s odd emissions are explained.

Ice Cube’s Neutrino Paper Appears

Some readers may remember that back in May, as I discussed in some detail, the IceCube experiment reported a new and exciting observation — possibly the first discovery of high-energy astrophysical neutrinos: neutrinos, with energies 5 – 50 times higher than those of the protons at the Large Hadron Collider, created in outer space and arriving on earth.  This is to be contrasted with most neutrinos measured at IceCube and other previous similar experiments, which have lower energy and are created in the earth’s atmosphere by other types of particles hitting atoms in the air (see Figure 4 of this article.)   Specifically, where the IceCube folks expected to measure 10 candidate events from non-neutrino backgrounds, they instead measured 28.  Well, these results were reported at a conference in May, but only now is the paper appearing in published form, in the journal Science.

Here’s the IceCube press release about the publication of their paper, http://icecube.wisc.edu/news/view/171 .  All indications are that there are few changes since May, except for greater confidence in the result; the numbers quoted all match the ones that I wrote about back in the spring.  If there is anything strikingly different from May, I haven’t yet noticed it; please let me know if you’re aware of something.

For those of you who missed this back in May, I wrote a few relevant posts back then that you may find useful.

Meanwhile, you may also remember that there was a big Gamma Ray Burst [GRB] observed in April — the most energetic ever measured.  [We were hoping that IceCube would observe neutrinos from that GRB, but it did not.] Science is also publishing papers about that event, and how measurements of it are making people rethink their understanding of how GRBs occur.  Once I’ve learned more about this, I’ll post something more detailed.

Curiously, both of these stories are appearing in the press with big headlines, as though they are new news… but if they sound familiar, it’s because they are indeed six months old.

Some Weird Twists and Turns

In my last post, I promised you some comments on a couple of other news stories you may have seen.  Promise kept! see below.

But before I go there, I should mention (after questions from readers) an important distinction.  Wednesday’s post was about the simple process by which a Bs meson (a hadron containing a bottom quark and a down[typo] strange anti-quark, or vice versa, along with the usual crowd of gluons and quark/antiquark pairs) decays to a muon and an anti-muon.  The data currently shows nothing out of the ordinary there.  This is not to be confused with another story, loosely related but with crucially different details. There are some apparent discrepancies (as much as 3.7 standard deviations, but only 2.8 after accounting for the look-elsewhere effect) cropping up in details of the intricate process by which a Bd meson (a hadron containing a bottom quark and a down antiquark, or vice versa, plus the usual crowd) decays to a muon, an anti-muon, and a spin-one Kaon (a hadron containing a strange quark and a down anti-quark, or vice versa, plus the usual crowd). The measurements made by the LHCb experiment at the Large Hadron Collider disagree, in some but not all features, with the (technically difficult) predictions made using the Standard Model (the equations used to describe the known particles and forces.)

Don't confuse these two processes!  (Top) The process B_s --> muon + anti-muon, covered in Wednesday's post, agrees with Standard Model predictions.   (Bottom) The process B_d --> muon + anti-muon + K* is claimed to deviate by nearly 3 standard deviations from the Standard Model, but (as far as I am aware) the prediction and associated claim has not yet been verified by multiple groups of people, nor has the measurement been repeated.

Don’t confuse these two processes! (Top) The process B_s –> muon + anti-muon, covered in Wednesday’s post, agrees with Standard Model predictions. (Bottom) The process B_d –> muon + anti-muon + K* is claimed to deviate by nearly 3 standard deviations from the Standard Model, but (as far as I am aware) the prediction and associated claim has not yet been verified by multiple groups of people, nor has the measurement been repeated.

A few theorists have even gone so far as to claim this discrepancy is clearly a new phenomenon — the end of the Standard Model’s hegemony — and have gotten some press people to write (very poorly and inaccurately) about their claim.  Well, aside from the fact that every year we see several 3 standard deviation discrepancies turn out to be nothing, let’s remember to be cautious when a few scientists try to convince journalists before they’ve convinced their colleagues… (remember this example that went nowhere? …) And in this case we have them serving as judge and jury as well as press office: these same theorists did the calculation which disagrees with the data.  So maybe the Standard Model is wrong, or maybe their calculation is wrong.  In any case, you certainly musn’t believe the news article as currently written, because it has so many misleading statements and overstatements as to be completely beyond repair. [For one thing, it’s a case study in how to misuse the word “prove”.] I’ll try to get you the real story, but I have to study the data and the various Standard Model predictions more carefully first before I can do that with complete confidence.

Ok, back to the promised comments: on twists and turns for neutrinos and for muons…   Continue reading

How IceCube Observes Neutrinos From The Cosmos

I’ve finished (more or less) a version of the promised article on IceCube — the giant neutrino experiment that may have made a major discovery, as announced last week, and that had an opportunity to make another a few weeks ago (though apparently nature didn’t provide).  The article is admittedly a bit rushed (darn computer trouble) and therefore a bit rough, and it also leaves out some more subtle points that may become important in the future — but I think it’s complete enough to help explain how IceCube made their most recent measurements.  As usual, please send comments and questions, and I’ll work on it further.

Here’s the link to the article.  You may also find it interesting to read more generally about how neutrinos are detected, and about the weird story of neutrino types, and how they can oscillate from one type to another as they travel.

A Few Items of Interest

I was sent or came across a few interesting links that relate to things covered on this blog and/or of general scientific interest.

It was announced yesterday that the European Physical Society 2013 High Energy Physics Prize was awarded to the collaboration of experimental physicists that operate the ATLAS and CMS experiments that discovered a type of Higgs particle, with special mention to Michel Della Negra, Peter Jenni, and Tejinder Virdee, for their pioneering role in the development of ATLAS and CMS.  Jenni and Virdee are both at the LHCP conference in Barcelona, which I’m also attending, and it has been a great pleasure for all of us here to be able to congratulate them in person .

One thing that came up a couple of times regarding weather forecasting (for instance, in forecasting the path of Hurricane Sandy) is that the European weather forecasters are doing a much better job of predicting storms a week in advance than U.S. forecasters are.  And I was surprised to learn that one of the the main reasons is simple: U.S. forecasters have less computing power than their European counterparts, which sounds (and is) ridiculous.  The new director of the U.S. National Weather Service, Louis Uccellini, has been successful in his goal of improving this situation, as reported here[Thanks to two readers for pointing me to this article.]

One of the possible interpretations of the new class of high-energy neutrinos reported by IceCube (see yesterday’s post) is that they come from the slow decay of a small fraction of the universe’s dark matter particles, assuming those particles have a mass of a couple of million GeV/c². [That’s much heavier than the types of dark matter particles that most people are currently looking for, in searches that I discussed in a recent article.]  I didn’t immediately mention this possibility (which is rather obvious to an expert) because I wanted a couple of days to think about it before generating a stampede or press articles.  But, not surprisingly, people who were paying more attention to what IceCube has been up to had recently written a paper on this subject[Here’s an older, related paper, but at much lower energy; maybe there are other similar papers that I don’t know about?]  At the time these authors wrote this paper, only the two highest energy neutrinos — which have energies that, within the uncertainties of the measurements, might be equal (see Figure 2 of yesterday’s post) — were publicly known.  In their paper, they predicted that (just as any expert would guess) in addition to a spike of neutrinos, all at about 1.1 million GeV, one would also find a population of lower-energy neutrinos, similar to those new neutrinos that IceCube has just announced. So yes, among many possibilities, it appears that it is possible that the new neutrinos are from decaying dark matter.  If more data reveals that there really is a spike of neutrinos with energy around 1.1 million GeV, and the currently-observed gap between the million-GeV neutrinos and the lower-energy ones barely fills in at all, then this will be extremely strong evidence in favor of this idea… though it will be another few years before the evidence could become convincing.  Conversely, if IceCube observes any neutrinos near but significantly above 1.1 million GeV, that would show there isn’t really a spike, disfavoring this particular version of the idea.

Regarding yesterday’s post, it was pointed out to me that when I wrote “The only previous example of neutrinos being used in astrophysics occurred with the discovery of neutrinos from the relatively nearby supernova, visible with the naked eye, that occurred in 1987,” I should also have noted that neutrinos were and are used to understand the interior of the sun (and vice versa).  And you could even perhaps say that atmospheric neutrinos have been used to understand cosmic rays (and vice versa.)

In sad news, in the “all-good-things-must-come-to-an-end” category, the Kepler spacecraft, which has brought us an unprecedented slew of discoveries of planets orbiting other stars, may have reached the end of the line (see for example here), at least as far as its main goals.  It’s been known for some time that its ability to orient itself precisely was in increasing peril, and it appears that it has now been lost.  Though this has occurred earlier than hoped, Kepler survived longer than its core mission was scheduled to do, and its pioneering achievements, in convincing scientists that small rocky planets not unlike our own are very common, will remain in the history books forever.  Simultaneous congratulations and condolences to the Kepler team, and good luck in getting as much as possible out of a more limited Kepler.

Possible Important Discovery at IceCube

IceCube [here’s my own description of the experiment], the big high-energy neutrino experiment cleverly embedded into the ice at the South Pole, announced a very interesting result yesterday, following on an already interesting result from a few weeks ago, one that I failed to cover properly. They have seen the highest-energy neutrinos ever observed, ones that, unlike previously observed high-energy neutrinos, appear not to be generated by cosmic rays hitting the top of the atmosphere. Instead, they apparently come from new sources far out in space. And as such, it tentatively appears that they’ve opened up, as long anticipated, a new era in neutrino astronomy, in which high-energy neutrinos will be used to understand astrophysical phenomena!

[The only previous example of neutrinos being used in astrophysics occurred with the discovery of neutrinos from the relatively nearby supernova, visible with the naked eye, that occurred in 1987. But those neutrinos had energies millions of times smaller than the ones discussed here.  And there was hope that IceCube might see neutrinos specifically from gamma-ray bursts, including the one that occurred just two weeks ago; but that appears not to have happened.]

I don’t understand certain details well enough yet to give you a careful explanation — that will probably come next week — but here’s an early description (and expert readers are strongly encouraged to correct any errors.) Continue reading