Neutrino Astronomy with IceCube and Beyond

Kevin J. Meagher on behalf of the IceCube Collaboration
The IceCube Neutrino Observatory is a cubic kilometer neutrino telescope located at the geographic South Pole. Cherenkov radiation emitted by charged secondary particles from neutrino interactions is observed by IceCube using an array of 5160 photomultiplier tubes embedded between a depth of 1.5 km to 2.5 km in the Antarctic glacial ice. The detection of astrophysical neutrinos is a primary goal of IceCube and has now been realized with the discovery of a diffuse, high-energy flux consisting of neutrino events from tens of TeV up to several PeV. Many analyses have been performed to identify the source of these neutrinos, including correlations with active galactic nuclei, gamma-ray bursts, and the Galactic plane. IceCube also conducts multi-messenger campaigns to alert other observatories of possible neutrino transients in real time. However, the source of these neutrinos remains elusive as no corresponding electromagnetic counterparts have been identified. This proceeding will give an overview of the detection principles of IceCube, the properties of the observed astrophysical neutrinos, the search for corresponding sources (including real-time searches), and plans for a next-generation neutrino detector, IceCube-Gen2.

The ghosts and the machine

Studying the diaphanous neutrino will be America’s contribution to a new generation of physics
neutrino1DEEP beneath the plains of Illinois, in a man-made cavern filled with racks of scientific equipment, someone has spray-painted a white circle onto the bare rock wall. Stand in front of it and you are standing in the path of the most powerful beam of neutrinos in the world, which is emerging from a nearby particle accelerator at Fermilab, America’s main particle-physics laboratory. With any other kind of accelerator, standing in the beam would have spectacular and fatal consequences. But your correspondent was not vapourised—nor, several weeks later, has he developed either cancer or superpowers.

And that is the point: neutrinos are ghostly things. Billions a second stream through every cubic centimetre of space. But because they feel only the two weakest of the four fundamental physical forces—gravity and the aptly named weak nuclear force, rather than electromagnetism and the strong nuclear force—they hardly interact with the rest of creation. Continue reading The ghosts and the machine

The Higgs boson and the neutrino

Massive thoughts
The Higgs boson and the neutrino fascinate the general public and particle physicists alike. Why is that?

Nigel Lockyer, Director of Fermilab

If there are two particles that everyone has read about in the news lately, it’s the Higgs boson and the neutrino. Why do we continue to be fascinated by these two particles?

As just about everyone now knows, the Higgs boson is integrally connected to the field that gives particles their mass. But the excitement of this discovery isn’t over; now we need to figure out how this actually works and whether it explains everything about how particles get their mass. With time, this knowledge is likely to affect daily life.

One way it could possibly bridge the gap between fundamental research and the commercial market, I believe, is in batteries. The ultimate battery in nature is mass. The expression E=mc2 is a statement of that fact. During the early moments of the universe, all particles were massless and traveling at the speed of light. Once the Higgs mechanism turned on, particles suddenly began interacting with the field and, in this process, converted their energy into what we now refer to as mass. In a recent address to the Canadian Nuclear Society, I suggested that if engineers of the future could learn how to manipulate the Higgs field (to “turn it on and off”), then we would have developed the ultimate energy source and the best battery nature has created. This idea definitely belongs in the science-fiction category, but remember that progress in science is driven by thinking “outside the box!”

This sort of thinking comes from looking at the Higgs from another angle. According to the Standard Model, many particles come in left-handed and right-handed versions (in the former, the particle’s direction of spin matches its direction of motion, while in the latter, they are opposite).

Keeping this fact in mind, let’s look at the mass of the familiar electron as an example. When we say that the mass of the electron is created by interactions with the Higgs field, we can think of this as the Higgs field rapidly changing a left-handed electron into a right-handed electron, and vice versa. This switching back and forth is energy and, through E=mc2, energy is mass. A heavier particle like the top quark would experience this flipping at a much higher frequency than a lighter particle like the electron. As we learn more about how this process works, I encourage physicists to also seek applications of that knowledge.

And what about neutrinos? Do they get their mass from the Higgs field or in a completely different way? Once thought to be massless, neutrinos are now known to have a tiny mass. If the Higgs mechanism is responsible for that mass, there must exist both a left-handed and a right-handed neutrino. A good number of physicists think that both are out there, but we do not yet know. That knowledge may help us understand why the neutrino mass is tiny, as well as why there is more matter than antimatter in the universe—one of the most important questions facing our field of particle physics.

But since the neutrino is a neutral particle, the story gets more interesting. It may instead be possible that there is another type of mass. Referred to as a Majorana mass, it is not a mass described by the flipping of left- and right-handed neutrinos back and forth, but it is “intrinsic,” not derived from any kind of “motional energy.” I expect that the efforts by our field of particle physics, in the collective sense, will pursue the questions associated with both the Higgs boson and the neutrino with enthusiasm, and that the results will lead to advancements we can’t even imagine today.

Neutrino Experiments Come Closer to Seeing CP Violation

The T2K experiment in Japan measures the appearance of electron neutrinos in a beam of muon neutrinos. A pure beam of muon neutrinos is sent from a source in Tokai to a detector 295 km away in Kamioka.

The T2K experiment in Japan measures the appearance of electron neutrinos in a beam of muon neutrinos. A pure beam of muon neutrinos is sent from a source in Tokai to a detector 295 km away in Kamioka.

Joseph A. Formaggio, Massachusetts Institute of Technology, Cambridge, MA 02139, USA

The T2K experiment has measured the largest number of events associated with muon neutrinos oscillating into electron neutrinos, an important step toward seeing CP violation in neutrino interactions.

Charge-parity (CP) violation—evidence that the laws of physics are different for particles and antiparticles—is often invoked as a “must” to explain why we observe more matter than antimatter in the universe. But the CP violation observed in interactions involving quarks is insufficient to explain this asymmetry. As a result, many theorists are looking toward leptons—and, specifically, neutrinos—for additional sources of CP violation. Researchers running the Tokai to Kamioka (T2K) experiment—a particle physics experiment at the Japan Proton Accelerator Research Complex (J-PARC)—have now made an important contribution toward the search for CP violation in neutrinos. Writing in Physical Review Letters, the T2K collaboration reports the strongest evidence to date for the appearance of electron neutrinos from a pure muon neutrino beam [1]. Their measurement allows them to determine a fundamental parameter of the standard model of particle physics, called θ13, which can in turn be used to make an early estimate of CP violation in neutrinos. Although this estimate has a large uncertainty, it will serve as a guide to future, more definitive neutrino experiments that are directly sensitive to CP violation.

Neutrinos come in three flavors: electron, muon, and tau. Each of these flavor states is known to be a mixture of the three allowed neutrino mass states, called m1, m2, and m3. This mixing is why one flavor state has a certain probability, over time, of transmuting, or “oscillating,” into another state. The oscillation rate depends, in part, on parameters called mixing angles and on the differences between the masses m1, m2, and m3.

T2K’s experiment is designed to gain information about one of these mixing angles, called θ13—named for where it sits in the so-called Pontecorvo-Maki-Nakagawa-Sakata (PMNS) matrix, a 3×3 matrix that describes neutrino oscillations. The collaboration performs what is called an “appearance” measurement. The experiment sends a beam of muon neutrinos over a long distance toward a detector that searches for the presence of electron neutrinos. Since the total number of electron neutrinos also depends on the propagation time of the initial state, T2K has to carefully select the initial state, energy, and distance traversed by the beam in order to extract θ13.

T2K’s measurement complements “disappearance” measurements, in which researchers look for a decrease in the flux of a particular neutrino state. Experiments using neutrinos produced in reactors fall into this later category. One advantage of an appearance experiment is that it is sensitive to other aspects of neutrino oscillations. In particular, appearance events depend on how the three neutrino mass states are ordered from lightest to heaviest (the “mass hierarchy”) and whether neutrinos respect the combined symmetry of charge and parity conjugation (the degree to which this process is violated is often referred to as the CP violating phase.) Both the mass hierarchy and the value of the CP violating phase are open questions in neutrino physics that have significant implications on our understanding of the standard model of particles.

T2K is the culmination of two impressive technological feats in particle physics (Fig. 1). On one end of the experiment, muon neutrinos are produced at the J-PARC beam facility in Tokai, Japan. Protons are accelerated and collided against a graphite target, producing copious quantities of unstable mesons (pions and kaons) that quickly decay and produce a shower of muon neutrinos with remarkably high purity. T2K works in what is known as an “off-axis” configuration, in which the axis of the primary beam of mesons is tilted a few degrees away from the detector. This configuration reduces unwanted background signals in the detector by narrowing the energy width and increasing the purity of the beam (specifically, it directs mesons with the appropriate sign and energy for producing muon neutrinos toward the detector).

The second half of the T2K experiment, the Super-Kamiokande (SK) detector, is located roughly 300km away, deep within the Kamioka mine. This immense 50 kiloton water Cherenkov detector was once used (and still is) to measure neutrino oscillations from the atmosphere—one of the first experiments to confirm that neutrinos could oscillate between flavor states, and hence possessed a finite mass [2] (see 1 September 1998 Focus). In its new role, the SK detector functions as the primary target for the Tokai neutrino beam. Instrumented with over 11,129photomultiplier tubes, SK detects the faint Cherenkov light created by high-speed charged particles, which are produced when neutrinos interact with water molecules.

When a neutrino arrives in the detector, its weak state—meaning whether it’s an electron, muon, or tau neutrino—can only be identified by which lepton (electron, muon, or tau) is created in the neutrino interaction. So T2K’s primary challenge is to distinguish electrons from muons and other types of background. One source of background is the constant flux of cosmic rays hitting Earth. T2K therefore uses a pulsed beam of muon neutrinos, which allows them to distinguish the timing of the signal of interest (electron neutrinos from appearance events) from that of the cosmic rays. In addition, to characterize the neutrinos closer to the source, they make use of a much smaller detector located less than 1km from J-PARC. Compared to the SK detector, this “near” detector permits them to make a more precise measurement of the energy profile of the neutrino beam and the probability of neutrinos interacting with matter. Finally, they have developed a careful analysis of the topology of the Cherenkov light cone and the energy of each detector event that allows them to further delineate candidate signal events from background events. (The background is typically dominated by neutral pion decays.)

Over three years of data taking, during which T2K collided over 1020 protons to produce the muon beam, the researchers recorded 28 candidate electron-neutrino appearance events. Only 4.92±0.55 of these events are expected to be background, providing a clear observation of muon neutrinos transmuting to electron neutrinos with a statistical significance of over 7 sigma. (Put differently, there is less than one billionth of a percent chance that these events are from the background.) Earlier measurements from T2K [3] and the MINOS experiment that stretches between Illinois and Minnesota in the US [4] had also indicated a neutrino appearance signal over their respective backgrounds, though with a lower statistical significance.

The results from T2K contribute to a steady march toward a full understanding of neutrino oscillations. At face value, the measurement provides a complementary verification of the value of θ13 to that measured by reactor neutrino experiments [5] (see 23 April 2012 Viewpoint). In addition, T2K combines their measurement of θ13 with that measured from disappearance experiments and feeds its value into the equation describing muon neutrino to electron neutrino oscillations. This allows them to solve for possible values of the CP violation phase. T2K’s preliminary analysis of the phase, which can vary between -π and π, seems to disfavor certain nonzero values. (The precise excluded values depend on what they assume about the neutrino mass state hierarchy.) The uncertainty of the excluded values is, however, still too large to make a definitive statement about CP violation in neutrinos.

The true telltale sign of whether neutrinos exhibit CP violation will come from comparing appearance events for neutrinos and antineutrinos. Larger experiments are being planned to perform exactly this measurement. In many ways, the results from T2K are reminiscent of when measurements of solar neutrinos were used to tease out preliminary information about θ13 before definitive measurements from reactor experiments became available. A similar path seems to be emerging here, and it does not seem that it will be long before experimentalists determine if CP violation exists in neutrinos.


  1. K. Abe et al. ((T2K Collaboration)), “Observation of Electron Neutrino Appearance in a Muon Neutrino Beam,” Phys. Rev. Lett. 112, 061802 (2014).
  2. Y. Fukuda and et al. (Super-Kamiokande Collaboration), “Evidence for Oscillation of Atmospheric Neutrinos,” Phys. Rev. Lett. 81, 1562 (1998).
  3. K. Abe and et al. (T2K Collaboration), “Evidence of Electron Neutrino Appearance in a Muon Neutrino Beam,” Phys. Rev. D 88, 032002 (2013).
  4. P. Adamson and et al. (MINOS Collaboration), “Electron Neutrino and Antineutrino Appearance in the Full MINOS Data Sample,” Phys. Rev. Lett. 110, 171801 (2013).
  5. F. P. An et al., “Observation of Electron-Antineutrino Disappearance at Daya Bay,” Phys. Rev. Lett. 108, 171803 (2012).


Oddball space neutrinos may be spawn of dark matter

Configuration of IceCube

Configuration of IceCube

by Anil Ananthaswamy
The first deep space neutrinos to be detected since the 1980s may be the spawn of mystery dark matter. That would explain puzzling features of these particles – and suggest an unusual identity for dark matter.

Neutrinos, ghostly subatomic particles, are routinely produced by the sun and on Earth, but apart from those seen after a 1987 supernova explosion, none had been detected from beyond the solar system.

Then, earlier this year, the IceCube collaboration, which monitors a cubic kilometre of ice at the South PoleMovie Camera, reported two deep-space neutrinos, dubbed Bert and Ernie, each with a mass of about 1 petaelectronvolt (1015 electronvolts). These were quickly followed by reports of a bunch more, with masses of tens of teraelectronvolts (1012 eV), mass and energy being equivalent for particles.

Deep-space neutrinos are prized because they could allow “neutrino astronomy” – using neutrinos to investigate mysterious cosmic objects. Being chargeless, neutrinos zip from a source direct to Earth without being waylaid.

Heavyweight particles

However, expected sources of such neutrinos, including energetic explosions called gamma-ray bursts or emissions from supermassive black holes called active galactic nuclei, should also produce neutrinos of energies different from those seen by IceCube so far.

Pasquale Serpico of the University of Savoy in Annecy-le-Vieux, France, and colleagues wondered if the lack of these other energies could be a sign of decaying dark matter – the invisible stuff thought to make up about 80 per cent of the universe’s matter.

They calculate that heavyweight dark matter particles of about 1 PeV would decay either directly into neutrinos of about 1 PeV, or into other particles and then into neutrinos with energies of tens of TeV. “It exactly reproduces the features that you see in IceCube,” says Serpico.

This comes hot on the heels of recent reports from several dark matter detectors, which have seen signs of much lighter particles, with masses of about 10 gigaelectronvolts.

Dark flavours

Tom Weiler of Vanderbilt University in Nashville, Tennessee, says there is no theoretical reason why dark matter shouldn’t be heavy. The production of such particles would require more complicated mechanisms in the early universe, so theorists tend to prefer lighter particle candidates. “But Nature is the arbiter, not theorists,” says Weiler.

Or the mysterious stuff might come in flavours – both light and heavy. “In general, the physics community prefers to have a single dominant dark matter [type], but it doesn’t have to be so,” says Serpico.

Francis Halzen, of the University of Wisconsin-Madison, and the principal investigator of the IceCube collaboration, isn’t convinced by the new theory. The neutrinos seen by IceCube can still be explained by standard sources if the gap in neutrino energies goes away as the experiment collects more particles. “I do not think that anything in the data requires a more exotic explanation at this point,” he says.

Dan Hooper, a theoretical physicist at Fermilab in Batavia, Illinois, agrees: “My money is on an astrophysical origin for these neutrinos, rather than dark matter.”

Whether the neutrinos come from dark matter will become clearer as IceCube amasses more neutrinos and the gaps in energies either persist or vanish.

“If the hypothesis is correct, the birth of neutrino astronomy coincides with the discovery of dark matter,” says Weiler.


News about ghost particles

This model of the GERDA experiment shows the onion-like structure which suppresses interfering signals from the environment. The germanium diodes in the center of the cryostat filled with liquid argon (–186°C) are to a larger scale. Credit: MPI for Nuclear Physics

This model of the GERDA experiment shows the onion-like structure which suppresses interfering signals from the environment. The germanium diodes in the center of the cryostat filled with liquid argon (–186°C) are to a larger scale. Credit: MPI for Nuclear Physics

( —Neutrinos are the most elusive particles having extremely weak interactions with all other particles. They have rather unusual properties and are even expected to be identical with their own antiparticles. So far this property is, however, not experimentally verified even though many studies of neutrinos over the last 60 years have already boosted our understanding of elementary particle physics.

Now scientists of the GERDA collaboration obtained new strong limits for the so-called neutrino-less double beta decay, which tests if neutrinos are their own antiparticles. The result rules out an earlier claim and has various important implications for cosmology, astrophysics and particle physics and it adds information about neutrino masses.
Besides photons, neutrinos are the most abundant particles in the Universe. They are often called `ghost particles’, because they interact extremely weakly with matter. They are therefore an invisible, but very important component of the Universe, which could carry altogether as much mass as all other known forms of matter, albeit traveling almost at the speed of light over fantastic distances. Their tiny masses have also important consequences for the structures in the Universe and they are the driving element in the explosion of Supernovae. But their most remarkable and important property has been proposed by Ettore Majorana in the 1930ies: Unlike all other particles that form the known matter around us, neutrinos may be their own antiparticles.

The GERDA (GERmanium Detector Array) experiment, which is operated at the Laboratori Nazionali del Gran Sasso underground laboratory of the Istituto Nazionale di Fisica Nucleare in Italy, is aiming to resolve the question whether neutrinos are in fact their own antiparticles, and to determine their mass. GERDA looks for so-called double beta decay processes in the germanium isotope Ge-76 with and without the emission of neutrinos, the latter being a consequence of the Majorana properties. In normal beta decay, a neutron inside a nucleus decays to a proton, an electron and an antineutrino. For nuclei like Ge-76, normal beta decay is energetically forbidden, but the simultaneous conversion of two neutrons with the emission of two antineutrinos is possible and has been measured by GERDA recently with unprecedented precision. This is one of the rarest decays ever observed with a half-life of about 2*1021 years, which is about 100 billion times longer than the age of the Universe. If neutrinos are Majorana particles, neutrino-less double beta decay should also occur at an even lower rate. In this case, the antineutrino from one beta decay is absorbed as neutrino by the second beta-decaying neutron, which becomes possible if neutrinos are their own antiparticle.

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