New window to the universe

Print edition : June 27, 2014

The IceCube Lab by night. It is a neutrino observatory whose dectectors are buried 1.5 km below the surface of the South Pole. Photo: Emanuel Jacobi/NSF

Figure 1: The cosmic neutrino spectrum: A low-energy background left over from the Big Bang is believed to suffuse the cosmos. Neutrinos have also been detected from the nearby explosion 1987A. Much of the spectrum of atmospheric neutrinos from cosmic-ray air showers has been measured by the Frejus underground detector (blue dots) near the South Pole. Not yet observed are neutrinos expected from cosmological point sources such as gamma-ray bursts and active galactic nuclei. The most energetic neutrinos are expected from the decay of pions created in collisions between cosmic-microwave-background photons and cosmic-ray protons with energies above 4x10(19) eV (the Greisen-Zatsepin-Kuz'min threshold).

Figure 2: This event display shows "Big Bird", the highest-energy neutrino at 2 PeV detected by the IceCube experiment. The colours show when the light arrived, with reds being the earliest, succeeded by yellows, greens and blues. The size of the circle indicates the number of photons observed. Photo: IceCube

With the recent discovery of very-high-energy cosmic neutrinos by the South Pole experiment IceCube, neutrino astronomy can be said to have finally arrived.

ON April 7, at a meeting of the American Physical Society (APS), scientists working at the Antarctic experiment called IceCube announced that they had detected a neutrino—the highly elusive, very weakly interacting and nearly massless elementary particle—with the highest energy ever. Its energy was above a whopping two million billion electronvolts (2 × 10 15 eV), or 2 peta eV (PeV). This’ is about 1,000 times the energy to which protons are accelerated at the Large Hadron Collider (LHC), the highest-energy man-made particle accelerator, which is 3.5 tera, or trillion (10 12), electronvolt (TeV).

The two high-energy neutrinos with more than 1 PeV energy that were detected by the same experiment in July 2012, the results of which were published last November in the journal Science, had already created quite a flutter in the high-energy particle physics community. They were detected along with 26 other neutrinos with energies above 30 TeV. With twice their energy, the new one, which is part of a new set of nine high-energy neutrino events observed, has added to the excitement. Since the earlier two PeV neutrinos had been named “Bert” and “Ernie” after two Muppet characters from the popular children’s TV show in the United States called Sesame Street, the new one has been named “Big Bird” to continue the tradition. The paper describing this new set of high-energy neutrino events was published on May 21 in the journal Physical Review Letters.

Nearly all of the lower-energy neutrinos detected thus far by terrestrial instruments were produced in the earth’s atmosphere as a result of interactions of cosmic rays, the energetic charged particles from space that constantly bombard the earth from all directions. The IceCube detector, with its array of 5,160 sensors located over a cubic kilometre of pure ice, and buried more than 1.5 km under the South Pole, has been designed and built specifically to detect high-energy neutrinos from space (“Astrophysics in polar ice”, Frontline, June 3, 2005). The origins of these 37 really high-energy neutrinos are not clear yet, but they seem to be of cosmic origin, coming from beyond the Milky Way galaxy. And with their detection, “‘neutrino astronomy”’ can be said to have well and truly arrived.

Almost all we know about the universe comes from observing photons with telescopes of various kinds. Besides the most familiar optical telescopes that use visible light, astronomers use telescopes that scan the sky in different wavelength (or frequency) regions of the electromagnetic spectrum: radio waves, infrared radiation, ultraviolet radiation, X-rays and gamma rays—all electromagnetic waves composed of photons with different energies.

As cosmic messengers, photons have many advantages. They are produced copiously in astronomical objects and are stable. Also, since photons are electrically neutral, their flight paths are not affected by galactic and intergalactic magnetic fields. So the direction from which photons are detected points to their source.

But this is not the case with cosmic rays, the other messengers of the cosmos from which scientists have begun to understand a great deal about the universe. They are largely made up of the positively charged protons or ions (atomic nuclei stripped of their orbiting electrons). Since they are all charged particles, they get deflected by cosmic magnetic fields and their trajectories do not trace back to their sources.

But the advantage photons have is only limited because the hot, dense regions of stars, active galactic nuclei (AGNs) and other objects of the cosmos, which constitute the central energy-producing engines, are totally opaque to them. The photons that the earth receives from the sun, for example, come from its photosphere, that is, its outer shell, and not from its core, where nuclear fusion of hydrogen occurs and powers the sun. Therefore, these regions are inaccessible for direct observations by nearly all astronomical techniques currently in use.

Another disadvantage of using photons as probes for distant astrophysical processes is their interaction with the cosmic microwave background (CMB), the relic radiation from the Big Bang with which the universe is awash. CMB photons have about a thousandth of an electronvolt of energy, and the wavelengths of these low-energy photons are in the microwave region. The spectral distribution of the CMB radiation corresponds to the emission of a black body at the ultra-cold temperature of about 2.7 Kelvin (−270.45 °C). Photons from astrophysical objects can interact with the CMB photons during their journey through intergalactic space and form electron-positron pairs (pair production) in accordance with the E = mc 2 relation and thus be lost to detection by earth-based instruments.

This interaction with the CMB effectively prevents any survey of the sky at distances beyond 100 megaparsecs (Mpc) even with high-energy photons (gamma rays) of over 10 TeV of energy (1 parsec = 3.26 light years, or 30.9 trillion km). All visible stars in the night sky lie within a distance of a few hundred parsecs, galactic objects lie within a few tens of kiloparsecs (kpc), and galaxies nearby and beyond lie at distances of the order of megaparsecs. (The above problem is actually the photonic analogue of the energy cut-off observed in cosmic rays, which is called the Greisen-Zatsepin-Kuz’min (GZK) effect and will be discussed a little later.)

Therefore, what is needed to study the inner workings of astrophysical objects and to gain an understanding of the large-scale universe over a larger range of energies and greater distances is a particle probe that is electrically neutral (so that intervening magnetic fields do not affect its trajectory), stable (for it to reach the earth from the far reaches of the universe without decaying), and weakly interacting (so that it traverses unscathed through hot and dense regions that, because of scattering, are opaque to photons). The neutrino is the only candidate that fits this bill.

The neutrino, just like the electron and the quark, is an elementary particle. According to the Standard Model of elementary particles, the fundamental building blocks of all matter are quarks and leptons. There are six quarks and six leptons. Both come in three families of two each. The electron is a lepton and has two heavier cousins, the muon and the tau, and each of them has a corresponding electrically neutral particle partner with a tiny mass: the electron-neutrino, the muon-neutrino and the tau-neutrino.

Other than photons, neutrinos, which are produced in nuclear interactions, are the most common particles in the universe. Trillions of them from the sun pass through us every second. They are also produced copiously in the atmosphere—–the so-called atmospheric neutrinos— –where they are produced by cosmic rays striking air molecules. Their interaction with matter is so weak that an average neutrino can pass through light years of lead without being impeded. But, because of the very small probability of neutrinos interacting with matter and radiation, they allow probing much larger distances. As they are chargeless and are, therefore, not bent by magnetic fields, their arrival directions unambiguously point back to their origins, thereby opening a new window of ‘neutrino astronomy’ to the universe, complementary to conventional photonic astronomy. But, even as they are ubiquitous, they are terribly elusive too because they interact so feebly with matter that it is very difficult to detect them.

Neutrinos were produced in huge numbers at the time of the Big Bang, and cosmology predicts that, like the low-energy photons of CMB, there should be a low-energy relic cosmic neutrino background (CNuB) because of the expansion of the universe. Just like the CMB temperature which is 2.7 K, the CNuB would have an effective temperature of about 1.9 K. But, such low-energy neutrinos would be very difficult to observe with current technology as the probability of interaction with a detector drops in inverse proportion to the square of the energy. And even in the rare case that such neutrinos might react, the signal will be too weak to be observed. So, at least for the near future, an all-sky neutrino map analogous to the CMB map produced by the Wilkinson Microwave Anisotropy Probe (WMAP) and Planck satellites ( Frontline, May 13, 2013) is unlikely.

The nuclear burning of stars, like the sun, also produces neutrinos. Hydrogen fusion produces electron-neutrinos, and the sun produces a huge flux (number per unit area per solid angle per unit time) of them, whose energies range from a few 100 keV to 10-20 MeV, and scientists have been studying these for over the past three decades. In this sense, scientists have been doing (low-energy) neutrino astronomy, but this is only limited to the sun as the high solar neutrino flux (about 7 ×10 10) swamps out neutrinos from outside the solar system in this energy range.

During the spectacular supernova event of February 1987 (SN 1987A), which occurred in the Large Magellanic Cloud at a distance of about 50 kpc (about 170,000 light years), neutrino detectors deep underground in Japan, Russia and the U.S. recorded a sudden burst of neutrinos: a total of 19 neutrinos (in the MeV energy range) over a span of 13 seconds. Such a burst of neutrinos is indeed expected when a star explodes, and with the conversion of iron nuclei to neutrons, a neutron star is formed in the heart of a supernova.

About 3 × 10 46 joules of energy, which is 1,000 times the solar energy release over its lifetime, was released in SN 1987A and about 10 58 neutrinos, carrying just about a thousandth of the total energy release, were produced over a few seconds. A lot of physics can be learnt from these violent events marking the death of a star, and indeed, a lot about the properties of the elusive neutrinos was extracted from the single SN 1987A event. There are now many underground detectors tuned to detect neutrinos from such events. In fact, SN 1987A was hailed as heralding the birth of neutrino astronomy. But the expected rate of visible supernovae in our galaxy is only about one in 200 years, and many cannot be observed because the galactic plane blocks the view.

Neutrinos from the sun and SN 1987A were the only extraterrestrial neutrinos detected until recently. Also, both the sun and supernovae produce only low-energy neutrinos. The existence of high-energy cosmic rays (HECRs), however, suggested that high-energy neutrinos should also be produced by these extremely energetic astrophysical objects that produce the former. In theory, the neutrino energy spectrum (variation of flux with energy) on the earth’s surface spans an enormous energy range (Figure 1)—from microwave energies (10 -4 eV) in the CNuB up to the highest cosmic ray energies (10 19 eV). However, until IceCube, the highest energy observed was about 10 14 eV in atmospheric neutrinos.

Primary cosmic rays, as mentioned before, are mostly protons with some admixture of heavier nuclei. Cosmic rays have an energy spectrum that extends to extremely high energies. They have been observed from 10 9 eV (GeV) to over 10 20 eV (100 million trillion, or 100 × 10 18 eV, or 100 exa eV (EeV)). Over this range, the flux appears to follow nearly a single smooth curve that falls off roughly as the cube of the particle energy. Cosmic rays with energies greater than an EeV, which greatly exceeds the energy of any terrestrial particle, are termed ultra-high-energy cosmic rays (UHECRs).

In accordance with this “power law" energy spectrum, the arrival rate of UHECRs above 10 19 eV is only 1 per km 2 a year, and above 10 20 eV the estimated arrival rate is only one per km 2 a century. So far hundreds of events of energy greater than 10 eV (about a joule, which is macroscopic scale energy in a microscopic particle) and about 20 events of energy greater than 10 19 eV have been seen. The most energetic cosmic ray ever observed had an energy of 3.2 × 10 20 eV, or 320 EeV, the kinetic-energy equivalent of a golf ball whizzing through at 47 m/s packed into a single proton, which was detected in 1991 by the University of Utah cosmic ray experiment called Fly’s Eye.

This is about 10 billion times the maximum energy of 3.5 TeV to which protons are accelerated at the LHC. An accelerator for protons with the same energy as the highest cosmic rays based on the LHC superconducting magnets would be larger than the earth’s trajectory around the sun (the LHC has a circumference of 27 km). Even the particles emitted in a typical supernova explosion, one of the most powerful sources of energy in the universe, have 100,000 times less energy. While galactic sources such as microquasars, supernova remnants and black holes can produce cosmic rays up to PeV scale (10 15 eV) energy, UHECRs are believed to be extragalactic.

This is because a cosmic ray proton carrying more than 10 19 eV, for example, would not be deflected significantly by the typically weak galactic magnetic fields, and even less by the much weaker intergalactic magnetic fields. So these extreme-energy particles would travel more or less in a straight line. If such particles came from the Milky Way, one should see them arriving preferentially from the disk of the galaxy or the part of the sky towards the galactic centre as the galaxy is not symmetric around us. Instead the distribution is essentially isotropic. But the exact sources of these remain a mystery. Scientists believe that the most powerful objects known, the AGNs and the enigmatic gamma-ray bursts (GRBs), which are the most violent phenomena in the universe, could be the sources of these highest-energy cosmic rays. But, more pertinently, these extreme-energy cosmic rays (>10 19 eV) seem to violate the present understanding of the universe in terms of the Big Bang model. In this model, there is a predicted maximum energy of 6 × 10 19 eV, which is called the GZK cut-off. This arises from the interaction of energetic protons, the dominant cosmic ray particles, with the CMB photons, the leftover radiation from the Big Bang that fills all space. While a CMB photon itself does not have much energy, a sufficiently energetic (high velocity) cosmic ray would see the CMB photon’s wavelength compressed because of the Doppler effect and would see it is a gamma ray photon rather than a microwave photon.

From accelerator studies, one knows that collisions between gamma rays and protons should produce particles called pions, which should cause the protons to lose energy. With each collision, a proton loses about 20 per cent of its energy. This only happens for protons that have at least 6 × 10 19 eV of energy. So if cosmic rays had an initial energy greater than that, they would lose energy through repeated collisions with CMB photons until the energy falls below this limit. During this time the proton can at most travel about 150 million light years.

But 150 million light years is only as far as the local supercluster of galaxies, which means that these extreme-energy particles should have come from nearby sources. The arrival direction, however, does not point to any known astrophysical object, notwithstanding the fact that the number of such events is not statistically large enough to draw definitive conclusions. These events have even led scientists to suggest that new physics may be at work.

High-energy cosmic ray protons and nuclei are also likely to be associated with a flux of high-energy neutrinos because of interactions of cosmic rays with ambient radiation or matter. HECR protons and nuclei will interact with light (photons) and gas in the galactic disk to give rise to neutrons, kaons and charged and neutral pions, which in turn decay to produce electron- and muon-neutrinos and gamma rays with energies proportional to the energy of the cosmic rays that produced them. These high-energy neutrinos, if detected, will point back to the direction of the source. The highest-energy neutrinos are produced in the collisions of UHECRs with low-energy CMB photons, the GZK phenomenon, and are called “‘cosmogenic neutrinos”’. Their energies are of the same order as those from GRBs and AGNs (Figure 1).

Therefore, for such enigmatic astrophysical sources that lie beyond the distances limited by the GZK cut-off, astronomy with the secondary neutrinos will be the only probe. Neutrinos are important probes for another important reason. Unlike photons, they are unambiguous signatures of interactions involving particles made of quarks that take part in strong nuclear (hadronic) interactions. They will thus give a direct proof of hadronic processes that provide the necessary particle acceleration to high energies in astrophysical objects. Neutrino astronomy thus offers the possibility of observing sources that correspond to the most powerful astrophysical phenomena in the universe.

The flip side of using neutrinos as cosmic probes is the very low rate at which secondary neutrinos from extragalactic cosmic rays will impact the earth. The low rate and the fact that neutrinos interact only weakly with matter make the detection of UHE neutrinos extremely difficult. To compensate for this, one needs not only detectors covering a very large area but also detectors with a large amount of target matter. The flux of cosmic neutrinos can be estimated from the observation of the rate of UHECRs, and one will find that one needs detectors the size of a cubic kilometre in order to catch these neutrinos! For a kilometre-scale detector, the flux rate is one event a year, though this estimate has uncertainties because of the still-unknown composition of UHECRs and because of the cosmological evolution of the sources.

Three facts make neutrino astronomy at higher energies more promising than at lower energies. One, the interaction probability of neutrinos with matter goes up with energy. While at lower energies one studies neutrinos coming from below (the far side of the earth) to ensure that everything but neutrinos are completely filtered out by the earth’s interior, at energies above 1 PeV, the earth becomes opaque to neutrinos, and therefore, one only looks for neutrinos coming downwards from the sky. Two, the chance of detecting neutrino interaction with target matter increases because, at higher energies, the energy released in the interaction is greater. Also, since cosmic neutrinos, on an average, have higher energies, they are easily distinguished from the large background of lower-energy atmospheric neutrinos. Atmospheric neutrino flux drops rapidly with increasing energy, and atmospheric neutrinos beyond 10 14 eV are extremely rare (Figure 1).

Neutrino detectors are generally placed deep underground or in water to avoid the unwanted background at ground level of other particles produced by the impact of cosmic rays on the atmosphere. Cosmic rays produce many muons (the heavier cousins of electrons) that can penetrate deep into the earth but with the flux decreasing with depth. The first detection of extraterrestrial neutrinos was, in fact, at the Kolar Gold Field Mines in the 1960s, when atmospheric neutrinos were observed. Also, since cost becomes an important factor at such huge scales, one chooses as the target matter existing in nature, such as water or ice.

When a neutrino interacts with matter, it can either continue as a neutrino after the interaction (“‘neutral current interaction”’) or create the corresponding charged particle (“‘charge current interaction”’). In the latter case, the electron-neutrino creates an electron, the muon-neutrino a muon, and the tau-neutrino a tau. The most common method to detect neutrinos is through the muons they produce as a result of their interaction with the target nuclei. These muons, which are charged, traverse a long distance in the earth—–about 1 km at an energy of a few TeV and even up to 10 km for the most energetic neutrino—in almost the same direction as the initial neutrino before stopping.

When a charged particle passes through any medium at a velocity greater than the velocity of light in the medium (in clear water or ice the velocity of light is three-fourths of its value in vacuum), it emits a short flash of blue light known as Cerenkov radiation in its wake, which is the electromagnetic version of a sonic boom. This flash is detectable at tens of metres by light sensors such as photomultiplier tubes (PMTs) in water or ice which are transparent to light. The light pattern reveals the direction of the muon. Its determination will give the direction of the muon-neutrino that produced it within a degree, which in turn will point back to its origin in astrophysical objects. This is the key to high-energy neutrino astronomy. The need for large volumes of a very transparent medium means that neutrino telescopes can be built only in very special places: as deep as 3-4 km in lakes or oceans or in the massive ice sheet in Antarctica. IceCube, the under-ice detector at the South Pole, is the first km 3-size neutrino telescope (see box).

As against 2,700 cosmic rays per second, IceCube actually sees one neutrino every six minutes, but these are all mostly atmospheric neutrinos. Truly cosmic neutrinos are expected only at the rate of about 10 a year. In fact, when the researchers began to look for signals of cosmic neutrinos in the initial analysis of the data, they found no high-energy neutrinos in the TeV-PeV range. But when they changed the strategy and began looking for UHE neutrinos in the PeV-EeV range, events “Bert” and “Ernie” stood out unmistakably from the data with energies of 1.04 PeV and 1.14 PeV respectively. Analysing the data with the changed filtering strategy, they found 26 more high-energy events that appeared to be of cosmic origin.

This marked the first direct observation of high-energy cosmic neutrinos in the last two decades and more since the highest-energy cosmic ray was detected, and the measurement of the cosmic ray energy spectrum up to the highest energies has since been greatly refined. This was the first indication of very high-energy neutrinos coming from outside the solar system with energies more than a million times those observed from SN 1987A. Neutrinos of such energy could only have come from distant highly energetic astrophysical sources.

This truly heralded the beginnings of neutrino astronomy. Indeed, the IceCube project was given the 2013 Breakthrough of the Year Award by the British journal Physics World. It was selected for making the first observation of cosmic neutrinos and for overcoming many of the challenges of creating and operating a mammoth but extremely sensitive detector deep under ice in the most inhospitable part of the world. Francis Halzen of the University of Wisconsin, who is the principal investigator of the IceCube experiment, had envisioned the project way back in 1988.

Now, with “‘Big Bird”’, the experiment has made another prized catch (Figure 2). The updated analysis also confirmed that neutrinos of higher energy than “‘Big Bird”’ were not seen. Some astrophysicists have speculated that there should be a sharp cut-off in neutrino energy at about a PeV. It is moot, therefore, if the “‘Big Bird”’ event, with more than 2 PeV neutrinos, contradicts that. Only more data can tell. But with the limited number of events—many of which, including “‘Big Bird”’, are diffuse where a shower of hundreds of particles are seen in the detector volume instead of a streaking muon—–the IceCube data are not yet good enough for one to trace them back to the cosmic accelerators from which these high-energy neutrinos originated. The evidence for a cosmic signal could be discerned only when the cascade events were included even though the original intent was only to focus on events with streaking muons.

But only about 15 of the total events were likely to have been spurious background of atmospheric neutrinos or muons, according to the IceCube researchers. The results have provided what in statistics parlance is known as a 5.7 sigma evidence that the neutrinos detected are indeed of cosmic origin, that is, there is only an one-in-about a billion chance that all the events can be attributed to purely atmospheric phenomena.

“With the current level of statistics, we did not observe significant clustering of these events in time or space, preventing the identification of their sources at this time,” the Science paper said. ‘’ “The observed flux is consistent with an isotropic and power-law spectrum varying inversely as the square of the energy, as expected for an astrophysical neutrino flux. The search for neutrino sources, either as significant clusters of observed neutrinos or as neutrinos observed in correlation with gamma-ray sources, yielded no significant evidence,” said the IceCube press release following the latest publication. “Regarding the origin of this flux,” the release added, “the lack of correlation with the galactic plane and the high galactic latitudes of some events suggest an extragalactic component.”

As Subir Sarkar of Oxford University and a member of the IceCube collaboration pointed out, one cannot even say unambiguously that these were extragalactic. “Most people might say that they are extragalactic because galactic sources should also be PeV gamma-ray sources (which have not been seen). However, half the events seen are in a part of the southern sky which has not been surveyed [for gamma rays],” he said. “But we need more data.” And more data are expected soon as the analysis is complete for twice the number of IceCube data samples than have been analysed so far. This data will also include up-going muon tracks (those due to neutrinos travelling from the far side of the earth) to ensure that the flux seen is an all-sky flux.

What seems to be at least clear is that these are not cosmogenic neutrinos a la GZK, that is, those produced as a result of interaction of UHECRs with the CMB photons. This is inferred from the non-observation of even higher-energy (up to 10 EeV) neutrinos as would be expected in a GZK-produced neutrino flux. The researchers have also statistically ruled out their source to GRBs from the fact that the neutrinos were not seen in coincidence with any of the hundreds of GRBs detected by satellites during the IceCube detection period. According to Sarkar, AGNs and hypernovae remain possible sources for the IceCube neutrinos. (Hypernovae are also supernovae but whose energies are substantially higher than standard supernovae.)

The IceCube findings have given renewed impetus to the plans for constructing KM3NeT, a similar detector in the Mediterranean (which will complement IceCube by being able to reconstruct muon tracks pointing towards the Southern sky). As the avenue of “‘neutrino astronomy”’ expands with more km 3-size neutrino telescopes being built, the sources of high-energy neutrinos, and therefore of UHECRs, should get unravelled in the very near future. The imminent growth of the field also provides the proposed India-based Neutrino Observatory (INO), being established at Bodi hills in Theni district in Tamil Nadu, an opportune period to do interesting physics.

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