Neutrino Astronomy Encyclopedia of Astronomy and Astrophysics Neutrino Astronomy The Sun generates energy deep in its core by nuclear fusion processes. A supernova (of type II) explodes and blows off the entire star by gravitational energy liberated when the central core collapses. These interesting phenomena of energy generation can never be observed with visible light or any other electromagnetic waves. The places where these phenomena take place are buried deep in dense material and the created heat is quickly thermalized before it comes out of the stellar surface. Relevant information is lost during thermal equilibration. Nevertheless, one may want to observationally probe the deep interior of the Sun or the gravitational collapse in the supernova core. One then has to find electrically neutral particles that have guaranteed straight travel paths in space. They obviously have to be produced in the regions and should not participate in thermal equilibration in order to bring the desired information, neutrinos are exactly the particles that possess these features. Neutrinos are produced in nuclear processes, but interact with matter so weakly that as soon as they are created they leave the region with very nearly the speed of light as their mass is very small. The universe about 1 s after the big bang had a temperature as high as the core of a supernova, and light particles such as electrons and positrons were in thermal equilibrium. Cosmic expansion in the meantime cooled down the universe and lowered the energy density. Electrons and positrons then annihilated, disappeared and created photons and neutrinos. These relic neutrinos should exist in the present universe and have important information on the early universe. cosmic rays are accelerated somewhere in our Galaxy or, if they are very energetic, perhaps in distant active galaxies, too. The extremely energetic cosmic rays (above 1020 eV) might even be produced by the decay of exotic objects created in the early universe and floating around even at the present, whose existence is conjectured in grand unified theories of elementary particles. Cosmic rays bend in galactic or intergalactic magnetic fields and cannot be traced back to their birthplaces. Even to this day, we do not know for sure where cosmic rays are actually born. Cosmic rays may interact with the medium surrounding the acceleration region and produce mesons, which decay in a short time and produce neutrinos. Neutrinos then escape freely to open space and may reach Earth with fruitful information. Neutrino astronomy is used to study the central engines of the Sun and supernova, to identify the origin of cosmic rays and to study the very early universe. Astronomical neutrinos can also be used as beams useful for investigating neutrino properties, such as mass and mixing. Indeed, only after having completed the study of neutrino properties can one begin real neutrino astronomy. Neutrinos from the Sun were first observed by R Davis Jr in the Homestake gold mine in the late 1960s. The experiment is still going on 30 yr after the initial operation. Since then four other experiments have joined this field and two more are coming into operation in the near future. All five experiments did observe solar neutrinos, thus confirming that the Sun is actually generating energy by nuclear fusion processes (see solar interior: neutrinos) . The reason why this field is still so active even now is that there are serious discrepancies between observations and expectations. What the experiments observed is only 30-60% of what the standard solar model predicts. It would be shocking if the Sun were not working as theory tells us, because all life on Earth depends on stable solar heat generation. The so-called solar neutrino problem has not been completely solved. The most popular idea is not that the solar engine works with an efficiency of only 30-60% but that the 'deficit' of solar neutrinos is caused by neutrino oscillations, namely electron neutrinos that are produced in the Sun are converted to a different kind of neutrino, probably muon neutrinos, either in the solar interior or on the way to Earth. Muon neutrinos are impossible or difficult to observe, hence leading to an apparent deficit of solar neutrinos. This idea will be tested by new-generation experiments in the near future. 23 February 1987 will be remembered as the day when a real instance of the gravitational collapse and the subsequent birth of a neutron star were first observed with neutrinos. Two large water Cherenkov detectors, Kamiokande and 1MB, observed 11 and eight neutrino events, respectively. Although the observed neutrinos were few, these observations beautifully demonstrated that the underlying theory of the explosion of a type II supernova is correct, namely that the explosion was initiated by the gravitational collapse and the total energy released was about 3 x 10s3 erg, which roughly corresponds to 500xsolar luminosityx4.6 billion years. These are the only successful observations of astronomical neutrinos. Large under-ice and under-water detectors are being built and tested for high-energy cosmic ray neutrinos with energies above 1012 eV. It will still take several more years before these neutrinos are first observed. There is absolutely no idea how to detect relic neutrinos born in the early universe. They are expected to have an average energy of only 0.00017 eV which is too low to have a meaningful interaction probability (cross section) with matter. What neutrinos are Particle physics tells us that the elementary particles (or simply called particles) are classified in four types: up-quark (u), down-quark (d), electron (e) and electron neutrino (ve). These particles are said to form a family. Three families have in fact been found in nature and hence there are 12 elementary particles altogether. They are listed in table 1. Three neutrino species are called the electron neutrino (ve), muon neutrino (vM), and tau neutrino (vr), corresponding to charged counterparts, electron (e), muon (fi) and tau lepton (t), respectively. The unique characteristics of neutrinos are as follows. Copyright © Nature Publishing Group 2001 Brunei Road, Houndmills, Basingstoke, Hampshire, RG21 6XS, UK Registered No. 785998 and Institute of Physics Publishing 2001 Dirac House, Temple Back, Bristol, BS1 6BE, UK 1 Neutrino Astronomy Encyclopedia of Astronomy and Astrophysics Table 1. Table of elementary particles. Rows in the second column correspond to u-quark, d-quark, electron and electron neutrino. They are classified by electric charge. The columns correspond to families. Families are distinguished only by the masses of the particles. Charge 2/3 u c t -1/3 d s b -1 e ß r 0 ve vß • They are electrically neutral and fly straight even in the presence of magnetic fields. • Only weak forces act on neutrinos and hence they have a great ability to penetrate matter. • Neutrinos are produced in high-temperature and high-density environments such as the center of stars and easily escape out of the star and reach Earth, bringing information on nuclear processes, energy generation etc at the production site. • Great penetrability in turn makes neutrino detection quite difficult and the target size of a detector should be as large as possible. Solar neutrinos According to the standard solar model the central temperature and density of the Sun are about 15 million kelvins (kT ~ 14 keV) and 150 g cm~3, respectively. These conditions allow protons to overcome the repelling force from their positive charges (Coulomb barrier) and to initiate nuclear reactions. The following nuclear fusion processes are taking place in the Sun: p + p —> d + e+ + ve (1) d + p —> 3He + y (2) 3He + 3He —► 4He + 2p (3) or 3He + 4He —► 7Be + y (4) 7Be + e" —► 7Li + ve (5) 7Be + p —> 8b + y (6) 8B —► 8Be* +e+ + ve. (7) The product 7Li further reacts with a proton and produces two 4He. The product 8Be* immediately decays into two 4He also. The reaction chain is now complete. One can see that electron neutrinos are produced by weak interactions (equation (1) and equation (5)) and a weak decay (equation (7)). These neutrinos are called pp neutrinos, 7Be neutrinos and 8B neutrinos, respectively, pp and 8B neutrinos have continuous energy spectra approximately given as ElEepe, where Ev, Ee and pe are neutrino energy, positron energy and positron momentum, respectively. Ev is given by Q — Ee, where Q is the available energy for the neutrino+positron pair (Q value); the maximum Ev are 0.42 MeV and 15 MeV for pp and 8B neutrinos, respectively. 7Be neutrinos have a line spectrum as equation (5) is a two-body reaction. Their energy is either 0.86 MeV (90%) or 0.38 MeV (10%). The standard solar model calculates the neutrino flux on Earth from each source. The fluxes obviously depend on the temperature, density of each chemical element, thermal conduction of the surrounding medium, etc. The calculated fluxes are enormous: 5.9 ± 0.06 x 1010 crrr2 s-1, 0.48 ± 0.04 x 1010 cm-2 s"1 and 5.2% x 10s cm"2 s"1 for pp, 7Be and 8B neutrinos, respectively. Neutrinos have to be detected on Earth. In general particle detectors utilize particles' electric charge to produce observable signals. Neutral particles such as neutrinos must first interact with target particles and produce charged particles that produce signals. For solar neutrinos the following reactions have been used: ve + e~ —> e~ + ve (8) ve + 37 CI —► e" + 37 Ar (9) ve + 71 Ga —► e" + 71 Ge. (10) In water Cherenkov experiments, Kamiokande and Super-Kamiokande (figure 1), water is the target material and electrons emitted from reaction (8) are detected. Only electrons with Ee > 5.5 MeV are detected owing to severe background problems. Thus the experiment is sensitive only to 8B neutrinos. For the latter two reactions, the end products 37Ar and 71 Ge are extracted by the radiochemical technique and their numbers counted. The pioneering experiment Homestake used reaction (9) with a threshold energy of 0.81 MeV, and was thus sensitive to 7Be and 8B neutrinos, and subsequent SAGE and GALLEX experiments adopted reaction (10) with a threshold energy of 0.23 MeV, sensitive to pp, 7Be and 8B neutrinos. The observational results are surprising. All five experiments observed much fewer neutrinos than what the standard solar model predicts. If one takes the ratio of observed to predicted numbers, the results were 0.27 ± 0.02, 0.42 ± 0.06, 0.47 ± 0.015, 0.56 ± 0.06, 0.50 ± 0.09 for Homestake, Kamiokande, Super-Kamiokande, GALLEX and SAGE, respectively. These serious discrepancies are called the solar neutrino problem and its solution has not been found yet, although neutrino oscillation is the most likely clue. Super-Kamiokande and coming experiments, SNO and BOREXINO, will challenge the problem further. Supernova neutrinos The theory of the type II supernova explosion can be summarized as follows. A star with mass larger than 8MQ (solar mass) rapidly burns out its nuclear fuels. Nuclear ashes form an onionskin-like structure: the iron core at the center, then successively silicon, oxygen, neon, carbon, helium layers Copyright © Nature Publishing Group 2001 Brunei Road, Houndmills, Basingstoke, Hampshire, RG21 6XS, UK Registered No. 785998 and Institute of Physics Publishing 2001 Dirac House, Temple Back, Bristol, BS1 6BE, UK 2 Neutrino Astronomy Encyclopedia of Astronomy and Astrophysics electronics hut to ENTRANCE SUPERKAMIOKANDE NSTITUTE FOR COSMIC RAY RESEARCH UNIVERSITY OF TOKYO NtKKEN SEKKEI Figure 1. The Super-Kamiokande detector. It consists of inner and outer parts, each of which contains 32 000 tons and 18 000 tons of pure water, surrounded by 11 200 50 cm diameter photomultiplier tubes (PMTs) and 1800 20 cm diameter PMTs, respectively. A charged particle moving faster than the speed of light in water emits blue light named after the discoverer P Cherenkov with an opening angle of about 42°. The energy, direction and type of the particle are determined by measuring the arrival time, intensity and ring pattern of the Cherenkov light. The Super-Kamiokande experiment discovered the finite mass of neutrinos from detailed observation of atmospheric neutrinos. and the hydrogen layer at the outermost layer. The central iron core becomes unstable when its mass exceeds the Chandrasekhar mass (~1.4MG) and eventually collapses gravitationally. The central core is pressurized by falling matter, stiffened and bounces back. How large the bounce is depends on the elasticity or equation of state of the matter. The temperature rises above 100 billion kelvins (kT ~ 10 MeV) and the inverse beta decay process sets in: p + e (11) An electron neutrino burst of about 10 ms duration thus follows. It is called a neutronization burst and signals the real onset of the supernova explosion. A neutron-rich high-density ball forms which is called a proto-neutron star as it will later become a neutron star. The matter density is now above 1011 g cm~3, which is high enough that even neutrinos frequently interact with matter and are trapped inside. Neutrinos are in approximate thermal equilibrium with electrons, protons and neutrons. Neutrinos of all kinds now evaporate from the surface of the neutron-rich, high-density sphere called a neutrinosphere, the radius of which was initially several tens of km. The underlying process for neutrino emission is the thermal one: (12) where v* denotes any of the three neutrino species and v; its antineutrino. The hot proto-neutron star quickly cools down with a time scale of about 5 s, where neutrino and antineutrino pairs carry off thermal energy. Note that the proto-neutron star is still surrounded by thick falling matter. Hence heat cannot be transported by electromagnetic waves. The proto-neutron star shrinks to a radius of about 10 km and the density reaches ~1014 g cm~3, at which neutrons are packed in contact with each other. It has become a neutron star. The reverse shock produced at the core bounce, and probably the reheat caused by the outgoing neutrinos, later blow off the whole star and the explosion becomes optically visible, thus the birth of a supernova. Copyright © Nature Publishing Group 2001 Brunei Road, Houndmills, Basingstoke, Hampshire, RG21 6XS, UK Registered No. 785998 and Institute of Physics Publishing 2001 Dirac House, Temple Back, Bristol, BS1 6BE, UK Neutrino Astronomy Encyclopedia of Astronomy and Astrophysics The total energy carried off by neutrinos, the luminosity, is equal to the gravitational potential of a neutron star with radius R ~ 10 km and mass M ~ 1AMQ: 3 GM2 „ L„ = --^~3xl053 erg. (13) The neutrino energy distribution approximately follows the Fermi-Dirac distribution, dNv dE ■■ An R (litficf exp[(£ - ß)/kT] + 1 4jtD2 (14) where £, T and R are the neutrino energy, temperature and radius of the neutrinosphere, respectively. g(= 1) is the helicity weight of the neutrino, fi the chemical potential and D the distance to the supernova. The average neutrino energy is, for zero chemical potential, (E) ~ 3.15fc7\ The cross sections satisfy the following inequalities: ffve > ffve > oVil = aVz > a-v 1 TeV, Fv forAGN3C279andF„ and 3C273. These numbers may be quite optimistic and could be smaller by several orders of magnitude. Muon neutrinos from astronomical objects interact with rock in Earth and produce muons: v„. + N [i + ■ (22) Detection of astronomical muon neutrinos adopts this reaction. A large-area muon detector is deployed deep underground in order to reduce the atmospheric muon background. Rock beneath the detector is the target. A high-energy muon produced in the rock travels a long distance upward, approximately Eli/(2 MeV) g cm~2, reaches the detector and is detected. Downward-going muons are completely swamped by atmospheric muons and cannot be detected. In order to overcome the huge background of atmospheric neutrinos, which are produced by cosmic rays in the atmosphere associated with atmospheric muons, a high-energy threshold must be set, say 1 TeV-100 TeV, depending on what objects are to be observed. A rough estimate of the observed rate is N,, 0.5 events^ > 1 TeV) F„(> 1 TeV) 10s m2 yr io- crrr (23) The largest detector under test is the AMANDA-B (figure 2) detector which is located 1500 m under Antarctic ice. The detector has cylindrical shape with 120 m radius and 400 m height. Cherenkov light emitted in the ice is detected with a number of photomultipliers. AMANDA-B may still be too small. There is a plan to enlarge it to a size of 1 km3, and underwater projects of similar size are being seriously considered. Depth surface 50 m snow layer 810 m 1000 m 1150 m AMANDA as of 1998 Eiffel Tower as comparison (true scaling) zoomed in on AMANDA-A (top) AMANDA-BIO (bottom) zoomed in on one optical module (OM) Figure 2. The AMANDA-B detector. A number of optical modules (20 cm diameter photomultipliers) are deployed within a volume 120 m in diameter and 400 m high. A high-energy upward-going muon produced by a cosmic ray muon neutrino in the rock passes through the region and emits Cherenkov light which is detected by optical modules. The direction and energy are obtained by measuring arrival time and intensity of the Cherenkov light. It could be extended to a km3 detector. Relic neutrinos In the hot and dense universe up to about 1 s after the big bang, electrons and neutrinos were in thermal equilibrium. They were interacting each other, e + e (24) where vt is a neutrino of any kind. Surprisingly, the environment at that time was much more modest than that in the core of the supernova. The temperature was 10 billion kelvins (kT ~ 1 MeV), which is about 5 times lower, and the energy (or mass) density was almost 9 orders of magnitude less than those of a hot neutron star. The universe expanded rapidly and hence cooled down (see universe: thermal history). The thermal equilibrium could not hold and eventually the reaction from right to left of equation (24) ceased. Neutrinos were left behind in the universe and their energies became lower and lower until the present day. Electrons and positrons Copyright © Nature Publishing Group 2001 Brunei Road, Houndmills, Basingstoke, Hampshire, RG21 6XS, UK Registered No. 785998 and Institute of Physics Publishing 2001 Dirac House, Temple Back, Bristol, BS1 6BE, UK 5 Neutrino Astronomy Encyclopedia of Astronomy and Astrophysics rapidly annihilated into two photons whose energies were also redshifted to a wavelength of a few mm. These photons are called the cosmic microwave background (CMB) and detailed studies are being carried out. Relic neutrinos and photons obey Fermi-Dirac and Planck distributions, respectively. The number densities are 1.5 ?(3) Tt2Qicf (kTvy f(3) ■(my (25) (26) y it2 Chef where f (3)/jt2 = 0.121.... The neutrino number density corresponds to one species only. There is a simple relation between the two temperatures, T T lY/3 11 j : 0.714. (27) The CMB temperature was measured to be 2.74 K. Hence nY = 420 cm~3 which is in good agreement with measurement. The neutrino temperature and number density per species should therefore be 1.96 K (0.00017 eV) and «, = 115 cm~3, respectively. These relic neutrinos must exist in the present universe. Recently the Super-Kamiokande experiment has discovered the finite neutrino mass from a detailed study of atmospheric neutrinos. However, the experiment is sensitive only to the mass-squared difference, not to each mass value. The measured value is Am2 (= m2 — m2 ) = (1.5-6) x 10~3 eV2. From this the tau neutrino mass is constrained as mVz > 0.04 eV, which is much larger than the temperature 0.00017 eV. Thus the relic tau neutrinos should be nonrelativistic. Their velocity is about 300 km s_1 which is determined from other astronomical factors such as the Earth's motion in the Galaxy. The neutrino mass could be as large as a few eV. If this is the case, the relic neutrinos are the hot dark matter component and could significantly contribute to the energy density of the universe. There is at present no idea how to detect such low-energy neutrinos. Bibliography General review: Totsuka Y 1992 Rep. Prog. Phys. 55 377 High-energy cosmic ray neutrinos: Gaisser T K, Halzen F and Stanev T 1995 Phys. Rep. 258173 Solar neutrinos and SN1987A: Bahcall J N 1989 Neutrino Astrophysics (Cambridge: Cambridge University Press) Neutrino physics and neutrino astronomy: Winter K (ed) 1991 Neutrino Physics (Cambridge: Cambridge University Press) Recent results on neutrino physics and astrophysics: Suzuki Y (ed) 1999 Proc. 18th Int. Conf. on Neutrino Physics and Astrophysics (Amsterdam: Elsevier) Yoji Totsuka Copyright © Nature Publishing Group 2001 Brunei Road, Houndmills, Basingstoke, Hampshire, RG21 6XS, UK Registered No. 785998 and Institute of Physics Publishing 2001 Dirac House, Temple Back, Bristol, BS1 6BE, UK 6