Neutrinos

Abstract

Neutrinos represent a new “window” to the Universe, spanning a large range of energy. We discuss the science of neutrino astrophysics and focus on two energy regimes. At “lower” energies (≈1 MeV), studies of neutrinos born inside the sun, or produced in interactions of cosmic rays with the atmosphere, have allowed the first incontrovertible evidence that neutrinos have mass. At energies typically one thousand to one million times higher, sources further than the sun (both within the Milky Way and beyond) are expected to produce a flux of particles that can be detected only through neutrinos.

The invention of the optical telescope 400 years ago fundamentally changed our view of the cosmos around us, as well as our view of ourselves in that cosmos. The enhanced photon-detection efficiency of the telescope, compared with the naked eye, revolutionized observational astronomy. One hundred years after burning Giordano Bruno at the stake, the Vatican, under the weight of an avalanche of experimental information obtained with optical telescopes, officially recognized heliocentrism as Catholic canon. Within the last 40 years, however, the limitations of the optical telescope have become increasingly apparent.

Detection of photons as tracers of astrophysical processes have two primary shortfalls: (i) they are sensitive only to those astrophysical processes that specifically produce visible light (an extremely small portion of the entire electromagnetic frequency range), and (ii) because the interstellar medium is not empty, and is rather a soup of dust (1 proton per cubic meter on average), low-energy microwave photons left over from the Big Bang (400/cm³), infrared starlight, etc., the likelihood of an optical photon penetrating the interstellar medium from the edge of the Cosmos and reaching our terrestrial telescopes without being perturbed, absorbed, or deflected en route, decreases with both the energy of the photon and the distance from Earth to source point. This motivates the search for alternative means of collecting astronomical information.

Neutrinos are “fundamental” particles in the sense that, like quarks or electrons, they are believed to have no constituents.¶ This is contrasted with protons, e.g., which are believed to contain three smaller quarks. Neutrinos are also the most inert of the presently known fundamental particles; because they have no electric charge, they do not particpate in either electric or magnetic interactions; unlike quarks, they do not participate in the “strong nuclear force” that holds the proton together and is responsible for the awesome explosive force of a hydrogen bomb. Neutrinos interact only by means of the “weak” interactions, which are, indeed, quite weak. A typical neutrino produced in the interior of the sun will travel unscathed through a light-year of lead before interacting. This represents a unique experimental opportunity—neutrinos from the sun carry direct information about the solar interior.‖ Alternatively, neutrinos from the edge of the Universe will easily penetrate the interstellar medium without being absorbed or deflected. However, the inertness of the neutrino immediately presents an experimental challenge: one must use an extremely large volume of material to detect the very rare neutrino interactions.

Because of its inertness, evidence for the neutrino was not uncovered until 1931, when Pauli posited the neutrino to solve a long-standing experimental problem of β decay. In the decay of a neutron into a proton and an electron, it was realized that the final state proton and electron energies did not add up to the initial neutron energy. Similarly, momentum appeared to be violated in the reaction. Pauli invoked the neutrino as an undetected, massless particle that carried away this missing energy and momentum. Since then, the neutrino has had an illustrious history: (i) the discovery of parity violation in 1957 by C. S. Wu at Columbia, which is intimately connected with the intrinsic “handedness” of the neutrino vs. the antineutrino; (ii) the discovery of different types of neutrinos [“electron neutrinos” (ν_e) and “muon neutrinos” (ν_μ)], which interacted very distinctly (1962); (iii) the detection of substantially fewer electron neutrinos from the sun than our model of the sun accommodated (beginning in 1968); and (iv) evidence for a nonzero neutrino mass (first unambiguously claimed in 1998). For the future, neutrinos may hold the key to solving the puzzles of Dark Matter (the fact that most of the mass in the universe is nonluminous; a large fraction of that mass may actually be in the form of massive neutrinos) and the the origin of the highest energy cosmic rays. The origin of these cosmic rays, a factor of 100 million times more energetic than particles capable of being produced by terrestrial accelerators, is unknown. If they are photons, protons, or any other “conventional” cosmic ray source, the nearest known high-energy cosmic accelerators are so far away that photons or protons produced by them should be absorbed before reaching the earth. Therefore, there is speculation that they may be caused by neutrinos. Neutrinos are also likely to be the primary experimental tool in understanding Active Galactic Nuclei, the extremely luminous, high mass (10 billion solar masses) black holes at the centers of many galaxies. Perhaps most spectacularly, neutrinos may provide essential information about the nature of γ-ray bursts. In these celestial explosions, detected at earth approximately once per day, a mass-energy equal to one solar mass is suddenly released in the space of approximately one second. During that second, the γ-ray burst outshines the entire remainder of the Universe.

Here on earth, the flux of neutrinos is summarized as shown in Table 1, arranged by increasing neutrino energies [in units of electron volts (eV)].

Table 1.

Neutrino flux on earth

Energy range, eV	Source	Local flux, 109/cm2·s
0.0004	Relic (Big Bang)	0.1
100	Terrestrial radioactivity	0.0075
106–107	Nuclear reactors	0.0075
107	Solar	50
100 → 109	Atmospheric	<0.001
100 → 1012	Man-made accelerator	<0.001
1012	Active galaxies, e.g.	<0.001

We now discuss in some detail two particular energy ranges of interest—solar/atmospheric neutrinos and neutrinos at the highest energy frontier. All solar neutrino experiments performed to date have detected fewer electron neutrinos than are expected from standard solar models. Similarly, experiments measuring the flux of atmospheric neutrinos have also seen disagreement with theoretical expectations. There are a number of possible reasons for these discrepancies, but by far the most favored solution is that neutrinos “oscillate” into a different type of neutrino by virtue of a heretofore unmeasureably tiny mass. In the case of solar neutrinos, this would allow a sizeable fraction of the neutrinos produced in the sun to oscillate into a type of neutrino to which previous experiments would have been insensitive; hence the observed electron solar neutrino flux deficit. In the case of atmospheric neutrinos, the deficit observed by previous experiments was not completely convincing because of lack of statistics. With the advent of second-generation neutrino detectors, collecting a much larger statistical sample of neutrinos, the observed deficit in atmospheric neutrinos is now viewed as convincing evidence for neutrino mass and oscillations. In the near future, the same second-generation devices may provide us with convincing evidence for neutrino mass and oscillations in solar neutrino sector, solving a long-standing puzzle in particle astrophysics.

There are two broad classes of detectors used to perform these experiments: radiochemical assay and real-time water Cherenkov detectors. Radiochemical devices use a vat of liquid containing atoms that neutrinos can render radioactive, and, after a certain period of time (on the order of one month), the radioactive atoms are swept out of the liquid and counted. This gives an estimate for the total incident flux of neutrinos, integrated over the period of time in question, and without any knowledge of the energy or direction of the interacting neutrino. Several experiments, including one that has been running for over 30 years, have used the radiochemical technique. In contrast, water Cherenkov devices detect each individual neutrino interaction in “real-time,” which means that the interaction is detected when it happens (not up to a month later, as with radiochemical devices), and they can also make an accurate determination of the neutrino energy and direction. Therefore, real-time devices produce a richer data set, making possible a much more detailed study of solar and atmospheric neutrinos. There are two real-time Cherenkov devices currently taking data: the SuperKamiokande (http://www.phys.washington.edu/∼superk/) experiment in Japan and the Sudbury Neutrino Observatory (SNO) experiment in Canada.

The SuperKamiokande detector is a 50-kiloton light-water Cherenkov device buried under a mountain in an active zinc mine in the Japanese Alps. It uses approximately 11,000 photomultiplier tubes in a cylindrical geometry to detect the Cherenkov light emitted by electrons. These electrons emit Cherenkov light because they have been hit hard enough by neutrinos to acquire velocities exceeding that of light in water. (Light moves slower in media such as water than it does in vacuum, so charged particles can move faster than photons in these media. When this happens, the charged particle emits a shock wave of so-called Cherenkov light, much like a supersonic plane produces a sound shock wave or “sonic boom.”) SuperKamiokande also detects the Cherenkov light emitted by upward-going charged muons produced by atmospheric neutrino interactions in the earth just below the detector and in the detector volume itself. In the summer of 1998, SuperKamiokande announced convincing evidence for neutrino oscillations (and hence neutrino mass) by using upward-going muons and high-energy electrons produced by atmospheric neutrinos.

The SNO (http://www.sno.phy.queensu.ca) has been designed primarily to study solar neutrinos. SNO is buried in an active nickel mine in northern Ontario, Canada. It uses about 10,000 photomultiplier tubes in a spherical geometry and is also sensitive to Cherenkov light. What makes the SNO experiment unique is that it utilizes one kiloton of heavy water, D₂O, as a target. Through a variety of interaction mechanisms, this permits SNO to make separate high-rate measurements of the flux of electron neutrinos and the flux of all neutrino flavors. SNO can thereby make a standard solar model-independent measurement that may provide us with the first conclusive evidence of solar neutrino oscillations in the years to come.

At a somewhat higher energy range (typical of Active Galactic Nuclei, γ-ray bursts, etc.), the incident flux of neutrinos is considerably smaller because of the difficulty in producing such high-energy neutrinos, and also the increased distance to the source relative to the lower-energy neutrinos produced by the Sun. Correspondingly, the size of the detector must be somewhat larger. The AMANDA (http://amanda.berkeley.edu) experiment at the South Pole uses cold polar ice as the neutrino target (analogous to the clean water of SuperKamiokande, or the D₂O of the SNO). To obtain appreciable event rates, AMANDA should be approximately 100 times the size of SuperKamiokande. Similar to SuperKamiokande and SNO, phototubes detect the Cherenkov light produced by the passage of a charged particle resulting from a neutrino interaction in the target material (here, the South Polar ice). Within the last several months, AMANDA has observed their first neutrinos (at energies approximately 10–100 times larger than those typical of SuperKamiokande), identifying 17 “gold-plated” neutrino interactions. Upgrades of AMANDA in the next several years should establish it as a premier neutrino astrophysics facility in the next decade.