Aspects of neutrino astronomy

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<ul><li><p>ASPECTS OF NEUTRINO ASTRONOMY </p><p>A. M. BAKICH School of Physics, University of Sydney, Sydney 2006 N.S. IV., Australia </p><p>(Received 18 August, 1988) </p><p>Abstract. A broad overview of the current status of experimental neutrino astronomy is presented. Particular emphasis is given to the major recent developments that have occurred during the last few years. It is concluded that these developments and the next generation of experiments currently being installed signifies the coming of age of neutrino astronomy. </p><p>I. Introduction </p><p>The aim of this paper is to provide a review of various aspects of experimental neutrino astronomy. This field represents a dramatic merging of concepts and techniques of submicroscopic elementary particle physics with the astrophysical theories and phenomena on a supermacroscopic scale. The field is not new, as most ideas have been debated for some time (Ruderman, 1965; Chiu, 1966; Lande, 1979). </p><p>Our motivation for this review stems from a number of very recent developments which appear to indicate that neutrino astronomy is gradually emerging from the phase of 'optimistic speculation and pilot experiments' into an active and exciting field of research. It is very likely that by the time review appears, new ideas and in particular new results will have been achieved. </p><p>1.1. SUMMARY OF RECENT DEVELOPMENTS </p><p>We begin our discussion with a brief summary of recent progress with reference to a schematic neutrino energy spectrum shown in Figure 1. </p><p>In the MeV energy range, there has been a long standing discrepancy between the predicted and the observed fluxes of the SB solar neutrinos, sometimes referred to as the 'solar neutrino problem'. Over the last couple of years, and for the first time, new data has been accumulating by a direct-counting electronic experiment. In addition, several other projects are in advanced stages of preparation, two of which will be measuring the flux of pp neutrinos (Section 2). </p><p>The registration of supernova SN1987A by several neutrino detectors, although widely expected on theoretical grounds, came as a bonus to the experimenters. Since the frequency of observable supernovae explosions is known to be rather low, the obtained neutrino data, however meagre, is unique. These results and the significance of their interpretation are discussed in Section 3. </p><p>At somewhat higher energies (,~ GeV) samples of 'fully contained' events have been recently accumulated by the nucleon decay detectors. These events can be directly attributed to the atmospheric (cosmic ray produced) neutrinos and represent the </p><p>Space Science Reviews 49 (1989) 259-310. 9 1989 by Kluwer Academic Publishers. </p></li><li><p>260 A.M. BAKICH </p><p>15 </p><p>10 </p><p>r&gt; </p><p>-5 To0 </p><p>O </p><p>~4 -~o J </p><p>2 -15 </p><p>-20 </p><p>-25 </p><p>-30 </p><p>i , </p><p>i I I I I I I I i I I I I I I </p><p>. ~ SN1987A </p><p>A </p><p>DIFFUSE </p><p>1MeV 1GeV 1TeV 1PeV ~ 1EeV ~ '~ I I | | I I | I I I I I I I </p><p>5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 </p><p>log (NEUTRINO ENERGY, eV) </p><p>Fig. 1. A schematic representation of neutrino fluxes. This figure shows the expected extraterrestrial and atmospheric neutrino fluxes as a function of neutrino energy (after Koshiba, 1987; unpublished). </p><p>background against which any extraterrestrial sources of neutrinos would have to be identified (Section 4). </p><p>At still higher energies (,,~ TeV) the same underground detectors have been collecting data on upward and horizontal through-going muons. Since these muons are known to be a characteristic signature of neutrino interactions, this information could provide a means of establishing the existence of discrete point sources of high energy neutrinos (Section 5). One such (albeit still very controversial) identification attempt is the Cygnus X-3 pulsar. </p><p>Finally, the long planned giant underwater experiments, intended to observe neutrino interactions at the ultra-high (~ PeV) energies, appear to be gradually moving past the preliminary prototyping stage. Interestingly, some estimates of the PeV neutrino flux limits have recently been obtained by a cosmic ray air shower detector (Section 6). </p></li><li><p>ASPECTS OF NEUTRINO ASTRONOMY 261 </p><p>The significance of these developments indicates that each one of them undoubtedly does deserve a separate and detailed review. However, our intention here is to provide a broad and global overview of the entire field of neutrino astronomy. It is the combined effect of all these recent developments that perhaps signifies the coming of age of neutrino astronomy. </p><p>1.2. NEUTRINO PROPERTIES AND INTERACTIONS </p><p>It is perhaps surprising how much and yet how little is known of the neutrino itself, sometimes referred to as 'the most elusive particle in nature'. </p><p>Neutrinos are point-like, neutral, spin -1 particles participating only in weak inter- actions. It is well established that at least two and almost certainly three flavours of neutrino (Ve, v,, and v~) exist in nature, in correspondence to their lepton partners, the electron, the muon, and the tan. However, whether the neutrino is its own antiparticle, or more specifically, what type of wave-function (two-component Majorana or four- component Dirac) actually describes the neutrino is not known and indeed is a subject of intensive theoretical and experimental investigations (Haxton and Stephenson, 1984). </p><p>Similarly, the upper limits on neutrino masses have been progressively reduced by a series of painstaking and perhaps still controversial experiments. These limits are </p><p>17 eV &lt; mve &lt; 40 eV (tritium decay) mve &lt; 18 eV (tritium decay) my, &lt; 250 keV (g/~ decay) mv~ &lt; 70MeV (zdecay) </p><p>Boris et al., 1987, Fritschi et al., 1986, Abela et al., 1984, Albrecht et al., 1985. </p><p>Hence, finite neutrino masses cannot be precluded, an uncertainty which has crucial implications for many theories and interpretation of experimental data (Vuilleumier, 1986). </p><p>The question of neutrino types and masses is closely related to the possibility of neutrino oscillations (Pontecorvo, 1958). This hypothesis does not rely on an arbitrary (and often tacit) assumption of the massive neutrino eigenstates being aiso the eigenstates of the weak interaction; instead it relates the two eigenstate vectors by means of a flavour mixing matrix. The resulting oscillations between neutrino types are then determined by the neutrino masses and the mixing strength (or Am 2 and sin220 in the case of two neutrinos ve and vu). </p><p>The above intrinsic or vacuum oscillations are modified on passage of neutrinos through matter (Wolfenstein, 1978) because the v,e (and the v,e) forward scattering amplitude is due to neutral current only, whereas vee does have an additional charged eurrent component. Recent realisation (Mikheev and Smimov, 1985, 1986) of the resonant character of this effect implies that even if the intrinsic mixing of neutrino types is very small, the oscillations can be dramatically enhanced under certain conditions, as determined by </p><p>~Am 2 cos 20 p- </p><p>E r </p></li><li><p>262 A .M. BAK ICH </p><p>where p is matter density, Ev is the neutrino energy and a is a medium and oscillation- type dependent parameter. This MSW effect has profound implications on the outcome of many neutrino experiments, as indicated throughout this review. In fact, because of the uniqueness of neutrino sources and the very large distances involved, neutrino astronomy experiments are particularly sensitive to these oscillation effects (Bilenky and Petcov, 1987). </p><p>It is, therefore, widely expected that the developing field of neutrino astronomy, apart from providing information of purely astrophysical significance, will help to resolve some of the above fundamental problems. </p><p>For the purposes of this review, the most important property of the neutrinos are the cross-sections of their interactions with other particles, such as elementary leptons and quarks as well as hadrons and nuclei. It is the characteristic relative weakness of these cross-sections and their dependence on energy that lead to uniqueness of neutrino physics and neutrino astronomy as a subject matter. </p><p>On the one hand, it makes much of the astronomical universe effectively transparent to neutrinos providing information unobtainable by any other means. As an example, a 1 PeV neutrino traversing galactic density of -,, 1 nucl. cm-3 would have an interaction length of &gt; 1015 light years, much greater than the radius of the Universe. On the other hand, it makes the very task of detecting these neutrinos extremely difficult, requiring vast and especially equiped detectors. This difficulty can be appreciated by considei-ing a 1 GeV neutrino to which the Earth diameter represents only 10-4 of an interaction length. </p><p>The energy dependence of cross-sections of some of the relevant neutrino reactions are schematically depicted in Figure 2. The important feature of this plot is the linear energy dependence of the cross-sections, characteristic of a point-like fermion-fermion interaction </p><p>G2M O'to t ,,~ - - E~, </p><p>7"C </p><p>where G is the Fermi coupling constant and M is the target mass. Low-energy reactions on proton targets exhibit a quadratic energy dependence before these exclusive processes saturate due to the form factor dependence upon Q2. At very high energies the linear energy dependence is expected to flatten due to the IVB propogator effects and QCD evolution of structure functions. </p><p>1.3. UNDERGROUND NEUTRINO DETECTORS </p><p>Throughout this review we will be referring to a number of underground detectors which have, in recent years, supplied the bulk of the data leading to our current understanding of neutrino astronomy. It is important to note that the initial and primary goal of most of these detectors has been to search for nucleon decay events; a task to which (perhaps ironically) atmospheric neutrino interactions themselves constitute a limiting background (Perkins, 1984; Meyer, 1986). These existing and operational detectors are listed in Table I. </p></li><li><p>ASPECTS OF NEUTRINO ASTRONOMY 263 </p><p>z 0 t- O iii o3 t.o </p><p>0 n- O v </p><p>0 </p><p>-30 </p><p>-31 </p><p>-32 </p><p>-33 </p><p>-34 </p><p>-35 </p><p>-36 </p><p>-37 </p><p>-38 </p><p>-39 </p><p>-40 </p><p>-41 </p><p>-42 </p><p>-43 </p><p>-44 </p><p>-45 </p><p>-46 -47 </p><p>-48 </p><p>' l l l l l l l l l l | l l l l </p><p>/ </p><p>/ J / ~ f f </p><p>f / </p><p>//J---/, </p><p>v G a ~ v C I </p><p>1MeV 1GeV 1TeV , 1PeV 1EeV .... i l i I i ~3 111 ' = 1 1.1 1'6 </p><p>log (NEUTRINO ENERGY, eV) </p><p>Fig, 2. Energy dependence of neutrino interaction cross-sections. The medium and high energy regions have been measured in many experiments (e.g., Eisele, 1986). The extrapolation to ultra-high energies </p><p>indicates the results of calculations by Quigg et al. (1986). </p><p>It is clear that any neutrino detector should have a very large target mass in order to obtain significant event yields. An additional but equally important consideration is that these neutrino events have to be extracted and identified from a substantial and in most cases overwhelming background. This background is due to far more prolific particles, which are likely to either directly or indirectly mimic the genuine neutrino signal. Therefore, most neutrino detectors have to be heavily shielded and/or located deep underground, in addition to being well instrumented and quite sophisticated (and expensive) installations. </p><p>Unfortunately the main aim of this review does not allow for a detailed description of either their design parameters or performance specifications, except in special cases where these considerations did have a significant effect on the interpretation of the obtained results. Similarly, although the experimental programs of most collaborations do include a diverse range of particle and/or cosmic-ray physics topics, we limit our discussion only to the issues and results directly relevant to neutrino astronomy. </p></li><li><p>264 A, M. BAKICH </p><p>TABLE I Existing underground neutrino detectors </p><p>Detector Depth Target Mass En~in AO Start, (location) (rowe) (tons) (MeV) (degrees) Upgrade </p><p>HOMESTAKE 4400 chlorine 133 0.814 - 1968 (South Dakota) G2CI 4 </p><p>KGF 7000 prop. tube 140 Oct. 1980 (India) calorimeter Dec. 1985 </p><p>ASD 570 liquid 105 5 - 1978 (Ukraine) scintillator </p><p>BAKSAN 850 modular 120 12 2 Aug. 1979 (Caucasus) scintillator (330) Jun. 1980 </p><p>SOUDAN I 1800 prop. tube 31 1.4 Oct. 1981 (Minnesota) calorimeter </p><p>NUSEX 5000 streamer 120 1.0 Jul. 1982 (Mont Blanc) calorimeter </p><p>IMB 1570 water 3300 25 8 Aug. 1982 (Ohio) Cherenkov (6800) Jun. 1986 </p><p>HPW 1450 water 900 3 Mar. 1983 (Utah) Cherenkov </p><p>KAMIOKANDE 2700 water 680 8.5 2.7 Jul. 1983 (Japan) Cherenkov (2140) Jan. 1986 </p><p>FREJUS 4400 flash tube 560 300 1.2 Mar. 1984 (Alps) calorimeter </p><p>LSD 5200 modular 90 6 - Oct. 1984 (Mont Blanc) scintillator </p><p>Specifically, we do not discuss such topics as nucleon decay, magnetic monopole searches, multiple muon bundles or neutrino geophysics. </p><p>An indication of the activity and interest in the field of neutrino astronomy should be particularly apparent from the number of newly approved detectors and pending proposals. These second generation experiments, listed in Table II, are in various stages of preparation ranging from being presently installed to being tested as a preliminary prototype. </p><p>Broadly speaking, and apart from the specialized radiochemical detectors, most of the neutrino detectors listed in Tables I and II can be classified into the following groups: </p><p>(i) Water Cherenkov detectors (Figure 3), usually very massive (&gt; 1000 tons) and capable of low-energy threshold levels (~ 10 MeV), The important parameter is the photosensitive area coverage, which determines both the spatial resolution (,-~ 1 m) and the angular resolution of a few degrees. </p></li><li><p>ASPECTS OF NEUTRINO ASTRONOMY </p><p>TABLE II </p><p>New detectors and proposals </p><p>265 </p><p>Detector Depth Target Mass /~min AO Status (location) (mwe) (tons) (MeV) (degrees) </p><p>GALLEX 4000 Gallium 30 0.233 - setting up (Gran Sasso) GaC13 </p><p>BAKSAN ~ 3250 Gallium 50 0.233 - setting up (Caucasus) metal </p><p>LVD 4000 modular 1840 5 0.2 setting up (Gran Sasso) scintillator </p><p>ICARUS 4000 liquid 6 500 5 prototype (Gran Sasso) argon </p><p>SOUDAN II 2200 drift tube 1 100 setting up (Minnesota) calorimeter </p><p>MACRO 4000 stream tube 700 0.2 setting up (Gran Sasso) scintillator </p><p>SUPER-KAMIOKA 2700 water 22000 5 2 proposal (Japan) Cherenkov (45 000) </p><p>SUDBURY SNO 6200 heavy water 1000 5 proposal (Ontario) Cherenkov </p><p>SUNLAB 3300 water 250 6 - prototype (Australia) Cherenkov </p><p>BAIKAL 1350 underwater - 0.5 prototype (Lake Baikal) Cherenkov </p><p>DUMAND 4500 underwater 3 107 - 0.5 prototype (Hawaii) Cherenkov </p><p>(ii) Liquid scintillator detectors (Figure 4), often of modular design and offering low-energy thresholds (few MeV) but without any signal directionality and limited angular resolution, unless supplemented with external tracking planes. </p><p>(iii) Tracking calorimeters (Figure 5), typically 100 to 1000 tons with the bulk of the material in the form of iron plates sandwiched between crossed planes of either propor- tional, drift, streamer or flash tubes. The spatial resolution of these detectors is deter- mined by the tube size and is typically of the order of few mm to few cm. Although very good angular resolution of &lt; 1 deg can be achieved for long tracks, the mi...</p></li></ul>