ЯДЕРНАЯ ФИЗИКА, 2012, том 75, № 9, с. 1141-1148
= ЭЛЕМЕНТАРНЫЕ ЧАСТИЦЫ И ПОЛЯ
NEUTRINO TELESCOPES
© 2012 H. Costantini*
Istituto Nazionale di Fisica Nucleare, Genova, Italia Received November 15, 2011
Neutrino astrophysics offers a new possibility to observe our Universe: high-energy neutrinos, produced by the most energetic phenomena in our Galaxy and in the Universe, carry complementary (if not exclusive) information about the cosmos: this young discipline extends in fact the conventional astronomy beyond the usual electromagnetic probe. The weak interaction of neutrinos with matter allows them to escape from the core of astrophysical objects and in this sense they represent a complementary messenger with respect to photons. However, their detection on Earth due to the small interaction cross section requires a large target mass. The aim of this article is to review the scientific motivations of the high-energy neutrino astrophysics, the detection principles together with the description of a running apparatus, the experiment ANTARES, the performance of this detector with some results, and the presentation of other neutrino telescope projects.
1. THE COSMIC RAYS AND HIGH-ENERGY NEUTRINO SOURCES
One of the main questions in astroparticle physics is the origin and nature of the high-energy cosmic rays (CR). CRs are mainly high-energy protons and heavier nuclei with a wide energy spectrum, which spans for more than 10 orders of magnitude up to 1020 eV. The measured power-law spectrum of CRs is characterized by the index a = 2.7 up to energies of roughly 3 x 1015 eV and a = 3.1 in the region above. This feature in the energy spectrum is known as the knee. At 1019 eV, a flattening in the spectrum is observed by most experiments, denoted as the ankle where it is generally assumed that these CRs are of extragalactic origin. The primary mechanism by which particles gain energy beyond the thermal energy is the Fermi mechanism [1] where the charged particle is accelerated by iterative scattering processes on a shock wave produced during exceptional events, like stellar gravitational collapses. Due to the magnetic fields confinement, the scattered particles are trapped inside the acceleration region and they have a small probability to escape. The Fermi mechanism in the supernova remnants predicts a power-law differential energy spectrum -2 and fits correctly to the energy power involved in the galactic CRs. Assuming that at these acceleration sites a fraction of the high-energy CRs interact with the ambient matter or photon fields, TeV y rays are produced by the n0 decay, while neutrinos are produced by charged pion decay. This is the so-called astrophysical hadronic model [2] where the
E-mail: costant@ge.infn.it
accelerated protons interact in the surroundings of the CRs emitter with photons predominantly via the A+ resonance:
p + y — A+ — n0 + p, (1)
p + y — A+ — + n,
or with the ambient matter (protons, neutrons, and nuclei), giving rise to the production of charged and neutral mesons. Neutral mesons decay in photons (observed at Earth as y rays):
n0 - YY, (2)
while charged mesons decay in neutrinos:
— Vp + (p+ — Vp + Ve + e+), (3) n- — Vp + p- (p- — Vp + Ve + e-).
Because the mechanisms that produce CRs can produce also neutrinos and high-energy photons through n0 decay, y-ray sources are in general also candidates for neutrino sources. Alternative mechanisms that produce high-energy photons like synchrotron radiation can be present [3]: only the coincident measurement of neutrinos from the source would give an uncontroversial proof of the discovery of the galactic CR acceleration sites. Various candidates of high-energy neutrino sources have been proposed both from galactic or extragalactic origin. Supernova remnants, where the strong variable magnetic field of the neutron star can accelerate charged particles, are considered to be the most likely sites of galactic CR production. In particular, the recent observations of y rays exceeding 10 TeV in the spectrum of the RX J0852.0-4622 supernova [4] have strengthened the hypothesis that the hadronic acceleration is the
Fig. 1. The cosmic-neutrino signal together with the background of the atmospheric neutrinos. In both cases the signal is an up-goingtrack produced in the surroundings of the detector. The down-going muons background produced by cosmic rays is also shown.
process that is needed to explain the hard and intense TeV 7-ray spectrum. Similarly, the measurement of the 7-ray spectrum from SNR RX J1713.7-3946, recently performed by the H.E.S.S. telescope disfavors purely electromagnetic processes as the only source of the observed 7-ray spectrum. In addition to SNR also Microquasars and our Galactic Center (GC) have been proposed as neutrino sources. The GC, in particular, is probably the most interesting region of our Galaxy. Here, as measured by the H.E.S.S. telescope, an intense diffuse emission of 7 rays with energies greater than 100 GeV has been observed which likely implies the presence of a source of CR protons and thus of neutrinos. This region is also interesting because it is in the full sky view of a neutrino telescope located in the Mediterranean sea. Extragalactic sources are very far and the possibility of an individual discovery in a 1 km3-scale neutrino telescope is expected only under particular assumptions. In this case, promising particle accelerators are the 7-ray bursters (GRBs) and, as for the origin of ultrahigh-energy CRs, the Active Galactic Nuclei (AGN) .
2. NEUTRINO DETECTION
The basic principle to build a large volume (~1 km3) neutrino telescope is a matrix of light detectors inside a transparent medium like water or ice to detect the Cherenkov photons emitted by relativistic particles produced by the neutrino interaction. The high-energy neutrino may interact, in fact, with a single nucleon in the medium
surrounding the detector and, in case of a charged current (CC) weak v^ interaction, the path length of the final muon may be of the same order or exceed the dimensions of the detector itself. The measurement of the number and arrival time of the Cherenkov photons produced by this muon in the 3D photomultipliers (PMTs) array of the detector allows the reconstruction of the track direction. Of course the initial v^ and the final n directions do not overlap but at energies higher than few TeV the difference between the two is below 0.5°, the typical intrinsic angular resolution of a neutrino telescope. These detectors are not background free. Showers induced by interactions of CRs with the atmosphere produce the so-called atmospheric muons and atmospheric neutrinos. Atmospheric muons can penetrate the atmosphere and up to several kilometers of ice/water. Neutrino detectors are therefore located deeply under a large amount of shielding in order to reduce the background. The flux of down-going atmospheric muons exceeds the flux induced by atmospheric neutrino interactions by many orders of magnitude, decreasing with increasing detector depth. Therefore the up-going muons can only be produced by interactions of up-going neutrinos and from the bottom hemisphere the neutrino signal is almost background free. Only atmospheric neutrinos that have traversed the Earth represent the irreducible background for the study of cosmic neutrinos (see Fig. 1). The rejection of this background depends both on the pointing capability of the telescope and on the uncertainty in the reconstruction of the parent neutrino energy. The evaluation of the expected event rate Nev depends on various factors. Namely, Nev is proportional to the neutrino flux v^ via the effective area Aff of the detector for neutrinos:
Nev = X AVff. (4)
Here the evaluation of the effective area Aff requires various ingredients: PEarth, the probability for the neutrino to reach the detector region after transmission through the Earth; av, the cross section per nucleon of a neutrino with energy Ev to produce a muon of energy E^ which survives with energy >E|thrs after the propagation from the interaction point to the detector; Ntarg, the number of target nucleons proportional to the density of the medium, the Avo-gadro number and the effective detection volume Veff corresponding to the product of the detector effective area to muons and the muon range RM. Aff is larger than the instrumented volume, it increases with increasing neutrino energy and it is strongly dependent on the geometry of the detector. Since it depends on many factors, Aff is computed using Monte-Carlo (MC) simulations. To have an idea of the expected
counting rate let us consider a reference neutrino flux equal to that of the gamma flux from the Crab nebula. This is only true if we assume that purely hadronic processes are involved in the Crab TeV 7-ray emission. It can be shown that for a neutrino telescope as big as 1 km3 the corresponding event rate is of the order of ^3 yr_1. This estimate explains why the neutrino telescopes must have at least such a large instrumented volume. This has already been achieved in the IceCube experiment in the South pole, while in the north hemisphere only the ANTARES experiment has realized a neutrino telescope in the Mediterranean sea with an instrumented volume of the order of 0.1 km3.
3. THE ANTARES DETECTOR
The ANTARES detector is located at a depth of 2475 m in the Mediterranean Sea, 42 km from La Seyne sur-Mer in the South of France (42°48N, 6°10E). It is equipped with 885 optical sensors arranged on 12 flexible lines. Each line comprises up to 25 detection storeys each equipped with three downward-looking 10" PMTs, oriented at 45° from the vertical. Each PMT is installed in an Optical Module (OM) that consists in a 17" glass sphere in which the optical connection between the PMT and the glass is assured by an optical gel. The lines are maintained straight by a buoy at the top of the 450-m long line. The spacing between storeys is 14.5 m. The dis
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