PDD - Science Motivation for Kilometer-Scale Detectors

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• 3.1 High-Energy Neutrinos Associated with Cosmic Particle Accelerators
  • 3.1.1 Energy Considerations
  • 3.1.2 Estimates Based on Models
  • 3.1.3 Neutrinos as a Diagnostic of TeV Gamma-Ray Sources
• 3.2 Other Science Opportunities
  • 3.2.1 WIMPs and Other Sources of Sub-TeV ν_μ
  • 3.2.2 Atmospheric Neutrinos
  • 3.2.3 PeV and EeV Neutrinos
  • 3.2.4 Tau Neutrino Detection
  • 3.2.5 Neutrinos from Supernovae
• 3.3 Summary

3 Science Motivation for Kilometer-Scale Detectors

The construction of neutrino telescopes is overwhelmingly motivated by their discovery potential in astronomy, astrophysics, cosmology and particle physics. To maximize this potential, one must design an instrument with the largest possible effective telescope area to overcome the small neutrino cross section with matter, and the best possible angular and energy resolution to address the wide diversity of possible signals. A well-designed neutrino telescope can

• search for high energy neutrinos from transient sources like Gamma Ray Bursts (GRB) or Supernova bursts;
• search for steady and variable sources of high energy neutrinos, e.g. Active Galactic Nuclei (AGN) or Supernova Remnants (SNR);
• search for the source(s) of the cosmic-rays;
• search for Weakly Interacting Massive Particles (WIMPs) which may constitute dark matter;
• search for neutrinos from the decay of superheavy particles related to topological defects;
• search for magnetic monopoles and other exotic particles like strange quark matter;
• monitor our Galaxy for MeV neutrinos from supernova explosions and operate within the worldwide SNEWS triangulation network;
• search for unexpected phenomena.

In practice, the observed fluxes of cosmic-rays and gamma rays set the scale of a neutrino telescope. With minimal model-dependence, one can estimate the very high energy cosmic neutrino fluxes by scaling to the observed energy density in high-energy cosmic-rays, or to the measured fluxes of non-thermal high-energy gamma rays. The basic assumption in calculating the expected neutrino fluxes is that some fraction of cosmic-rays will interact in their sources to produce neutrinos.

Although they are not yet identified, the sources of the highest energy cosmic radiation very likely involve extremely dense regions with exceptional gravitational forces such as supermassive black holes, collapse of massive stars or mergers of black holes and neutron stars. With accretion and intense radiation fields as ingredients, some fraction of the particles accelerated in such environments will likely produce pions in hadronic collisions with ambient gas and/or by photoproduction. In either case, the neutral pions decay to photons, while charged pions include neutrinos among their decay products with spectra related to the observed gamma-ray spectra. In the first part of this section, we discuss estimates based on this relationship and conclude that a km-scale detector is needed to observe neutrino signals from known classes of high energy astrophysical sources.

The baseline design of the detector maximizes sensitivity to ν_μ-induced muons from below with energies in the TeV to PeV range, where the acceptance is enhanced by the increasing neutrino cross section and muon range but the Earth is still largely transparent to neutrinos. Good angular resolution is required to distinguish possible point sources from background, while energy resolution is needed to enhance the signal from astrophysical sources, which are expected to have flatter energy spectra than that of the atmospheric neutrino background.

A standard technique to search for high energy neutrinos of astrophysical origin is to look for upgoing muons induced by ν_μ that have penetrated the Earth. The signal is given by the convolution

Signal ∼ Area ⊗ R_μN_A⊗σ_ν⊗φ_ν,

where R_μ is the muon range in g/cm² and N_A is Avogadro's number. The range and cross section both increase linearly with energy into the TeV region, after which the rate of increase slows. Neutrinos with E_ν < 100 TeV are not strongly attenuated by the Earth, and much of the solid angle away from the nadir remains accessible up to 1 PeV [16]. Thus the optimum range for ν_μ-induced upgoing muons is from a TeV to a PeV. Also in this energy range the muon energy loss is greater than minimum ionizing, which is a potential way to discriminate against the background of atmospheric neutrinos, which have a steeply falling spectrum. We will return to the importance of energy measurement further on.

The generic cosmic accelerator is believed to produce neutrinos in the flux ratio ν_e : ν_μ : ν_τ :: 1 : 2 : 0. With neutrino oscillations, however, at the detection point this ratio becomes 1 : 1 : 1. This is especially interesting for neutrino telescopes because the ν_τ is not absorbed in the Earth like the ν_e and ν_μ due to the charged current regeneration effect (as discussed below). Instead, ν_τ's with energies exceeding roughly 1 PeV pass through the Earth and emerge with an energy of roughly 1 PeV. IceCube is well-suited to detecting neutrinos in this energy range and will have full 4π sensitivity to this potential signal.

In the second part of this section we discuss several other important scientific goals which depend on the sensitivity of the detector to neutrinos of much higher energies and of much lower energies. On the one hand, we will discuss detecting and measuring the energy of PeV–EeV ν_e, ν_μ and ν_τ interactions. On the other, we will consider the detection of low energy muon-neutrinos from the annihilation of dark matter particles and the detection of MeV electron-antineutrinos from galactic supernovae. We describe how the baseline design can address these objectives as well.

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