IceCube – research
IceCube is a multi-purpose detector, covering topics of astrophysics and particle physics. We are studying:
ACTIVE GALACTIC NUCLEI (AGN)
Active galactic nuclei are believed to be powered by a supermassive black hole. Some AGN present jets of matter accelerated at relativistic energies. High energy photons are produced by synchrotron radiation by electrons. Protons should be accelerated in the jets, as well as electrons, and neutrino emission from the jets should result from the decay of pions produced in p+γ interactions. We are looking for neutrinos from AGN to confirm such scenario.
GAMMA RAY BURSTS (GRB)
The most powerful explosions detected in the sky, their origin is still unclear. A GRB can emit ~1050 ergs/s (our entire galaxy produces 1043 ergs/s in all wavelengths). Current models explain GRBs as a relativistically expanding fireball produced in an explosion. What can produce such explosions is still unknown, but the expanding matter is shock-accelerated and electromagnetic radiation and neutrinos are supposed to be produced by similar physical processes as in the AGN case. IceCube is also looking for neutrino emission from GRBs.
Particle physics provides several candidates for dark matter, in the form of weakly interacting massive particles that have survived as relics from the Big Bang. The Minimal SuperSymmetric extension to the Standard Model (MSSM) neutralino is one of them, but the lightest Kaluza-Klein mode, that arises in models of extra space-time dimensions, is also a viable candidate. If they exist, such particles should cluster gravitationally as halos in galaxies, and by the same principle accumulate in the center of heavy objects, like the Sun or Earth. If a high enough concentration is achieved, they could annihilate pair-wise, and neutrinos be produced as a by-product. An excess of neutrinos from the center of the Sun or the Earth would be a clear signature of such a process, that IceCube is looking for.
MICRO BLACK HOLES
One of the manifestations of low-scale gravity is the production of micro black holes in the collision of high energy neutrinos with the partons in matter nuclei. If the CMS energy of the interaction exceeds the Planck scale, a microscopic black hole can be produced in the interaction. However, in our 4-dimensional world, the Planck scale lies at energies MP~1019 GeV, and the best man-made accelerators only reach TeV CMS energies. But in 4+D space-time dimensions the Planck scale may be much lower, and a 1010 GeV neutrino interacting with a nucleus inside the detector can do the job. Although that might seem an extreme high energy, it is expected that such neutrinos are guaranteed by interactions of the known flux of cosmic rays with the all-permeating cosmic microwave relic photons.
The immediate Hawking evaporation of the black hole in a burst of Standard Model particles (in ~10-27 s) can be detected in a neutrino telescope through the emission of Cherenkov light of such products.
Monopoles are predicted in Grand Unified theories to be copiously produced as topological defects in symmetry breaking phase transitions in the early Universe, although their density must have been strongly diluted by inflation. Magnetic monopoles are massive (1017 GeV, depending on the symmetry group and unification scale of the underlying theory) and topologically stable, so they should still be present in today's Universe.
Monopoles with masses below 1014 GeV can have been accelerated to relativistic velocities over long distances along the magnetic field lines of cosmic objects (galaxies, AGNs and the like), and could be detected in neutrino telescopes through their direct Cherenkov emission. Cherenkov emission for a monopole is enhanced by a factor of 8300 in water or ice with respect to a particle with a single electric charge. The passage of a relativistic monopole through a Cherenkov detector will therefore produce a very luminous event.
In some GUTs, non-relativistic monopoles can act as ”catalysts” in interactions that violate baryon number conservation. In a neutrino telescope, the signature of these ”catalyzing” monopoles would be a series of closely spaced light bursts from nucleon decay products produced along the monopole trajectory.
IceCube has the capability of searching for these type of events in order to set competitive limits on the cosmic monopole flux.
In a class of supersymmetric theories with interacting scalar fields φ that carry some conserved global charge, the ground state is a Q-ball, coherent states of squarks, sleptons and Higgs fields, that can be described semiclassically as a non-topological soliton.
Q-balls can interact with matter nuclei converting them into pions whose decay can produce a signal in various detectors. In this sense Q-balls will have a similar experimental signature in a neutrino telescope as catalyzing monopoles, although the underlying process is different.
TESTS OF FUNDAMENTAL LAWS
Neutrino oscillations is a typical quantum mechanical superposition effect between propagation (mass) and flavour states. However, there can be other causes of oscillations if certain fundamental physics laws are broken at some scale. These include violation of the equivalence principle (VEP), where the different neutrino flavours couple differently to the gravitational potential, violation of Lorentz invariance (VLI), where the different flavours can achieve different asymptotic velocities giving rise to velocity-induced oscillations, or non standard neutrino interactions with matter at very high energies. The handle that makes such processes interesting for large-scale neutrino telescopes is their dependence on the energy of the neutrino. While the wavelength of standard oscillations is proportional to the energy of the neutrino, in the case of VEP or VLI the oscillation wavelength is proportional to 1/E. Neutrino telescopes can detect such effect by, for example, looking for distortions of the angular dependence of the high energy tail of the atmospheric neutrino flux.
COSMIC RAY PHYSICS
Cosmic Rays hitting the atmosphere produce abundant fluxes of muons. The IceTop air shower array in coincidence with IceCube allow to carry on studies of the composition and the energy spectrum of cosmic rays.
Neutrinos produced in the same cosmic ray interactions in the atmosphere constitute the background to all the physics topics mentioned above, but they are of interest by themselves as a ’test beam’ for IceCube.
Low energy (MeV) neutrinos produced in a Supernova explosion can be also detected in IceCube through a coherent increase in the rate of all the DOMs during the duration of the burst. Though in this case we do not have tracking (i.e. pointing) capabilities, a detector like IceCube can sense the onset of a SN in our galaxy. IceCube is a member of the SuperNova Early Warning System, SNEWS.