Astroparticle Physics

High-energy phenomena naturally occurring in the universe provide a wealth of data and new valuable insights into particle physics and cosmology. Kavli IPMU researchers use the universe as a laboratory for testing new theories of dark matter and new physics beyond the Standard Model, and for understanding the basic properties of the universe. In the past year, several exciting developments in particle astrophysics were initiated by IPMU members.

Dark Matter

Its existence is supported by a substantial body of astrophysical evidence, but the identity of the dark matter particle (or particles) remains a mystery. Since one does not know the interactions of dark matter particles, besides their gravitational interactions, one must pursue a broad range of possibilities.

One necessary condition is that the dark matter particles must be stable on cosmological time scales. Stability is often associated with a symmetry. So, one may approach the question of dark matter identity by asking what symmetries might guarantee the stability of dark matter. A very powerful symmetry is the conservation of (B − L), the difference between the baryon and lepton numbers. Ibe, Matsumoto, and Yanagida have pointed out that, if dark matter takes advantage of this formidable symmetry, then the dark matter particle must have a well defined mass, independent of the details of new physics. The researchers provided two very appealing examples, one of which can explain an unusual signal recently reported by one of the experiments.

Matsumoto and collaborators explored another possible connection between a symmetry and dark matter. Weakly interacting massive particle (WIMP) is one of the most popular darkmatter candidates, which emerges in many models of new physics at the TeV scale. Matsumoto and collaborators have pointed out that, if the WIMP is a vector particle associated with some gauge symmetry broken at the TeV scale, then the Higgs mass is often predicted to be 120–125 GeV range, consistent with Large Hadron Collider results.

Another well-motivated dark matter candidate is a very light scalar particle, called axion. Kawasaki and collaborators have described the production of these particles from the collapse of domain walls, which could have formed in the early universe because of topological properties of the underlying field theory. This production channel can generate a substantial quantity of relic axions, which has implications for axion properties and for understanding of dark matter.

An aesthetically appealing possibility is that both dark matter and ordinary matter might have emerged from the same process in the early universe. It is remarkable that the amounts of these two components are not very different: they are within one order of magnitude of each other, which would have to be regarded as fortuitous if they arise from unrelated processes. Kawasaki and collaborators have proposed a scenario in which axino dark matter arises from decays of supersymmetric Q-balls. In a separate paper, Kasuya and Kawasaki studied gravitino dark matter produced in Q-ball decays.

Dark matter may also be related to neutrinos. Neutrino masses are most elegantly explained by the famous seesaw mechanism, which employs right-handed counterparts of the usual lefthanded neutrinos. If one of the right-handed neutrinos has a small Majorana mass, which is natural in the split seesaw and in some other models, then the resulting low-mass sterile neutrino can play the role of dark matter. Kusenko and Loewenstein have continued the search for dark matter in the form of sterile neutrinos using dedicated observations on X-ray telescopes. They have published new limits based on observations of dwarf spheroidal galaxies.

Supermassive black holes then and now.

Supermassive black holes exist in the centers of galaxies, including Milky Way, but there is no compelling theory of their formation. Evolution of massive stars can produce black holes with masses of the order of a few solar masses, but the origin of million-solar-mass black holes remains a mystery. Furthermore, observations of quasars imply that such objects have already existed at some very high redshifts, suggesting the possibility of their primordial origin. Observational evidence points to a population of black holes with a narrow mass distribution around 105 solar masses at early times, from which the observed distribution of masses could emerge via mergers and accretion. Kawasaki, Kusenko, and Yanagida have described a plausible scenario for the primordial origin of supermassive black holes. Cosmological inflation occurs when the energy density is dominated by a scalar field, slowly moving in a relatively flat potential. Theories beyond the standard model predict a plethora of such scalar fields, and the inflaton can follow a non-trivial path in the labyrinth of flat directions. Therefore, a two-stage or multi-stage inflation can be a generic feature of supersymmetry and string theory. When the inflaton switches between these directions, the spectrum of density perturbations can acquire a feature that can give rise to primordial black holes. The latest event of this kind could have produced a narrow spectrum of black holes with the requisite characteristic mass of 105 solar masses, thus explaining the origin of supermassive black holes.

The largest supermassive black holes, with masses of hundred million solar masses and beyond, are the most powerful sources of radiation in the universe. These giant black holes absorb gas and stellar matter in the centers of active galaxies, spewing very high energy gamma rays and cosmic rays, which are accelerated in their powerful jets. TeV gamma rays cannot travel large distances because they lose energy in interactions with starlight and infrared light re-emitted by dust. Yet, some very energetic gamma rays have been observed from some very distant objects. This created a puzzle. Lorentz invariance violation, as well as the existence of axion-like particles mixed with the photon, have been considered as possible explanations.

However, Kusenko and collaborators showed that the observed spectra, as well as their relatively mild dependence on the redshift, can be explained by the secondary gamma-rays, which did not originate at the source, but were produced in the cosmic ray interactions along the line of sight. This interpretation paves the way for measuring magnetic fields deep in the voids between galaxies, where the primordial seed fields may have existed from the time of Big Bang. Furthermore, one can measure correctly the extragalactic background light, which reveals the history of star formation in the universe.

Group Members