Executive Summary of Workshop ResultsThe workshop followed an unusual but very successful format. Short talks were made parts of extended discussion sessions, in which the participants took ample opportunities to ask questions and discuss important points in detail. Moreover, the plentiful unscheduled time dedicated to individual interactions proved fruitful for exploration of new collaborations, several of which are coming to fruition several months later. The small number of participants also contributed to the success of the workshop. The rest of this document describes the topics addressed in each discussion session.
Radiative corrections play an important role in shaping the energy spectrum and strength of WIMP annihilation signals. One prominent example is virtual internal Bremsstrahlung (IB), which leads to pronounced signatures that can be searched for with gamma-ray telescopes like the Fermi-LAT or HESS. Strong IB signals require the existence of additional charged particles not much heavier than the dark matter particle itself. Such compressed spectra are difficult to probe at particle colliders, but can lead to a resonant enhancement of nucleon scattering cross-sections at direct search experiments like XENON100. Electroweak and gluon bremsstrahlung enhance the expected antimatter signal. The complementarity of the different signatures can be well studied in minimal toy models for dark matter that concentrate on the relevant low-mass states [M. Garny]. Significant progress has also been made in a full computation of electroweak corrections for MSSM neutralino dark matter, including all diagrams, and aiming at a full implementation in the software package DarkSUSY [F. Calore]. In the high-mass regime of the inert double model for dark matter, small mass-splittings are rather generic, and the corresponding IB emission enhances the detectability of such models with current and future Air Cherenkov Telescopes [C. Garcia].
Monochromatic photons are the prototypical example of gamma-ray spectral features from dark matter annihilation. In most models for dark matter, they are however only produced at one-loop level and hence significantly suppressed. The optical theorem can be used to relate the underlying tree-level processes to the line signal strength in a rather model independent way. More specifically, it allows to constrain the imaginary part of the loop diagram leading to gamma-ray line emission with complementary antiproton and radio probes [M. Vollmann]. An interesting class of models with enhanced annihilation into gamma-ray lines is magnetic and Rayleigh dark matter, which again requires the existence of charged states close to the dark matter mass [I. Yavin].
Secondary photons may be emitted in the particle cascades following the decay of the primary annihilation products, typically quark-antiquark, lepton-antilepton, and gauge-boson pairs. The energy spectrum of the first secondary photons has a characteristic "box-like" shape that can in principle be easily distinguished in observational data. A lack of box-like features places constraints on the annihilation strength into the primary channel just mentioned [M. Pato]. The energy spectrum of secondaries produced further down the parcel cascade requires Monte-Carlo modeling. However systematic uncertainties associated to these Mote-Carlo generators, such as PYTHIA or HERWIG, can be really important [J.A.R. Cembranos].
The rise in the positron fraction of cosmic-rays that was detected by PAMELA and Fermi LAT, and now confirmed by AMS-02 with high precision up to energies of 350 GeV, could by due to dark matter annihilation (although less exotic sources like pulsar wind nebulae are a viable alternative). The strong observational constraints on such scenarios -- e.g. from gamma-ray and radio measurements of the Galactic center and halo -- could be weakened if the dark matter annihilation is locally enhanced by an increased dark matter density [A. Hektor]. One of the few indirect probes for dark matter annihilation that are almost background free are anti-deuterons, with new results expected in the near future by AMS-02 and GAPS. The theoretical uncertainties in the signal prediction, which are related to different Monte Carlo codes and the validity of the adopted coalescence model, are under current investigation [C. Savage].
The dark matter density in galactic halos is of paramount importance in determining the intensity of all indirect signals from dark matter annihilation in the galactic halo, and galactic halos in general. Perhaps the most difficult question in this context is figuring out the number, location, and interior density of the smallest dark objects; it is their properties that determine the overall indirect emission. Numerical N-body simulations, which are the workhorse for studying larger objects, do not reach into the small scale regime. A powerful analytical idea (stable clustering) is a promising method, especially when tuned to N-body simulations at large scales [J. Zavala Franco]. Upcoming astrometric surveys of the position and proper motion of millions of stars (GAIA) may reveal the presence of invisible dark matter sub-halos [D. Spolyar]. Densities in dwarf spheroidal galaxies may be better determined, in particular the dark matter velocity anisotropy, using fourth moments of the dark matter phase-space density [T. Richardson].
Particle physics models beyond the Standard Model (SM) are necessary to explain dark matter as an elementary particle, since no SM particle has the characteristics of dark matter. Supersymmetric extensions of the SM are still viable and popular, although constraints coming from the Large Hadron Collider (LHC) tend to exclude the simplest supersymmetric models and encourage the study of non-supersymmetric extensions. Among the latter, models with Higgs bosons playing the role of mediator between the SM and the dark matter sector (Higgs portal extensions) have received a lot of attention. Models in which the Higgs boson receives its mass radiatively (Coleman-Weinberg Higgs bosons) can account for dark matter cross sections of interest in direct dark matter searches [H.D. Kim]. Systematic studies of scalar, fermion, and vector dark matter in Higgs portal extensions leads to constraints, predictions, and an understanding of the limitations of simplified approaches such as effective operators [P. Ko]. Models with two electroweak doublets of Higgs bosons can accommodate both the SM Higgs boson with its mass as measured at the LHC, and a dark matter phenomenology that is accessible to current and planned experiments [Y. Jiang]. The simplest SM extension, containing just an additional electroweak Higgs singlet, provides a dark matter candidate still compatible with all constraints and within reach of the next generation of direct and indirect searches [P. Scott].
Other indirect methods to search for dark matter annihilation include microwaves and radio waves. Ionization of the cosmic plasma by dark matter annihilation in the early universe affects cosmic microwave background maps, and constitutes a dark matter probe competitive with gamma-ray and positron searches [F. Iocco]. Synchrotron radiation from the motion of annihilation electrons in magnetic fields is effective in testing models around the galactic center, but not in galaxy clusters [F. Huang].