9th Week Paper List

An anomalous positron abundance in cosmic rays with energies 1.5–100 GeV

O. Adriani^1,2, G. C. Barbarino^3,4, G. A. Bazilevskaya⁵, R. Bellotti^6,7, M. Boezio⁸, E. A. Bogomolov⁹, L. Bonechi^1,2, M. Bongi², V. Bonvicini⁸, S. Bottai², A. Bruno^6,7, F. Cafagna⁷, D. Campana⁴, P. Carlson¹⁰, M. Casolino¹¹, G. Castellini¹², M. P. De Pascale^11,13, G. De Rosa⁴, N. De Simone^11,13, V. Di Felice^11,13, A. M. Galper¹⁴, L. Grishantseva¹⁴, P. Hofverberg¹⁰, S. V. Koldashov¹⁴, S. Y. Krutkov⁹, A. N. Kvashnin⁵, A. Leonov¹⁴, V. Malvezzi¹¹, L. Marcelli¹¹, W. Menn¹⁵, V. V. Mikhailov¹⁴, E. Mocchiutti⁸, S. Orsi^10,11, G. Osteria⁴, P. Papini², M. Pearce¹⁶, P. Picozza^11,13, M. Ricci¹⁷, S. B. Ricciarini², M. Simon¹⁵, R. Sparvoli^11,13, P. Spillantini^1,2, Y. I. Stozhkov⁵, A. Vacchi⁸, E. Vannuccini², G. Vasilyev⁹, S. A. Voronov¹⁴, Y. T. Yurkin¹⁴, G. Zampa⁸, N. Zampa⁸ & V. G. Zverev¹⁴

Antiparticles account for a small fraction of cosmic rays and are known to be produced in interactions between cosmic-ray nuclei and atoms in the interstellar medium1, which is referred to as a 'secondary source'. Positrons might also originate in objects such as pulsars2 and microquasars3 or through dark matter annihilation4, which would be 'primary sources'. Previous statistically limited measurements5, 6, 7 of the ratio of positron and electron fluxes have been interpreted as evidence for a primary source for the positrons, as has an increase in the total electron+positron flux at energies between 300 and 600 GeV (ref. 8). Here we report a measurement of the positron fraction in the energy range 1.5–100 GeV. We find that the positron fraction increases sharply over much of that range, in a way that appears to be completely inconsistent with secondary sources. We therefore conclude that a primary source, be it an astrophysical object or dark matter annihilation, is necessary.

Links : http://www.nature.com/nature/journal/v458/n7238/full/nature07942.html

http://www.nature.com/nature/journal/v458/n7238/extref/nature07942-s1.pdf(supplementary information)

A Selection Rule for Enhanced Dark Matter Annihilation

Anirban Das (Tata Inst., Mumbai), Basudeb Dasgupta (Tata Inst., Mumbai)

We point out a selection rule for enhancement (suppression) of odd (even) partial waves of dark matter co/annihilation using Sommerfeld effect. Using this, the usually velocity-suppressed p-wave annihilation can dominate the annihilation signals in the present Universe. The selection mechanism is a manifestation of an exchange symmetry, and generic for multi-state DM with off-diagonal long-range interactions. As a consequence, the relic and late-time annihilation rates are parametrically different and a distinctive phenomenology, with large but strongly velocity-dependent annihilation rates, is predicted.

Links : https://arxiv.org/abs/1611.04606

Realistic estimation for the detectability of dark matter sub-halos with Fermi-LAT

Francesca Calore, Valentina De Romeri, Mattia Di Mauro, Fiorenza Donato, Federico Marinacci

Numerical simulations of structure formation have recorded a remarkable progress in the recent years, in particular due to the inclusion of baryonic physics evolving with the dark matter component. We generate Monte Carlo realizations of the dark matter sub-halo population based on the results of the recent hydrodynamical simulation suite of Milky Way-sized galaxies. We then simulate the gamma-ray sky for both the setup of the 3FGL and 2FHL Fermi Large Area Telescope (LAT) catalogs, including the contribution from the annihilation of dark matter in the sub-halos. We find that the flux sensitivity threshold strongly depends on the particle dark matter mass, and more mildly also on its annihilation channel and the observation latitude. The results differ for the 3FGL and 2FHL catalogs, given their different energy thresholds. We also predict that the number of dark matter sub-halos among the unassociated sources is very small. A null number of detectable sub-halos in the Fermi-LAT 3FGL catalog would imply upper limits on the dark matter annihilation cross section into bb¯ of 2⋅10−26 (5⋅10−25) cm3/s with MDM= 50 (1000) GeV. We find less than one extended sub-halo in the Fermi-LAT 3FGL catalog. As a matter of fact, the differences in the spatial and mass distribution of sub-halos between hydrodynamic and dark matter-only runs do not have significant impact on the gamma-ray dark matter phenomenology.

Links : https://arxiv.org/abs/1611.03503

Cross-correlation of weak lensing and gamma rays: implications for the nature of dark matter

Tilman Tröster, Stefano Camera, Mattia Fornasa, Marco Regis, Ludovic van Waerbeke, Joachim Harnois-Déraps, Shin'ichiro Ando, Maciej Bilicki, Thomas Erben, Nicolao Fornengo, Catherine Heymans, Hendrik Hildebrandt, Henk Hoekstra, Konrad Kuijken, Massimo Viola

We measure the cross-correlation between Fermi-LAT gamma-ray photons and over 1000 deg2 of weak lensing data from the Canada-France-Hawaii Telescope Lensing Survey (CFHTLenS), the Red Cluster Sequence Lensing Survey (RCSLenS), and the Kilo Degree Survey (KiDS). We present the first measurement of tomographic weak lensing cross-correlations and the first application of spectral binning to cross-correlations between gamma rays and weak lensing. The measurements are performed using an angular power spectrum estimator while the covariance is estimated using an analytical prescription. We verify the accuracy of our covariance estimate by comparing it to two internal covariance estimators. Based on the non-detection of a cross-correlation signal, we derive constraints on weakly interacting massive particle (WIMP) dark matter. We compute exclusion limits on the dark matter annihilation cross-section ⟨σannv⟩, decay rate Γdec, and particle mass mDM. We find that in the absence of a cross-correlation signal, tomography does not significantly improve the constraining power of the analysis. Assuming a strong contribution to the gamma-ray flux due to small-scale clustering of dark matter and accounting for known astrophysical sources of gamma rays, we exclude the thermal relic cross-section for masses of mDM≲20 GeV

Links : https://arxiv.org/abs/1611.03554