S. Caroff

The pinching method for Galactic cosmic ray positrons: implications in the light of precision measurements

Two years ago, the AMS collaboration released the most precise measurement of the cosmic ray positron flux. It confirms that pure secondary predictions fall below the data above 10 GeV, suggesting the presence of a primary component, e.g. annihilations of WIMPs dark matter. Most analyses have focused on the high-energy part of the spectrum, disregarding the GeV energy region where cosmic ray transport is harder to model and solar modulation comes into play. Given the high quality of AMS measurements, it is timely to re-examine the positron anomaly over the entire energy range, taking into account transport processes so far neglected, e.g. convection or diffusive re-acceleration. We devise a new semi-analytical method to take into account transport processes so far neglected, but important below a few GeV. It is based on the pinching of inverse Compton and synchrotron energy losses inside the Galactic disc. It allows to carry out extensive scans over the cosmic ray propagation parameters, which we strongly constrain by requiring that the secondary component does not overshoot the AMS measurements. Only models with large diffusion coefficients survive this test. The positron flux is a powerful and independent probe of cosmic ray propagation, complementary to the boron-to-carbon ratio. We then scan over WIMP mass to fit the annihilation cross section and branching ratios, exploring both direct annihilations into standard model particles or through light mediators. In the former case, the best fit yields a p-value of 0.4\% for a mass of 264 GeV, a value that does not allow to reproduce the highest energy data points. Worse quality fits are found in the latter case. The interpretation of the positron excess in terms of single DM species annihilations is strongly disfavored. This conclusion is based solely on the positron data, and no other observation needs to be invoked.

What do Galactic electrons and positrons tell us about dark matter

We devise a new semi-analytical method dedicated to the propagation of Galactic electrons and positrons from MeV to TeV energies: the pinching method. It is essentially based on the pinching of inverse Compton and synchrotron energy losses from the magnetic halo, where they take place, inside the Galactic disc. This new tool is fast and allows to carry out extensive scans over parameters. We strongly constrain the cosmic ray propagation parameters by requiring that the secondary component of positrons does not overshoot the AMS-02 measurements. We find that only models with a large diffusion coefficient and a large magnetic halo size are selected by this test. Therefore, we find that the positron excess appears from 1 GeV. We then explore the possibility to explain the positron excess with a component coming from the annihilation of dark matter particles. We show that the pure dark matter interpretation of the AMS-02 positron data is strongly disfavoured. This conclusion is based solely on the positron data, and no other observation, such as the antiproton and gamma ray fluxes or the CMB anisotropies, needs to be invoked. MeV dark matter particles annihilating or decaying to electron-positron pairs cannot, in principle, be observed via local cosmic ray measurements because of the shielding solar magnetic field. We take advantage of spacecraft Voyager-Is capacity for detecting interstellar cosmic rays since it crossed the heliopause in 2012. This opens up a new avenue to probe dark matter particles in the sub-GeV energy/mass range that we exploit here for the first time.

Indications for a high-rigidity break in the cosmic-ray diffusion coefficient

Using cosmic-ray boron to carbon ratio (B/C) data recently released by the AMS-02 experiment, we find tantalizing indications (decisive evidence, in Bayesian terms) in favor of a diffusive origin for the broken power-law spectra found in protons (

A new look at the cosmic ray positron fraction

p

Uncertainties on propagation parameters: impact on the interpretation of the positron fraction

) and helium nuclei (He). The result is robust with respect to currently estimated uncertainties in the cross sections, and in the presence of a small component of primary boron, expected because of spallation at the acceleration site. Reduced errors at high energy as well as further cosmic ray nuclei data (as absolute spectra of C, N, O, Li, Be) may definitively confirm this scenario.

High Statistics Measurement of the Positron Fraction in Primary Cosmic Rays of 0.5–500 GeV with the Alpha Magnetic Spectrometer on the International Space Station

Mathieu Boudaud∗a, Sandy Aupetita, Sami Caroffb, Antje Putzea, Geneviève Bélangera, Yoann Genolinia, Corrine Goyb, Vincent Poireaub, Vivian Poulina, Sylvie Rosierb, Pierre Salatia, Li Taob, and Manuela Vecchic aLAPTh, Université Savoie Mont Blanc & CNRS, 9 Chemin de Bellevue, B.P.110 Annecy-le-Vieux, F-74941, France bLAPP, Université Savoie Mont Blanc & CNRS, 9 Chemin de Bellevue, B.P.110 Annecy-le-Vieux, F-74941, France cInstituto de Fisica de Saõ Carlos Av. Trabalhador saõ-carlense, 400 CEP: 13566-590 Saõ Carlos (SP), Brazil

Experimental method to measure the positron fraction in AMS-02

Mathieu Boudauda, Sandy Aupetita, Sami Caroffb, Antje Putzea, Genevieve Belangera, Yoann Genolini∗a, Corine Goyb, Vincent Poireaub, Vivian Poulina, Sylvie Rosierb, Pierre Salatia, Li Taob, and Manuela Vecchic aLAPTh, Universite Savoie Mont Blanc & CNRS, 9 Chemin de Bellevue, B.P.110 Annecy-le-Vieux, F-74941, France bLAPP, Universite Savoie Mont Blanc & CNRS, 9 Chemin de Bellevue, B.P.110 Annecy-le-Vieux, F-74941, France cInstituto de Fisica de Sao Carlos Av. Trabalhador sao-carlense, 400 CEP: 13566-590 Sao Carlos (SP), Brazil