Confronting recent AMS-02 positron fraction and Fermi-LAT Extragalactic Gamma-Ray Background measurements with gravitino dark matter

TL;DR

This paper investigates whether gravitino dark matter can explain the AMS-02 positron excess and examines gamma-ray constraints, ultimately ruling out gravitino decay as the sole source of high-energy positrons.

Contribution

It provides a detailed analysis of gravitino decay channels fitting AMS-02 data and sets gamma-ray observational limits that challenge this dark matter explanation.

Findings

Gravitino masses of 1-2 TeV fit AMS-02 positron data.

Gamma-ray constraints exclude gravitino decay as the primary positron source.

Gravitino dark matter is ruled out as the sole explanation for the positron excess.

Abstract

Recent positron flux fraction measurements in cosmic-rays (CR) made by the AMS-02 satellite confirm and extends the evidence on the existence of a new (yet unknown) source of high energy positrons. To explain this excess, we use the gravitino of bilinear R-parity violating SUSY models as a decaying Dark Matter candidate, as the source of those high energy particles. Being a long lived weak-interacting and spin 3/2 particle, it offers several particularities which makes it an attractive candidate. We compute the electron, positron and $\gamma$-ray\ fluxes produced by each gravitino decay channel at the Earth. Combining the flux from the different decay modes we can fit AMS-02 measurements of the positron fraction, as well as the electron and positron fluxes, with a gravitino mass in the range $1-2$ TeV and lifetimes of $\sim 1.0-0.8\times 10^{26}$ s. . Then, we study the viability of these scenarios through their implications in $\gamma$-ray observations. We set limits on the gravitino lifetime using the Extragalactic $\gamma$-ray Background recently reported by the {\it Fermi}-LAT Collaboration and a state-of-the-art model of its known contributors. These limits exclude the gravitino parameter space which provides an acceptable explanation of the AMS-02 data. Therefore, we conclude that the gravitino of bilinear R-parity violating models is ruled out as the unique primary source of electrons and positrons needed to explain the rise in the positron fraction.