The electron plus positron spectrum from annihilation of Kaluza–Klein dark matter in the Galaxy

Abstract

The lightest Kaluza–Klein particle (LKP), which appears in the theory of universal extra dimensions (UED), is one of the good candidates for cold dark matter (CDM). When LKP pairs annihilate around the center of the Galaxy where CDM is concentrated, there are some modes which produce electrons and positrons as final products, and we categorize them into two components. One of them is the “line” component, which directly annihilates into electron–positron pair. Another one is the “continuum” component, which consists of secondarily produced electrons and positrons via some decay modes. Before reaching Earth, directions of electrons and positrons are randomized by the Galactic magnetic field, and their energies are reduced by energy loss mechanisms. We assume the LKP is in the mass range from 300[Formula: see text]GeV to 1500[Formula: see text]GeV. We calculate the electron plus positron spectrum after propagation in the Galactic halo to Earth, and we analyze the resulting spectrum and positron fraction. We also point out that the energy dependence of observed positron fraction is well reproduced by the mixture of “line” and “continuum” components. We can fit the electron plus positron spectrum and the positron fraction by assuming appropriate boost factors describing dark matter concentration in the Galactic halo. However, it is difficult to explain both the electron plus positron spectrum and the positron fraction by a single boost factor if we take account of observational data obtained by AMS-02 only.