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Description
Summary
The Calorimetric Electron Telescope (CALET) will be installed on the external facility of the Japanese Kibo Module at the ISS within the Japanese fiscal year 2014, and measure the energy and direction distribution of electron and positron cosmic rays. Complementary to the charge sensitive AMS-02, CALET is focused on the total electron and positron flux, which it will determine well into to the TeV energy range thanks to its large aperture of 1200 m$^2$sr. Featuring a total absorption calorimeter of 30 radiation lengths thickness, it has a proton rejection capability of $1:10^5$ and an energy resolution of 2$\%$, which makes it well suited to investigate features in the energy spectrum for testing the hypotheses of Dark Matter annihilation and/or local accelerators being responsible for the positron excess observed by AMS-02.
A parameterization of the total flux including both a component from Dark Matter annihilation $\Phi_{DM}$ scaled by a Boost Factor $BF$ and a power law local accelerator source with index $\gamma_{s}$, coefficient $C_{s}$ and cutoff energy $E_{cut_s}$ above a diffuse power law background with indices $\gamma_{e}$ and $\gamma_{e^{+}}$ and coefficients $C_{e}$ and $C_{e^{+}}$ for total and positron only flux respectively, and a cutoff energy $E_{cut_d}$ caused by propagation is given by
$ \Phi_{e}(E) = 2 \Phi_{DM}(E) \cdot BF + C_{e} E^{\gamma_{e}} \left(2 {C_{s} \over C_{e}} E^{\gamma_{e}-\gamma_{s}} \cdot \exp\left({-E \over E_{cut_s}}\right) + \left({C_{e^{+}} \over C_{e}} \cdot E^{\gamma_{e^{+}}-\gamma_{e}}+1\right) \cdot \exp\left({-E \over E_{cut_d}}\right)\right) $
With the current constraints on above parameters from the positron fraction measured by AMS-02 and the total flux data from Fermi-LAT, the additional component could be both solely the power law accelerator term, or solely the Dark Matter annihilation term, if the Dark Matter consists of the Lightest Kaluza Klein Particle (LKP). For the case that primarily local accelerators are the cause of the excess, the sensitivity of CALET to exclude a possible minor contribution from Dark Matter annihilation has been calculated for several Dark Matter candidates and Dark Matter particle masses, for five years of taking data. To obtain this result, a $\chi^2$ analysis using above formula was performed on simulated CALET data in conjunction with the present AMS-02 positron fraction data, increasing $BF$ starting from zero until the resultant $\chi^2$ exceeded the 95$\%$ CL threshold, with the other parameters optimized in each step.
If LKP Dark Matter is assumed responsible for the excess, a large Boost Factor in the range of 10$^2$ to 10$^4$ due to a nearby subhalo, or an significantly increased speed averaged annihilation crossection $<\sigma v>$ compared to the expected value of 3$\cdot$10$^{-26}$ cm$^{3}$s$^{-1}$ predicted by the relic density is required. Using an equivalent method to the sensitivity calculation, the expected region in $BF$ vs. $m_{LKP}$ and the corresponding $<\sigma v>$ vs. $m_{LKP}$ to which CALET could constrain these parameters after five years of taking data was determined.
It is shown that CALET will be able to clearly distinguish between the cases of a power law spectrum from a nearby pulsar and the spectrum from Dark Matter annihilation with a significant lepton-channel branching ratio.
For selected cases the predictions from the parameterization were reproduced with the numerical simulation code DRAGON to validate the analytical parameterization and allowed parameter ranges.
