The PAMELA excess from neutralino annihilation in the NMSSM

TL;DR

This paper proposes that the PAMELA positron excess can be explained by neutralino annihilation in the NMSSM, leveraging an s-channel resonance via the heavier CP-odd state $a_2$ to boost the cross section while ensuring leptonic final states through the decay $a_2\to a_1 h_1$ with $a_1\to\mu^+\mu^-$. The authors identify a narrow NMSSM parameter region with a light $a_1$ (sub-GeV) and a lightest neutralino around 160 GeV, yielding predominantly leptonic annihilation products and a large annihilation rate without conflicting with antiproton constraints. They compute positron, gamma-ray, and antiproton spectra, compare to PAMELA and Fermi LAT data, and discuss constraints from LEP, Tevatron, and rare decays, finding a viable muon-favored point that fits PAMELA well but faces some tension with gamma-ray observations depending on the DM profile. The study makes three distinctive predictions: a turnover in the PAMELA positron spectrum near 70 GeV, a DM mass below the top quark mass, and the potential discovery of a sub-GeV pseudoscalar at colliders, offering a tight set of experimental tests for this NMSSM explanation.

Abstract

We examine whether the cosmic ray positron excess observed by PAMELA can be explained by neutralino annihilation in the Next-to-Minimal Supersymmetric Standard Model (NMSSM). The main dark matter annihilation products are the lightest CP-even scalar h1 plus the lightest CP-odd scalar a1, with the a1 decaying into two muons. The energetic positrons needed to explain PAMELA are thus obtained in the NMSSM simply from kinematics. The required large annihilation cross section is obtained from an s-channel resonance with the heavier CP-odd scalar a2. Various experiments constrain the PAMELA-favored NMSSM parameter space, including collider searches for a light a1. These constraints point to a unique corner of the NMSSM parameter space, having a lightest neutralino mass around 160 GeV and a very light pseudoscalar mass less than a GeV. A simple parameterized formula for the charge-dependent solar modulation effects reconciles the discrepancy between the PAMELA data and the estimated background at lower energies. We also discuss the electron and gamma ray spectra from the Fermi LAT observations, and point out the discrepancy between the NMSSM predictions and Fermi LAT preliminary results and possible resolution. An NMSSM explanation of PAMELA makes three striking and uniquely correlated predictions: the rise in the PAMELA positron spectrum will turn over at around 70 GeV, the dark matter particle mass is less than the top quark mass, and a light sub-GeV pseudoscalar will be discovered at colliders.

Figures (9)

• Figure 1: The Feynman diagram of the dominant neutralino annihilation channel. The upper panel is for the muon-favored case, while the lower panel is for the tau-favored case.
• Figure 2: Upper panel: the positron energy spectrum from inclusive tau decays. The red and the black points are for 10 GeV and 300 GeV $\tau^+$ energy, respectively. The blue line is from a fitted analytic function in the text. Lower panel: similar as the upper panel, but for electrons. On average, 1.16 positrons and 0.16 electrons are generated from one $\tau^+$ decay.
• Figure 3: The positron fraction from the $\mu$-favored model point in the NMSSM. The solid black line is the background without considering solar modulation. The dotdashed black line is the background with the solar modulation effects. The dashed blue line is the positron fraction from neutralino ($m_\chi=162$ GeV) annihilations plus the modulated background. The red points are data from PAMELA with one standard deviation errors. The dark matter annihilation cross section is $6.0 \times 10^{-24}$ cm$^3$ s$^{-1}$. The M2 propagation model is used here.
• Figure 4: The electron plus positron spectrum from the $\mu$-favored model. The solid black line is the background. The dashed blue line is from the neutralino annihilation plus the background. The red crossed points are the results from Fermi Large Area Telescope (Fermi LAT) with the gray band for systematic errors. The dark matter annihilation cross section is $6.0\times 10^{-24}$ cm$^2$ s$^{-1}$. The M2 propagation model is used here.
• Figure 5: Upper panel: the positron fraction from one $\tau$-favored model point in the NMSSM. The dark matter ($m_\chi=160$ GeV) annihilation cross section is $9.0\times 10^{-24}$ cm$^2$ s$^{-1}$. The M2 propagation model is used here. Lower panel: a comparison of positron energy spectra from $\tau^+$ decays and from $\mu^+$ decays.