Looking for a Dark Matter Signature in the Sun’s Interior

Ilídio Lopes

[This is an invited article based on the author's work in collaboration with Joseph Silk of the University of Oxford -- 2Physics.com]

Author: Ilídio Lopes
Affiliation: Centro Multidisciplinar de Astrofísica, Instituto Superior Técnico, Lisboa, Portugal;
Departamento de Física, Universidade de Évora, Évora, Portugal.

The standard concordance cosmological model of the Universe firmly established that 85% of its mass is constituted by cold, non-baryonic particles which are almost collisionless. During its evolution, the Universe formed a complex network of dark matter haloes, where baryons are gravitationally trapped, leading to the formation of galaxies and stars, including our own Galaxy and our Sun. There are many particle physics candidates for dark matter, for which their specific mass and other properties are still unknown. Among these candidates, the neutralino, a fundamental particle proposed by supersymmetric particle physics models, seems to be the more suitable candidate. The neutralino is a weak interacting massive particle with a present day relic thermal abundance determined by the annihilating dark matter freeze-out in the primordial universe.

Among other celestial’s bodies, the Sun is a privileged place to look for dark matter particles, due to its proximity to the Earth. More significantly, its large mass – which constitutes 99% of the mass of the solar system - creates a natural local trap for the capture of dark matter particles. Present day simulations show that dark matter particles in our local dark matter halo, depending on their mass and other intrinsic properties, can be gravitationally captured by the Sun and accumulate in significant amounts in its core. By means of helioseismology and solar neutrinos we are able to probe the physics in the Sun’s interior, and by doing so, we can look for a dark matter signature.

Neutrinos, once produced in the nuclear reactions of the solar core, will leave the Sun travelling to Earth in less than 8 minutes. These neutrinos stream freely to Earth, subject only to interactions with baryons in a weak scale with a typical scattering cross section of the order of 10^-44 cm², and hence are natural “messengers” of the physical processes occurring in the Sun’s deepest layers. In a paper to be published in the scientific journal “Science” [1], Ilidio Lopes (from Évora University and Instituto Superior Técnico) and Joseph Silk (from Oxford University) suggest that the presence of dark matter particles in the Sun’s interior, depending upon their mass among other properties, can cause a significant drop in its central temperature, leading to a decrease in the neutrino fluxes being produced in the Sun’s core. The calculations have shown that, in some dark matter scenarios, an isothermal solar core is formed. In another paper published in “The Astrophysical Journal Letters” [2], the same authors suggest that, through the detection of gravity waves in the Sun’s interior, Helioseismology can also independently test the presence of dark matter in the Sun’s core.

The new generation of solar neutrino experiments will be able to measure the neutrino fluxes produced in different locations of the Sun’s core. The Borexino and SNO experiments are starting to measure the neutrino fluxes produced at different depths of the Sun’s interior by means of the nuclear reactions of the proton-proton chain. Namely these are pp-ν, ⁷Be-ν and ⁸B-ν electronic neutrinos, among others. The high precision measurements expected to be obtained by such neutrino experiments will provide an excellent tool for testing the existence of dark matter in the Sun’s core. In the near future, it is expected that the measurements of pep-ν neutrino fluxes and neutrinos from the CNO cycle will also be measured by the Borexino detector or by the upcoming experiments SNO+ or LENA.

This work is supported in part by Fundação para a Ciência e a Tecnologia and Fundação Calouste Gulbenkian.

References:
[1] Ilídio Lopes, Joseph Silk, ''Neutrino Spectroscopy Can Probe the Dark Matter Content in the Sun'', Science, DOI: 10.1126/science.1196564, in press. Abstract.
[2] Ilídio Lopes, Joseph Silk, ''Probing the Existence of a Dark Matter Isothermal Core Using Gravity Modes'', The Astrophysical Journal Letters, Volume 722, Issue 1, pp. L95-L99 (2010), DOI:10.1088/2041-8205/722/1/L95. Abstract.