Cointeracting dark matter and conformally coupled light scalars

The increasing observational pressure on the standard cosmological model motivates analyses going beyond the paradigm of the collisionless cold dark matter (DM). Since the only clear evidence for the existence of DM is based on gravitational interactions, it seems particularly fitting to study them in this sector where extensions to the standard model can be naturally introduced. A promising avenue can be obtained using modifications of the space-time metric coupled to DM and induced by the presence of a new ultralight scalar field $\varphi$. The $\varphi$ field can contribute to the DM density and can couple all the matter species universally, including additional heavy DM particles. We present a simple two-component DM model employing derivative conformal interactions between the two DM species. This can simultaneously (1) guarantee the necessary symmetries to stabilize the dark species, (2) predict a subdominant thermal relic density of the heavy DM component, and (3) alleviate small-scale structure tensions of the cold DM scenario due to the possible cointeractions in the dark sector. The scenario is highly predictive with future observational prospects ranging from the Large Hadron Collider to gravitational-wave searches and can be generalized to more rich and realistic dark sector models.

Figure 1

The effective interaction rate ${\mathrm{\Gamma }}_{\varphi }$ of the light scalars $\varphi$ due to their scatterings with the heavy DM component $\chi$ as a function of the scalar mass $m_{\varphi }$. We show with the black solid (dotted) line the value of ${\mathrm{\Gamma }}_{\varphi }$ for $m_{\chi }=150\mathrm{GeV}$ (300 GeV) and the DM velocities characteristic for dwarf galaxies $v_r\simeq v_0\simeq 10\mathrm{km}/\mathrm{s}$. The blue dash-dotted line corresponds to $m_{\chi }=300\mathrm{GeV}$ and larger velocities $v_r\simeq v_0\simeq 1000\mathrm{km}/\mathrm{s}$ relevant for clusters of galaxies. All the lines are shown for ${\rho }_{\mathrm{DM}}^{\mathrm{tot}}=0.1M_{\odot }/{\mathrm{pc}}^3$. The horizontal red dashed line indicates the value of ${\mathrm{\Gamma }}_{\varphi }=0.1{\mathrm{Gyr}}^{-1}$.

Figure 2

Left: the preferred region in the parameter space of the simplified model presented in the $(m_{\chi },M)$ plane, in which the cointeracting DM regime is found with ${\mathrm{\Gamma }}_{\varphi }\sim 0.1{\mathrm{Gyr}}^{-1}$ in dwarf galaxies. The relevant values of the light scalar mass $m_{\varphi }$ are also indicated with vertical lines. The colorful regions correspond to the approximate EFT, LHC bounds, as indicated in the plot. In the yellow-shaded region, the heavy DM component $\chi$ has a sizable abundance that exceeds 10% of the total DM density and the standard cold DM regime is reproduced (see text). In the orange region, the thermal relic density of the $\chi$ DM component overcloses the Universe. Right: similar to the left panel, but focusing on the future observational prospects. The cointeracting DM regime is obtained in the white region in the plot. Expected future bounds on the maximum value of the conformal mass $M$ are shown with the dashed brown lines. The black dashed line corresponds to the effective number of additional, $\varphi$-induced, relativistic degrees of freedom in the early Universe equal to $\mathrm{\Delta }{N}_{\mathrm{eff}}=0.04$. The colorful diagonal arrows describe the expected sensitivity of the future gravitational-wave detectors to ultralight scalars with the mass $m_{\varphi }$, which is related to $m_{\chi }$ and $M$ by Eq. (17). We show the scale relevant for $m_{\varphi }$ on top of the plot. These bounds are not sensitive to the precise value of the conformal mass $M$ (see text for details).

Figure 3

The Feynman diagram for the loop-induced self-interactions of ultralight scalars $\varphi$ with the exchange of the heavy DM component $\chi$.