One bump or two peaks: The 750 GeV diphoton excess and dark matter with a complex mediator

We consider the possibility that the recently observed excess in the diphoton invariant mass spectrum around 750 GeV is due to two narrow scalars. We demonstrate that there is no need to introduce invisible decay modes to enlarge the widths as long as the two scalars exhibit an appropriate mass splitting. Nevertheless, we investigate the possible connection of these two scalars to the dark matter in a toy model with a Majorana fermionic dark-matter candidate. We explore how large the widths of the two scalar widths could be, taking into account various constraints from collider and dark-matter experiments. Introducing two scalars alleviates the tension between the diphoton production and dark-matter constraints.

Figure 1

Contour plots of $\mathrm{\Delta }{\chi }^2$ as a function of $\mathrm{\Delta }M$ and $\mathrm{\Gamma }$ with (a) $M=740\mathrm{GeV}$ and (b) $M=760\mathrm{GeV}$.

Figure 2

Diphoton invariant mass distributions, where the red and blue dashed curves represent each individual scalar contribution and the black curves denote their sums. The top row shows the narrow scalars and the bottom row displays the broad scalars.

Figure 3

(a) The ${\mathrm{\Gamma }}_{\chi \chi }{\mathrm{\Gamma }}_{\mathrm{vis}}$ needed to explain the relic abundance as a function of $M_{\chi }$; (b) the allowed parameter space of ${\mathrm{\Gamma }}_{gg}/{\mathrm{\Gamma }}_{\mathrm{vis}}$ by the LUX spin-independent (SI) detection; (c) the LUX-ZEPLIN projection of the SI detection.

Figure 4

The parameter space projected in the plane of $M_{\chi }$ and ${⟨\sigma v⟩}_{\gamma \gamma }$. All the points satisfy the constraints from the collider searches, the dark-matter relic abundance, and the LUX bound on the spin-independent direct-detection cross section. The black curve denotes the Fermi-LAT gamma-ray line limit from region R3 while the red curve represents the limit from region R41.

Figure 5

The parameter space allowed by the collider searches, the dark-matter relic abundance, the LUX direct-detection bound, and the Fermi-LAT gamma-ray limit from region R41 in the $M_{\chi }-{\sigma }_{\mathrm{SI}}$ plane. The green points can be probed by the LZ experiment in the near future, while the red points are not testable by the foreseeable direct-detection experiments.

Figure 6

The parameter space allowed by the collider searches, the dark-matter relic abundance, the LUX direct-detection bound, and the Fermi-LAT gamma-ray limit from region R41 in the $M_{\chi }-{\sigma }_{\mathrm{SI}}$ plane. The green points can be probed by the LZ experiment in the near future, while the red points are not testable by the foreseeable direct detection experiments.