Measuring the Dark Force at the LHC

Abstract

A long-range ``dark force'' has recently been proposed to mediate the dark matter (DM) annihilation. If DM particles are copiously produced at the Large Hadron Collider (LHC), the light dark force mediator will also be produced through radiation. We demonstrate how and how precise we can utilize this fact to measure the coupling constant of the dark force. The light mediator's mass is measured from the ``lepton jet'' it decays to. In addition, the mass of the DM particle is determined using the MT2 technique. Knowing these quantities is critical for calculating the DM relic density.

Figures (4)

• Figure 1: The schematic Feynman diagram of the dark matter radiating a light mediator $a_{\rm dark}$, which decays to two leptons.
• Figure 2: Contour plot of the number of events consistent with the mass pair $(\mu_{\widetilde{\chi}_{\rm dark}}, \,\mu_{\widetilde{\chi}^0_1})$. Starting from the top contour, the number of events descends by 1000 from the adjacent contour. Above the top contour, the masses are consistent with all 5941 events. The dashed straight line has fixed mass difference, which is tangent to the top contour.
• Figure 3: Number of consistent events along the line with fixed mass difference and tangent to the $m_{T2}^{\max}$ contour.
• Figure 4: The ratio $R$ as a function of $M_{a_{\rm dark}}$ for different values of $M_{h_{\rm dark}}/M_{a_{\rm dark}}$ and fixed values $m_{\widetilde{\chi}^0_1}=700$ GeV and $m_{\widetilde{\chi}_{\rm dark}}=600$ GeV. The ratios are proportional to the gauge coupling square, which is chosen to be $g=1$ here. The black dashed line is calculated using the approximate formula in Eq. (\ref{['eq:ratio2']}).