Probing the pseudoscalar portal to dark matter via $\overline{b}bZ(\to \ell \ell )+{\cancel{E}}_T$: From the LHC to the Galactic Center excess

We propose a new search for dark matter at the LHC, characteristic of scenarios beyond the Standard Model with a pseudoscalar portal between the visible and dark sectors. This search, leading to a $\overline{b}bZ(\to \ell \ell )+{\cancel{E}}_T$ final state, reaches large regions of parameter space not probed by dark matter searches via $\mathrm{multijet}+{\cancel{E}}_T$, searches for new scalars and flavor bounds. We show that this search could be used to test the dark matter origin of the gamma-ray Galactic Center excess with LHC Run 2 data.

Figure 1

$m_{H_0}=600\mathrm{GeV}$, $m_a=150\mathrm{GeV}$ signal (top) and $t\overline{t}$ background (bottom) events after event selection with $300{\mathrm{fb}}^{-1}$, in the ($m_{T2}$, ${\cancel{E}}_T$) plane. The dashed line corresponds to the signal region ${\cancel{E}}_T,m_{T2}>110\mathrm{GeV}$.

Figure 2

Current (solid) and projected LHC with $\mathcal{L}=300{\mathrm{fb}}^{-1}$ (dashed lines) 95% C.L. exclusion regions in the ($s_{\theta }$, $\tan \beta$) plane for $(m_{H_0},m_a)=(600,150)\mathrm{GeV}$ (left) and (800,150) GeV (right) with a DM mass $m_{\chi }=45\mathrm{GeV}$, from $H_0\to {\tau }^{+}{\tau }^{-}$ (blue), $a\to {\tau }^{+}{\tau }^{-}$ (brown), $\mathrm{multijet}+{\cancel{E}}_T$ (black) and ATLAS mono-$Z$ GF (yellow). Exclusion from CMS/LHCb 8 TeV $B_s\to {\mu }^{+}{\mu }^{-}$ measurements is shown in red. The dashed region corresponds to the 95% C.L. sensitivity for our proposed search, $pp\to b\overline{b}\ell \ell +{\cancel{E}}_T$, with $\mathcal{L}=300{\mathrm{fb}}^{-1}$. The coupling $y_{\chi }$ is fixed at each point to match the DM relic density. The perturbativity excluded region $y_{\chi }>4\pi$ is depicted in grey. The lines $y_{\chi }=1$ (dotted grey) and $y_{\chi }=0.1$ (dot-dashed grey) are shown for guidance.

Figure 3

95% C.L. sensitivity of the proposed search $pp\to b\overline{b}\ell \ell +{\cancel{E}}_T$ with $\mathcal{L}=300{\mathrm{fb}}^{-1}$ (dashed region) in the ($m_a$, $s_{\theta }$) plane for $m_{H_0}=600\mathrm{GeV}$, $\tan \beta =10$, $y_{\chi }=1$. The red region is excluded by $B_s\to {\mu }^{+}{\mu }^{-}$, while the green band yields the observed DM relic density. The DM mass is $m_{\chi }=45\mathrm{GeV}$.