Improved Limits for Higgs-Portal Dark Matter from LHC Searches

Searches for invisible Higgs decays at the Large Hadron Collider constrain dark matter Higgs-portal models, where dark matter interacts with the standard model fields via the Higgs boson. While these searches complement dark matter direct-detection experiments, a comparison of the two limits depends on the coupling of the Higgs boson to the nucleons forming the direct-detection nuclear target, typically parametrized in a single quantity $f_N$. We evaluate $f_N$ using recent phenomenological and lattice-QCD calculations, and include for the first time the coupling of the Higgs boson to two nucleons via pion-exchange currents. We observe a partial cancellation for Higgs-portal models that makes the two-nucleon contribution anomalously small. Our results, summarized as $f_N=0.308(18)$, show that the uncertainty of the Higgs-nucleon coupling has been vastly overestimated in the past. The improved limits highlight that state-of-the-art nuclear physics input is key to fully exploiting experimental searches.

Figure 1

Higgs-mediated interaction of the WIMP $\chi$ with the light quarks $q=u$, $d$, $s$ (left) and the gluon field $g$ by closing a heavy-quark loop $Q=c$, $b$, $t$ (right).

Figure 2

Two-body contributions to WIMP-nucleus scattering. Solid (dashed) lines refer to nucleons (pions), the crosses to the coupling of the external current.

Figure 3

Exclusion limits for scalar (blue), fermion (red), and vector (green) Higgs-portal WIMPs. The gray-shaded bands refer to the range $f_N=0.260\dots 0.629$ from the most recent ATLAS [12] and CMS [13] analyses, the dashed lines to the central value $f_N=0.326$ considered therein, and the colored bands to our improved limits using Eq. (14). For comparison, we show the direct-detection limits from SuperCDMS [62], PandaX-II [66], LUX [67], and XENON1T [68].