Update on scalar singlet dark matter

TL;DR

This paper reexamines the Higgs-portal scalar singlet dark matter model with a $Z_2$ symmetry, updating constraints from Higgs invisible decays, the thermal relic density, and indirect/direct detection using recent hadronic and lattice inputs. It computes the full thermal-averaged annihilation cross section near the Higgs resonance and combines CMB, dwarf-galaxy gamma rays, and CTA data to derive a comprehensive indirect-detection likelihood, while also refining the Higgs-nucleon coupling $f_N$ to reduce hadronic uncertainties in direct detection. The results indicate that nearly all of the viable parameter space will be probed by upcoming experiments, particularly XENON1T/LUX, with a small region around $m_S\approx 57-62$ GeV remaining under tight scrutiny; the work also discusses implications for electroweak phase transition, vacuum stability, and possible extensions (complex singlet, curvaton, and new sectors). Overall, direct-detection experiments are poised to decisively test this simplest Higgs-portal DM scenario in the near term, while indirect signals provide valuable cross-checks and insights into broader phenomenology.

Abstract

One of the simplest models of dark matter is that where a scalar singlet field S comprises some or all of the dark matter, and interacts with the standard model through an HHSS coupling to the Higgs boson. We update the present limits on the model from LHC searches for invisible Higgs decays, the thermal relic density of S, and dark matter searches via indirect and direct detection. We point out that the currently allowed parameter space is on the verge of being significantly reduced with the next generation of experiments. We discuss the impact of such constraints on possible applications of scalar singlet dark matter, including a strong electroweak phase transition, and the question of vacuum stability of the Higgs potential at high scales.

Figures (10)

• Figure 1: Contours of fixed relic density, labelled in terms of their fraction of the full dark matter density. Dark-shaded lower regions are ruled out because they produce more than the observed relic density of dark matter. Left: a close-up of the mass region $m_{ S} \sim m_h/2$, where annihilations are resonantly enhanced. The region ruled out by the Higgs invisible width at $2\sigma$ CL is indicated by the darker-shaded region in the upper left-hand corner. The projected $1\sigma$ constraint from 300 fb$^{-1}$ of luminosity at the 14 TeV LHC is shown as the lighter-shaded region, corresponding to a limit of 5% on the Higgs branching fraction to invisible states Peskin:2012we. Right: relic density contours for the full range of $m_{ S}$.
• Figure 2: Branching fractions for $SS$ to annihilate at threshold into various SM final states, versus the DM mass. We have chosen $\lambda_{h{ S}}$ at each dark matter mass such that the $S$ relic density exactly matches the observed value; these $\lambda_{h{ S}}$ values can be seen along the $\Omega_{ S} = \Omega_{\rm DM}$ curve in Fig. \ref{['fig:relden']}.
• Figure 3: Limits on scalar singlet dark matter from indirect searches for dark matter annihilation. The lowermost shaded region is ruled out because these models exceed the observed relic density. Regions below the other curves are in tension with indirect searches, or will be in the future: at more than $1\sigma$ according to current data from Fermi dwarf galaxy observations and WMAP 7-year CMB data (solid), at $\ge$90% CL (dashes) and $\ge1\sigma$ CL (dots) with CTA, Planck polarization data and future Fermi observations. The area ruled out by the Higgs invisible width at $2\sigma$ CL is indicated by the shaded region in the upper left-hand corner of both plots. Note that all indirect detection signals are scaled for the thermal relic density of the scalar singlet, regardless of whether that density is greater than or less than the observed density of dark matter. Left: a close-up of the resonant annihilation region. Right: the full mass range.
• Figure 4: Contributions of different searches for dark matter annihilation to the combined future 90% CL exclusion curve. The limit from future Fermi searches for annihilation in dwarf galaxies alone are shown by the dotted line, assuming 10 years of exposure and the discovery of a further 10 southern dwarfs. The impact of Planck alone, including polarization data, can be seen from the solid line, and the projected impact of CTA is shown as a dashed line. The parameter space excluded by the relic density appears once more as a dark shaded area in the lower part of the plot.
• Figure 5: Predicted distributions (in arbitrary units) of the strangeness content $y$ of the nucleon (left), the nucleon matrix element $\sigma_0$ (centre) and the Higgs-nucleon coupling factor $f_N$ (right). These are drawn from a random sample generated using experimental and theoretical constraints, as explained in the text.