Composite Scalar Dark Matter

TL;DR

This work investigates a light scalar dark matter candidate η arising as a pseudo Nambu-Goldstone boson in a TeV-scale composite sector with an SO(6)/SO(5) symmetry breaking pattern. The Higgs is also a composite pNGB, and η interacts with the Standard Model through derivative couplings fixed by symmetry and explicit $U(1)_η$ breaking, controlled by four parameters $f$, $\,\lambda$, $c_t$, and $c_b$. The authors compute the relic density via the Boltzmann equation, analyze Higgs-width and coupling constraints from LHC, and compare to dark-matter direct-detection bounds, finding three predictive mass regions for viable DM: near the Higgs resonance at $m_η \approx m_h/2$, a heavier region $m_η \approx 100$–$500$ GeV dominated by derivative interactions, and a light region disfavored by theory and collider bounds. The model is highly predictive, tying EWSB and DM phenomenology, and will be decisively tested by upcoming direct-detection experiments and LHC precision Higgs measurements.

Abstract

We show that the dark matter (DM) could be a light composite scalar $η$, emerging from a TeV-scale strongly-coupled sector as a pseudo Nambu-Goldstone boson (pNGB). Such state arises naturally in scenarios where the Higgs is also a composite pNGB, as in $O(6)/O(5)$ models, which are particularly predictive, since the low-energy interactions of $η$ are determined by symmetry considerations. We identify the region of parameters where $η$ has the required DM relic density, satisfying at the same time the constraints from Higgs searches at the LHC, as well as DM direct searches. Compositeness, in addition to justify the lightness of the scalars, can enhance the DM scattering rates and lead to an excellent discovery prospect for the near future. For a Higgs mass $m_h\simeq 125$ GeV and a pNGB characteristic scale $f \lesssim 1$ TeV, we find that the DM mass is either $m_η\simeq 50-70$ GeV, with DM annihilations driven by the Higgs resonance, or in the range 100-500 GeV, where the DM derivative interaction with the Higgs becomes dominant. In the former case the invisible Higgs decay to two DM particles could weaken the LHC Higgs signal.

Figures (8)

• Figure 1: Relic density of the scalar singlet DM as a function of its mass $m_{\eta}$. We take $m_h=125$ GeV and $\lambda=10^{-1}$ ($10^{-3}$) in the left (right) panel. The dashed curves correspond to the non-composite case, the red (blue) curves to the composite case with $f=1$ TeV ($500$ GeV). The red and blue bands describe the variation of the DM$-$bottom coupling $c_b$ in Eq. (\ref{['param']}), between $a=b=0$ (dark lines) and $a=b=1$ (light lines). Above the threshold for annihilation into $t\bar{t}$, the result depends also on the value of the DM$-$top coupling: the dotted and solid lines correspond to $c_t=1/2$ (Case 1) and $c_t=0$ (Case 2), respectively.
• Figure 2: The strength $\mu_{VV}$ of the Higgs signal in the gauge channels, defined in Eq. (\ref{['muvv']}), as a function of the $h-\eta$ quartic coupling $\lambda$ (negative in the left panel, positive in the right panel). We chose $m_h=125$ GeV and two values for the scalar singlet mass $m_\eta$, one larger than $m_h/2$ and the other smaller. The dotted curves correspond to the non-composite case. The dashed (solid) curves correspond to compositeness with $f=1$ TeV (500 GeV). We took into account the order $\xi$ corrections to the Higgs couplings to vector bosons and fermions, see Eqs. (\ref{['hVV']})-(\ref{['hpp']}). The shaded region is disfavoured by the LHC Higgs searches (we roughly extracted the 95% C.L. lower bound on $\mu_{VV}$ from Ref. ATLAS).
• Figure 3: The same as in Fig. \ref{['fig:lam125']}, except for $m_h=145$ GeV. For this mass ATLAS and CMS put an upper bound on $\mu_{VV}$.
• Figure 4: The spin-independent cross section for the elastic scattering of the DM candidate $\eta$ off nuclei. The green shaded region is excluded by XENON100 Aprile:2011hi, while the orange shaded region roughly corresponds to the excess events reported by DAMA, CoGeNT and CRESST-II Hooper:2012ft. The predictions of our scenario are labeled in the same way as in Fig. \ref{['fig:BoltzmannRelic']}. The two panels correspond to two different values of $\lambda$, and we took Case 2 for the DM-quark couplings $c_q$. In the left panel, the band for $f=1$ TeV (not shown) is very similar to the one for $f=500{\rm \,GeV}$.
• Figure 5: The contour $\Omega_\eta =\Omega_{DM}$ (solid dark purple line) in the plane $(m_{\eta},\lambda)$, for $m_h=125$ GeV, $f=500$ GeV, assuming Case 2 with $c_b=1/2$. The green shaded region is disfavoured by XENON100, the region delimited by a blue line is favoured by DAMA/CoGeNT/CRESST-II, and the red shaded region is disfavoured by the Higgs signal at the LHC. The solid light purple/green/blue lines correspond to the same observables for maximal $c_b$ ($a=b=1$ in Eq. (\ref{['param']})). The dashed purple/green/blue/red lines correspond to the same observables in the non-composite case, $f=\infty$. Finally, the region below the yellow dot-dashed line corresponds to the theoretical preferred region defined by Eq. (\ref{['expectTheo']}).