From oversimplified to overlooked: the case for exploring Rich Dark Sectors

TL;DR

This paper argues that exploring rich dark sectors (RDS) below the electroweak scale offers a more complete framework to address neutrino masses, dark matter, and the baryon asymmetry than minimal models. It promotes a bottom-up approach, introducing a prototype 3-portal model with a dark photon, a dark scalar, and dark fermions that generate a multi-portal connection to the SM, enabling diverse production and decay pathways, including semi-visible cascades and short lifetimes. The work details distinctive phenomenology for heavy neutral leptons, dark photons, and dark scalars, and surveys experimental opportunities across colliders, fixed targets, and neutrino detectors, while stressing the need for inclusive, recastable bounds and robust simulation tools. It also highlights early-universe implications, such as first-order phase transitions and gravitational waves, linking DS dynamics to PTA observations like NANOGrav. Overall, the paper advocates a coordinated program combining theory, phenomenology, and diverse experiments to fully exploit rich dark-sector signatures and their cosmological consequences.

Abstract

The Standard Model (SM) of particle physics provides a very successful description of fundamental particles and their interactions but it is incomplete, as neutrino masses, dark matter and the baryon asymmetry of the Universe indicate. In addition, the origin of masses and of the approximate fundamental symmetries call out for deeper explanations. The quest for a New SM Theory, that extends the SM to a more general theory, is ongoing. For decades the main focus has been on the TeV scale, but despite an impressive theoretical and experimental effort, no hints of new physics at such scale has been found in experiments. Dark sectors provide an interesting alternative to TeV scale extensions of the SM to explain the open questions in particle and astroparticle physics. Going beyond minimal models, rich dark sectors extend the SM to a complex theory with multiple particles and interactions, in analogy to the SM itself. They have a wealth of theoretical and astrophysical/cosmological consequences and can lead to phenomenological signatures that can be markedly different to that of minimal ones. These include short-lived particles and semi-visible decay signatures, as opposed to minimal models where new states are typically long-lived and purely visible or invisible resonances. Given the experimental configurations and analysis strategies, current dark sector searches might miss such signatures. We advocate a dedicated programme of searches for rich dark sectors that overcomes the assumptions on minimality and on the long lifetime of particles and encompasses a broader range of possibilities. Here, we discuss a prototype model that includes a complex structure akin to the SM: multiple generations of fermions charged under a new spontaneously-broken gauge symmetry.

Figures (6)

• Figure 1: An illustration of the portal approach to searching for dark sectors at the high-intensity frontier. We include renormalizable portals as well as the possibility of non-renormalizable ones.
• Figure 2: A few examples of experimental signatures, where dark blue lines represent some visible charged final states and dotted lines a neutrino or other invisible particles. For instance, in the top left diagram would represent decays such as $K^+ \to \mu^+ ( N\to \nu e^+e^-$). In rich dark sectors, the new particles often decay semi-visibly and are not targeted by simpler searches for visible or fully invisible resonances.
• Figure 3: An illustration of the impact of new forces on the signatures of a HNL at a neutrino experiment. From left to right, the new force strenght $G_X \sim g_X^2/M_X^2$ increases and the HNL lifetime decreases. In the presence of this new mediator, new production channels for HNLs open up, including new meson decays, but eventually, the new particles are short-lived enough that neutrino upscattering becomes the most promising avenue to search for these states.
• Figure 4: The parameter space of a HNL $\nu_h$ mixing with the muon flavor and coupled to a dark photon that kinematically mixes with the SM photon with $\chi \equiv \varepsilon / c_W = 10^{-4}$. On the left we vary the HNL mass for fixed $m_{Z^\prime} = 150$ MeV and on the right we vary the dark photon mass for a fixed $m_{\nu_h} = 300$ MeV. For NA62, we show the zero-background event rate sensitivity. Reproduced from Ballett:2019pyw.
• Figure 5: Slices of the parmeter space of semi-visible dark photons; on the left, we show the recast of existing limits on inelastic dark matter. On the right, we show the same, but for a model with three heavy neutral fermions (HNF), which may or may not mix with neutrinos. The various shades of blue in the BaBar recasted limits represent different assumptions on the veto thresholds of the original analysis. Reproduced from Abdullahi:2023tyk.