Probing Dark Forces and Light Hidden Sectors at Low-Energy e+e- Colliders

TL;DR

The paper investigates the potential for low-energy $e^+e^-$ colliders to probe light dark sectors with gauge group $G_D$ that are Higgsed or confining at the GeV scale and coupled to the SM through kinetic mixing of a dark $U(1)_D$ with hypercharge. By linking DAMA/LIBRA and other dark-matter hints to a concrete cross-section normalization, it provides quantitative expectations for dark-sector production at B-factories via off-shell $A'$ and on-shell $A'$ with a photon, predicting ${\cal O}$(fb) to ${\cal O}(10^{2})$ events for plausible parameters. The work then systematically analyzes the decays of Higgsed and confined dark sectors, detailing metastable states, lifetimes, and event topologies ranging from prompt multi-lepton signatures to displaced decays and invisible final states, including the impact of additional mixing interactions. Finally, it proposes a comprehensive set of collider search strategies—spanning exclusive $4\ell$ and higher-multiplicity channels, displaced-vertex cuts, high track multiplicities, and $\gamma$-tagged missing energy—to exploit existing BaBar/BELLE data and uncover or constrain such dark sectors with realistic couplings and masses.

Abstract

A dark sector -- a new non-Abelian gauge group Higgsed or confined near the GeV scale -- can be spectacularly probed in low-energy e+e- collisions. A low-mass dark sector can explain the annual modulation signal reported by DAMA/LIBRA and the PAMELA, ATIC, and INTEGRAL observations by generating small mass splittings and new interactions for weak-scale dark matter. Some of these observations may be the first signs of a low-mass dark sector that collider searches can definitively confirm. Production and decay of O(GeV)-mass dark states is mediated by a Higgsed Abelian gauge boson that mixes kinetically with hypercharge. Existing data from BaBar, BELLE, CLEO-c, and KLOE may contain thousands of striking dark-sector events with a high multiplicity of leptons that reconstruct mass resonances and possibly displaced vertices. We discuss the production and decay phenomenology of Higgsed and confined dark sectors and propose e+e- collider search strategies. We also use the DAMA/LIBRA signal to estimate the production cross-sections and decay lifetimes for dark-sector states.

Figures (19)

• Figure 1: We consider a dark sector with non-Abelian gauge group $G_D$, which is Higgsed or confined at $\mathcal{O}(\hbox{MeV}-10~\hbox{GeV})$. We assume that $G_D$ contains a Higgsed Abelian factor $U(1)_D$, so that the dark sector interacts with Standard Model matter through kinetic mixing of hypercharge with the $U(1)_D$ gauge boson $A'$, of mass in the same range. Either the Higgsed or confined phases of $G_D$ necessarily include new states that can be produced through $A'$ interactions.
• Figure 2: Left: Cartoon of an event in which non-Abelian gauge bosons in a Higgsed dark sector are produced through an off-shell $A'$: the gauge bosons may decay into other dark bosons (or fermions if present), which in turn decay to light Standard Model fermions. Right: a similar final state recoils off a hard photon in $A'+\gamma$ radiative return production.
• Figure 3: Left: Cartoon of an event in which quarks in a simple confined dark sector are produced through an off-shell $A'$. The quarks shower and hadronize into mesons, which decay into Standard Model particles. Final states frequently contain many leptons, but can also include hadrons and long-lived dark states that escape the detector unobserved. Right: Phase space structure of different kinds of events. An off-shell $A'$ produces jet-like structure if $\Lambda_D \ll \sqrt{s}$ (top), and approximately spherical final states if $\Lambda_D \lesssim \sqrt{s}$ (middle). In $A'/\gamma$ radiative return production, the dark-sector final state recoils against a hard photon.
• Figure 4: Production modes of light dark-sector particles $X\bar{X}$ at B-factories. Left: Production through an off-shell $A'$. Right: Production of an on-shell $A'$ and a photon --- we assume the $A'$ subsequently decays into lighter dark-sector particles.
• Figure 5: Left: Inclusive cross-section at the B-factories $(E_{\rm cm}=10.58\hbox{GeV})$ for production of dark-sector states, $X$, via an off-shell $A'$ as a function of $\alpha_D\epsilon^2$ and $m_{A'}$, for $m_X=1$ GeV, $N_f=1$, $N_c=3$, and unit charges $q_i$ (equation \ref{['eq:off-shell']}). Note that the cross-section scales linearly with the number of dark flavors $N_f$ and dark colors $N_c$. Right: Cross-section at the B-factories for production of an on-shell $A'$ and a photon as a function of $\epsilon^2$ and $m_{A'}$ (equation \ref{['eq:onshellXsectionnoapprox']}). Black lines correspond to fixed cross-sections. Also shown are the constraints on the couplings of a new $U(1)_D$ mixing with hypercharge from measurements of the muon anomalous magnetic dipole moment (shaded regions) Pospelov:2008zw. The green dotted lines correspond to the lower bounds on the range of couplings that could be probed by a search at LEPI for rare $Z$-decays to various exotic final states, assuming that branching ratios as low as $10^{-5}$ can be probed. More details are given in Appendix \ref{['app:Contraints']}.