Stopping Dark Mesons in Their Tracks with Long-Lived Particle and Resonant Signatures

Abstract

Dark sectors with confining gauge interactions can provide both simple dark matter candidates and striking signals at colliders. We recast Large Hadron Collider searches for two different signatures of dark mesons that arise from a strongly-coupled theory with vector-like dark quarks that are in some non-trivial representation of Standard Model SU(2)$_L$. For any such electroweak representation, there is a 3-plet of dark mesons whose charged components are long-lived, and we reinterpret searches for disappearing tracks to place a lower bound on their mass of $\sim 1.2$ TeV. When the dark quarks are in SU(2)$_L$ representations larger than the fundamental, there is also a 5-plet of dark mesons that interacts with the electroweak gauge bosons via a chiral anomaly. We show that the 5-plet is the unique non-trivial meson multiplet with this anomaly and recast searches for the resulting diboson resonances to place bounds on model parameters. With additional measurements, the anomaly also enables one to reconstruct some ultraviolet parameters (the numbers of dark flavors and colors) while only measuring states in the infrared. Each of these signals represents an exciting opportunity for future searches using higher luminosity.

Figures (10)

• Figure 1: Feynman diagram corresponding to the anomaly-induced resonant production of the $\hat{\eta}_5$. The $\otimes$ symbols denote insertions of the anomaly operator in \ref{['eq:5pletAnomaly']}. The $\hat{\eta}_5$ species are the unique pNGBs of dark chiral symmetry breaking with this operator. Observation of this signal provides sensitivity to UV parameters such as $N_c$ and $N_f$, which set the strength of the coupling (see \ref{['eq:5pletAnomaly', 'eq:c_value']}). LHC implications are discussed in \ref{['sec:resonances']}.
• Figure 2: Diagrams depicting decay of the $\hat{\pi}_3^\pm$ via cascade (left), $\hat{\eta}_5$ via the anomaly (middle), and higher multiplets via hopping (right). For the $\hat{\pi}_3^\pm$, the radiated off-shell $W$ boson effectively mixes with the SM pion. For the $\hat{\eta}_5$, the decay products are pairs of electroweak bosons, and the $\otimes$ denotes insertion of the anomaly operator discussed in \ref{['sec:anomaly']}. For the higher multiplets, the meson hops down to a lower multiplet of the same $G$-parity by emitting two electroweak bosons, and the blob denotes insertion of the loop-induced vertex shown in \ref{['sec:mass']}.
• Figure 3: Diagrams representing pair production of the $\hat{\pi}_3$ mesons via Drell-Yan (top) and vector boson fusion (bottom). The VBF processes also get contributions from diagrams with three-point vertices coupling the mesons to vector bosons. Diagrams with production of two neutral mesons via VBF are not pictured, as we do not consider such processes.
• Figure 4: Total LHC production cross sections of the dark meson triplet $\hat{\pi}_3$ pairs we consider. The production rate for exclusively Drell-Yan processes falls much faster as a function of the $\hat{\pi}_3$ mass $m_\pi$ than the rate of processes that include vector boson fusion.
• Figure 5: The visible cross section for production of disappearing tracks due to decays of the long-lived $\hat{\pi}_3^\pm$ meson. The dashed lines indicate model-independent upper bounds on this cross section based on the ATLAS searches in Refs. ATLAS:2017oalATLAS:2022rme and using the re-interpretation methods provided by Ref. Belyaev:2020wok. This shows a strong lower bound on the $\hat{\pi}_3$ meson mass $m_\pi$ of $\sim\!1.2\,\text{TeV}$.