“Is it possible that the universe’s most mysterious substance could finally leave a visible trace?” For decades, this question has been at the center of particle physics. Now, it is being framed in a new way using a novel theoretical and experimental framework: internal pair production as a direct detection path for dark matter.
Dark matter, estimated to comprise 80% of the universe’s total mass, has persistently evaded direct observation. Traditional detection strategies have focused on searching for weak interactions with nuclei, but for light dark matter candidates, transferring enough energy to produce a detectable nuclear recoil is kinematically prohibitive. As Bhaskar Dutta and colleagues at Texas A&M University explain, The particle nature of DM can be revealed when a DM particle scatters off a nucleus and produces a visible recoil signal. However, for light DM, transferring sufficient energy to a heavy nucleus is kinematically challenging, even if the DM is energetic. To overcome this limitation, we developed a framework where additional particles are produced in the final state, allowing the DM’s energy to be shared among them, while the nucleus remains largely at rest.
The innovation here is to use internal pair production, a phenomenon based on quantum electrodynamics. Here, in the scattering of an energetic dark matter particle off of a nucleus, it may swap a virtual photon, causing emission of a lepton-antilepton pair most importantly, electron-positron or muon-antimuon pairs. Such a process, similar to the neutrino trident process, enables the energy of the dark matter to be shared between the outgoing leptons, evading nuclear recoil limits and producing energetic, observable signatures distinguishable from background events. As described in recent theory work, “When energetic dark matter scatters in a material, it can create a lepton-antilepton pair by exchanging a virtual photon with the nucleus, akin to the neutrino trident process. We demonstrate this process for dark matter coupled to dark photons in experiments such as DarkQuest, SBND, and DUNE ND.”
This detection approach is especially attractive to short-baseline neutrino facilities like the Fermilab and the future DUNE experiment. In these settings, high-intensity proton-target collisions produce substantial fluxes of both neutrinos and candidate dark matter particles. The challenge has been separating unusual dark matter interactions from the ubiquitous background neutrino population. The Dutta group’s strategy takes advantage of the characteristic kinematic and topological signatures of muon and electron pairs that are created through IPP. “These energetic final states provide distinctive signatures to separate dark matter signals from neutrino backgrounds and offer new ways to probe the underlying dark matter models,” the authors say.
The experimental implementation of this technique is inextricably linked with developments in detector technology, most notably the liquid argon time projection chamber. These detectors, which are now at the core of experiments such as DUNE and ICARUS, provide high-resolution three-dimensional reconstruction of particle tracks reconstruction and superior calorimetry. When lepton pair production occurs in the active volume of the detector, the resultant ionization and scintillation are measurable to high precision, allowing for stable event tagging and background rejection. It is indeed crucial to establish the IPP signature that the LArTPC can disentangle overlapping events and reconstruct the spatial and energy distribution of lepton pairs.
Beyond accelerator-based experiments, the internal pair production mechanism opens new frontiers to astrophysical dark matter searches. Large-volume neutrino observatories may be triggered by energetic dark matter particles originating from galactic sources or astrophysical events, such as blazars and supernovae. These huge detection volumes and multi-GeV energy sensitivity make facilities like Hyper-Kamiokande, JUNO, and IceCube well suited to search for these exotic signals. Recent research has highlighted that core-collapse supernovae can be powerful sources of dark mattercore-collapse supernovae may act as intense sources of boosted dark matter, boosted to high energies and can generate relativistic particles detectable through electron or nuclear interactions in these observatories. The combination of neutrino and dark matter detection in these environments not only maximizes discovery potential but also offers a multi-messenger method of investigating the foundations of physics.
At the theoretical level, the quantum electrodynamics basis of IPP guarantees that the process is thoroughly understood and computationally calculable, while experimental realization takes advantage of several decades of advancements in detector design and data analysis. The framework of Dutta et al. is versatile, allowing for a broad range of dark matter models such as those involving dark photons or leptophilic couplings to be probed. As they explain, “We show that DIPP is highly effective at probing various dark matter models, particularly at DUNE ND and DarkQuest, by searching for electron-positron and muon-antimuon signatures.”
The confluence of creative theory, state-of-the-art detector technology, and the increasing reach of neutrino and astrophysical observatories represents a turning point in the quest for dark matter. By distributing the subtle particle’s energy among leptons via internal pair production, scientists are opening a new door a door that perhaps will finally shed light on the universe’s darkest mysteries.
