Searching for millicharged particles

Ever since the oil-drop experiment to measure the electron charge and, later, the discovery of the quark, scientists have postulated electric charge to come in discrete units, and the minimal electric charge has been believed to be carried by quarks. Yet theories still postulate that particles can carry much smaller charges — significantly smaller than that of quarks.

Scientists, including Fermilab researchers, have proposed a new experiment to help search for these “millicharged particles.” The proposal is inspired by analyses based on results from several neutrino experiments. The potential discovery would shatter the current Standard Model paradigm and open a window to new physics.

The new proposed experiment is called FerMINI, which has the ability to search for millicharged particles in the MeV/c2 to few GeV/c2 mass range.

This illustration shows how millicharged particles (labeled Χ) can be produced in a proton beam facility and detected by various different detectors, including MiniBooNE, MicroBooNE, DUNE and FerMINI. Image courtesy of the DUNE, FerMINI, MicroBooNE, MilliQan, MiniBooNE, and MINOS collaborations

FerMINI builds on previous analyses. A group of theoretical physicists showed that data from neutrino experiments MiniBooNE at Fermilab, the Liquid Scintillator Neutrino Detector at Los Alamos National Laboratory, and Super-Kamiokande Observatory in Japan limits the possible range of mass and electric charge that millicharged particles can have. Their findings narrow the region where scientists should look for millicharged particles. Independent and detailed millicharge analyses were studied for the ArgoNeuT neutrino experiment and conducted by the ArgoNeuT collaboration.

The search could extend beyond MiniBooNE and LSND to other Fermilab neutrino experiments, including MicroBooNE and the Short-Baseline Near Detector. Further, experiments such as the international, Fermilab-hosted Deep Underground Neutrino Experiment, or DUNE, and CERN’s proposed experiment, the Search for Hidden Particles, called SHiP, have the potential to discover millicharged particles in mass ranges that have yet to be experimentally tested. This research may have implications for their detector designs and analysis techniques.

The FerMINI detector can sense millicharged particles produced in the Fermilab proton beam when it hits a fixed target. It detects multiple scintillation hits in a small time window as the millicharged-particle signature. The detector technology is inspired by the milliQan experiment, a proposed search at the Large Hadron Collider at CERN, some of whose collaborators are also involved in the FerMINI project.

This shows an example of a simulation of a background event for the demonstrator of the millicharged particle detector. Image: milliQan collaboration

The search could potentially help explain the nature of dark matter, as the hypothetical particle could contribute to a fraction of the universe’s dark matter abundance. For example, scientists on the Experiment to Detect the Global EoR Signature, or EDGES, recently reported an anomaly in the 21-centimeter hydrogen absorption spectrum from the early universe. The discovery of millicharged particles as a fraction of dark matter might explain the anomaly.

This type of fractional dark matter candidate, with sizable coupling to Standard Model particles, would be hard for underground direct-detection experiments to detect, because the dark matter particles would lose their kinetic energy through their interaction with Earth’s atmosphere and crust before they reach the underground detectors. The Fermilab experiments thus have advantages in detecting such particles since they can directly produce these particles from the proton beam with a high energy.

We now know where we can look in searching for these millicharged particles, given available capabilities. By combining detector technology with existing and planned high-intensity proton beams provided by Fermilab, we can advance our search for these mysterious particles, overturning our understanding of the structure of nature’s fundamental constituents.

The FerMINI collaboration, based at Fermilab, comprises 10 institutions.

Yu-Dai Tsai is a postdoctoral researcher at Fermilab and the University of Chicago.

This work is supported by the DOE Office of Science.

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