Peering lnto the Dark: A Search for Dark Matter in Opportune Places
Benjamin Knepper stands with GigaBREAD members from the University of Chicago and Fermilab in front of the solenoid magnet at Argonne National Laboratory, where the experiment collected axion dark matter data.
Bejamin Knepper.
How can you observe something that you cannot see? Particle physicists must contend with this question when researching dark matter. Experimentally observing dark matter is essential to a better understanding of the structure of the cosmos, as it constitutes 85% of the total mass content of the universe. However, dark matter is called “dark” precisely because it does not interact with light, making it extremely difficult to detect, despite existing all around us.
One promising dark matter particle candidate is called the axion, which—if it exists—likely has a mass less than a single electronvolt! The discovery of this elusive particle could help explain large-scale phenomena such as the formation of galaxies and a key issue in nuclear physics known as the strong CP problem. As such, many experiments are searching for it, including the Broadband Reflector Experiment for Axion Detection (BREAD).
BREAD acts like a telescope for dark matter: inside of a magnetic field, cosmic axions flying by a metal cylinder would excite electromagnetic waves of a corresponding frequency on the surface of the cylinder. These photons would be emitted toward the cylinder’s center and redirected by a coaxial parabolic reflector up to a single focal spot, where they would be detected by a photosensor. BREAD’s cylinder is about the size of a wine barrel. Its geometry means it can fit inside of a large solenoid magnet (the kind used for MRIs).
Following my second year of university, I embarked on a gap year in order to pursue full-time physics research with BREAD. My research experience began through a Science Undergraduate Laboratory Internship (SULI) at Fermi National Accelerator Laboratory (Fermilab) in January 2023, where I worked under Cosmic Physics Center director Andrew Sonnenschein and postdoc Stefan Knirck (now a Harvard University professor). There, I performed simulations to improve the experimental detection efficiency of InfraBREAD (the version of BREAD looking for infrared frequency axions), with particle accelerators and a herd of bison as my backdrop.
A ray-tracing simulation of BREAD. Axion-generated photos would enter the detector perpendicular to the cylinder’s side walls. As they travel toward the center, a parabolic reflector would redirect them to its focal point for detection.
At this frequency scale, photons are emitted incoherently inside the experimental apparatus, which results in a smeared focal spot beyond the area of the photon detector. I quantified the severity of this effect through geometric ray-tracing simulations and designed a novel configuration of optical components to refocus the smeared signal that could increase the signal efficiency maximally by 55%.(1)
Motivated to continue working on BREAD, I was then hired through the summer by the University of Chicago Enrico Fermi Institute to work in Professor David Miller’s experimental particle physics group and help conduct the first-ever dark matter search by GigaBREAD (the version looking for gigahertz-frequency axions). I spent many late evenings with fellow students in a radio-frequency-sealed basement taking calibration measurements and analyzing data. After many Python scripts, signal processing, cross-checks, worries, and cheers later, this work eventually led to the most sensitive result of a dish antenna experiment excluding dark photons in a 10.7–12.5 GHz range.(2)
The BREAD experiment continues to rapidly grow and advance, with a collaboration of physicists now spanning Fermilab, Argonne National Lab, SLAC National Accelerator Lab, Lawrence Livermore National Lab, the University of Chicago, University of California, Berkeley, and Harvard University, among others. Just recently, GigaBREAD published its first axion dark matter results.3 There is much exciting scientific work to be done and many opportunities for discovery building off these results in the future!
Through this transformative research, I discovered a passion for areas of physics I had not even known existed—in particular, quantum sensing, and more broadly, the intersection of quantum information science with high-energy physics. BREAD relies on precise measurements of particles, which are enabled and enhanced by quantum sensors that can detect, for instance, single photons. Many discoveries in the next few decades will likely emerge from quantum measurements.
The GigaBREAD cylinder in a radio frequency shielded chamber at the University of Chicago.
Moreover, this experience opened up exciting and unexpected opportunities. I have since transferred to UC Berkeley, where I am working at Lawrence Berkeley National Lab on both theoretical and experimental quantum measurement projects.
Given the relatively small collaboration size of BREAD, I also had the opportunity to work directly with incredibly hardworking and motivated students and professional physicists, in both national lab and university research settings. I acquired fundamental research skills that cannot be obtained from solving problem sets: learning how to write a scientific paper, presenting the status of my research to fellow physicists, and collaborating with colleagues to problem solve when a code or measurement does not go as expected.
When I decided to embark on the gap year that generated these experiences, I did not know where it would lead. A few people even discouraged me from this path. However, taking advantage of an academic opportunity often requires a leap of faith and trusting your own drive. There is great excitement that comes with peering into the dark—for therein is the possibility of discovering something that you cannot yet see!
References
1. B. Knepper, A. Sonnenschein, and S. Knirck, “Focusing Optics for Axion Detection: Simulating Enhancements of Photons in InfraBREAD” (US Department of Energy Office of Science and Technical Information, 2024), osti.gov/biblio/2377355.
2. S. Knirck, G. Hoshino, M. H. Awida, et al. (BREAD Collaboration), “First Results from a Broadband Search for Dark Photon Dark Matter in the 44 to 52 μeV Range with a Coaxial Dish Antenna,” Phys. Rev. Lett., 132 (2024): 131004, doi.org/10.1103/PhysRevLett.132.131004.
3. G. Hoshino, S. Knirck, M. H. Awida, et al. (GigaBREAD Collaboration). “First Axion-Like Particle Results from a Broadband Search for Wave-Like Dark Matter in the 44 to 52 μeV Range with a Coaxial Dish Antenna,” arXiv:2501.17119 (2024), arxiv.org/abs/2501.17119.
