Search by the SENSEI Experiment for Millicharged Particles Produced in the NuMI Beam.

Millicharged particles appear in several extensions of the standard model, but have not yet been detected. These hypothetical particles could be produced by an intense proton beam striking a fixed target. We use data collected in 2020 by the SENSEI experiment in the MINOS cavern at the Fermi National Accelerator Laboratory to search for ultrarelativistic millicharged particles produced in collisions of protons in the NuMI beam with a fixed graphite target. The absence of any ionization events with 3 to 6 electrons in the SENSEI data allow us to place world-leading constraints on millicharged particles for masses between 30 to 380 MeV. This work also demonstrates the potential of utilizing low-threshold detectors to investigate new particles in beam-dump experiments, and motivates a future experiment designed specifically for this purpose.

SENSEI: Direct-Detection Results on sub-GeV Dark Matter from a New Skipper CCD.

We present the first direct-detection search for sub-GeV dark matter using a new ∼2-gram high-resistivity Skipper CCD from a dedicated fabrication batch that was optimized for dark matter searches. Using 24 days of data acquired in the MINOS cavern at the Fermi National Accelerator Laboratory, we measure the lowest rates in silicon detectors of events containing one, two, three, or four electrons, and achieve world-leading sensitivity for a large range of sub-GeV dark matter masses. Data taken with different thicknesses of the detector shield suggest a correlation between the rate of high-energy tracks and the rate of single-electron events previously classified as "dark current." We detail key characteristics of the new Skipper CCDs, which augur well for the planned construction of the ∼100-gram SENSEI experiment at SNOLAB.

SENSEI: Direct-Detection Constraints on Sub-GeV Dark Matter from a Shallow Underground Run Using a Prototype Skipper CCD.

We present new direct-detection constraints on eV-to-GeV dark matter interacting with electrons using a prototype detector of the Sub-Electron-Noise Skipper-CCD Experimental Instrument. The results are based on data taken in the MINOS cavern at the Fermi National Accelerator Laboratory. We focus on data obtained with two distinct readout strategies. For the first strategy, we read out the Skipper CCD continuously, accumulating an exposure of 0.177 g day. While we observe no events containing three or more electrons, we find a large one- and two-electron background event rate, which we attribute to spurious events induced by the amplifier in the Skipper-CCD readout stage. For the second strategy, we take five sets of data in which we switch off all amplifiers while exposing the Skipper CCD for 120 ks, and then read out the data through the best prototype amplifier. We find a one-electron event rate of (3.51±0.10)×10^{-3} events/pixel/day, which is almost 2 orders of magnitude lower than the one-electron event rate observed in the continuous-readout data, and a two-electron event rate of (3.18_{-0.55}^{+0.86})×10^{-5} events/pixel/day. We again observe no events containing three or more electrons, for an exposure of 0.069 g day. We use these data to derive world-leading constraints on dark matter-electron scattering for masses between 500 keV and 5 MeV, and on dark-photon dark matter being absorbed by electrons for a range of masses below 12.4 eV.

Constraints on axion-like dark matter from a SERF comagnetometer.

Ultralight axion-like particles are well-motivated relics that might compose the cosmological dark matter and source anomalous time-dependent magnetic fields. We report on terrestrial bounds from the Noble And Alkali Spin Detectors for Ultralight Coherent darK matter (NASDUCK) collaboration on the coupling of axion-like particles to neutrons and protons. The detector uses nuclei of noble-gas and alkali-metal atoms and operates in the Spin-Exchange Relaxation-Free (SERF) regime, achieving high sensitivity to axion-like dark matter fields. Conducting a month-long search, we cover the mass range of 1.4 × 10-12 eV/c2 to 2 × 10-10 eV/c2 and provide limits which supersede robust astrophysical bounds, and improve upon previous terrestrial constraints by over two orders of magnitude for many masses within this range for protons, and up to two orders of magnitude for neutrons. These are the sole reliable terrestrial bounds reported on the coupling of protons with axion-like dark matter, covering an unexplored terrain in its parameter space.

New constraints on axion-like dark matter using a Floquet quantum detector.

Dark matter is one of the greatest mysteries in physics. It interacts via gravity and composes most of our universe, but its elementary composition is unknown. We search for nongravitational interactions of axion-like dark matter with atomic spins using a precision quantum detector. The detector is composed of spin-polarized xenon gas that can coherently interact with a background dark matter field as it traverses through the galactic dark matter halo. Conducting a 5-month-long search, we report on the first results of the Noble and Alkali Spin Detectors for Ultralight Coherent darK matter (NASDUCK) collaboration. We limit ALP-neutron interactions in the mass range of 4 × 10-15 to 4 × 10-12 eV/c2 and improve upon previous terrestrial bounds by up to 1000-fold for masses above 4 × 10-13 eV/c2. We also set bounds on pseudoscalar dark matter models with quadratic coupling.