A search for the rare decay K L → π 0 νν was performed. With the data collected in 2015, corresponding to 2.2 × 10 19 protons on target, a single event sensitivity of ð1.30 AE 0.01 stat AE 0.14 syst Þ × 10 −9 was achieved and no candidate events were observed. We set an upper limit of 3.0 × 10 −9 for the branching fraction of K L → π 0 νν at the 90% confidence level (C.L.), which improved the previous limit by almost an order of magnitude. An upper limit for K L → π 0 X 0 was also set as 2.4 × 10 −9 at the 90% C.L., where X 0 is an invisible boson with a mass of 135 MeV=c 2 .

We introduce the Broadband Reflector Experiment for Axion Detection (BREAD) conceptual design and science program. This haloscope plans to search for bosonic dark matter across the ½10 −3 ; 1 eV ([0.24, 240] THz) mass range. BREAD proposes a cylindrical metal barrel to convert dark matter into photons, which a novel parabolic reflector design focuses onto a photosensor. This unique geometry enables enclosure in standard cryostats and high-field solenoids, overcoming limitations of current dish antennas. A pilot 0.7 m 2 barrel experiment planned at Fermilab is projected to surpass existing dark photon coupling constraints by over a decade with one-day runtime. Axion sensitivity requires < 10 −20 W= ffiffiffiffiffiffi Hz p sensor noise equivalent power with a 10 T solenoid and 10 m 2 barrel. We project BREAD sensitivity for various sensor technologies and discuss future prospects.

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Dedicated spectrometers for terahertz radiation with [0.3, 30] THz frequencies using traditional optomechanical interferometry are substantially less common than their infrared and microwave counterparts. This paper presents public documentation for the design and initial performance measurements of a tabletop Fourier transform spectrometer (FTS) for terahertz spectral analysis using infrared optics in a Michelson arrangement. This is coupled to a broadband pyroelectric photodetector designed for [0.1, 30] THz frequencies. We measure spectra of narrowband and broadband input radiation to characterize the performance of this instrument above 10 THz, where signal-to-noise is high. This device is constructed in the context of research-and-development for the recently-proposed Broadband Reflector Experiment for Axion Detection (BREAD), where the optical components can be applied to ongoing efforts for testing the pilot experiment.

We report the first search for the K L → π 0 γ decay, which is forbidden by Lorentz invariance, using the data from 2016 to 2018 at the J-PARC KOTO experiment. With a single event sensitivity of ð7.1 AE 0.3 stat AE 1.6 syst Þ × 10 −8 , no candidate event was observed in the signal region. The upper limit on the branching fraction was set to be 1.7 × 10 −7 at the 90% confidence level.