Scalar-magnetometer search for ultralight dark photon dark matter with a single-site, two-sensor array: A 6-channel discrete-time Fourier transform likelihood analysis with scalar optically pumped magnetometers

We report a laboratory search for ultralight dark photon dark matter using a , array of commercial scalar optically pumped magnetometers. In the low-frequency regime where the Earth–ionosphere system acts as an electromagnetic transducer, the expected magnetic signal is a narrow-band triplet of frequencies. This signature consists of a central peak at the dark photon’s Compton frequency, accompanied by two sidebands shifted by Earth’s sidereal rotation frequency. Because scalar magnetometers measure field magnitude, the observable signal is the projection of the oscillating dark photon magnetic field onto the direction of the large, local geomagnetic field. This preserves the crucial triplet signature in the resulting time series data. Analyzing 10.5 h of continuous data, we construct a six-channel complex data vector by evaluating the discrete-time Fourier transform for both sensors directly at the three physical frequencies of the signal triplet. Assuming complex-Gaussian noise, we develop a likelihood framework to set robust, frequency-resolved upper limits on the kinetic-mixing parameter <a:math xmlns:a="http://www.w3.org/1998/Math/MathML" display="inline"> <a:mrow> <a:mi>ϵ</a:mi> </a:mrow> </a:math> , which governs the coupling between Standard Model photons and dark photons. Within the mass range <c:math xmlns:c="http://www.w3.org/1998/Math/MathML" display="inline"> <c:mn>4</c:mn> <c:mo>×</c:mo> <c:msup> <c:mn>10</c:mn> <c:mrow> <c:mo>−</c:mo> <c:mn>15</c:mn> </c:mrow> </c:msup> <c:mtext> </c:mtext> <c:mtext> </c:mtext> <c:mi>eV</c:mi> <c:mo>≲</c:mo> <c:msub> <c:mi>m</c:mi> <c:msup> <c:mi>A</c:mi> <c:mo>′</c:mo> </c:msup> </c:msub> <c:mo>≲</c:mo> <c:mn>3</c:mn> <c:mo>×</c:mo> <c:msup> <c:mn>10</c:mn> <c:mrow> <c:mo>−</c:mo> <c:mn>14</c:mn> </c:mrow> </c:msup> <c:mtext> </c:mtext> <c:mtext> </c:mtext> <c:mi>eV</c:mi> </c:math> , we obtain the most stringent direct laboratory limits to date on the kinetic-mixing parameter, which are complementary to existing astrophysical bounds, including those inferred from observations of the Leo T dwarf galaxy.