New limit on dark photon kinetic mixing in the 0.2–1.2 μeV mass range from the Dark E-field Radio experiment

We report new limits on the kinetic mixing strength of the dark photon spanning the mass range <a:math xmlns:a="http://www.w3.org/1998/Math/MathML" display="inline"><a:mrow><a:mn>0.21</a:mn><a:mo>−</a:mo><a:mn>1.24</a:mn><a:mtext> </a:mtext><a:mtext> </a:mtext><a:mi mathvariant="normal">μ</a:mi><a:mi>eV</a:mi></a:mrow></a:math> corresponding to a frequency span of 50–300 MHz. The Dark E-field Radio experiment is a wideband search for dark photon dark matter. In this paper we detail changes in calibration and upgrades since our proof-of-concept pilot run. Our detector employs a wide-bandwidth E-field antenna moved to multiple positions in a shielded room, a low noise amplifier, wideband analog-to-digital converter, followed by a <d:math xmlns:d="http://www.w3.org/1998/Math/MathML" display="inline"><d:msup><d:mn>2</d:mn><d:mn>24</d:mn></d:msup></d:math>-point fast Fourier transform. An optimal filter searches for signals with <f:math xmlns:f="http://www.w3.org/1998/Math/MathML" display="inline"><f:mrow><f:mi mathvariant="normal">Q</f:mi><f:mo stretchy="false">≈</f:mo><f:msup><f:mn>10</f:mn><f:mn>6</f:mn></f:msup></f:mrow></f:math>. In nine days of integration, this system is capable of detecting dark photon signals corresponding to a kinetic mixing strength <j:math xmlns:j="http://www.w3.org/1998/Math/MathML" display="inline"><j:mi>ε</j:mi></j:math> several orders of magnitude lower than previous limits. We find a 95% exclusion limit on <l:math xmlns:l="http://www.w3.org/1998/Math/MathML" display="inline"><l:mi>ε</l:mi></l:math> over this mass range between <n:math xmlns:n="http://www.w3.org/1998/Math/MathML" display="inline"><n:mn>6</n:mn><n:mo>×</n:mo><n:msup><n:mn>10</n:mn><n:mrow><n:mo>−</n:mo><n:mn>15</n:mn></n:mrow></n:msup></n:math> and <p:math xmlns:p="http://www.w3.org/1998/Math/MathML" display="inline"><p:mn>6</p:mn><p:mo>×</p:mo><p:msup><p:mn>10</p:mn><p:mrow><p:mo>−</p:mo><p:mn>13</p:mn></p:mrow></p:msup></p:math>, tracking the complex resonant mode structure in the shielded room. Published by the American Physical Society 2024