Axion and dark fermion electromagnetic form factors in superfluid <mml:math xmlns:mml="http://www.w3.org/1998/Math/MathML" display="inline"><mml:mrow><mml:mmultiscripts><mml:mrow><mml:mi>He</mml:mi></mml:mrow><mml:mprescripts/><mml:none/><mml:mn>4</m

Condensed matter materials have shown great potential in searching for light dark matter (DM) via detecting the phonon or magnon signals induced by the scattering of DM off the materials. In this paper, we study the possibility of detecting electromagnetic form factors of fermionic DM and axionlike particles (ALPs) using superfluid <a:math xmlns:a="http://www.w3.org/1998/Math/MathML" display="inline"><a:mrow><a:mmultiscripts><a:mrow><a:mi>He</a:mi></a:mrow><a:mprescripts/><a:none/><a:mrow><a:mn>4</a:mn></a:mrow></a:mmultiscripts></a:mrow></a:math>. The phonon induced by a sub-GeV fermionic DM scattering off the superfluid can be described using the effective field theory with the interaction between DM and the bulk <c:math xmlns:c="http://www.w3.org/1998/Math/MathML" display="inline"><c:mrow><c:mmultiscripts><c:mrow><c:mi>He</c:mi></c:mrow><c:mprescripts/><c:none/><c:mrow><c:mn>4</c:mn></c:mrow></c:mmultiscripts></c:mrow></c:math>. Signals arising from the electromagnetic form factors of light DM in the presence of an external electric field are calculated. Projected constraints on the charge radius, anapole moment, and magnetic moment of the DM are derived with <e:math xmlns:e="http://www.w3.org/1998/Math/MathML" display="inline"><e:mrow><e:mn>1</e:mn><e:mtext> </e:mtext><e:mtext> </e:mtext><e:mi>kg</e:mi><e:mo>·</e:mo><e:mi>yr</e:mi></e:mrow></e:math> exposure. The phonon signal induced by the scattering of ALPs off the superfluid is also calculated, which can put competitive and the first direct detection bounds on ALP-photon-dark photon couplings in the projected experiments. Published by the American Physical Society 2024