Enhancing the sensitivity to seesaw mechanism predictions in gauged <mml:math xmlns:mml="http://www.w3.org/1998/Math/MathML" display="inline"><mml:mi>B</mml:mi><mml:mo>−</mml:mo><mml:mi>L</mml:mi></mml:math> scenarios

New gauge bosons coupled to heavy neutral leptons (HNLs) are simple and well-motivated extensions of the Standard Model. In searches for HNLs in proton fixed-target experiments, we find that a large population of gauge bosons (<a:math xmlns:a="http://www.w3.org/1998/Math/MathML" display="inline"><a:msup><a:mi>Z</a:mi><a:mo>′</a:mo></a:msup></a:math>) produced by proton bremsstrahlung may decay to HNLs, leading to a significant improvement in existing bounds on the (<c:math xmlns:c="http://www.w3.org/1998/Math/MathML" display="inline"><c:mrow><c:msub><c:mrow><c:mi>m</c:mi></c:mrow><c:mrow><c:mi>HNL</c:mi></c:mrow></c:msub><c:mo>,</c:mo><c:msub><c:mrow><c:mi>U</c:mi></c:mrow><c:mrow><c:mi>α</c:mi></c:mrow></c:msub></c:mrow></c:math>), where <e:math xmlns:e="http://www.w3.org/1998/Math/MathML" display="inline"><e:msub><e:mi>U</e:mi><e:mi>α</e:mi></e:msub></e:math> represent the mixing between HNL and the active neutrinos with flavor <g:math xmlns:g="http://www.w3.org/1998/Math/MathML" display="inline"><g:mi>α</g:mi></g:math>. We study this possibility in fixed target experiments with the 8 GeV proton beams, including SBND, MicroBooNE, and ICARUS, as well as DUNE and DarkQuest at 120 GeV. We find the projected sensitivities to additional <i:math xmlns:i="http://www.w3.org/1998/Math/MathML" display="inline"><i:msup><i:mi>Z</i:mi><i:mo>′</i:mo></i:msup></i:math>-mediated HNL production can bring the seesaw mechanism of the neutrino masses within a broadened experimental reach.