Exotic-interaction searches on small scales via near-threshold enhancement

Exotic interactions between fermions mediated by new bosons beyond the Standard Model may hold the key to several fundamental conundrums on the frontier of physics. However, laboratory searching for exotic interactions on small length scales is fundamentally held back by the short force range. Here we propose that the force range of exotic interactions tends to infinity when the oscillation frequency of exotic interactions approaches the mass of the new bosons, i.e., the new bosons become nearly on-shell. This is named as the near-threshold enhancement. Through the near-threshold enhancement, even fermions at distances larger than the original force range can make considerable contributions to exotic interactions. Therefore, the size of the experimental apparatus can break the limitation of the force range, and thus both the signal of exotic interactions and the sensitivity of sensors can be greatly enhanced. We also propose a method to search for the exotic interactions in the mass range between <a:math xmlns:a="http://www.w3.org/1998/Math/MathML" display="inline"> <a:mrow> <a:mn>80</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> and <d:math xmlns:d="http://www.w3.org/1998/Math/MathML" display="inline"> <d:mrow> <d:mn>800</d:mn> <d:mtext> </d:mtext> <d:mtext> </d:mtext> <d:mi mathvariant="normal">μ</d:mi> <d:mi>eV</d:mi> </d:mrow> </d:math> taking advantage of the near-threshold enhancement. For the coupling <g:math xmlns:g="http://www.w3.org/1998/Math/MathML" display="inline"> <g:msubsup> <g:mi>g</g:mi> <g:mi mathvariant="normal">A</g:mi> <g:mi mathvariant="normal">e</g:mi> </g:msubsup> <g:msubsup> <g:mi>g</g:mi> <g:mi mathvariant="normal">A</g:mi> <g:mi mathvariant="normal">e</g:mi> </g:msubsup> </g:math> , we expect an improvement on its upper bounds of ten orders of magnitude at <m:math xmlns:m="http://www.w3.org/1998/Math/MathML" display="inline"> <m:mrow> <m:mn>800</m:mn> <m:mtext> </m:mtext> <m:mtext> </m:mtext> <m:mi mathvariant="normal">μ</m:mi> <m:mi>eV</m:mi> </m:mrow> </m:math> . This method can be further extended to enhance the search for other types of exotic interactions and boost the study of new physics beyond the Standard Model.