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

Abstract

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 80 $μ$eV and 800 $μ$eV taking advantage of the near-threshold enhancement. For the coupling $g_{\rm A}^{\rm e}g_{\rm A}^{\rm e}$, we expect an improvement on its upper bounds of 10 orders of magnitude at 800 $μ$eV. 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.

Figures (6)

• Figure 1: (a) Feynman diagram of the hypothesized exotic interactions. (b) Schematic diagram of the time-dependent exotic interaction. Two fermions in the standard model (orange and purple) interact with each other through exchanging new bosons beyond the standard model. Since the new bosonic field is driven periodically by the precessing fermion, a new bosonic wave emerges and spreads out (black).
• Figure 2: (a) The distance dependence of the exponential factor $e^{-r/\mu}$ at $t=0$ with $\gamma\omega = m_{\rm X}$ (blue dotted line) and $\gamma\omega = 0$ (red solid line). Here $m_{\rm X}$ is set to 2 meV, corresponding to a force range of $\lambda = 0.1$ mm. (b) An illustration of the concept of the near-threshold enhancement. At the static scenario, interacting fermions are limited to a thin layer with the thickness $\lambda$, while at the near-threshold scenario, all fermions within the distance $\mu$, which is orders of magnitude larger than $\lambda$, can contribute to the total interaction signal.
• Figure 3: Schematic diagram of the proposed experiment. (a) The spin source. YIG bulks (orange) are driven by a pure-tone microwave to generate an oscillating exotic interaction. They are cut into slices and equally spaced in a resonant cavity (gray), so that all spins contribute positively to the exotic interaction. (b) The driving system. A microwave is generated, amplified, and fed into the source cavity. (c) The spin sensor. YIG bulks (purple) in another cavity (gray) perform as the sensor. When the spins sense the exotic interaction, they will precess and radiate microwave photons. Both the spin source and the spin sensor are placed in a homogeneous magnetic field $B_{0}$, as the yellow area shows. (d) The readout system. The microwave signal from the sensor cavity is amplified, down-converted, and finally collected by a data acquisition card.
• Figure 4: Expected constraints on $g_{\rm A}^{\rm e}g_{\rm A}^{\rm e}$ as the driving frequencies are swept from 20 GHz to 200 GHz. At 800 $\mu$eV the expected constraint surpasses existing results by 10 orders of magnitude.
• Figure 5: The distance dependence of the exponential factor $e^{-r/\mu}$ at $t=0$ with $m_{\rm X} = 2\ \rm meV$ and (a) $\omega = 0$; (b) $0<\gamma\omega<m_{\rm X}$; (c) $\gamma\omega = m_{\rm X}$; (d) $\gamma\omega > m_{\rm X}$. The red solid lines refer to the real part while the blue dotted lines refer to the imaginary part.