Searching for millicharged particles with superconducting radio-frequency cavities

We demonstrate that superconducting radio-frequency cavities can be used to create and detect millicharged particles and are capable of extending the reach to couplings several orders of magnitude beyond other laboratory-based constraints. Millicharged particles are Schwinger pair produced in driven cavities and quickly accelerated out of the cavity by the large electric fields. The electric current generated by these particles is detected by a receiver cavity. A light-shining-through-walls experiment may only need to reanalyze future data to provide new constraints on millicharged particles.

Figure 1

A cartoon picture of our setup. The large electric fields in the emitter cavity produce millicharged particles via Schwinger pair production. The electric field of the cavity is arranged such that the particles are accelerated toward the shielded receiver cavity, where the current of millicharged particles excites the resonant modes to detectable levels.

Figure 2

The projected reach of future cavity experiments (shaded blue) to millicharged fermions for various volumes of the emitter/receiver cavities, $V_{\mathrm{cav}}$ (the reach for millicharged scalars is weaker by a factor of $2^{1/3}$). In each case, we take the amplitude of the driven cavity field to be $E_{\mathrm{em}}=50\mathrm{MV}{\mathrm{m}}^{-1}$, the quality factor to be $Q={10}^{12}$, the receiver cavity temperature to be $T=10\mathrm{mK}$, and the integration time to be $t_{\mathrm{int}}=\mathrm{year}$. We take the emitter and receiver cavities to both be cylinders of equal radius and length, such that the resonant frequency of the ${\mathrm{TM}}_{010}$ mode is fixed to be $\omega ={\alpha }_{01}{(\pi /V_{\mathrm{cav}})}^{1/3}$, where ${\alpha }_{01}$ is the first zero of $J_0$. The shaded gray region corresponds to the best existing laboratory bound from the PVLAS Collaboration [26, 27]. Above the solid red line, the amplitude of the driven emitter cavity’s electric field is larger than the critical field strength for Schwinger pair production of millicharged particles. Not shown are astrophysical and cosmological limits derived from considerations of, e.g., stellar cooling, SN1987A, big bang nucleosynthesis, and the cosmic microwave background [28]. Models in which these constraints are mitigated are discussed in Appendix pp3.