Accelerator Cavities as a Probe of Millicharged Particles

Abstract

We investigate Schwinger pair production of millicharged fermions in the strong electric field of cavities used for particle accelerators. Even without a direct detection mechanism at hand, millicharged particles, if they exist, contribute to the energy loss of the cavity and thus leave an imprint on the cavity's quality factor. Already conservative estimates substantially constrain the electric charge of these hypothetical particles; the resulting bounds are competitive with the currently best laboratory bounds which arise from experiments based on polarized laser light propagating in a magnetic field. We propose an experimental setup for measuring the electric current comprised of the millicharged particles produced in the cavity.

Figures (2)

• Figure 1: Laboratory limits on the fractional electric charge $\epsilon\equiv Q_\epsilon/e$ of a millicharged fermion of mass $m_\epsilon$. The "Orthopositronium" limit stems from a limit on the branching fraction of invisible orthopositronium decay Mitsui:1993ha. The green "BFRT" upper limits arise Gies:2006ca from the upper limit on vacuum magnetic dichroism and birefringence placed by the laser polarization experiment BFRT Cameron:1993mr. The red (thin solid) line corresponds to the (too) naive bound obtained from Eq. \ref{['sens_est']} (${\mathcal{E}}_{0}=25$ MV/m). The solid red "Cavity (TESLA)" upper limit arises from the bound on the energy loss caused by Schwinger pair production of millicharged particles in accelerator cavities developed for TESLA Lilje:2004ib (${\mathcal{E}}_{0}=25$ MV/m, $L_{\rm{cav}}=10\,\rm{cm}$, $Q^{\rm min}_{\rm{MCP}}= 10^{10}$). The red dashed upper limit demonstrates the possible bounds obtainable in the near future (${\mathcal{E}_0}=50$ MV/m, $L_{\rm{cav}}=10\,\rm{cm}$, $Q^{\rm min}_{\rm{MCP}}= 10^{12}$).
• Figure 2: Schematic set up for a "dark current shining through a wall" experiment. The alternating dark current (frequency $\nu$), comprised of the produced millicharged particles (dashed line), escapes from the accelerator cavity and traverses also a thick shielding ("wall"), in which the conventional dark current of electrons is stopped. The dark current induces a magnetic field in a resonant (frequency $\nu$) detector cavity behind the wall, which is detected by a SQUID Vodel:2005ma.