Light dark states with electromagnetic form factors

Understand

New particles $\chi$ that are electrically neutral but couple to the electromagnetic current via higher-dimensional operators and that are sufficiently light, at or below the GeV-mass scale, can be produced in pairs in a number of dedicated high-intensity experiments.

• In this work we consider the production of $\chi$ through magnetic- and electric-dipole moments as well as through anapole moment and charge radius interactions in electron beams.
• We derive new constraints from BaBar, NA64 and mQ and forecast the future sensitivity on the existence of such states, from Belle-II, LDMX and BDX.
• We present for the first time a detailed treatment of the off-shell production of photons in electron beams with subsequent decay into a $\chi\bar\chi$ pair in a 2-to-4 process.