Precision Measurement of Trident Production in Strong Electromagnetic Fields

We demonstrate experimentally that the trident process $e^{-}\to e^{-}{e}^{+}{e}^{-}$ in a strong external field, with a spatial extension comparable to the effective radiation length, is well understood theoretically. The experiment, conducted at CERN, probes values for the strong field parameter $\chi$ up to 2.4. Experimental data and theoretical expectations using the local constant field approximation show remarkable agreement over almost 3 orders of magnitude in yield.

Figure 1

Experimental setup. A schematic representation of the experimental setup in the H4 beamline in the SPS North Area at CERN. The symbols “$\mathrm{M}i$,” with $i=0,\dots ,7$, denote “Mimosa-26” position sensitive detectors.

Figure 2

Positron spectrum of reconstructed trident events. Solid lines are simulations while squares and triangles are experimental data points. Blue and red show the trident spectrum in aligned orientation, magenta and cyan show the amorphous yield while black and green is the background. The right figure is the same as the left but with a linear vertical axis. The inset shows a zoom of the low energy part, which largely overlaps with the energy range measured in 2007 [29].