Direct Measurement of the Chudakov Effect

Experimental results for the restricted energy loss of pairs created from 1–178 GeV photons in a thin Au target and subsequently passing a CCD detector are presented. It is shown that pairs—when detected close to the creation vertex—suffer a reduced energy loss due to the internal screening of the charges constituting the pair. Furthermore, the ability to measure directly the energy of the pair by calorimetry enables a comparison with theory as a function of energy. The observed phenomenon is in good qualitative agreement with general expectations from the Chudakov effect but indicates a quantitative disagreement with either of two mutually disagreeing theories.

Figure 1

Schematic setup of the Chudakov experiment at CERN. For details, see the text.

Figure 2

A photograph of the CCD setup inside the vacuum chamber. To reduce dark current, the CCD was cooled to $\simeq 200\mathrm{K}$ by thermal contact to a liquid nitrogen reservoir through the Cu blocks shown.

Figure 3

A restricted energy loss spectrum as recorded by the CCD (squares with error bars) and a fit by a Landau distribution (line). The shown data set is for electrons penetrating the CCD.

Figure 4

The MPEL for the gold foil positioned at an average distance of $16\mu \mathrm{m}$ from the CCD divided by the MPEL for the gold foil positioned at an average distance of $116\mu \mathrm{m}$ from the CCD. The solid squares show the values obtained in the experiment, the dashed line shows the expected ionization reduction based on Eq. (3) with ${\xi }_{\pm }=1/2$, and the contour plot represents simulated values as described in the text. The solid line represents the expected ionization reduction from a quantum treatment [24].