Deviation from Simple Quantum Electrodynamics

Abstract

Measurements of the photoproduction from carbon of wide-angle electron-positron pairs in the energy range from 1 to 6 BeV were made at the Cambridge electron accelerator. This experiment, which was proposed by Drell 1 and discussed in detail by Bjorken, Drell, and Frautschi, 2 is a new test of quantum electrodynamics at high energies and small distances. The object of the experiment is to study the behavior of the electron propagator for large spacelike virtual momenta by measuring the cross section for the photoproduction of symmetrical electron-positron pairs. The experimental results do not agree with the predictions of quantum electrodynamics for pair production; they suggest a breakdown of the theory or the presence of other processes.The apparatus is shown in Fig. 1. A 10-mil tungsten ribbon was used to produce the bremsstrahlung beam. A circular magnet bent charged particles away from the gamma-ray beam so that pairs with small opening angles could be detected. The electrons and positrons were detected and momentum analyzed by two mirror-image magnet-counter systems• The momentum-analyzing magnet was one-half of a conventional quadrupole to which was bolted an iron plate as an image plane. The half quadrupoles focused in the vertical plane, and they were used as spectrometers by placing lead obstacles in their centers. The smallest production angle which could be accepted was determined by the height of the lead obstacle• A brass mask at the front of each half-quadrupole defined the entrance aperture.The particle trajectories were defined by scintillation counters placed behind the quadrupoles. On each arm a large, threshold-type, gas Cerenkov counter set below the thresholds of pions and muons was used to detect electrons. Behind the Cerenkov counters were scintillator-lead sandwiches in which electrons showered; these counters provided additional discrimination against pions and muons. The Cerenkov counter, shower counter, and certain of the fast-coincidence pulses were displayed on a fast oscilloscope and photographed for many of the events. Two methods were used to determine the contamination due to pion pairs; both methods yielded the same result. The total energy of the bremsstrahlung beam was measured with a quantameter. The calibration of the quantameter was checked against two Cornell quantameters as well as other quantameters in this laboratory. Table I summarizes the experimental and theoretical electron pair yields. All of the yields have been normalized to the rate for a |-in.-thick carbon target per unit charge collected by the quantameter. The experimental data have been corrected for random coincidences, counting-rate loss, and pion contamination. The target thickness ranged from $ to \ in. and was selected so that the chance electron-pair coincidences were always less than 10% of the real rate. The average count -FIG. 1. A drawing showing the general layout of the apparatus and its relationship to the electron synchrotron.660

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