C. Bula

POSITRON PRODUCTION IN MULTIPHOTON LIGHT-BY-LIGHT SCATTERING

A signal of 106 14 positrons above background has been observed in collisions of a low-emittance 46.6-GeV electron beam with terawatt pulses from a Nd:glass laser at 527 nm wavelength in an experiment at the Final Focus Test Beam at SLAC. The positrons are interpreted as arising from a two-step process in which laser photons are backscattered to GeV energies by the electron beam followed by a collision between the high-energy photon and several laser photons to produce an electron-positron pair. These results are the rst laboratory evidence for inelastic light-by-light scattering involving only real photons. Submitted to Physical Review Letters Work supported by Department of Energy contract DE{AC03{76SF00515 and grants DE{FG02{ 91ER40671, DE{FG02{91ER40685 and DE{FG05{91ER40627. Present address: Hughes Leitz Optical Technologies Ltd., Midland, Ontario, Canada L4R 2H2. Present address: Lawrence Livermore National Laboratory, Livermore, CA 94551. also Department of Mechanical Engineering Present address: Panoramastrasse 8, 78589 Durbheim, Germany The production of an electron-positron pair in the collision of two real photons was rst considered by Breit and Wheeler [1] who calculated the cross section for the reaction !1 + !2 ! e e (1) to be of order r e , where re is the classical electron radius. While pair creation by real photons is believed to occur in astrophysical processes [2] it has not been observed in the laboratory up to the present. After the invention of the laser the prospect of intense laser beams led to reconsideration of the Breit-Wheeler process by Reiss [3] and others [4, 5]. Of course, for production of an electron-positron pair the center-of-mass (CM) energy of the scattering photons must be at least 2mc 1 MeV. While this precludes pair creation by a single electromagnetic wave, the necessary CM energy can be achieved by colliding a laser beam against a highenergy photon beam created, for example, by backscattering the laser beam o a high-energy electron beam. With laser light of wavelength 527 nm (energy 2.35 eV), a photon of energy 111 GeV would be required for reaction (1) to proceed. However, with an electron beam of energy 46.6 GeV as available at the Stanford Linear Accelerator Center (SLAC) the maximum Compton-backscattered photon energy from a 527-nm laser is only 29.2 GeV. In strong electromagnetic elds the interaction need not be limited to initial states with two photons [3], but rather the number of interacting photons becomes large as the dimensionless, invariant parameter = e q hA A i=mc 2 = eErms=m!0c = eErms 0=mc approaches or exceeds unity. Here the laser beam has laboratory frequency !0, reduced wavelength 0, root-mean-square electric eld Erms, and four-vector potential A ; e and m are the charge and mass of the electron, respectively, and c is the speed of light. For photons of wavelength 527 nm a value of = 1 corresponds to laboratory eld strength of Elab = 6 10 V/cm and intensity I = 10 W/cm. Such intensities are now practical in tabletop laser systems based on chirped-pulse ampli cation [6]. Then the multiphoton Breit-Wheeler reaction

Picosecond timing of terawatt laser pulses with the SLAC 46 GeV electron beam

Abstract We report on the collision of 1.5 ps (FWHM) laser pulses traversing at 17° a short ∼7 ps (FWHM) 46.6 GeV electron bunch. The phase-locked system used to maintain the correct timing of the laser pulses and the appropriate diagnostics are described. The jitter between the laser and electron pulses is determined from the stability of the observed rate of Compton scatters and can be described by a Gaussian distribution with σ j ⋍ 2.2 ps .