A. C. Melissinos

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

Search for photon regeneration in a magnetic field

We have searched for the regeneration of photons propagating in a transverse magnetic field. Such an effect would reveal the existence of light scalar or pseudoscalar particles such as the axion that couple to two photons. We obtain for this coupling the limitgaγγ<(1.3×106 GeV)−1, provided the axion massma≲10−3 eV.

The CLEO detector

Abstract The construction and performance of a large aperture magnetic detector designed for use at the Cornell Electon Storage Ring is descr bed.

Measurement of the magnetic birefringence of neon gas

We constructed a sensitive ellipsometer to study extremely small ellipticities that are acquired by laser light when it traverses a multipass optical cavity in a 9-m-long superconducting magnet. This setup was used to measure the Cotton–Mouton constant (CCM) of neon at 514.5 nm. We find for 760 Torr (1 atm) and 25°C that CCM(Ne) = (5.5 ± 0.3) × 10−20 G−2 cm−1.

Proton-proton diffraction dissociation at incident energies from 175 to 400 GEV

Abstract Differential cross sections d 2 σ d t d M X 2 are presented for the inclusive reaction p + p → X in the range 0.01 t c ) 2 , 1.3 m X 2 2 , and incident energies from 175 to 400 GeV. The results are compared with data in the 20 GeV energy region.

Muon pair production in 16 and 22 GeV πCu collisions

Abstract We have measured inclusive muon pair production by 16 and 22 GeV π − on a copper target in a wide aperture magnetic spectrometer. We interpret the data in terms of quark annihilation models and find approximate scaling in the variable τ = M 2 / s over a wide energy range. At large values of τ the yield of μ-pairs produced by π − is ∼ 100 times that produced by protons. This effect can be understood by assuming appropriate quark distributions for π − and p and thus provides experimental insight into the inner structure of these particles.

Quasi-two-body reactions in 13 GeV/c K+p interactions

Abstract We have studied the final states pK ∗+ (890), K O N ∗++ (1236), and K ∗O (890) N ∗++ (1236) as produced in 12.7 GeV/ c K + p interactions. The density matrix elements and t -distributions have been obtained and are compared to theoretical models.

Searching for photon-sector Lorentz violation using gravitational-wave detectors

Abstract We study the prospects for using interferometers in gravitational-wave detectors as tools to search for photon-sector violations of Lorentz symmetry. Existing interferometers are shown to be exquisitely sensitive to tiny changes in the effective refractive index of light occurring at frequencies around and below the microhertz range, including at the harmonics of the frequencies of the Earths sidereal rotation and annual revolution relevant for tests of Lorentz symmetry. We use preliminary data obtained by the Laser Interferometer Gravitational-Wave Observatory (LIGO) in 2006–2007 to place constraints on coefficients for Lorentz violation in the photon sector exceeding current limits by about four orders of magnitude.

Measurement of the thermal contribution to the nonlinear refractive index of air at 1064 nm

The thermal contribution to the nonlinear refractive index of air at 1.064mum was measured with a high-finesse Fabry-Perot cavity and a 500-mW cw laser beam. At room temperature and pressure, the nonlinear refractive-index coefficient of air was found to be n(2)((th))=(-1.9+/-0.2)x10 (-14) cm(2)/W for a beam waist radius of 0.23 mm and was found to be independent of the relative humidity. The thermal nonlinearities of N(2) , O(2) , and CO(2) were also measured, and it was found that the dominant contribution to air is its O(2) content.

Parametric converters for detection of small harmonic displacements

Abstract We discuss the construction and operation of a superconducting microwave cavity parametric converter suitable for the measurement of small harmonic displacements. Displacements of one endwall of the cavity produced by a piezoelectric ceramic and of order δx = 3 × 10−17 cm have been detected. Such a device is presently used to search for anomalous long range interactions of a beam of stored high energy particles.