E. Prebys

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

Aerogel Cherenkov counter for the BELLE detector

In the BELLE experiment at the KEK B-factory, a threshold aerogel Cherenkov counter, with refractive index from 1.010 to 1.030, is used to provide p/K separation in the momentum region up to 3.5 GeV/c. The detector system has been constructed, installed into the BELLE detector, and then commissioned with cosmic rays and beams. This paper presents a brief explanation of the detector system and its performance obtained in early BELLE physics runs. ( 2000 Elsevier Science B.V. All rights reserved.

The KL/μ detector subsystem for the BELLE experiment at the KEK B-factory

Abstract The KL and muon detection subsystem (KLM) of the BELLE experiment at the KEK-B asymmetric B-factory is described. The system consists of glass-electrode resistive plate counters installed within the segmented flux return iron of the BELLE superconducting solenoid. The design and construction of the detectors, including the gas distribution system and readout electronics, are described in detail. The operating characteristics and performance with cosmic rays and e+e− collision data are presented.

The Cherenkov correlated timing detector: beam test results from quartz and acrylic bars

Abstract Several prototypes of a Cherenkov correlated timing (CCT) detector have been tested at the KEK-PS test beam line. We describe the results for Cherenkov light yields and timing characteristics from quartz and acrylic bar prototypes. A Cherenkov angle resolution is found to be 15 mrad at a propagation distance of 100 cm with a 2 cm thick quartz bar prototype.

Prototype studies of a fast RICH detector with a CsI photocathode

A prototype fast RICH detector with a CsI photocathode coupled to a wire chamber filled with atmospheric pressure ethane has been studied in a 3.5 GeV/c π− beam. Using a 1 cm thick C6F14 radiator, 8.5 photoelectrons per ring were detected. Measurements were made of the CsI quantum efficiency, photoelectron position resolution, and Cherenkov ring radius resolution. UV photon feedback appears to have degraded the resolution of the detector. Techniques to improve the photon detection efficiency and reduce backgrounds are discussed.

RPC systems for BELLE detector at KEKB

Abstract We constructed glass RPC modules for detecting K L and muon in BELLE experiment. The modules have been installed in the instrumented iron yoke outside of the solenoid coil and cover about 2000 m 2 of detectors. We report the design and construction of the detectors, including the gas distribution and exhaust system, the HV system, the readout electronics and DAQ system. The performance with cosmic ray and the e + e − collision data are presented.

Physics prospects for Belle

Abstract In this paper, we present the physics prospects for BELLE, an experiment dedicated to the study of CP violation in the B meson system. A brief introduction will give the physics motivation for the experiment. The current status will be given, and the prospects for the next running period and beyond will be outlined.

The Cherenkov correlated timing detector: materials, geometry and timing constraints

Abstract The key parameters of Cherenkov correlated timing (CCT) detectors are discussed. Measurements of radiator geometry, optical properties of radiator and coupling materials, and photon detector timing performance are presented.

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 .

Characterization of CsI photocathodes for use in a fast RICH detector

We have completed a series of measurements that provide a basic understanding of the properties of CsI photocathodes for use in ring imaging Cherenkov (RICH) detectors. The quantum efficiency of CsI has been measured relative to an NIST-calibrated photodiode and is in excellent agreement with a similar measurement by Breskin et al. A representative value of the quantum efficiency is 20% at 180 nm. The quantum efficiency of a fresh photocathode is unaffected by temperature, but heating the photocathode can be helpful if it has been exposed to water vapor, or has been aged by a large integrated photocurrent. Detailed studies of aging show a “fast” component that appears to be associated with a rise in the work function, and a “slow” component associated with conversion of the bulk CsI to Cs. We judge that a practical lifetime of a CsI photocathode is until it has lost 20% of its initial quantum efficiency, which process is dominated by the “fast” rise in the work function. This rise occurs both due to photoelectron transport with an effective lifetime of 0.1 μC/mm2 and due to positive-ion bombardment with an effective lifetime of 15 μC/mm2. When the CsI photocathode is used in a chamber with gas gain greater than 150 the latter lifetime is the relevant one. This lifetime should be sufficient for use of a RICH detector at an e+e− B factory. The reduction of quantum efficiency of a CsI photocathode in a gas-filled chamber has been studied for several gases over a wide range of reduced electric field. This effect can be minimized by use of atmospheric-pressure methane in a chamber with anode wires rather than a mesh. We have also demonstrated that excellent spatial resolution for the location of the photoelectrons can be obtained using a coarse cathode-pad readout if the anode-cathode spacing is similar to the pad width.