C. Field

Precision measurement of the weak mixing angle in Møller scattering

We report on a precision measurement of the parity-violating asymmetry in fixed target electron-electron (Møller) scattering: A(PV) = [-131 +/- 14(stat) +/- 10(syst)] x 10(-9), leading to the determination of the weak mixing angle sin2(thetaW(eff) = 0.2397 +/- 0.0010(stat) +/- 0.0008(syst), evaluated at Q2 = 0.026 GeV2. Combining this result with the measurements of sin2(thetaW(eff) at the Z0 pole, the running of the weak mixing angle is observed with over 6sigma significance. The measurement sets constraints on new physics effects at the TeV scale.

Observation of the Askaryan Effect: Coherent Microwave Cherenkov Emission from Charge Asymmetry in High-Energy Particle Cascades

We present the first direct experimental evidence for the charge excess in high-energy particle showers and corresponding radio emission predicted nearly 40 years ago by Askaryan. We directed picosecond pulses of GeV bremsstrahlung photons at the SLAC Final Focus Test Beam into a 3.5 ton silica sand target, producing electromagnetic showers several meters long. A series of antennas spanning 0.3 to 6 GHz detected strong, subnanosecond radio-frequency pulses produced by the showers. Measurements of the polarization, coherence, timing, field strength vs shower depth, and field strength vs frequency are completely consistent with predictions. These measurements thus provide strong support for experiments designed to detect high-energy cosmic rays such as neutrinos via coherent radio emission from their cascades.

Measurement of Pressure Dependent Fluorescence Yield of Air: Calibration Factor for UHECR Detectors

In a test experiment at the Final Focus Test Beam of the Stanford Linear Accelerator Center, the fluorescence yield of 28.5 GeV electrons in air and nitrogen was measured. The measured photon yields between 300 and 400 nm at 1 atm and 29 deg C are Y(760 Torr, air) = 4.42 +/- 0.73 and Y(760 Torr, nitrogen) = 29.2 +/- 4.8 photons per electron per meter. Assuming that the fluorescence yield is proportional to the energy deposition of a charged particle traveling through air, good agreement with measurements at lower particle energies is observed.

A high resolution wire scanner for micron-size profile measurements at the SLC

Abstract Fine conductive fibers have been used to measure transverse beam dimensions of a few microns at the Stanford Linear Collider (SLC). The beam profile is obtained by scanning a fiber across the beam in steps as small as 1 μm, and recording the secondary emission signal at each step, using a charge sensitive amplifier. We first outline the mechanical construction and the analogue electronics of the wire scanner. We then describe its performance in test beams and in actual operation. The article closes with a brief discussion of performance limitations of such a beam profile monitor.

Comparison of air fluorescence and ionization measurements of EM shower depth profiles: Test of a UHECR detector technique

Measurements are reported on the fluorescence of air as a function of depth in electromagnetic showers initiated by bunches of 28.5 GeV electrons. The light yield is compared with the expected and observed depth profiles of ionization in the showers. It validates the use of atmospheric fluorescence profiles in measuring ultra high energy cosmic rays.

The wire scanner system of the final focus test beam at SLAC

Abstract The system of wire scanners in use at the FFTB at SLAC is described. In addition to the scanners themselves, there is a discussion of detectors for the scattering from the wires, and of the procedure for handling beam spots of large aspect ratio.

A compact beam profile probe using carbon fibres

Abstract A beam profile monitor based on the use of carbon fibres is described. It has been designed to be compatible with the operation of the Mark II particle detector at the SLC, including high-resolution secondary vertex detectors mounted 25 mm radially from the beam.

Retractable carbon fibre targets for measuring beam profiles at the SLC collision point

Abstract Retractable targets of carbon fibres with nominal diameters of 30, 7 and 4.5 μm have been in use at the collision point of the SLC (the SLAC Linear Collider). The target mechanism is compatible with the Mark II detector now in place at the collision point. Beam profiles are measured in horizontal and vertical axes by using magnetic dipoles to scan the beams across the fibres. Two signal detection mechanisms are available. Electron emission from the fibres is measured with the help of charge-sensitive amplifiers. Bremsstrahlung from the fibres is detected by downstream counters. Examples of measurements are given, and limits on the use of the techniques are discussed.

A fast luminosity monitor system for PEP II

The PEP II fast luminosity system provides a measurement of luminosity to the control system with a time constant of 0.3 seconds and fluctuations less than 0.1% for this interval, adequate for use in feedback systems. Continuous visual updates of luminosity are provided. The alignment of the positron beam at the collision point can also be monitored, and there is a visual display of the luminosity associated with each bunch-pair in the machine, sampled approximately every two seconds.

A monitor for gamma radiation at zero degrees from the SLC collision point

Abstract Bremsstrahlung gamma rays are emitted in a narrow forward cone while the electron and positron beams are being measured with carbon fibers at the SLC (the linear collider at the Stanford Linear Accelerator Center). Beam-beam deflections which occur at the collision point also emit softer gamma rays, called beamstrahlung. A device to detect these gammas is described. It has a converter to produce electron-positron pairs and a gas Cherenkov volume to detect them in the presence of a large, low energy, radiation background. The system is used for beam diagnostics at the SLC.