Improved Measurement of the Left-RightZ0Cross Section Asymmetry

Abstract

We present a new measurement of the left-right cross section asymmetry (A LR ) for Z boson production by e + e − collisions. The measurement was performed at a center-of-mass energy of 91.28 GeV with the SLD detector at the SLAC Linear Collider (SLC). The luminosity-weighted average polarization of the SLC electron beam was (77.23±0.52)%. Using a sample of 93,644 Z decays, we measure the pole-value of the asymmetry, A 0 LR , to be 0.1512±0.0042(stat.)±0.0011(syst.) which is equivalent to an effective weak mixing angle of sin 2 θ eff W = 0.23100 ± 0.00054(stat.) ± 0.00014(syst.). Submitted to Physical Review LettersM. Carpinelli,A. Chou,S. Fahey,T. Gillman,J.L. Harton, T.W. Markiewicz,A.K. McKemey, (4) B.T. Meadows,T.B. Moore,M. Nussbaum,J. Quigley,O.H. Saxton,S. Sen,D.J. Sherden,M.B. Smy,T. Takahashi,T. Usher,A.W. Weidemann, R.W. Zdarko, (28) and J. Zhou,(1) Adelphi University, Garden City, New York 11530 The left-right asymmetry is defined as, where σ L and σ R are the e + e − production cross sections for Z bosons at the Z-pole energy with left-handed and right-handed electrons, respectively. The Standard Model predicts that this quantity depends upon the effective vector (v e ) and axial-vector (a e ) couplings of the Z boson to the electron current,where the effective electroweak mixing parameter is defined 5The operation of the SLC with a polarized electron beam has been described previously [5]. In 1994, the beam polarization at the SLC source [6] was increased from 63% to ∼ 80% by the use of a thinner (0.1 µm) strained-lattice GaAs photocathode [7] which was illuminated by a pulsed Ti:Sapphire laser operating at 845 nm. The circular polarization state of each laser pulse (and hence, the helicity of each electron pulse) was chosen randomly. The electron spin orientation was manipulated in the SLC North Arc by a pair of large amplitude betatron oscillations to achieve longitudinal polarization at the SLC interaction point (IP) The counting rates in each detector channel are measured for three combinations of electron and photon beam parameters: parallel electron and photon helicities, antiparallel helicities, and photon beam absent. The latter combination is used to measure detector background. The asymmetry formed from the background-subtracted counting rates is

Complete O(α) QED corrections to polarized Compton scattering

“…Usually, in a Compton polarimeter, circularly polarized laser light is brought to collision with the high-energetic e ± beam, and the degree of beam polarization is deduced from the left-right asymmetry with respect to the polarization of the laser photons. The high precision of such polarization measurements, which is typically of the order of 1% [1][2][3][4], sets the necessary level of accuracy in theoretical predictions to a few per mille. Therefore, it is indispensable to control radiative corrections.…”

Supersymmetry withoutRparity: Constraints from leptonic phenomenology

“…These are listed in Table II and used to derive the 3σ bounds on the ∆A aτ 's shown in Table I. The A e and A τ used in the ∆A eτ bounds of Table I are obtained from LEP measurements of τ polarization (note however that the SLD group at SLAC [17] measured A e directly with their polarized electron beam and obtained a substantially higher value for A e ). The ∆A µτ bounds are obtained from LEP forward-backward asymmetry measurements (which provide the best A µ value but larger uncertainty in A τ ).…”

A study of the charged scalar in the Zee model

“…where A 0 LR is the SM left-right asymmetry. The most stringent bounds are obtained by combined LEP [16]and SLC measurements [19] given in terms of the axial and vector couplings. This is shown in Fig.…”