A Direct Measurement of Parity Violation in the Coupling of Z Bosons to b Quarks Using a Mass Tag and Momentum-Weighted Track Charge

We present a direct measurement of the parity-violation parameter Ab using a self-calibrating track-charge technique. In the SLD experiment we observe hadronic decays of Z bosons produced in collisions between longitudinally polarized electrons and unpolarized positrons at the SLAC Linear Collider. A sample of b b events is selected using the topologically reconstructed mass of B hadrons. From our 1993{ 1995 sample of approximately 150 thousands hadronic Z decays, we obtain Ab = 0:911 0:045(stat) 0:045(syst). To be submitted to Physical Review Letters Measurements of b quark production asymmetries at the Z pole determine the extent of parity violation in the Zb b coupling. At Born level, the di erential cross section for the process ee ! Z ! b b can be expressed as a function of the polar angle of the b quark relative to the electron beam direction, ( ) d b=d / (1 AePe)(1 + ) + 2Ab(Ae Pe) ; (1) where Pe is the longitudinal polarization of the electron beam, = cos . The parameters Af = 2vfaf=(v 2 f + a 2 f ), (f = e or b) where vf (af) is the vector (axial vector) coupling of the fermion f to the Z boson, express the extent of parity violation in the Zf f coupling. From the conventional forward-backward asymmetries formed with an unpolarized electron beam (Pe = 0), such as used by the LEP experiments, only the product of parityviolaton parameters AeAb can be measured [1]. For a polarized electron beam, it is possible to measure Ab directly by forming the left-right forward-backward asymmetry [2] ~ AbFB( ) = [ b L( ) b L( )] [ b R( ) b R( )] b L( ) + b L( ) + b R( ) + b R( ) = jPejAb 2 1 + 2 ; (2) where L;R refers to Z ! bb decays produced with a predominantly left-handed (negative helicity) or right-handed (positive helicity) electron beam, respectively. The measurement of the double asymmetry eliminates the dependence on the initial state coupling. The quantity Ab is largely independent of propagator e ects that modify the e ective weak mixing angle and thus is complementary to other electroweak asymmetry measurements performed at the Z 0 pole. In this Letter we present a direct measurement of Ab from data collected in the SLC Large Detector (SLD) between 1993 and 1995. We use an inclusive vertex mass tag to select a sample of Z ! bb events, and the net momentum-weighted track charge, rst suggested by Feynman and Field [3], to identify the sign of the charge of the underlying quark. The analysis presented in this paper uses an improved track charge calibration technique which greatly reduces the model dependence of the result. The operation of the SLAC Linear Collider (SLC) with a polarized electron beam has been described previously [4]. During the 1994-5 (1993) run, SLD recorded 3.6 pb 1 (1.8 pb ) of ee annihilation data at a mean center-of-mass energy of 91.28 0.02 GeV, with a mean electron beam longitudinal polarization of 77:2 0:5% (63:0 1:1%). A detailed description of the SLD can be found elsewhere [5]. Charged particles are tracked in the Central Drift Chamber (CDC) in a uniform axial magnetic eld of 0.6T. In addition, a pixel-based CCD vertex detector (VXD) provides an accurate measure of particle trajectories close to the beam axis. The measured r (rz) track impact parameter resolution approaches 11 m (37 m) for high momentum tracks, and is 76 m (80 m) at p? p sin = 1 GeV/c, where z is the coordinate parallel to the beam axis and p? is the momentum in GeV/c perpendicular to the beamline. The momentum resolution of the combined SLD tracking systems is ( p?=p?) 2 = (:01) + (:0026p?) . The thrust axis is reconstructed using the liquid argon calorimeter, which covers a range of j cos j < 0:98. The uncertainty in the position of the primary vertex (PV ) is 7 m transverse to the beam axis and 35 m (52 m for b b events) along the beam axis. Events are classi ed as hadronic Z decays if they contain: (1) at least seven wellmeasured tracks (as described in Ref. [5]), (2) a visible charged energy of at least 20 GeV,