1995 Preliminary SLD ALR Measurement

The SLD Collaboration has completed its preliminary Left-Right Asymmetry (ALR) analysis of new data taken in 1994-95 (a final result is expected by Autumn 1995). Using a simple calorimeter-only event selection, a total of 92,261 Z events are identified:
51,446 (e- left-handed) and 40,815 (e- right-handed).
The luminosity-weighted average beam polarization over the course of the 1994-95 run is evaluated to be
P_e = 0.7734+-0.0062(syst),
where the systematic uncertainty includes effects associated with beam transport (including the 'chromaticity effect' that dominated the 1993 systematic uncertainty).

After correcting for the usual small residual effects (the fractional correction is 0.327% mostly from background contamination and left-right luminosity asymmetry), the measured value of ALR is

ALR(91.270 GeV) = 0.1495 +- 0.0042 +- 0.0012
which is corrected slightly for Z-gamma interference (and pure gamma exchange) to arrive at a pole value of
ALR^0 = 0.1524 +- 0.0042 +- 0.0012
and is equivalent to the follwing value of the effective weak mixing angle,
sin**2(theta_w^eff) = 0.23084 +- 0.00054 +- 0.00015.
Combining these numbers with the 1992 and 1993 measurements (assuming that the systematic uncertainties are completely correlated), the following values are obtained: 
      **********************************************
      *                                            *
      *       ALR^0 = 0.1551 +- 0.0040             *
      *                                            *
      *  sin**2(theta_w^eff) = 0.23049 +- 0.00050  *
      *                                            *
      **********************************************
The following figure compares the preliminary SLD result with LEP results presented at Moriond 95.

The sources of the LEP results are tabulated separately. The following figure compares the SLD result with those from individual LEP experiments.

The systematic uncertainties on the 1994-1995 result are summarized below: 

   Item                                  Delta P/P (%)  Delta A/A(%)

1) Laser Polarization                     0.2
2) Laser Pickup                          <0.1
3) Electronic Crosstalk                   0.2
4) Detector Linearity                     0.5*
5) Analyzing Power Calibration            0.29
6) Cross Checks                           0.4*
7) Polarization Transport                 0.2**
   (Chromaticity and beam phase space)
-------------------------------------------------------------------
A) Beam Polarization Total                0.8             0.8
B) Asymmetry Corrections                                  0.1
-------------------------------------------------------------------
  Total Uncertainty                                       0.8%
*
Note: The evaluation of the detector linearity uncertainty 4) is in progress. The quoted value is the taken from the worst linearity measurement and has known problems. We expect it to improve substantially. Similarly, a number of cross checks have been performed and all but one (a comparison of polarization measurements made with different thickness radiators) are consistent with the other uncertainties quoted. Work is in progress to understand this (0.4%) discrepancy and it is expected to vanish.
**
Note: The operation of the SLC with narrow energy spreads in 1994-5 (RMS <= 0.12%) and the reduction of the dependence of P on beam energy has substantially reduced the size of the chromaticity correction from the 1993 value (1.7% in 93 to 0.2% in 94-5). The systematic uncertainty associated with the correction is correspondingly smaller.