Electroweak Results from the Sld Experiment

We present an overview of the electroweak physics program of the SLD experiment at the Stanford Linear Accelerator Center (SLAG). A data sample of 550K 2’ decays has been collected. This experiment utilizes a highly polarized electron beam, a small interaction volume, and a very precise pixel vertex detector. It is the first experiment at a linear electron collider. We present a preliminary result for the weak mixing angle, sin2(0e&f) = 0.23110 f0.00029. We also present a preliminary result for the parity violating parameter, A6 = 0.898 f0.029. These measurements are used to test for physics beyond the Standard Model. Presented at the Lake Louise Winter Institute: Electroweak Physics Lake Louise, Alberta, Canada February 14-20, 1999 *Work supported by Department of Energy contract DEAC03-76SF00515 1 Electroweak Physics program of the SLD Experiment The SLD Experiment began its physics program at the SLAC Linear Collider (SLC) in 1992, and has accumulated a total data sample of approximately 550K hadronic Z” decays between 1992 and 1998. This data sample is a factor 30 smaller than the Z” sample available from the combined data of the 4 LEP experiments, ALEPH, DELPHI, L3 and OPAL. Yet the SLD physics results in many areas are competitive with the combined LEP result, and for some measurements SLD has the world’s most precise results. There are 3 features that distinguish the SLD experiment at the SLC: a small, stable interaction volume; a precision vertex detector; and a highly polarized electron beam. SLD is the first experiment at an electron linear collider. The collision volume is small and stable, measuring 1.5 microns by 0.7 microns in the transverse dimensions by 700 microns longitudinally. These key features for the SLD experiment result in the world’s best measurement of the weak mixing angle, a precise direct measurement of parity violation at the Zb& vertex, At,, and a good measurement of the Zbb coupling strength, &,. The weak mixing angle measurement provides an excellent means to search for new physics that may enter through oblique (or loop) corrections, while the Ab and &, measurements are excellent means to search for new physics that may enter through a correction at the Zbb vertex. In its near (analysis) future, SLD is also exploiting its capabilities to search for B, mixing. The analysis for this is evolving to take full advantage of the precise vertexing information, and by the time of the summer 1999 conferences SLD should have a measurement of B, mixing comparable in sensitivity with the combined LEP result. SLD estimates it should have a reach for Am, of 12 15~s~l, in the region where it is predicted in the SM. 2 Z” Coupling Parameters At the ZfT vertex, the SM gives the vector and axial vector couplings to be Vf = I; 2&f sin2(8gf), and ap = I;, where If is the fermion isospin and &f is the fermion charge. Radiative corrections are significant and are treated as follows. First, vacuum polarization and vertex corrections are included in the coupling constants, and an effective weak mixing angle is defined to be sin2(8gf) E $ (1 ~,/a,). Second, experimental measurements need to be corrected for initial state radiation and for Z 7 interference to extract the Z-pole contribution. One can define a parity-violating fermion asymmetry parameter, Af = $$. The cross-section for e+e+ Z” + j7 can be expressed by dof yg cx b; + a”rl (l+ cos2 fI)(l + PA,)+ 2cosBAf (P + A,) (1) where 0 is the angle of the outgoing fermion with respect to the incident electron, and P is the polarization of the electron beam (the positron beam is assumed to be unpolarized). We can then define forward, backwal, and left, right cross-sections as follows: OF = s,’ gd(cose); cqj = j+, gd(cos8); CrL = f, %d(cose); 411 = J; da r -$d(cosO). Here, QL (cry) is the cross-section for left (right) polarized electrons colliding with unpolarized positrons. At the SLC, the availability of a highly polarized electron beam allows for direct determinations of the Af parameters via measurements of the left-right forward-backward asymmetry, Afg, defined by Additionally, a very precise determination of A, is achieved from the measurement of the left-right asymmetry, ALR, which is defined as All Z decay modes can be used, and this allows for a sim P le analysis with good statistical power for a precise determination of sin2(0$ ).