Measurement of the weak mixing angle in Moller scattering.

摘要

The weak mixing parameter, sin2q w , is one of the fundamental parameters of the Standard Model. Its tree-level value has been measured with high precision at energies near the Z0 pole; however, due to radiative corrections at the one-loop level, the value of sin2q w is expected to change with the interaction energy. As a result, a measurement of sin2q w at low energy (Q2 ≪ mZ, where Q2 is the momentum transfer and mZ is the Z boson mass), provides a test of the Standard Model at the one-loop level, and a probe for new physics beyond the Standard Model.; One way of obtaining sin2q w at low energy is from measuring the left-right, parity-violating asymmetry in electron-electron (Moller) scattering: APV = sR-sLs R+sL , where sigmaR and sigma L are the cross sections for right- and left-handed incident electrons, respectively. The parity violating asymmetry is proportional to the pseudo-scalar weak neutral current coupling in Moller scattering, gee. At tree level gee = (¼ - sin2q w ). A precision measurement of the parity-violating asymmetry in Moller scattering was performed by Experiment E158 at the Stanford Linear Accelerator Center (SLAC). During the experiment, ∼50 GeV longitudinally polarized electrons scattered off unpolarized atomic electrons in a liquid hydrogen target, corresponding to an average momentum transfer Q 2 ∼ 0.03 (GeV/c)2. The tree-level prediction for APV at such energy is ≃300 ppb. However one-loop radiative corrections reduce its value by ∼40%.; This document reports the E158 results from the 2002 data collection period. The parity-violating asymmetry was found to be APV = -160 +/- 21 (stat.) +/- 17 (syst.) ppb, which represents the first observation of a parity-violating asymmetry in Moller scattering. This value corresponds to a weak mixing angle at Q 2 = 0.026 (GeV/c)2 of sin2q wMS = 0.2379 +/- 0.0016 (stat.) +/- 0.0013 (syst.), which is -0.3 standard deviations away from the Standard Model prediction: sin2q predictedw MS = 0.2385 +/- 0.0006 (theory). The E158 measurement of sin2q w at a precision of delta( sin2q w ) = 0.0020 provides new physics sensitivity at the TeV scale.