Spin alignment of leading K*(892)^0 mesons in hadronic Z^0 decays

TL;DR

This paper measures the spin alignment of K*(892)^0 mesons produced in Z^0 decays, finding a preference for helicity zero states and confirming theoretical predictions about meson polarization and fragmentation.

Contribution

It provides the first comprehensive measurement of helicity density matrix elements for K*(892)^0 mesons in Z^0 decays, testing Standard Model predictions of meson polarization.

Findings

Helicity zero state is preferred at high momentum fractions.

The off-diagonal matrix element rho_1-1 is negative at large x_p.

The total K*^0 production rate per Z^0 decay is measured as 0.74.

Abstract

Helicity density matrix elements for inclusive K*(892)^0 mesons from hadronic Z^0 decays have been measured over the full range of K^*0 momentum using data taken with the OPAL experiment at LEP. A preference for occupation of the helicity zero state is observed at all scaled momentum x_p values above 0.3, with the matrix element rho_00 rising to 0.66 +/- 0.11 for x_p > 0.7. The values of the real part of the off-diagonal element rho_1-1 are negative at large x_p, with a weighted average value of -0.09 +/- 0.03 for x_p > 0.3, in agreement with new theoretical predictions based on Standard Model parameters and coherent fragmentation of the qq(bar) system from the Z^0 decay. All other helicity density matrix elements measured are consistent with zero over the entire x_p range. The K^*0 fragmentation function has also been measured and the total rate determined to be 0.74 +/- 0.02 +/- 0.02 K*(892)^0 mesons per hadronic Z^0 decay.