## Publications [#246131] of Haiyan Gao

Papers Published
1. with Ackerstaff et al., K, Measurement of angular distributions and $R=\sigma_L/\sigma_T$ in diffractive electroproduction of $\rho^0$ mesons, The European Physical Journal C, vol. 18 no. 2 (December, 2000), pp. 303-316, Springer Nature [doi] .

Abstract:
Production and decay angular distributions were extracted from measurements of exclusive electroproduction of the Ï0(770) meson over a range in the virtual photon negative four-momentum squared 0.5 < Q2 < 4 GeV2 and the photon-nucleon invariant mass range 3.8 < W < 6.5 GeV. The experiment was performed with the HERMES spectrometer, using a longitudinally polarized positron beam and a 3He gas target internal to the HERA eÂ± storage ring. The event sample combines Ï0 mesons produced incoherently off individual nucleons and coherently off the nucleus as a whole. The distributions in one production angle and two angles describing the Ï0 â†’ Ï€+Ï€- decay yielded measurements of eight elements of the spin-density matrix, including one that had not been measured before. The results are consistent with the dominance of helicity conserving amplitudes and natural parity exchange. The improved precision achieved at 4 < W < 7 GeV, in combination with other data at W > 7 GeV, reveals evidence for an energy dependence in the ratio R of the longitudinal to transverse cross sections at constant Q2.