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Register a new userRatios of helicity amplitudes for exclusive $rho^0$ electroproduction on transversely polarized protons
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Exclusive $rho^0$-meson electroproduction is studied by the HERMES experiment, using the 27.6 GeV longitudinally polarized electron/positron beam of HERA and a transversely polarized hydrogen target, in the kinematic region 1.0 GeV$^2$<Q$^2$<7.0 GeV$^2$, 3.0 GeV<W<6.3 GeV, and -t<0.4 GeV$^2$. Using an unbinned maximum-likelihood method, 25 parameters are extracted. These determine the real and imaginary parts of the ratios of several helicity amplitudes describing $rho^0$-meson production by a virtual photon. The denominator of those ratios is the dominant amplitude, the nucleon-helicity-non-flip amplitude $F_{0frac{1}{2}0frac{1}{2}}$, which describes the production of a longitudinal $rho^{0}$-meson by a longitudinal virtual photon. The ratios of nucleon-helicity-non-flip amplitudes are found to be in good agreement with those from the previous HERMES analysis. The transverse target polarization allows for the first time the extraction of ratios of a number of nucleon-helicity-flip amplitudes to $F_{0frac{1}{2}0frac{1}{2}}$. Results obtained in a handbag approach based on generalized parton distributions taking into account the contribution from pion exchange are found to be in good agreement with these ratios. Within the model, the data favor a positive sign for the $pi-rho$ transition form factor. By also exploiting the longitudinal beam polarization, a total of 71 $rho^0$ spin-density matrix elements is determined from the extracted 25 parameters, in contrast to only 53 elements as directly determined in earlier analyses.
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Exclusive rho^0-meson electroproduction is studied in the HERMES experiment, using a 27.6 GeV longitudinally polarized electron/positron beam and unpolarized hydrogen and deuterium targets in the kinematic region 0.5 GeV^2 < Q^2 < 7.0 GeV^2, 3.0 GeV < W < 6.3 GeV, and -t < 0.4 GeV^2. Real and imaginary parts of the ratios of the natural-parity-exchange helicity amplitudes T_{11} (gamma^*_T --> rho_T), T_{01} (gamma^*_T --> rho_L), T_{10} (gamma^*_L --> rho_T), and T_{1-1} (gamma^*_{-T} -->rho_T) to T_{00} (gamma^*_L --> rho_L) are extracted from the data. For the unnatural-parity-exchange amplitude U_{11}, the ratio |U_{11}/T_{00}| is obtained. The Q^2 and t dependences of these ratios are presented and compared with perturbative QCD predictions.
The transverse target spin azimuthal asymmetry A_UT in hard exclusive production of rho^0 mesons was measured at COMPASS by scattering 160 GeV/c muons off transversely polarised protons and deuterons. The measured asymmetry is sensitive to the nucleon helicity-flip generalised parton distributions E^q, which are related to the orbital angular momentum of quarks in the nucleon. The Q^2, x_B and p_t^2 dependence of A_UT is presented in a wide kinematic range. Results for deuterons are obtained for the first time. The measured asymmetry is small in the whole kinematic range for both protons and deuterons, which is consistent with the theoretical interpretation that contributions from GPDs E^u and E^d approximately cancel.
Exclusive production of $omega$ mesons was studied at the COMPASS experiment by scattering $160~mathrm{GeV}/mathit{c}$ muons off transversely polarised protons. Five single-spin and three double-spin azimuthal asymmetries were measured in the range of photon virtuality $1~(mathrm{GeV}/mathit{c})^2 < Q^2 < 10~(mathrm{GeV}/mathit{c})^2$, Bjorken scaling variable $0.003 < x_{mathit{Bj}} < 0.3$ and transverse momentum squared of the $omega$ meson $0.05~(mathrm{GeV}/mathit{c})^2 < p_{T}^{2} < 0.5~(mathrm{GeV}/mathit{c})^2$. The measured asymmetries are sensitive to the nucleon helicity-flip Generalised Parton Distributions (GPD) $E$ that are related to the orbital angular momentum of quarks, the chiral-odd GPDs $H_{T}$ that are related to the transversity Parton Distribution Functions, and the sign of the $piomega$ transition form factor. The results are compared to recent calculations of a GPD-based model.
Exclusive production of $rho^0$ and $J/psi$ mesons in e^+ p collisions has been studied with the ZEUS detector in the kinematic range $0.25 < Q^2 < 50 GeV^2, 20 < W < 167 GeV$ for the $rho^0$ data and $2 < Q^2 < 40 GeV^2, 50 < W < 150 GeV$ for the $J/psi$ data. Cross sections for exclusive $rho^0$ and $J/psi$ production have been measured as a function of $Q^2, W$ and $t$. The spin-density matrix elements $r^{04}_{00}, r^1_{1-1}$ and $Re r^{5}_{10}$ have been determined for exclusive $rho^0$ production as well as $r^{04}_{00}$ and $r^{04}_{1-1}$ for exclusive $J/psi$ production. The results are discussed in the context of theoretical models invoking soft and hard phenomena.
Exclusive photoproduction of $rho^0(770)$ mesons is studied using the H1 detector at the $ep$ collider HERA. A sample of about 900000 events is used to measure single- and double-differential cross sections for the reaction $gamma p to pi^{+}pi^{-}Y$. Reactions where the proton stays intact (${m_Y{=}m_p}$) are statistically separated from those where the proton dissociates to a low-mass hadronic system ($m_p{<}m_Y{<}10$ GeV). The double-differential cross sections are measured as a function of the invariant mass $m_{pipi}$ of the decay pions and the squared $4$-momentum transfer $t$ at the proton vertex. The measurements are presented in various bins of the photon-proton collision energy $W_{gamma p}$. The phase space restrictions are $0.5 < m_{pipi} < 2.2$ GeV, ${vert tvert < 1.5}$ GeV${}^2$, and ${20 < W_{gamma p} < 80}$ GeV. Cross section measurements are presented for both elastic and proton-dissociative scattering. The observed cross section dependencies are described by analytic functions. Parametrising the $m_{pipi}$ dependence with resonant and non-resonant contributions added at the amplitude level leads to a measurement of the $rho^{0}(770)$ meson mass and width at $m_rho = 770.8 {}^{+2.6}_{-2.7}$ (tot) MeV and $Gamma_rho = 151.3 {}^{+2.7}_{-3.6}$ (tot) MeV, respectively. The model is used to extract the $rho^0(770)$ contribution to the $pi^{+}pi^{-}$ cross sections and measure it as a function of $t$ and $W_{gamma p}$. In a Regge asymptotic limit in which one Regge trajectory $alpha(t)$ dominates, the intercept $alpha(t{=}0) = 1.0654 {}^{+0.0098}_{-0.0067}$ (tot) and the slope $alpha^prime(t{=}0) = 0.233 {}^{+0.067 }_{-0.074 }$ (tot) GeV${}^{-2}$ of the $t$ dependence are extracted for the case $m_Y{=}m_p$.
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