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Register a new userMeasurement of Exclusive $pi^0$ Electroproduction Structure Functions and their Relationship to Transversity GPDs
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Exclusive $pi^0$ electroproduction at a beam energy of 5.75 GeV has been measured with the Jefferson Lab CLAS spectrometer. Differential cross sections were measured at more than 1800 kinematic values in $Q^2$, $x_B$, $t$, and $phi_pi$, in the $Q^2$ range from 1.0 to 4.6 GeV$^2$, $-t$ up to 2 GeV$^2$, and $x_B$ from 0.1 to 0.58. Structure functions $sigma_T +epsilon sigma_L, sigma_{TT}$ and $sigma_{LT}$ were extracted as functions of $t$ for each of 17 combinations of $Q^2$ and $x_B$. The data were compared directly with two handbag-based calculations including both longitudinal and transversity GPDs. Inclusion of only longitudinal GPDs very strongly underestimates $sigma_T +epsilon sigma_L$ and fails to account for $sigma_{TT}$ and $sigma_{LT}$, while inclusion of transversity GPDs brings the calculations into substantially better agreement with the data. There is very strong sensitivity to the relative contributions of nucleon helicity flip and helicity non-flip processes. The results confirm that exclusive $pi^0$ electroproduction offers direct experimental access to the transversity GPDs.
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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 $m_Y{=}m_p$.
Exclusive photoproduction cross sections have been measured for the process $gamma p rightarrow ppi^0(e^+e^-(gamma))$ with the Dalitz decay final state using tagged photon energies in the range of $E_{gamma} = 1.275-5.425$ GeV. The complete angular distribution of the final state $pi^0$, for the entire photon energy range up to large values of $t$ and $u$, has been measured for the first time. The data obtained show that the cross section $dsigma/dt$, at mid to large angles, decreases with energy as $s^{-6.89pm 0.26} $. This is in agreement with the perturbative QCD quark counting rule prediction of $s^{-7} $. Paradoxically, the size of angular distribution of measured cross sections is greatly underestimated by the QCD based Generalized Parton Distribution mechanism at highest available invariant energy $s=11$ GeV$^2$. At the same time, the Regge exchange based models for $pi^0$ photoproduction are more consistent with experimental data.
In this talk various spin effects in hard exclusive electroproduction of mesons are briefly reviewed and the data discussed in the light of recent theoretical calculations within the frame work of the handbag approach. For electroproduction of positively charged pions it is shown that there is a strong contribution from photon-pion transitions which can be modeled by the transversity GPD H_T accompanied by the twist-3 pion wave function.
The results on the photo- and electroexcitation amplitudes of most nucleon resonances in the mass range up to 2.0 GeV determined from the CLAS experimental data on exclusive $pi^+pi^-p$ photo-/electroproduction off protons in collaboration between the Jefferson Lab and Moscow State University are presented. The first and only available results on electroexcitation amplitudes from CLAS in a wide range of photon virtualities $Q^2$ $<$ 5.0 GeV$^2$ revealed the nucleon resonance structure as a complex interplay between the inner core of three dressed quarks and external meson-baryon cloud. These results shed light on the strong QCD dynamics which underlines the generation of excited nucleon states of different structural features from confined quarks and gluons. The future prospects of these studies in the new era of experiments with the CLAS12 detector, which started successfully in Spring of 2018, are outlined.
Exclusive neutral-pion electroproduction ($epto e^prime p^prime pi^0$) was measured at Jefferson Lab with a 5.75-GeV electron beam and the CLAS detector. Differential cross sections $d^4sigma/dtdQ^2dx_Bdphi_pi$ and structure functions $sigma_T+epsilonsigma_L, sigma_{TT}$ and $sigma_{LT}$ as functions of $t$ were obtained over a wide range of $Q^2$ and $x_B$. The data are compared with Regge and handbag theoretical calculations. Analyses in both frameworks find that a large dominance of transverse processes is necessary to explain the experimental results. For the Regge analysis it is found that the inclusion of vector meson rescattering processes is necessary to bring the magnitude of the calculated and measured structure functions into rough agreement. In the handbag framework, there are two independent calculations, both of which appear to roughly explain the magnitude of the structure functions in terms of transversity generalized parton distributions.
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