Abstract:In this contribution we summarize recent investigations on the deeply virtual Compton Scattering (DVCS) within the color dipole approach. The color dipole cross section is implemented through the phenomenological saturation model. The role played by its QCD evolution and skewedness effects in the DVCS cross section are discussed. The results are compared with the recent H1 and ZEUS Collaborations data. The skewing factor, defined as the ratio of the imaginary parts of the amplitudes Im A(γ * p → γ * p)/Im A(γ … Show more
“…In this article we restrict ourselves to DVCS at small x B , which has been measured at DESY on the electron/positron-proton collider HERA experiments H1 and ZEUS [8,9,7,10]. This has been previously studied in the spirit of the aligned-jet model [47], from high-energy/Regge perspective [48][49][50], in color dipole model [51][52][53][54][55], and in collinear factorization approach at leading order (LO) [56][57][58], next-leading order (NLO) [59][60][61], and next-to-next-leading order (NNLO) [62]. As said above, the view on the partonic content of the proton varies with the approach.…”
We give a partonic interpretation for the deeply virtual Compton scattering (DVCS) measurements of the H1 and ZEUS Collaborations in the small-x B region in terms of generalized parton distributions. Thereby we have a closer look at the skewness effect, parameterization of the t-dependence, revealing the chromomagnetic pomeron, and at a model-dependent access to the anomalous gravitomagnetic moment of nucleon. We also quantify the reparameterization of generalized parton distributions resulting from the inclusion of radiative corrections up to next-to-next-to-leading order. Beyond the leading order approximation, our findings are compatible with a 'holographic' principle that would arise from a (broken) SO(2,1) symmetry. Utilizing our leading-order findings, we also perform a first model-dependent "dispersion relation" fit of HERMES and JLAB DVCS measurements. From that we extract the generalized parton distribution H on its cross-over line and predict the beam charge-spin asymmetry, measurable at COMPASS. This is nothing but the RDDA model in the limit b → ∞; practically, a large value b ≫ 1 is sufficient. This ansatz implies that the skewness function is set r(η/x) = 1 for all x. With such an initial condition, evolution, starting at a rather low input scale, will rapidly lead to an increase of the r-ratio. Thus, this GPD model fails to describe data, too. We will not go into details here, 7 Usage of term "dual" was motivated by the fact that in dual models [98] the s-channel amplitude is described by the t-channel exchanges. We add that this feature is more general and arises from crossing and the Sommerfeld-Watson transform of the t-channel SO(3) partial wave expansion. In Regge theory/phenomenology the resummation of t-channel exchanges is encoded in the Regge trajectory.
“…In this article we restrict ourselves to DVCS at small x B , which has been measured at DESY on the electron/positron-proton collider HERA experiments H1 and ZEUS [8,9,7,10]. This has been previously studied in the spirit of the aligned-jet model [47], from high-energy/Regge perspective [48][49][50], in color dipole model [51][52][53][54][55], and in collinear factorization approach at leading order (LO) [56][57][58], next-leading order (NLO) [59][60][61], and next-to-next-leading order (NNLO) [62]. As said above, the view on the partonic content of the proton varies with the approach.…”
We give a partonic interpretation for the deeply virtual Compton scattering (DVCS) measurements of the H1 and ZEUS Collaborations in the small-x B region in terms of generalized parton distributions. Thereby we have a closer look at the skewness effect, parameterization of the t-dependence, revealing the chromomagnetic pomeron, and at a model-dependent access to the anomalous gravitomagnetic moment of nucleon. We also quantify the reparameterization of generalized parton distributions resulting from the inclusion of radiative corrections up to next-to-next-to-leading order. Beyond the leading order approximation, our findings are compatible with a 'holographic' principle that would arise from a (broken) SO(2,1) symmetry. Utilizing our leading-order findings, we also perform a first model-dependent "dispersion relation" fit of HERMES and JLAB DVCS measurements. From that we extract the generalized parton distribution H on its cross-over line and predict the beam charge-spin asymmetry, measurable at COMPASS. This is nothing but the RDDA model in the limit b → ∞; practically, a large value b ≫ 1 is sufficient. This ansatz implies that the skewness function is set r(η/x) = 1 for all x. With such an initial condition, evolution, starting at a rather low input scale, will rapidly lead to an increase of the r-ratio. Thus, this GPD model fails to describe data, too. We will not go into details here, 7 Usage of term "dual" was motivated by the fact that in dual models [98] the s-channel amplitude is described by the t-channel exchanges. We add that this feature is more general and arises from crossing and the Sommerfeld-Watson transform of the t-channel SO(3) partial wave expansion. In Regge theory/phenomenology the resummation of t-channel exchanges is encoded in the Regge trajectory.
“…A priori, one cannot say how important this contribution is in a particular process. As a first approximation, one can just ignore the nonperturbative effects, as was done in [19,20,21,22], and evaluate everything perturbatively. While for DIS this approximation is partially justified due to high virtualities Q 2 in both vertices, for DVCS its validity is questionable, since the final photon is real.…”
We study the DVCS amplitude within the color dipole approach. The light-cone wave function of a real photon is evaluated in the instanton vacuum model. Our parameter free calculations are able to describe H1 data, both the absolute values and the t-dependences, at medium-high values of Q 2 . The Q 2 dependence is found to be sensitive to the choice of the phenomenological cross section fitted to DIS data.
“…A priori, one cannot say how important this contribution is in a particular process. As a first approximation, one can just ignore the nonperturbative effects, as was done in [19,20,21,22], and evaluate everything perturbatively. While for DIS this approximation is partially justified due to high virtualities Q 2 in both vertices, for DVCS its validity is questionable, since the final photon is real.…”
We study the DVCS amplitude within the color dipole approach. The light-cone wave function of a real photon is evaluated in the instanton vacuum model. Our parameter free calculations are able to describe H1 data, both the absolute values and the t-dependences, at medium-high values of Q 2 . The Q 2 dependence is found to be sensitive to the choice of the phenomenological cross section fitted to DIS data.
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