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The authors calculate the cross section of the deeply virtual Compton scattering at large energies and intermediate momentum transfers. In recent years the study of the deeply virtual Compton scattering (DVCS) became one of the most popular topics in QCD due to the fact that it is determined by skewed parton distributions which generalize usual parton densities introduced by Feynman. These new probes of the nucleon structure are accessible in exclusive processes such as DVCS and potentially they can give more information than the traditional parton densities. In this paper the authors consider the small-x DVCS where the energy of the incoming virtual photon E is very large in comparison to its virtuality Q2. To be specific, they calculate the DVCS amplitude in the region s” Q2” -t” m2 where s = 2mE, m is the nucleon mass, and t is the momentum transfer. The DVCS in this region is a semihard process which can be described by the BFKL (Balitsky-Fadin-Kuraev-Lipatov) pomeron. It turns out that at large momentum transfer the coupling of the BFKL pomeron to the nucleon is essentially equal to the Dirac form factor of the nucleon F1(t), so the DVCS amplitude in the region can be calculated without any model assumptions. The results obtained in this region can be used for the estimates of the amplitude at experimentally accessible energies where one or more conditions in Eq. (1) are relaxed. To be specific, they have in mind the HERA kinematics where x H"10−2 ̦ 10−4, Q2 e"6 GeV2, and -t H"1 ̦ 5 GeV2. Since there are only model predictions for the small-x DVCS in current literature, even the approximate calculations of the cross section in QCD are very timely.
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The authors calculate the cross section of the deeply virtual Compton scattering at large energies and intermediate momentum transfers. In recent years the study of the deeply virtual Compton scattering (DVCS) became one of the most popular topics in QCD due to the fact that it is determined by skewed parton distributions which generalize usual parton densities introduced by Feynman. These new probes of the nucleon structure are accessible in exclusive processes such as DVCS and potentially they can give more information than the traditional parton densities. In this paper the authors consider the small-x DVCS where the energy of the incoming virtual photon E is very large in comparison to its virtuality Q2. To be specific, they calculate the DVCS amplitude in the region s” Q2” -t” m2 where s = 2mE, m is the nucleon mass, and t is the momentum transfer. The DVCS in this region is a semihard process which can be described by the BFKL (Balitsky-Fadin-Kuraev-Lipatov) pomeron. It turns out that at large momentum transfer the coupling of the BFKL pomeron to the nucleon is essentially equal to the Dirac form factor of the nucleon F1(t), so the DVCS amplitude in the region can be calculated without any model assumptions. The results obtained in this region can be used for the estimates of the amplitude at experimentally accessible energies where one or more conditions in Eq. (1) are relaxed. To be specific, they have in mind the HERA kinematics where x H"10−2 ̦ 10−4, Q2 e"6 GeV2, and -t H"1 ̦ 5 GeV2. Since there are only model predictions for the small-x DVCS in current literature, even the approximate calculations of the cross section in QCD are very timely.
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Languages : en
Pages : 13357
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Author: Elena Kuchina
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Category : Nuclear reactions
Languages : en
Pages : 142
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Languages : en
Pages : 17
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Http://dx.doi.org/10.1140/epjc/s10052-008-0655-x Within the framework of the Color Glass Condensate model, we evaluate quark and gluon Generalized Parton Distributions (GPDs) and the cross section of Deeply Virtual Compton Scattering (DVCS) in the small-$x_{B}$ region. We demonstrate that the DVCS cross section becomes independent of energy in the limit of very small $x_{B}$, which clearly indicates saturation of the DVCS cross section. Our predictions for the GPDs and the DVCS cross section at high-energies can be tested at the future Electron-Ion Collider and in ultra-peripheral nucleus-nucleus collisions at the LHC.
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The authors propose a measurement of the Deep Virtual Compton Scattering process (DVCS) ep → ep[gamma] in Hall A at Jefferson Lab with a 6 GeV beam. The authors are able to explore the onset of Q2 scaling, by measuring a beam helicity asymmetry for Q2 ranging from 1.5 to 2.5 GeV2 at x{sub B} ≈ 0.35. At this kinematics, the asymmetry is dominated by the DVCS Bethe-Heitler (BH) interference, which is proportional to the imaginary part of the DVCS amplitude amplified by the full magnitude of the BH amplitude. The imaginary part of the DVCS amplitude is expected to scale early. Indeed, the imaginary part of the forward Compton amplitude measured in deep inelastic scattering (via the optical theorem) scales at Q2 as low as 1 GeV2. If the scaling regime is reached, they make an 8% measurement of the skewed parton distributions (SPD) contributing to the DVCS amplitude. Also, this experiment allows them to separately estimate the size of the higher-twist effects, since they are only suppressed by an additional factor 1/Q compared to the leading-twist term, and have a different angular dependence. They use a polarized electron beam and detect the scattered electron in the HRSe, the real photon in an electromagnetic calorimeter (under construction) and the recoil proton in a shielded scintillator array (to be constructed). This allows them to determine the difference in cross-sections for electrons of opposite helicities. This observable is directly linked to the SPD's. The authors estimate that 25 days of beam (600 hours) are needed to achieve this goal.
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An overview is given about the capabilities provided by the JLab 12 GeV Upgrade to measure deeply virtual exclusive processes with high statistics and covering a large kinematics range in the parameters that are needed to allow reconstruction of a spatial image of the nucleon's quark structure. The measurements planned with CLAS12 will cross section asymmetries with polarized beams and with longitudinally and transversely polarized proton targets in the constrained kinematics $x = \pm \xi$. In addition, unpolarized DVCS cross sections, and doubly polarized beam target asymmetries will be measured as well. In this talk only the beam and target asymmetries will be discussed.
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The authors give a complete representation of virtual Compton scattering gamma*p --> gamma-p at large initial photon virtuality Q2 and small momentum transfer squared t in terms of the light-cone wavefunctions of the target proton. They verify the identities between the skewed parton distributions H(x, zeta, t) and E(x, zeta, t) which appear in deeply virtual Compton scattering and the corresponding integrands of the Dirac and Pauli form factors F1(t) and F2(t) and the gravitational form factors A{sub q}(t) and B{sub q}(t) for each quark and anti-quark constituent. The authors illustrate the general formalism for the case of deeply virtual Compton scattering on the quantum fluctuations of a fermion in quantum electrodynamics at one loop.
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Applications of perturbative QCD to deeply virtual Compton scattering process require a generalization of usual parton distributions for the case when long-distance information is accumulated in nonforward matrix elements of quark and gluon operators. The authors discuss two types of functions parameterizing such matrix elements: double distributions F(x, y;t) and nonforward distribution functions F{sub zeta}(X;t) and also their relation to usual parton densities f{sub a}(x).
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Author: Ken Takase
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Category :
Languages : en
Pages : 29
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