Deeply Virtual Compton Scattering at Next-to-Leading Order

DVCS (a) and BH (b and c)

Contents

  • 1. Introduction
  • 2. Codes for NLO DVCS: evolution, amplitudes and observables
  • 3. References for the forward PDFs we've used so far
  • 4. References for the NLO QCD analysis and our other papers on DVCS

    • 1. Introduction

    Welcome to the NLO DVCS homepage of Andreas Freund and Martin McDermott!

    We have recently (2001) completed a next-to-leading order (NLO) analysis of the deeply virtual Compton scattering (DVCS) process shown in fig a) above. This included writing a NLO skewed evolution program for generalised parton distributions (GPDs), a convolution code (now also fro LO twist-3 amplitudes) to produce NLO DVCS amplitudes and a NLO DVCS observables code (now new and extended version). On the lepton level DVCS interferes with the QED Compton or Bethe-Heitler process shown in figs b) and c). These programs allowed us to produce numbers for physical cross sections and asymmetries in both HERA and HERMES kinematics for the ep -> ep gamma process. Links to our papers on hep-ph may be found below.

    We have proposed a new GPD model ( hep-ph/0208160 and hep-ph/0306012 ), fullfilling all mathematical requirements, which describes the available data on DVCS from H1, ZEUS, HERMES and CLAS experiments well. The model is inspired by the aligned jet model and uses the following ansatz:

    F^{S,NS,G}(X,\zeta) = f^{S,NS,G}((X-\zeta/2)/(1-\zeta/2)) for the DGLAP region

    and a simple analytical form securing the polynomiality of the moments and the symmetries of the GPDs in the ERBL region (see hep-ph/0208160. and hep-ph/0306012 for more details!).

    These models are employed in our updated code for generating the input GPDs.

    There are now NUCLEAR GPD models avaliable ( hep-ph/0307211 ) based on two different parameterizations of nuclear shadowing. Send email to get the necessary input files and instructions!

    This web page now contains all of the fortran code for our NLO DVCS analysis.

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    2. Codes

  • Current Readme for the codes. Please read carefully since it explains the usage of the various programms below!

  • Version 3 Source code for the LO and NLO evolution of GPDs (requires input code file, see below!). You'll also need the GRSV LO and NLO gridfiles for the polarised case (valence scenario) and table files for CTEQ6 in LO and NLO . We'll send you an input code (see foot of the page for our email addresses), with 9 choices of input PDFs, to be used in conjunction with the evolution code to generate LO/NLO GPDs as appropriate.

  • Convolution code for the DVCS amplitude to convolute LO/NLO GPDs with LO/NLO coefficient functions of DVCS with sample program . Now extended to compute LO Twist-3 amplitudes (you also need this program. See readme file.)

  • A new DVCS observables package (v2: with improved interpolation routine using a 2D Spline) with a global amplitudes grid . With this package (module plus sample program plus readme) you can compute any DVCS observables you want or write your own Monte Carlo program with the module supplying the theoretical input. The global amplitude grid is based on GPDs which allows one to reproduce the current world DVCS data in both LO and NLO. This package also allows you to compute DVCS observables for four nuclei (O-16, Ca-40, Pd-110 and Pb-206) (spin-0 case) using these files for the t-dependence.

  • An updated (more flexible) code to calculate the unpolarised DVCS cross section on the photon level from the unpolarised amplitude in HERA kinematics (assuming single photon exchange)

    • Coming at one point

  • Global grid for the nuclear DVCS amplitudes.

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    4. References

    Our papers on the NLO DVCS analysis

    A. Freund, M. McDermott and M. Strikman, "Modelling generalized parton distributions to describe deeply virtual Compton scattering data", in Phys. Rev D.


    A. Freund and M. McDermott, 'A detailed next-to-leading order QCD analysis of deeply virtual Compton scattering observables' in Eur. Phys. J. C . Note the published version is substantially shorter than the original preprint hep-ph/0111472 (v1)


    A. Freund and M. McDermott, 'A next-to-leading order QCD analysis of deeply virtual Compton scattering amplitudes', in Phys. Rev. D .


    A. Freund and M. McDermott, "A next-to-leading order analysis of Deeply Virtual Compton Scattering', in Phys. Rev. D


    A. Freund and M. McDermott, 'Next-to-leading order evolution of generalised parton distributions for HERA and HERMES', in Phys. Rev. D


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    Our other papers on DVCS


    M. McDermott, R. Sandapen and G. Shaw, "Colour dipoles and virtual Compton scattering", hep-ph/0107224 , Eur. Phys. J. C 22 (2002) 655.


    A. Freund and M. Strikman, "Single Spin asymmetry in DVCS", hep-ph/9906205 , Phys Rev. D60 (1999) 071501.


    L. Frankfurt, A. Freund and M. Strikman, "Deeply Virtual Compton Scattering at HERA - A probe of Asymptopia", hep-ph/9806535 , Phys. Lett. B460 (1999) 417.


    A. Freund and J.C. Collins, "Proof of Factorization for Deeply Virtual Compton Scattering in QCD ", hep-ph/9801262 , Phys. Rev. D59 (1999) 074009.


    L. Frankfurt, A. Freund and M. Strikman, "Diffractive Exclusive Photon Production in DIS at HERA", hep-ph/9710356 , Phys. Rev. D58 (1998) 114001; Erratum-ibid. D59 (1999) 119901.

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    Andreas Freund and Martin McDermott 15.12.03 (from this date onward the site will be handled by Vadim Guzey vadimg@tp2.ruhr-uni-bochum.de

    Email : martinmc@sune.amtp.liv.ac.uk

    andreas.freund@physik.uni-regensburg.de

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