DFTT xx/96 August 1996 Longitudinal and Transverse Nuclear Shadowing

نویسندگان

  • V. Barone
  • M. Genovese
چکیده

Nuclear shadowing arises from multiple scattering of the hadronic uctuations (jq qi; jq qg:::i) of the virtual photon in a nucleus. We predict di erent longitudinal and transverse shadowing and an A{dependence of R L= T which can be up to a 50% e ect. The possibility of detecting nuclear e ects on R at HERA is discussed. To be published in the Proceedings of the Workshop on \Future Physics at HERA". Supported by EU Contract ERBFMBICT 950427. While there are very di erent ideas on the nature of the EMC e ect at intermediate and large values of x, the situation seems to be less controversial at small x, the region of nuclear shadowing (NS). There is a general consensus that NS is due to the recombination of partons belonging to di erent nucleons. A concrete and quantitative realization of this old idea has been o ered in [1] where NS is attributed to the multiple scattering of the hadronic uctuations of the virtual photon in nuclei. One can thus speak of parton recombination in that the multiple scattering diagrams involve quarks and gluons which do not belong to a single nucleon. This approach leads rather naturally to the prediction of di erent shadowing e ects in the longitudinal and transverse channels and consequently to an enhancement of R L= T in nuclei [2]. In the following we shall sketch the derivation of this result and present some quantitative estimates. We shall also study the possibility of measuring nuclear e ects on R at HERA. In virtual-photon{nucleus scattering, using Glauber formalism, the nuclear cross section is given by A = A N 4 A 1 A d D dt t=0 Z d~b T (~b) + : : : ; (1) where d =dt is the N di raction dissociation cross section integrated over the mass M of the excited hadronic states d D dt t=0 = Z dM d D dtdM2 t=0 F(k L) : (2) The longitudinal form factor of the nucleus F(k L) appearing in (2) suppresses heavy mass excitations corresponding to non negligible longitudinal momenta of the recoil proton, kL = (Q 2 +M)=2 = xmN (1 +M =Q). Eq. (1) establishes a link between the leading nuclear shadowing correction and the pomeron structure function which is proportional to d =dt. This allows relating the di ractive DIS currently under experimental study at HERA to the small-x nuclear phenomena which will hopefully be a future chapter of the HERA program. In the Nikolaev{Zakharov picture of small{x DIS [3, 4] the double scattering term in (1) can be calculated on the basis of the Fock structure of the virtual photon interacting with the nucleus. At small x, the hadronic states into which the uctuates

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Longitudinal and Transverse Nuclear Shadowing

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تاریخ انتشار 1996