1,529 research outputs found

### Mass dependence of nuclear shadowing at small Bjorken-x from diffractive scattering

We calculate the nuclear shadowing ratio for a wide range of nuclei at small
Bjorken-x in the framework of Gribov theory. The coherent contribution to the
(virtual) photon-nucleon cross section is obtained in terms of the diffractive
dissociation cross section. Information on diffraction from FNAL and HERA is
used. Our results are compared to available experimental data from the NMC and
E665 experiments at x ~ 10^{-4}.Comment: To be published in Phys. Rev.

### Photon parton distributions in nuclei and the EMC effect

Photons as well as quarks and gluons are constituents of the infinite
momentum frame (IMF) wave function of an energetic particle. They are mostly
equivalent photons whose amplitude follows from the Lorentz transformation of
the particle rest frame Coulomb field into the IMF and from the conservation of
the electromagnetic current. We evaluate in a model independent way the
dominant photon contribution \propto \alpha_{em}(Z^2/A^{4/3})\ln(1/R_{A}m_{N}x)
to the nuclear structure functions as well as the term \propto \alpha_{em}Z/A.
In addition we show that the definition of x consistent with the exact
kinematics of eA scattering (with exact sum rules) works in the same direction
as the nucleus field of equivalent photons. Combined, these effects account for
the bulk of the EMC effect for x\le 0.5 where Fermi motion effects are small.
In particular for these x the hadronic mechanism contribution to the EMC effect
does not exceed \sim 3% for all nuclei. Also the A-dependence of the hadronic
mechanism of the EMC effect for x > 0.5 is significantly modified.Comment: 14 pages, 3 figures, final version published in Phys.Rev.

### Nuclear shadowing in inclusive and tagged deuteron structure functions and extraction of F_2^p-F_2^n at small x from electron-deuteron collider data

We review predictions of the theory of leading twist nuclear shadowing for
inclusive unpolarized and polarized deuteron structure functions F_2^D, g_1^D
and b_1^D and for the tagged deuteron structure function F_2^D(x,Q^2,\vec{p}).
We analyze the possibility to extract the neutron structure function F_2^n from
electron-deuteron data and demonstrate that an account of leading twist nuclear
shadowing leads to large corrections for the extraction of F_2^n from the
future deuteron collider data both in the inclusive and in the tagged structure
function modes. We suggest several strategies to address the extraction of
F_2^n and to measure at the same time the effect of nuclear shadowing via the
measurement of the distortion of the proton spectator spectrum in the
semi-inclusive e D \to e^{\prime}NX process. We address the issue of the final
state interactions in the e D \to e^{\prime}NX process and examine how they
affect the extraction of F_2^n.Comment: 19 pages, 7 figures. Published in Mod. Phys. Lett.

### A unitarized model of inclusive and diffractive DIS with Q2-evolution

We discuss the interplay of low-x physics and QCD scaling violations by
extending the unified approach describing inclusive structure functions and
diffractive production in $\gamma* p$ interactions proposed in previous papers,
to large values of Q2. We describe the procedure of extracting, from the
non-perturbative model, initial conditions for the QCD evolution that respect
unitarity. Assuming Regge factorization of the diffractive structure function,
a similar procedure is proposed for the calculation of hard diffraction. The
results are in good agreement with experimental data on the proton structure
function $F_2$ and the most recent data on the reduced diffractive cross
section, x_P \sigma_r^{\D(3)}. Predictions for both $F_2$ and $F_L$ are
presented in a wide kinematical range and compared to calculations within
high-energy QCD.Comment: 22 pages, 12 figure

### Probing coherent charmonium photoproduction off light nuclei at medium energies

We demonstrate how the elementary amplitudes $\gamma N\to \Psi N$, the
amplitude of the nondiagonal $J/\psi N\Leftrightarrow \psi' N$ transition, and
the total $J/\psi N$ and $\psi' N$ cross sections can be determined from
measurements of the coherent $J/\psi$ and $\psi'$ photoproduction off light
nuclei at moderate energies. For this purpose we provide a detailed numerical
analysis of the coherent charmonium photoproduction off silicon within the
generalized vector dominance model (GVDM) adjusted to account for the physics
of charmonium models and color transparency phenomenon.Comment: 8 pages, 5 figures (color

### Transverse target spin asymmetry in inclusive DIS with two-photon exchange

We study the transverse target spin dependence of the cross section for
inclusive electron-nucleon scattering with unpolarized beam. Such dependence is
absent in the one-photon exchange approximation (Christ-Lee theorem) and arises
only in higher orders of the QED expansion, from the interference of one-photon
and absorptive two-photon exchange amplitudes as well as from real photon
emission (bremsstrahlung). We demonstrate that the transverse spin-dependent
two-photon exchange cross section is free of QED infrared and collinear
divergences. We argue that in DIS kinematics the transverse spin dependence
should be governed by a "parton-like" mechanism in which the two-photon
exchange couples mainly to a single quark. We calculate the normal spin
asymmetry in an approximation where the dominant contribution arises from quark
helicity flip due to interactions with non-perturbative vacuum fields
(constituent quark picture) and is proportional to the quark transversity
distribution in the nucleon. Such helicity-flip processes are not significantly
Sudakov-suppressed if the infrared scale for gluon emission in the photon-quark
subprocess is of the order of the chiral symmetry breaking scale, mu_chiral^2
>> Lambda_QCD^2. We estimate the asymmetry in the kinematics of the planned
Jefferson Lab Hall A experiment to be of the order 10^{-4}, with different sign
for proton and neutron. We also comment on the spin dependence in the limit of
soft high-energy scattering.Comment: 22 pages, 14 figures; uses revtex

### Active-to-absorbing state phase transition in the presence of fluctuating environments: Weak and strong dynamic scaling

We investigate the scaling properties of phase transitions between survival
and extinction (active-to-absorbing state phase transition, AAPT) in a model,
that by itself belongs to the directed percolation (DP) universality class,
interacting with a spatio-temporally fluctuating environment having its own
non-trivial dynamics. We model the environment by (i) a randomly stirred fluid,
governed by the Navier-Stokes (NS) equation, and (ii) a fluctuating surface,
described either by the Kardar-Parisi-Zhang (KPZ) or the Edward-Wilkinson (EW)
equations. We show, by using a one-loop perturbative field theoretic set up,
that depending upon the spatial scaling of the variance of the external forces
that drive the environment (i.e., the NS, KPZ or EW equations), the system may
show {\em weak} or {\em strong dynamic scaling} at the critical point of active
to absorbing state phase transitions. In the former case AAPT displays scaling
belonging to the DP universality class, whereas in the latter case the
universal behavior is different.Comment: 17 pages, 2 figures, accepted in PR

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