Related papers: Low-x scaling in $\gamma^*p$ total cross sections
We investigate the possibility to constrain the pion distribution amplitude from the gamma* gamma* -> pi transition. For a surprisingly large range in the two photon virtualities we find that the transition form factor is essentially…
The DIS diffractive cross section, $d\sigma^{diff}_{\gamma^* p \to XN}/dM_X$, has been measured in the mass range $M_X < 15$ GeV for $\gamma^*p$ c.m. energies $60 < W < 200$ GeV and photon virtualities $Q^2 = 7$ to 140 GeV$^2$. For fixed…
We study inclusive electroproduction on the proton at low $x$ and low $Q^2$ using a soft and a hard Pomeron. The contribution of the soft Pomeron is based on the Stochastic Vacuum Model, in which a nonperturbative dipole-dipole cross…
The scale dependence of the evolution of photoproduction cross sections with the photon-proton centre of mass energy W is studied using low Q^2 < 0.01 GeV^2 e^+p interactions collected by the H1 experiment at HERA. The value of the largest…
The photon-proton total cross section has been measured in the process e+ p -> e+ gamma p -> e+ X with the ZEUS detector at HERA. Events were collected with photon virtuality Q^2 < 0.02 GeV^2 and average gamma-p center-of-mass energy…
We discuss $(\pi^0,\eta,\eta')\to\gamma^*\gamma$ transition form factors using the light-front quark model. Our discussion includes the analysis of the mixing angles for $\eta-\eta'$. Our results for $Q^2…
Spin dependent structure function $g_1^{\gamma}(x,Q^2)$ of the polarised photon is analysed within the formalism based upon the unintegrated spin dependent parton distributions incorporating the LO Altarelli-Parisi evolution and the double…
For photon virtualities of $Q^2 \gsim 20 {\rm GeV}^2$, the results of the (hard Pomeron) fit in the tensor-Pomeron model in terms of the variables $Q^2$ and the virtual-photon-proton energy squared, $W^2$, provide empirical evidence for the…
We extend the QCD sum rule analysis of the \gamma^*\gamma^* -->\pi^o form factor into the region where one of the photons has small virtuality: q^2 << Q^2 > 1 GeV^2. In this kinematics, one should perform an additional factorization of…
The general structure of the cross section of $\gamma N$ scattering with polarized photon and/or nucleon in initial and/or final state is systematically described and exposed through invariant amplitudes. A low-energy expansion of the cross…
Scalar mesons are a key expression of the infrared regime of QCD. The lightest of these is the $\sigma$. Now that its pole in the complex energy plane has been precisely located, we can ask whether this state is transiently ${\bar q}q$ or…
The dijet cross section as a function of both the fraction of the photon momentum participating in the hard process, x_\gamma^{OBS}, and the photon's virtuality, Q^2, is compared with the predictions of Herwig 5.9 for various photon parton…
An analysis of all experimental data of 50's - 90's on elastic gamma-p scattering at photon energies below 150 MeV (10 experimental works) is performed in order to determine the electric (alpha_p) and magnetic (beta_p) polarizabilities of…
We discuss the relevance of a dedicated measurement of exclusive production of a pair of neutral pions in a hard gamma* gamma scattering at small momentum transfer. In this case, the virtuality of one photon provides us with a hard scale in…
We study the scaling regime of nucleon-anti-nucleon annihilation into a deeply virtual photon and a photon or meson, \bar p N -> gamma ^* pi, \bar p N -> gamma ^* gamma, in the forward direction. The leading twist amplitude factorizes into…
We generalize our previous model for $\gamma^* p$ scattering to $\gamma \gamma$ scattering. Performing a new simultaneous fit to $\gamma^* p$ and $\gamma \gamma$ total cross section we find an optimal set of parameters to describe both…
Virtual Compton Scattering is studied at the Thomas Jefferson National Accelerator Facility in the energy domain below pion threshold and in the Delta(1232) resonance region. The data analysis is based on the Dispersion Relation (DR)…
We present one- and two-jet inclusive cross sections for gamma*gamma scattering and virtual photoproduction in ep collisions. The hard cross sections are calculated in next-to-leading order QCD. Soft and collinear singularities are…
We discuss total cross-sections within the context of the QCD calculable mini-jet model, highlighting its successes and failures. In particular we show its description of $\gamma \gamma \to hadrons$ and compare it with OPAL and L3 data. We…
The meson-photon transition form factors $\gamma\gamma^*\to P$ ($P$ stands for $\pi$, $\eta$ and $\eta'$) provide strong constraints on the distribution amplitudes of the pseudoscalar mesons. In this paper, these transition form factors are…