arXiv subjects

G. Zardo Becker

Publications and source records attributed to G. Zardo Becker.

2 recordsLinked to original sources

arXiv2026

Orbital angular momentum at small $x$ in the large $N_c\&N_f$ limit

We extend the small-$x$ analysis of the quark and gluon orbital angular momentum (OAM) distributions in the proton from the large-$N_c$ limit considered in our earlier works to the large-$N_c\&N_f$ limit, in which the numbers of quark colors $N_c$ and flavors $N_f$ are large with their ratio held fixed. Working in the double-logarithmic approximation (DLA), summing powers of $α_s \ln^2(1/x)$ with $α_s$ the strong coupling and $x$ the proton's momentum fraction carried by a parton, we correct the small-$x$ operator expression for the quark OAM distribution suggested earlier in \cite{Kovchegov:2024wjs} and relate both the quark and gluon OAM distributions to the impact-parameter moments of the polarized dipole amplitudes. We derive the large-$N_c\&N_f$ evolution equations for these moment amplitudes in the DLA; these include a new equation for the moment amplitude governing the quark OAM. We then solve these evolution equations numerically together with the helicity evolution for $N_f = 2,3,4,5,6$ and $N_c =3$. We find that, similar to the large-$N_c$ case, the OAM distributions share a common small-$x$ intercept with the helicity distributions, $L_{q+\bar{q}}(x,Q^2) \sim L_G(x,Q^2) \sim ΔΣ(x,Q^2) \sim ΔG(x,Q^2) \sim \left(\frac{1}{x} \right)^{α_h}$ with the intercept $α_h \approx {3.48 \sqrt{α_s N_c/2π}}$ for $N_f = N_c = 3$ (cf.~\cite{Borden:2025ehe}): this result, along with $α_h$ for other values of $N_f \neq 0$ that we studied, is smaller than the intercept of $3.66\sqrt{α_s N_c/2π}$ found in the large-$N_c$ limit. We also compute the ratios of the OAM distributions to the helicity parton distribution functions as $x\to 0$, obtaining $L_{q+\bar{q}}(x,Q^2)/ΔΣ(x,Q^2) \approx -1.01$ and $L_G(x,Q^2)/ΔG(x,Q^2) \approx -1.94$ at $Q^2=10\, \mathrm{GeV}^2$, with both ratios being nearly independent of $N_f$.

hep-ph↗

arXiv2025

Phenomenology of two-photon interaction at high energies: accessing dilute and high parton density of the photon structure

In this work, $γ^{(\ast)}γ^{(\ast)}$ interactions in electron-positron collisions are studied across both low- and high-energy regimes. The analysis includes contributions from the Vector Meson Dominance (VMD) model (via Reggeon exchange), the Quark Parton Model (via box diagrams), and the gluonic component (described using the dipole formalism), which becomes dominant at high energies. A key feature of the dipole picture is that a photon can fluctuate into a quark-antiquark ($q\bar{q}$) pair, forming a color dipole. The dipole-dipole cross section is modeled using two different prescriptions. We analyze the impact of these models on several key observables: the total cross section for real photons ($σ^{γγ}$), including heavy quark production $γγ\rightarrow c\bar{c}X, b\bar{b}X$; for virtual photons ($σ^{γ^{\ast}γ^{\ast}}$); and the photon structure function ($F_2^γ$). Both prescriptions express the dipole-dipole interaction in terms of the dipole-proton scattering amplitude, used in Deep Inelastic Scattering (DIS). This amplitude is obtained by solving the Balitsky-Kovchegov (BK) non-linear evolution equation, incorporating running coupling and various models that differ in their treatment of the transition between the dilute and saturation regimes. These approaches exhibit distinct behaviors in photon-photon interactions at high energies: while one prescription describes the photon as a smaller and denser system, the other treats it as a larger and more dilute configuration. Accordingly, they predict greater and lesser hadron production in the final state, respectively, at the energies of future colliders. These characteristics become more significant with increasing photon virtuality.

hep-ph↗