Semihard interactions at TeV energies

TL;DR

The paper investigates the high-energy behavior of $σ_{ ext{tot}}$ and $ρ$ in $pp$ and $ar{p}p$ scattering using a QCD-inspired eikonal model in which semihard gluon-driven processes (minijets) set the rise of forward observables. The eikonal is split into soft and semihard pieces, with the semihard part modeled by $χ^{+}_{ ext{SH}}(s,b)=rac{1}{2}{ ext N} ext{σ}_{ ext{QCD}}(s)W_{ ext{SH}}(b;ν_{ ext{SH}})$ and the soft piece describing lower-energy data; PDFs (NNPDF4.0, CT18, MSHT20) and a phenomenological $K$-factor are used to stabilize predictions for $ ext{σ}_{ ext{QCD}}(s)$ and to mitigate scale dependence. Global forward-data fits are performed for two data ensembles reflecting ATLAS/ALFA and TOTEM results, revealing that an even-dominant semihard amplitude can describe $ ext{σ}_{ ext{tot}}$ and $ρ$ reasonably well, but central 13 TeV $ρ$ values prefer including an asymptotically surviving crossing-odd component (Odderon). The results underscore the impact of PDF choice and normalization and motivate future work on odd-under-crossing contributions and unitarization schemes to fully reconcile high-energy forward data. Key methodological elements include the impact-parameter eikonal formalism, the separation into $ ext{χ}^{+}$ and $ ext{χ}^{-}$ with dispersion relations, the dipole-based overlap densities $W_{ ext{SH}}$, and the use of LO QCD cross sections augmented by a $K$-factor to mimic higher-order effects, all aimed at a cohesive description of forward hadronic scattering across energies. The study provides potential evidence for an Odderon in the semihard regime and highlights the necessity of further theoretical refinements to capture precise 13 TeV forward-data central values, with implications for our understanding of high-energy QCD dynamics in hadronic collisions.

Abstract

We investigate the high-energy behavior of the total cross section, $σ_{\text{tot}}$, and the ratio of the real to imaginary parts of the scattering amplitude, $ρ$, in both proton-proton and antiproton-proton channels. Our analysis is based on a QCD-inspired model in which the rise of the cross sections is predominantly driven by semihard processes involving gluons. We address the tension between measurements from the ATLAS/ALFA and TOTEM Collaborations, showing that independent analyses of their datasets can provide statistically consistent descriptions of the overall data, even though they do not fully reproduce the central values of the $ρ$ parameter at $\sqrt{s} = 13$ TeV. The slight discrepancy between these central values and the model's predicted values, obtained using an asymptotic dominant crossing-even elastic scattering amplitude, points to the potential presence of an odd component in the semihard amplitude at high energies.

Figures (2)

• Figure 1: Description of $\sigma_{\text{tot}}(s)$ and $\rho(s)$ obtained from the global fit to Ensemble A. The solid and and dashed curves correspond to the results using CT18 and NNPDF4.0, respectively.
• Figure 2: Description of $\sigma_{\text{tot}}(s)$ and $\rho(s)$ obtained from the global fit to Ensemble T. The solid and and dashed curves correspond to the results using CT18 and NNPDF4.0, respectively.