Parton distributions and small-x QCD at the Large Hadron Electron Collider

TL;DR

The work analyzes the physics potential of the Large Hadron Electron Collider (LHeC) to probe nucleon structure and small-x QCD using the NNPDF framework. It shows that combining F2 and, critically, F_L pseudo-data can sharply constrain the low-x gluon distribution beyond what F2 alone achieves. The study explores whether LHeC data can reveal departures from fixed-order DGLAP by testing non-DGLAP models (saturation/dipole) and by exploiting small-x resummation, proposing a distance-based method to identify deviations. Overall, the results underscore the pivotal role of accurate F_L measurements and outline a path to integrating full LHeC data and assessing impacts on LHC phenomenology.

Abstract

The proposed Large Hadron Electron Collider (LHeC) at CERN would bring Deep-Inelastic scattering into the unexplored TeV regime. The LHeC rich physics program, among other topics, includes both precision SM measurements to complement LHC physics as well as studies of QCD in the high energy limit. The present contribution reports on ongoing studies within the NNPDF framework towards the LHeC CDR. We study the impact of LHeC simulated data on PDF uncertainties, in particular the small-x gluon. We also assess the LHeC potential to disentangle between various scenarios of small-x QCD, including saturation models and small-x resummation. Finally, we explore how deviations from DGLAP can be quantified in inclusive measurements.

Figures (5)

• Figure 1: The kinematical coverage of the LHeC pseudo-data used in the present studies, together with the data already included in the reference NNPDF1.2 dataset.
• Figure 2: Left: the gluon which results from a fit which includes on top of the NNPDF1.2 dataset nnpdf12 only $F_2$ LHeC pseudo-data. Right: the gluon PDF determined from the same dataset together with $F_2$ and $F_L$ LHeC pseudo-data, generated from the central and $\pm$1-$\sigma$ NNPDF1.0 gluons.
• Figure 3: The results of the combined DGLAP analysis of the NNPDF1.2 data set and the LHeC pseudo-data for $F_L(x,Q^2)$ in various $Q^2$ bins generated with the AAMS09 model.
• Figure 4: A comparison of various approximations to linear low-$x$ QCD for $F_2$ at the LHeC: the NNPDF1.0 prediction which includes PDF uncertainties (green lines) and the NNPDF1.0 result corrected with the NNLO (black, dot-dashed) and NLOres (violet, short-dashed) K-factors. The expected experimental precision at the LHeC is also shown for illustration.
• Figure 5: Left: distances, Eq. \ref{['eq:dist']}, in the $Q^2< 1.5~x^{-0.3}$ HERA region computed using a global fit with these data points included. Right: distances in the same region computed when these points are excluded from the global fit.