Photon-induced jet production at future lepton colliders

TL;DR

The paper addresses photon-induced jet production at future $e^+e^-$ colliders as a means to probe the parton structure of photons, focusing on the balance between direct and resolved photon contributions. It models lepton-photon scattering using the Weizsäcker-Williams photon flux and includes both direct and resolved photon processes, employing SAS1M photon PDFs and MEPS@LO merging to coherently combine channels with a democratic QCD+QED evolution to account for anomalous $\gamma\to q\bar q$ components. The results show that the resolved component dominates across the accessible phase space, with forward lepton tagging essential to reach the small-$Q^2$ and small-$x_\gamma$ regime, while direct contributions emerge at high $Q^2$ and near $x_\gamma\to1$, and a $Z^0$ peak appears in the direct hadronic mass distribution. Breit-frame jets are not ideal for resolving photon structure, but Breit-frame event shapes like thrust $\tau$ and jet broadening $B$ provide complementary information; the lab frame remains the preferred basis for jets, and cuts on jet multiplicity or kinematics can help suppress $W$-boson background, although the large-$Q^2$ region remains challenging for photon-PDF fits. The work highlights experimental and theoretical considerations for constraining photon PDFs at future facilities (FCC, CEPC, ILC) and underscores the need for forward detector coverage and refined final-state modeling.

Abstract

The production of hadronic final states in electron-positron or electron-hadron collisions is induced predominantly by quasi-real photons that were emitted off incoming leptons. In these processes, the photon either enters directly or through its resolved parton content, which is at present only loosely constrained by experimental data. We perform a detailed phenomenological study of photon-induced jet production processes in high-energy $e^+e^-$ collisions, investigating in particular their potential to assess contributions from the resolved photon structure.

Figures (8)

• Figure 1: Examples of diagrams contributing to deep-inelastic scattering of an electron on a collinear quasi-real photon. (a) The photon can be resolved and interact hadron-like. (b) The direct contribution contributes at the level of two out-going partons.
• Figure 2: Distributions in virtuality $Q^2$ (top left), parton-to-photon momentum ratio $x_\gamma$ (top right), hadronic mass $W^2_\mathrm{h}$ (bottom left) and photon-to-lepton momentum ratio $z$ (bottom right) for $e \gamma \to e X$, with and without cut on the rapidity of the out-going lepton, and separating the direct and resolved contributions to the cross-section.
• Figure 3: Distributions in leading jet transverse momentum $p_T^{(1)}$ (top row) and jet multiplicity $N$ (bottom row) in the laboratory (left column) and Breit frame (right column) for $e \gamma \to e X$, with and without cut on the rapidity of the out-going lepton, and separating the direct and resolved contributions to the cross-section.
• Figure 4: Event shape distributions in the variables thrust $\tau = 1 - T$ (left) and jet broadening $B$ (right) for $e \gamma \to e X$, separating the direct and resolved contributions to the cross-section.
• Figure 5: Distributions in virtuality $Q^2$ (left) and Bjorken variable $x_\gamma$ (right), for the $e \gamma \to e X$ and $e^+e^- \to W^\ast_{\to l \bar{\nu}} W^\ast_{\to jj}$ processes, with and without cut on the rapidity of the out-going lepton.