Transverse nucleon structure and multiparton interactions

TL;DR

The work develops a coherent impact-parameter framework to study the transverse structure of the nucleon and its role in high-energy pp and pA collisions. By tying GPDs and the two-gluon form factor to the spatial distribution of gluons, it predicts narrower b distributions for dijet-triggered events and substantial multiparton interaction effects that depend on correlations and fluctuations of the gluon field. It introduces postselection in the black-disk regime as a mechanism for effective energy losses, explaining forward hadron suppression and forward–central correlations observed at RHIC, and extends the analysis to double-forward pion production, where double-parton scattering can dominate in certain kinematic regimes. The findings have significant implications for modeling the underlying event and forward physics at the LHC, highlighting the need to include transverse structure, parton correlations, and nonlinear QCD dynamics in event generators and phenomenology.

Abstract

The transverse structure of the nucleon as probed in hard exclusive processes plays critical role in the understanding of the structure of the underlying event in hard collisions at the LHC, and multiparton interactions. We summarize results of our recent studies of manifestation of transverse nucleon structure in the hard collisions at the LHC, new generalized parton distributions involved in multiparton interactions, presence of parton fluctuations. The kinematic range where interaction of fast partons of the projectile with the target reach black disk regime (BDR) strength is estimated. We demonstrate that in the BDR postselection effect leads to effective fractional energy losses. This effect explains regularities of the single and double forward pion production in $ dAu$ collisions at RHIC and impacts on the forward physics in $pp$ collisions at the LHC.

Figures (10)

• Figure 1: $pp$ collisions at small and large impact parameters. Transverse and side views. Dark (gray) disks correspond to the areas occupied by hard (soft) partons.
• Figure 2: Transverse geometry of hard collisions.
• Figure 3: $t$-dependence of the exclusive $J/\psi$ photoproduction data from the FNAL E401/458 experiment Binkley:1981kv. Solid line: $t$--dependence obtained with the exponential parametrization of the two-gluon form factor, Eq.5 (the slope of the $J/\psi$ cross section is $B_{J/\psi} = B_g + \Delta B$, where $\Delta B \approx 0.3 \, \hbox{GeV}^{-2}$ accounts for the finite size of the $J/\psi$; see Frankfurt:2010ea for details). Dashed line: $t$--dependence obtained with the dipole parametrization, Eq.5. Dotted line: $t$--dependence obtained with PYTHIA, effectively corresponding to a dipole form factor with $m^2 \approx 2 \, \hbox{GeV}^2$.
• Figure 4: Overlap integral of the transverse spatial parton distributions, defining the impact parameter distribution of $pp$ collisions with a hard parton--parton process, Eq. (\ref{['P_2_def']}).
• Figure 5: (a) Impact parameter distributions of inelastic $pp$ collisions at $\sqrt{s} = 7 \, \hbox{TeV}$. Solid (dashed) line: Distribution of events with a dijet trigger at zero rapidity, $y_{1, 2} = 0$, for $p_T = 100 \, (10) \, \hbox{GeV}$ cf. Eq. (\ref{['P_2_exp_dip']}). Dotted line: Distribution of minimum--bias inelastic events,cf. Eq. (\ref{['P_in_def']}). (b) Dependence of median $b$ on $p_T$ for different rapidities of the dijets.