Parton Ladder Splitting and the Rapidity Dependence of Transverse Momentum Spectra in Deuteron-Gold Collisions at RHIC

TL;DR

The paper develops EPOS, an energy-conserving, parton-ladder-based multiple-scattering framework that includes off-shell remnants and ladder splitting. It identifies elastic and inelastic ladder splitting as the key nuclear effects, with inelastic splitting producing a collective hadronization of parallel ladders and resulting transverse-momentum broadening, especially in dAu collisions. The authors parameterize these effects with Z and related exponents, implement them in a Monte Carlo scheme, and demonstrate good agreement with pp and dAu data from all four RHIC experiments across rapidity and centrality. The findings support a picture where ladder splitting drives the observed spectra and modest rapidity dependence, offering a unified description of RHIC transverse-momentum phenomena.

Abstract

We present a phenomenological approach (EPOS), based on the parton model, but going much beyond, and try to understand proton-proton and deuteron-gold collisions, in particular the transverse momentum results from all the four RHIC experiments. It turns out that elastic and inelastic parton ladder splitting is the key issue. Elastic splitting is in fact related to screening and saturation, but much more important is the inelastic contribution, being crucial to understand the data. We investigate in detail the rapidity dependence of nuclear effects, which is actually relatively weak in the model, in perfect agreement with the data, if the latter ones are interpreted correctly.

Figures (28)

• Figure 1: Elementary parton-parton scattering: the hard scattering in the middle is preceded by parton emissions (initial state space-like cascade); these partons being usually off-shell, they emit further partons (final state time-like cascade). For all this we use a symbolic parton ladder.
• Figure 2: The complete picture, including remnants. The remnants are an important source of particle production at RHIC energies.
• Figure 3: Inner contributions, from the parton ladder (full lines), and "outer" contributions, from the remnants (dashed lines), to the rapidity distribution of hadrons. (Artists view)
• Figure 4: The two elements of the multiple scattering theory: open ladders, representing inelastic interactions, and closed ladders, representing elastic interactions.
• Figure 5: Basic parton-parton interaction in nucleus-nucleus collisions: a projectile parton always interacts with exactly one parton on the other side, elastically (closed parton ladder) or inelastically (open parton ladder).