Breakdown of QCD factorization at large Feynman x

TL;DR

The paper argues that nuclear suppression at large Feynman x (xF) across diverse reactions is governed by energy-conservation–driven Sudakov suppression and enhanced resolution of higher Fock states in nuclei, leading to a breakdown of QCD factorization rather than reliance on coherence-based explanations like the color glass condensate. Using a rest-frame light-cone dipole framework and Glauber-Gribov formalism with AGK rules, the authors quantify suppression for leading hadrons, high-pT forward hadrons (BRAHMS data), dileptons, and charmonium, reproducing observed xF scaling with minimal reliance on gluon shadowing. They show that shadowing effects vanish or are small at the relevant kinematics, while Sudakov suppression and effective energy loss predict universal xF-dependent suppression across reactions, aligning with data and providing a cohesive, testable mechanism. The work emphasizes factorization breaking as a leading-twist effect in these regimes and offers predictions for forward production at current and future colliders, while acknowledging areas for quantitative refinement in hadronization and charm/anticharm channels.

Abstract

Recent measurements by the BRAHMS collaboration of high-pT hadron production at forward rapidities at RHIC found the relative production rate(d-Au)/(p-p) to be suppressed, rather than enhanced. Examining other known reactions (forward production of light hadrons, the Drell-Yan process, heavy flavor production, etc.), one notes that all of these display a similar property, namely, their cross sections in nuclei are suppressed at large xF. Since this is the region where x2 is minimal, it is tempting to interpret this as a manifestation of coherence, or of a color glass condensate, whereas it is actually a simple consequence of energy conservation and takes place even at low energies. We demonstrate that in all these reactions there is a common suppression mechanism that can be viewed, alternatively, as a consequence of a reduced survival probability for large rapidity gap processes in nuclei, Sudakov suppression, an enhanced resolution of higher Fock states by nuclei, or an effective energy loss that rises linearly with energy. Our calculations agree with data.

Figures (8)

• Figure 1: Shadowing in DY reaction on carbon, iron and tungsten as function of $x_2$ at $M_{\bar{l}l}=4.5\,GeV$ and $s=1600\,\hbox{GeV}^2$. Nuclear shadowing disappears both at large and small $x_2$, since the coherence length, Eq. (\ref{['10']}), vanishes in both limits.
• Figure 2: The exponent describing the $A$-dependence ($\propto A^\alpha$) of the ratio for the production of different hadrons in $p-Au$ relative to $pp$ collisions as function of $x_F$. The collection of data and references can be found in bartongeistdata. The curve is a result of a parameter free calculation using Eq. (\ref{['120']}).
• Figure 3: Ratio of negative particle production rates in $d-Au$ and $pp$ collisions as function of $p_T$. Data are from brahms, solid and dashed curves correspond to calculations with the diquark size $0.3\,\hbox{fm}$ and $0.4\,\hbox{fm}$ respectively.
• Figure 4: Number of negative hadrons versus $p_T$ produced in $pp$ collisions at $\sqrt{s}=200\,\hbox{GeV}$ and pseudorapidity $\eta=3.2$. Our calculations, given by the solid curve, are compared with BRAHMS data brahms.
• Figure 5: Ratio of negative particle production in central (0-20%) and semi-central (30-50%) to peripheral (60-80%) $d-Au$ collisions, shown by closed and open points respectively. The results of the corresponding calculations are depicted by solid and dashed curves.