Limiting fragmentation in high-energy nuclear collisions at the CERN Large Hadron Collider

The hypothesis of limiting fragmentation (LF), or, as it has been otherwise recently called, longitudinal scaling, is an interesting phenomena in the high-energy multiparticle production process. This paper discusses different regions of phase space and their importance in hadron production, giving special emphasis on the fragmentation region. Although it was conjectured as a universal phenomenon in high-energy physics, with the advent of higher center-of-mass energies, it has become prudent to analyze and understand the validity of such a hypothesis in view of the increasing inelastic nucleon-nucleon cross section (${\sigma }_{\mathrm{in}}$). In this work, we revisit the phenomenon of limiting fragmentation for nucleus-nucleus ($A$+$A$) collisions in the pseudorapidity distribution of charged particles at various energies. We use energy-dependent ${\sigma }_{\mathrm{in}}$ to transform the charged-particle pseudorapidity distributions ($d{N}_{\mathrm{ch}}^{AA}/d\eta$) into differential cross section per unit pseudorapidity ($d{\sigma }^{AA}/d\eta$) of charged particles and study the phenomenon of LF. We find that in $d{\sigma }^{AA}/d\eta$ LF seems to be violated at Large Hadron Collider (LHC) energies while considering the energy-dependent ${\sigma }_{\mathrm{in}}$. We also perform a similar study using the A Multi-Phase Transport model with a string melting scenario and also find that LF is violated at LHC energies.

Figure 1

A schematic of (pseudo)rapidity distribution showing the pionization and fragmentation regions.

Figure 2

The inelastic cross section as a function of $\sqrt{s}$. The symbols are experimental data [34, 35, 36, 37] and the fitted lines are phenomenologically motivated functions.

Figure 3

The number of participant pair normalized pseudorapidity distribution of charged particles ($d{N}_{\mathrm{ch}}^{AA}/d\eta$) in heavy-ion collisions versus $\eta -y_{\mathrm{beam}}$ for various energies. The symbols are experimental data [35, 43, 44, 45] and the lines are the double Gaussian fits.

Figure 4

The differential cross section per unit pseudorapidity ($d{\sigma }^{AA}/d\eta$) as a function of $\eta -y_{\mathrm{beam}}$ for various collision energies. The symbols are experimental points and the lines are double Gaussian fits.

Figure 5

The comparison of AMPT model predictions with experimental data on $d{N}_{\mathrm{ch}}^{AA}/d\eta$ versus $\eta -y_{\mathrm{beam}}$ for various energies.

Figure 6

$d{\sigma }^{AA}/d\eta$ versus $\eta -y_{\mathrm{beam}}$ using AMPT results.