Inclusive and multiplicity-dependent pseudorapidity densities of charged particles in pp collisions at $\mathbf{\sqrt{s} = 13.6}$ TeV

TL;DR

This work presents a high-precision measurement of the inclusive and multiplicity-dependent charged-particle pseudorapidity density $\langle \mathrm{d}N_{\mathrm{ch}}/\mathrm{d}\eta \rangle$ in pp collisions at $\sqrt{s}=13.6$ TeV using ALICE's Run 3 detectors. Leveraging the upgraded ITS and TPC with continuous readout and the forward FT0M-based multiplicity classifier, the study analyzes INEL$>$0 events across $|\eta|<1.5$, obtaining $\langle \mathrm{d}N_{\mathrm{ch}}/\mathrm{d}\eta \rangle = 7.10 \pm 0.18$ for $|\eta|<0.5$ and establishing a power-law energy scaling $\langle \mathrm{d}N_{\mathrm{ch}}/\mathrm{d}\eta \rangle \propto s^{\delta}$ with $\delta = 0.115 \pm 0.003$ up to $\sqrt{s}=13.6$ TeV. The analysis also explores multiplicity dependence, showing a strong correlation between forward activity and midrapidity density, with high-multiplicity events yielding significantly elevated densities; comparisons to PYTHIA 8 Monash 2013 and EPOS4 reveal that the former describes the data well while the latter overestimates central yields, underscoring the need for improved MPI, color reconnection, and hadronization modeling. These results provide a crucial reference for charged-particle production at the highest LHC energy and offer stringent constraints for tuning event generators in small systems. The work emphasizes how forward detectors and detector upgrades enable deeper insight into event activity and particle production mechanisms in high-energy pp collisions.

Abstract

The distribution of the pseudorapidity density of primary charged particles (${\rm d}N_{\rm ch}/ {\rm d}η$) in $|η|<1.5$ is measured in pp collisions at a center-of-mass energy of 13.6 TeV using the ALICE detector. Tracks are reconstructed with the upgraded Inner Tracking System and Time Projection Chamber, and results are reported for the INEL$>$0 event class (events having at least one charged particle in $|η|<1$). The multiplicity dependence of the pseudorapidity density of charged particles is also explored with the multiplicity determined by the forward detectors. The average charged-particle pseudorapidity density at midrapidity ($|η|<0.5$) is measured to be $\langle \mathrm{d}N_{\mathrm{ch}}/\mathrm{d}η\rangle = 7.10 \pm 0.18$. Combined with lower-energy measurements, the INEL$>$0 results follow a power-law scaling with the center-of-mass energy. These measurements provide a new reference for charged-particle production at the highest proton$-$proton collision energy available at the LHC.

Figures (4)

• Figure 1: The pseudorapidity distribution of primary charged particles for INEL>0 events in pp collisions at $\sqrt{s} = 13.6$ TeV. Data are compared to model predictions from PYTHIA 8 Monash 2013 and EPOS4 with their variations. The systematic uncertainties in the data are represented by gray bands. The lower panel shows the ratio of model predictions to the data.
• Figure 2: The values of $\langle \mathrm{d}N_\mathrm{ch} / \mathrm{d}\eta \rangle$ in $|\eta|<0.5$ for the INEL, NSD, and INEL>0 event classes as a function of center-of-mass energy ALICE:2009wplALICE:2010cinALICE:2010mtyALICE:2015olqALICE:2015qqjALICE:2020swjUA1:1989bouUA5:1986yefSTAR:2008medCDF:1989nknCMS:2010tjhCMS:2010wcxCMS:2015zrmAmes-Bologna-CERN-Dortmund-Heidelberg-Warsaw:1983cqwPHOBOS:2004xnp. The lines depict a power-law fit for each event class. The gray bands show one standard deviation of the fit where the errors are quite small. The $\langle \mathrm{d}N_\mathrm{ch} / \mathrm{d}\eta \rangle$ value for INEL>0 events in pp collisions at $\sqrt{s}=13.6$ TeV is included (the red star symbol). Note that the newly added data point is not included in the fit.
• Figure 3: Charged-particle pseudorapidity density as a function of $\eta$ for the 0--1 to 70--100% multiplicity classes measured with the forward multiplicity estimator, illustrating the dependence of particle production on forward event activity. Higher event activity classes exhibit an overall increase in charged-particle multiplicity, while the shape of the distributions remains consistent across different multiplicity classes.
• Figure 4: $\mathrm{d}N_\mathrm{ch}/\mathrm{d}\eta$ distributions for different multiplicity classes in pp collisions at $\sqrt{s}=13.6$ TeV. The top, third and fifth rows present $\mathrm{d}N_\mathrm{ch}/\mathrm{d}\eta$ distributions from the 0--1% to the 70--100% multiplicity classes, and the other rows display the model-to-data ratios of the rows above. The distributions are compared with theoretical predictions from PYTHIA 8 and EPOS4 with their variations. Note that the multiplicity classes in the models correspond to INEL>0, and multiplicity classes in the data correspond to MB>0 classification as listed in Tab. \ref{['tab:multdependentavdndeta']}.