Charged-particle multiplicity measurement in proton-proton collisions at $\sqrt{s}$ = 0.9 and 2.36 TeV with ALICE at LHC (1004.3034v3)

Abstract: Charged-particle production was studied in proton-proton collisions collected at the LHC with the ALICE detector at centre-of-mass energies 0.9 TeV and 2.36 TeV in the pseudorapidity range |$\eta$| < 1.4. In the central region (|$\eta$| < 0.5), at 0.9 TeV, we measure charged-particle pseudorapidity density dNch/deta = 3.02 $\pm$ 0.01 (stat.) $^{+0.08}_{-0.05}$ (syst.) for inelastic interactions, and dNch/deta = 3.58 $\pm$ 0.01 (stat.) $^{+0.12}_{-0.12}$ (syst.) for non-single-diffractive interactions. At 2.36 TeV, we find dNch/deta = 3.77 $\pm$ 0.01 (stat.) $^{+0.25}_{-0.12}$ (syst.) for inelastic, and dNch/deta = 4.43 $\pm$ 0.01 (stat.) $^{+0.17}_{-0.12}$ (syst.) for non-single-diffractive collisions. The relative increase in charged-particle multiplicity from the lower to higher energy is 24.7% $\pm$ 0.5% (stat.) $^{+5.7}_{-2.8}$% (syst.) for inelastic and 23.7% $\pm$ 0.5% (stat.) $^{+4.6}_{-1.1}$% (syst.) for non-single-diffractive interactions. This increase is consistent with that reported by the CMS collaboration for non-single-diffractive events and larger than that found by a number of commonly used models. The multiplicity distribution was measured in different pseudorapidity intervals and studied in terms of KNO variables at both energies. The results are compared to proton-antiproton data and to model predictions.

Analysis of Charged-Particle Multiplicity in Proton-Proton Collisions at the LHC

The paper investigates charged-particle production in proton-proton collisions at the LHC using the ALICE detector at two center-of-mass energies: 0.9 TeV and 2.36 TeV. The paper covers the pseudorapidity range up to 1.4, with a specific focus on the central region (|η| < 0.5), providing quantitative insights essential for understanding particle multiplicity in high-energy collisions.

Key Findings

1. Pseudorapidity Density Measurements:
   • At 0.9 TeV, the charged-particle pseudorapidity density for inelastic interactions is reported as 3.02 ± 0.01 (stat.) +0.08–0.05 (syst.), while for non-single-diffractive interactions, it is 3.58 ± 0.01(stat.) ±0.12(syst.).
   • At 2.36 TeV, the densities are 3.77 ± 0.01(stat.) +0.25−0.12(syst.) for inelastic interactions and 4.43 ± 0.01(stat.) ±0.17(syst.) for non-single-diffractive interactions.
2. Energy Dependence:
   • The relative increase in charged-particle multiplicity from 0.9 to 2.36 TeV is approximately 24.7% ±0.5%(stat.) +5.7–2.8%(syst.) for inelastic interactions and 23.7% ±0.5%(stat.) +4.6–1.1%(syst.) for non-single-diffractive interactions.
3. Comparison with Models:
   • The increase is consistent with results from the CMS collaboration for non-single-diffractive events but exceeds predictions by several commonly used models, indicating discrepancies in current theoretical approximations.
4. Multiplicity Distribution:
   • The paper measures the multiplicity distribution in various pseudorapidity intervals, analyzing KNO scaling variables. The results reveal some deviations from scaling with increasing pseudorapidity intervals, paralleling findings in historical collider data.

Implications

The measurements provide critical constraints for phenomenological models of QCD in the non-perturbative regime, essential for accurately simulating background events in searches for rare and hard processes at high-energy particle colliders. The deviation between experimental data and model predictions highlights a potential area for refinement in theoretical approaches, particularly regarding energy scaling and diffractive processes.

Future Perspectives

The insights gained from these measurements set the stage for further exploration as the LHC probes even higher energy regimes. Improved models incorporating these findings could enhance our understanding of hadronic interactions and better predict experimental results, contributing to advancements in particle physics.

The paper's analyses also pave the way for subsequent studies focusing on particle yield, event structure, and other global characteristics that determine the dynamics of proton-proton collisions, each of which holds potential for discovery in fundamental physics exploration.