Charged-particle multiplicities in pp interactions measured with the ATLAS detector at the LHC (1012.5104v2)

Abstract: Measurements are presented from proton-proton collisions at centre-of-mass energies of sqrt(s) = 0.9, 2.36 and 7 TeV recorded with the ATLAS detector at the LHC. Events were collected using a single-arm minimum-bias trigger. The charged-particle multiplicity, its dependence on transverse momentum and pseudorapidity and the relationship between the mean transverse momentum and charged-particle multiplicity are measured. Measurements in different regions of phase-space are shown, providing diffraction-reduced measurements as well as more inclusive ones. The observed distributions are corrected to well-defined phase-space regions, using model-independent corrections. The results are compared to each other and to various Monte Carlo models, including a new AMBT1 PYTHIA 6 tune. In all the kinematic regions considered, the particle multiplicities are higher than predicted by the Monte Carlo models. The central charged-particle multiplicity per event and unit of pseudorapidity, for tracks with pT >100 MeV, is measured to be 3.483 +- 0.009 (stat) +- 0.106 (syst) at sqrt(s) = 0.9 TeV and 5.630 +- 0.003 (stat) +- 0.169 (syst) at sqrt(s) = 7 TeV.

• The paper provides a detailed measurement of charged-particle multiplicities across various energy levels using the ATLAS detector.
• It employs model-independent corrections and analyzes distinct phase-space regions defined by transverse momentum and pseudorapidity.
• The findings reveal significantly higher multiplicities than predicted by Monte Carlo models, prompting the need for refined QCD simulations.

A Detailed Examination of Charged-Particle Multiplicities in Proton-Proton Collisions at the LHC

The paper under examination, "Charged-particle multiplicities in $pp$ interactions measured with the ATLAS detector at the LHC," offers a comprehensive investigation of charged-particle multiplicities resulting from proton-proton collisions, leveraging the capabilities of the ATLAS detector at the Large Hadron Collider (LHC). The paper's scope spans center-of-mass energies at $\sqrt{s} = 0.9$ TeV, $2.36$ TeV, and $7$ TeV, captured via the single-arm minimum-bias trigger. This analysis is crucial for advancing our understanding of soft Quantum Chromodynamics (QCD) processes at these energy scales.

Methodological Framework

Key methodological approaches include measuring the charged-particle multiplicity, its dependency on variables such as transverse momentum ($p_T$) and pseudorapidity ($\eta$), and examining the relationship between mean $p_T$ and charged-particle multiplicity. These measurements were conducted across various phase-space regions, allowing for both diffraction-reduced and more inclusive analyses. Importantly, the paper applies model-independent corrections to address potential biases introduced by reconstructive methodologies, thus ensuring the integrity of the findings.

Results and Observations

In detailed comparisons against a range of Monte Carlo (MC) models, the paper reveals systematic discrepancies, with the data indicating consistently higher particle multiplicities than those predicted by the MC models across all kinematic regions analyzed. Specifically, the central charged-particle multiplicity per event and unit of pseudorapidity for tracks with $p_T > 100$ MeV is notably higher than model predictions at both $\sqrt{s} = 0.9$ TeV and $\sqrt{s} = 7$ TeV. This divergence underscores the need for refined tuning of MC models to more accurately reflect empirical observations.

Furthermore, the analysis delineates three distinct phase-space regions that exhibit variations in charged-particle multiplicity. These include: (1) inclusive measurements with at least one charged particle with $p_T > 500$ MeV; (2) inclusive spectra measurements with at least two charged particles with $p_T > 100$ MeV; and (3) diffraction-suppressed regions with at least six charged particles with $p_T > 500$ MeV. The results across these regions demonstrate significant variations with regard to predictions, challenging existing theoretical frameworks.

Implications and Future Directions

This paper presents significant implications for the refinement of QCD-inspired models and their application in high-energy physics. The outlined discrepancies between observed data and model predictions point to potential areas for improvement in the theoretical treatment of soft QCD processes. Additionally, the higher multiplicities observed in the central region suggest that existing models may be overlooking key dynamic factors in proton-proton collisions at these energy scales.

Future work should aim to incorporate these findings into the development of more sophisticated models that can accommodate the nuances of observed multiplicities in high-energy collisions. Enhancements in simulation fidelity are necessary to ensure that predictive models align more closely with empirical data, thereby advancing our understanding of fundamental particle interactions under LHC conditions.

In summary, this paper makes substantial contributions to the field of particle physics by providing a rigorous analysis of charged-particle multiplicities at the LHC, thereby laying the groundwork for future investigations aimed at refining theoretical models and enhancing our understanding of proton-proton interactions.