Transverse-momentum and pseudorapidity distributions of charged hadrons in pp collisions at sqrt(s) = 7 TeV (1005.3299v2)

Abstract: Charged-hadron transverse-momentum and pseudorapidity distributions in proton-proton collisions at sqrt(s) = 7 TeV are measured with the inner tracking system of the CMS detector at the LHC. The charged-hadron yield is obtained by counting the number of reconstructed hits, hit-pairs, and fully reconstructed charged-particle tracks. The combination of the three methods gives a charged-particle multiplicity per unit of pseudorapidity, dN(charged)/d(eta), for |eta| < 0.5, of 5.78 +/- 0.01 (stat) +/- 0.23 (syst) for non-single-diffractive events, higher than predicted by commonly used models. The relative increase in charged-particle multiplicity from sqrt(s) = 0.9 to 7 TeV is 66.1% +/- 1.0% (stat) +/- 4.2% (syst). The mean transverse momentum is measured to be 0.545 +/- 0.005 (stat) +/- 0.015 (syst) GeV/c. The results are compared with similar measurements at lower energies.

Overview of Transverse-Momentum and Pseudorapidity Distributions in Proton-Proton Collisions at 7 TeV

This paper presents detailed measurements of transverse momentum (\pt) and pseudorapidity ($\eta$) distributions for charged hadrons in proton-proton ($pp$) collisions conducted at the Large Hadron Collider (LHC) at a center-of-mass energy of 7 TeV. The experimental data was acquired using the inner tracking system of the Compact Muon Solenoid (CMS) detector. These measurements offer insights not only into the particle production dynamics at this unprecedented energy scale but also serve as benchmarks for adjusting phenomenological models and event generators widely employed in high-energy physics.

Key Experimental Findings

The paper employs three independent methods to derive the charged-particle multiplicity: reconstruction of hits within the pixel detector, cluster-pair construction termed as pixel tracklets, and full track reconstruction within the tracking system. These methods collectively provide a charged-particle multiplicity per unit pseudorapidity $$\text{d}N_{\text{ch}}/\text{d}\eta|_{|\eta| < 0.5} = 5.78\pm 0.01\text{ (stat.)}\pm 0.23\text{ (syst.)}$$ for non-single-diffractive (NSD) events. Notably, this result surpasses predictions from several commonly used simulation frameworks, indicating potential inadequacies in the underlying physics representations of these models at higher collision energies.

The increase in charged-particle multiplicity from $\sqrt{s} = 0.9$ to 7 TeV is quantified at $$66.1 \pm 1.0\text{ (stat.)} \pm 4.2\text{ (syst.)}\%$$, aligning with measurements reported by other collider experiments such as ALICE. Furthermore, the mean transverse momentum ($\langle \pt \rangle$) of charged particles is determined to be $$0.545\pm 0.005\text{ (stat.)}\pm 0.015\text{ (syst.)}$$ GeV/c. The paper compares these findings with past measurements at lower energies, thereby illustrating the energetic progression and its implications on charged particle dynamics.

Implications and Future Directions

The results from this paper have substantial implications for the understanding of soft and hard scattering processes in hadronic collisions. Since most particle production in $pp$ collisions arises from soft interactions, these measurements provide crucial empirical data to refine phenomenological models and tune event generators. Discrepancies between the measured and predicted particle yields signal the necessity for improved simulations that accurately account for the energy-dependent behavior of diffractive processes and particle multiplicities.

The paper sets a foundation for further investigation into nuclear-medium effects in heavy ion collisions, such as those in Pb-Pb interactions, by establishing baseline measurements for proton-proton collisions at high energy. The success in achieving higher precision at 7 TeV opens avenues for theoretical developments in quantum chromodynamics (QCD) and particle production models, with potential impacts on the broader field of particle physics.

As experimental capabilities at the LHC advance, continued measurements at varying energy scales and in diverse collision environments are anticipated to provide deeper insights into particle physics phenomena, which, combined with theoretical innovations, will contribute to a more comprehensive understanding of the universe’s fundamental constituents and their interactions.