Measurement of pseudorapidity distributions of charged particles in proton-proton collisions at sqrt(s) = 8 TeV by the CMS and TOTEM experiments

TL;DR

The paper reports a concurrent CMS-TOTEM measurement of charged-particle pseudorapidity densities in pp collisions at $\sqrt{s}=8$ TeV over $|\eta|<2.2$ and $5.3<|\eta|<6.4$, using three event classes to separate inclusive, NSD-, and SD-dominated topologies. It combines central CMS tracking with forward TOTEM T2 data, applying extensive corrections to obtain $p_T$-unbiased distributions and extrapolations to $p_T=0$, and compares the results to multiple MC models (including Pythia6/8, Herwig++, EPOS, QGSJETII). The study finds that none of the models can describe all measured distributions across all regions and event categories, highlighting strong tensions in forward production and diffractive components. It also provides the energy-dependence of central charged-particle production, showing a modest rise with energy consistent with $s^{0.23}$, and offers critical data to tune hadronic interaction and cosmic-ray models. Overall, the measurements deliver the broadest LHC pseudorapidity coverage to date and illuminate correlations between central and forward particle production.

Abstract

Pseudorapidity (eta) distributions of charged particles produced in proton-proton collisions at a centre-of-mass energy of 8 TeV are measured in the ranges abs(eta) < 2.2 and 5.3 < abs(eta) < 6.4 covered by the CMS and TOTEM detectors, respectively. The data correspond to an integrated luminosity of 45 inverse microbarns. Measurements are presented for three event categories. The most inclusive category is sensitive to 91-96% of the total inelastic proton-proton cross section. The other two categories are disjoint subsets of the inclusive sample that are either enhanced or depleted in single diffractive dissociation events. The data are compared to models used to describe high-energy hadronic interactions. None of the models considered provide a consistent description of the measured distributions.

Figures (7)

• Figure 1: Definition of the $z_\text{impact}$ parameter.
• Figure 2: The $z_\text{impact}$ parameter distribution measured in the data, for tracks reconstructed in one T2 half-arm in the range $-5.8<\eta<-5.75$. A global (double-Gaussian + exponential function) fit, performed in the range from $-8$ to 15$\text{\,m}$ is shown by the solid curve. The dotted curve represents the exponential component from secondary particles, while the dashed curve is the double-Gaussian component, mainly due to primary tracks.
• Figure 3: Trigger efficiency as a function of the total track multiplicity in the T2 telescopes for single-sided events with primary candidates in only the $z>0$ ($+$) or $z<0$ ($-$) telescope and for double-sided events with primary candidates in both telescopes.
• Figure 4: Top: tracking efficiency, $\epsilon_\text{trk}$, as a function of $p_{\mathrm{T}}$ and $\eta$ and averaged over all multiplicity bins ($n_{\text{CMS}}$), for tracks with $p_{\mathrm{T}}\xspace>0.1$${\,\text{Ge\spaceV\space/\space}c}$ and $\lvert \eta \rvert<2.2$. Bottom: correction factor, $f_{\mathrm{np}}$, for non-primary tracks as a function of $p_{\mathrm{T}}$ and $\eta$ and averaged over all multiplicity bins ($n_{\text{CMS}}$), for tracks with $p_{\mathrm{T}}\xspace>0.1$${\,\text{Ge\spaceV\space/\space}c}$ and $\lvert \eta \rvert<2.2$.
• Figure 5: Primary track efficiency as a function of $\eta$ and average pad cluster multiplicity (APM) in one T2 half-arm for the inclusive pp sample. The efficiency includes the track primary-candidate condition. Only particles with $p_{\mathrm{T}}\xspace>40$${\,\text{Me\spaceV\space/\space}c}$ are considered.