Transverse momentum and pseudorapidity distributions of charged hadrons in pp collisions at sqrt(s) = 0.9 and 2.36 TeV

TL;DR

CMS measured inclusive charged-hadron $p_T$ and $\eta$ distributions in pp collisions at $\sqrt{s}=0.9$ and 2.36 TeV during LHC commissioning. Using three independent reconstruction approaches, the study provides $dN_{ch}/d\eta$ and $dN_{ch}/d p_T$ spectra, fits the $p_T$ distributions with a Tsallis form to extract mean $p_T$, and reports central densities with high precision. The 2.36 TeV results show a larger-than-expected rise in charged-hadron multiplicity compared with model tunes, extending the energy frontier and offering a crucial baseline for heavy-ion comparisons. Overall, the work demonstrates the CMS tracker performance and delivers baseline soft-particle production measurements at the highest collider energy to date.

Abstract

Measurements of inclusive charged-hadron transverse-momentum and pseudorapidity distributions are presented for proton-proton collisions at sqrt(s) = 0.9 and 2.36 TeV. The data were collected with the CMS detector during the LHC commissioning in December 2009. For non-single-diffractive interactions, the average charged-hadron transverse momentum is measured to be 0.46 +/- 0.01 (stat.) +/- 0.01 (syst.) GeV/c at 0.9 TeV and 0.50 +/- 0.01 (stat.) +/- 0.01 (syst.) GeV/c at 2.36 TeV, for pseudorapidities between -2.4 and +2.4. At these energies, the measured pseudorapidity densities in the central region, dN(charged)/d(eta) for |eta| < 0.5, are 3.48 +/- 0.02 (stat.) +/- 0.13 (syst.) and 4.47 +/- 0.04 (stat.) +/- 0.16 (syst.), respectively. The results at 0.9 TeV are in agreement with previous measurements and confirm the expectation of near equal hadron production in p-pbar and pp collisions. The results at 2.36 TeV represent the highest-energy measurements at a particle collider to date.

Figures (7)

• Figure 1: (a) Generated multiplicity distributions of primary charged hadrons in the range $|\eta| < 2.5$ for $\sqrt{s} =$ 0.9 TeV (solid dots and histogram) and 2.36 TeV (open circles and dashed histogram) after the event selection is applied to the reconstructed events, using inelastic events from the pythia (histograms) and phojet (symbols) event generators. (b) The event selection efficiency expected for NSD events from the pythia (histograms) and phojet (symbols) event generators as a function of generated charged hadron multiplicity in the region $|\eta|<2.5$.
• Figure 2: \ref{['vz_npix_1']} The distribution of the reconstructed $z$ position of the primary vertex in the data (dots), compared to that from the pythia simulation (histogram). \ref{['landau']} Distribution of the cluster charge multiplied by $|\sin\theta|$ in the data (dots) and the simulation (histogram), for the clusters selected for analysis.
• Figure 3: \ref{['background_1']} Pixel cluster length along $z$ as a function of $\eta$ for the 900 GeV data. The solid line illustrates the cut applied in the cluster counting method. \ref{['background_2']} The $\Delta\eta$ distribution of clusters on tracklets in the data (dots and circles), together with the distribution obtained from the pythia simulation (solid and dotted lines), for both 0.9 and 2.36 TeV collision energy. The dashed line shows the $\Delta\eta$ distribution of clusters for primary charged-particle tracks in the Monte Carlo simulation at 0.9 TeV. The tail of the $\Delta\eta$ distribution comes from the combinatorial background.
• Figure 4: \ref{['vz_npix_2']} The distribution of the number of pixel hits attached to reconstructed tracks in the region of $|\eta|<1$ (closed circles), compared to the CMS detector simulation (histogram). \ref{['nstrip']} The distribution of the number of hits in the SST detector attached to reconstructed tracks in the region of $|\eta|<1$ (closed circles), compared to the CMS detector simulation (histogram).
• Figure 5: (a) Measured differential yield of charged hadrons in the range $|\eta| < 2.4$ in $0.2$-unit-wide bins of $|\eta|$ for the 2.36 TeV data. The measured values with systematic uncertainties (symbols) and the fit functions (Eq. \ref{['eq:tsallis']}) are displayed. The values with increasing $\eta$ are successively shifted by four units along the vertical axis. (b) Measured yield of charged hadrons for $|\eta| < 2.4$ with systematic uncertainties (symbols), fit with the empirical function (Eq. \ref{['eq:tsallis']}).