Charged particle transverse momentum spectra in pp collisions at sqrt(s) = 0.9 and 7 TeV

TL;DR

This study provides precise measurements of charged-particle pT spectra in pp collisions at 0.9 and 7 TeV with the CMS detector, testing perturbative QCD predictions and exploiting xT scaling to interpolate a 2.76 TeV pp reference for PbPb analyses. It employs a combination of minimum-bias and jet-triggered data, sophisticated vertexing and track-selection, and careful jet-based event classification to extend the high-pT reach. The results show general agreement with parton-level expectations and reveal that xT scaling holds across energies with a modest NLO-driven residual; the 7 TeV data favor pythia8 tunes while 0.9 TeV data align better with ProQ20. The two-pronged interpolation to 2.76 TeV produces a robust reference spectrum with ~12–13% uncertainty, facilitating accurate measurements of high-pT particle suppression in heavy-ion collisions. Overall, the work strengthens the pp baseline essential for interpreting QCD medium effects at the LHC and provides methodological advances in combining energy scaling with fixed-energy interpolation.

Abstract

The charged particle transverse momentum (pT) spectra are presented for pp collisions at sqrt(s)=0.9 and 7 TeV. The data samples were collected with the CMS detector at the LHC and correspond to integrated luminosities of 231 inverse microbarns and 2.96 inverse picobarns, respectively. Calorimeter-based high-transverse-energy triggers are employed to enhance the statistical reach of the high-pT measurements. The results are compared with both leading-order QCD and with an empirical scaling of measurements at different collision energies using the scaling variable xT = 2 pT/sqrt(s) over the pT range up to 200 GeV/c. Using a combination of xT scaling and direct interpolation at fixed pT, a reference transverse momentum spectrum at sqrt(s)=2.76 TeV is constructed, which can be used for studying high-pT particle suppression in the dense QCD medium produced in heavy-ion collisions at that centre-of-mass energy.

Figures (7)

• Figure 1: (a) The efficiency ($\varepsilon_{\mathrm{NSD}}^{\mathrm{selected}}$ in Eq. (\ref{['eqn:evtWeight']})) for selecting non-single-diffractive (NSD) events as a function of the multiplicity of reconstructed charged particles in the tracker acceptance ($|\eta|<2.4$) after applying the full event selection described in the text, including a single pixel-track vertex (filled circles) and additionally requiring a fully-reconstructed-track vertex with $Ndof>4$ (open circles) as described in Section \ref{['sec:vtx']}. Also, the remaining single-diffractive (SD) fraction ($f^{\mathrm{selected}}_{\mathrm{SD}}$ in Eq. (\ref{['eqn:evtWeight']})) as a function of charged particle multiplicity for the same selections (solid and dashed lines). (b) Correlation between the $z$ positions, $z^0_{\mathrm{PV}}$ and $z^1_{\mathrm{PV}}$, of the two vertices with the most associated tracks for measured events with more than one fully-reconstructed-track vertex satisfying the quality selections.
• Figure 2: (a) The algorithmic tracking efficiency for two different momentum ranges as a function of $\eta$. (b) The product of geometrical acceptance (A) with tracking efficiency ($\varepsilon^{\mathrm{tr}}$) (upper points) and the misidentification ('fake') rate (lower points) as a function of transverse momentum for tracks with $|\eta|<1$ in bins of corrected leading-jet transverse energy.
• Figure 3: (a) Upper panel: distributions of the corrected transverse energy of leading jets normalised by the number of selected minimum bias events $N^{\mathrm{Evt}}_{\mathrm{MB}}$. Lower panel: the efficiency turn-on curves for the jet triggers with uncorrected energy thresholds of 15 and 50$\,\text{Ge\spaceV}$. (b) Upper panel: the three contributions to the charged particle transverse momentum spectrum and their sum (solid circles). Open squares show the minimum bias spectrum for all values of leading-jet $E_{\mathrm{T}}$; open triangles show the spectrum with the addition of only the lower threshold jet trigger. Lower panel: the ratio of the combined spectrum to minimum bias only (solid circles) and with the addition of only the lower threshold jet trigger (open triangles).
• Figure 4: (a) Upper panel: the invariant charged particle differential yield from the present analysis (solid circles) and the previous CMS measurements at $\sqrt{s}=0.9$$\,\text{Te\spaceV}$ (stars) over the limited $p_{\mathrm{T}}$ range of the earlier result. Lower panel: the ratio of the new (solid circles) and previous (stars) CMS results to a Tsallis fit of the earlier measurement. Error bars on the earlier measurement are the statistical plus systematic uncertainties added in quadrature. The systematic uncertainty band around the new measurement consists of all contributions, except for the common event selection uncertainty. (b) The same for $\sqrt{s}=7$$\,\text{Te\spaceV}$.
• Figure 5: (a) Upper panel: the invariant charged particle differential yield at $\sqrt{s}=0.9$$\,\text{Te\spaceV}$ compared with the predictions of four tunes of the pythia MC generator. Lower panel: the ratio of the new CMS measurement to the four pythia tunes. The grey band corresponds to the statistical and systematic uncertainties added in quadrature. (b) The same for $\sqrt{s}=7$$\,\text{Te\spaceV}$.