Charged particle transverse momentum spectra
in pp collisions at √s = 0.9 and 7 TeV

The charged particle transverse momentum (pT) spectra are presented for pp collisions at √s = 0.9 and 7 TeV, collected with the CMS detector during the first months of the 2010 LHC run. To extend the statistical reach of the measurements, calorimeter-based high-transverse-energy triggers are employed to enhance yields at high pT. The results are compared with both leading-order QCD and with an empirical scaling of different collision energies using the scaling variable xT≡ 2pT/√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 √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 the intermediate center-of-mass energy.

Approved Plots from QCD-10-008 (click for pdf)

Figure 1 Abbreviated Caption
(a) The efficiency for selecting non-single-diffractive (NSD) events as a function of the multiplicity of reconstructed charged particles in the tracker acceptance (|η|<2.4) after applying the full event selection, including a single pixel track (filled circles) and additionally requiring a fully-reconstructed-track vertex with Ndof>4 (open circles). Also, the remaining single-diffractive (SD) fraction as a function of charged particle multiplicity for the same selections (solid and dashed lines).

(b) For data events with more than one fully-reconstructed-track vertex satisfying the quality selections, the correlation between the z positions of the two vertices with the most associated tracks.
(a) The algorithmic tracking efficiency for two different momentum ranges as a function of η.

(b) The product of geometrical acceptance with tracking efficiency (upper points) and the misidentification ('fake') rate (lower points) as a function of transverse momentum for tracks with |η|<1 in bins of corrected leading-jet transverse energy.
(a) Distributions of the corrected transverse energy of leading jets normalized by the number of selected minimum bias events.
Lower panel: the efficiency turn-on curves for the jet triggers with uncorrected energy thresholds of 15 and 50 GeV.

(b) The three contributions to the charged particle transverse momentum spectrum and their sum.
Lower panel: the ratio of the combined spectrum to minimum bias only (solid circles) and adding only the lower threshold jet trigger (open triangles).
(a) The invariant charged particle differential yield from the present analysis (solid circles) and the previous CMS 0.9 TeV measurement (stars) over the limited pT 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 errors 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 √s = 7 TeV.
(a) The invariant charged particle differential yield at √s = 0.9 TeV 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 gray band corresponds to the statistical and systematic uncertainties added in quadrature.

(b) The same for √s = 7 TeV.
(a) Inclusive charged particle invariant differential cross sections, scaled by √s4.9, for |η|<1 as a function of the scaling parameter xT.

(b) Upper panel: ratios of the scaled differential cross sections to the global power-law xT fit described in the text (colored markers) and fits to these ratios (similarly colored lines). The expected ratio for √s = 2.76 TeV after applying NLO-based corrections to each of the three measurements (solid blue lines). The corresponding uncertainty from the NLO parameters is represented by the shaded band.
Lower panel: ratios of the NLO-calculated cross sections at three different energies, scaled by √s4.9, to the cross section calculated at √s = 2.75 TeV. The width of the bands represents the variation of the factorization scale by a factor of two.
(a) Interpolations between measured charged particle differential cross sections at different √s for the two example values of pT = 3 and 9 GeV/c. Second-order polynomial fits to the measured data are shown by the solid lines. The open squares show the resulting interpolated cross sections for √s = 2.76 TeV. The open circle on the lower panel represents the corresponding estimate from the xT-scaling approach in the overlap region where both can be estimated.

(b) The predicted 2.76 TeV charged particle differential transverse momentum cross section, based on the combined direct pT interpolation and NLO-corrected xT-scaling techniques described in the text.
Lower panel: ratios of combined interpolation to predictions from several PYTHIA tunes, an NLO-based rescaling approach External site, and the ALICE interpolation External site.}

Link to paper on arXiv

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