Study of the inclusive production of charged pions, kaons, protons
in pp collisions at √s = 0.9, 2.76 and 7 TeV

Eur Phys J C 72 (2012) 2164 arXiv:1207.4724 HepData

Abstract

Measured spectra of identified charged hadrons produced in pp collisions at √s = 0.9, 2.76 and 7 TeV are presented in the transverse momentum range p_T ≈ 0.1-1.7 GeV/c at midrapidity (|y|<1). The charged pions, kaons, and protons are identified using the measured energy loss in the silicon tracks and the calculated goodness-of-fit value of the reconstructed track. The fully corrected primary p_T spectra and integrated yields are compared to various tunes of the Pythia6 and Pythia8 event generators. The average p_T for pions, kaons and protons increases rapidly with the mass of the hadron and with the event charged-particle multiplicity. This increase does not seem to depend on the center-of-mass energy. The average p_T of protons, as a function of √s and multiplicity, is not reproduced by the models.

Data

Data files are available: cms_pid_spectra.tgz

Figures

1 Introduction

Figure	Caption
	Figure 1-left Values of the most probable energy deposit ε, at the reference path length of 450 μm, in silicon for electrons, pions, kaons and protons. The insert shows the region 1 < p < 5 GeV/c.

2 CMS detector

Figure	Caption
	Figure 1-right The accessible (y, pT) range for electrons, pions, kaons and protons. It is limited from below according to the η acceptance of the tracker and due to tracking inefficiencies at low momentum; from above because of limitations in particle identification at high momentum.

3 Data analysis

Figure	Caption
	Figure 2 Event selection: ratio of selected to DS events (ratio of the corresponding efficiencies in the inelastic sample), according to Pythia6 tunes (0.9 TeV -- D6T, 2.76 TeV -- Z2, 7 TeV -- Z1), as a function of reconstructed primary charged track multiplicity.

4 Energy deposits and estimation of energy loss rate

Figure	Caption
	Figure 3 Validation of energy deposit model, in case of the 7 TeV dataset. Measured energy deposit distributions of identified hadrons at given βγ values PXB (left) and TIB (right) are shown. Values are given at path lengths of l = 270, 300, 450, 600, 750, 900 and 1050 μm silicon, shown together with model predictions (curves) already containing the hit-level corrections (scale factors and shifts). The average cluster noise σn is also indicated.
	Figure 4 Distribution of logε values as a function of total momentum p in case of the 2.76 TeV dataset, for positives (left) and negatives (right). Note that the color scale is linear. The curves show logε for electrons, pions, kaons and protons.

5 Fitting logε distributions

Figure	Caption
	Figure 5-left Invariant mass distribution of K0S, Λ/Λ and γ candidates. In case of K0S the histograms are multiplied by 0.2. Vertical arrows denote the chosen mass limits for candidate selection.
	Figure 5-right Example distribution of logε in a narrow total momentum slice at p = 0.80 GeV/c, for the clean pion sample. Curves are template fits to the data, scale factors (α) and shifts (δ) are indicated. Both plots are given for 7 TeV center-of-mass energy.
	Figure 6 Examples of logε distributions (symbols) for the 7 TeV dataset at η = 0.35, pT = 0.675 GeV/c, and corresponding template fits (curves). The most probable values for pions (π), kaons (K) and protons (p) are indicated. Left: distributions in nhits slices. The points and the curves were scaled down by factors of 10 for better visibility, with nhits=1 at the top. Right: distributions in track-fit &chi2/ndf slices. The points and the curves were scaled down by factors of 10 for better visibility, with the lowest χ2/ndf slice at the top.
	Figure 7-left Example of nhits distributions (symbols) of pions, kaons and protons, for the 2.76 TeV dataset at η = 0.35, pT = 0.875 GeV/c, and corresponding fits (curves, see Sec. 5.1, Difference of hit losses).
	Figure 7-right Probability of additional hit loss q with respect to pions as function of total momentum p in the range | η | < 1 for positive kaons and protons, for the 2.76 TeV dataset, if the track-fit &chi2/ndf value is in the lowest slice. In order to exclude regions of crossing logε bands, values are not shown if p > 1.1 GeV/c for kaons, and 1.1 < p < 1.3 GeV/c for protons. These points were also omitted during the fit with double-exponential.
	Figure 8 Example logε distributions at η = 0.35 in some selected pT bins, for the 7 TeV inclusive dataset. The details of the template fits are discussed in the text. Scale factors (α) and shifts (δ) are indicated.

7 Results

7.1 Inclusive measurements

	Figure 9 Transverse momentum distributions of identified charged hadrons (pions, kaons, protons) in the range | y | < 1, for positives (left) and negatives (right), at √s = 0.9, 2.76 and 7 TeV (from top to bottom). Kaons and protons are scaled as shown in the legends. Fits to Eq. (4) are superimposed. Error bars indicate the uncorrelated statistical, while bands show the uncorrelated systematic uncertainties. The fully correlated normalization uncertainty (not shown) is 3.0%.
	Figure 10 Transverse momentum distributions of identified charged hadrons (pions, kaons, protons) in the range | y | < 1, for positives (left) and negatives (right), at √s = 0.9, 2.76 and 7 TeV (from top to bottom). Measured values (same as in Fig. 9) are plotted together with Pythia predictions. Error bars indicate the uncorrelated statistical, while bands show the uncorrelated systematic uncertainties. The fully correlated normalization uncertainty (not shown) is 3.0%.
	Figure 11 Ratios of particle yields as a function of transverse momentum, at √s = 0.9, 2.76 and 7 TeV (from top to bottom). Error bars indicate the uncorrelated statistical, while boxes show the uncorrelated systematic uncertainties. Curves indicate predictions from Pythia6 (D6T and Z2 tunes) and the 4C tune of Pythia8.

7.2 Multiplicity-dependent measurements

	Figure 12 Normalized transverse momentum distributions of charged pions in multiplicity classes, in the range | y | < 1, at √s = 0.9, 2.76 and 7 TeV, fitted to the Tsallis-Pareto parametrization. For visibility, the values with increasing multiplicity are successively shifted by 0.5 units along the vertical axis. Error bars indicate the uncorrelated statistical, while bands show the uncorrelated systematic uncertainties.
	Figure 13 Normalized transverse momentum distributions of charged kaons in multiplicity classes, in the range | y | < 1, at √s = 0.9, 2.76 and 7 TeV, fitted to the Tsallis-Pareto parametrization. For visibility, the values with increasing multiplicity are successively shifted by 0.5 units along the vertical axis. Error bars indicate the uncorrelated statistical, while bands show the uncorrelated systematic uncertainties.
	Figure 14 Normalized transverse momentum distributions of charged protons in multiplicity classes, in the range | y | < 1, at √s = 0.9, 2.76 and 7 TeV, fitted to the Tsallis-Pareto parametrization. For visibility, the values with increasing multiplicity are successively shifted by 0.5 units along the vertical axis. Error bars indicate the uncorrelated statistical, while bands show the uncorrelated systematic uncertainties.
	Figure 15 Ratios of particle yields as function as function of particle multiplicity in | η | < 2.4 at √s = 0.9, 2.76 and 7 TeV (from top to bottom). Error bars indicate the uncorrelated combined, while boxes show the uncorrelated systematic uncertainties. Curves indicate predictions from Pythia6 (D6T and Z2 tunes) and the 4C tune of Pythia8.
	Figure 16 Average transverse momentum of identified charged hadrons (pions, kaons, protons) in the range | y | < 1, for positives (left) and negatives (right), as a function of the true track multiplicity in | η | < 2.4, at √s = 0.9, 2.76 and 7 TeV (from top to bottom). Error bars indicate the uncorrelated combined, while boxes show the uncorrelated systematic uncertainties. The fully correlated normalization uncertainty (not shown) is 1.0%. Curves indicate predictions from Pythia6 (D6T and Z2 tunes) and the 4C tune of Pythia8.
	Figure 17 Center-of-mass energy dependence of dN/dy, average transverse momentum <pT> and ratios of particle yields. Error bars indicate the uncorrelated combined, while boxes show the uncorrelated systematic uncertainties. In case of dN/dy (<pT>) the fully correlated normalization uncertainty (not shown) is 3.0% (1.0%). Curves indicate predictions from Pythia6 (D6T and Z2 tunes) and the 4C tune of Pythia8.
	Figure 18 Left: Average transverse momentum of identified charged hadrons (pions, kaons, protons) in the range | y | < 1, for all particle types, as a function of the true track multiplicity in | η | < 2.4, for all energies. Right: Ratios of particle yields as function as function of particle multiplicity in | η | < 2.4, for all energies. Error bars indicate the uncorrelated combined, while boxes show the uncorrelated systematic uncertainties. In case of <pT> the fully correlated normalization uncertainty (not shown) is 1.0%. Lines are drawn to guide the eye (solid -- 0.9 TeV, dotted -- 2.76 TeV, dash-dotted -- 7 TeV).
	Figure 19 Comparison of transverse momentum distributions of identified charged hadrons (pions, kaons, protons) at central rapidity (| y | < 1 for CMS, | y | < 0.5 for ALICE), for positives (left) and negatives (right), at √s = 0.9 TeV. Kaon and proton points were scaled, for clarity, by the quoted factors. Error bars indicate the uncorrelated statistical, while bands show the uncorrelated systematic uncertainties. In case of CMS the fully correlated normalization uncertainty (not shown) is 3.0%. ALICE values were corrected to inelastic pp collisions. CMS points are scaled down by a factor of 0.78.
	Figure 20 Comparison of center-of-mass energy dependences of central rapidity density dN/dy (left) and average transverse momentum <pT> (right). Measurements from low energies, ISR, PHENIX are shown with LHC (ALICE and CMS) data. In case of CMS error bars indicate the uncorrelated combined, while boxes show the uncorrelated systematic uncertainties. The fully correlated normalization uncertainty (not shown) is around 3.0% (left plot) and 1.0% (right plot).
	Figure 21 Comparison of central rapidity p/p ratio as a function of the rapidity loss Δy. Measurements at low energies [31], and from [32], NA49 [33], BRAHMS [34], PHENIX [35], PHOBOS [36] and STAR [37] are shown with LHC (ALICE and CMS) data.

B Extra plots

Figure	Caption
	Figure 22 Comparison of pT spectra. CMS data (symbols) are compared with Pythia6 Z2 tune predictions (curves).
	Figure 23 Analysis flowchart.

-- FerencSikler - 27-Mar-2012