Mid-rapidity production of $\pi^{\pm}$, $\rm{K}^{\pm}$ and ($\bar{\rm{p}}$)p measured by the ALICE experiment at the LHC, in Pb-Pb and inelastic pp collisions at $\sqrt{s_{\rm{NN}}}$ = 5.02 TeV, is presented. The invariant yields are measured over a wide transverse momentum ($p_{\rm{T}}$) range from hundreds of MeV/$c$ up to 20 GeV/$c$. The results in Pb-Pb collisions are presented as a function of the collision centrality, in the range 0$-$90%. The comparison of the $p_{\rm{T}}$-integrated particle ratios, i.e. proton-to-pion (p/$\pi$) and kaon-to-pion (K/$\pi$) ratios, with similar measurements in Pb-Pb collisions at $\sqrt{s_{\rm{NN}}}$ = 2.76 TeV show no significant energy dependence. Blast-wave fits of the $p_{\rm{T}}$ spectra indicate that in the most central collisions radial flow is slightly larger at 5.02 TeV with respect to 2.76 TeV. Particle ratios (p/$\pi$, K/$\pi$) as a function of $p_{\rm{T}}$ show pronounced maxima at $p_{\rm{T}}$ $\approx$ 3 GeV/$c$ in central Pb-Pb collisions. At high $p_{\rm{T}}$, particle ratios at 5.02 TeV are similar to those measured in pp collisions at the same energy and in Pb-Pb collisions at $\sqrt{s_{\rm{NN}}}$ = 2.76 TeV. Using the pp reference spectra measured at the same collision energy of 5.02 TeV, the nuclear modification factors for the different particle species are derived. Within uncertainties, the nuclear modification factor is particle species independent for high $p_{\rm{T}}$ and compatible with measurements at $\sqrt{s_{\rm{NN}}}$ = 2.76 TeV. The results are compared to state-of-the-art model calculations, which are found to describe the observed trends satisfactorily.
$p_{T}$-distributions of pions ($\pi^{+}+\pi^{-}$) measured in Pb-Pb collisions at $\sqrt{s_{NN}}$ = 5.02 TeV.
$p_{T}$-distributions of pions ($\pi^{+}+\pi^{-}$) measured in pp collisions at $\sqrt{s}$ = 5.02 TeV.
$p_{T}$-distributions of kaons ($K^{+}+K^{-}$) measured in Pb-Pb collisions at $\sqrt{s_{NN}}$ = 5.02 TeV.
The momentum-weighted sum of the electric charges of particles inside a jet, known as jet charge, is sensitive to the electric charge of the particle initiating the parton shower. This paper presents jet charge distributions in $\sqrt{s_\mathrm{NN}} =$ 5.02 TeV lead-lead (PbPb) and proton-proton (pp) collisions recorded with the CMS detector at the LHC. These data correspond to integrated luminosities of 404 $\mu$b$^{-1}$ and 27.4 pb$^{-1}$ for PbPb and pp collisions, respectively. Leveraging the sensitivity of the jet charge to fundamental differences in the electric charges of quarks and gluons, the jet charge distributions from simulated events are used as templates to extract the quark- and gluon-like jet fractions from data. The modification of these jet fractions is examined by comparing pp and PbPb data as a function of the overlap of the colliding Pb nuclei (centrality). This measurement tests the color charge dependence of jet energy loss due to interactions with the quark-gluon plasma. No significant modification between different centrality classes and with respect to pp results is observed in the extracted fractions of quark- and gluon-like jet fractions.
Unfolded jet charge measurements for the $p_{T}$-weighting factor $\kappa = 0.5$ and a minimum track $p_{T}$ of 1 GeV for inclusive jets in pp and PbPb data. The PbPb results are shown for different centrality regions.
The standard deviation of the jet charge distributions with different track $p_{T}$ thresholds and $\kappa$ value of 0.5 for pp collisions and in the various event centrality bins for PbPb collisions compared with the PYTHIA6 prediction.
The standard deviation of the jet charge distributions with different track $p_{T}$ thresholds and $\kappa$ value of 0.3 for pp collisions and in the various event centrality bins for PbPb collisions compared with the PYTHIA6 prediction.
Anisotropic flow coefficients, $v_n$, non-linear flow mode coefficients, $\chi_{n,mk}$, and correlations among different symmetry planes, $\rho_{n,mk}$ are measured in Pb-Pb collisions at $\sqrt{s_\rm{NN}}=5.02$ TeV. Results obtained with multi-particle correlations are reported for the transverse momentum interval $0.2<p_\rm{T}<5.0$ GeV/$c$ within the pseudorapidity interval $0.4<|\eta|<0.8$ as a function of collision centrality. The $v_n$ coefficients and $\chi_{n,mk}$ and $\rho_{n,mk}$ are presented up to the ninth and seventh harmonic order, respectively. Calculations suggest that the correlations measured in different symmetry planes and the non-linear flow mode coefficients are dependent on the shear and bulk viscosity to entropy ratios of the medium created in heavy-ion collisions. The comparison between these measurements and those at lower energies and calculations from hydrodynamic models places strong constraints on the initial conditions and transport properties of the system.
Centrality dependence of flow harmonics from $v_2$ to $v_9$.
Centrality dependence of flow harmonics from $v_2$ to $v_9$.
Centrality dependence of flow harmonics from $v_2$ to $v_9$.
We report high statistics measurements of inclusive charged hadron production in Au+Au and p+p collisions at \sqrtsNN=200 GeV. A large, approximately constant hadron suppression is observed in central Au+Au collisions for $5\lt\pT\lt12$ GeV/c. The collision energy dependence of the yields and the centrality and \pT dependence of the suppression provide stringent constraints on theoretical models of suppression. Models incorporating initial-state gluon saturation or partonic energy loss in dense matter are largely consistent with observations. We observe no evidence of \pT-dependent suppression, which may be expected from models incorporating jet attentuation in cold nuclear matter or scattering of fragmentation hadrons.
Inclusive invariant pT distributions of (h+ + h−)/2 for centrality-selected Au+Au and p+p NSD interactions. Hash marks at the top indicate bin boundaries for pT>4 GeV/c.The invariant cross section for p+p is indicated on the right vertical axis.
R200/130(pT ) vs. pT for (h+ + h−)/2 for four different centrality bins. The overall normalization uncertainty is +6−14% for the 40-60% bin and is negligible for the other panels. Calculations are described in the text.
RAA(pT) (Eq. 1) for (h+ + h−)/2 in |η|<0.5, for centrality-selected Au+Au spectra relative to the measured p+p spectrum. The p+p spectrum is common to all panels. Calculations are described in the text.
The Quark Gluon Plasma (QGP) produced in ultra relativistic heavy-ion collisions at the Large Hadron Collider (LHC) can be studied by measuring the modifications of jets formed by hard scattered partons which interact with the medium. We studied these modifications via angular correlations of jets with charged hadrons for jets with momenta 20 < $p_{\rm{T}}^{\rm{jet}}$ < 40 GeV/$c$ as a function of the associated particle momentum. The reaction plane fit (RPF) method is used in this analysis to remove the flow modulated background. The analysis of angular correlations for different orientations of the jet relative to the second order event plane allows for the study of the path length dependence of medium modifications to jets. We present the dependence of azimuthal angular correlations of charged hadrons with respect to the angle of the axis of a reconstructed jet relative to the event plane in Pb-Pb collisions at $\sqrt{s_{\rm{NN}}}$ = 2.76 TeV. The dependence of particle yields associated with jets on the angle of the jet with respect to the event plane is presented. Correlations at different angles relative to the event plane are compared through ratios and differences of the yield. No dependence of the results on the angle of the jet with respect to the event plane is observed within uncertainties, which is consistent with no significant path length dependence of the medium modifications for this observable.
The near-side and away-side yield vs $p_{T}^{assoc}$ for $20<p_T^{jet}<40$ GeV/$c$ full jets of 30-50% centrality in Pb-Pb collisions. The background uncertainty is non-trivially correlated point-to-point. The correlated systematic uncertainties come from the shape uncertainty of the acceptance correction. There is an additional 5% global scale uncertainty.
The differences between out-of-plane and in-plane yields and mid-plane and in-plane yields on near-side and away-side vs $p_{T}^{assoc}$ for $20<p_T^{jet}<40$ GeV/$c$ full jets of 30-50% centrality in Pb-Pb collisions. The background uncertainty is non-trivially correlated point-to-point. The correlated systematic uncertainties come from the shape uncertainty of the acceptance correction. There is an additional 5% global scale uncertainty.
The ratios of out-of-plane to in-plane yields and mid-plane to in-plane yields on near-side and away-side vs $p_{T}^{assoc}$ for $20<p_T^{jet}<40$ GeV/$c$ full jets of 30-50% centrality in Pb-Pb collisions. The background uncertainty is non-trivially correlated point-to-point. The correlated systematic uncertainties come from the shape uncertainty of the acceptance correction.
In this Letter, the ALICE Collaboration presents the first measurements of the charged-particle multiplicity density, $\rm{d}N_{\rm{ch}}/\rm{d}\eta$, and total charged-particle multiplicity, $N_{\rm{ch}}^{\rm{tot}}$, in Xe-Xe collisions at a centre-of-mass energy per nucleon--nucleon pair of $\sqrt{s_{\rm NN}}$ = 5.44 TeV. The measurements are performed as a function of collision centrality over a wide pseudorapidity range of $-3.5 < \eta < 5$. The values of $\rm{d}N_{\rm{ch}}/\rm{d}\eta$ at mid-rapidity and $N_{\rm{ch}}^{\rm{tot}}$ for central collisions, normalised to the number of nucleons participating in the collision ($N_{\rm{part}}$) as a function of $\sqrt{s_{\rm NN}}$, follow the trends established in previous heavy-ion measurements. The same quantities are also found to increase as a function of $N_{\rm{part}}$, and up to the 10% most central collisions the trends are the same as the ones observed in Pb-Pb at a similar energy. For more central collisions, the Xe-Xe scaled multiplicities exceed those in Pb-Pb for a similar $N_{\rm{part}}$. The results are compared to phenomenological models and theoretical calculations based on different mechanisms for particle production in nuclear collisions. All considered models describe the data reasonably well within 20%.
Charged-particle pseudorapidity density for 12 centrality classes over a broad $\eta$ range in Xe--Xe collisions at $\sqrt{s_{_{\mathrm{NN}}}} = 5.44\,\mathrm{TeV}$. Statistical errors are generally insignificant.
Charged-particle pseudorapidity density for 12 centrality classes over a broad $\eta$ range in Xe--Xe collisions at $\sqrt{s_{_{\mathrm{NN}}}} = 5.44\,\mathrm{TeV}$. Statistical errors are generally insignificant.
Values of $2/\langle N_\mathrm{part} \rangle \langle \mathrm{d}N_\mathrm{ch}/\mathrm{d}\eta\rangle$ and $2/\langle N_\mathrm{part} \rangle N^\mathrm{tot}_\mathrm{ch}$ in Xe--Xe collisions at $\sqrt{s_{_{\mathrm{NN}}}} = 5.44\,\mathrm{TeV}$ for the top 5$\%$ central collisions.
Modifications of the properties of jets in PbPb collisions, relative to those in pp collisions, are studied at a nucleon-nucleon center-of-mass energy of $\sqrt{s_\mathrm{NN}} =$ 5.02 TeV via correlations of charged particles with the jet axis in relative pseudorapidity ($\Delta \eta$), relative azimuth ($\Delta \phi$), and relative angular distance from the jet axis $\Delta \mathrm{r} = \sqrt{{(\Delta\eta)^{2}+(\Delta\phi)^{2}}}$. This analysis uses data collected with the CMS detector at the LHC, corresponding to integrated luminosities of 404 $\mu$b$^{-1}$ and 27.4 pb$^{-1}$ for PbPb and pp collisions, respectively. Charged particle number densities, jet fragmentation functions, and jet shapes are presented as a function of PbPb collision centrality and charged-particle track transverse momentum, providing a differential description of jet modifications due to interactions with the quark-gluon plasma.
The distribution of jet-correlated charged-particle tracks with $|{\Delta\phi}|<1.0$ as a function of $\Delta\eta$ in pp and PbPb collisions. The PbPb results are shown for different centrality regions.
The difference between the PbPb and pp measurements from Table 1.
The distribution of jet-correlated charged-particle tracks with $|{\Delta\eta}|<1.0$ as a function of $\Delta\phi$ in pp and PbPb collisions. The PbPb results are shown for different centrality regions.
We report the measured transverse momentum ($p_{\rm T}$) spectra of primary charged particles from pp, p-Pb and Pb-Pb collisions at a center-of-mass energy $\sqrt{s_{\rm NN}} = 5.02$ TeV in the kinematic range of $0.15<p_{\rm T}<50$ GeV/$c$ and $|\eta|< 0.8$. A significant improvement of systematic uncertainties motivated the reanalysis of data in pp and Pb-Pb collisions at $\sqrt{s_{\rm NN}} = 2.76$ TeV, as well as in p-Pb collisions at $\sqrt{s_{\rm NN}} = 5.02$ TeV, which is also presented. Spectra from Pb-Pb collisions are presented in nine centrality intervals and are compared to a reference spectrum from pp collisions scaled by the number of binary nucleon-nucleon collisions. For central collisions, the $p_{\rm T}$ spectra are suppressed by more than a factor of 7 around 6-7 GeV/$c$ with a significant reduction in suppression towards higher momenta up to 30 GeV/$c$. The nuclear modification factor $R_{\rm pPb}$, constructed from the pp and p-Pb spectra measured at the same collision energy, is consistent with unity above 8 GeV/$c$. While the spectra in both pp and Pb-Pb collisions are substantially harder at $\sqrt{s_{\rm NN}} = 5.02$ TeV compared to 2.76 TeV, the nuclear modification factors show no significant collision energy dependence. The obtained results should provide further constraints on the parton energy loss calculations to determine the transport properties of the hot and dense QCD matter.
Transverse momentum spectra of charged particles in PbPb collisions in nine centrality classes.
Transverse momentum spectra of charged particles in PbPb collisions in nine centrality classes.
Transverse momentum spectra of charged particles in pPb collisions.
Measurements of the azimuthal anisotropy in lead-lead collisions at $\sqrt{s_\mathrm{NN}} = 5.02$ TeV are presented using a data sample corresponding to 0.49 $\mathrm{nb}^{-1}$ integrated luminosity collected by the ATLAS experiment at the LHC in 2015. The recorded minimum-bias sample is enhanced by triggers for "ultra-central" collisions, providing an opportunity to perform detailed study of flow harmonics in the regime where the initial state is dominated by fluctuations. The anisotropy of the charged-particle azimuthal angle distributions is characterized by the Fourier coefficients, $v_{2}-v_{7}$, which are measured using the two-particle correlation, scalar-product and event-plane methods. The goal of the paper is to provide measurements of the differential as well as integrated flow harmonics $v_{n}$ over wide ranges of the transverse momentum, 0.5 $
The V2 harmonic measured with the scalar product method as a funtion of transverse momentum in centrality bin 0-0.1%
The V2 harmonic measured with the scalar product method as a funtion of transverse momentum in centrality bin 0-1%
The V2 harmonic measured with the scalar product method as a funtion of transverse momentum in centrality bin 0-5%
The azimuthal anisotropy Fourier coefficients ($v_n$) in 8.16 TeV pPb data are extracted via long-range two-particle correlations as a function of event multiplicity and compared to corresponding results in pp and PbPb collisions. Using a four-particle cumulant technique, $v_n$ correlations are measured for the first time in pp and pPb collisions. The $v_2$ and $v_4$ coefficients are found to be positively correlated in all collision systems. For high multiplicity pPb collisions an anticorrelation of $v_2$ and $v_3$ is observed, with a similar correlation strength as in PbPb data at the same multiplicity. The new correlation results strengthen the case for a common origin of the collectivity seen in pPb and PbPb collisions in the measured multiplicity range.
The $v_{n}$ result from 2-particle correlation as a function of multiplicity ($N_{trk}^{offline}$) in PbPb collisions at $\sqrt{s_{NN}}$ = 5.02 TeV. Results after low-multiplicity subtraction are denoted as $v_{n}^{sub}$.
The $v_{n}$ result from 2-particle correlation as a function of multiplicity ($N_{trk}^{offline}$) in pPb collisions at $\sqrt{s_{NN}}$ = 8.16 TeV. Results after low-multiplicity subtraction are denoted as $v_{n}^{sub}$.
The $v_{4}$ result from 2-particle correlation as a function of multiplicity ($N_{trk}^{offline}$) in pp collisions at $\sqrt{s}$ = 13.00 TeV. Results after low-multiplicity subtraction are denoted as $v_{4}^{sub}$.
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