The elliptic, $v_2$, triangular, $v_3$, and quadrangular, $v_4$, azimuthal anisotropic flow coefficients are measured for unidentified charged particles, pions and (anti-)protons in Pb-Pb collisions at $\sqrt{s_{\rm NN}} = 2.76$ TeV with the ALICE detector at the Large Hadron Collider. Results obtained with the event plane and four-particle cumulant methods are reported for the pseudo-rapidity range $|\eta|<0.8$ at different collision centralities and as a function of transverse momentum, $p_{\rm T}$, out to $p_{\rm T}=20$ GeV/$c$. The observed non-zero elliptic and triangular flow depends only weakly on transverse momentum for $p_{\rm T}>8$ GeV/$c$. The small $p_{\rm T}$ dependence of the difference between elliptic flow results obtained from the event plane and four-particle cumulant methods suggests a common origin of flow fluctuations up to $p_{\rm T}=8$ GeV/$c$. The magnitude of the (anti-)proton elliptic and triangular flow is larger than that of pions out to at least $p_{\rm T}=8$ GeV/$c$ indicating that the particle type dependence persists out to high $p_{\rm T}$.
Elliptic flow (v2) estimated with Event Plane method (with eta gap of 2.0) measured for unidentified charged particles as a function of transverse momentum for various centrality classes.
Elliptic flow (v2) estimated with four-particle cumulants measured for unidentified charged particles as a function of transverse momentum for various centrality classes.
Triangular flow (v3) estimated with Event Plane method (with eta gap of 2.0) measured for unidentified charged particles as a function of transverse momentum for various centrality classes.
The transverse momentum ($p_{\mathrm T}$) distribution of primary charged particles is measured at midrapidity in minimum-bias p-Pb collisions at $\sqrt{s_{\mathrm{NN}}}=5.02$ TeV with the ALICE detector at the LHC in the range $0.15<p_{\mathrm T}<50$ GeV/$c$. The spectra are compared to the expectation based on binary collision scaling of particle production in pp collisions, leading to a nuclear modification factor consistent with unity for $p_{\mathrm T}$ larger than 2 GeV/$c$, with a weak indication of a Cronin-like enhancement for $p_{\rm T}$ around 4 GeV/$c$. The measurement is compared to theoretical calculations and to data in Pb-Pb collisions at $\sqrt{s_{\mathrm{NN}}}=2.76$ TeV.
Transverse momentum distributions of charged particles in minimum-bias (NSD) p-Pb collisions for different pseudorapidity ranges.
The histogram represents the reference spectrum (cross section scaled by the nuclear overlap function, T(pPb)) in inelastic pp collisions, determined in |eta(cms)| < 0.8.
The ratio of spectra in p-Pb at backward pseudorapidities to that at |eta(cms)| < 0.3.
Transverse momentum spectra of $\pi^{\pm}$, $\rm K^{\pm}$ and p($\bar{\rm p}$) up to $p_{\rm T}$ = 20 GeV/$c$ at mid-rapidity in pp and Pb-Pb collisions at $\sqrt{s_{\rm NN}}=$ 2.76 TeV have been measured using the ALICE detector at the Large Hadron Collider. The proton-to-pion and the kaon-to-pion ratios both show a distinct peak at $p_{\rm T} \approx 3 GeV/c$ in central Pb-Pb collisions. Below the peak, $p_{\rm T}$ < 3 GeV/$c$, both ratios are in good agreement with hydrodynamical calculations, suggesting that the peak itself is dominantly the result of radial flow rather than anomalous hadronization processes. For $p_{\rm T}$ > 10 GeV/$c$ particle ratios in pp and Pb-Pb collisions are in agreement and the nuclear modification factors for $\pi^{\pm}$, $\rm K^{\pm}$ and $\rm p$($\bar{\rm p}$) indicate that, within the systematic and statistical uncertainties, the suppression is the same. This suggests that the chemical composition of leading particles from jets in the medium is similar to that of vacuum jets.
Invariant yields of identified pions in central and peripheral Pb-Pb collisions, together with the unscaled pp reference yields.
Invariant yields of identified kaons in central and peripheral Pb-Pb collisions, together with the unscaled pp reference yields.
Invariant yields of identified protons in central and peripheral Pb-Pb collisions, together with the unscaled pp reference yields.
The production of J/$\psi$ and $\psi(2S)$ was measured with the ALICE detector in Pb-Pb collisions at the LHC. The measurement was performed at forward rapidity ($2.5 < y < 4 $) down to zero transverse momentum ($p_{\rm T}$) in the dimuon decay channel. Inclusive J/$\psi$ yields were extracted in different centrality classes and the centrality dependence of the average $p_{\rm T}$ is presented. The J/$\psi$ suppression, quantified with the nuclear modification factor ($R_{\rm AA}$), was studied as a function of centrality, transverse momentum and rapidity. Comparisons with similar measurements at lower collision energy and theoretical models indicate that the J/$\psi$ production is the result of an interplay between color screening and recombination mechanisms in a deconfined partonic medium, or at its hadronization. Results on the $\psi(2S)$ suppression are provided via the ratio of $\psi(2S)$ over J/$\psi$ measured in pp and Pb-Pb collisions.
Values of $\langle p_{\rm T}\rangle$ and $\langle p^2_{\rm T}\rangle$ of inclusive J/$\psi$ measured in $0<p_{\rm T}<8$ GeV/$c$ and $2.5<y<4$. Statistical and systematic uncertainties are also reported.
Inclusive J/$\psi$ yields in $p_{\rm T}$ intervals for the 0-20%, 20-40% and 40-90% most central Pb-Pb collisions. The rapidity range is $2.5<y<4$. Statistical and systematic uncertainties are also reported. A global systematic uncertainty of 4% affects all the values. A 2%, 1% and 2% systematic uncertainty, independent of $p_{\rm T}$, affects the centrality classes 0-20%, 20-40% and 40-90%, respectively.
Inclusive J/$\psi$ $R_{\rm AA}$ and Pb-Pb yields as a function of centrality, $p_{\rm T}<8$ GeV/$c$ and $2.5<y<4.0$. Statistical and systematic uncertainties are also reported. A global systematic uncertainty of 15% (12%) affects all the $R_{\rm AA}$ (yields) values.
Direct photon production at mid-rapidity in Pb-Pb collisions at $\sqrt{s_{_{\mathrm{NN}}}} = 2.76$ TeV was studied in the transverse momentum range $0.9 < p_\mathrm{T} < 14$ GeV$/c$. Photons were detected with the highly segmented electromagnetic calorimeter PHOS and via conversions in the ALICE detector material with the $e^+e^-$ pair reconstructed in the central tracking system. The results of the two methods were combined and direct photon spectra were measured for the 0-20%, 20-40%, and 40-80% centrality classes. For all three classes, agreement was found with perturbative QCD calculations for $p_\mathrm{T} \gtrsim 5$ GeV$/c$. Direct photon spectra down to $p_\mathrm{T} \approx 1$ GeV$/c$ could be extracted for the 20-40% and 0-20% centrality classes. The significance of the direct photon signal for $0.9 < p_\mathrm{T} < 2.1$ GeV$/c$ is $2.6\sigma$ for the 0-20% class. The spectrum in this $p_\mathrm{T}$ range and centrality class can be described by an exponential with an inverse slope parameter of $(297 \pm 12^\mathrm{stat}\pm 41^\mathrm{syst})$ MeV. State-of-the-art models for photon production in heavy-ion collisions agree with the data within uncertainties.
Invariant differential yields of direct GAMMA produced in 0-20% central inelastic PbPb collisions at center-of-mass energy per nucleon 2.76 TeV.
Invariant differential yields of direct GAMMA produced in 20-40% central inelastic PbPb collisions at center-of-mass energy per nucleon 2.76 TeV. The quoted upper limits correspond to a 95% confidence level.
Invariant differential yields of direct GAMMA produced in 40-80% central inelastic PbPb collisions at center-of-mass energy per nucleon 2.76 TeV. The quoted upper limits correspond to a 95% confidence level.
We present measurements of two-particle correlations with neutral pion trigger particles of transverse momenta $8 < p_{\mathrm{T}}^{\rm trig} < 16~\mathrm{GeV}/c$ and associated charged particles of $0.5 < p_{\mathrm{T}}^{\rm assoc} < 10~\mathrm{GeV}/c$ versus the azimuthal angle difference $\Delta\varphi$ at midrapidity in pp and central Pb-Pb collisions at $\sqrt{s_{\mathrm{NN}}}=2.76$ TeV with ALICE. The new measurements exploit associated charged hadrons down to $0.5~\mathrm{GeV}/c$, which significantly extends our previous measurement that only used charged hadrons above $3~\mathrm{GeV}/c$. After subtracting the contributions of the flow background, $v_2$ to $v_5$, the per-trigger yields are extracted for $|\Delta\varphi|<0.7$ on the near and for $|\Delta\varphi-\pi| < 1.1$ on the away side. The ratio of per-trigger yields in Pb--Pb to those in pp collisions, $I_{\mathrm{AA}}$, is measured on the near and away side for the $0$--$10$\% most central Pb--Pb collisions. On the away side, the per-trigger yields in Pb--Pb are strongly suppressed to the level of $I_{\mathrm{AA}} \approx 0.6$ for $p_{\mathrm{T}}^{\rm assoc} > 3~\mathrm{GeV}/c$, while with decreasing momenta an enhancement develops reaching about $5$ at low $p_{\mathrm{T}}^{\rm assoc}$. On the near side, an enhancement of $I_{\mathrm{AA}}$ between $1.2$ at the highest to $1.8$ at the lowest $p_{\mathrm{T}}^{\rm assoc}$ is observed. The data are compared to parton-energy-loss predictions of the JEWEL and AMPT event generators, as well as to a perturbative QCD calculation with medium-modified fragmentation functions. All calculations qualitatively describe the away-side suppression at high $p_{\mathrm{T}}^{\rm assoc}$. Only AMPT captures the enhancement at low $p_{\mathrm{T}}^{\rm assoc}$, both on the near and away side. However, it also underpredicts $I_{\mathrm{AA}}$ above $5$ GeV/$c$, in particular on the near-side.
Charged-particle associated yields relative to $\pi^{0}$ trigger particles versus $\Delta\varphi$ in pp collisions at $\sqrt{s_{NN}}$=2.76 TeV. The $\pi^{0}$ trigger momentum range is $8< p_{T}^{trig} < 16~GeV/c$, and associated charged particle ranges are $0.5 < p_{T}^{assoc} < 1$, $1 < p_{T}^{assoc} < 2$, $2 < p_{T}^{assoc} < 4$ and $4 < p_{T}^{assoc} < 6~GeV/c$.
Charged-particle associated yields relative to $\pi^{0}$ trigger particles versus $\Delta\varphi$ in Pb-Pb collisions at $\sqrt{s_{NN}}$=2.76 TeV. The $\pi^{0}$ trigger momentum range is $8< p_{T}^{trig} < 16~GeV/c$, and associated charged particle ranges are $0.5 < p_{T}^{assoc} < 1$, $1 < p_{T}^{assoc} < 2$, $2 < p_{T}^{assoc} < 4$ and $4 < p_{T}^{assoc} < 6~GeV/c$.
Per-trigger yield modification, $I_{\rm AA}$, on the near side with trigger $\pi^{0}$ particle at $8 < p_{T}^{trig} < 16~GeV/c$ for $0$-$10$$\%$ Pb-Pb collisions at $\sqrt{s_{NN}}$= 2.76 TeV.
The inclusive production of the $\psi$(2S) charmonium state was studied as a function of centrality in p-Pb collisions at the nucleon-nucleon center of mass energy $\sqrt{s_{\rm NN}}$ = 5.02 TeV at the CERN LHC. The measurement was performed with the ALICE detector in the center of mass rapidity ranges $-4.46<y_{\rm cms}<-2.96$ and $2.03<y_{\rm cms}<3.53$, down to zero transverse momentum, by reconstructing the $\psi$(2S) decay to a muon pair. The $\psi$(2S) production cross section $\sigma_{\psi(\rm 2S)}$ is presented as a function of the collision centrality, which is estimated through the energy deposited in forward rapidity calorimeters. The relative strength of nuclear effects on the $\psi$(2S) and on the corresponding 1S charmonium state J/$\psi$ is then studied by means of the double ratio of cross sections $[\sigma_{\psi(\rm 2S)}/\sigma_{\rm J/\psi}]_{\rm pPb}/[\sigma_{\psi(\rm 2S)}/\sigma_{\rm J/\psi}]_{\rm pp}$ between p-Pb and pp collisions, and by the values of the nuclear modification factors for the two charmonium states. The results show a large suppression of $\psi$(2S) production relative to the J/$\psi$ at backward (negative) rapidity, corresponding to the flight direction of the Pb-nucleus, while at forward (positive) rapidity the suppressions of the two states are comparable. Finally, comparisons to results from lower energy experiments and to available theoretical models are presented.
Centrality-differential cross section dsigma_JPsi/dy in the backward and forward rapidity ranges (-4.46<y_cms<-2.96 and 2.03<y_cms<3.53). The first uncertainty is statistical, the second is a systematic one. The third uncertainty is a systematic uncertainty fully correlated over centrality.
Centrality dependence of the Psi(2S)/J/Psi ratio in the backward and forward rapidity ranges (-4.46<y_cms<-2.96 and 2.03<y_cms<3.53). The first uncertainty is statistical, the second is a systematic one. The third systematic uncertainty is fully correlated over centrality.
Centrality dependence of the (Psi(2S)/J/Psi)_pA/(Psi(2S)/J/Psi)_pp double ratio in the backward and forward rapidity range (-4.46<y_cms<-2.96 and 2.03<y_cms<3.53). The first uncertainty is statistical, the second one is a systematic one. The third systematic uncertainty is fully correlated over centrality, but uncorrelated versus rapidity, while the fourth uncertainty is fully correlated over centrality and over rapidity.
The production of charged pions, kaons and (anti)protons has been measured at mid-rapidity ($-0.5 y 0$) in p-Pb collisions at $\sqrt{s_{\rm NN}}=5.02$ TeV using the ALICE detector at the LHC. Exploiting particle identification capabilities at high transverse momentum ($p_{\rm T}$), the previously published $p_{\rm T}$ spectra have been extended to include measurements up to 20 GeV/$c$ for seven event multiplicity classes. The $p_{\rm T}$ spectra for pp collisions at $\sqrt{s}=7$ TeV, needed to interpolate a pp reference spectrum, have also been extended up to 20 GeV/$c$ to measure the nuclear modification factor ($R_{\rm pPb}$) in non-single diffractive p-Pb collisions. At intermediate transverse momentum ($2 p_{\rm T} 10$\,GeV/$c$) the proton-to-pion ratio increases with multiplicity in p-Pb collisions, a similar effect is not present in the kaon-to-pion ratio. The $p_{\rm T}$ dependent structure of such increase is qualitatively similar to those observed in pp and heavy-ion collisions. At high $p_{\rm T}$ ($>10$ GeV/$c$), the particle ratios are consistent with those reported for pp and Pb-Pb collisions at the LHC energies. At intermediate $p_{\rm T}$ the (anti)proton $R_{\rm pPb}$ shows a Cronin-like enhancement, while pions and kaons show little or no nuclear modification. At high $p_{\rm T}$ the charged pion, kaon and (anti)proton $R_{\rm pPb}$ are consistent with unity within statistical and systematic uncertainties.
pT-differential invariant yield of charged pions in p-Pb collisions with centre-of-mass energy/nucleon=5.02 TeV, measured for different V0A multiplicity classes. The first uncertainty is statistical, the second one is the total systematic uncertainty, while the third one is the uncorrelated systematic uncertainty which is multiplicity dependent.
pT-differential invariant yield of charged pions in p-Pb collisions with centre-of-mass energy/nucleon=5.02 TeV, measured for NSD events. The first uncertainty is statistical, the second one is the total systematic uncertainty, while the third one is the uncorrelated systematic uncertainty which is multiplicity dependent.
pT-differential invariant yield of charged kaons in p-Pb collisions with centre-of-mass energy/nucleon=5.02 TeV, measured for different V0A multiplicity classes. The first uncertainty is statistical, the second one is the total systematic uncertainty, while the third one is the uncorrelated systematic uncertainty which is multiplicity dependent.
At sufficiently high temperature and energy density, nuclear matter undergoes a transition to a phase in which quarks and gluons are not confined: the Quark-Gluon Plasma (QGP) [1]. Such an extreme state of strongly-interacting QCD (Quantum Chromo-Dynamics) matter is produced in the laboratory with high-energy collisions of heavy nuclei, where an enhanced production of strange hadrons is observed [2-6]. Strangeness enhancement, originally proposed as a signature of QGP formation in nuclear collisions [7], is more pronounced for multi-strange baryons. Several effects typical of heavy-ion phenomenology have been observed in high-multiplicity proton-proton (pp) collisions [8,9]. Yet, enhanced production of multi-strange particles has not been reported so far. Here we present the first observation of strangeness enhancement in high-multiplicity pp collisions. We find that the integrated yields of strange and multi-strange particles relative to pions increases significantly with the event charged-particle multiplicity. The measurements are in remarkable agreement with p-Pb collision results [10,11] indicating that the phenomenon is related to the final system created in the collision. In high-multiplicity events strangeness production reaches values similar to those observed in Pb-Pb collisions, where a QGP is formed.
$K^{0}_{S}$ transverse momentum spectrum: V0M Class I (pp at $\sqrt{s}=7$ TeV).
$K^{0}_{S}$ transverse momentum spectrum: V0M Class II (pp at $\sqrt{s}=7$ TeV).
$K^{0}_{S}$ transverse momentum spectrum: V0M Class III (pp at $\sqrt{s}=7$ TeV).
Neutral pion and $\eta$ meson invariant differential yields were measured in non-single diffractive p-Pb collisions at $\sqrt{s_{\rm NN}} = 5.02$ TeV with the ALICE experiment at the CERN LHC. The analysis combines results from three complementary photon measurements, utilizing the PHOS and EMCal calorimeters and the Photon Conversion Method. The invariant differential yields of $\pi^0$ and $\eta$ meson inclusive production are measured near mid-rapidity in a broad transverse momentum range of $0.3<p_{\rm T}< 20$ GeV/$c$ and $0.7<p_{\rm T}<20$ GeV/$c$, respectively. The measured $\eta$/$\pi^{0}$ ratio increases with $p_{\rm T}$ and saturates for $p_{\rm T} > 4$ GeV/$c$ at $0.483\pm 0.015_{\rm stat}\pm 0.015_{\rm sys}$. A deviation from $m_{\rm T}$ scaling is observed for $p_{\rm T}<2$ GeV/$c$. The measured $\eta$/$\pi^{0}$ ratio is consistent with previous measurements from proton-nucleus and pp collisions over the full $p_{\rm T}$ range. The measured $\eta$/$\pi^{0}$ ratio at high $p_{\rm T}$ also agrees within uncertainties with measurements from nucleus-nucleus collisions. The $\pi^0$ and $\eta$ yields in p-Pb relative to the scaled pp interpolated reference, $R_{\rm pPb}$, are presented for $0.3<p_{\rm T}<20$ GeV/$c$ and $0.7<p_{\rm T}< 20$ GeV/$c$, respectively. The results are compared with theoretical model calculations. The values of $R_{\rm pPb}$ are consistent with unity for transverse momenta above 2 GeV/$c$. These results support the interpretation that the suppressed yield of neutral mesons measured in Pb-Pb collisions at LHC energies is due to parton energy loss in the hot QCD medium.
Invariant differential yield of PI0 produced in NSD p-Pp collisions with centre-of-mass energy/nucleon 5.02 TeV.
Invariant differential yields of ETA produced in NSD p-Pb collisions with centre-of-mass energy/nucleon 5.02 TeV.
The measured ratio of invariant yields for inclusive ETA to PI0 production at a centre-of-mass energy/nucleon 5.02 TeV.
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