Midrapidity production of π±, K±, and (¯p)p measured by the ALICE experiment at the CERN Large Hadron Collider, in Pb-Pb and inelastic pp collisions at √sNN=5.02 TeV, is presented. The invariant ...yields are measured over a wide transverse momentum (pT) 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 pT-integrated particle ratios, i.e., proton-to-pion (p/π) and kaon-to-pion (K/π) ratios, with similar measurements in Pb-Pb collisions at √sNN=2.76 TeV show no significant energy dependence. Blast-wave fits of the pT 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/π, K/π) as a function of pT show pronounced maxima at pT≈3GeV/c in central Pb-Pb collisions. At high pT, particle ratios at 5.02 TeV are similar to those measured in pp collisions at the same energy and in Pb-Pb collisions at √sNN=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 pT and compatible with measurements at √sNN=2.76 TeV. The results are compared to state-of-the-art model calculations, which are found to describe the observed trends satisfactorily.
The first evidence of spin alignment of vector mesons ( K*0 and ϕ ) in heavy-ion collisions at the Large Hadron Collider (LHC) is reported. The spin density matrix element ρ 00 is measured at ...midrapidity ( | y | < 0.5 ) in Pb-Pb collisions at a center-of-mass energy ( √sNN ) of 2.76 TeV with the ALICE detector. ρ 00 values are found to be less than 1/3 (1/3 implies no spin alignment) at low transverse momentum ( p T < 2 GeV / c ) for K*0 and ϕ at a level of 3 σ and 2 σ , respectively. No significant spin alignment is observed for the K0S meson ( spin = 0 ) in Pb-Pb collisions and for the vector mesons in p p collisions. The measured spin alignment is unexpectedly large but qualitatively consistent with the expectation from models which attribute it to a polarization of quarks in the presence of angular momentum in heavy-ion collisions and a subsequent hadronization by the process of recombination.
The pseudorapidity density of charged particles, dNch / d η , at midrapidity in Pb-Pb collisions has been measured at a center-of-mass energy per nucleon pair of √ s N N = 5.02 TeV . For the 5% ...most central collisions, we measure a value of 1943 ± 54 . The rise in dNch / d η as a function of √ s N N is steeper than that observed in proton-proton collisions and follows the trend established by measurements at lower energy. The increase of dNch / d η as a function of the average number of participant nucleons, ⟨ N part ⟩ , calculated in a Glauber model, is compared with the previous measurement at √ s N N = 2.76 TeV . A constant factor of about 1.2 describes the increase in dNch / d η from √ s N N = 2.76 to 5.02 TeV for all centrality classes, within the measured range of 0%–80% centrality. The results are also compared to models based on different mechanisms for particle production in nuclear collisions.
Comprehensive results on the production of unidentified charged particles, π±, K±, K$_S^0$, K*(892)0, $p, \bar{p}, ϕ$(1020), Λ, Λ, Ξ-, Ξ+, Ω-, and $\bar{Ω}^+$ hadrons in proton-proton ...(pp) collisions at $ \sqrt{s}$=7 TeV at midrapidity (|y|<0.5) as a function of charged-particle multiplicity density are presented. In order to avoid autocorrelation biases, the actual transverse momentum (pT) spectra of the particles under study and the event activity are measured in different rapidity windows. In the highest multiplicity class, the charged-particle density reaches about 3.5 times the value measured in inelastic collisions. While the yield of protons normalized to pions remains approximately constant as a function of multiplicity, the corresponding ratios of strange hadrons to pions show a significant enhancement that increases with increasing strangeness content. Furthermore, all identified particle-to-pion ratios are shown to depend solely on charged-particle multiplicity density, regardless of system type and collision energy. The evolution of the spectral shapes with multiplicity and hadron mass shows patterns that are similar to those observed in p-Pb and Pb-Pb collisions at Large Hadron Collider energies. The obtained pT distributions and yields are compared to expectations from QCD-based pp event generators as well as to predictions from thermal and hydrodynamic models. These comparisons indicate that traces of a collective, equilibrated system are already present in high-multiplicity pp collisions.
A
bstract
We report the measured transverse momentum (
p
T
) spectra of primary charged particles from pp, p-Pb and Pb-Pb collisions at a center-of-mass energy
s
N
N
=
5.02
TeV in the kinematic range ...of 0
.
15
< p
T
<
50 GeV/
c
and |
η
|
<
0
.
8. A significant improvement of systematic uncertainties motivated the reanalysis of data in pp and Pb-Pb collisions at
s
N
N
=
2.76
TeV, as well as in p-Pb collisions at
s
N
N
=
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
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
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
s
N
N
=
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.
This Letter presents the first experimental observation of the attractive strong interaction between a proton and a multistrange baryon (hyperon) Ξ−. The result is extracted from two-particle ...correlations of combined p−Ξ−⊕p¯−Ξ¯+ pairs measured in p−Pb collisions at sNN=5.02 TeV at the LHC with ALICE. The measured correlation function is compared with the prediction obtained assuming only an attractive Coulomb interaction and a standard deviation in the range 3.6, 5.3 is found. Since the measured p−Ξ−⊕p¯−Ξ¯+ correlation is significantly enhanced with respect to the Coulomb prediction, the presence of an additional, strong, attractive interaction is evident. The data are compatible with recent lattice calculations by the HAL-QCD Collaboration, with a standard deviation in the range 1.8, 3.7. The lattice potential predicts a shallow repulsive Ξ− interaction within pure neutron matter and this implies stiffer equations of state for neutron-rich matter including hyperons. Implications of the strong interaction for the modeling of neutron stars are discussed.
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