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.
The prompt production of the charm baryon Λ+c and the Λ+c/D0 production ratios were measured at midrapidity with the ALICE detector in pp and p-Pb collisions at √sNN=5.02 TeV. These new measurements ...show a clear decrease of the Λ+c/D0 ratio with increasing transverse momentum (pT) in both collision systems in the range 2<pT<12 GeV/c, exhibiting similarities with the light-flavor baryon-to-meson ratios p/π and Λ/K0S. At low pT, predictions that include additional color-reconnection mechanisms beyond the leading-color approximation, assume the existence of additional higher-mass charm-baryon states, or include hadronization via coalescence can describe the data, while predictions driven by charm-quark fragmentation processes measured in e+e− and e−p collisions significantly underestimate the data. The results presented in this Letter provide significant evidence that the established assumption of universality (colliding-system independence) of parton-to-hadron fragmentation is not sufficient to describe charm-baryon production in hadronic collisions at LHC energies.
Measurements of anisotropic flow coefficients (vn) and their cross-correlations using two- and multiparticle cumulant methods are reported in collisions of pp at s=13 TeV, p−Pb at a center-of-mass ...energy per nucleon pair sNN=5.02 TeV, Xe-Xe at sNN=5.44 TeV, and Pb-Pb at sNN=5.02 TeV recorded with the ALICE detector. The multiplicity dependence of vn is studied in a very wide range from 20 to 3000 particles produced in the midrapidity region |η|<0.8 for the transverse momentum range 0.2<pT<3.0 GeV/c. An ordering of the coefficients v2>v3>v4 is found in pp and p−Pb collisions, similar to that seen in large collision systems, while a weak v2 multiplicity dependence is observed relative to nucleus-nucleus collisions in the same multiplicity range. Using a novel subevent method, v2 measured with four-particle cumulants is found to be compatible with that from six-particle cumulants in pp and p−Pb collisions. The magnitude of the correlation between vn2 and vm2, evaluated with the symmetric cumulants SC(m,n) is observed to be positive at all multiplicities for v2 and v4, while for v2 and v3 it is negative and changes sign for multiplicities below 100, which may indicate a different vn fluctuation pattern in this multiplicity range. The observed long-range multiparticle azimuthal correlations in high multiplicity pp and p−Pb collisions can neither be described by pythia 8 nor by impact-parameter-Glasma, music, and ultrarelativistic quantum molecular dynamics model calculations, and hence, provide new insights into the understanding of collective effects in small collision systems.
Invariant differential yields of deuterons and antideuterons in pp collisions at s = 0.9, 2.76 and 7 TeV and the yields of tritons, He3 nuclei, and their antinuclei at s = 7 TeV have been measured ...with the ALICE detector at the CERN Large Hadron Collider. The measurements cover a wide transverse momentum (pT) range in the rapidity interval |y|
The azimuthal anisotropy coefficient v2 of prompt D0, D+, D∗+, and Ds+ mesons was measured in midcentral (30%-50% centrality class) Pb-Pb collisions at a center-of-mass energy per nucleon pair ...sNN=5.02 TeV, with the ALICE detector at the LHC. The D mesons were reconstructed via their hadronic decays at midrapidity, |y|
The production rates and the transverse momentum distribution of strange hadrons at mid-rapidity (
y
<
0.5
) are measured in proton-proton collisions at
s
= 13 TeV as a function of the charged ...particle multiplicity, using the ALICE detector at the LHC. The production rates of
K
S
0
,
Λ
,
Ξ
, and
Ω
increase with the multiplicity faster than what is reported for inclusive charged particles. The increase is found to be more pronounced for hadrons with a larger strangeness content. Possible auto-correlations between the charged particles and the strange hadrons are evaluated by measuring the event-activity with charged particle multiplicity estimators covering different pseudorapidity regions. When comparing to lower energy results, the yields of strange hadrons are found to depend only on the mid-rapidity charged particle multiplicity. Several features of the data are reproduced qualitatively by general purpose QCD Monte Carlo models that take into account the effect of densely-packed QCD strings in high multiplicity collisions. However, none of the tested models reproduce the data quantitatively. This work corroborates and extends the ALICE findings on strangeness production in proton-proton collisions at 7 TeV.
Transverse momentum (pT ) spectra of charged particles at mid-pseudorapidity in Xe–Xe collisions at sNN=5.44TeV measured with the ALICE apparatus at the Large Hadron Collider are reported. The ...kinematic range 0.15<pT<50GeV/c and |η|<0.8 is covered. Results are presented in nine classes of collision centrality in the 0–80% range. For comparison, a pp reference at the collision energy of s=5.44 TeV is obtained by interpolating between existing pp measurements at s=5.02 and 7 TeV. The nuclear modification factors in central Xe–Xe collisions and Pb–Pb collisions at a similar center-of-mass energy of sNN=5.02 TeV, and in addition at 2.76 TeV, at analogous ranges of charged particle multiplicity density 〈dNch/dη〉 show a remarkable similarity at pT>10GeV/c. The centrality dependence of the ratio of the average transverse momentum 〈pT〉 in Xe–Xe collisions over Pb–Pb collision at s=5.02 TeV is compared to hydrodynamical model calculations.
In this Letter, the ALICE Collaboration presents the first measurements of the charged-particle multiplicity density, dNch/dη, and total charged-particle multiplicity, Nchtot, in Xe–Xe collisions at ...a centre-of-mass energy per nucleon–nucleon pair of sNN=5.44TeV. The measurements are performed as a function of collision centrality over a wide pseudorapidity range of −3.5<η<5. The values of dNch/dη at mid-rapidity and Nchtot for central collisions, normalised to the number of nucleons participating in the collision (Npart) as a function of sNN follow the trends established in previous heavy-ion measurements. The same quantities are also found to increase as a function of Npart, and up to the 5% 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 Npart. 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 15%.
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