A(c)(+) Production and Baryon-to-Meson Ratios in pp and p-Pb Collisions at root S-NN=5.02 TeV at the LHC

The prompt production of the charm baryon \u39bc+ and the \u39bc+/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 \u39bc+/D0 ratio with increasing transverse momentum (pT) in both collision systems in the range 2<12 GeV/c, exhibiting similarities with the light-flavor baryon-to-meson ratios p/\u3c0 and \u39b/KS0. 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

A(c)(+) Production and Baryon-to-Meson Ratios in pp and p-Pb Collisions at root S-NN=5.02 TeV at the LHC

The prompt production of the charm baryon Λ_{c}^{+} and the Λ_{c}^{+}/D^{0} production ratios were measured at midrapidity with the ALICE detector in pp and p-Pb collisions at sqrt[s_{NN}]=5.02  TeV. These new measurements show a clear decrease of the Λ_{c}^{+}/D^{0} ratio with increasing transverse momentum (p_{T}) in both collision systems in the range 2<p_{T}<12  GeV/c, exhibiting similarities with the light-flavor baryon-to-meson ratios p/π and Λ/K_{S}^{0}. At low p_{T}, 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

Measurement of isolated photon-hadron correlations in √ s NN = 5.02 TeV p p and p -Pb collisions

This paper presents isolated photon-hadron correlations using pp and p-Pb data collected by the ALICE detector at the LHC. For photons with vertical bar eta vertical bar < 0.67 and 12 < p(T) < 40 GeV/c, the associated yield of charged particles in the range vertical bar eta vertical bar < 0.80 and 0.5 < p(T) < 10 GeV/c is presented. These momenta are much lower than previous measurements at the LHC. No significant difference between pp and p-Pb is observed, with PYTHIA 8.2 describing both data sets within uncertainties. This measurement constrains nuclear effects on the parton fragmentation in p-Pb collisions, and provides a benchmark for future studies of Pb-Pb collisions

Transverse-momentum and event-shape dependence of D-meson flow harmonics in Pb-Pb collisions at root(S)(NN)=5 . 02 TeV

The elliptic and triangular flow coefficients v(2) and v(3) of prompt D-0 , D+ , and D*(+) mesons were measured at midrapidity (|y| < 0.8) in Pb-Pb collisions at the centre-of-mass energy per nucleon pair of root(S)(NN) = 5.02 TeV with the ALICE detector at the LHC. The D mesons were reconstructed via their hadronic decays in the transverse momentum interval 1 < p(T) < 36 GeV/c in central (0-10%) and semi-central (30-50%) collisions. Compared to pions, protons, and J/psi mesons, the average D-meson v(n) harmonics are compatible within uncertainties with a mass hierarchy for p(T) less than or similar to 3 GeV/c , and are similar to those of charged pions for higher p(T). The coupling of the charm quark to the light quarks in the underlying medium is further investigated with the application of the event-shape engineering (ESE) technique to the D-meson v(2) and p(T)-differential yields. The D-meson v(2) is correlated with average bulk elliptic flow in both central and semi-central collisions. Within the current precision, the ratios of per-event D meson yields in the ESE-selected and unbiased samples are found to be compatible with unity. All the measurements are found to be reasonably well described by theoretical calculations including the effects of charm-quark transport and the recombination of charm quarks with light quarks in a hydrodynamically expanding medium. (C) 2020 The Author(s). Published by Elsevier B.V

Elliptic Flow of Electrons from Beauty-Hadron Decays in Pb-Pb Collisions at root s(NN)=5.02 TeV

The elliptic flow of electrons from beauty hadron decays at midrapidity (|y|<0.8) is measured in Pb-Pb collisions at sNN=5.02 TeV with the ALICE detector at the LHC. The azimuthal distribution of the particles produced in the collisions can be parametrized with a Fourier expansion, in which the second harmonic coefficient represents the elliptic flow, v2. The v2 coefficient of electrons from beauty hadron decays is measured for the first time in the transverse momentum (pT) range 1.3-6 GeV/c in the centrality class 30%-50%. The measurement of electrons from beauty-hadron decays exploits their larger mean proper decay length c\u3c4 48500 \u3bcm compared to that of charm hadrons and most of the other background sources. The v2 of electrons from beauty hadron decays at midrapidity is found to be positive with a significance of 3.75 \u3c3. The results provide insights into the degree of thermalization of beauty quarks in the medium. A model assuming full thermalization of beauty quarks is strongly disfavored by the measurement at high pT, but is in agreement with the results at low pT. Transport models including substantial interactions of beauty quarks with an expanding strongly interacting medium describe the measurement within uncertainties

First measurement of quarkonium polarization in nuclear collisions at the LHC

The polarization of inclusive J/\u3c8 and \u3d2(1S) produced in Pb\u2013Pb collisions at sNN=5.02 TeV at the LHC is measured with the ALICE detector. The study is carried out by reconstructing the quarkonium through its decay to muon pairs in the rapidity region 2.5<4 and measuring the polar and azimuthal angular distributions of the muons. The polarization parameters \u3bb\u3b8, \u3bb\u3d5 and \u3bb\u3b8\u3d5 are measured in the helicity and Collins-Soper reference frames, in the transverse momentum interval 2<10 GeV/c and pT<15 GeV/c for the J/\u3c8 and \u3d2(1S), respectively. The polarization parameters for the J/\u3c8 are found to be compatible with zero, within a maximum of about two standard deviations at low pT, for both reference frames and over the whole pT range. The values are compared with the corresponding results obtained for pp collisions at s=7 and 8 TeV in a similar kinematic region by the ALICE and LHCb experiments. Although with much larger uncertainties, the polarization parameters for \u3d2(1S) production in Pb\u2013Pb collisions are also consistent with zero

First measurement of quarkonium polarization in nuclear collisions at the LHC

The polarization of inclusive J/ψ and ϒ(1S) produced in Pb–Pb collisions at sNN=5.02 TeV at the LHC is measured with the ALICE detector. The study is carried out by reconstructing the quarkonium through its decay to muon pairs in the rapidity region 2.5<y<4 and measuring the polar and azimuthal angular distributions of the muons. The polarization parameters λθ, λϕ and λθϕ are measured in the helicity and Collins-Soper reference frames, in the transverse momentum interval 2<pT<10 GeV/c and pT<15 GeV/c for the J/ψ and ϒ(1S), respectively. The polarization parameters for the J/ψ are found to be compatible with zero, within a maximum of about two standard deviations at low pT, for both reference frames and over the whole pT range. The values are compared with the corresponding results obtained for pp collisions at s=7 and 8 TeV in a similar kinematic region by the ALICE and LHCb experiments. Although with much larger uncertainties, the polarization parameters for ϒ(1S) production in Pb–Pb collisions are also consistent with zero

Multiplicity dependence of

This paper presents the measurements of $$\pi ^{\pm }$$, $$\mathrm {K}^{\pm }$$, $$\text {p}$$ and $$\overline{\mathrm{p}}$$ transverse momentum ($$p_{\text {T}}$$) spectra as a function of charged-particle multiplicity density in proton–proton (pp) collisions at $$\sqrt{s}\ =\ 13\ \text {TeV}$$ with the ALICE detector at the LHC. Such study allows us to isolate the center-of-mass energy dependence of light-flavour particle production. The measurements reported here cover a $$p_{\text {T}}$$ range from 0.1 to 20 $$\text {GeV}/c$$ and are done in the rapidity interval $$|y|<0.5$$. The $$p_{\text {T}}$$-differential particle ratios exhibit an evolution with multiplicity, similar to that observed in pp collisions at $$\sqrt{s}\ =\ 7\ \text {TeV}$$, which is qualitatively described by some of the hydrodynamical and pQCD-inspired models discussed in this paper. Furthermore, the $$p_{\text {T}}$$-integrated hadron-to-pion yield ratios measured in pp collisions at two different center-of-mass energies are consistent when compared at similar multiplicities. This also extends to strange and multi-strange hadrons, suggesting that, at LHC energies, particle hadrochemistry scales with particle multiplicity the same way under different collision energies and colliding systems

Global baryon number conservation encoded in net-proton fluctuations measured in Pb-Pb collisions at root s(NN)=2.76TeV

Experimental results are presented on event-by-event net-proton fluctuation measurements in Pb–Pb collisions at sNN=2.76 TeV, recorded by the ALICE detector at the CERN LHC. These measurements have as their ultimate goal an experimental test of Lattice QCD (LQCD) predictions on second and higher order cumulants of net-baryon distributions to search for critical behavior near the QCD phase boundary. Before confronting them with LQCD predictions, account has to be taken of correlations stemming from baryon number conservation as well as fluctuations of participating nucleons. Both effects influence the experimental measurements and are usually not considered in theoretical calculations. For the first time, it is shown that event-by-event baryon number conservation leads to subtle long-range correlations arising from very early interactions in the collisions