Abstract Measurements of the production cross sections of prompt D0, D+, D*+, D s + $$ {\textrm{D}}_{\textrm{s}}^{+} $$ , Λ c + $$ {\Lambda}_{\textrm{c}}^{+} $$ , and Ξ c + $$ {\Xi}_{\textrm{c}}^{+} ...$$ charm hadrons at midrapidity in proton-proton collisions at s $$ \sqrt{s} $$ = 13 TeV with the ALICE detector are presented. The D-meson cross sections as a function of transverse momentum (p T) are provided with improved precision and granularity. The ratios of p T-differential meson production cross sections based on this publication and on measurements at different rapidity and collision energy provide a constraint on gluon parton distribution functions at low values of Bjorken-x (10 −5–10 −4). The measurements of Λ c + $$ {\Lambda}_{\textrm{c}}^{+} $$ ( Ξ c + $$ {\Xi}_{\textrm{c}}^{+} $$ ) baryon production extend the measured p T intervals down to p T = 0(3) GeV/c. These measurements are used to determine the charm-quark fragmentation fractions and the c c ¯ $$ \textrm{c}\overline{\textrm{c}} $$ production cross section at midrapidity (|y| < 0.5) based on the sum of the cross sections of the weakly-decaying ground-state charm hadrons D0, D+, D s + $$ {\textrm{D}}_{\textrm{s}}^{+} $$ , Λ c + $$ {\Lambda}_{\textrm{c}}^{+} $$ , Ξ c 0 $$ {\Xi}_{\textrm{c}}^0 $$ and, for the first time, Ξ c + $$ {\Xi}_{\textrm{c}}^{+} $$ , and of the strongly-decaying J/ψ mesons. The first measurements of Ξ c + $$ {\Xi}_{\textrm{c}}^{+} $$ and Σ c 0 , + + $$ {\Sigma}_{\textrm{c}}^{0,++} $$ fragmentation fractions at midrapidity are also reported. A significantly larger fraction of charm quarks hadronising to baryons is found compared to e+e − and ep collisions. The c c ¯ $$ \textrm{c}\overline{\textrm{c}} $$ production cross section at midrapidity is found to be at the upper bound of state-of-the-art perturbative QCD calculations.
Abstract $$\phi $$ ϕ meson measurements provide insight into strangeness production, which is one of the key observables for the hot medium formed in high-energy heavy-ion collisions. ALICE measured ...$$\phi $$ ϕ production through its decay in muon pairs in Pb–Pb collisions at $$\sqrt{s_\mathrm {NN}} = 2.76$$ sNN=2.76 TeV in the intermediate transverse momentum range $$2< p_\mathrm {T}< 5$$ 2<pT<5 GeV/c and in the rapidity interval $$2.5<y<4$$ 2.5<y<4 . The $$\phi $$ ϕ yield was measured as a function of the transverse momentum and collision centrality. The nuclear modification factor was obtained as a function of the average number of participating nucleons. Results were compared with the ones obtained via the kaon decay channel in the same $$p_\mathrm {T}$$ pT range at midrapidity. The values of the nuclear modification factor in the two rapidity regions are in agreement within uncertainties.
Abstract A measurement of beauty hadron production at mid-rapidity in proton-lead collisions at a nucleon–nucleon centre-of-mass energy $$\sqrt{s_\text {NN}}=5.02$$ sNN=5.02 TeV is presented. The ...semi-inclusive decay channel of beauty hadrons into $$\hbox {J}/\psi $$ J/ψ is considered, where the $$\hbox {J}/\psi $$ J/ψ mesons are reconstructed in the dielectron decay channel at mid-rapidity down to transverse momenta of 1.3 GeV/c. The $$\hbox {b}\bar{\hbox {b}}$$ bb¯ production cross section at mid-rapidity, $$\hbox {d}\sigma _{\hbox {b}\bar{\hbox {b}}}/\hbox {d}y$$ dσbb¯/dy , and the total cross section extrapolated over full phase space, $$\sigma _{\text {b}\bar{\text {b}}}$$ σbb¯ , are obtained. This measurement is combined with results on inclusive $$\hbox {J}/\psi $$ J/ψ production to determine the prompt $$\hbox {J}/\psi $$ J/ψ cross sections. The results in p–Pb collisions are then scaled to expectations from pp collisions at the same centre-of-mass energy to derive the nuclear modification factor $$R_{\text {pPb}}$$ RpPb , and compared to models to study possible nuclear modifications of the production induced by cold nuclear matter effects. $$R_{\text {pPb}}$$ RpPb is found to be smaller than unity at low $$p_{\mathrm{T}}$$ pT for both $$\hbox {J}/\psi $$ J/ψ coming from beauty hadron decays and prompt $$\hbox {J}/\psi $$ J/ψ .
Abstract The fractions of non-prompt (i.e. originating from beauty-hadron decays) D0 and D+ mesons with respect to the inclusive yield are measured as a function of the charged-particle multiplicity ...in proton-proton collisions at a centre-of-mass energy of s $$ \sqrt{s} $$ = 13 TeV with the ALICE detector at the LHC. The results are reported in intervals of transverse momentum (p T) and integrated in the range 1 < p T < 24 GeV/c. The fraction of non-prompt D0 and D+ mesons is found to increase slightly as a function of p T in all the measured multiplicity intervals, while no significant dependence on the charged-particle multiplicity is observed. In order to investigate the production and hadronisation mechanisms of charm and beauty quarks, the results are compared to PYTHIA 8 as well as EPOS 3 and EPOS 4 Monte Carlo simulations, and to calculations based on the colour glass condensate including three-pomeron fusion.
Abstract The cross section for coherent photonuclear production of J/ψ is presented as a function of the electromagnetic dissociation (EMD) of Pb. The measurement is performed with the ALICE detector ...in ultra-peripheral Pb-Pb collisions at a centre-of-mass energy per nucleon pair of s NN $$ \sqrt{{\textrm{s}}_{\textrm{NN}}} $$ = 5.02 TeV. Cross sections are presented in five different J/ψ rapidity ranges within |y| < 4, with the J/ψ reconstructed via its dilepton decay channels. In some events the J/ψ is not accompanied by EMD, while other events do produce neutrons from EMD at beam rapidities either in one or the other beam direction, or in both. The cross sections in a given rapidity range and for different configurations of neutrons from EMD allow for the extraction of the energy dependence of this process in the range 17 < W γ Pb,n < 920 GeV, where W γ Pb,n is the centre-of-mass energy per nucleon of the γPb system. This range corresponds to a Bjorken-x interval spanning about three orders of magnitude: 1.1 × 10 −5 < x < 3.3 × 10 −2. In addition to the ultra-peripheral and photonuclear cross sections, the nuclear suppression factor is obtained. These measurements point to a strong depletion of the gluon distribution in Pb nuclei over a broad, previously unexplored, energy range. These results, together with previous ALICE measurements, provide unprecedented information to probe quantum chromodynamics at high energies.
Abstract The azimuthal ( $$\Delta \varphi $$ Δ φ ) correlation distributions between heavy-flavor decay electrons and associated charged particles are measured in pp and p–Pb collisions at ...$$\sqrt{s_{\mathrm{{NN}}} = 5.02$$ s NN = 5.02 TeV. Results are reported for electrons with transverse momentum $$4<p_{\textrm{T}}<16$$ 4 < p T < 16 $$\textrm{GeV}/c$$ GeV / c and pseudorapidity $$|\eta |<0.6$$ | η | < 0.6 . The associated charged particles are selected with transverse momentum $$1<p_{\textrm{T}}<7$$ 1 < p T < 7 $$\textrm{GeV}/c$$ GeV / c , and relative pseudorapidity separation with the leading electron $$|\Delta \eta | < 1$$ | Δ η | < 1 . The correlation measurements are performed to study and characterize the fragmentation and hadronization of heavy quarks. The correlation structures are fitted with a constant and two von Mises functions to obtain the baseline and the near- and away-side peaks, respectively. The results from p–Pb collisions are compared with those from pp collisions to study the effects of cold nuclear matter. In the measured trigger electron and associated particle kinematic regions, the two collision systems give consistent results. The $$\Delta \varphi $$ Δ φ distribution and the peak observables in pp and p–Pb collisions are compared with calculations from various Monte Carlo event generators.
Abstract A newly developed observable for correlations between symmetry planes, which characterize the direction of the anisotropic emission of produced particles, is measured in Pb–Pb collisions at ...$$\sqrt{s_\text {NN}}$$ s NN = 2.76 TeV with ALICE. This so-called Gaussian Estimator allows for the first time the study of these quantities without the influence of correlations between different flow amplitudes. The centrality dependence of various correlations between two, three and four symmetry planes is presented. The ordering of magnitude between these symmetry plane correlations is discussed and the results of the Gaussian Estimator are compared with measurements of previously used estimators. The results utilizing the new estimator lead to significantly smaller correlations than reported by studies using the Scalar Product method. Furthermore, the obtained symmetry plane correlations are compared to state-of-the-art hydrodynamic model calculations for the evolution of heavy-ion collisions. While the model predictions provide a qualitative description of the data, quantitative agreement is not always observed, particularly for correlators with significant non-linear response of the medium to initial state anisotropies of the collision system. As these results provide unique and independent information, their usage in future Bayesian analysis can further constrain our knowledge on the properties of the QCD matter produced in ultrarelativistic heavy-ion collisions.
Abstract The production of inclusive, prompt and non-prompt J/ψ was studied for the first time at midrapidity (−1.37 < y cms < 0.43) in p-Pb collisions at s NN $$ \sqrt{s_{\textrm{NN}}} $$ = 8.16 TeV ...with the ALICE detector at the LHC. The inclusive J/ψ mesons were reconstructed in the dielectron decay channel in the transverse momentum (p T) interval 0 < p T < 14 GeV/c and the prompt and non-prompt contributions were separated on a statistical basis for p T > 2 GeV/c. The study of the J/ψ mesons in the dielectron channel used for the first time in ALICE online single-electron triggers from the Transition Radiation Detector, providing a data sample corresponding to an integrated luminosity of 689 ± 13 μb −1. The proton-proton reference cross section for inclusive J/ψ was obtained based on interpolations of measured data at different centre-of-mass energies and a universal function describing the p T-differential J/ψ production cross sections. The p T-differential nuclear modification factors R pPb of inclusive, prompt, and non-prompt J/ψ are consistent with unity and described by theoretical models implementing only nuclear shadowing.
Abstract This article reports measurements of the angle between differently defined jet axes in pp collisions at s $$ \sqrt{s} $$ = 5.02 TeV carried out by the ALICE Collaboration. Charged particles ...at midrapidity are clustered into jets with resolution parameters R = 0.2 and 0.4. The jet axis, before and after Soft Drop grooming, is compared to the jet axis from the Winner-Takes-All (WTA) recombination scheme. The angle between these axes, ∆R axis, probes a wide phase space of the jet formation and evolution, ranging from the initial high-momentum-transfer scattering to the hadronization process. The ∆R axis observable is presented for 20 < p T ch jet $$ {p}_{\textrm{T}}^{\textrm{ch}\ \textrm{jet}} $$ < 100 GeV/c, and compared to predictions from the PYTHIA 8 and Herwig 7 event generators. The distributions can also be calculated analytically with a leading hadronization correction related to the non-perturbative component of the Collins-Soper-Sterman (CSS) evolution kernel. Comparisons to analytical predictions at next-to-leading-logarithmic accuracy with leading hadronization correction implemented from experimental extractions of the CSS kernel in Drell-Yan measurements are presented. The analytical predictions describe the measured data within 20% in the perturbative regime, with surprising agreement in the non-perturbative regime as well. These results are compatible with the universality of the CSS kernel in the context of jet substructure.
Abstract The production of strange hadrons ( K S 0 $$ {\textrm{K}}_{\textrm{S}}^0 $$ , Λ, Ξ ± , and Ω ± ), baryon-to-meson ratios (Λ/ K S 0 $$ {\textrm{K}}_{\textrm{S}}^0 $$ , Ξ/ K S 0 $$ ...{\textrm{K}}_{\textrm{S}}^0 $$ , and Ω/ K S 0 $$ {\textrm{K}}_{\textrm{S}}^0 $$ ), and baryon-to-baryon ratios (Ξ/Λ, Ω/Λ, and Ω/Ξ) associated with jets and the underlying event were measured as a function of transverse momentum (p T) in pp collisions at s $$ \sqrt{s} $$ = 13 TeV and p Pb collisions at s NN $$ \sqrt{s_{\textrm{NN}}} $$ = 5.02 TeV with the ALICE detector at the LHC. The inclusive production of the same particle species and the corresponding ratios are also reported. The production of multi-strange hadrons, Ξ ± and Ω ± , and their associated particle ratios in jets and in the underlying event are measured for the first time. In both pp and p–Pb collisions, the baryon-to-meson and baryon-to-baryon yield ratios measured in jets differ from the inclusive particle production for low and intermediate hadron p T (0.6–6 GeV/c). Ratios measured in the underlying event are in turn similar to those measured for inclusive particle production. In pp collisions, the particle production in jets is compared with Pythia 8 predictions with three colour-reconnection implementation modes. None of them fully reproduces the data in the measured hadron p T region. The maximum deviation is observed for Ξ ± and Ω ± which reaches a factor of about six. The event multiplicity dependence is further investigated in p−Pb collisions. In contrast to what is observed in the underlying event, there is no significant event-multiplicity dependence for particle production in jets. The presented measurements provide novel constraints on hadronisation and its Monte Carlo description. In particular, they demonstrate that the fragmentation of jets alone is insufficient to describe the strange and multi-strange particle production in hadronic collisions at LHC energies.
PDF