Abstract The transverse momentum (p T) and centrality dependence of the nuclear modification factor R AA of prompt and non-prompt J/ψ, the latter originating from the weak decays of beauty hadrons, ...have been measured by the ALICE collaboration in Pb–Pb collisions at s NN $$ \sqrt{s_{\textrm{NN}}} $$ = 5.02 TeV. The measurements are carried out through the e+e − decay channel at midrapidity (|y| < 0.9) in the transverse momentum region 1.5 < p T < 10 GeV/c. Both prompt and non-prompt J/ψ measurements indicate a significant suppression for p T > 5 GeV/c, which becomes stronger with increasing collision centrality. The results are consistent with similar LHC measurements in the overlapping p T intervals, and cover the kinematic region down to p T = 1.5 GeV/c at midrapidity, not accessible by other LHC experiments. The suppression of prompt J/ψ in central and semicentral collisions exhibits a decreasing trend towards lower transverse momentum, described within uncertainties by models implementing J/ψ production from recombination of c and c ¯ $$ \overline{\textrm{c}} $$ quarks produced independently in different partonic scatterings. At high transverse momentum, transport models including quarkonium dissociation are able to describe the suppression for prompt J/ψ. For non-prompt J/ψ, the suppression predicted by models including both collisional and radiative processes for the computation of the beauty-quark energy loss inside the quark-gluon plasma is consistent with measurements within uncertainties.
Abstract Correlations in azimuthal angle extending over a long range in pseudorapidity between particles, usually called the “ridge” phenomenon, were discovered in heavy-ion collisions, and later ...found in pp and p–Pb collisions. In large systems, they are thought to arise from the expansion (collective flow) of the produced particles. Extending these measurements over a wider range in pseudorapidity and final-state particle multiplicity is important to understand better the origin of these long-range correlations in small collision systems. In this Letter, measurements of the long-range correlations in p–Pb collisions at s NN $$ \sqrt{s_{\textrm{NN}}} $$ = 5.02 TeV are extended to a pseudorapidity gap of ∆η ~ 8 between particles using the ALICE forward multiplicity detectors. After suppressing non-flow correlations, e.g., from jet and resonance decays, the ridge structure is observed to persist up to a very large gap of ∆η ~ 8 for the first time in p–Pb collisions. This shows that the collective flow-like correlations extend over an extensive pseudorapidity range also in small collision systems such as p–Pb collisions. The pseudorapidity dependence of the second-order anisotropic flow coefficient, v 2(η), is extracted from the long-range correlations. The v 2(η) results are presented for a wide pseudorapidity range of –3.1 < η < 4.8 in various centrality classes in p–Pb collisions. To gain a comprehensive understanding of the source of anisotropic flow in small collision systems, the v 2(η) measurements are compared with hydrodynamic and transport model calculations. The comparison suggests that the final-state interactions play a dominant role in developing the anisotropic flow in small collision systems.
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 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.
Celotno besedilo
Dostopno za:
DOBA, IZUM, KILJ, NUK, PILJ, PNG, SAZU, SIK, UILJ, UKNU, UL, UM, UPUK
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 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.
Celotno besedilo
Dostopno za:
DOBA, IZUM, KILJ, NUK, PILJ, PNG, SAZU, SIK, UILJ, UKNU, UL, UM, UPUK
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.