19 Search Results

The nuclear modification factor R AA of charm jets, identified by the presence of a D0 meson among the jet constituents, has been measured for the first time in Pb–Pb collisions at a centre-of-mass energy per nucleon pair s NN = 5.02 TeV with the ALICE detector at the LHC. The D0 mesons and their charge conjugates are reconstructed from the hadronic decay D 0 → K − π + . Jets are reconstructed from D0-meson candidates and charged particles using the anti-k T algorithm with jet resolution parameter R = 0.3 , in the jet transverse momentum (p T)more » range 5 < p T ch jet < 50 GeV/c and pseudorapidity |η ch jet| < 0.6. A hint of reduced suppression in the charm-jet R AA is observed in comparison to inclusive jets in central Pb–Pb collisions with a significance of about 2σ in 20 < p T ch jet < 50 GeV/c, suggesting the in-medium energy loss to depend on both the difference between quark and gluon coupling strength (Casimir colour-charge effect) and quark mass (dead-cone effect). The data are compared with model calculations that include mass effects in the in-medium energy loss. Several state-of-the-art models are consistent with the data, with the LIDO model providing the best description of the data in the common kinematic range of inclusive and D0-tagged jets, highlighting the role of mass effects in interpreting the results.« less

The dependence of f₀ (980) production on the final-state charged-particle multiplicity is reported for proton–proton (pp) collisions at the centre-of-mass energy, $$\mathbf{\sqrt{s}}$$ = 13 TeV. The production of (980) is measured with the ALICE detector via the decay channel in a midrapidity region of |y| < 0.5. The evolution of the integrated yields and mean transverse momentum of f₀ (980) as a function of charged-particle multiplicity measured in pp at $$\mathbf{\sqrt{s}}$$ = 13 TeV follows the trends observed in pp at $$\mathbf{\sqrt{s}}$$ = 5.02 TeV and in proton–lead (p–Pb) collisions at$$\mathbf{\sqrt{s_{NN}}}$$ = 5.02 TeV. Particle yield ratios of f₀ (980)more » to π^± and K* (892) are found to decrease with increasing charged-particle multiplicity. These particle ratios are compared with calculations from the canonical statistical thermal model as a function of charged-particle multiplicity. The thermal model calculations provide a better description of the decreasing trend of particle ratios when no strange or antistrange quark composition for f₀ (980) is assumed, which suggests that the data do not support significant hidden strangeness in the f₀ (980).« less

Quantum Chromodynamics predicts a phase transition from hadronic matter to quark–gluon plasma (QGP) at high temperatures and energy densities, where quarks and gluons (partons) are no longer confined within hadrons. The QGP forms in ultrarelativistic heavy-ion collisions. Anisotropic flow coefficients, quantifying the azimuthal expansion of produced matter, probe QGP properties. Flow measurements in high-energy heavy-ion collisions show a distinctive grouping of anisotropic flow for baryons and mesons at intermediate transverse momentum – a feature associated with flow imparted at the quark level, confirming QGP existence. The observation of QGP-like features in proton–proton and proton–ion collisions has sparked debate about QGPmore » formation in smaller systems. For the first time, we demonstrate the distinctive grouping of anisotropic flow for baryons and mesons in high-multiplicity proton–lead and proton–proton collisions at the Large Hadron Collider (LHC). These results are described by a model including hydrodynamic flow followed by hadron formation via quark coalescence, consistent with the formation of partonic flowing systems in these collisions.« less

The intense photon fluxes from relativistic nuclei provide an opportunity to study photonuclear interactions in ultraperipheral collisions. In particular, it allows for the investigations of excited, light-flavour vector mesons. The measurement of coherently photoproduced 𝜋⁺𝜋⁻𝜋⁺𝜋⁻ final states in ultraperipheral Pb–Pb collisions at $$\sqrt{s_{NN}}$$ = 5.02 TeV is presented for the first time. The cross section, d𝜎/d𝑦, times the branching ratio (𝜌 → 𝜋⁺𝜋⁺𝜋⁻𝜋⁻) is found to be 47.8 ± 2.3 (stat.) ± 7.7 (syst.) mb in the rapidity interval |𝑦| < 0.5. The invariant mass distribution is not well described with a single Breit-Wigner resonance without an interference term. Includingmore » in- terference with a non-resonant contribution results in the mass and width values being too far from those reported in PDG, while the production of two interfering resonances, 𝜌(1450) and 𝜌(1700), also provides a good description of the data. The values of the masses (𝑚) and widths (Γ) of the resonances extracted from the fit assuming two interfering resonances are 𝑚₁ = 1385 ± 14 (stat.) ± 3 (syst.) MeV/𝑐², Γ₁ = 431 ± 36 (stat.) ± 82 (syst.) MeV/𝑐², 𝑚₂ = 1663 ± 13 (stat.) ± 22 (syst.) MeV/𝑐² and Γ₂ = 357 ± 31 (stat.) ± 49 (syst.) MeV/𝑐², respectively. The measured cross sections times the branching ratios are compared to recent theoretical predictions.« less

This work reports femtoscopic correlations of p – – p ($$\overline{p}$$ – – $$\overline{p}$$) and p – – d ($$\overline{p}$$ – – $$\overline{d}$$) pairs measured in Pb–Pb collisions at center-of-mass energy per nucleon $$\sqrt{s_{NN}}$$ = 5.02 TeV in the ALICE Collaboration. A fit to the measured proton-proton correlation functions allows one to extract the dependence of the nucleon femtoscopic radius of the particle-emitting source on the pair transverse mass (m_T) and on the average charge particle multiplicity $$\langle$$dN_ch/dη$$\rangle$$^1/3 for three centrality intervals (0–10%, 10 – – 30 %, 30 – – 50 %). In both cases, the expected power-law andmore » linear scalings are observed, respectively. The measured p–d correlations can be described by both two- and three-body calculations, indicating that the femtoscopy observable is not sensitive to the short-distance features of the dynamics of the p-(p-n) system, due to the large inter-particle distances in Pb–Pb collisions at the LHC. Indeed, in this study, the minimum measured femtoscopic source sizes for protons and deuterons have a minimum value at 2.73$$^{+0.05}_{–0.05}$$ and 3.10$$^{+1.04}_{–0.86}$$ fm, respectively, for the 30–50% centrality collisions. Moreover, the m_T-scaling obtained for the p–p and p–d systems is compatible within 1σ of the uncertainties. These findings provide new input for fundamental studies on the production of light (anti)nuclei under extreme conditions.« less

The distribution of angular correlations between prompt charm hadrons and primary charged particles in pp collisions is sensitive to the charm-quark hadronisation process. In this letter, charm-baryon correlations are measured for the first time by studying the azimuthal-angle difference between charged particles and prompt Λ c + baryons produced in pp collisions at a centre-of-mass energy of s = 13 TeV, with the ALICE detector. Λ c + baryons are reconstructed at midrapidity ( | y | < 0.5 ) in the transverse-momentum interval 3 < p T < 16 GeV/c, and correlated with charged particles with p T >more » 0.3 GeV/c and pseudorapidity | η | < 0.8 . For 3 < p T Λ c + , D < 5 GeV/c, the comparison with published measurements of D-meson and charged-particle correlations in the same collision system hints at a larger number of low-momentum particles associated with Λ c + -baryon triggers than with D-meson triggers, both in the collinear and opposite directions with respect to the trigger particle. These differences can be quantified by the comparison of the properties of the near- and away-side correlation peaks, and are not reproduced by predictions of various Monte Carlo event generators, generally underpredicting the associated particle yields at p T assoc < 1 GeV/c. This tension between Λ c + -baryon and D-meson associated peak yields could suggest a modified fragmentation of the charm quark, or a different hadronisation process, when a charm baryon is produced in the final state.« less

The ALICE Collaboration at the CERN LHC has measured the inclusive production cross section of isolated photons at midrapidity as a function of the photon transverse momentum (p$$^{γ}_{T}$$), in Pb–Pb collisions in different centrality intervals, and in pp collisions, at centre-of-momentum energy per nucleon pair of $$\sqrt{s_{NN}}$$ = 5.02 TeV. The photon transverse momentum range is between 10–14 and 40–140 GeV/c, depending on the collision system and on the Pb–Pb centrality class. The result extends to lower p$$^{γ}_{T}$$ than previously published results by the ATLAS and CMS experiments at the same collision energy. The covered pseudorapidity range is |η^γ| <more » 0.67. The isolation selection is based on a charged particle isolation momentum threshold p$$^{iso, ch}_{T}$$ GeV/c within a cone of radii R = 0.2 and 0.4. The nuclear modification factor is calculated and found to be consistent with unity in all centrality classes, and also consistent with the HG-PYTHIA model, which describes the event selection and geometry biases that affect the centrality determination in peripheral Pb–Pb collisions. The measurement is compared to next-to-leading order perturbative QCD calculations and to the measurements of isolated photons and Z⁰ bosons from the CMS experiment, which are all found to be in agreement.« less

The production of J/ψ is measured at midrapidity (|y| < 0.9) in proton-proton collisions at \( \sqrt{s} \) = 5.02 and 13 TeV, through the dielectron decay channel, using the ALICE detector at the Large Hadron Collider. The data sets used for the analyses correspond to integrated luminosities of \( \mathcal{L} \)_int = 19.4 ± 0.4 nb^-1 and \( \mathcal{L} \)_int = 32.2 ± 0.5 nb^-1 at \( \sqrt{s} \) = 5.02 and 13 TeV, respectively. The fraction of non-prompt J/ψ mesons, i.e. those originating from the decay of beauty hadrons, is measured down to a transverse momentum p_T =more » 2 GeV/c (1 GeV/c) at \( \sqrt{s} \) = 5.02 TeV (13 TeV). The p_T and rapidity (y) differential cross sections, as well as the corresponding values integrated over p_T and y, are carried out separately for prompt and non-prompt J/ψ mesons. The results are compared with measurements from other experiments and theoretical calculations based on quantum chromodynamics (QCD). The shapes of the p_T and y distributions of beauty quarks predicted by state-of-the-art perturbative QCD models are used to extrapolate an estimate of the \( \mathrm{b}\overline{\mathrm{b}} \) pair cross section at midrapidity and in the total phase space. The total \( \mathrm{b}\overline{\mathrm{b}} \) cross sections are found to be \( {\sigma}_{\mathrm{b}\overline{\mathrm{b}}} \) = 541 ± 45 (stat.) ± 69 \( {\left(\mathrm{syst}.\right)}_{-12}^{+10} \) (extr.) μb and \( {\sigma}_{\mathrm{b}\overline{\mathrm{b}}} \) = 218±37 (stat.)±31 \( {\left(\mathrm{syst}.\right)}_{-9.1}^{+8.2} \) (extr.) μb at \( \sqrt{s} \) = 13 and 5.02 TeV, respectively. The value obtained from the combination of ALICE and LHCb measurements in pp collisions at \( \sqrt{s} \) = 13 TeV is also provided.« less

The production of \(\pi ^{\pm }\), \(\mathrm{K}^{\pm }\), \(\mathrm{K}^{0}_{S}\), \(\mathrm{K}^{*}(892)^{0}\), \(\mathrm{p}\), \(\phi (1020)\), \(\Lambda \), \(\Xi ^{-}\), \(\Omega ^{-}\), and their antiparticles was measured in inelastic proton–proton (pp) collisions at a center-of-mass energy of \(\sqrt{s}\) = 13 TeV at midrapidity (\(|y|<0.5\)) as a function of transverse momentum (\(p_{\mathrm{T}}\)) using the ALICE detector at the CERN LHC. Furthermore, the single-particle \(p_{\mathrm{T}}\) distributions of \(\mathrm{K}^{0}_{S}\), \(\Lambda \), and \(\overline{\Lambda }\) in inelastic pp collisions at \(\sqrt{s} = 7\) TeV are reported here for the first time. The \(p_{\mathrm{T}}\) distributions are studied at midrapidity within the transverse momentum range \(0\le p_{\mathrm{T}}\le 20\) GeV/c,more » depending on the particle species. The \(p_{\mathrm{T}}\) spectra, integrated yields, and particle yield ratios are discussed as a function of collision energy and compared with measurements at lower \(\sqrt{s}\) and with results from various general-purpose QCD-inspired Monte Carlo models. A hardening of the spectra at high \(p_{\mathrm{T}}\) with increasing collision energy is observed, which is similar for all particle species under study. The transverse mass and \(x_{\mathrm{T}}\equiv 2p_{\mathrm{T}}/\sqrt{s}\) scaling properties of hadron production are also studied. As the collision energy increases from \(\sqrt{s}\) = 7–13 TeV, the yields of non- and single-strange hadrons normalized to the pion yields remain approximately constant as a function of \(\sqrt{s}\), while ratios for multi-strange hadrons indicate enhancements. The \(p_\mathrm{{T}}\)-differential cross sections of \(\pi ^{\pm }\), \(\mathrm {K}^{\pm }\) and \(\mathrm {p}\) (\(\overline{\mathrm{p}}\)) are compared with next-to-leading order perturbative QCD calculations, which are found to overestimate the cross sections for \(\pi ^{\pm }\) and \(\mathrm{p}\) (\(\overline{\mathrm{p}}\)) at high \(p_\mathrm{{T}}\).« less

The invariant differential cross section of inclusive \(\omega (782)\) meson production at midrapidity (\(|y|<0.5\)) in pp collisions at \(\sqrt{s}=7\,\hbox {TeV}\) was measured with the ALICE detector at the LHC over a transverse momentum range of \(2< p_{\mathrm {T}}< 17\,\hbox {GeV}/c\). The \(\omega \) meson was reconstructed via its \(\omega \rightarrow \pi ^+\pi ^-\pi ^0\) decay channel. The measured \(\omega \) production cross section is compared to various calculations: PYTHIA 8.2 Monash 2013 describes the data, while PYTHIA 8.2 Tune 4C overestimates the data by about 50%. A recent NLO calculation, which includes a model describing the fragmentation of the wholemore » vector-meson nonet, describes the data within uncertainties below \(6\,\hbox {GeV}/c\), while it overestimates the data by up to 50% for higher \(p_{\mathrm {T}}\). The \(\omega /\pi ^0\) ratio is in agreement with previous measurements at lower collision energies and the PYTHIA calculations. In addition, the measurement is compatible with transverse mass scaling within the measured \(p_{\mathrm {T}}\) range and the ratio is constant with \(C^{\omega /\pi ^{0}}= 0.67 \pm 0.03 \text {~(stat)~} \pm 0.04 \text {~(sys)~}\) above a transverse momentum of \(2.5\,\hbox {GeV}/c\).« less