94 research outputs found

    Measurement of (anti)alpha production in central Pb-Pb collisions at sNN\sqrt{s_{\rm NN}} = 5.02 TeV

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    International audienceIn this letter, measurements of (anti)alpha production in central (0-10%) Pb-Pb collisions at a center-of-mass energy per nucleon-nucleon pair of sNN\sqrt{s_{\rm NN}} = 5.02 TeV are presented, including the first measurement of an antialpha transverse-momentum spectrum. Owing to its large mass, (anti)alpha production yields and transverse-momentum spectra are of particular interest because they provide a stringent test of particle production models. The averaged antialpha and alpha spectrum is included into a common blast-wave fit with lighter particles, indicating that the (anti)alpha also participates in the collective expansion of the medium created in the collision. A blast-wave fit including only protons, (anti)alpha, and other light nuclei results in a similar flow velocity as the fit that includes all particles. A similar flow velocity, but a significantly larger kinetic freeze-out temperature is obtained when only protons and light nuclei are included in the fit. The coalescence parameter B4B_4 is well described by calculations from a statistical hadronization model but significantly underestimated by calculations assuming nucleus formation via coalescence of nucleons. Similarly, the (anti)alpha-to-proton ratio is well described by the statistical hadronization model. On the other hand, coalescence calculations including approaches with different implementations of the (anti)alpha substructure tend to underestimate the data

    Investigating the nature of the K0(700)^*_0(700) state with π±\pi^\pmKS0^0_{\rm S} correlations at the LHC

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    International audienceThe first measurements of femtoscopic correlations with the particle pair combinations π±\pi^\pmKS0^0_{\rm S} in pp collisions at s=13\sqrt{s}=13 TeV at the Large Hadron Collider (LHC) are reported by the ALICE experiment. Using the femtoscopic approach, it is shown that it is possible to study the elusive K0(700)^*_0(700) particle that has been considered a tetraquark candidate for over forty years. Boson source parameters and final-state interaction parameters are extracted by fitting a model assuming a Gaussian source to the experimentally measured two-particle correlation functions. The final-state interaction is modeled through a resonant scattering amplitude, defined in terms of a mass and a coupling parameter, decaying into a π±\pi^\pmKS0^0_{\rm S} pair. The extracted mass and Breit-Wigner width, derived from the coupling parameter, of the final-state interaction are found to be consistent with previous measurements of the K0(700)^*_0(700). The small value and increasing behavior of the correlation strength with increasing source size support the hypothesis that the K0(700)^*_0(700) is a four-quark state, i.e. a tetraquark state. This latter trend is also confirmed via a simple geometric model that assumes a tetraquark structure of the K0(700)^*_0(700) resonance

    Measurement of Ωc0\Omega^0_{\rm c} baryon production and branching-fraction ratio BR(Ωc0Ωe+νe)/BR(Ωc0Ωπ+){\rm BR(\Omega^0_c \rightarrow \Omega^- e^+\nu_e)} / {\rm BR(\Omega^0_c \rightarrow \Omega^- \pi^+)} in pp collisions at s\sqrt{s} = 13 TeV

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    International audienceThe inclusive production of the charm-strange baryon Ωc0\Omega^{0}_{\rm c} is measured for the first time via its semileptonic decay into Ωe+νe\Omega^{-}\rm e^{+}\nu_{e} at midrapidity (y<0.8|y|<0.8) in proton-proton (pp) collisions at the centre-of-mass energy s=13\sqrt{s}=13 TeV with the ALICE detector at the LHC. The transverse momentum (pTp_{\rm T}) differential cross section multiplied by the branching ratio is presented in the interval 2<pT<12 GeV/c2<p_{\rm T}<12~{\rm GeV}/c. The branching-fraction ratio BR(Ωc0Ωe+νe)/BR(Ωc0Ωπ+){\rm BR}(\Omega^0_{\rm c} \rightarrow \Omega^{-}{\rm e}^{+}\nu_{\rm e})/ {\rm BR}(\Omega^0_{\rm c} \rightarrow \Omega^{-}{\pi}^{+}) is measured to be 1.12 ±\pm 0.22 (stat.) ±\pm 0.27 (syst.). Comparisons with other experimental measurements, as well as with theoretical calculations, are presented

    Multiplicity dependence of charged-particle intra-jet properties in pp collisions at √s = 13 TeV

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    The first measurement of the multiplicity dependence of intra-jet properties of leading charged-particle jets in proton-proton (pp) collisions is reported. The mean charged-particle multiplicity and jet fragmentation distributions are measured in minimum-bias and high-multiplicity pp collisions at s√ = 13 TeV using the ALICE detector. Jets are reconstructed from charged particles produced in the midrapidity region (|η|<0.9) using the sequential recombination anti-kT algorithm with jet resolution parameters R = 0.2, 0.3, and 0.4 for the transverse momentum (pT) interval 5−110 GeV/c. High-multiplicity events are selected by the forward V0 scintillator detectors. The mean charged-particle multiplicity inside the leading jet cone rises monotonically with increasing jet pT in qualitative agreement with previous measurements at lower energies. The distributions of jet fragmentation functions zch and ξch are measured for different jet-pT intervals. Jet-pT independent fragmentation of leading jets is observed for wider jets except at high- and low-zch. The observed "hump-backed plateau" structure in the ξch distribution indicates suppression of low-pT particles. In high-multiplicity events, an enhancement of the fragmentation probability of low-zch particles accompanied by a suppression of high-zch particles is observed compared to minimum-bias events. This behavior becomes more prominent for low-pT jets with larger jet radius. The results are compared with predictions of QCD-inspired event generators, PYTHIA 8 with Monash 2013 tune and EPOS LHC. It is found that PYTHIA 8 qualitatively reproduces the jet modification in high-multiplicity events except at high jet pT. These measurements provide important constraints to models of jet fragmentation

    Systematic study of flow vector decorrelation in sNN=5.02\mathbf{\sqrt{\textit{s}_{_{\bf NN}}}=5.02} TeV Pb-Pb collisions

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    International audienceMeasurements of the pTp_{\rm T}-dependent flow vector fluctuations in Pb-Pb collisions at sNN=5.02 TeV\sqrt{s_{_{\rm NN}}} = 5.02~\mathrm{TeV} using azimuthal correlations with the ALICE experiment at the LHC are presented. A four-particle correlation approach [1] is used to quantify the effects of flow angle and magnitude fluctuations separately. This paper extends previous studies to additional centrality intervals and provides measurements of the pTp_{\rm T}-dependent flow vector fluctuations at sNN=5.02 TeV\sqrt{s_{_{\rm NN}}} = 5.02~\mathrm{TeV} with two-particle correlations. Significant pTp_{\rm T}-dependent fluctuations of the V2\vec{V}_{2} flow vector in Pb-Pb collisions are found across different centrality ranges, with the largest fluctuations of up to \sim15% being present in the 5% most central collisions. In parallel, no evidence of significant pTp_{\rm T}-dependent fluctuations of V3\vec{V}_{3} or V4\vec{V}_{4} is found. Additionally, evidence of flow angle and magnitude fluctuations is observed with more than 5σ5\sigma significance in central collisions. These observations in Pb-Pb collisions indicate where the classical picture of hydrodynamic modeling with a common symmetry plane breaks down. This has implications for hard probes at high pTp_{\rm T}, which might be biased by pTp_{\rm T}-dependent flow angle fluctuations of at least 23% in central collisions. Given the presented results, existing theoretical models should be re-examined to improve our understanding of initial conditions, quark--gluon plasma (QGP) properties, and the dynamic evolution of the created system

    Studying the interaction between charm and light-flavor mesons

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    The two-particle momentum correlation functions between charm mesons (D±\mathrm{D^{*\pm}} and D±\mathrm{D}^\pm) and charged light-flavor mesons (π±\pi^{\pm} and K±^{\pm}) in all charge-combinations are measured for the first time by the ALICE Collaboration in high-multiplicity proton-proton collisions at a center-of-mass energy of s=13\sqrt{s} =13 TeV. For DK\mathrm{DK} and DK\mathrm{D^*}\mathrm{K} pairs, the experimental results are in agreement with theoretical predictions of the residual strong interaction based on quantum chromodynamics calculations on the lattice and chiral effective field theory. In the case of Dπ\mathrm{D}\pi and Dπ\mathrm{D^*}\pi pairs, a tension between the calculations including strong interactions and the measurement is observed. For all particle pairs, the data can be adequately described by Coulomb interaction only, indicating a shallow interaction between charm and light-flavor mesons. Finally, the scattering lengths governing the residual strong interaction of the Dπ\mathrm{D}\pi and Dπ\mathrm{D^*}\pi systems are determined by fitting the experimental correlation functions with a model that employs a Gaussian potential. The extracted values are small and compatible with zero.The two-particle momentum correlation functions between charm mesons (D±\mathrm{D^{*\pm}} and D±\mathrm{D}^\pm) and charged light-flavor mesons (π±\pi^{\pm} and K±^{\pm}) in all charge-combinations are measured for the first time by the ALICE Collaboration in high-multiplicity proton-proton collisions at a center-of-mass energy of s=13\sqrt{s} =13 TeV. For DK\mathrm{DK} and DK\mathrm{D^*K} pairs, the experimental results are in agreement with theoretical predictions of the residual strong interaction based on quantum chromodynamics calculations on the lattice and chiral effective field theory. In the case of Dπ\mathrm{D}\pi and Dπ\mathrm{D^*}\pi pairs, tension between the calculations including strong interactions and the measurement is observed. For all particle pairs, the data can be adequately described by Coulomb interaction only, indicating a shallow interaction between charm and light-flavor mesons. Finally, the scattering lengths governing the residual strong interaction of the Dπ\mathrm{D}\pi and Dπ\mathrm{D^*}\pi systems are determined by fitting the experimental correlation functions with a model that employs a Gaussian potential. The extracted values are small and compatible with zero

    Multiplicity dependence of charged-particle intra-jet properties in pp collisions at s\sqrt{s} = 13 TeV

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    International audienceThe first measurement of the multiplicity dependence of intra-jet properties of leading charged-particle jets in proton-proton (pp) collisions is reported. The mean charged-particle multiplicity and jet fragmentation distributions are measured in minimum-bias and high-multiplicity pp collisions at s\sqrt{s} = 13 TeV using the ALICE detector. Jets are reconstructed from charged particles produced in the midrapidity region (η<0.9|\eta| < 0.9) using the sequential recombination anti-kTk_{\rm T} algorithm with jet resolution parameters RR = 0.2, 0.3, and 0.4 for the transverse momentum (pTp_{\rm T}) interval 5-110 GeV/cc. High-multiplicity events are selected by the forward V0 scintillator detectors. The mean charged-particle multiplicity inside the leading jet cone rises monotonically with increasing jet pTp_{\rm T} in qualitative agreement with previous measurements at lower energies. The distributions of jet fragmentation functions zchz^{\rm ch} and ξch\xi^{\rm ch} are measured for different jet-pTp_{\rm T} intervals. Jet-pTp_{\rm T} independent fragmentation of leading jets is observed for wider jets except at high- and low-zchz^{\rm ch}. The observed "hump-backed plateau" structure in the ξch\xi^{\rm ch} distribution indicates suppression of low-pTp_{\rm T} particles. In high-multiplicity events, an enhancement of the fragmentation probability of low-zchz^{\rm ch} particles accompanied by a suppression of high-zchz^{\rm ch} particles is observed compared to minimum-bias events. This behavior becomes more prominent for low-pTp_{\rm T} jets with larger jet radius. The results are compared with predictions of QCD-inspired event generators, PYTHIA 8 with Monash 2013 tune and EPOS LHC. It is found that PYTHIA 8 qualitatively reproduces the jet modification in high-multiplicity events except at high jet pTp_{\rm T}. These measurements provide important constraints to models of jet fragmentation

    Measurement of the impact-parameter dependent azimuthal anisotropy in coherent ρ0 photoproduction in Pb–Pb collisions at √sNN = 5.02 TeV

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    The first measurement of the impact-parameter dependent angular anisotropy in the decay of coherently photoproduced ρ0 mesons is presented. The ρ0 mesons are reconstructed through their decay into a pion pair. The measured anisotropy corresponds to the amplitude of the cos(2ϕ) modulation, where ϕ is the angle between the two vectors formed by the sum and the difference of the transverse momenta of the pions, respectively. The measurement was performed by the ALICE Collaboration at the LHC using data from ultraperipheral Pb−Pb collisions at a center-of-mass energy of sNN−−−√ = 5.02 TeV per nucleon pair. Different impact-parameter regions are selected by classifying the events in nuclear-breakup classes. The amplitude of the cos(2ϕ) modulation is found to increase by about one order of magnitude from large to small impact parameters. Theoretical calculations, which describe the measurement, explain the cos(2ϕ) anisotropy as the result of a quantum interference effect at the femtometer scale that arises from the ambiguity as to which of the nuclei is the source of the photon in the interaction

    Investigating the nature of the K∗0(700) state with π±K0S correlations at the LHC

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    The first measurements of femtoscopic correlations with the particle pair combinations π±K0S in pp collisions at s√=13 TeV at the Large Hadron Collider (LHC) are reported by the ALICE experiment. Using the femtoscopic approach, it is shown that it is possible to study the elusive K∗0(700) particle that has been considered a tetraquark candidate for over forty years. Boson source parameters and final-state interaction parameters are extracted by fitting a model assuming a Gaussian source to the experimentally measured two-particle correlation functions. The final-state interaction is modeled through a resonant scattering amplitude, defined in terms of a mass and a coupling parameter, decaying into a π±K0S pair. The extracted mass and Breit-Wigner width, derived from the coupling parameter, of the final-state interaction are found to be consistent with previous measurements of the K∗0(700). The small value and increasing behavior of the correlation strength with increasing source size support the hypothesis that the K∗0(700) is a four-quark state, i.e. a tetraquark state. This latter trend is also confirmed via a simple geometric model that assumes a tetraquark structure of the K∗0(700) resonance

    Photoproduction of K+^{+}K^{-} pairs in ultra-peripheral collisions

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    International audienceK+^{+}K^{-} pairs may be produced in photonuclear collisions, either from the decays of photoproduced ϕ(1020)\phi (1020) mesons, or directly as non-resonant K+^{+}K^{-} pairs. Measurements of K+^{+}K^{-} photoproduction probe the couplings between the ϕ(1020)\phi (1020) and charged kaons with photons and nuclear targets. We present the first measurement of coherent photoproduction of K+^{+}K^{-} pairs on lead ions in ultra-peripheral collisions using the ALICE detector, including the first investigation of direct K+^{+}K^{-} production. There is significant K+^{+}K^{-} production at low transverse momentum, consistent with coherent photoproduction on lead targets. In the mass range 1.1<MKK<1.41.1 < M_{\rm{KK}} < 1.4 GeV/c2c^2 above the ϕ(1020)\phi (1020) resonance, for rapidity yKK<0.8|y_{\rm{KK}}|<0.8 and pT,KK<0.1p_{\rm T,KK} < 0.1 GeV/cc, the measured coherent photoproduction cross section is dσ/dy\mathrm{d}\sigma/\mathrm{d}y = 3.37 ± 0.61\pm\ 0.61 (stat.) ± 0.15\pm\ 0.15 (syst.) mb. The centre-of-mass energy per nucleon of the photon-nucleus (Pb) system WγPb,nW_{\gamma \mathrm{Pb, n}} ranges from 33 to 188 GeV, far higher than previous measurements on heavy-nucleus targets. The cross section is larger than expected for ϕ(1020)\phi (1020) photoproduction alone. The mass spectrum is fit to a cocktail consisting of ϕ(1020)\phi (1020) decays, direct K+^{+}K^{-} photoproduction, and interference between the two. The confidence regions for the amplitude and relative phase angle for direct K+^{+}K^{-} photoproduction are presented
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