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Institute
Measurement of ϒ(1S) elliptic flow at forward rapidity in Pb–Pb collisions at √sNN = 5.02 TeV
(2019)
The first measurement of the Υ(1S) elliptic flow coefficient (v2) is performed at forward rapidity (2.5 < y < 4) in Pb-Pb collisions at sNN−−−√=5.02 TeV with the ALICE detector at the LHC. The results are obtained with the scalar product method and are reported as a function of transverse momentum (pT) up to 15 GeV/c in the 5-60% centrality interval. The measured Υ(1S) v2 is consistent with zero and with the small positive values predicted by transport models within uncertainties. The v2 coefficient in 2 < pT < 15 GeV/c is lower than that of inclusive J/ψ mesons in the same pT interval by 2.6 standard deviations. These results, combined with earlier suppression measurements, are in agreement with a scenario in which the Υ(1S) production in Pb-Pb collisions at LHC energies is dominated by dissociation limited to the early stage of the collision whereas in the J/ψ case there is substantial experimental evidence of an additional regeneration component.
Measurement of ϒ(1S) elliptic flow at forward rapidity in Pb–Pb collisions at √sNN = 5.02 TeV
(2019)
The first measurement of the Υ(1S) elliptic flow coefficient (v2) is performed at forward rapidity (2.5 < y < 4) in Pb-Pb collisions at sNN−−−√=5.02 TeV with the ALICE detector at the LHC. The results are obtained with the scalar product method and are reported as a function of transverse momentum (pT) up to 15 GeV/c in the 5-60% centrality interval. The measured Υ(1S) v2 is consistent with zero and with the small positive values predicted by transport models within uncertainties. The v2 coefficient in 2 < pT < 15 GeV/c is lower than that of inclusive J/ψ mesons in the same pT interval by 2.6 standard deviations. These results, combined with earlier suppression measurements, are in agreement with a scenario in which the Υ(1S) production in Pb-Pb collisions at LHC energies is dominated by dissociation limited to the early stage of the collision whereas in the J/ψ case there is substantial experimental evidence of an additional regeneration component.
An improved value for the lifetime of the (anti-)hypertriton has been obtained using the data sample of Pb-Pb collisions at sNN−−−√= 5.02 TeV collected by the ALICE experiment at the LHC. The (anti-)hypertriton has been reconstructed via its charged two-body mesonic decay channel and the lifetime has been determined from an exponential fit to the dN/d(ct) spectrum. The measured value, τ = 242+34−38 (stat.) ± 17 (syst.) ps, is compatible with all the available theoretical predictions, thus contributing to the solution of the longstanding hypertriton lifetime puzzle.
An improved value for the lifetime of the (anti-)hypertriton has been obtained using the data sample of Pb-Pb collisions at sNN−−−√= 5.02 TeV collected by the ALICE experiment at the LHC. The (anti-)hypertriton has been reconstructed via its charged two-body mesonic decay channel and the lifetime has been determined from an exponential fit to the dN/d(ct) spectrum. The measured value, τ = 242+34−38 (stat.) ± 17 (syst.) ps, is compatible with all the available theoretical predictions, thus contributing to the solution of the longstanding hypertriton lifetime puzzle.
The production rates and the transverse momentum distribution of strange hadrons at mid-rapidity ( |y |<0.5) are measured in proton-proton collisions at s√ = 13 TeV as a function of the charged particle multiplicity, using the ALICE detector at the LHC. The production rates of K0S, Λ, Ξ, and Ω increase with the multiplicity faster than what is reported for inclusive charged particles. The increase is found to be more pronounced for hadrons with a larger strangeness content. Possible auto-correlations between the charged particles and the strange hadrons are evaluated by measuring the event-activity with charged particle multiplicity estimators covering different pseudorapidity regions. When comparing to lower energy results, the yields of strange hadrons are found to depend only on the mid-rapidity charged particle multiplicity. Several features of the data are reproduced qualitatively by general purpose QCD Monte Carlo models that take into account the effect of densely-packed QCD strings in high multiplicity collisions. However, none of the tested models reproduce the data quantitatively. This work corroborates and extends the ALICE findings on strangeness production in proton-proton collisions at 7 TeV.
The first measurement at the LHC of charge-dependent directed flow (v1) relative to the spectator plane is presented for Pb-Pb collisions at sNN−−−√ = 5.02 TeV. Results are reported for charged hadrons and D0 mesons for the transverse momentum intervals pT>0.2 GeV/c and 3<pT< 6 GeV/c in the 5-40% and 10-40% centrality classes, respectively. The difference between the positively and negatively charged hadron v1 is found to have a positive slope as a function of pseudorapidity η, dΔv1/dη=[1.68 ± 0.49 (stat.) ± 0.41 (syst.)] ×10−4, with a 2.6σ significance. The same measurement for D0 and D¯0 mesons yields a positive value dΔv1/dη= [4.9 ± 1.7 (stat.) ± 0.6 (syst.)]×10−1, which is about three orders of magnitude larger than the one of the charged hadrons, and is larger than zero with significance of 2.7σ. These measurements can provide new insights into the effects of the strong electromagnetic field and the initial tilt of matter created in non-central heavy-ion collisions on the dynamics of light (u, d, and s) and heavy (c) quarks. The large difference between the observed Δv1 of charged hadrons and D0 mesons may reflect different sensitivity of the charm and light quarks to the early time dynamics of a heavy-ion collision. These observations challenge some of the recent theoretical calculations incorporating effects of the strong electromagnetic field, which predicted a negative and an order of magnitude smaller value of dΔv1/dη for both light-flavour and charmed hadrons.
Υ production in p-Pb interactions is studied at the centre-of-mass energy per nucleon-nucleon collision sNN−−−√ = 8.16 TeV with the ALICE detector at the CERN LHC. The measurement is performed reconstructing bottomonium resonances via their dimuon decay channel, in the centre-of-mass rapidity intervals 2.03<ycms<3.53 and −4.46<ycms<−2.96, down to zero transverse momentum. In this work, results on the inclusive Υ(1S) production cross section as a function of rapidity and transverse momentum are presented. The corresponding nuclear modification factor shows a suppression of the Υ(1S) yields with respect to pp collisions, both at forward and backward rapidity. This suppression is stronger in the low transverse momentum region and shows no significant dependence on the centrality of the interactions. Furthermore, the Υ(2S) nuclear modification factor is also evaluated, suggesting a suppression similar to that of the Υ(1S). A first measurement of the Υ(3S) has also been performed. Finally, results are compared with previous measurements performed by ALICE in p-Pb collisions at sNN−−−√ = 5.02 TeV and with theoretical calculations.
Υ production in p-Pb interactions is studied at the centre-of-mass energy per nucleon-nucleon collision sNN−−−√ = 8.16 TeV with the ALICE detector at the CERN LHC. The measurement is performed reconstructing bottomonium resonances via their dimuon decay channel, in the centre-of-mass rapidity intervals 2.03<ycms<3.53 and −4.46<ycms<−2.96, down to zero transverse momentum. In this work, results on the inclusive Υ(1S) production cross section as a function of rapidity and transverse momentum are presented. The corresponding nuclear modification factor shows a suppression of the Υ(1S) yields with respect to pp collisions, both at forward and backward rapidity. This suppression is stronger in the low transverse momentum region and shows no significant dependence on the centrality of the interactions. Furthermore, the Υ(2S) nuclear modification factor is also evaluated, suggesting a suppression similar to that of the Υ(1S). A first measurement of the Υ(3S) has also been performed. Finally, results are compared with previous measurements performed by ALICE in p-Pb collisions at sNN−−−√ = 5.02 TeV and with theoretical calculations.
Inclusive Υ production in p-Pb interactions is studied at the centre-of-mass energy per nucleon-nucleon collision sNN−−−√ = 8.16 TeV with the ALICE detector at the CERN LHC. The measurement is performed reconstructing bottomonium resonances via their dimuon decay channel, in the centre-of-mass rapidity intervals 2.03<ycms<3.53 and −4.46<ycms<−2.96, down to zero transverse momentum. In this work, results on the inclusive Υ(1S) production cross section as a function of rapidity and transverse momentum are presented. The corresponding nuclear modification factor shows a suppression of the Υ(1S) yields with respect to pp collisions, both at forward and backward rapidity. This suppression is stronger in the low transverse momentum region and shows no significant dependence on the centrality of the interactions. The inclusive Υ(2S) nuclear modification factor is also evaluated, suggesting a suppression similar to that of the Υ(1S). Finally, results are compared with previous measurements performed by ALICE in p-Pb collisions at sNN−−−√ = 5.02 TeV and with theoretical calculations.
Measurements of identified hadrons as a function of the charged-particle multiplicity in pp collisions enable a search for the onset of collective effects in small collision systems. With such measurements, it is possible to study the mechanisms that determine the shapes of hadron transverse momentum (pT) spectra, to search for possible modifications of the yields of short-lived hadronic resonances due to scattering effects in the hadron-gas phase, and to investigate different explanations for the multiplicity evolution of strangeness production provided by phenomenological models. In this paper, these topics are addressed through measurements of the K∗(892)0 and ϕ(1020) mesons at midrapidity in pp collisions at s√ = 13 TeV as a function of the charged-particle multiplicity. The results include the pT spectra, pT-integrated yields, mean transverse momenta, and the ratios of the yields of these resonances to those of longer-lived hadrons. Comparisons with results from other collision systems and energies, as well as predictions from phenomenological models, are also discussed.
Production cross sections of muons from semi-leptonic decays of charm and beauty hadrons were measured at forward rapidity (2.5<y<4) in proton--proton (pp) collisions at a centre-of-mass energy s√=5.02 TeV with the ALICE detector at the CERN LHC. The results were obtained in an extended transverse momentum interval, 2<pT<20 GeV/c, and with an improved precision compared to previous measurements performed in the same rapidity interval at centre-of-mass energies s√=2.76 and 7 TeV. The pT- and y-differential production cross sections as well as the pT-differential production cross section ratios between different centre-of-mass energies and different rapidity intervals are described, within experimental and theoretical uncertainties, by predictions based on perturbative QCD.
Production cross sections of muons from semi-leptonic decays of charm and beauty hadrons were measured at forward rapidity (2.5<y<4) in proton--proton (pp) collisions at a centre-of-mass energy s√=5.02 TeV with the ALICE detector at the CERN LHC. The results were obtained in an extended transverse momentum interval, 2<pT<20 GeV/c, and with an improved precision compared to previous measurements performed in the same rapidity interval at centre-of-mass energies s√=2.76 and 7 TeV. The pT- and y-differential production cross sections as well as the pT-differential production cross section ratios between different centre-of-mass energies and different rapidity intervals are described, within experimental and theoretical uncertainties, by predictions based on perturbative QCD.
The jet radial structure and particle transverse momentum (pT) composition within jets are presented in centrality-selected Pb-Pb collisions at sNN−−−√ = 2.76 TeV. Track-based jets, which are also called charged jets, were reconstructed with a resolution parameter of R=0.3 at midrapidity |ηchjet|<0.6 for transverse momenta pT,chjet=30-120 GeV/c. Jet-hadron correlations in relative azimuth and pseudorapidity space (Δφ, Δη) are measured to study the distribution of the associated particles around the jet axis for different pT,assoc-ranges between 1 and 20 GeV/c. The data in Pb-Pb collisions are compared to reference distributions for pp collisions, obtained using embedded PYTHIA simulations. The number of high-pT associate particles (4<pT,assoc<20 GeV/c) in Pb-Pb collisions is found to be suppressed compared to the reference by 30 to 10%, depending on centrality. The radial particle distribution relative to the jet axis shows a moderate modification in Pb-Pb collisions with respect to PYTHIA. High-pT associate particles are slightly more collimated in Pb-Pb collisions compared to the reference, while low-pT associate particle tend to be broadened. The results, which are presented for the first time down to pT,chjet=30 GeV/c in Pb-Pb collisions, are compatible with both previous jet-hadron-related measurements from the CMS Collaboration and jet shape measurements from the ALICE Collaboration at higher pT, and add further support for the established picture of in-medium parton energy loss.
The jet radial structure and particle transverse momentum (pT) composition within jets are presented in centrality-selected Pb−Pb collisions at sNN−−−√ = 2.76 TeV. Track-based jets, which are also called charged jets, were reconstructed with a resolution parameter of R=0.3 at midrapidity |ηchjet|<0.6 for transverse momenta pT,chjet=30-120 GeV/c. Jet−hadron correlations in relative azimuth and pseudorapidity space (Δφ, Δη) are measured to study the distribution of the associated particles around the jet axis for different pT,assoc-ranges between 1 and 20 GeV/c. The data in Pb−Pb collisions are compared to reference distributions for pp collisions, obtained using embedded PYTHIA simulations. The number of high-pT associate particles (4<pT,assoc<20 GeV/c) in Pb−Pb collisions is found to be suppressed compared to the reference by 30 to 10\%, depending on centrality. The radial particle distribution relative to the jet axis shows a moderate modification in Pb−Pb collisions with respect to PYTHIA. High-pT associate particles are slightly more collimated in Pb−Pb collisions compared to the reference, while low-pT associate particle tend to be broadened. The results, which are compatible with both previous jet−hadron-related measurements from the CMS Collaboration and jet shape measurements from the ALICE Collaboration at higher pT, support the currently established picture of in-medium parton energy loss.
This work presents the first experimental observation of the attractive strong interaction between a proton and a multi-strange baryon (hyperon) Ξ−. The result is extracted from two-particle correlations of combined p−Ξ−⊕p¯¯¯−Ξ¯¯¯¯+ pairs measured in p-Pb collisions at sNN−−−√=5.02 TeV at the LHC with ALICE. The measured correlation function is compared with the prediction obtained assuming only an attractive Coulomb interaction and a standard deviation in the range [3.6,5.3] is found. Since the measured p−Ξ−⊕p¯¯¯−Ξ¯¯¯¯+ correlation is significantly enhanced with respect to the Coulomb prediction, the presence of an additional, strong, attractive interaction is evident. The data are compatible with recent lattice calculations by the HAL-QCD Collaboration, with a standard deviation in the range [1.8,3.7]. The lattice potential predicts a shallow repulsive Ξ− interaction within pure neutron matter at saturation densities and this implies stiffer equations of state for neutron-rich matter including hyperons. Implications of the strong interaction for the modeling of neutron stars are discussed.
This Letter presents the first experimental observation of the attractive strong interaction between a proton and a multistrange baryon (hyperon) Ξ−. The result is extracted from two-particle correlations of combined p−Ξ−⊕¯p−¯Ξ+ pairs measured in p−Pb collisions at √sNN=5.02 TeV at the LHC with ALICE. The measured correlation function is compared with the prediction obtained assuming only an attractive Coulomb interaction and a standard deviation in the range [3.6, 5.3] is found. Since the measured p−Ξ−⊕¯p−¯Ξ+ correlation is significantly enhanced with respect to the Coulomb prediction, the presence of an additional, strong, attractive interaction is evident. The data are compatible with recent lattice calculations by the HAL-QCD Collaboration, with a standard deviation in the range [1.8, 3.7]. The lattice potential predicts a shallow repulsive Ξ− interaction within pure neutron matter and this implies stiffer equations of state for neutron-rich matter including hyperons. Implications of the strong interaction for the modeling of neutron stars are discussed.
This work presents the first experimental observation of the attractive strong interaction between a proton and a multi-strange baryon (hyperon) Ξ−. The result is extracted from two-particle correlations of combined p−Ξ−⊕p¯−Ξ¯+ pairs measured in p-Pb collisions at sNN−−−√=5.02 TeV at the LHC with ALICE. The measured correlation function is compared with the prediction obtained assuming only an attractive Coulomb interaction and a standard deviation in the range [3.6,5.3] is found. Since the measured p−Ξ−⊕p¯−Ξ¯+ correlation is significantly enhanced with respect to the Coulomb prediction, the presence of an additional, strong, attractive interaction is evident. The data are compatible with recent lattice calculations by the HAL-QCD Collaboration, with a standard deviation in the range [1.8,3.7]. The lattice potential predicts a shallow repulsive Ξ− interaction within pure neutron matter at saturation densities and this implies stiffer equations of state for neutron-rich matter including hyperons. Implications of the strong interaction for the modeling of neutron stars are discussed.
Measurement of the production of charm jets tagged with D0 mesons in pp collisions at √s = 7 TeV
(2019)
The production of charm jets in proton-proton collisions at a center-of-mass energy of s√=7 TeV was measured with the ALICE detector at the CERN Large Hadron Collider. The measurement is based on a data sample corresponding to a total integrated luminosity of 6.23 nb−1, collected using a minimum-bias trigger. Charm jets are identified by the presence of a D0 meson among their constituents. The D0 mesons are reconstructed from their hadronic decay D0→K−π+. The D0-meson tagged jets are reconstructed using tracks of charged particles (track-based jets) with the anti-kT algorithm in the jet transverse momentum range 5<pchT,jet<30 GeV/c and pseudorapidity |ηjet|<0.5. The fraction of charged jets containing a D0-meson increases with pchT,jet from 0.042±0.004(stat)±0.006(syst) to 0.080±0.009(stat)±0.008(syst). The distribution of D0-meson tagged jets as a function of the jet momentum fraction carried by the D0 meson in the direction of the jet axis (zch||) is reported for two ranges of jet transverse momenta, 5<pchT,jet<15 GeV/c and 15<pchT,jet<30 GeV/c in the intervals 0.2<zch||<1.0 and 0.4<zch||<1.0, respectively. The data are compared with results from Monte Carlo event generators (PYTHIA 6, PYTHIA 8 and Herwig 7) and with a Next-to-Leading-Order perturbative Quantum Chromodynamics calculation, obtained with the POWHEG method and interfaced with PYTHIA 6 for the generation of the parton shower, fragmentation, hadronisation and underlying event.
Measurement of the production of charm jets tagged with D0 mesons in pp collisions at √s = 7 TeV
(2019)
The production of charm jets in proton-proton collisions at a center-of-mass energy of s√=7 TeV was measured with the ALICE detector at the CERN Large Hadron Collider. The measurement is based on a data sample corresponding to a total integrated luminosity of 6.23 nb−1, collected using a minimum-bias trigger. Charm jets are identified by the presence of a D0 meson among their constituents. The D0 mesons are reconstructed from their hadronic decay D0→K−π+. The D0-meson tagged jets are reconstructed using tracks of charged particles (track-based jets) with the anti-kT algorithm in the jet transverse momentum range 5<pchT,jet<30 GeV/c and pseudorapidity |ηjet|<0.5. The fraction of charged jets containing a D0-meson increases with pchT,jet from 0.042±0.004(stat)±0.006(syst) to 0.080±0.009(stat)±0.008(syst). The distribution of D0-meson tagged jets as a function of the jet momentum fraction carried by the D0 meson in the direction of the jet axis (zch||) is reported for two ranges of jet transverse momenta, 5<pchT,jet<15 GeV/c and 15<pchT,jet<30 GeV/c in the intervals 0.2<zch||<1.0 and 0.4<zch||<1.0, respectively. The data are compared with results from Monte Carlo event generators (PYTHIA 6, PYTHIA 8 and Herwig 7) and with a Next-to-Leading-Order perturbative Quantum Chromodynamics calculation, obtained with the POWHEG method and interfaced with PYTHIA 6 for the generation of the parton shower, fragmentation, hadronisation and underlying event.
Study of the Λ–Λ interaction with femtoscopy correlations in pp and p–Pb collisions at the LHC
(2019)
This work presents new constraints on the existence and the binding energy of a possible Λ-Λ bound state, the H-dibaryon, derived from Λ-Λ femtoscopic measurements by the ALICE collaboration. The results are obtained from a new measurement using the femtoscopy technique in pp collisions at s√=13 TeV and p-Pb collisions at sNN−−−√=5.02 TeV, combined with previously published results from p-Pb collisions at s√=7 TeV. The Λ-Λ scattering parameter space, spanned by the inverse scattering length f−10 and the effective range d0, is constrained by comparing the measured Λ-Λ correlation function with calculations obtained within the Lednicky model. The data are compatible with hypernuclei results and lattice computations, both predicting a shallow attractive interaction, and permit to test different theoretical approaches describing the Λ-Λ interaction. The region in the (f−10,d0) plane which would accommodate a Λ-Λ bound state is substantially restricted compared to previous studies. The binding energy of the possible Λ-Λ bound state is estimated within an effective-range expansion approach and is found to be BΛΛ=3.2+1.6−2.4(stat)+1.8−1.0(syst) MeV.