We present a new measurement of $J/\psi$ production in Pb-Pb collisions at 158 GeV/nucleon, from the data sample collected in year 2000 by the NA50 Collaboration, under improved experimental ...conditions with respect to previous years. With the target system placed in vacuum, the setup was better adapted to study, in particular, the most peripheral nuclear collisions with unprecedented accuracy. The analysis of this data sample shows that the ($J/\psi$)/Drell-Yan cross-sections ratio measured in the most peripheral Pb-Pb interactions is in good agreement with the nuclear absorption pattern extrapolated from the studies of proton-nucleus collisions. Furthermore, this new measurement confirms our previous observation that the ($J/\psi$)/Drell-Yan cross-sections ratio departs from the normal nuclear absorption pattern for semi-central Pb-Pb collisions and that this ratio persistently decreases up to the most central collisions.
Recent pT-integrated cross-section measurements of the ground-state charm mesons and baryons, D0, D+, D+s, Λ+c, and Ξ0c, are used to evaluate the charm fragmentation fractions and production cross ...section per unit of rapidity at midrapidity (|y|<0.5), in pp collisions at √s=5.02 TeV at the LHC. The latter is dσc¯c/dy||y|<0.5=1165±44(stat)+134−101(syst) μb. These measurements were obtained for the first time in hadronic collisions at the LHC, including the charm baryon states, recently measured by ALICE at midrapidity. The charm fragmentation fractions differ significantly from the values measured in e+e− and ep collisions, providing evidence of the dependence of the parton-to-hadron fragmentation fractions on the collision system, indicating that the assumption of their universality is not supported by the measured cross sections. An increase of a factor of about 3.3 for the fragmentation fraction for the Λ+c with a significance of 5σ between the values obtained in pp collisions and those obtained in e+e− (ep) collisions is reported. The fragmentation fraction for the Ξ0c was obtained for the first time in any collision system. The measured fragmentation fractions were used to update the c¯c cross sections per unit of rapidity at |y|<0.5 at √s=2.76 and 7 TeV, which are about 40% higher than the previously published results. The data were compared with perturbative-QCD calculations and lie at the upper edge of the theoretical bands.
The first measurement at the LHC of charge-dependent directed flow (v1) relative to the spectator plane is presented for Pb-Pb collisions at √ s NN = 5.02 TeV . Results are reported for charged ...hadrons and D 0 mesons for the transverse momentum intervals p T > 0.2 GeV / c and 3 < p T < 6 GeV / c in the 5%–40% and 10%–40% centrality classes, respectively. The difference between the positively and negatively charged hadron v 1 has a positive slope as a function of pseudorapidity η , dΔv1/dη = 1.68 ± 0.49 ( stat ) ± 0.41 ( syst ) × 10−4 . The same measurement for D0 and ¯ D0 mesons yields a positive value d Δv1/dη = 4.9 ± 1.7 ( stat ) ± 0.6 ( syst ) × 10−1, which is about 3 orders of magnitude larger than the one of the charged hadrons. These measurements can provide new insights into the effects of the strong electromagnetic field and the initial tilt of matter created in noncentral 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 recent theoretical calculations, which predicted a negative and an order of magnitude smaller value of d Δv1/dη for both light flavor and charmed hadrons.
This Letter presents the first experimental evidence of the attractive strong interaction between a proton and a ϕ meson. The result is obtained from two-particle correlations of combined p − ϕ ⊕ ¯ ...p − ϕ pairs measured in high-multiplicity p p collisions at √ s = 13 TeV by the ALICE Collaboration. The spin-averaged scattering length and effective range of the p − ϕ interaction are extracted from the fully corrected correlation function employing the Lednický-Lyuboshits approach. In particular, the imaginary part of the scattering length vanishes within uncertainties, indicating that inelastic processes do not play a prominent role for the p − ϕ interaction. These data demonstrate that the interaction is dominated by elastic p − ϕ scattering. Furthermore, an analysis employing phenomenological Gaussian- and Yukawa-type potentials is conducted. Under the assumption of the latter, the N − ϕ coupling constant is found to be gN−ϕ = 0.14 ± 0.03 ( stat ) ± 0.02 ( syst ) . This work provides valuable experimental input to accomplish a self-consistent description of the N − ϕ interaction, which is particularly relevant for the more fundamental studies on partial restoration of chiral symmetry in nuclear medium.
The study of the strength and behavior of the antikaon-nucleon (Kover ¯N) interaction constitutes one of the key focuses of the strangeness sector in low-energy quantum chromodynamics (QCD). In this ...Letter a unique high-precision measurement of the strong interaction between kaons and protons, close and above the kinematic threshold, is presented. The femtoscopic measurements of the correlation function at low pair-frame relative momentum of (K^{+}p⊕K^{-}pover ¯) and (K^{-}p⊕K^{+}pover ¯) pairs measured in pp collisions at sqrts=5, 7, and 13 TeV are reported. A structure observed around a relative momentum of 58 MeV/c in the measured correlation function of (K^{-}p⊕K^{+}pover ¯) with a significance of 4.4σ constitutes the first experimental evidence for the opening of the (Kover ¯^{0}n⊕K^{0}nover ¯) isospin breaking channel due to the mass difference between charged and neutral kaons. The measured correlation functions have been compared to Jülich and Kyoto models in addition to the Coulomb potential. The high-precision data at low relative momenta presented in this work prove femtoscopy to be a powerful complementary tool to scattering experiments and provide new constraints above the Kover ¯N threshold for low-energy QCD chiral models.
This article presents the first measurement of the interaction between charm hadrons and nucleons. The two-particle momentum correlations of pD− and ¯pD+ pairs are measured by the ALICE Collaboration ...in high-multiplicity pp collisions at √s=13 TeV. The data are compatible with the Coulomb-only interaction hypothesis within (1.1–1.5)σ. The level of agreement slightly improves if an attractive nucleon (N)¯D strong interaction is considered, in contrast to most model predictions which suggest an overall repulsive interaction. This measurement allows for the first time an estimation of the 68% confidence level interval for the isospin I=0 inverse scattering length of the N¯D state f−10,I=0∈−0.4,0.9 fm−1, assuming negligible interaction for the isospin I=1 channel.
We study the polarization of inclusive J/ψ produced in Pb-Pb collisions at sqrts_{NN}=5.02 TeV at the LHC in the dimuon channel, via the measurement of the angular distribution of its decay ...products. We perform the study in the rapidity region 2.5<y<4, for three transverse momentum intervals (2<p_{T}<4, 4<p_{T}<6, 6<p_{T}<10 GeV/c) and as a function of the centrality of the collision for 2<p_{T}<6 GeV/c. For the first time, the polarization is measured with respect to the event plane of the collision, by considering the angle between the positive-charge decay muon in the J/ψ rest frame and the axis perpendicular to the event-plane vector in the laboratory system. A small transverse polarization is measured, with a significance reaching 3.9σ at low p_{T} and for intermediate centrality values. The polarization could be connected with the behavior of the quark-gluon plasma, formed in Pb-Pb collisions, as a rotating fluid with large vorticity, as well as with the existence of a strong magnetic field in the early stage of its formation.
The most precise measurements to date of the $^{3}_{Λ}$H lifetime τ and Λ separation energy BΛ are obtained using the data sample of Pb–Pb collisions at $\sqrt{s_{NN}}$ = 5.02 TeV collected by ALICE ...at the LHC. The $^{3}_{Λ}$H is reconstructed via its charged two-body mesonic decay channel ($^{3}_{Λ}$H → 3He + π– and the charge-conjugate process). The measured values τ = 253 ± 11 (stat.) ± 6(syst.) ps and BΛ = 102 ± 63 (stat.) ± 67 (syst.) keV are compatible with predictions from effective field theories and confirm that the $^{3}_{Λ}$H structure is consistent with a weakly-bound system.
The transverse-momentum (p_{T}) spectra and coalescence parameters B_{2} of (anti)deuterons are measured in p-p collisions at sqrts=13 TeV for the first time in and out of jets. In this measurement, ...the direction of the leading particle with the highest p_{T} in the event (p_{T}^{lead}>5 GeV/c) is used as an approximation for the jet axis. The event is consequently divided into three azimuthal regions, and the jet signal is obtained as the difference between the toward region, that contains jet fragmentation products in addition to the underlying event (UE), and the transverse region, which is dominated by the UE. The coalescence parameter in the jet is found to be approximately a factor of 10 larger than that in the underlying event. This experimental observation is consistent with the coalescence picture and can be attributed to the smaller average phase-space distance between nucleons in the jet cone as compared with the underlying event. The results presented in this Letter are compared to predictions from a simple nucleon coalescence model, where the phase-space distributions of nucleons are generated using pythia8 with the Monash 2013 tuning, and to predictions from a deuteron production model based on ordinary nuclear reactions with parametrized energy-dependent cross sections tuned on data. The latter model is implemented in pythia8.3. Both models reproduce the observed large difference between in-jet and out-of-jet coalescence parameters, although the almost flat trend of the B_{2}^{Jet} is not reproduced by the models, which instead give a decreasing trend.
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