Search for: All records

The first measurement of the cross section for incoherent photonuclear production of $J/\psi$vector mesons as a function of the Mandelstam $\left|t\right|$variable is presented. The measurement was carried out with the ALICE detector at midrapidity, $\left|y\right|<0.8$, using ultraperipheral collisions of Pb nuclei at a center-of-mass energy per nucleon pair of $\sqrt{s_{\mathrm{NN}}}=5.02\mathrm{TeV}$. This rapidity interval corresponds to a Bjorken- $x$range $\left(0.3–1.4\right)\times {10}^{-3}$. Cross sections are given in five $\left|t\right|$intervals in the range $0.04<\left|t\right|<1{\mathrm{GeV}}^2$and compared to the predictions by different models. Models that ignore quantum fluctuations of the gluon density in the colliding hadron predict a $\left|t\right|$dependence of the cross section much steeper than in data. The inclusion of such fluctuations in the same models provides a better description of the data. © 2024 CERN, for the ALICE Collaboration2024CERN 

A<sc>bstract</sc> 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$$ \sqrt{s_{\textrm{NN}}} $$ $\sqrt{s_{\mathrm{NN}}}$= 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,v₂(η), is extracted from the long-range correlations. Thev₂(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, thev₂(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. 

A<sc>bstract</sc> Measurements of the production cross sections of prompt D⁰, D⁺, D^*+,$$ {\textrm{D}}_{\textrm{s}}^{+} $$ $D_s^{+}$,$$ {\Lambda}_{\textrm{c}}^{+} $$ ${\Lambda }_c^{+}$, and$$ {\Xi}_{\textrm{c}}^{+} $$ ${\Xi }_c^{+}$charm hadrons at midrapidity in proton-proton collisions at$$ \sqrt{s} $$ $\sqrt{s}$= 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 ofp_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$$ {\Lambda}_{\textrm{c}}^{+} $$ ${\Lambda }_c^{+}$($$ {\Xi}_{\textrm{c}}^{+} $$ ${\Xi }_c^{+}$) baryon production extend the measuredp_Tintervals down top_T= 0(3) GeV/c. These measurements are used to determine the charm-quark fragmentation fractions and the$$ \textrm{c}\overline{\textrm{c}} $$ $c\overline{c}$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⁺,$$ {\textrm{D}}_{\textrm{s}}^{+} $$ $D_s^{+}$,$$ {\Lambda}_{\textrm{c}}^{+} $$ ${\Lambda }_c^{+}$,$$ {\Xi}_{\textrm{c}}^0 $$ ${\Xi }_c^0$and, for the first time,$$ {\Xi}_{\textrm{c}}^{+} $$ ${\Xi }_c^{+}$, and of the strongly-decaying J/ψmesons. The first measurements of$$ {\Xi}_{\textrm{c}}^{+} $$ ${\Xi }_c^{+}$and$$ {\Sigma}_{\textrm{c}}^{0,++} $$ ${\Sigma }_c^{0,++}$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$$ \textrm{c}\overline{\textrm{c}} $$ $c\overline{c}$production cross section at midrapidity is found to be at the upper bound of state-of-the-art perturbative QCD calculations. 

Abstract The elliptic flow$$(v_2)$$ $\left(v_2\right)$of$${\textrm{D}}^{0}$$ ${\text{D}}^0$mesons from beauty-hadron decays (non-prompt$${\textrm{D}}^{0})$$ ${\text{D}}^0)$was measured in midcentral (30–50%) Pb–Pb collisions at a centre-of-mass energy per nucleon pair$$\sqrt{s_{\textrm{NN}}} = 5.02$$ $\sqrt{s_{\text{NN}}}=5.02$ TeV with the ALICE detector at the LHC. The$${\textrm{D}}^{0}$$ ${\text{D}}^0$mesons were reconstructed at midrapidity$$(|y|<0.8)$$ $\left(\right|y|<0.8)$from their hadronic decay$$\mathrm {D^0 \rightarrow K^-\uppi ^+}$$ $D^0\to K^{-}{\pi }^{+}$, in the transverse momentum interval$$2< p_{\textrm{T}} < 12$$ $2<p_{\text{T}}<12$ GeV/c. The result indicates a positive$$v_2$$ $v_2$for non-prompt$${{\textrm{D}}^{0}}$$ ${\text{D}}^0$mesons with a significance of 2.7$$\sigma $$ $\sigma$. The non-prompt$${{\textrm{D}}^{0}}$$ ${\text{D}}^0$-meson$$v_2$$ $v_2$is lower than that of prompt non-strange D mesons with 3.2$$\sigma $$ $\sigma$significance in$$2< p_\textrm{T} < 8~\textrm{GeV}/c$$ $2<p_{\text{T}}<8\text{GeV}/c$, and compatible with the$$v_2$$ $v_2$of beauty-decay electrons. Theoretical calculations of beauty-quark transport in a hydrodynamically expanding medium describe the measurement within uncertainties. 

A<sc>bstract</sc> The Chiral Magnetic Wave (CMW) phenomenon is essential to provide insights into the strong interaction in QCD, the properties of the quark-gluon plasma, and the topological characteristics of the early universe, offering a deeper understanding of fundamental physics in high-energy collisions. Measurements of the charge-dependent anisotropic flow coefficients are studied in Pb-Pb collisions at center-of-mass energy per nucleon-nucleon collision$$ \sqrt{s_{\textrm{NN}}} $$ $\sqrt{s_{\mathrm{NN}}}$= 5.02 TeV to probe the CMW. In particular, the slope of the normalized difference in elliptic (v₂) and triangular (v₃) flow coefficients of positively and negatively charged particles as a function of their event-wise normalized number difference, is reported for inclusive and identified particles. The slope$$ {r}_3^{\textrm{Norm}} $$ $r_3^{\text{Norm}}$is found to be larger than zero and to have a magnitude similar to$$ {r}_2^{\textrm{Norm}} $$ $r_2^{\text{Norm}}$, thus pointing to a large background contribution for these measurements. Furthermore,$$ {r}_2^{\textrm{Norm}} $$ $r_2^{\text{Norm}}$can be described by a blast wave model calculation that incorporates local charge conservation. In addition, using the event shape engineering technique yields a fraction of CMW (f_CMW) contribution to this measurement which is compatible with zero. This measurement provides the very first upper limit forf_CMW, and in the 10–60% centrality interval it is found to be 26% (38%) at 95% (99.7%) confidence level. 

A<sc>bstract</sc> The cross section for coherent photonuclear production of J/ψis 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$$ \sqrt{{\textrm{s}}_{\textrm{NN}}} $$ $\sqrt{s_{\mathrm{NN}}}$= 5.02 TeV. Cross sections are presented in five different J/ψrapidity ranges within |y| < 4, with the J/ψreconstructed via its dilepton decay channels. In some events the J/ψis 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_γPb,n< 920 GeV, whereW_γPb,nis the centre-of-mass energy per nucleon of theγPb system. This range corresponds to a Bjorken-xinterval 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.