Measurements of J/ψ→e+e−\mathrm{J/\psi \rightarrow e^{+}e^{-}} with ALICE at the LHC

The ALICE detector provides excellent capabilities to study quarkonium production at the Large Hadron Collider (LHC). Heavy quarkonia, bound states of charm or beauty quark anti-quark pairs such as the $\mathrm{J/\psi}$, are expected to be produced by initial hard processes. Thus they will provide insight into the earliest and hottest stages of AA collisions where the formation of a Quark-Gluon Plasma (QGP) is expected. Furthermore, high-precision data from pp collisions represent an essential baseline for the measurement of nuclear modifications in heavy-ions and serve also as a crucial test for several models of quarkonium hadroproduction. In addition, the study of pA collisions allows to investigate nuclear modifications due to Cold Nuclear Matter (CNM) effects. In ALICE, $\mathrm{J/\psi}$ were measured in pp and Pb--Pb collisions down to $p_{\mathrm T}$ = 0 via their di-electron decay channel in the central barrel ($|y| <$ 0.8). Results on the nuclear modification factor ($R_{\mathrm{AA}}$) at central rapidities in Pb--Pb collisions at $\sqrt{s_{\mathrm{NN}}} = 2.76$ TeV will be shown and their implications discussed. A separation of the prompt and non-prompt components is also possible down to $p_{\mathrm T}$ of the $\mathrm{J/\psi}$ of 2 GeV/$c$

Closing in on critical net-baryon fluctuations at LHC energies: Cumulants up to third order in Pb-Pb collisions

Fluctuation measurements are important sources of information on the mechanism of particle production at LHC energies. This article reports the first experimental results on third-order cumulants of the net-proton distributions in Pb–Pb collisions at a center-of-mass energy sNN=5.02 TeV recorded by the ALICE detector. The results on the second-order cumulants of net-proton distributions at sNN=2.76 and 5.02 TeV are also discussed in view of effects due to the global and local baryon number conservation. The results demonstrate the presence of long-range rapidity correlations between protons and antiprotons. Such correlations originate from the early phase of the collision. The experimental results are compared with HIJING and EPOS model calculations, and the dependence of the fluctuation measurements on the phase-space coverage is examined in the context of lattice quantum chromodynamics (LQCD) and hadron resonance gas (HRG) model estimations. The measured third-order cumulants are consistent with zero within experimental uncertainties of about 4% and are described well by LQCD and HRG predictions

Energy dependence of coherent photonuclear production of J/ψ mesons in ultra-peripheral Pb-Pb collisions at √sNN = 5.02 TeV

The cross section for coherent photonuclear production of J/ψ is 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 sNN = 5.02 TeV. Cross sections are presented in five different J/ψ rapidity ranges within |y| &lt; 4, with the J/ψ reconstructed via its dilepton decay channels. In some events the J/ψ is 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 &lt; Wγ Pb,n &lt; 920 GeV, where Wγ Pb,n is the centre-of-mass energy per nucleon of the γPb system. This range corresponds to a Bjorken-x interval spanning about three orders of magnitude: 1.1 × 10 −5 &lt; x &lt; 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

Measurements of Groomed-Jet Substructure of Charm Jets Tagged by D0 Mesons in Proton-Proton Collisions at √s=13 TeV

Understanding the role of parton mass and Casimir color factors in the quantum chromodynamics parton shower represents an important step in characterizing the emission properties of heavy quarks. Recent experimental advances in jet substructure techniques have provided the opportunity to isolate and characterize gluon emissions from heavy quarks. In this Letter, the first direct experimental constraint on the charm-quark splitting function is presented, obtained via the measurement of the groomed shared momentum fraction of the first splitting in charm jets, tagged by a reconstructed D0 meson. The measurement is made in proton-proton collisions at s=13 TeV, in the low jet transverse-momentum interval of 15≤pTjet ch&lt;30 GeV/c where the emission properties are sensitive to parton mass effects. In addition, the opening angle of the first perturbative emission of the charm quark, as well as the number of perturbative emissions it undergoes, is reported. Comparisons to measurements of an inclusive-jet sample show a steeper splitting function for charm quarks compared with gluons and light quarks. Charm quarks also undergo fewer perturbative emissions in the parton shower, with a reduced probability of large-angle emissions

Multiplicity and rapidity dependence of K∗(892)0 and φ(1020) production in p–Pb collisions at √sNN = 5.02 TeV

The transverse-momentum (pT) spectra of K ∗(892)0 and φ(1020) measured with the ALICE detector up to pT = 16 GeV/c in the rapidity range - 1.2 &lt; y&lt; 0.3 , in p–Pb collisions at the center-of-mass energy per nucleon–nucleon collision sNN=5.02 TeV are presented as a function of charged particle multiplicity and rapidity. The measured pT distributions show a dependence on both multiplicity and rapidity at low pT whereas no significant dependence is observed at high pT . A rapidity dependence is observed in the pT -integrated yield (dN/dy), whereas the mean transverse momentum (⟨ pT⟩) shows a flat behavior as a function of rapidity. The rapidity asymmetry (Yasym) at low pT (&lt; 5 GeV/c) is more significant for higher multiplicity classes. At high pT , no significant rapidity asymmetry is observed in any of the multiplicity classes. Both K ∗(892)0 and φ(1020) show similar Yasym . The nuclear modification factor (QCP) as a function of pT shows a Cronin-like enhancement at intermediate pT , which is more prominent at higher rapidities (Pb-going direction) and in higher multiplicity classes. At high pT (&gt; 5 GeV/ c), the QCP values are greater than unity and no significant rapidity dependence is observed

Study of the p-p-K+ and p-p-K- dynamics using the femtoscopy technique

The interactions of kaons (K) and antikaons (K ̄) with few nucleons (N) were studied so far using kaonic atom data and measurements of kaon production and interaction yields in nuclei. Some details of the three-body KNN and K ̄ NN dynamics are still not well understood, mainly due to the overlap with multi-nucleon interactions in nuclei. An alternative method to probe the dynamics of three-body systems with kaons is to study the final state interaction within triplet of particles emitted in pp collisions at the Large Hadron Collider, which are free from effects due to the presence of bound nucleons. This Letter reports the first femtoscopic study of p–p–K + and p–p–K - correlations measured in high-multiplicity pp collisions at s = 13 TeV by the ALICE Collaboration. The analysis shows that the measured p–p–K + and p–p–K - correlation functions can be interpreted in terms of pairwise interactions in the triplets, indicating that the dynamics of such systems is dominated by the two-body interactions without significant contributions from three-body effects or bound states

Accessing the strong interaction between baryons and charged kaons with the femtoscopy technique at the LHC

The interaction between Λ baryons and kaons/antikaons is a crucial ingredient for the strangeness S=0 and S=−2 sector of the meson–baryon interaction at low energies. In particular, the ΛK ̅ might help in understanding the origin of states such as the Ξ(1620), whose nature and properties are still under debate. Experimental data on Λ–K and Λ–K ̅ systems are scarce, leading to large uncertainties and tension between the available theoretical predictions constrained by such data. In this Letter we present the measurements of Λ–K⊕+Λ ̅–K− and Λ–K⊕−Λ ̅–K+ correlations obtained in the high-multiplicity triggered data sample in pp collisions at s=13 TeV recorded by ALICE at the LHC. The correlation function for both pairs is modeled using the Lednický–Lyuboshits analytical formula and the corresponding scattering parameters are extracted. The Λ–K⊕−Λ ̅–K+ correlations show the presence of several structures at relative momenta k⁎ above 200 MeV/c, compatible with the Ω baryon, the Ξ(1690), and Ξ(1820) resonances decaying into Λ–K− pairs. The low k⁎ region in the Λ–K⊕−Λ ̅–K+ also exhibits the presence of the Ξ(1620) state, expected to strongly couple to the measured pair. The presented data allow to access the ΛK+ and ΛK− strong interaction with an unprecedented precision and deliver the first experimental observation of the Ξ(1620) decaying into ΛK−

Data-driven precision determination of the material budget in ALICE

The knowledge of the material budget with a high precision is fundamental for measurements of direct photon production using the photon conversion method due to its direct impact on the total systematic uncertainty. Moreover, it influences many aspects of the charged-particle reconstruction performance. In this article, two procedures to determine data-driven corrections to the material-budget description in ALICE simulation software are developed. One is based on the precise knowledge of the gas composition in the Time Projection Chamber. The other is based on the robustness of the ratio between the produced number of photons and charged particles, to a large extent due to the approximate isospin symmetry in the number of produced neutral and charged pions. Both methods are applied to ALICE data allowing for a reduction of the overall material budget systematic uncertainty from 4.5% down to 2.5%. Using these methods, a locally correct material budget is also achieved. The two proposed methods are generic and can be applied to any experiment in a similar fashion

Measurement of beauty-strange meson production in Pb-Pb collisions at √sNN=5.02 TeV via non-prompt Ds+ mesons

The production yields of non-prompt Ds+ mesons, namely Ds+ mesons from beauty-hadron decays, were measured for the first time as a function of the transverse momentum (pT) at midrapidity (|y|&lt;0.5) in central and semi-central Pb–Pb collisions at a centre-of-mass energy per nucleon pair sNN=5.02TeV with the ALICE experiment at the LHC. The Ds+ mesons and their charge conjugates were reconstructed from the hadronic decay channel Ds+→φπ+, with φ→K−K+, in the 4&lt;36GeV/c and 2&lt;24GeV/c intervals for the 0–10% and 30–50% centrality classes, respectively. The measured yields of non-prompt Ds+ mesons are compared to those of prompt Ds+ and non-prompt D0 mesons by calculating the ratios of the production yields in Pb–Pb collisions and the nuclear modification factor RAA. The ratio between the RAA of non-prompt Ds+ and prompt Ds+ mesons, and that between the RAA of non-prompt Ds+ and non-prompt D0 mesons in central Pb–Pb collisions are found to be on average higher than unity in the 4&lt;12GeV/c interval with a statistical significance of about 1.6σ and 1.7σ, respectively. The measured RAA ratios are compared with the predictions of theoretical models of heavy-quark transport in a hydrodynamically expanding QGP that incorporate hadronisation via quark recombination

Probing the chiral magnetic wave with charge-dependent fow measurements in Pb-Pb collisions at the LHC

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 sNN = 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 r3Norm is found to be larger than zero and to have a magnitude similar to r2Norm, thus pointing to a large background contribution for these measurements. Furthermore, r2Norm 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 (fCMW) contribution to this measurement which is compatible with zero. This measurement provides the very first upper limit for fCMW, and in the 10–60% centrality interval it is found to be 26% (38%) at 95% (99.7%) confidence level