Search for: All records

This paper presents the first measurement of the angle between different jet axes (denoted as $\mathrm{\Delta }{R}_{\mathrm{axis}}$) in $\text{Pb-Pb}$collisions. The measurement is carried out in the 0–10 % most-central events at $\sqrt{s_{NN}}=5.02$TeV. Jets are assembled by clustering charged particles at midrapidity using the $\text{anti-}{k}_{\mathrm{T}}$algorithm with resolution parameters $R=0.2$and 0.4 and transverse momenta in the intervals $40<p_{\mathrm{T}}^{\mathrm{ch}\mathrm{jet}}<140\mathrm{GeV}/c$and $80<p_{\mathrm{T}}^{\mathrm{ch}\mathrm{jet}}<140\mathrm{GeV}/c$, respectively. Measurements at these low transverse momenta enhance the sensitivity to quark-gluon plasma (QGP) effects. A comparison to models implementing various mechanisms of jet energy loss in the QGP suggests that the observed narrowing of the $\text{Pb-Pb}$distribution relative to $\mathit{pp}$can be explained if quark-initiated jets are more likely to emerge from the medium than gluon-initiated jets. These new measurements disfavor intrajet $p_{\mathrm{T}}$broadening as implemented in a simple model calculation with the Baier-Dokshitzer-Mueller-Peigne-Schiff formalism for energy loss in the QGP. The comparison of $\text{Pb-Pb}$and $\mathit{pp}$collisions shows sensitivity to the angular scale at which the QGP can resolve two independent splittings, favoring mechanisms that incorporate incoherent energy loss. 

The first measurement of the $e^{+}{e}^{-}$pair production at midrapidity and low invariant mass in central Pb-Pb collisions at $\sqrt{s_{NN}}=5.02\mathrm{TeV}$at the Large Hadron Collider is presented. The yield of $e^{+}{e}^{-}$pairs is compared with a cocktail of expected hadronic decay contributions in the invariant mass ( $m_{ee}$) and pair transverse momentum ( $p_{\mathrm{T},ee}$) ranges $m_{ee}<3.5\mathrm{GeV}/c^2$and $p_{\mathrm{T},ee}<8\mathrm{GeV}/c$. For $0.18<m_{ee}<0.5\mathrm{GeV}/c^2$the ratio of data to the cocktail of hadronic contributions amounts to $1.40\pm 0.11\left(\mathrm{stat}.\right)\pm 0.23\left(\mathrm{syst}.\right)\pm 0.16\left(\mathrm{cocktail}\right)$and $1.42\pm 0.11\left(\mathrm{stat}.\right)\pm 0.23{\left(\mathrm{syst}.\right)}_{-0.29}^{+0.24}\left(\mathrm{cocktail}\right)$, including or not including medium effects in the estimation of the heavy-flavor background, respectively. It is consistent with predictions from two different models for an additional contribution of thermal $e^{+}{e}^{-}$pairs from the hadronic and partonic phases. In the intermediate-mass range ( $1.2<m_{ee}<2.6\mathrm{GeV}/c^2$), the pair transverse impact parameter of the $e^{+}{e}^{-}$pairs ( ${\mathrm{DCA}}_{ee}$, where “DCA” denotes “distance of closest approach”) is used for the first time in Pb-Pb collisions to separate displaced dielectrons from heavy-flavor hadron decays from a possible (thermal) contribution produced at the interaction point. The data are consistent with a suppression of $e^{+}{e}^{-}$pairs from $c\overline{c}$and an additional prompt component. Finally, the first direct-photon measurement in the 10% most central Pb-Pb collisions at $\sqrt{s_{NN}}=5.02\mathrm{TeV}$is reported via the study of virtual direct photons in the transverse momentum range $1<p_{\mathrm{T}}<5\mathrm{GeV}/c$. A model including prompt photons, as well as photons from the preequilibrium and fluid-dynamic phases, can reproduce the result, while being at the upper edge of the data uncertainties. 

The ALICE Collaboration reports measurements of the large relative transverse momentum ( $k_T$) component of jet substructure in $pp$and Pb-Pb collisions at center-of-mass energy per nucleon pair $\sqrt{s_{\mathrm{NN}}}=5.02\mathrm{TeV}$. Enhancement in the yield of such large- $k_{\mathrm{T}}$emissions in head-on Pb-Pb collisions is predicted to arise from partonic scattering with quasiparticles of the quark-gluon plasma. The analysis utilizes charged-particle jets reconstructed by the anti- $k_{\mathrm{T}}$algorithm with resolution parameter $R=0.2$in the transverse-momentum interval $60<p_{\mathrm{T},\mathrm{ch},\mathrm{jet}}<80\mathrm{GeV}/c$. The soft drop and dynamical grooming algorithms are used to identify high transverse momentum splittings in the jet shower. Comparison of measurements in Pb-Pb and $pp$collisions shows medium-induced narrowing, corresponding to yield suppression of high- $k_T$splittings, in contrast to the expectation of yield enhancement due to quasiparticle scattering. The measurements are compared to theoretical model calculations incorporating jet modification due to jet-medium interactions (“jet quenching”), both with and without quasiparticle scattering effects. These measurements provide new insight into the underlying mechanisms and theoretical modeling of jet quenching. 

A<sc>bstract</sc> We report on the measurement of inclusive, non-prompt, and prompt J/ψ-hadron correlations by the ALICE Collaboration at the CERN Large Hadron Collider in pp collisions at a center-of-mass energy of 13 TeV. The correlations are studied at midrapidity (|y| <0.9) in the transverse momentum rangesp_T<40 GeV/cfor the J/ψand 0.15< p_T<10 GeV/cand |η|<0.9 for the associated hadrons. The measurement is based on minimum bias and high multiplicity data samples corresponding to integrated luminosities ofL_int= 34 nb⁻¹andL_int= 6.9 pb⁻¹, respectively. In addition, two more data samples are employed, requiring, on top of the minimum bias condition, a threshold on the tower energy ofE= 4 and 9 GeV in the ALICE electromagnetic calorimeters, which correspond to integrated luminosities ofL_int= 0.9 pb⁻¹andL_int= 8.4 pb⁻¹, respectively. The azimuthally integrated near and away side yields of associated charged hadrons per J/ψtrigger are presented as a function of the J/ψand associated hadron transverse momentum. The measurements are discussed in comparison to PYTHIA calculations. 

Abstract Differential two-particle normalized cumulants ($$R_2$$ $R_2$) and transverse momentum correlations ($$P_2$$ $P_2$) are measured as a function of the relative pseudorapidity and azimuthal angle difference$$( \Delta \eta , \Delta \varphi )$$ $(\Delta \eta ,\Delta \phi )$of charged particle pairs in minimum bias pp collisions at$$\sqrt{\textit{s}}$$ $\sqrt{s}$= 13 TeV. The measurements use charged hadrons in the pseudorapidity region of$$|\eta | < 0.8$$ $\left|\eta \right|<0.8$and the transverse momentum range 0.2$$< \textit{p}_{\mathrm T} < $$ $<p_T<$2.0$$\textrm{GeV}/\textit{c}$$ $\text{GeV}/c$in order to focus on soft multiparticle interactions and to complement prior measurements of these correlation functions in p–Pb and Pb–Pb collisions. The correlation functions are reported for both unlike-sign and like-sign pairs and their charge-independent and charge-dependent combinations. Both the$$R_2$$ $R_2$and$$P_2$$ $P_2$measured in pp collisions exhibit features qualitatively similar to those observed in p–Pb and Pb–Pb collisions. The$$\Delta \eta $$ $\Delta \eta$and$$\Delta \varphi $$ $\Delta \phi$root mean square widths of the near-side peak of the correlation functions are evaluated and compared with those observed in p–Pb and Pb–Pb collisions and show smooth evolution with the multiplicity of charged particles produced in the collision. The comparison of the measured correlation functions with predictions from PYTHIA8 shows that this model qualitatively captures their basic structure and characteristics but feature important differences. In addition, the$$R_2^{\textrm{CD}}$$ $R_2^{\text{CD}}$is used to determine the charge balance function of hadrons produced within the detector acceptance of the measurements. The integral of the balance function is found to be compatible with those reported by a previous measurement in Pb–Pb collisions. 

The production yields of the orbitally excited charm-strange mesons ${\mathrm{D}}_{\mathrm{s}1}\left(1^{+}\right)(2536{)}^{+}$and ${\mathrm{D}}_{\mathrm{s}2}^{*}\left(2^{+}\right)(2573{)}^{+}$were measured for the first time in proton-proton (pp) collisions at a center-of-mass energy of $\sqrt{s}=13\mathrm{TeV}$with the ALICE experiment at the LHC. The ${\mathrm{D}}_{\mathrm{s}1}^{+}$and ${\mathrm{D}}_{\mathrm{s}2}^{*+}$mesons were measured at midrapidity ( $\left|y\right|<0.5$) in minimum-bias and high-multiplicity pp collisions in the transverse-momentum interval $2<p_{\mathrm{T}}<24\mathrm{GeV}/c$. Their production yields relative to the ${\mathrm{D}}_{\mathrm{s}}^{+}$ground-state yield were found to be compatible between minimum-bias and high-multiplicity collisions, as well as with previous measurements in ${\mathrm{e}}^{\pm }\mathrm{p}$and ${\mathrm{e}}^{+}{\mathrm{e}}^{-}$collisions. The measured ${\mathrm{D}}_{\mathrm{s}1}^{+}/{\mathrm{D}}_{\mathrm{s}}^{+}$and ${\mathrm{D}}_{\mathrm{s}2}^{*+}/{\mathrm{D}}_{\mathrm{s}}^{+}$yield ratios are described by statistical hadronization models and can be used to tune the parameters governing the production of excited charm-strange hadrons in Monte Carlo generators, such as 8. 

The correlations between event-by-event fluctuations of symmetry planes are measured in Pb-Pb collisions at a center-of-mass energy per nucleon pair $\sqrt{s_{NN}}=5.02\mathrm{TeV}$recorded by the ALICE detector at the Large Hadron Collider. This analysis is conducted using the Gaussian estimator technique, which is insensitive to biases from correlations between different flow amplitudes. The study presents, for the first time, the centrality dependence of correlations involving up to five different symmetry planes. The correlation strength varies depending on the harmonic order of the symmetry plane and the collision centrality. Comparisons with measurements from lower energies indicate no significant differences within uncertainties. Additionally, the results are compared with hydrodynamic model calculations. Although the model predictions provide a qualitative explanation of the experimental results, they overestimate the data for some observables. This is particularly true for correlators that are sensitive to the nonlinear response of the medium to initial-state anisotropies in the collision system. As these new correlators provide unique information—independent of flow amplitudes—their usage in future model developments can further constrain the properties of the strongly interacting matter created in ultrarelativistic heavy-ion collisions.