Catalin Ristea

A bstract: The measurement of hyperon (Λ and $$\overline{\Lambda }$$ ) polarization along the beam axis in Pb-Pb collisions at $$\sqrt{{s}_{\text{N}\text{N}}}=5.36$$ TeV using the ALICE detector is presented. The polarization, arising from vorticity components induced by anisotropic flow, is studied relative to the second- and third-order event planes. The measured polarization exhibits clear azimuthal sine modulations, providing the first observation of polarization along the beam direction measured relative to the third-order event plane at the Large Hadron Collider. The values of the polarization measured with respect to the second-order event plane are consistent with previous measurements at $$\sqrt{{s}_{\text{N}\text{N}}}=5.02$$ TeV and show improved statistical precision owing to the larger data sample. Comparisons with hydrodynamic calculations indicate that the measured polarization is sensitive to the bulk viscosity and the vortical structure of the quark-gluon plasma, offering new constraints on its transport properties. In particular, the polarization measured relative to the third-order event plane is expected to provide an additional and independent input to constrain both bulk and shear viscosities of the quark-gluon plasma.

A bstract: This Paper presents rapidity-differential measurements of coherent J /ψ and ψ (2S) photoproduction, as well as rapidity- and mass-differential measurements of exclusive dimuon production, in the forward rapidity region −4 < y < −2 . 5 in ultra-peripheral Pb–Pb collisions at $$\sqrt{{s}_{\text{NN}}}=5.36$$ TeV using data recorded by the ALICE detector at the LHC in 2023, corresponding to an integrated luminosity of ℒ = 1170 ± 50 μ b ^{− 1} . The J /ψ and ψ (2S) results reveal the significant role of nuclear shadowing effects. The square root of the ratio of the measured quarkonium cross section to the impulse approximation prediction is about 0.76 for J /ψ and 0.71 for ψ (2S), at y ≈ −3, corresponding to typical Bjorken- x values of 10 ^{− 2} . The exclusive dimuon results highlight the sensitivity of such measurements to precise modeling of the photon flux, particularly at impact parameters near the nuclear radius.

The ALICE Collaboration has measured the azimuthal correlation between trigger isolated-prompt photons and associated charged hadrons in Pb–Pb collisions at the CERN LHC, at a centre-of-mass energy per nucleon pair of $$\sqrt{s_{\textrm{NN}}}~=~5.02$$ s NN = 5.02 TeV. The trigger isolated-prompt photons are measured in the transverse-momentum range $$18<p_{\textrm{T}}^{\gamma }<40$$ 18 < p T γ < 40 GeV/ c and pseudorapidity range $$|\eta ^{\gamma }| <0.67$$ | η γ | < 0.67 . The isolation selection is based on a charged particle isolation momentum threshold $$p_{\textrm{T}}^{\mathrm{iso,\,ch}} = 1.5$$ p T iso , ch = 1.5 GeV/ c within a cone of radius $$R=0.2$$ R = 0.2 . The associated charged particles are measured in the transverse-momentum ranges $$p_{\textrm{T}}^{\textrm{h}} > 1.8$$ p T h > 1.8 GeV/ c and pseudorapidity $$|\eta ^{{\textrm{h}}}| <0.9$$ | η h | < 0.9 . The yield D( $$z_{\textrm{T}}$$ z T ) of associated hadrons per trigger, with $$z_{\textrm{T}} = p_{\textrm{T}}^{{\textrm{h}}}/p_{\textrm{T}}^{\gamma }$$ z T = p T h / p T γ , is measured in three Pb–Pb collision centrality classes: central (0–30%), semicentral (30–50%), and peripheral (50–90%). The D( $$z_{\textrm{T}}$$ z T ) measurement in pp collisions with the same $$p_{\textrm{T}}^{\gamma }$$ p T γ interval was too limited statistically, but at a lower $$p_{\textrm{T}}^{\gamma }$$ p T γ interval, showing agreement with theoretical predictions. Therefore, an approximation to the standard $$I_{\textrm{AA}}$$ I AA is computed from the D( $$z_{\textrm{T}}$$ z T ) conditional yields, using NLO pQCD predictions as pp reference, the $$I_{\textrm{pQCD}}$$ I pQCD . A strong suppression of this ratio is observed in central collisions compared to peripheral collisions. The result extends to a lower $$p_{\textrm{T}}^{\gamma }$$ p T γ relative to those reported in previously published Pb–Pb collisions measurements at $$\sqrt{s_{\textrm{NN}}}~=~5.02$$ s NN = 5.02 TeV. The $$I_{\textrm{pQCD}}$$ I pQCD measurement is compared to $$I_{\textrm{AA}}$$ I AA calculations with an NLO pQCD model that includes energy loss, and to the CoLBT-hydro model. The $$I_{\textrm{pQCD}}$$ I pQCD results from central collisions are also compared with $$I_{\textrm{AA}}$$ I AA measurements of jets correlated with isolated-prompt photons and of hadrons correlated with $$\hbox {Z}^{0}$$ Z 0 bosons, both reported by the CMS Collaboration at the LHC, as well as with direct photon–hadron correlation $$I_{\textrm{AA}}$$ I AA measurements reported by the PHENIX and STAR Collaborations at RHIC.

This paper presents the measurement of $$\pi ^{\pm }$$ π ± , $$\text {K}^{\pm }$$ K ± , and p( $$\bar{\text {p}}$$ p ¯ ) production in high-multiplicity proton–proton collisions at $$\sqrt{s}=13$$ s = 13 TeV at midrapidity ( $$|y| < 0.5$$ | y | < 0.5 ) using the ALICE detector at the LHC. The transverse-momentum ( $$p_\textrm{T} $$ p T ) spectra of these particles are reported for three high-multiplicity classes. The results show a mass- and multiplicity-dependent hardening of the $$p_\textrm{T} $$ p T spectra and an enhancement of the p/ $$\pi $$ π ratio at intermediate $$p_\textrm{T} $$ p T . These features are similar to those observed in heavy-ion collisions, where quark–gluon plasma formation is expected. The new measurements have extended the highest average charged-particle multiplicity density per unit of pseudorapidity achieved in pp collisions, roughly a factor five higher than that in average inelastic pp collisions, thereby reducing the multiplicity gap between small and large collision systems. In addition, the results are further compared with previously published measurements and with model calculations obtained using distinct tunes of the PYTHIA 8 Monte Carlo generator, as well as with predictions from the EPOS4. The comparison of the $$p_\textrm{T}$$ p T -integrated K/ $$\pi $$ π and p/ $$\pi $$ π ratios across different collision systems and energies suggests that particle production scales with charged-particle multiplicity, rather than with collision energy or system size. While the PYTHIA 8 tunes and the EPOS4 model are able to reproduce some of these measurements, either quantitatively or qualitatively, none of them consistently describes all observed features of the data.

Measurements of the production of the W $$^{\pm }$$ ± and Z $$^0$$ 0 bosons at midrapidity in pp collisions at $$\sqrt{s} = 13$$ s = 13 TeV with ALICE at the Large Hadron Collider (LHC) are presented. The W $$^{\pm }$$ ± and Z $$^0$$ 0 bosons are detected via their (di)electronic decay channels, with the electron reconstruction performed in the midrapidity region $$(|y|< 0.6).$$ ( | y | < 0.6 ) . The $$p_{{\text {T}}}$$ p T -integrated and $$p_{\textrm{T}}$$ p T -differential production cross sections of electrons from W $$^{\pm }$$ ± decays in the interval $$30<p_{{\text {T}}}< 60$$ 30 < p T < 60 GeV/ c , as well as the $$p_{{\text {T}}}$$ p T -integrated production cross section of Z $$^0$$ 0 bosons, are measured. The results are described by perturbative QCD calculations using different sets of parton distribution functions. The production of W $$^{\pm }$$ ± bosons and azimuthally correlated associated hadrons is also measured as a function of the charged-particle multiplicity for the first time at the LHC. The former increases approximately linearly with the charged-particle multiplicity, while for the latter, there are hints of a faster-than-linear increase. These observations are compared with theoretical calculations .

Particle identification (PID) is a fundamental aspect of the ALICE detector system, central to its heavy-ion and proton–proton physics programs. Among the different PID strategies, ALICE uses the time-of-flight (TOF) detector to identify particles at intermediate momenta ( $$0.5< p_{\mathrm{T}} <4\ {\,{\textrm{GeV}}/c} $$ 0.5 < p T < 4 GeV / c ). The ALICE TOF detector performed successfully during the first 10 years of LHC operations. During the Long Shutdown 2, many ALICE sub-detectors, including TOF, were upgraded to fully leverage the targeted 50 kHz interaction rate of Pb–Pb collisions, which required the implementation of a continuous readout scheme. The TOF detector electronics were upgraded and refurbished, while processing algorithms for data quality control, reconstruction, calibration, and analysis were rewritten. This paper presents the upgraded TOF detector operation and calibration procedures and its performance in terms of timing resolution, a key factor for particle separation in ALICE analyses. Using 2022 pp collision data at $$\sqrt{s} = 13.6\,\textrm{TeV}$$ s = 13.6 TeV from Run 3, the time resolution of the detector was estimated with two independent methods, both yielding consistent results, better than 80 ps. Despite the excellent performance already achieved, further improvements are expected after additional detector commissioning and refined calibration procedures, thus enhancing the ALICE PID capabilities for Run 3 and beyond.