Inclusive quarkonium production in pp collisions at √s = 5.02 TeV

This article reports on the inclusive production cross section of several quarkonium states, $\textrm{J}/\psi$, $\psi \mathrm{(2S)}$, $\Upsilon \mathrm (1S)$, $\Upsilon \mathrm{(2S)}$, and $\Upsilon \mathrm{(3S)}$, measured with the ALICE detector at the LHC, in pp collisions at $\sqrt{s} = 5.02$ TeV. The analysis is performed in the dimuon decay channel at forward rapidity ($2.5< y < 4$). The integrated cross sections and transverse-momentum ($p_{\textrm{T}}$) and rapidity ($y$) differential cross sections for $\textrm{J}/\psi$, $\psi \mathrm{(2S)}$, $\Upsilon \mathrm (1S)$, and the $\psi \mathrm{(2S)}$-to-$\textrm{J}/\psi$ cross section ratios are presented. The integrated cross sections, assuming unpolarized quarkonia, are: $\sigma _{\textrm{J}/\psi }$ ($p_{\textrm{T}} <20$ GeV/c) = 5.88 ± 0.03 ± 0.34$~\mu$b, $\sigma _{\psi \mathrm{(2S)}}$ ($p_{\textrm{T}} <12$ GeV/c) = 0.87 ± 0.06 ± 0.10$~\mu$b, $\sigma _{\Upsilon \mathrm (1S)}$ ($p_{\textrm{T}} <15$ GeV/c) = 45.5 ± 3.9 ± 3.5 nb, $\sigma _{\Upsilon \mathrm{(2S)}}$ ($p_{\textrm{T}} <15$ GeV/c) = 22.4 ± 3.2 ± 2.7 nb, and $\sigma _{\Upsilon \mathrm{(3S)}}$ ($p_{\textrm{T}} <15$ GeV/c) = 4.9 ± 2.2 ± 1.0 nb, where the first (second) uncertainty is the statistical (systematic) one. For the first time, the cross sections of the three $\Upsilon$ states, as well as the $\psi \mathrm{(2S)}$ one as a function of $p_{\textrm{T}}$ and $y$, are measured at $\sqrt{s} = 5.02$ TeV at forward rapidity. These measurements also significantly extend the $\textrm{J}/\psi$ $p_{\textrm{T}}$ reach and supersede previously published results. A comparison with ALICE measurements in pp collisions at $\sqrt{s} = 2.76$, 7, 8, and 13 TeV is presented and the energy dependence of quarkonium production cross sections is discussed. Finally, the results are compared with the predictions from several production models.publishedVersio

First measurements of N-subjettiness in central Pb-Pb collisions at √sNN = 2.76 TeV

The ALICE Collaboration reports the first fully-corrected measurements of the N-subjettiness observable for track-based jets in heavy-ion collisions. This study is performed using data recorded in pp and Pb-Pb collisions at centre-of-mass energies of s√ = 7 TeV and sNN−−−√ = 2.76 TeV, respectively. In particular the ratio of 2-subjettiness to 1-subjettiness, τ2/τ1, which is sensitive to the rate of two-pronged jet substructure, is presented. Energy loss of jets traversing the strongly interacting medium in heavy-ion collisions is expected to change the rate of two-pronged substructure relative to vacuum. The results are presented for jets with a resolution parameter of R = 0.4 and charged jet transverse momentum of 40 ≤ pT,jet ≤ 60 GeV/c, which constitute a larger jet resolution and lower jet transverse momentum interval than previous measurements in heavy-ion collisions. This has been achieved by utilising a semi-inclusive hadron-jet coincidence technique to suppress the larger jet combinatorial background in this kinematic region. No significant modification of the τ2/τ1 observable for track-based jets in Pb-Pb collisions is observed relative to vacuum PYTHIA6 and PYTHIA8 references at the same collision energy. The measurements of τ2/τ1, together with the splitting aperture angle ∆R, are also performed in pp collisions at s√ = 7 TeV for inclusive jets. These results are compared with PYTHIA calculations at s√ = 7 TeV, in order to validate the model as a vacuum reference for the Pb-Pb centre-of-mass energy. The PYTHIA references for τ2/τ1 are shifted to larger values compared to the measurement in pp collisions. This hints at a reduction in the rate of two-pronged jets in Pb-Pb collisions compared to pp collisions.publishedVersio

Anisotropic flow of identified hadrons in Xe-Xe collisions at √sNN = 5.44 TeV

Measurements of elliptic (v2) and triangular (v3) flow coefficients of π±, K±, p+$\overline{\mathrm{p}}$, ${\mathrm{K}}_{\mathrm{S}}^0$, and Λ+$\overline{\Lambda}$ obtained with the scalar product method in Xe-Xe collisions at $\sqrt{s_{\mathrm{NN}}}$ = 5.44 TeV are presented. The results are obtained in the rapidity range |y| < 0.5 and reported as a function of transverse momentum, pT, for several collision centrality classes. The flow coefficients exhibit a particle mass dependence for pT < 3 GeV/c, while a grouping according to particle type (i.e., meson and baryon) is found at intermediate transverse momenta (3 < pT < 8 GeV/c). The magnitude of the baryon v2 is larger than that of mesons up to pT = 6 GeV/c. The centrality dependence of the shape evolution of the pT-differential v2 is studied for the various hadron species. The v2 coefficients of π±, K±, and p+$\overline{\mathrm{p}}$ are reproduced by MUSIC hydrodynamic calculations coupled to a hadronic cascade model (UrQMD) for pT < 1 GeV/c. A comparison with vn measurements in the corresponding centrality intervals in Pb-Pb collisions at $\sqrt{s_{\mathrm{NN}}}$ = 5.02 TeV yields an enhanced v2 in central collisions and diminished value in semicentral collisions.publishedVersio

Inclusive J/ψ production at midrapidity in pp collisions at √s=13 TeV

We report on the inclusive $\text {J}/\psi$ production cross section measured at the CERN Large Hadron Collider in proton–proton collisions at a center-of-mass energy $\sqrt{s}~=~13$ TeV. The $\text {J}/\psi$ mesons are reconstructed in the $\text {e}^{+}\text {e}^{-}$ decay channel and the measurements are performed at midrapidity ($|y|<0.9$) in the transverse-momentum interval $0<p_{\mathrm{T}} <40$ GeV/$c$, using a minimum-bias data sample corresponding to an integrated luminosity $L_{\text {int}} = 32.2~\text {nb}^{-1}$ and an Electromagnetic Calorimeter triggered data sample with $L_{\text {int}} = 8.3~\mathrm {pb}^{-1}$. The $p_{\mathrm{T}}$-integrated $\text {J}/\psi$ production cross section at midrapidity, computed using the minimum-bias data sample, is $\text {d}\sigma /\text {d}y|_{y=0} = 8.97\pm 0.24~(\text {stat})\pm 0.48~(\text {syst})\pm 0.15~(\text {lumi})~\mu \text {b}$. An approximate logarithmic dependence with the collision energy is suggested by these results and available world data, in agreement with model predictions. The integrated and $p_{\mathrm{T}}$-differential measurements are compared with measurements in pp collisions at lower energies and with several recent phenomenological calculations based on the non-relativistic QCD and Color Evaporation models.publishedVersio

KS0 - and (anti-) Λ -hadron correlations in pp collisions at √s=13 TeV

Two-particle Azimuthal correlations are measured with the ALICE apparatus in pp collisions at s√=13s=13 TeV to explore strangeness- and multiplicity-related effects in the fragmentation of jets and the transition regime between bulk and hard production, probed with the condition that a strange meson (KS0KS0) or baryon (ΛΛ) with transverse momentum pT>3pT>3 GeV/cc is produced. Azimuthal correlations between kaons or ΛΛ hyperons with other hadrons are presented at midrapidity for a broad range of the trigger (3<ptriggT<203<pTtrigg<20 GeV/cc) and associated particle pTpT (1 GeV/cc <passocT<ptriggT<pTassoc<pTtrigg), for minimum-bias events and as a function of the event multiplicity. The near- and away-side peak yields are compared for the case of either KS0KS0 or ΛΛ(Λ¯¯¯¯Λ¯) being the trigger particle with that of inclusive hadrons (a sample dominated by pions). In addition, the measurements are compared with predictions from PYTHIA 8 and EPOS LHC event generators.publishedVersio

Experimental Evidence for an Attractive p-φ Interaction

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 pp 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.publishedVersio

Energy dependence of ϕ meson production at forward rapidity in pp collisions at the LHC

The production of $\phi$ mesons has been studied in pp collisions at LHC energies with the ALICE detector via the dimuon decay channel in the rapidity region $2.5< y < 4$. Measurements of the differential cross section $\mathrm{d}^2\sigma /\mathrm{d}y \mathrm{d}p_{\mathrm {T}}$ are presented as a function of the transverse momentum ($p_{\mathrm {T}}$) at the center-of-mass energies $\sqrt{s}=5.02$, 8 and 13 TeV and compared with the ALICE results at midrapidity. The differential cross sections at $\sqrt{s}=5.02$ and 13 TeV are also studied in several rapidity intervals as a function of $p_{\mathrm {T}}$, and as a function of rapidity in three $p_{\mathrm {T}}$ intervals. A hardening of the $p_{\mathrm {T}}$-differential cross section with the collision energy is observed, while, for a given energy, $p_{\mathrm {T}}$ spectra soften with increasing rapidity and, conversely, rapidity distributions get slightly narrower at increasing $p_{\mathrm {T}}$. The new results, complementing the published measurements at $\sqrt{s}=2.76$ and 7 TeV, allow one to establish the energy dependence of $\phi$ meson production and to compare the measured cross sections with phenomenological models. None of the considered models manages to describe the evolution of the cross section with $p_{\mathrm {T}}$ and rapidity at all the energies.publishedVersio

Prompt and non-prompt J/Psi cross-sections in proton-proton collisions at sqrt(s) = 5.02 TeV with ALICE

The study presented in this thesis is about the measurement of \jpsi\ production in proton-proton collisions at center-of-mass energy $\sqrt{s}$ = 5.02 TeV with the ALICE detector at the LHC, with a focus on the separation between prompt \jpsi\ and non-prompt \jpsi, the latter originated from beauty-hadron decays. Statistics collected during 2017 with the minimum bias trigger $L_{\rm int}$ = 19.4 nb\).{-1}\) is the base for the measurement described in this thesis that is performed at central rapidity |y| < 0.9 in the dielectron channel \jpsi\ \to e^+e^- . The fraction of non-prompt \jpsi\ is determined on a statistical basis down to \pt\ = 2 GeV/\).\). through the use of a maximum-likelihood fit procedure. The value of the non-prompt \jpsi\ fraction integrated over \pt\ is: f_{\rm b} (2 < \pt\ < 10 {\rm GeV}/c) = 0.103 $\pm$ 0.022(stat) $\pm$ 0.011 (syst). The non-prompt \jpsi\ fraction is further measured in four bins of transverse momentum and compared to similar measurements from other experiments. The non-prompt \jpsi\ fractions, both \pt-differential and \pt-integrated, are combined with the corresponding inclusive \jpsi\ cross-section measurements, providing prompt and non-prompt \jpsi\ cross-sections. The \pt-integrated cross section for prompt \jpsi\ is: \sigma_{\rm prompt \jpsi}(2 < \pt < 10~{\rm GeV}/c, |y| < 0.9) = 5.09~\pm~0.69~(\rm stat.) \pm 0.49 ~(\rm syst.)~{\rm \mu b}. For non-prompt \jpsi\ the corresponding cross section is: \sigma_{\jpsi \leftarrow h_{\rm B}}(2 < \pt < 10~{\rm GeV}/c, |y| < 0.9)~ = ~ 0.58 ~\pm ~0.15~(\rm stat.) \pm ~0.08 (\rm syst.)~{\rm \mu b}. Prompt and non-prompt \jpsi\ cross-section measurements are finally compared to several QCD-based models

Charmonium Production in Pb–Pb Collisions with ALICE

Charmonium production is a direct probe of the quark–gluon plasma (QGP), a deconfined state of nuclear matter formed in heavy-ion collisions. For J/ψ, a bound state of c¯c quarks, its (re-)generation within the QGP or at the phase boundary, is found to be the dominant production mechanism at low transverse momentum (pT) and in central Pb–Pb collisions at the LHC energies. The relative production of the ψ(2S) excited state with respect to the J/ψ is one possible discriminator between the two different regeneration scenarios. In addition, the non-prompt component of J/ψ production from b-hadron decays allows one to access the interaction of beauty quarks inside the QGP down to low pT. In these proceedings, we present, for the first time, results on the ψ(2S)-to-J/ψ double ratio in Pb–Pb collisions at forward rapidity and √ sNN = 5.02 TeV with respect to a new pp reference with an improved precision compared to the earlier publications. The combined Run 2 data set of ALICE allows the extraction of a signficant ψ(2S) signal in central Pb–Pb collisions at forward rapidity down to zero transverse momentum. The ψ(2S) nuclear modfication factor RAA as a function of pT and centrality will also be shown, as well as the inclusive J/ψ RAA at forward rapidity. At midrapidity, the inclusive, prompt and non-prompt J/ψ RAA as a function of centrality and pT will be presented, based on the full Run 2 statistics. The extraction of the nonprompt J/ψ fraction extends other LHC measurements down to very low pT and its precision is improved signficantly compared to previous ALICE publications. Results will be compared with available theoretical model calculations