Understanding the nature of f0(980) with ALICE at the LHC

The f0(980) resonance had been observed years ago in ππ scattering experiments and is expected to be one of the scalar mesons. Since its first observation in the 1970s, the nature of light scalar mesons is far from the understanding, and no consensus on its internal structure has been reached, raising different suggestions regarding the structure of the f0(980), such as tetraquark, mesonic molecule, and conventional diquark structure. The extreme environment, such as high temperature and density, de- confines quarks and gluons, freeing them and forming the Quark-Gluon Plasma (QGP), which is expected to be created immediately after the Big Bang. Such an early stage of our universe can be reproduced in relativis- tic heavy ion collisions, providing chances to study the properties of QGP. Extensive studies have been conducted on the geometry, evolution, and particle production of QGP, and its modification, with many scientific ac- ceptances for the existence of QGP. Surprisingly, phenomena exhibiting the existence of QGP are also observed in high-multiplicity proton–proton and proton–ion collisions, leading to further questioning QGP formation. In this respect, the f0(980) resonance is measured with the ALICE detector via the f0(980) → π+π− decay channel in relativistic nucleus– nucleus collisions. The present thesis describes the entire methodology to measure the invariant yield in different multiplicity classes at midra- pidity. The particle yield ratios of the f0(980) resonance are measured to discuss the properties of the late hadronic phase throughout differ- ent collision systems and to explore the internal structure of the f0(980). Furthermore, the measurement of the nuclear modification factor and the elliptic flow and model predictions with different assumptions for the f0(980) strengthen the physics messages in the present thesis

Initial Stages 2021

In this contribution we will present the latest results on two-particle number and transverse momentum correlations from the ALICE Collaboration in order to study the initial stages and dynamic evolution of nucleus-nucleus collisions from small to large systems. In pp and p-Pb collisions, the physical origin of long-range flow-like correlations remains an open question, with implications for our understanding of collective dynamics in both small and large systems. We will present recent measurements of the second Fourier harmonic $v_2$ as a function of multiplicity in pp collisions using the Forward Multiplicity Detector, which makes it possible to measure the correlations between particles which are separated by up to eight units of pseudorapidity, the largest $\Delta\eta$ gap at the LHC. To further probe the origin of long-range correlations in small systems, we will present a quantitative study of the ridge in high-multiplicity pp collisions which contain a high-momentum charged particle or reconstructed jet, in order to determine whether long-range correlations are correlated with hard processes. The experimental results are compared to the Pythia and EPOS Monte Carlo models which employ different mechanisms to generate ridge-like features, in order to draw conclusions about the underlying physical processes that produce long-range correlations. We will also present new measurements of the transverse momentum correlator $G_2$ in pp and p-Pb collisions, and discuss the evolution of the correlation function with multiplicity from small to large collision systems. Measurements of these correlations in Pb-Pb collisions have been recently published by the ALICE Collaboration, and demonstrate features attributed to radial flow, delayed hadronization, momentum transfer due to viscous effects, and system properties like $\eta/s$. It is thus of high interest to elucidate how those transverse momentum correlators behave in small collision systems

Investigation of suppression of Υ(nS)\Upsilon(nS) in relativistic heavy-ion collisions at RHIC and LHC energies

The primary purpose of studying quarkonium production in relativistic heavy-ion collisions is to understand the properties of the quark-gluon plasma. At various collision systems, measurements of quarkonium states of different binding energies, such as $\Upsilon(nS)$, can provide comprehensive information. A model study has been performed to investigate the modification of $\Upsilon(nS)$ production in Pb-Pb collisions at $\sqrt{s_{\mathrm{NN}}}=$ 5.02 TeV and Au-Au collisions at $\sqrt{s_{\mathrm{NN}}}=$ 200 GeV. The Monte-Carlo simulation study is performed with a publicly available hydrodynamic simulation package for the quark-gluon plasma medium and a theoretical calculation of temperature-dependent thermal width of $\Upsilon(nS)$ considering the gluo-dissociation and inelastic parton scattering for dissociation inside the medium. In addition, we perform a systematic study with different descriptions of initial collision geometry and formation time of $\Upsilon(nS)$ to investigate their impacts on yield modification. The model calculation with a varied parameter set can describe the experimental data of $\Upsilon(nS)$ in Pb-Pb collisions at 5.02 TeV and $\Upsilon(2S)$ in Au-Au collisions at 200 GeV but underestimates the modification of $\Upsilon(1S)$ at the lower collision energy. The nuclear absorption mechanism is explored to understand the discrepancy between the data and simulation

Multiplicity dependence of light (anti-)nuclei production in p–Pb collisions at sNN=5.02 TeV

The measurement of the deuteron and anti-deuteron production in the rapidity range −1 < y < 0 as a function of transverse momentum and event multiplicity in p–Pb collisions at √sNN = 5.02 TeV is presented. (Anti-)deuterons are identified via their specific energy loss dE/dx and via their time-of- flight. Their production in p–Pb collisions is compared to pp and Pb–Pb collisions and is discussed within the context of thermal and coalescence models. The ratio of integrated yields of deuterons to protons (d/p) shows a significant increase as a function of the charged-particle multiplicity of the event starting from values similar to those observed in pp collisions at low multiplicities and approaching those observed in Pb–Pb collisions at high multiplicities. The mean transverse particle momenta are extracted from the deuteron spectra and the values are similar to those obtained for p and particles. Thus, deuteron spectra do not follow mass ordering. This behaviour is in contrast to the trend observed for non-composite particles in p–Pb collisions. In addition, the production of the rare 3He and 3He nuclei has been studied. The spectrum corresponding to all non-single diffractive p-Pb collisions is obtained in the rapidity window −1 < y < 0 and the pT-integrated yield dN/dy is extracted. It is found that the yields of protons, deuterons, and 3He, normalised by the spin degeneracy factor, follow an exponential decrease with mass number

Elliptic Flow of Electrons from Beauty-Hadron Decays in Pb-Pb Collisions at sNN\sqrt {s_{NN}} = 5.02 TeV

The elliptic flow of electrons from beauty hadron decays at midrapidity (|y|<0.8) is measured in Pb-Pb collisions at $\sqrt {s_{NN}}$ = 5.02 TeV with the ALICE detector at the LHC. The azimuthal distribution of the particles produced in the collisions can be parametrized with a Fourier expansion, in which the second harmonic coefficient represents the elliptic flow, $v_2$. The $v_2$ coefficient of electrons from beauty hadron decays is measured for the first time in the transverse momentum ($p_T$) range 1.3–6 GeV/c in the centrality class 30%–50%. The measurement of electrons from beauty-hadron decays exploits their larger mean proper decay length cτ ≈ 500 μm compared to that of charm hadrons and most of the other background sources. The $v_2$ of electrons from beauty hadron decays at midrapidity is found to be positive with a significance of 3.75 σ. The results provide insights into the degree of thermalization of beauty quarks in the medium. A model assuming full thermalization of beauty quarks is strongly disfavored by the measurement at high $p_T$, but is in agreement with the results at low $p_T$. Transport models including substantial interactions of beauty quarks with an expanding strongly interacting medium describe the measurement within uncertainties

J/ψ\psi production as a function of charged-particle multiplicity in p-Pb collisions at sNN = 8.16\sqrt{\textit{s}_{\rm NN}}~=~8.16 TeV

Inclusive J/ψ yields and average transverse momenta in p-Pb collisions at a center-of-mass energy per nucleon pair $\sqrt{s_{\mathrm{NN}}}$ = 8.16 TeV are measured as a function of the charged-particle pseudorapidity density with ALICE. The J/ψ mesons are reconstructed at forward (2.03 < y$_{cms}$< 3.53) and backward (−4.46 < y$_{cms}$< −2.96) center-of-mass rapidity in their dimuon decay channel while the charged-particle pseudorapidity density is measured around midrapidity. The J/ψ yields at forward and backward rapidity normalized to their respective average values increase with the normalized charged-particle pseudorapidity density, the former showing a weaker increase than the latter. The normalized average transverse momenta at forward and backward rapidity manifest a steady increase from low to high charged-particle pseudorapidity density with a saturation beyond the average value

Production of charged pions, kaons, and (anti-)protons in Pb-Pb and inelastic pppp collisions at sNN\sqrt {s_{NN}} = 5.02 TeV

International audienceMid-rapidity production of $\pi^{\pm}$, $\rm{K}^{\pm}$ and ($\bar{\rm{p}}$)p measured by the ALICE experiment at the LHC, in Pb-Pb and inelastic pp collisions at $\sqrt{s_{\rm{NN}}}$ = 5.02 TeV, is presented. The invariant yields are measured over a wide transverse momentum ($p_{\rm{T}}$) range from hundreds of MeV/$c$ up to 20 GeV/$c$. The results in Pb-Pb collisions are presented as a function of the collision centrality, in the range 0$-$90%. The comparison of the $p_{\rm{T}}$-integrated particle ratios, i.e. proton-to-pion (p/$\pi$) and kaon-to-pion (K/$\pi$) ratios, with similar measurements in Pb-Pb collisions at $\sqrt{s_{\rm{NN}}}$ = 2.76 TeV show no significant energy dependence. Blast-wave fits of the $p_{\rm{T}}$ spectra indicate that in the most central collisions radial flow is slightly larger at 5.02 TeV with respect to 2.76 TeV. Particle ratios (p/$\pi$, K/$\pi$) as a function of $p_{\rm{T}}$ show pronounced maxima at $p_{\rm{T}}$ $\approx$ 3 GeV/$c$ in central Pb-Pb collisions. At high $p_{\rm{T}}$, particle ratios at 5.02 TeV are similar to those measured in pp collisions at the same energy and in Pb-Pb collisions at $\sqrt{s_{\rm{NN}}}$ = 2.76 TeV. Using the pp reference spectra measured at the same collision energy of 5.02 TeV, the nuclear modification factors for the different particle species are derived. Within uncertainties, the nuclear modification factor is particle species independent for high $p_{\rm{T}}$ and compatible with measurements at $\sqrt{s_{\rm{NN}}}$ = 2.76 TeV. The results are compared to state-of-the-art model calculations, which are found to describe the observed trends satisfactorily

Multiplicity dependence of (multi-)strange hadron production in proton-proton collisions at s\sqrt{s} = 13 TeV

The production rates and the transverse momentum distribution of strange hadrons at mid-rapidity ($\left| y\right| < 0.5$) are measured in proton-proton collisions at $\sqrt{s}$ = 13 TeV as a function of the charged particle multiplicity, using the ALICE detector at the LHC. The production rates of $\mathrm{K}^{0}_{S}$, $\Lambda$, $\Xi$, and $\Omega$ increase with the multiplicity faster than what is reported for inclusive charged particles. The increase is found to be more pronounced for hadrons with a larger strangeness content. Possible auto-correlations between the charged particles and the strange hadrons are evaluated by measuring the event-activity with charged particle multiplicity estimators covering different pseudorapidity regions. When comparing to lower energy results, the yields of strange hadrons are found to depend only on the mid-rapidity charged particle multiplicity. Several features of the data are reproduced qualitatively by general purpose QCD Monte Carlo models that take into account the effect of densely-packed QCD strings in high multiplicity collisions. However, none of the tested models reproduce the data quantitatively. This work corroborates and extends the ALICE findings on strangeness production in proton-proton collisions at 7 TeV

Measurement of the low-energy antideuteron inelastic cross section

In this Letter, we report the first measurement of the inelastic cross section for antideuteron-nucleus interactions at low particle momenta, covering a range of 0.3 ≤ p < 4 GeV/c. The measurement is carried out using p-Pb collisions at a center-of-mass energy per nucleon–nucleon pair of $\sqrt{s_{\rm NN}}$ = 5.02 TeV, recorded with the ALICE detector at the CERN LHC and utilizing the detector material as an absorber for antideuterons and antiprotons. The extracted raw primary antiparticle-to-particle ratios are compared to the results from detailed ALICE simulations based on the geant4 toolkit for the propagation of (anti)particles through the detector material. The analysis of the raw primary (anti)proton spectra serves as a benchmark for this study, since their hadronic interaction cross sections are well constrained experimentally. The first measurement of the inelastic cross section for antideuteron-nucleus interactions averaged over the ALICE detector material with atomic mass numbers ⟨A⟩ = 17.4 and 31.8 is obtained. The measured inelastic cross section points to a possible excess with respect to the Glauber model parametrization used in geant4 in the lowest momentum interval of 0.3 ≤ p < 0.47 GeV/c up to a factor 2.1. This result is relevant for the understanding of antimatter propagation and the contributions to antinuclei production from cosmic ray interactions within the interstellar medium. In addition, the momentum range covered by this measurement is of particular importance to evaluate signal predictions for indirect dark-matter searches

Pseudorapidity distributions of charged particles as a function of mid- and forward rapidity multiplicities in pp collisions at s\sqrt{s} = 5.02, 7 and 13 TeV

The multiplicity dependence of the pseudorapidity density of charged particles in proton–proton (pp) collisions at centre-of-mass energies $\sqrt{s}~=~5.02$, 7 and 13 TeV measured by ALICE is reported. The analysis relies on track segments measured in the midrapidity range ($|\eta | < 1.5$). Results are presented for inelastic events having at least one charged particle produced in the pseudorapidity interval $|\eta |<1$. The multiplicity dependence of the pseudorapidity density of charged particles is measured with mid- and forward rapidity multiplicity estimators, the latter being less affected by autocorrelations. A detailed comparison with predictions from the PYTHIA 8 and EPOS LHC event generators is also presented. The results can be used to constrain models for particle production as a function of multiplicity in pp collisions