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.Comment: 9 pages, 11 figure

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

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

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

Sampling calorimeter to measure the photon’s incident angle

A three-dimensional fine-segmented sampling calorimeter enables us to measure the profiles of generated shower particles along the photon’s direction. For a feasibility study, a toy detector was designed via Geant4 simulation. It consists of alternating layers of a 1-mm-thick lead absorber and a 5-mm-thick plastic scintillator. The plastic scintillator is segmented into 15-mm-wide strips, alternately oriented in the vertical and horizontal directions. The energy deposits of each strip are used to train the machine learning algorithm (XGboost) to deduce the given angle, and the resolution of its angle reconstruction is expected to be 1.3 degrees for 1 GeV photon. We fabricate a small sampling calorimeter to validate the simulation results. We use 0.15-mm-thick tungsten strips instead of lead plates and 1mm-square scintillating fibers instead of plastic scintillators for better energy resolution. This updated configuration indicates no significant difference in the angular resolution, while the energy resolution significantly improves. Its performance shows a reasonable agreement between the Monte Carlo expectation and the obtained data with a positron beam. A detailed study is underway to understand the measured data thoroughly

Medium-induced modification of azimuthal correlations of electrons from heavy-flavor hadron decays with charged particles in Pb--Pb collisions at sNN=5.02\mathbf{\sqrt{s_{\rm{NN}}} = 5.02} TeV

The azimuthal-correlation distributions between electrons from the decays of heavy-flavor hadrons and associated charged particles in Pb-Pb collisions at $\sqrt{s_{\mathrm{NN}}} = 5.02$ TeV are reported for the 0-10% and 30-50% centrality classes. This is the first measurement to provide access to the azimuthal-correlation observables in the heavy-flavor sector in Pb-Pb collisions. The analysis is performed for trigger electrons from heavy-flavor hadron decays with transverse momentum $4 4$ GeV/$c$ on the away side. The $I_\mathrm{AA}$ for electron triggers from heavy-flavor hadron decays is compared with that for light-flavor and strange-particle triggers to investigate the dependence on different fragmentation processes and parton-medium dynamics, and is found to be the same within uncertainties.The azimuthal-correlation distributions between electrons from the decays of heavy-flavor hadrons and associated charged particles in Pb-Pb collisions at $\sqrt{s_{\rm NN}} = 5.02$ TeV are reported for the 0-10% and 30-50% centrality classes. This is the first measurement to provide access to the azimuthal-correlation observables in the heavy-flavor sector in Pb-Pb collisions. The analysis is performed for trigger electrons from heavy-flavor hadron decays with transverse momentum $4 4$ GeV/$c$ on the away side. The $I_{\rm AA}$ for electron triggers from heavy-flavor hadron decays is compared with that for light-flavor and strange-particle triggers to investigate the dependence on different fragmentation processes and parton-medium dynamics, and is found to be the same within uncertainties

Femtoscopic study of the proton-proton and proton-deuteron systems in heavy-ion collisions at the LHC

This work reports femtoscopy correlations of p$-$p (p$-$p) and p$-$d (p$-$d) pairs measured in Pb$-$Pb collisions at center-of-mass energy $\sqrt{s_{\rm NN}} = 5.02$ TeV by the ALICE Collaboration. A fit to the measured proton-proton correlation functions allows one to extract the dependence of the nucleon femtoscopic radius of the particle-emitting source as a function of the pair transverse mass ($m_{\rm T}$) and of the average charge particle multiplicity $\langle {\rm d}N_{\rm ch}/{\rm d}\eta \rangle^{1/3}$ for three centrality intervals (0$-$10%, 10$-$30%, 30$-$50%). In both cases, the expected power-law and liner scalings are observed, respectively. The measured p$-$d correlations can be described by both two- and three-body calculations, indicating that the femtoscopy observable is not sensitive to the short-distance features of the dynamics of the p$-$(p$-$n) system, due to the large inter-particle distances in Pb$-$Pb collisions at the LHC. Indeed, in this study, the minimum measured femtoscopic source sizes for protons and deuterons start at $2.73^{+0.05}_{−0.05}$ and $3.10^{+1.04}_{−0.86}$ fm, respectively, for the 30$-$50% centrality of the collisions. Moreover, the $m_{\rm T}$-scaling obtained for the p$-$p and p$-$d systems is compatible within $1\sigma$ of the uncertainties. These findings provide new input for fundamental studies on the production of light (anti)nuclei under extreme conditions.This work reports femtoscopic correlations of p$-$p ($\bar{\rm p}-\bar{\rm p}$) and p$-$d ($\bar{\rm p}-\bar{\rm d}$) pairs measured in Pb$-$Pb collisions at center-of-mass energy $\sqrt{s_{\rm NN}}$ = 5.02 TeV by the ALICE Collaboration. A fit to the measured proton-proton correlation functions allows one to extract the dependence of the nucleon femtoscopic radius of the particle-emitting source on the pair transverse mass ($m_\text{T}$) and on the average charge particle multiplicity $\langle\text{dN}_\text{ch}/\text{d}\eta\rangle^{1/3}$ for three centrality intervals (0$-$10$\%$, 10$-$30$\%$, 30$-$50$\%$). In both cases, the expected power-law and linear scalings are observed, respectively. The measured p$-$d correlations can be described by both two- and three-body calculations, indicating that the femtoscopy observable is not sensitive to the short-distance features of the dynamics of the p$-$(p$-$n) system, due to the large inter-particle distances in Pb$-$Pb collisions at the LHC. Indeed, in this study, the minimum measured femtoscopic source sizes for protons and deuterons have a minimum value at $2.73^{+0.05}_{-0.05}$ and $3.10^{+1.04}_{-0.86}$ fm, respectively, for the 30$-$50$\%$ centrality collisions. Moreover, the $m_{\rm{T}}$-scaling obtained for the p$-$p and p$-$d systems is compatible within 1$\sigma$ of the uncertainties. These findings provide new input for fundamental studies on the production of light (anti)nuclei under extreme conditions

Multiplicity dependence of K∗(892)±\mathbf{^*(892)^{\pm}} production in pp collisions at s=13\mathbf{\sqrt{{s}} = 13} TeV

The first results of K$^*$(892)$^{\pm}$ production at midrapidity ($|y| < 0.5$) in pp collisions at $\sqrt{s} = 13$ TeV as a function of the event multiplicity are presented. The K$^*$(892)$^{\pm}$ has been reconstructed via its hadronic decay channel K$^*$(892)$^{\pm} \rightarrow~\pi^{\pm}~+~{\rm K}^0_{\rm S}$ using the ALICE detector at the LHC. For each multiplicity class the differential transverse momentum ($p_{\rm T}$) spectrum, the mean transverse momentum $\langle p_{\rm T} \rangle$, the $p_{\rm T}$-integrated yield (d$N$/d$y$), and the ratio of the K$^*$(892)$^{\pm}$ to K$^0_{\rm S}$ yields are reported. These are consistent with previous K$^*$(892)$^0$ resonance results with a higher level of precision. Comparisons with phenomenological models such as PYTHIA6, PYTHIA8, EPOS-LHC, and DIPSY are also discussed. A first evidence of a significant K$^*$(892)$^{\pm}$/${\rm K}^0_{\rm S}$ suppression in pp collisions is observed at a 7$\sigma$ level passing from low to high multiplicity events. The ratios of the $p_{\rm T}$-differential yields of K$^*$(892)$^{\pm}$ and K$^0_{\rm S}$ in high and low multiplicity events are also presented along with their double ratio. For $p_{\rm T} \lesssim 2$ GeV/$c$ this double ratio persists below unity by more than $3\sigma$ suggesting that the suppression affects mainly low $p_{\rm T}$ resonances. The measured decreasing trend of the K$^*$(892)$^{\pm}$/K$^0_{\rm S}$ ratio with increasing multiplicity, which in heavy-ion collisions is typically attributed to the rescattering of decay particles of the short-lived resonances, is reproduced by the EPOS-LHC model without the use of hadronic afterburners.The first results of K$^*$(892)$^{\pm}$ production at midrapidity ($|y| < 0.5$) in pp collisions at $\sqrt{s} = 13$ TeV as a function of the event multiplicity are presented. The K$^*$(892)$^{\pm}$ has been reconstructed via its hadronic decay channel K$^*$(892)$^{\pm} \rightarrow π^{\pm} + K_{\rm S}^0$ using the ALICE detector at the LHC. For each multiplicity class the differential transverse momentum ($p_{\rm T}$) spectrum, the mean transverse momentum $\langle p_{\rm T} \rangle$, the $p_{\rm T}$-integrated yield (d$N$/d$y$), and the ratio of the K$^*$(892)$^{\pm}$ to $K_{\rm S}^0$ yields are reported. These are consistent with previous K$^*$(892)$^0$ resonance results with a higher level of precision. Comparisons with phenomenological models such as PYTHIA6, PYTHIA8, EPOS-LHC, and DIPSY are also discussed. A first evidence of a significant K$^*$(892)$^{\pm}$/$K_{\rm S}^0$ suppression in pp collisions is observed at a 7$σ$ level passing from low to high multiplicity events. The ratios of the $p_{\rm T}$-differential yields of K$^*$(892)$^{\pm}$ and $K_{\rm S}^0$ in high and low multiplicity events are also presented along with their double ratio. For $p_{\rm T} \lesssim 2$ GeV/$c$ this double ratio persists below unity by more than $3σ$ suggesting that the suppression affects mainly low $p_{\rm T}$ resonances. The measured decreasing trend of the K$^*$(892)$^{\pm}$/$K_{\rm S}^0$ ratio with increasing multiplicity, which in heavy-ion collisions is typically attributed to the rescattering of decay particles of the short-lived resonances, is reproduced by the EPOS-LHC model without the use of hadronic afterburners

Revealing the microscopic mechanism of deuteron formation at the LHC

The formation of light (anti)nuclei with mass number A of few units (e.g., d, $^3$He, and $^4$He) in high-energy hadronic collisions presents a longstanding mystery in nuclear physics [1, 2]. It is not clear how nuclei bound by a few MeV can emerge in environments characterized by temperatures above 100 MeV [3–5], about 100,000 times hotter than the center of the Sun. Despite extensive studies, this question remained unanswered. The ALICE Collaboration now addresses it with a novel approach using deuteron–pion momentum correlations in proton-proton (pp) collisions at the Large Hadron Collider (LHC). Our results provide model-independent evidence that about 80% of the observed (anti)deuterons are produced in nuclear fusion reactions [6] following the decay of short-lived resonances, such as the $\Delta(1232)$. These findings resolve a crucial gap in our understanding of nucleosynthesis in hadronic collisions. Beyond answering the fundamental question on how nuclei are formed in hadronic collisions, the results can be employed in the modeling of the production of light and heavy nuclei in cosmic rays [7] and dark matter decays [8, 9].The formation of light (anti)nuclei with mass number A of a few units (e.g., d, $^3$He, and $^4$He) in high-energy hadronic collisions presents a longstanding mystery in nuclear physics [1,2]. It is not clear how nuclei bound by a few MeV can emerge in environments characterized by temperatures above 100 MeV [3-5], about 100,000 times hotter than the center of the Sun. Despite extensive studies, this question remained unanswered. The ALICE Collaboration now addresses it with a novel approach using deuteron-pion momentum correlations in proton-proton (pp) collisions at the Large Hadron Collider (LHC). Our results provide model-independent evidence that about 80% of the observed (anti)deuterons are produced in nuclear fusion reactions [6] following the decay of short-lived resonances, such as the $\Delta (1232)$. These findings resolve a crucial gap in our understanding of nucleosynthesis in hadronic collisions. Beyond answering the fundamental question on how nuclei are formed in hadronic collisions, the results can be employed in the modeling of the production of light and heavy nuclei in cosmic rays [7] and dark matter decays [8,9]