Prabhat Ranjan Pujahari

Correlation functions measured as a function of $\ensuremath{\Delta}\ensuremath{\eta},\ensuremath{\Delta}\ensuremath{\phi}$ have emerged as a powerful tool to study the dynamics of particle production in nuclear collisions at high energy. They are however subject, like any other observables, to instrumental effects which must be properly accounted for to extract meaningful physics results. We compare the merits of several techniques used towards measurement of these correlation functions in nuclear collisions. We discuss and distinguish the effects of finite acceptance, and detection efficiency that may vary with collision parameters such as the position of the event in the detector and the instantaneous luminosity of the beam. We focus in particular on instrumental effects which break the factorization of the particle pair detection efficiency, and describe a technique to recover the robustness of correlation observables. We finally introduce a multidimensional weight method to correct for efficiencies that vary simultaneously with particle pseudo rapidity, azimuthal angle, transverse momentum, and the collision vertex position. The method can be generalized to account for any number of ``event variables'' that may break the factorability of the pair efficiency.

The discovery of hot and dense quantum chromodynamics (QCD) matter, known as Quark–Gluon Plasma (QGP), is an essential milestone in understanding the finite temperature QCD medium. Experimentalists around the world collect an unprecedented amount of data in heavy ion collisions, at Relativistic Heavy Ion Collider (RHIC), at Brookhaven National Laboratory (BNL) in New York, USA, and at the Large Hadron Collider (LHC), at CERN in Geneva, Switzerland. The experimentalists analyze these data to unravel the mystery of this new phase of matter that filled a few microseconds old universe just after the Big Bang. Recent advancements in theory, experimental techniques, and high computing facilities help us to better interpret experimental observations in heavy ion collisions. The exchange of ideas between experimentalists and theorists is crucial for the characterization of QGP. The motivation of this first conference, named Hot QCD Matter 2022 is to bring the community together to have a discourse on this topic. In this paper, there are 36 sections discussing various topics in the field of relativistic heavy ion collisions and related phenomena that cover a snapshot of the current experimental observations and theoretical progress. This paper begins with the theoretical overview of relativistic spin-hydrodynamics in the presence of the external magnetic field, followed by the Lattice QCD results on heavy quarks in QGP. Finally, it concludes with an overview of experimental results.

We report studies of charge-independent (CI) and charge-dependent (CD) two-particle differential-number correlation functions, $\rm{R_{2}}(\Delta\eta, \Delta\varphi)$, and transverse momentum ($p_{\rm T}$) correlation functions, $\rm{P_{2}}(\Delta\eta, \Delta\varphi)$, of charged particles in $\sqrt{\textit{s}}$ = 2.76 TeV pp collisions with the PYTHIA and HERWIG models. Model predictions are presented for inclusive charged hadrons ($h^\pm$), as well as pions ($\pi^\pm$), kaons (K$^\pm$), and (anti-)protons ($\rm \bar{p}$/p) in the ranges $0.2 < \textit{p}_{\rm T} \le 2.0~\rm{GeV}/\textit{c}$, $2.0 < \textit{p}_{\rm T} \le 5.0~\rm{GeV}/\textit{c}$, and $5.0 < \textit{p}_{\rm T} \le 30.0~\rm{GeV}/\textit{c}$, with full azimuthal coverage in the range $|\eta|< 1.0$. We compare the model predictions for the strength and shape of the $\rm{R_{2}}$ and $\rm{P_{2}}$ correlators as these pertain to recent measurements by the ALICE collaboration. The $\rm{R_{2}}$ and $\rm{P_{2}}$ correlation functions estimated with PYTHIA and HERWIG exhibit qualitatively similar near-side and away-side correlation structures but feature important differences. Our analysis indicates that comparative studies of $\rm{R_{2}}$ and $\rm{P_{2}}$ correlation functions would provide valuable insight towards the understanding of particle production in pp collisions, and by extension, should also be useful in studies of heavy-ion collisions. Comparison of the $\Delta \eta$ dependence of $\rm{R_{2}}$ and $\rm{P_{2}}$ could contribute, in particular, to a better understanding and modeling of the angular ordering of particles produced by hadronization in jets, as well as a better description of jet fragmentation functions of identified species at low momentum fraction $(z)$.

Two-particle correlations provide information about particle production mechanisms in heavy-ion collisions. We report on the study of two particle number correlations (R2) and transverse momentum correlations (ΔpTΔpT) in Pb-Pb collisions at = 2.76 TeV and in p-Pb collisions at = 5.02 TeV measured with ALICE detector at the LHC. We study the evolution of both correlation functions with collision centrality and particle multiplicity and find they exhibit a strong dependence on either the centrality in Pb-Pb collisions or the multiplicity in p-Pb collisions. We present studies of pseudorapidity dependent Fourier decompositions of these correlation functions. Given that the details of the collision dynamics may depend on the charge of the particles, we analyze like-sign and unlike-sign particle pairs separately. We combine the correlation functions of like-sign and unlike-sign to obtain a charge independent function. The positive correlation of ΔpTΔpT indicates that both particles from a pair are more likely to have momentum above or below the average transverse momentum in an event ensemble.

The first measurement of the ρ 0 -vector meson elliptic flow v2 at mid-rapidity (|y| < 0.5) in 40 -80 % centrality in Au + Au collisions at √ sNN = 200 GeV from the STAR experiment at RHIC is presented.The study is through the π + π -hadronic decay channel of ρ 0 which has a branching ratio of ∼ 100 %.The analysis is being carried out in two different methods.The v2 results obtained in these methods are consistent.Number of Constituent Quark (NCQ) scaling of v2 of ρ 0 meson with respect to other hadrons at intermediate pT is observed.The ρ 0 v2 favors N CQ = 2 scaling, supporting the coalescence being the dominant mechanism of hadronization in the intermediate pT region at RHIC.

Two-particle correlations provide information about particle production mechanisms in heavy-ion collisions.We report on the study of two particle transverse momentum correlations (∆pT∆pT) in Pb-Pb collisions at √ sNN = 2.76 TeV and in p-Pb collisions at √ sNN = 5.02 TeV measured with the ALICE detector.Unlike the particle number correlation functions, ∆pT∆pT measures transverse momentum correlations between particle pairs.The pT dependence of this correlation measure may make it amenable to probe the fluctuations in temperature, average momentum, flow, hardness of spectrum and hardness of correlations (jet vs non-jets).Correlation functions for ++, --, +-, and -+ charged particle pairs as a function of pair azimuthal and pseudo-rapidity separation (∆ϕ, ∆η) are measured.The information from the above charged pairs are combined into a charge independent (CI) correlation.We study their evolution with collision centrality and particle multiplicity per event.We find that ∆pT∆pT correlation shapes exhibit a strong centrality dependence in Pb-Pb collisions and multiplicity dependence in p-Pb collisions.The correlation function is everywhere positive, indicating that both particles from a pair are more likely to have momentum above or below the average transverse momentum in an event ensemble.We further study the Fourier decomposition of the correlation functions dependence on ∆ϕ as a function of ∆η.

The discovery and characterization of hot and dense QCD matter, known as Quark Gluon Plasma (QGP), remains the most international collaborative effort and synergy between theorists and experimentalists in modern nuclear physics to date. The experimentalists around the world not only collect an unprecedented amount of data in heavy-ion collisions, at Relativistic Heavy Ion Collider (RHIC), at Brookhaven National Laboratory (BNL) in New York, USA, and the Large Hadron Collider (LHC), at CERN in Geneva, Switzerland but also analyze these data to unravel the mystery of this new phase of matter that filled a few microseconds old universe, just after the Big Bang. In the meantime, advancements in theoretical works and computing capability extend our wisdom about the hot-dense QCD matter and its dynamics through mathematical equations. The exchange of ideas between experimentalists and theoreticians is crucial for the progress of our knowledge. The motivation of this first conference named "HOT QCD Matter 2022" is to bring the community together to have a discourse on this topic. In this article, there are 36 sections discussing various topics in the field of relativistic heavy-ion collisions and related phenomena that cover a snapshot of the current experimental observations and theoretical progress. This article begins with the theoretical overview of relativistic spin-hydrodynamics in the presence of the external magnetic field, followed by the Lattice QCD results on heavy quarks in QGP, and finally, it ends with an overview of experiment results.

We presented a study of charge-independent (CI) and charge-dependent (CD) two-particle differential number correlation functions $$\textrm{R}_2$$ and transverse momentum correlation functions $$\textrm{P}_2$$ in pp collisions at $$\sqrt{s} = 2.76$$ TeV with the PYTHIA and HERWIG models. Calculations were presented for unidentified hadrons in three $$p_\textrm{T}$$ ranges $$0.2 < p_\textrm{T} \le 2.0$$ GeV/c, $$2.0 < p_\textrm{T} \le 5.0$$ GeV/c, and $$5.0 < p_\textrm{T} \le 30.0$$ GeV/c. PYTHIA and HERWIG both qualitatively reproduce the near-side peak and away-side ridge correlation features reported by experiments. At low $$p_\textrm{T}$$ , both models produce narrower near-side peaks in $$\textrm{P}_2$$ correlations than in $$\textrm{R}_2$$ as reported by the ALICE collaboration in p–Pb and Pb–Pb collisions. This suggests that the narrower shape of the $$\textrm{P}_2$$ near-side peak is largely determined by the $$p_\textrm{T}$$ dependent angular ordering of hadrons produced in jets. Both PYTHIA and HERWIG predict widths that decrease with increasing $$p_\textrm{T}$$ . Widths extracted for $$\textrm{P}_2$$ correlators are typically significantly narrower than those of the $$\textrm{R}_2$$ counterparts [1].

The observation of a wide variety of physical phenomena in the context of the formation of a strongly interacting QCD matterQCD matter in heavy-ion nuclear collisions at the LHC has drawn significant attention to the high energy heavy-ion physics community. The appearance of a varieties of similar phenomena as in heavy-ion in the high multiplicity proton-proton and proton-nucleus collisions at the LHC energies has triggered further investigation to understand the dynamics of particle production mechanism in a highly dense and small QCD medium. The CMS collaboration uses many different probes in these studies ranging from the particle production cross section to multi-particle correlationsMulti-particle correlations. In this proceeding, I report a few selected recent CMS results from the small systems with the main focus on the measurement of collective phenomena in high multiplicity pp and pPb collisions.

The observation of a wide variety of physical phenomena in the context of the formation of a strongly interacting QCD matter in heavy-ion nuclear collisions at the LHC has drawn significant attention to the high energy heavy-ion physics community. The appearance of a varieties of similar phenomena as in heavy-ion in the high multiplicity proton-proton and proton-nucleus collisions at the LHC energies has triggered further investigation to understand the dynamics of particle production mechanism in a highly dense and small QCD medium. The CMS collaboration uses many different probes in these studies ranging from the particle production cross section to multi-particle correlations. In this proceeding, I report a few selected recent CMS results from the small systems with the main focus on the measurement of collective phenomena in high multiplicity pp and pPb collisions.