Flow In Small Systems Research Articles

Symmetric cumulant sc2,4{4} and asymmetric cumulant ac2{3} from transverse momentum conservation and flow

Multiparticle cumulants method can be used to reveal long-range collectivity in small and large colliding systems. The four-particle symmetric cumulant sc2,4{4}, three-particle asymmetric cumulant ac2{3}, and the normalized cumulants nsc2,4{4} and nac2{3} from the transverse momentum conservation and flow are calculated. The interplay between the two effects is also investigated. Our results are in a good agreement with the recent ATLAS measurements of multiparticle azimuthal correlations with the subevent cumulant method, which provides insight into the origin of collective flow in small systems. Published by the American Physical Society 2024

The role of longitudinal decorrelations for measurements of anisotropic flow in small collision systems

Within a (3+1)D viscous hydrodynamic model we compute anisotropic flow in small system collisions as performed at the Relativistic Heavy Ion Collider and measured by the STAR and PHENIX Collaborations. We emphasize the importance of the rapidity dependence of the geometry for interpreting the differences encountered in measurements by the two collaborations.

New theoretical developments on the early-time dynamics and approach to equilibrium in Heavy-Ion collisions

We discuss recent theoretical developments in understanding the early pre-equilibrium dynamics and onset of hydrodynamic behavior in high-energy heavy-ion collisions. We highlight possible experimental signatures of the pre-equilibrium phase, and present recent progress in developing a consistent theoretical description of collective flow in small systems.

Pinning down the origin of collective flow in small systems with ALICE

In this proceeding, the new preliminary results of anisotropic flow in pp, p–Pb, and Pb–Pb collisions measured with ALICE are presented. The highlights include the flow of charged particles, correlations and fluctuations of flow vectors, and correlations between the mean transverse momentum and flow coefficients, ρ(vn2,[pT]). We compare our results to state-of-the-art theoretical models, allowing us to study contributions from initial momentum anisotropies to final anisotropic flow and understand how anisotropic flow is developed from the initial geometry through the system’s dynamic evolution in hadron collisions of different sizes.

Quarkonium production and elliptic flow in small systems measured with ALICE

Abstract. Quarkonium production in hadronic collisions provides a unique testing ground for understanding quantum chromodynamics (QCD) since it involves both perturbative and non-perturbative regimes of this theory. A variety of experimental observables, such as the production cross-section, polarization, particle correlations, serve as insights into the phenomenology of the quarkonium production and help constrain the theoretical models. Measurements of quarkonia as a function of multiplicity probe multiple parton interactions (MPI), i.e., several parton-parton interactions occurring in a single hadron-hadron collision. Finally, measurements of the azimuthal correlation structure of emitted particles in high multiplicity pp collisions can probe the existence of collective behaviour in small systems. The ALICE detector can reconstruct inclusive quarkonia over a broad kinematical range, spanning from mid-rapidity up to forward rapidity, and down to zero transverse momentum. In addition, at midrapidity, the non-prompt charmonium contribution can be separated from the prompt contribution. In these proceedings, we present new results on the inclusive, prompt and non-prompt J/ψ production cross sections measured by ALICE in pp collisions at different collision energies. The self-normalized ψ(2S)-to-J/ψ yield ratio measurement is investigated as a function of the charged particle multiplicity in pp collisions at √S = 13. TeV. Finally, the first measurement of the J/ψ elliptic flow (v2) in high multiplicity pp collisions at √S = 13. TeV is presented. Results will be compared to theoretical models.

Highlights from the PHENIX Collaboration

PHENIX presented several new results at Quark Matter 2019 in 7 parallel talks and 7 posters. These proceedings highlight a sampling of the measurements presented during the conference. PHENIX results on flow in small systems, published in Nature Physics, indicate that the flow in small systems is driven by the initial state geometry. Additional measurements exploring QGP effects in small systems have also been explored. Two-particle correlations in d+Au and 3He+Au suggest potential modification compared to those in p+p collisions. PHENIX has also measured J/Ψ in the forward and backward rapidity regions in d+Au and 3He+Au collisions. Thermal photon measurements are also consistent with QGP formation in small systems. A new measurement of thermal photons using the Au+Au data set collected by PHENIX in 2014 verifies the scaling relationship previously observed for thermal photons. New results for a variety of hadron species with increasing precsion are now available and provide both insight into hadronization as well as crucial information for energy loss models that use RAA to tune their parameters. Jet modification is also studied via a new observable extracted from π0-hadron correlations in Au+Au collisions.

Flow in collisions of light nuclei

In this talk we discuss several interesting aspects of collective flow in small systems in reference to planned future experiments. First, we bring in the novel possibility of exploring elliptic flow with heavy nuclei collisions on polarized deuteron targets, accessible in AFTER@LHC program after Long Shutdown 3. Then we discuss the Glauber model predictions for the presently considered 16O −16O program at RHIC and the LHC. Finally, we turn to our previous proposal concerning specific flow signatures of intrinsic correlations (such as the predicted α clusterization) in the structure of light nuclei in collisions with heavy nuclei. In particular, 12C–heavy nucleus ultrarelativistic reactions would provide insight into the ground-state 12C structure, complementary to traditional nuclear structure measures.

Signatures of collectivity and flow in small systems observed by PHENIX

A wide variety of recent measurements from small systems indicate that there are collective flow phenomena in these systems, which are well described by hydrodynamics. However, it is widely understood that all flow measurements are affected by nonflow correlations. In order to disentangle the genuine hydrodynamical flow from the nonflow contributions, we have systematically studied the v2 as a function of pT and η in p+Au and d+Au collisions at sNN=200GeV by comparing several nonflow subtraction methods and by combining different detectors. We have also measured 4-particle cumulants in p+Au at sNN=200GeV with subevents, which kinematically suppress nonflow via removal of short-range correlations.

Hydrodynamic flow in small systems or: “How the heck is it possible that a system emitting only a dozen particles can be described by fluid dynamics?”

The “unreasonable effectiveness” of relativistic fluid dynamics in describing high energy heavy-ion and even proton-proton collisions will be demonstrated and discussed. Several recent ideas of optimizing relativistic fluid dynamics for the specific challenges posed by such collisions will be presented, and some thoughts will be offered why the framework works better than originally expected. I will also address the unresolved question where exactly hydrodynamics breaks down, and why.

Elliptic flow in small systems due to elliptic gluon distributions?

We investigate the contributions from the so-called elliptic gluon Wigner distributions to the rapidity and azimuthal correlations of particles produced in high energy pp and pA collisions by applying the double parton scattering mechanism. We compute the ‘elliptic flow’ parameter v2 as a function of the transverse momentum and rapidity, and find qualitative agreement with experimental observations. This shall encourage further developments with more rigorous studies of the elliptic gluon distributions and their applications in hard scattering processes in pp and pA collisions.

Small systems – hydrodynamics

The scenario assuming a collective expansion stage in collisions of small systems, p-A, d-Au, and 3He-Au is discussed. A review of the observables predicted in relativistic hydrodynamic models in comparison with experimental data is presented, with arguments indicating the presence of collective expansion. The limits of applicability of the hydrodynamic model are addressed. We briefly indicate possible applications of the collective flow in small systems to study the space-time dynamics at very small scales in relativistic collisions.

Flow in small systems from parton scatterings

We show that the incoherent elastic scattering of partons, as present in a multi-phase transport model (AMPT), with a modest parton-parton cross-section of σ=1.5–3 mb, naturally explains the long-range two-particle azimuthal correlations as observed in p+Pb collisions at the LHC. The elliptic, v2, and triangular, v3, Fourier coefficients are in good agreement with the CMS data. An escape mechanism was proposed recently to explain the AMPT results. We demonstrate that parton scatterings are directly responsible for generating the azimuthal anisotropy.

Relativistic Hydrodynamics in Heavy-Ion Collisions: General Aspects and Recent Developments

Relativistic hydrodynamics has been quite successful in explaining the collective behaviour of the QCD matter produced in high energy heavy-ion collisions at RHIC and LHC. We briefly review the latest developments in the hydrodynamical modeling of relativistic heavy-ion collisions. Essential ingredients of the model such as the hydrodynamic evolution equations, dissipation, initial conditions, equation of state, and freeze-out process are reviewed. We discuss observable quantities such as particle spectra and anisotropic flow and effect of viscosity on these observables. Recent developments such as event-by-event fluctuations, flow in small systems (proton-proton and proton-nucleus collisions), flow in ultracentral collisions, longitudinal fluctuations, and correlations and flow in intense magnetic field are also discussed.

Mean [formula omitted] scaling with [formula omitted] at the LHC: Absence of (hydro) flow in small systems?

In this work, a study of the average transverse momentum (pT) as a function of the mid-rapidity charged hadron multiplicity (Nch) and hadron mass (m) in p–Pb and Pb–Pb collisions at LHC energies is presented. For the events producing low Nch, the average pT is found to scale with the reduced hadron mass, i.e., mass divided by the number of quark constituents (m/nq), this scaling also holds for inelastic pp collisions at RHIC and LHC energies. The scaling is broken in high multiplicity p–Pb and Pb–Pb collisions, where, for 〈dNch/dη〉≲60 the average pT is higher for baryons than that for mesons, though they increase linearly with m/nq. This behavior is qualitatively well reproduced by Pythia 8, but not by hydro calculations, where an universal scaling with the hadron mass (and not with m/nq) is predicted for all the multiplicity event classes. Only the central (0–60%) Pb–Pb collisions behave as expected from hydro.

Collective flow in small systems

The large density of matter in the interaction region of the proton-nucleus or deuteron-nucleus collisions enables the collective expansion of the fireball. Predictions of the hydrodynamic model for the asymmetric transverse flow are presented and compared to experimental data.

Identified particle production in p–Pb collisions measured with the ALICE detector

ALICE has unique capabilities among the LHC experiments for particle identification (PID) at mid-rapidity ($|\eta| < 0.9$) over a wide range of transverse momentum ($p_{\rm T}$). In this proceeding recent measurements of $p_{\rm T}$ spectra for $\pi$, K, ${\rm K}^0_s$, p, and $\Lambda$ in p-Pb collisions are presented and compared to results from Pb-Pb and pp. In particular the implications for the question of the existence of radial flow in small systems is discussed.

On the probability of violations of Fourier’s law for heat flow in small systems observed for short times

We study the statistical mechanics of thermal conduction in a classical many-body system that is in contact with two thermal reservoirs maintained at different temperatures. The ratio of the probabilities, that when observed for a finite time, the time averaged heat flux flows in and against the direction required by Fourier's Law for heat flow, is derived from first principles. This result is obtained using the transient fluctuation theorem. We show that the argument of that theorem, namely, the dissipation function is, close to equilibrium, equal to a microscopic expression for the entropy production. We also prove that if transient time correlation functions of smooth zero mean variables decay to zero at long times, the system will relax to a unique nonequilibrium steady state, and for this state, the thermal conductivity must be positive. Our expressions are tested using nonequilibrium molecular dynamics simulations of heat flow between thermostated walls.