This report summarizes the presentations and discussions during the Rapid Reaction Task Force “Dynamics of critical fluctuations: Theory – phenomenology – heavy-ion collisions”, which was organized ...by the ExtreMe Matter Institute EMMI and held at GSI, Darmstadt, Germany in April 2019. We address the current understanding of the dynamics of critical fluctuations in QCD and their measurement in heavy-ion collision experiments. In addition, we outline what might be learned from studying correlations in other physical systems, such as cold atomic gases.
Recent theoretical explanations for how hydrodynamic-like flow can build up quickly in small collision systems (hydrodynamization) has led to a microscopic picture of flow building up in a ...gluon-dominated phase before chemical equilibrium between quarks and gluons has been attained. The goal of this contribution to Offshell-2021 is to explore consequence of assuming a long-lived gluon-dominated phase, which we shall denote a gluon plasma (GP). As these consequences are naturally enhanced in a large systems, we assume and explore the extreme scenario in which a GP would be created in AA collisions and exist for significant time before the formation of a chemically-equilibrated quark-gluon plasma (QGP). The GP and its formation would be impossible to probe with light-quark hadrons, which are first produced later in this scenario. As charm quarks are produced early in the collision, they can circumvent the limitations of light quarks and we propose charm balance functions as an effective tool to test this idea and constrain the dynamics of the GP.
Measurements made by the ALICE Collaboration of single- and two-particle distributions in high-energy pp and p–Pb collisions are used to characterize the interactions in small collision systems, tune ...models of particle production in QCD, and serve as a baseline for heavy-ion observables. The measurements of charged-particle multiplicity density, (dNch / dη), and multiplicity distributions are shown in pp and p–Pb collisions, including data from the top center-of-mass energy achieved at the Large Hadron Collider (LHC), √S = 13 TeV. Two-particle angular correlations in p–Pb collisions are studied in detail to investigate long-range correlations in pseudorapidity which are reminiscent of structures previously thought unique to heavy-ion collisions.
The presence of correlations between particles significantly separated in pseudorapidity in proton-proton and proton-nucleus collisions has raised questions about whether collective effects are ...observed in small collision systems as well as in heavy-ion collisions. The quantification of these long-range correlations by vn coefficients is of particular interest. A selection of the latest vn measurements is presented, including results from the recent d+Au beam energy scan at RHIC where a significant non-zero v2 is measured down to low center-of-mass energies (sNN=39GeV). Results from a collision system scan – comprising p+Au, d+Au, and 3He+Au collisions – are also shown to address the role of the initial nuclear geometry in the final state anisotropy. Finally, the challenge of measuring multi-particle cumulants, particularly c2{4}, in p+p collisions is discussed, and new methods for reducing the effects of non-flow are shown to produce a more robust measurement of v2{4} in p+p collisions.
The discovery of correlations between particles separated by several units of pseudorapidity in high-multiplicity pp and p-Pb collisions, reminiscent of structures observed in Pb-Pb collisions, was a ...challenge to traditional ideas about collectivity in heavy ion collisions. In order to further explore long-range correlations and provide information to theoretical models, correlations between forward trigger muons and mid-rapidity associated hadrons were measured in p-Pb collisions at √sNN = 5.02 TeV. The results demonstrate that the nearside and awayside ridges extend to Δη ∼ ±5 and that the v2 of muons, obtained from subtracting the correlation functions in high- and low-multiplicity events, is (16 ± 6)% higher in the Pb-going than in the p-going direction. The results are compared with AMPT simulations.
The propagation and modification of hard-scattered partons in the QGP can be studied using various types of jet and jet-like correlation measurements. The STAR detector with its full azimuthal and ...large pseudorapidity acceptance, as well as its wide transverse momentum (pT) coverage, is well-suited for these measurements. At mid-rapidity, azimuthal correlations of charged hadrons with the axis of a reconstructed trigger jet are used to study the modification of jet shapes and associated hadron yields from p+p to Au+Au. Dihadron correlations with back-to-back high-pT hadron pairs are used to investigate dijets and fragmentation biases. STARʼs increased particle identification capabilities due to the Time-Of-Flight detector are utilized to investigate the differences between jet-related and bulk-related particle production. Dihadron correlations with identified trigger particles provide experimental tests of simple recombination theories. The comprehensive set of STAR jet-quenching measurements can be used to further constrain theories of parton energy loss at RHIC.
The relationship between jet properties and the underlying geometry of the medium produced in heavy ion collisions can be explored through a measurement of the correlation between the axes of ...reconstructed jets and the reaction plane, defined as “jet v2”. Such a measurement provides information on the pathlength dependence of medium-induced parton energy loss and may also be used to assess biases in jet-finding methods.
We present first measurements of jet v2 in sNN=200 GeV Au + Au collisions in the STAR experiment at RHIC. In order to reduce the artificial jet – event plane bias, which results from jet fragments being included in the event plane calculation, detectors at forward pseudorapidity are used to determine the event plane when measuring the v2 of reconstructed jets at mid-rapidity. These measurements demonstrate a non-zero jet v2, which is indicative of pathlength-dependent parton energy loss.
Jet-hadron correlations in STAR Ohlson, Alice
Journal of physics. Conference series,
09/2011, Letnik:
316, Številka:
1
Journal Article
Recenzirano
Odprti dostop
In recent years, the study of dihadron correlations has been one of the primary methods used to investigate the propagation and modification of hard-scattered partons through the QGP. Due to recent ...advances in jet-finding algorithms, it is now possible to use reconstructed jets in these correlation studies, extending the kinematic reach compared to dihadron analyses. The results of the jet-hadron correlation analysis indicate a broadening and softening of jets that interact with the medium. Jet-hadron correlations can also be used to assess the systematics of other jet-like correlation analyses, such as 2+1 correlations. It is shown that the jets selected in 2+1 correlations are relatively unmodified. Future work will include an analysis of jet-hadron correlations with respect to the event plane to measure the pathlength dependence of parton energy loss. The first steps in this analysis indicate that complications arise when calculating the event plane in the presence of a jet as well as in calculating jet v2. The data analyzed were collected by the STAR detector in 200 GeV Au-Au collisions at the Relativistic Heavy Ion Collider (RHIC).
Event-by-event fluctuations of conserved charges – such as electric charge, strangeness, and baryon number – in ultrarelativistic heavy-ion collisions provide insight into the properties of the ...quark-gluon plasma and the QCD phase diagram. They can be related to the higher moments of the multiplicity distributions of identified particles, such as the Λ baryon which carries both strangeness and baryon number and is thus of particular interest. We present the first measurement of net-Λ fluctuations in Pb–Pb collisions at sNN=5.02TeV as a function of centrality and the pseudorapidity acceptance of the measurement. The results are compared to expectations of the effects of global baryon number conservation as well as to predictions from the HIJING Monte Carlo event generator. In this analysis the Identity Method is applied in a novel way to account for the combinatoric background in the invariant mass distribution.
PDF
