Periodic Reporting for period 3 - QGP-MYSTERY (Demystifying the Quark-Gluon Plasma)
Période du rapport: 2021-01-01 au 2022-06-30
In the explorations of the phase diagram of strong nuclear force, some of the most intriguing questions are associated with the phase dubbed Quark-Gluon Plasma (QGP), in which under large temperatures and/or baryon densities quarks and gluons are deconfined. Properties of this extreme state of matter have been investigated with the plethora of different observables and across different collision systems. One of the most important programs in these studies is the analyses of the anisotropic flow phenomenon, which primarily have been carried out with the two- and multi-particle correlation techniques. When two heavy ions collide at ultrarelativistic energies a very rich and non-trivial sequence of stages emerges in the evolution of the produced fireball. Since each of these stages typically involves different underlying physics, ideally they would be described separately in theoretical models and probed one at a time in an experiment. To date, however, most of the analyzed heavy-ion observables are final-state observables in the momentum space, which pick up cumulatively the contributions from all stages in the heavy-ion evolution, starting all the way down from the details of the initial collision geometry. To leading order, these stages can be divided into the following categories: initial conditions, deconfined QGP stage, hadronization, chemical freeze-out, rescatterings, kinetic freeze-out, and finally free streaming. An important program in the field is the development of new observables which would be sensitive only to one particular stage at a time in the heavy-ion evolution.
The collective dynamics of QGP is sensitive to shear viscosity, bulk viscosity and entropy density. The overall success of hydrodynamic models to describe the heavy-ion data implied that shear viscosity to entropy density ratio of QGP is lower than that of any other liquid found in nature. This conclusion established the perfect liquid paradigm of QGP properties, which is one of the most striking recent discoveries in high-energy physics. One of the most important remaining open questions in the field is the location of the transition temperature between the QGP phase and ordinary matter in the phase diagram of the strong nuclear force. Due to collision geometry, in mid-central heavy-ion collisions the resulting overlapping volume is to leading order ellipsoidal. The isotropic fluctuations which change the volume size of such ellipsoidal are mostly sensitive to the bulk viscosity, while the fluctuations of its shape to the shear viscosity. Theoretical calculations indicate that the observation of non-vanishing values of bulk viscosity to entropy density ratio can pinpoint the location of the transition temperature. Since in heavy-ion collisions shear viscous effects dominate over bulk viscous ones by a factor of approximately 3 to 10, it is becoming increasingly important to study observables which can separate their contributions, or equivalently, to separate the fluctuations which change only the volume size from the ones which affect only the volume shape.
The collective dynamics of QGP is sensitive to shear viscosity, bulk viscosity and entropy density. The overall success of hydrodynamic models to describe the heavy-ion data implied that shear viscosity to entropy density ratio of QGP is lower than that of any other liquid found in nature. This conclusion established the perfect liquid paradigm of QGP properties, which is one of the most striking recent discoveries in high-energy physics. One of the most important remaining open questions in the field is the location of the transition temperature between the QGP phase and ordinary matter in the phase diagram of the strong nuclear force. Due to collision geometry, in mid-central heavy-ion collisions the resulting overlapping volume is to leading order ellipsoidal. The isotropic fluctuations which change the volume size of such ellipsoidal are mostly sensitive to the bulk viscosity, while the fluctuations of its shape to the shear viscosity. Theoretical calculations indicate that the observation of non-vanishing values of bulk viscosity to entropy density ratio can pinpoint the location of the transition temperature. Since in heavy-ion collisions shear viscous effects dominate over bulk viscous ones by a factor of approximately 3 to 10, it is becoming increasingly important to study observables which can separate their contributions, or equivalently, to separate the fluctuations which change only the volume size from the ones which affect only the volume shape.
The project has yielded so far two publications in peer-reviewed journals. In arXiv:1901.06968 we have introduced a new paradigm in the field, according to which correlations among flow amplitudes can be studied reliably with the general mathematical formalism of cumulants only if that formalism is applied directly on the flow amplitudes, and not on azimuthal angles, as it was done in all publications by now. Based on this novel approach, we have introduced in the field new observables for the studies of correlated fluctuations of different anisotropic flow amplitudes, dubbed higher-order symmetric cumulants. These new observables have the potential to separate for the first time the effects of bulk and shear viscosities, and these studies are currently underway.
In arXiv:2004.01066 we have introduced the new estimator for symmetry plane correlations in anisotropic flow analyses, which unlike any previous estimator can probe the true symmetry plane correlations (i.e. without any systematic biases from flow amplitudes). By separating the symmetry planes from the flow amplitudes, we might be able to independently constrain the effects of shear and bulk viscosity separately, and further studies are currently ongoing.
In arXiv:2004.01066 we have introduced the new estimator for symmetry plane correlations in anisotropic flow analyses, which unlike any previous estimator can probe the true symmetry plane correlations (i.e. without any systematic biases from flow amplitudes). By separating the symmetry planes from the flow amplitudes, we might be able to independently constrain the effects of shear and bulk viscosity separately, and further studies are currently ongoing.
The most important new results which go beyond the state of the art are summarized next.
In arXiv:1901.06968 we have introduced the shift of paradigm in the usage of multi-variate cumulants in flow analyses. Contrary to the common belief, we have demonstrated for the first time that correlations among flow amplitudes can be studied reliably with the general mathematical formalism of cumulants only if that formalism is applied directly to the flow amplitudes, and not to the azimuthal angles. This work was accepted for publication in Physical Review C.
In arXiv:2004.01066 we have developed the first estimator for the true symmetry plane correlations. The currently available observables in the field cannot separate the flow magnitudes and symmetry planes, and by overcoming these limitations this work paves the road for separating the effects of bulk and shear viscosities, in the studies of Quark-Gluon Plasma properties. This work was accepted for publication in Physical Review C.
Until the end of the project, we expect that recently submitted arXiv:2007.06851 arXiv:2005.04742 and arXiv:1907.12140 will be accepted for publication as well. Next, based on the new paradigm introduced in arXiv:1901.06968 we have written an experimental paper for ALICE Collaboration (the paper is written and as of June 3rd, 2020 under internal review within ALICE Collaboration) and we expect to submit it to Physical Review Letters by the end of 2020.
In the fall of 2020, the new experimental paper based on theoretical work in arXiv:2004.01066 will be initiated within ALICE Collaboration. Feasibility study for the usage of multi-particle correlations within CBM Collaboration is well advanced and will be published by the end of the project. An interdisciplinary project on the usage of multi-particle correlations in the femtoscopic studies is currently underway, and it is expected that by the end of the project this undertaking will yield both theoretical and experimental publication.
In arXiv:1901.06968 we have introduced the shift of paradigm in the usage of multi-variate cumulants in flow analyses. Contrary to the common belief, we have demonstrated for the first time that correlations among flow amplitudes can be studied reliably with the general mathematical formalism of cumulants only if that formalism is applied directly to the flow amplitudes, and not to the azimuthal angles. This work was accepted for publication in Physical Review C.
In arXiv:2004.01066 we have developed the first estimator for the true symmetry plane correlations. The currently available observables in the field cannot separate the flow magnitudes and symmetry planes, and by overcoming these limitations this work paves the road for separating the effects of bulk and shear viscosities, in the studies of Quark-Gluon Plasma properties. This work was accepted for publication in Physical Review C.
Until the end of the project, we expect that recently submitted arXiv:2007.06851 arXiv:2005.04742 and arXiv:1907.12140 will be accepted for publication as well. Next, based on the new paradigm introduced in arXiv:1901.06968 we have written an experimental paper for ALICE Collaboration (the paper is written and as of June 3rd, 2020 under internal review within ALICE Collaboration) and we expect to submit it to Physical Review Letters by the end of 2020.
In the fall of 2020, the new experimental paper based on theoretical work in arXiv:2004.01066 will be initiated within ALICE Collaboration. Feasibility study for the usage of multi-particle correlations within CBM Collaboration is well advanced and will be published by the end of the project. An interdisciplinary project on the usage of multi-particle correlations in the femtoscopic studies is currently underway, and it is expected that by the end of the project this undertaking will yield both theoretical and experimental publication.