Periodic Reporting for period 4 - QGP-MYSTERY (Demystifying the Quark-Gluon Plasma)
Période du rapport: 2022-07-01 au 2024-02-29
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 a 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, which primarily have been carried out with the two- and multi-particle correlation techniques. The collective dynamics of QGP is sensitive to shear viscosity, bulk viscosity and entropy density. The overall success of hydrodynamic models in describing the heavy-ion data implied that the shear viscosity to entropy density ratio of QGP is lower than that of any other substance 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 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.
The project has yielded 13 publications in peer-reviewed journals: 9 theoretical and 4 experimental papers for ALICE Collaboration at the LHC.
In the first theoretical paper (arXiv:1901.06968) a new paradigm was introduced, according to which correlations among anisotropic 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 approach, new observables dubbed higher-order symmetric cumulants (SC) were developed for the studies of correlated fluctuations of different anisotropic flow amplitudes.
Symmetry plane correlations (SPC) are important observables used to quantify anisotropic flow phenomenon and independently constrain the properties of nuclear matter at the highest energies. In arXiv:2004.01066 a new estimator for symmetry plane correlations in anisotropic flow analyses was introduced, which, unlike any previous estimator, can probe the true symmetry plane correlations (i.e. without any systematic biases from flow amplitudes).
In the technical paper (arXiv:2101.05619) for the first time, the strict mathematical formalism of multivariate cumulants was reconciled with the usage of cumulants in anisotropic flow analyses in high-energy nuclear collisions. It was demonstrated that properties of cumulants are preserved only for the stochastic variables on which the cumulant expansion has been performed directly, and if there are no underlying symmetries due to which some terms in the cumulant expansion are identically zero. The new estimators introduced in this paper are: event-by-event cumulants of azimuthal angles, asymmetric cumulants of flow amplitudes, and cumulants of symmetry plane correlations.
In arXiv:2106.05760 the first analytic solutions for the contribution of combinatorial background in the measured 2- and 3-particle correlations were obtained. It was demonstrated that these solutions for the combinatorial background are universal as they can be written generically in terms of multiplicity-dependent combinatorial weights and marginal probability density functions of starting multivariate distribution.
In parallel to the development and measurements of new flow observables, techniques like Bayesian optimization, where the model parameters are systematically varied to find the best fit for all selected experimental measurements, have been applied. In arXiv:2111.08145 it was demonstrated that uncertainties in extracted parameters from Bayesian studies can be reduced by increasing the scope of the input experimental data, either from various beam-energy ranges or from additional independent observables sensitive to the initial conditions and system properties.
In the remaining theoretical papers, group members have contributed to hydrodynamics studies of nuclear matter (arXiv:1901.0696 arXiv:1907.12140 arXiv:2005.04742) and to the development of multiparticle cumulants for femtoscopy (arXiv:2107.10227).
In the first experimental paper (arXiv:2101.02579) multiharmonic correlations of different flow amplitudes using higher-order symmetric cumulants (arXiv:1901.0696) were published for ALICE Collaboration. As the most important new result, it was found that genuine three-harmonic correlations develop during the collective evolution of the medium, when compared to the corresponding correlations that exist in the initial state. These results for higher-order SC provide new and independent constraints on the non-linear response in flow amplitudes, which for the first time does not require any assumption in the derivation on the nature of lower-order two-harmonic SC.
In arXiv:2302.01234 symmetry plane correlations were measured by ALICE Collaboration, using the new and improved estimator developed in arXiv:2004.01066. The results obtained are strikingly different for some observables when compared to previous measurements, in the sense that their nonvanishing signal in previous measurements can be attributed to the systematic bias, which the new estimator can remove.
The success of the SC observables has prompted a natural quest towards further generalizing the SC to a new and independent set of multiharmonic observables involving different moments of the flow amplitudes. One example of new higher-order estimators are the asymmetric cumulants (AC) (arXiv:2101.05619). In arXiv:2303.13414 the first measurements of the AC observables, in their simplest realization that involves two flow amplitudes, evaluated for three different pairs of harmonics, were published. By utilizing Bayesian studies, it was demonstrated that higher-order cumulants of flow amplitudes exhibit better sensitivity to QGP properties when compared to the traditional lower-order flow observables.
The group members have contributed in arXiv:2206.03344 to the first measurements by ALICE Collaboration of three-body interaction in the p–p–p and p–p–Λ systems using the formalism of multivariate cumulants. Three-body nuclear forces play an important role in the structure of nuclei and hypernuclei and are also incorporated in models to describe the dynamics of dense baryonic matter, such as in neutron stars.
Finally, the first feasibility studies for the usage of multiparticle correlations in flow analysis in the future CBM Collaboration at GSI were performed.
In the first theoretical paper (arXiv:1901.06968) a new paradigm was introduced, according to which correlations among anisotropic 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 approach, new observables dubbed higher-order symmetric cumulants (SC) were developed for the studies of correlated fluctuations of different anisotropic flow amplitudes.
Symmetry plane correlations (SPC) are important observables used to quantify anisotropic flow phenomenon and independently constrain the properties of nuclear matter at the highest energies. In arXiv:2004.01066 a new estimator for symmetry plane correlations in anisotropic flow analyses was introduced, which, unlike any previous estimator, can probe the true symmetry plane correlations (i.e. without any systematic biases from flow amplitudes).
In the technical paper (arXiv:2101.05619) for the first time, the strict mathematical formalism of multivariate cumulants was reconciled with the usage of cumulants in anisotropic flow analyses in high-energy nuclear collisions. It was demonstrated that properties of cumulants are preserved only for the stochastic variables on which the cumulant expansion has been performed directly, and if there are no underlying symmetries due to which some terms in the cumulant expansion are identically zero. The new estimators introduced in this paper are: event-by-event cumulants of azimuthal angles, asymmetric cumulants of flow amplitudes, and cumulants of symmetry plane correlations.
In arXiv:2106.05760 the first analytic solutions for the contribution of combinatorial background in the measured 2- and 3-particle correlations were obtained. It was demonstrated that these solutions for the combinatorial background are universal as they can be written generically in terms of multiplicity-dependent combinatorial weights and marginal probability density functions of starting multivariate distribution.
In parallel to the development and measurements of new flow observables, techniques like Bayesian optimization, where the model parameters are systematically varied to find the best fit for all selected experimental measurements, have been applied. In arXiv:2111.08145 it was demonstrated that uncertainties in extracted parameters from Bayesian studies can be reduced by increasing the scope of the input experimental data, either from various beam-energy ranges or from additional independent observables sensitive to the initial conditions and system properties.
In the remaining theoretical papers, group members have contributed to hydrodynamics studies of nuclear matter (arXiv:1901.0696 arXiv:1907.12140 arXiv:2005.04742) and to the development of multiparticle cumulants for femtoscopy (arXiv:2107.10227).
In the first experimental paper (arXiv:2101.02579) multiharmonic correlations of different flow amplitudes using higher-order symmetric cumulants (arXiv:1901.0696) were published for ALICE Collaboration. As the most important new result, it was found that genuine three-harmonic correlations develop during the collective evolution of the medium, when compared to the corresponding correlations that exist in the initial state. These results for higher-order SC provide new and independent constraints on the non-linear response in flow amplitudes, which for the first time does not require any assumption in the derivation on the nature of lower-order two-harmonic SC.
In arXiv:2302.01234 symmetry plane correlations were measured by ALICE Collaboration, using the new and improved estimator developed in arXiv:2004.01066. The results obtained are strikingly different for some observables when compared to previous measurements, in the sense that their nonvanishing signal in previous measurements can be attributed to the systematic bias, which the new estimator can remove.
The success of the SC observables has prompted a natural quest towards further generalizing the SC to a new and independent set of multiharmonic observables involving different moments of the flow amplitudes. One example of new higher-order estimators are the asymmetric cumulants (AC) (arXiv:2101.05619). In arXiv:2303.13414 the first measurements of the AC observables, in their simplest realization that involves two flow amplitudes, evaluated for three different pairs of harmonics, were published. By utilizing Bayesian studies, it was demonstrated that higher-order cumulants of flow amplitudes exhibit better sensitivity to QGP properties when compared to the traditional lower-order flow observables.
The group members have contributed in arXiv:2206.03344 to the first measurements by ALICE Collaboration of three-body interaction in the p–p–p and p–p–Λ systems using the formalism of multivariate cumulants. Three-body nuclear forces play an important role in the structure of nuclei and hypernuclei and are also incorporated in models to describe the dynamics of dense baryonic matter, such as in neutron stars.
Finally, the first feasibility studies for the usage of multiparticle correlations in flow analysis in the future CBM Collaboration at GSI were performed.
1. A new paradigm was introduced for the usage of multivariate cumulants in flow analyses. It was demonstrated that correlations among flow amplitudes can be studied reliably with the mathematical formalism of cumulants only if it is applied directly to the flow amplitudes, and not to azimuthal angles of reconstructed particles;
2. The first estimator for the true symmetry plane correlations was developed, free of systematic biases originating from correlations of flow amplitudes;
3. New flow observables developed and introduced in the field: higher-order symmetric cumulants, asymmetric cumulants of flow amplitudes, event-by-event cumulants of azimuthal angles, cumulants of symmetry plane correlations;
4. First analytic solution for combinatorial background in 2- and 3-particle correlations.
2. The first estimator for the true symmetry plane correlations was developed, free of systematic biases originating from correlations of flow amplitudes;
3. New flow observables developed and introduced in the field: higher-order symmetric cumulants, asymmetric cumulants of flow amplitudes, event-by-event cumulants of azimuthal angles, cumulants of symmetry plane correlations;
4. First analytic solution for combinatorial background in 2- and 3-particle correlations.