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Register a new userAcoustic scaling of linear and mode-coupled anisotropic flow; implications for precision extraction of the specific shear viscosity
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The $mathrm{n^{th}}$-order linear flow coefficients $mathrm{v^L_n , (n=2,3,4,5)}$, and the corresponding nonlinear mode-coupled ($mathrm{mc}$) coefficients $mathrm{v^{mc}_{4,(2,2)}}$, $mathrm{v^{mc}_{5,(2,3)}}$, $mathrm{v^{mc}_{6,(3,3)}}$ and $mathrm{v^{mc}_{6,(2,2,2)}}$, are studied for Pb+Pb collisions at $sqrt{s_{_{rm NN}}} = 2.76$ TeV. Both sets of coefficients indicate a common acoustic scaling pattern of exponential viscous modulation, with a rate proportional to the square of the harmonic numbers and the mean transverse momenta (respectively), and inversely proportional to the cube root of the charged particle multiplicity ($mathrm{(N_{ch})^{1/3}}$), that characterizes the dimensionless size of the systems produced in the collisions. These patterns and their associated scaling parameters, provide new stringent constraints for eccentricity independent estimates of the specific shear viscosity ($eta/s$) and the viscous correction to the thermal distribution function for the matter produced in the collisions. They also give crucial constraints for extraction of the initial-state eccentricity spectrum.
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The specific shear viscosity $bareta$ of a classically rotating system of nucleons that interact via a monopole pairing interaction is calculated including the effects of thermal fluctuations and coupling to pair vibrations within the selfconsistent quasiparticle random-phase approximation. It is found that $bareta$ increases with angular momentum $M$ at a given temperature $T$. In medium and heavy systems, $bareta$ decreases with increasing $T$ at $Tgeq$ 2 MeV and this feature is not affected much by angular momentum. But in lighter systems (with the mass number $Aleq$ 20), $bareta$ increases with $T$ at a value of $M$ close to the maximal value $M_{max}$, which is defined as the limiting angular momentum for each system. The values of $bareta$ obtained within the schematic model as well as for systems with realistic single-particle energies are always larger than the universal lower-bound conjecture $hbar/(4pi k_B)$ up to $T$=5 MeV.
The Chiral Magnetic Wave (CMW) [1] predicts a dependence of the positive and negative particle elliptic flow on the event charge asymmetry. Such a dependence has been observed by the STAR Collaboration [2]. However, it is rather difficult to interpret the results of this measurement, as well as to perform cross-experiment comparisons, due to the dependence of the observable on experimental inefficiencies and the kinematic acceptance used to determine the net asymmetry. We propose another observable that is free from these deficiencies. It also provides possibilities for differential measurements clarifying the interpretation of the results. We use this new observable to study the effect of the local charge conservation that can mimic the effect of the CMW in charge dependent flow measurements.
We calculate the shear viscosity $eta$ and thermal conductivity $kappa$ of a nuclear pasta phase in neutron star crusts. This involves complex non-spherical shapes. We use semiclassical molecular dynamics simulations involving 40,000 to 100,000 nucleons. The viscosity $eta$ can be simply expressed in terms of the height $Z^*$ and width $Delta q$ of the peak in the static structure factor $S_p(q)$. We find that $eta$ increases somewhat, compared to a lower density phase involving spherical nuclei, because $Z^*$ decreases from form factor and ion screening effects. However, we do not find a dramatic increase in $eta$ from non-spherical shapes, as may occur in conventional complex fluids.
We evaluate the viscous damping of anisotropic flow in heavy-ion collisions for arbitrary temperature-dependent shear and bulk viscosities. We show that the damping is solely determined by effective shear and bulk viscosities, which are weighted averages over the temperature. We determine the relevant weights for nucleus-nucleus collisions at $sqrt{s_{rm NN}}=5.02$ TeV and 200 GeV, corresponding to the maximum LHC and RHIC energies, by running ideal and viscous hydrodynamic simulations. The effective shear viscosity is driven by temperatures below $210$ MeV at RHIC, and below $280$ MeV at the LHC, with the largest contributions coming from the lowest temperatures, just above freeze-out. The effective bulk viscosity is driven by somewhat higher temperatures, corresponding to earlier stages of the collision. We show that at a fixed collision energy, the effective viscosity is independent of centrality and system size, to the same extent as the mean transverse momentum of outgoing hadrons. The variation of viscous damping is determined by Reynolds number scaling.
Many features of multiparticle production in ultra-relativistic nuclear collisions reflect the collision geometry and other collision characteristics determining the initial conditions. As the initial conditions affect to a different degree all the particles, it leads to truly multiparticle effects often referred to as anisotropic collective flow. Studying anisotropic flow in nuclear collisions provides unique and invaluable information about the system evolution and the physics of multiparticle production in general. Being not able to cover all aspects of anisotropic flow in one lecture, I decided in the first part of the lecture to discuss briefly a few important and established results, and in the second part, to focus, in a little more detail, on one recent development -- a recent progress in our understanding of the role of fluctuations in the initial conditions. I also discuss some future measurements that might reveal further details of the multiparticle production processes.
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