Azimuthal anisotropies in p+Pb collisions from classical Yang-Mills dynamics

We compute single and double inclusive gluon distributions in classical Yang-Mills simulations of proton-lead collisions and extract the associated transverse momentum dependent Fourier harmonics $v_2(p_T)$ and $v_3(p_T)$. Gluons have a large $v_2$ in the initial state, while odd harmonics such as $v_3$ vanish identically at the initial time $\tau=0^{+}$. By the time $\tau \lesssim 0.4\,{\rm fm/c}$ final state effects in the classical Yang-Mills evolution generate a non-zero $v_{3}$ and only mildly modify the gluon $v_{2}$. Unlike hydrodynamic flow, these momentum space anisotropies are uncorrelated with the global spatial anisotropy of the collision. A principal ingredient for the generation of $v_2$ and $v_3$ in this framework is the event-by-event breaking of rotational invariance in domains the size of the inverse of the saturation scale $Q_s$. In contrast to our findings in p+Pb collisions Yang-Mills simulations of lead-lead collisions generate much smaller values of $v_{2,3} (p_T)$ and additional collective flow effects are needed to explain experimental data. This is because the locally generated anisotropy due to the breaking of rotational invariance is depleted with the increase in the number of uncorrelated domains.Comment: Minor modifications; Version to appear in Phys. Lett.

Over-populated gauge fields on the lattice

We study nonequilibrium dynamics of SU(2) pure gauge theory starting from initial over-population, where intense classical gauge fields are characterized by a single momentum scale Q_s. Classical-statistical lattice simulations indicate a quick evolution towards an approximate scaling behavior with exponent 3/2 at intermediate times. Remarkably, the value for the scaling exponent may be understood as arising from the leading O(g^2) contribution in the presence of a time-dependent background field. The phenomenon is associated to weak wave turbulence describing an energy cascade towards higher momenta. This particular aspect is very similar to what is observed for scalar theories, where an effective cubic interaction arises because of the presence of a time-dependent Bose condensate.Comment: 15 pages, 4 figure

Chiral instabilities & the fate of chirality imbalance in non-Abelian plasmas

We present a first principles study of chiral plasma instabilities and axial charge transfer in non-Abelian plasmas with a strong gauge-matter coupling $g^2N_f=64$, by performing $3+1$ D real-time classical-statistical lattice simulation with dynamical fermions. We explicitly demonstrate for the first time that -- unlike in an Abelian plasma -- the transfer of chirality from the matter sector to the gauge fields occurs predominantly due to topological sphaleron transitions. We elaborate on the similarities and differences of the axial charge dynamics in cold Abelian $U(1)$ and non-Abelian $SU(2)$ plasmas, and comment on the implications of our findings for the study of anomalous transport phenomena, such as the Chiral Magnetic Effect in QCD matter.Comment: 7 pages, 3 figure

Hydrodynamic and Non-hydrodynamic Excitations in Kinetic Theory -- A Numerical Analysis in Scalar Field Theory

Viscous hydrodynamics serves as a successful mesoscopic description of the Quark-Gluon Plasma produced in relativistic heavy-ion collisions. In order to investigate, how such an effective description emerges from the underlying microscopic dynamics we calculate the hydrodynamic and non-hydrodynamic modes of linear response in the sound channel from a first-principle calculation in kinetic theory. We do this with a new approach wherein we discretize the collision kernel to directly calculate eigenvalues and eigenmodes of the evolution operator. This allows us to study the Green's functions at any point in the complex frequency space. Our study focuses on scalar theory with quartic interaction and we find that the analytic structure of Green's functions in the complex plane is far more complicated than just poles or cuts which is a first step towards an equivalent study in QCD kinetic theory.Comment: 30 pages and 8 figures. v2 changes: Added a more nuanced discussion about hydrodynamization in introduction. Clarified the potential differences between the QFT scalar \phi^4 theory and kinetic theory derived from it. Added some additional discussion of a hydrodynamic breakdown scale k_c to the conclusion of the paper. Fixed some typo

Sphaleron damping and effects on vector and axial charge transport in high-temperature QCD plasmas

We modify the anomalous hydrodynamic equations of motion to account for dissipative effects due to QCD sphaleron transitions. By investigating the linearized hydrodynamic equations, we show that sphaleron transitions lead to nontrivial effects on vector and axial charge transport phenomena in the presence of a magnetic field. Due to the dissipative effects of sphaleron transitions, a wavenumber threshold $k_{\rm CMW}$ emerges characterizing the onset of chiral magnetic waves. Sphaleron damping also significantly impacts the time evolution of both axial and vector charge perturbations in a QCD plasma in the presence of a magnetic field. Based on our analysis of the linearized hydrodynamic equations, we also investigate the dependence of the vector charge separation on the sphaleron transition rate, which may have implications for the experimental search for the Chiral Magnetic Effect in Heavy Ion Collisions.Comment: 19 pages, 9 figure

Collectivity in small systems - Initial state vs. final state effects

Observations of long rang azimuthal correlations in small collision systems (p+p/A) have triggered an enormous excitement in the heavy-ion community. However, it is presently unclear to what extent the experimentally observed correlations should be attributed to initial state momentum correlations and/or the final state response to the initial state geometry. We discuss how a consistent theoretical description of the non-equilibrium dynamics is important to address both effects within a unified framework and present first results from weakly coupled non-equilibrium simulations in \cite{Greif:2017bnr} to quantify the relative importance of initial state and final state effects based on theoretical calculations.Comment: 7 pages, 3 figures; Proceedings of the XLVII International Symposium on Multiparticle Dynamics, Tlaxcala City, Mexico, September 11--15, 2017; to be published in EPJ-Conference