Strongly Interacting Matter Under Rotation: An Introduction

Abstract

Ultrarelativistic collisions between heavy nuclei briefly generate the quark-gluon plasma (QGP), a new state of matter characterized by deconfined partons last seen microseconds after the Big Bang. The properties of the QGP are of intense interest, and a large community has developed over several decades, to produce, measure and understand this primordial plasma. The plasma is now recognized to be a strongly-coupled fluid with remarkable properties, and hydrodynamics is commonly used to quantify and model the system. An important feature of any fluid is its vorticity, related to the local angular momentum density; however, this degree of freedom has received relatively little attention because no experimental signals of vorticity had been detected. Thanks to recent high-statistics datasets from experiments with precision tracking and complete kinemetic coverage at collider energies, hyperon spin polarization measurements have begun to uncover the vorticity of the QGP created at the Relativistic Heavy Ion Collider. The injection of this new degree of freedom into a relatively mature field of research represents an enormous opportunity to generate new insights into the physics of the QGP. The community has responded with enthusiasm, and this book (to be published as a volume of Lecture Notes in Physics series by Springer) represents some of the diverse lines of inquiry into aspects of strongly interacting matter under rotation.

Probing early-time longitudinal dynamics with the $Λ$ hyperon's spin polarization in relativistic heavy-ion collisions

“…Extensive theoretical and phenomenological investigations have been devoted to the effects of fluid vorticity on spin polarization [2][3][4][5][6][7][8][9][10][11][12][13][14] as well as the related transport phenomenon involving spin [15][16][17][18][19][20][21][22][23][24]. Hydrodynamics + hadronic transport hybrid models and pure transport approaches can provide good descriptions of the global polarization for Λ and Λ.…”

“…The grey arrows in panel (b) indicate the nonzero initial longitudinal flow u η with yL = yCM in Eqs. (13) and(14). u η = 0 in panel (a).…”

Implications of correlations and fluctuations in small systems

“…However, while comparing with the case in heavy-ion collisions where we study QGP, we are restricted to correlations in momenta and also the association to spatial correlations is also non-trivial [45]. Since fluctuations also assume paramount importance for studying perturbation of a thermodynamic system, some similarities also exist between the characterization methodologies, regarding the early universe and the heavy-ion collisions [46,47,48,49,50,51] . A QGP system [52,53,54,55,56,57,58] in a small span of time cools-down and eventually re-hadronizes.…”

“…To investigate the first order we observe and infer upon the thermal spectra and radial flow [62,63,64]. Furthermore, a large quadrupole correlation due to elliptic flow (v 2 ) is also perceived [45,49,51]. The pertinent question then remains, whether we can pick out the smaller correlations due to a possible phase transition and eventually the QCD critical point [12] after subtracting thermal emission and v 2 , which are two ruling backgrounds [15,45,62,63,64,11].…”

Helicity polarization in relativistic heavy ion collisions

“…Much development has been made along this direction on either experimental aspect [1][2][3][4][5][6][7][8] or theoretical aspect . Some relevant review on spin effects in relativistic heavy ion collisions are available in Refs [44][45][46][47][48][49][50]. Most of these works concentrate on the global or local polarization along the global angular momentum (transverse polarization) or beam momentum (longitudinal polarization) for hyperons or vector mesons.…”