비만아 부모의 돌봄 경험: q 방법론

© 2017 Macmillan Publishers Limited, part of Springer Nature. All rights reserved. The extreme energy densities generated by ultra-relativistic collisions between heavy atomic nuclei produce a state of matter that behaves surprisingly like a fluid, with exceptionally high temperature and low viscosity. Non-central collisions have angular momenta of the order of 1,000h, and the resulting fluid may have a strong vortical structure that must be understood to describe the fluid properly. The vortical structure is also of particular interest because the restoration of fundamental symmetries of quantum chromodynamics is expected to produce novel physical effects in the presence of strong vorticity. However, no experimental indications of fluid vorticity in heavy ion collisions have yet been found. Since vorticity represents a local rotational structure of the fluid, spin-orbit coupling can lead to preferential orientati on of particle spins along the direction of rotation. Here we present measurements of an alignment between the global angular momentum of a non-central collision and the spin of emitted particles (in this case the collision occurs between gold nuclei and produces Λ baryons), revealing that the fluid produced in heavy ion collisions is the most vortical system so far observed. (At high energies, this fluid is a quark-gluon plasma.) We find that Λ and hyperons show a positive polarization of the order of a few per cent, consistent with some hydrodynamic predictions. (A hyperon is a particle composed of three quarks, at least one of which is a strange quark; the remainder are up and down quarks, found in protons and neutrons.) A previous measurement that reported a null result, that is, zero polarization, at higher collision energies is seen to be consistent with the trend of our observations, though with larger statistical uncertainties. These data provide experimental access to the vortical structure of the nearly ideal liquid created in a heavy ion collision and should prove valuable in the development of hydrodynamic models that quantitatively connect observations to the theory of the strong force.

http://inspirehep.net/record/1510474/files/arXiv:1701.06657.pdf

Global Λ hyperon polarization in nuclear collisions

1. Opening remarks 2. A heavy ion phenomenology primer 3. Results from lattice QCD at nonzero temperature 4. Introducing the gauge/string duality 5. A duality toolbox 6. Bulk properties of strongly coupled plasma 7. From hydrodynamics for far-from-equilibrium dynamics 8. Probing strongly coupled plasma 9. Quarkonium mesons in strongly coupled plasma 10. Concluding remarks and outlook Appendixes References Index.

/pdf/gauge-string-duality-hot-qcd-and-heavy-ion-collisions-3dz9i1vbsu.pdf

Gauge/String Duality, Hot QCD and Heavy Ion Collisions

http://inspirehep.net/record/1347138/files/arXiv:1503.00732.pdf

Quantum field theory in a magnetic field: From quantum chromodynamics to graphene and Dirac semimetals

Chiral magnetic and vortical effects in high-energy nuclear collisions—A status report

Produced partons have large local relative orbital angular momentum along the direction opposite to the reaction plane in the early stage of non-central heavy-ion collisions. Parton scattering is shown to polarize quarks along the same direction due to spin-orbital coupling.Such global quark polarization will lead to many observable consequences,such as left-right asymmetry of hadron spectra, global transverse polarization of thermal photons, dileptons and hadrons. Hadrons from the decay of polarized resonances will have azimuthal asymmetry similar to the elliptic flow. Global hyperon polarization is predicted with indifferent hadronization scenarios and can be easily tested.

/pdf/globally-polarized-quark-gluon-plasma-in-noncentral-a-a-3cq5julxcj.pdf

Globally polarized quark-gluon plasma in noncentral A + A collisions.

A power expansion scheme is set up to determine the Wigner function that satisfies the quantum kinetic equation for spin-$1/2$ charged fermions in a background electromagnetic field. Vector and axial-vector current induced by magnetic field and vorticity are obtained simultaneously from the Wigner function. The chiral magnetic and vortical effect and chiral anomaly are shown as natural consequences of the quantum kinetic equation. The axial-vector current induced by vorticity is argued to lead to a local polarization effect along the vorticity direction in heavy-ion collisions.

Chiral anomaly and local polarization effect from the quantum kinetic approach.

https://escholarship.org/content/qt7337s72x/qt7337s72x.pdf

Spin alignment of vector mesons in non-central A+A collisions

We present the complete first order relativistic quantum kinetic theory with spin for massive fermions derived from the Wigner function formalism in a concise form that shows explicitly how the 32 Wigner equations reduce in practice to 4 independent transport equations. We solve the modified on-shell conditions to obtain the general solution and present the corresponding transport equations in three different forms that are suitable for different purposes. We demonstrate how different spin effects arise from the kinetic theory by calculating the chiral separation effect with mass correction, the chiral anomaly from the axial current and the quantum magnetic moment density induced by vorticity and magnetic field. We also show how to generate the global polarization effect due to spin vorticity coupling. The formalism presented may serve as a practical theoretical framework to study different spin effects in relativistic fermion systems encountered in different areas such as heavy ion, astro-particle and condensed matter physics as well.

Relativistic quantum kinetic theory for massive fermions and spin effects

We develop a general formalism for the quantum kinetics of chiral fermions in a background electromagnetic field based on a semiclassical expansion of covariant Wigner functions in the Planck constant $\ensuremath{\hbar}$. We demonstrate to any order of $\ensuremath{\hbar}$ that only the time-component of the Wigner function is independent while other components are explicit derivative. We further demonstrate to any order of $\ensuremath{\hbar}$ that a system of quantum kinetic equations for multiple-components of Wigner functions can be reduced to one chiral kinetic equation involving only the single-component distribution function. These are remarkable properties of the quantum kinetics of chiral fermions and will significantly simplify the description and simulation of chiral effects in heavy ion collisions and Dirac/Weyl semimetals. We present the unintegrated chiral kinetic equations in four-momenta up to $O({\ensuremath{\hbar}}^{2})$ and the integrated ones in three-momenta up to $O(\ensuremath{\hbar})$. We find that some singular terms emerge in the integration over the time component of the four-momentum, which result in a new source term contributing to the chiral anomaly, in contrast to the well-known scenario of the Berry phase term. Finally we rewrite our results in any Lorentz frame with a reference four-velocity and show how the non-trivial transformation of the distribution function in different frames emerges in a natural way.

/pdf/disentangling-covariant-wigner-functions-for-chiral-fermions-471c7x885a.pdf

Zuo-tang Liang

Papers

Globally polarized quark-gluon plasma in noncentral A + A collisions.

Chiral anomaly and local polarization effect from the quantum kinetic approach.

Spin alignment of vector mesons in non-central A+A collisions

Relativistic quantum kinetic theory for massive fermions and spin effects

Disentangling covariant Wigner functions for chiral fermions