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In high energy nuclear collisions, the phase predicted by Quantum Chromo-Dynamics is created for a short time: it is the so-called Quark Gluon Plasma (QGP), where quarks and gluons are deconfined from their hadronic bounds. The QGP, as a relativistic fluid, has remarkable features such as a very low viscosity, a very high temperature and a huge initial acceleration. Quantum relativistic effects are also relevant.
A fascinating phenomenon which is genuinely quantum is the polarization of particles emitted in the QGP hadronization in nuclear collisions at finite impact parameter. It was quantitatively predicted based on the idea of local equilibrium and a formula connecting spin and the gradients of the hydrodynamic field (thermal vorticity) was put forward here. In 2017 the STAR experiment published a paper in Nature where the polarization of the Λ hyperon was measured, confirming the quantitative prediction of the local equilibrium model and hydrodynamics.
These results and some later observed discrepancies in the momentum dependence of the polarization triggered an intense and exciting theoretical work which is still ongoing and which we are deeply involved in. Spin physics is becoming an important chapter of the QGP.
For a general review paper on the spin physics in relativistic nuclear collisions, see here. A theoretical summary of the formulae of spin polarization in QFT can be found here. For an introductory video lecture, click on the window on the right.
Lecture2.mp4Video Lecture
Lecture on spin polarization in HIC.pdfRECENT RESULTS
Spin and local parity violation
Local parity violation is a long-sought phenomenon in QCD at finite temperature. It has been proposed to observe it via another fascinating phenomenon, the Chiral Magnetic Effect (CME): the generation of an electric current directed along a magnetic field in the presence of an axial charge imbalance. The observation of local parity violation with the CME requires a sufficiently large magnetic field, but there is a large uncertainty on its magnitude. We recently proposed to probe local parity violations by using the spin polarization along the momentum direction, that is helicity. In the presence of a fluctuating axial charge, the helicity of Λ hyperons has an additional, parity-violating contributing term which may be detected by studying azimuthal Λ-Λ helicity correlation. The point of this method is its independence of an electromagnetic field mediation, unlike CME.
Spin-shear coupling and the longitudinal polarization puzzle
While the global spin polarization predicted by hydrodynamics is in excellent agreement with the data, the local spin polarization - that is, as a function of the momentum of the particle - revealed large discrepancy with respect to hydrodynamic predictions. Several ideas were proposed to solve this puzzle over the past few years. Lately, we realized (and another group independently) that at local equilibrium there is another contributing term to the spin polarization of a fermion which is proportional to the symmetric part of the four-temperature gradient, the thermal shear. We worked out the formula connecting spin vector to thermal shear - independently found by another group - here. We showed that, under the assumption of constant temperature freeze-out hypersurface - what is expected in high energy collisions -, the agreement is restored between theory and experiment, see here.
Exact formulae for the spin density matrix and spin polarization at global equilibrium
The exact formula relating spin to thermal vorticity, at global equilibrium, can be obtained in the Botlzmann statistics limit, that is for a single quantum relativistic particle. The inclusion of quantum statistics effects, that is a full quantum field theoretical calculation, is hard even in the free case and only the linear approximation in thermal vorticity was known (see here). While the linear approximation is sufficient in most phenomonological applications, there might be cases where one needs to go beyond it. In a recent paper, we obtained the exact field theoretical formula for the spin density matrix and the spin polarization vector of a spin 1/2 particle, in the form of a series, at global equilibrium. The numerical impact of the corrections are still to be evaluated.
ONGOING PROJECTS
Inclusion of decays in the numerical computation of polarization
Dissipative effects in the spin polarization formulae
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