Current version:

Title: Global polarization of Lambda hyperons in Au+Au Collisions at RHIC BES

Non-central heavy-ion collisions have large ($\sim 10^{5} \hbar$) angular momentum which may be transferred, in part, to the quark-gluon plasma through shear forces that generate a vortical substructure in the hydrodynamic flow field. The vortical nature of the system is expected to polarize emitted hadrons along the direction of system angular momentum. $\Lambda$ and $\overline{\Lambda}$ hyperons, which reveal their polarization through decay topology, should be polarized similarly. The vorticity of the fluid is an important parameter for the generation of a Chiral Vortical Effect (CVE).

These same collisions are also characterized by dynamic magnetic fields with magnitude as large as $10^{14}$ Tesla. A splitting between $\Lambda$ and $\overline{\Lambda}$ polarization may signal a magnetic coupling and provide a quantitative estimate of the field strength at freeze out. Details of the magnetic field and its evolution are of particular interest to other novel phenomena, e.g. the Chiral Magnetic Effect (CME).

The STAR Collaboration has made the first observation of global hyperon polarization in non-central Au+Au collisions at Beam Energy Scan energies. A magnetic splitting is hinted at, but the improved statistics and resolution achievable with future runs are required to make a definitive measurement of the magnetic field.

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v1:
Non-central collisions between ultra-relativistic heavy ions involve thousands of $\hbar$ of
angular momentum. Some of this angular momentum may be transferred to the quark-gluon
plasma through shear forces that generate a vortical substructure in the hydrodynamic flow
field. Understanding this fundamental femtoscopic substructure may be crucial, as we move
beyond boost-invariant scenarios and rely more on sophisticated three-dimensional viscous
models of the plasma. The vortical nature of the system is expected to polarize the spins of
hadrons that eventually emerge. $\Lambda$ and $\overline{\Lambda}$ hyperons, which reveal their
polarization through their decay topology, should be polarized similarly in the direction of the
system’s angular momentum.

These same collisions are also characterized by dynamic magnetic fields with magnitude
as large as $10^{14}$ Tesla. Magnetic effects have been the focus of intense study in recent
years due to their relevance to the Chiral Magnetic Effect (CME) and other novel phenomena.
A splitting between $\Lambda$ and $\overline{\Lambda}$ polarization may signal a magnetic coupling
and provide a quantitative estimate of the field strength at freeze out. Physically, this strength
depends on the conductivity of the QGP.

The STAR Collaboration has made the first observation of global hyperon polarization along
the direction of the angular momentum in non-central Au+Au collisions at Beam Energy Scan
energies. Our preliminary results indicate that the QGP created at RHIC is the highest-vorticity
fluid ever created in the laboratory. A magnetic splitting is hinted at, but the improved statistics
and resolution achievable with future runs are required to make a definitive measurement of
the magnetic field.

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v2:
Non-central heavy ion collisions have large ($\sim 10^{5} \hbar$) angular momentum which may be transferred, in part, to the quark-gluon plasma through shear forces that generate a vortical substructure in the hydrodynamic flow field. The vortical nature of the system is expected to polarize emitted hadrons along the direction of system angular momentum. $\Lambda$ and $\overline{\Lambda}$ hyperons, which reveal their polarization through decay topology, should be polarized similarly. The vorticity of the fluid is an important parameter for the generation of a Chiral Vortical Effect (CVE).

These same collisions are also characterized by dynamic magnetic fields with magnitude as large as $10^{14}$ Tesla. A splitting between $\Lambda$ and $\overline{\Lambda}$ polarization may signal a magnetic coupling and provide a quantitative estimate of the field strength at freeze out. Details of the magnetic field and it's evolution are of particular interest to other novel phenomena, e.g. the Chiral Magnetic Effect (CME).

The STAR Collaboration has made the first observation of global hyperon polarization in non-central Au+Au collisions at Beam Energy Scan energies. A magnetic splitting is hinted at, but the improved statistics and resolution achievable with future runs are required to make a definitive measurement of the magnetic field.

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v3: