HET-Discovery seminar by Michael Lisa – University of Copenhagen

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HET-Discovery seminar by Michael Lisa

Global Hyperon Polarization in Semicentral Heavy Ion Collisions Measured by STAR 

Non-central collisions between ultra-relativistic heavy ions involve thousands of h-bar 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 Anti-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 Anti-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.