Nettsider med emneord «QGP»

Publisert 15. mars 2021 14:18

Heavy nuclei at ultrarelativistic speeds are sources of intense electromagnetic fields, interacting with the other nucleus and leading to photoproduction of charmonium states at very low transverse momenta. A very strong excess of J/psi mesons in this transverse momentum region has been observed by ALICE in peripheral (hadronic) to ultraperipheral Pb-Pb collisions, with and without nuclear overlap.

Publisert 15. mars 2021 14:17

The QGP behaves like a low-viscosity liquid and exhibits a collective, multi-harmonic azimuthally anisotropic expansion pattern (flow), which can be described in terms of a Fourier series. The various Fourier components v_n originate from pressure gradients, due to the collision geometry in non-central collisions (typically the quadrupole component v_2, called elliptic flow) or to fluctuations in the initial nucleon density (typically triangular flow v_3 and higher order components). Non-zero elliptic flow has been observed for the J/psi meson, providing further evidence that the charm quarks are thermalized and participate in the collective flow of the plasma, as expected in a recombination scenario. The elliptic flow at higher transverse momenta, where a significant contribution from primordial J/psi is expected, is not well described by model calculations and presently not well understood.