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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.
Ultra-relativistic heavy-ion collisions offer a unique opportunity to study the nuclear phase diagram at high temperatures and densities. The matter under such extreme conditions probably has existed in the early Universe within the first few fm/c after the Big Bang. Therefore, it is very tempting to investigate the properties of the Little Big Bang in the laboratory, and to search for a new state of matter, predicted by the fundamental theory of strong interactions - Quantum Chromodynamics (QCD), namely, a plasma of deconfined quarks and gluons or quark-gluon plasma (QGP).
To describe such complex phenomenon one has to rely on phenomenological models, which can be subdivided into macroscopic, i.e. thermal and hydrodynamic, and microscopic Monte Carlo models, incorporating partonic and hadronic degrees of freedom in a consistent fashion. These models are indispensable for the comparison with the experimental data coming from current heavy-ion accelerators and for planning the new machines such as FAIR at GSI, NICA at JINR, and FCC at CERN, which is widely discussed nowadays.
In Oslo we use several MC models at our disposal, namely, Ultra-relativistic Quantum Molecular Dynamics (UrQMD) and Quark-Gluon String Model (QGSM) for description of various hadronic and nuclear collisions, and HYDrodynamics with JETs (HYDJET++) model for simulation of heavy-ion collisions.
Late autumn 2018, the ALICE experiment collected a high-statistics data set with Pb-Pb collisions at a centre-of-mass energy of 5.02 TeV. High-quality reference data from p-p collisions at the same energy are also available.
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.
We study the universe at the smallest distance scales (corresponding to the highest energy scales). After the Higgs boson was discovered at CERN in 2012, one of the hottest goals of our research is to reveal the nature of dark matter.
The main objective is to enable the study of fundamental particles and interactions and the characterizaton of high-temperature strongly interacting matter at the extreme energies and collision rates of the upgraded Large Hadron Collider (HL-LHC) at CERN in the years 2017-2037.
The mission of the HENP project is the exploration of the behaviour of nuclear matter under extreme conditions and in particular the Quark-Gluon Plasma, which existed for about a microsecond after the Big Bang, and which can be recreated in ultra-relativistic heavy-ion collisions in the laboratory. The HENP project uses the Large Hadron Collider (LHC) at CERN and the dedicated heavy-ion experiment ALICE to produce and study the Quark-Gluon Plasma.