Mr
Moritz Greif
(Goethe University Frankfurt)
The question behind our work is the origin of the observed large momentum anisotropies in high energy pA collisions. It has been shown that initial state momentum correlations are sizeable in such small systems, and evidently, final state interactions of some form are also present and important. Because of the large gradients present in the spatial gluon distribution and the small number of particles present, these final state interactions are unlikely to be properly described by hydrodynamics, despite many attempts in this direction.
We establish a novel, unique methodology, combining a state-of-the-art initial state model (Yang-Mills dynamics of pre-equilibrium gluon fields, “IP-GLASMA”) with an explicit and exact non-equilibrium solution of the Boltzmann equation (parton cascade “BAMPS”) which is far more general than hydrodynamic calculations. In this study we focus on high and low multiplicity p+Pb collisions at top LHC energies.
The average parton number per rapidity in pA collisions is on the order of 5-30, which is very little compared to rather well-understood heavy-ion collision systems where parton numbers reach a few thousand, and hydrodynamics can be successful.
So far, our framework is the first and only one giving quantitative insights of both the initial state correlations and final state interactions in azimuthal correlation observables. The results show the interplay of classical Yang-Mills dynamics and the potential onset of thermalization quantitatively in high and low multiplicity pA systems. It is the dynamics on the border of initial state dominated to final state dominated - but not yet fully developed hydrodynamic - behavior.
More precise, we find that signatures of both the
initial state correlations and final state interactions are seen in azimuthal correlation observables,
such as $v_2\{2P C\} (p_T)$, their strength depending on the event multiplicity and transverse momentum.
Initial state correlations dominate $v_2\{2P C\} (p_T)$ in low multiplicity events for transverse momenta
p T > 2 GeV. While final state interactions are dominant in high multiplicity events, initial state
correlations affect $v_2\{2P C\} (p_T)$ for $p_T > 2~\mathrm{GeV}$ as well as the pT integrated $v_2\{2P C\} (p_T)$.
Preprint [arxiv:1708.02076][1]
[1]: https://arxiv.org/abs/1708.02076
Summary
We investigate the relative importance of initial and final state effects on azimuthal correlations of gluons in low and high multiplicity p+Pb collisions. To achieve this, we couple Yang-Mills dynamics of pre-equilibrium gluon fields (IP-GLASMA) to a perturbative QCD based parton cascade for the final state evolution (BAMPS) on an event-by-event basis. We find that signatures of both the initial state correlations and final state interactions are seen in azimuthal correlation observables, such as $v_2\left\lbrace2PC\right\rbrace(p_T)$, their strength depending on the event multiplicity and transverse momentum.
Initial state correlations dominate $v_2\left\lbrace2PC\right\rbrace(p_T)$ in low multiplicity events for transverse momenta $p_T>2~{\rm GeV}$. While final state interactions are dominant in high multiplicity events, initial state correlations affect $v_2\left\lbrace2PC\right\rbrace(p_T)$ for $p_T>2~{\rm GeV}$ as well as the pT integrated $v_2\left\lbrace2PC\right\rbrace$.
Preprint arxiv:1708.02076
