Directed flow of D mesons at RHIC and LHC: non-perturbative dynamics, longitudinal bulk matter asymmetry and electromagnetic fields

Abstract

We present a study of the directed flow v1 for D mesons discussing both the impact of initial vorticity and electromagnetic field. Recent studies predicted that v1 for D mesons is expected to be surprisingly much larger than that of light charged hadrons; we clarify that this is due to a different mechanism leading to the formation of a directed flow with respect to the one of the bulk matter at both relativistic and non-relativistic energies. We point out that the very large v1 for D mesons can be generated only if there is a longitudinal asymmetry between the bulk matter and the charm quarks and if the latter have a large non-perturbative interaction in the QGP medium. A quite good agreement with the data of STAR and ALICE is obtained if the diffusion coefficient able to correctly predict the RAA(pT), v2(pT) and v3(pT) of D meson is employed. Furthermore, the mechanism for the build-up of the v1(y) is associated to a quite small formation time that can be expected to be more sensitive to the initial high-temperature dependence of the charm diffusion coefficient.We discuss also the splitting of v1 for D0 and $$ {\overline{D}}^0 $$ D ¯ 0 due to the electromagnetic field that is again much larger than the one observed for charged particles and in agreement with the data by STAR that have however still error bars comparable with the splitting itself, while at LHC standard electromagnetic profile assuming a constant conductivity is not able to account for the huge splitting observed.

Longitudinal distribution of initial energy density and directed flow of charged particles in relativistic heavy-ion collisions

“…It takes into account the asymmetry along the impact parameter direction (x) at different spacetime rapidity (η s ) which generates a counter-clockwise tilt of the initial fireball in the x-η s plane. This initialization method has been found successful in understanding the directed flow of not only the soft hadron, but also the heavy flavor mesons [40,[48][49][50] after coupling the heavy quark evolution model to the titled QGP medium. Following the idea of Bozėk-Wyskiel, we developed an alternative initialization ansatz (CCNU parametrization) of the longitudinal distribution of the nuclear matter in an earlier study [51].…”

Probing the initial longitudinal density profile and electromagnetic field in ultrarelativistic heavy-ion collisions with heavy quarks

“…In this work, we will employ two different model calculations of the spacetime profiles of the electromagnetic field and compare their impacts on the heavy flavor v 1 . Following earlier studies [38,41,51,52,68], only the two dominant components, E x and B y , are included in our calculation.…”

“…More detailed studies have later been performed in Refs. [41,42] using transport models of heavy quarks coupled to a QGP medium that takes into account the initial longitudinal tilt [43] in the reaction plane with respect to the beam axis.…”

“…Interestingly, while the STAR measurement [40] observes decreasing v 1 with respect to rapidity (y) for both D 0 and D0 , with very small difference between their magnitudes, the ALICE measurement [50] presents apparent splitting of the directed flow (∆v 1 ) between opposite charges, with D 0 increasing and D0 decreasing with y. This puzzling observation has attracted a series of investigations on heavy quark dynamics in the presence of electromagnetic field [38,41,51,52].…”

Splitting of elliptic flow in non-central relativistic heavy-ion collisions

“…Current studies are focusing on the initial vortical configuration of the QCD matter in non-central heavy-ion collisions. The directed flow v 1 of the light charged hadrons [22][23][24] and heavy flavor hadrons [25][26][27][28] are suggested as probes to the initial tilted shape of matter. Still, there exists a spin sign problem [29][30][31] between the experimental and theoretical results on the local polarization such as azimulthal angle dependence, which demands deeper understanding to the initial vortical configuration.…”