Heavy-flavour production in high-energy d-Au and p-Pb collisions

Abstract

Soft-hadron measurements in high-energy collisions of small systems like p-Pb and d-Au show peculiar qualitative features (long-range rapidity correlations, flattening of the p T -spectra with increasing hadron mass and centrality, non-vanishing Fourier harmonics in the azimuthal particle distributions) suggestive of the formation of a strongly-interacting medium displaying a collective behaviour, with a hydrodynamic flow as a response to the pressure gradients in the initial conditions. Hard observables (high-p T jet and hadron spectra) on the other hand, within the current large systematic uncertainties, appear only midly modified with the respect to the benchmark case of minimum-bias p-p collisions. What should one expect for heavy-flavour particles, initially produced in hard processes but tending, in the nucleus-nucleus case, to approach kinetic equilibrium with the rest of the medium? This is the issue we address in the present study, showing how the current experimental findings are compatible with a picture in which the formation of a hot medium even in proton-nucleus collisions modifies the propagation and hadronization of heavyflavour particles.

Development of heavy-flavour flow-harmonics in high-energy nuclear collisions

“…The initial state is simply taken from the Glauber model, either in its optical or Monte Carlo implementation. As in our past publications [9][10][11][12], the system is initialized via the entropy-density at the longitudinal proper-time τ 0 ranging, depending on the center-of-mass energy of the collision, from τ 0 = 1 fm/c at √ s NN = 200 GeV to τ 0 = 0.5 fm/c at √ s NN = 5.02 TeV. The hydrodynamic equations describing its evolution are solved through the ECHO-QGP code [33] in 2+1 dimensions, assuming longitudinal boost-invariance, which is a reasonable approximation to describe observables around mid-rapidity.…”

“…On the other hand, for the study of observables arising from event-by-event fluctuations of the initial geometry like the triangular flow, this is not enough: smooth initial conditions would lead to v 3 = 0 for any impact parameter of the colliding nuclei and only the granularity of the initial condition can give rise to a non-vanishing triangular flow. Here, as done in [12], we assume that the above lumpiness arises mainly from event-by-event fluctuations in the positions of the nucleons inside the colliding nuclei. We proceed as follows, generalizing to the nucleus-nucleus case the Monte Carlo approach adopted in [12] for proton(deuteron)-nucleus collisions.…”

“…Here, as done in [12], we assume that the above lumpiness arises mainly from event-by-event fluctuations in the positions of the nucleons inside the colliding nuclei. We proceed as follows, generalizing to the nucleus-nucleus case the Monte Carlo approach adopted in [12] for proton(deuteron)-nucleus collisions. We generate several thousands (∼ 6000) of Pb-Pb collisions at random impact parameter and…”

“…of soft-hadron azimuthal distributions are measured (providing information on event-by-event fluctuations and granularity of the initial conditions), one would like to address this issue also in the heavy-flavour sector (for first experimental results, see ref. [7]): this will be one of the major topics dealt with in this paper, based on the POWLANG transport setup developed by the authors over the last years [9][10][11][12]. A similar theoretical study was performed in [13] and, accounting only for the path-length dependence of parton energy-loss, in [14].…”

Measurement of the relative yields of ψ(2S) to ψ(1S) mesons produced at forward and backward rapidity in Adare1, Aidala2, Ajitanand3 et al. 2017Phys. Rev. C

“…In collisions involving nuclei, the initial heavy quark production can be affected by modifications of the parton distribution functions [6], energy loss in the nucleus [7], and scattering with other partons [8]. Effects which may be of hydrodynamic origin are also present in small systems [9][10][11], and may further alter the heavy quark final state [12,13]. If these flow effects are due to quarkgluon-plasma formation, the presence of deconfined colored partons can inhibit coalescence into a bound state or dissolve fully-formed bound states [14].…”