Colour coherence in small collision systems

TL;DR

This work probes whether colour coherence in jet-medium interactions can explain the coexistence of sizeable high-$p_ op$ azimuthal anisotropy and the apparent absence of jet quenching in small collision systems. By implementing a colour-coherence mechanism in the Jewel MC and comparing coherent versus incoherent scenarios across temperature profiles and medium models (TGlauberMC and Trajectum), the study finds that coherence modestly enhances $R_{AA}$ and can slightly modify $v_2$, especially when $R_{AA}$ is held fixed. A key result is that, at the same multiplicity, O+O and Pb+Pb show similar $R_{AA}$ for hadrons and jets despite different geometries, suggesting limited nonlinearities in translating geometry to suppression. Overall, colour coherence emerges as a plausible contributor to the observed $v_2$ in small systems without requiring large jet quenching signals, with implications for interpreting high-$p_ op$ observables in light-heavy ion collisions.

Abstract

The observation of collectivity in collisions of small systems has constituted a challenge for the heavy-ion community for over a decade now. The absence of jet quenching in those systems presents an apparent contradiction to the presence of an azimuthal anisotropy of high-$p_\perp$ particles. In the present work, we investigate the role of colour coherence in this puzzle. For that, we use the \textsc{Jewel} Monte Carlo model in its latest version, which includes effects of colour coherence in the jet-medium interactions. We then compare the two scenarios, with and without colour coherence, and quantify the effect on hadron and jet $R_{AA}$ as well as on high-$p_\perp$ $v_2$. The results indicate that, although coherence effects do account for an increase in $R_{AA}$, they do not affect $v_2$ to the same extent. Using hydrodynamic profiles generated with \textit{Trajectum} we compare O+O and Pb+Pb collisions at the same charged particle multiplicity and find that the nuclear modification factors are the same in both systems despite their different shapes.

Figures (8)

• Figure 1: Hadron spectra and nuclear modification factors for the different temperature profiles with and without colour coherence. The initial temperatures for the different profiles are given in Table \ref{['tab:tivalues']}.
• Figure 2: Jet spectra and nuclear modification factors for the different temperature profiles with and without colour coherence. The initial temperatures for the different profiles are given in Table \ref{['tab:tivalues']}.
• Figure 3: Hadron $v_2$ for the different temperature profiles with and without colour coherence in bins of hadron transverse momentum. $v_2$ is calculated as $v_2 = \langle \langle \cos(2(\phi - \Psi_2)) \rangle \rangle$, where $\Psi_2$ is the participant plane angle obtained from the Glauber model. The averages are taken first over the hadrons in a given event, and then over the events.
• Figure 4: Hadron spectra and nuclear modification factors for the linear temperature profiles with and without colour coherence, where the initial temperature of the incoherent simulation was adjusted to yield the same nuclear modification factor as the coherent one.
• Figure 5: Jet spectra and nuclear modification factors for the linear temperature profiles with and without colour coherence, where the initial temperature of the incoherent simulation was adjusted to yield the same nuclear modification factor as the coherent one.