Jet quenching without energy loss

TL;DR

The paper addresses the onset of jet quenching in small systems by exploring colour-coherence loss as a mechanism independent of energy loss. It uses a minimal Monte Carlo setup where the first splitting is unconstrained in angular ordering (AO-F) to mimic coherence breaking, contrasted with a full angular ordering plus small energy loss to simulate a kinematic effect; the formation time $\tau$ computed via the $\tau$-algorithm from a large-radius jet's first unclustering serves as the key observable. The results demonstrate that disabling angular ordering for the first splitting significantly alters the parton-shower formation-time distribution, an effect that persists after hadronization, while jet yields show a 5–10% suppression without actual energy loss. Importantly, an explicit energy-loss scenario yields similar jet suppression but leaves the formation-time distribution effectively unchanged, establishing the formation-time observable as a robust discriminator of colour-coherence dynamics in jet evolution and motivating future lighter-ion measurements to probe QGP droplets. The analysis combines a vacuum-like evolution with selective angular constraints and a reclustering-based formation-time estimator, offering a new, measurable handle on coherence effects in jet quenching.

Abstract

The onset of jet quenching, i.e. the suppression of high transverse momentum particles and jets, is an important question in the context of understanding the onset of collective behaviour and small collision systems. We investigate a minimal scenario where a hard parton experiences a single soft re-scattering that leaves the kinematics unmodified, but the colour exchange leads to a loss of colour coherence that is observable in the final distribution of fragments. In particular, the formation time of the first splitting as reconstructed at hadron level from jets using the formation time clustering algorithm is sensitive the loss of colour coherence. Moreover, it can distinguish the coherence loss scenario from a corresponding energy loss scenario, since small energy loss effects leave the formation time distribution unchanged.

Figures (4)

• Figure 1: Formation time distribution of jets with radius $R=1.0$ for events with and without angular ordering at the first splitting. \ref{['fig:taudist_PS']} Formation time computed at the parton shower level. \ref{['fig:taudist_REC']} Formation time computed at the hadron level using the tau-algorithm unclustering procedure.
• Figure 2: The original $\tau_{\text{PS}}$ distribution for the \ref{['fig:tausmearing-T']} AO-T and the \ref{['fig:tausmearing-F']} AO-F configuration along with the corresponding smeared distributions. Solid grey lines show the smeared distribution resulting from the AO-T reconstruction resolution whereas dashed grey lines correspond to smearing from the AO-F resolution. The vertical lines show the value of the median of the corresponding distributions.
• Figure 3: Jet $p_\perp$ spectra for events with and without angular ordering in the first splitting for jets of radius $R=1.0$.
• Figure 4: \ref{['fig:eloss-raa']} Jet $p_\perp$ spectra and \ref{['fig:eloss-tau']} reconstructed formation time distribution for the two angular ordering configurations without energy loss and for the configuration with energy loss and angular ordering turned on in the entire evolution.