Open quantum system approach to the transverse momentum broadening of a colour dipole

by Pietro Benzoni (Subatech)

Thursday 18 Jun 2026, 10:00 → 12:00 Europe/Zurich

4/2-037 - TH meeting room (CERN)

Using the open quantum systems formalism, we study the propagation of a quark-antiquark pair propagating through a dense QCD plasma of size $L$ and transverse momentum broadening transport coefficient $\hat q$, and we derive the Lindblad evolution equation for the density matrix of the system.

We focus on the boosted regime where the opening angle $\theta_{q\bar q}$ of this effective colour dipole satisfies $\theta_{q\bar q}\ll 1$. In the back-to-back limit where the quark-antiquark relative transverse momentum $p_\perp$ is much larger than the imbalance $q_\perp$ as well as the medium typical transverse momentum scale $Q_s=\sqrt{\hat q L}$, we demonstrate that the resulting Wigner distribution displays quasi factorisation between a hard factor describing the hard splitting producing the $q\bar q$ pair and the transverse momentum imbalance $q_\perp$-distribution encoding the broadening induced by the medium. The factorisation is violated by a "colour decoherence'' factor that controls the $\theta_{q\bar q}$ dependence of the $q_\perp$-distribution through the ratio $\theta_{q\bar q}/\theta_c$, with $\theta_c \sim (\hat q L^3)^{-1/2}$. The open quantum systems approach enables us to clarify the role of this critical angle $\theta_c$ and its associated critical time $t_c$ in the genuine quantum decoherence of the density matrix in colour and kinematic space.

Finally, we investigate the corrections beyond the quasi-factorised picture due to the quantum diffusion term in $p_\perp$ of the Lindblad equation and we find that these corrections are mild.

 

Based on [F. Arleo, P. Benzoni, P. Caucal, P. B. Gossiaux, https://arxiv.org/abs/2606.07744].