The fast life of holographic mesons

TL;DR

The paper uses gauge/gravity duality to analyze meson quasiparticles in a thermal ${\cal N}=2$ SYM plasma with fundamental flavors, showing a universal limiting velocity $v_{\rm lim}<1$ at low momentum due to redshift and a rapid rise of widths near a critical momentum $q_{\rm crit}$. By examining D7-brane embeddings in BH backgrounds with nonzero quark density, it demonstrates how BH embeddings yield metastable mesons whose lifetimes can be tuned via the throat geometry, and how spectral functions reveal dispersion relations $\Omega(q)$ and $\Gamma(q)$ up to $q_{\rm crit}$, with $\Omega_i(q) \simeq v_{\rm lim} q + a_i$ and decreasing residues with $q$. Beyond $q_{\rm crit}$, spectral peaks dissolve and a Schrödinger-picture analysis of quasinormal modes suggests an asymptotic approach to light-speed propagation ($\partial_q \Omega \to 1$) at very large $q$, consistent with a high-momentum decoupling from the plasma. The findings imply a robust, redshift-driven mechanism for slow-moving bound states, and predict qualitative high-momentum behavior with potential implications for heavy-quark phenomenology in QCD-like plasmas, including possible signatures in photon production and jet quenching observables.

Abstract

We use holographic techniques to study meson quasiparticles moving through a thermal plasma in N=2 super-Yang-Mills theory, with gauge group SU(N_c) and coupled to N_f flavours of fundamental matter. This holographic approach reliably describes the system at large N_c, large 't Hooft coupling and N_f/N_c<<1. The meson states are destabilized by introducing a small quark density n_q. Spectral functions are used to examine the dispersion relations of these quasiparticles. In a low-momentum regime, the quasiparticles approach a limiting velocity which can be significantly less than the speed of light. In this regime, the widths of the quasiparticles also rise dramatically as their momentum approaches a critical value q_crit. While the spectral functions do not display isolated resonances for q>q_crit, the dispersion relations can be extended into this high-momentum regime by studying the dual quasinormal modes. A preliminary qualitative analysis of these modes suggests that the group velocity rises to the speed of light for q>>q_crit.