Thermal excitation spectrum from entanglement in an expanding quantum string

Understand

A surprising result in $e^+ e^-$ collisions is that the particle spectra from the string formed between the expanding quark-antiquark pair have thermal properties even though scatterings appear not to be frequent enough to explain this.

• We address this problem by considering the finite observable interval of a relativistic quantum string in terms of its reduced density operator by tracing over the complement region.
• We show how quantum entanglement in the presence of a horizon in spacetime for the causal transfer of information leads locally to a reduced mixed-state density operator.
• For very early proper time $\tau$, we show that the entanglement entropy becomes extensive and scales with the rapidity.