Electron-Positron Plasma Drop Formed by Ultra-Intense Laser Pulses
  We study the initial properties and positron annihilation within a small electron-positron plasma drop formed by intense laser pulse. Such QED cascade initiated plasma is, in general, far below the chemical (particle yield) equilibrium. We find that the available electrons and positrons equilibrate kinetically, yet despite relatively high particle density, the electron-positron annihilation is very slow, suggesting a rather long lifespan of the plasma drop. 
Inga Kuznetsova and Johann Rafelski
physics.plasm-ph hep-ph