The Upper Critical Dimension of the KPZ Equation
  The strong-coupling regime of Kardar-Parisi-Zhang surface growth driven by short-ranged noise has an upper critical dimension d_> less or equal to four (where the dynamic exponent z takes the value z (d_>) = 2). To derive this, we use the mapping onto directed polymers with quenched disorder. Two such polymers coupled by a small contact attraction are shown to form a bound state at all temperatures below the roughening temperature of a single polymer. Comparing the singularities of the localization length at and below the critical temperature yields d_> \leq 4. 
Michael Lassig and Harald Kinzelbach