The KPZ equation with flat initial condition and the directed polymer with one free end

We study the directed polymer (DP) of length t in a random potential in dimension 1 + 1 in the continuum limit, with one end fixed and one end free. This maps onto the Kardar–Parisi–Zhang growth equation in time t, with flat initial conditions. We use the Bethe ansatz solution for the replicated problem, which is an attractive bosonic model. The problem is more difficult than the previous solution of the fixed endpoint problem as it requires regularization of the spatial integrals over the Bethe eigenfunctions. We use either a large fixed system length or small finite slope KPZ initial conditions (wedge). The latter allows one to take properly into account non-trivial contributions, which appear as deformed strings in the former. By considering a half-space model in a proper limit we obtain an expression for the generating function of all positive integer moments of the directed polymer partition function. We obtain the generating function of the moments of the DP partition sum as a Fredholm Pfaffian. At large time, this Fredholm Pfaffian, valid for all time t, exhibits convergence of the free energy (i.e. KPZ height) distribution to the GOE Tracy–Widom distribution.