Stochastic reconstructions of spectral functions: Application to lattice QCD

We present a detailed study of the applications of two stochastic approaches, Stochastic Optimization Method (SOM) and Stochastic Analytical Inference (SAI), to extract spectral functions from Euclidean correlation functions. SOM has the advantage that it does not require prior information. On the other hand, SAI is a more generalized method based on Bayesian inference. Under mean field approximation SAI reduces to the often-used Maximum Entropy Method (MEM), and for a specific choice of the prior SAI becomes equivalent to SOM. To test the applicability of these two stochastic methods to lattice QCD, firstly, we apply these methods to various reasonably chosen model correlation functions, and present detailed comparisons of the reconstructed spectral functions obtained from SOM, SAI and MEM. Next, we present similar studies for charmonia correlation functions obtained from lattice QCD computations using clover-improved Wilson fermions on large, fine, isotropic lattices. We find that SAI and SOM give consistent results that suggest dissociation of $\eta_c$ and $J/\psi$ in the gluon plasma already at $1.5T_c$. These findings reinforce the previous conclusions of Ref.~\cite{Ding:2012sp} that was solely based on MEM.