Speaker
Description
A first-order phase transition at the electroweak scale is a common feature of many extensions of the Standard Model. The bubble nucleation rate is one of the key parameters describing this process and its lattice computation involves determining the statistical probability of critical bubbles. With the help of multicanonical Markov chain Monte Carlo the probability distribution can be sampled uniformly over some range of a chosen order parameter, an observable distinguishing the two phases. In practice, for large lattices the autocorrelation time of the Markov chain becomes impractically long since tunneling between the homogeneous configurations and those containing bubbles becomes increasingly suppressed.
One way of tackling this issue is by finding a better order parameter that more carefully distinguishes the bubbles from mere fluctuations in the homogeneous configurations. In this work we investigate an alternative approach; pretending that the system is smaller when it suits us. Applying the method of Expanded Ensembles, essentially a generalisation of simulated tempering, we can use multiple order parameters to improve the tunneling between the two types of configurations. These interpolating systems also use the standard order parameter, except that its value is computed in smaller subsets of the lattice. While the interpolating systems take a narrow and unrepresentative path through the configuration space, the full Markov chain still yields correct relative probabilities for the configurations of interest.
