Rooted staggered fermions: good, bad or ugly?

Abstract

I give a status report on the validity of the so-called “fourt h-root trick”, i.e. the procedure of representing the determinant for a single fermion by the fourth root of the staggered fermion determinant. This has been used by the MILC collaboration to create a large ensemble of lattices using which many quantities of physical interest have been and are being calculated. It is also used extensively in studies of QCD thermodynamics. The main question is whether the theory so defined has the correct continuum limit. There has been sig nificant recent progress towards answering this question. After recalling the issue, and put ting it into a broader context of results from statistical mechanics, I critically review the new wor k. I also address the related issue of the impact of treating valence and sea quarks differently in rooted simulations, discuss whether rooted simulations at finite temperature and density are sub ject to additional concerns, and briefly update results for quark masses using the MILC configuration s. An answer to the question in the title is proposed in the summary.