Flesh and Blood, or Merely Ghosts? Some Comments on the Multi-Muon Study   at CDF
  A recent paper by the CDF collaboration suggests (but does not claim) an anomalous event sample containing muons produced with large impact parameter, often with high multiplicity and at small angles from one another. This curious hint of a signal is potentially consistent with the hidden valley scenario, as well as with some other classes of models. Despite its tenuous nature, this hint highlights the experimental difficulties raised by such signals, and merits some consideration. Some of the simplest interpretations of the data, such as a light neutral particle decaying to muon and/or tau pairs, are largely disfavored; three-body decays to $\tau\tau\nu$ appear slightly better. An alternative speculative possibility -- a "micro-cascade decay" -- might be consistent with the data. It is suggested that the experimentalists involved provide additional plots showing invariant mass distributions of same- and opposite-sign dimuon pairs, invariant masses of various classes of displaced vertices, and spatial correlations among vertices within a cone. 
Matthew J. Strassler (Rutgers U.)