We use sampling techniques to find robust constraints on the masses of a possible fourth sequential fermion generation from electroweak oblique variables. We find that in the case of a light (115 GeV) Higgs from a single electroweak symmetry breaking
doublet, inverted mass hierarchies are possible for both quarks and leptons, but a mass splitting more than M(W) in the quark sector is unlikely. We also find constraints in the case of a heavy (600 GeV) Higgs in a single doublet model. As recent data from the Large Hadron Collider hints at the existence of a resonance at 124.5 GeV and a single Higgs doublet at that mass is inconsistent with a fourth fermion generation, we examine a type II two Higgs doublet model. In this model, there are ranges of parameter space where the Higgs sector can potentially counteract the effects of the fourth generation. Even so, we find that such scenarios produce qualitatively similar fermion mass distributions.
This presentation reports on the first experimental search for the decay Klong to $pi^0 pi^0 mu^+ mu^-$ based on data collected by the KTeV experiment. Although this decay mode is possible within the standard model, its rate is phase space suppressed
. The HyperCP experiment has recently observed 3 $Sigma^+ to p mu^+ mu^-$ events within a narrow di-muon mass range, suggesting that the process may occur via a new neutral state: $Sigma^+ to p X^0, X^0 tomu^+ mu^-$ with $m(X^0) = 214.3mev$. The $X^0$ would create an $s$- to $d$- quark transition at a rate that would cause Klong to $pi^0 pi^0 X, ^0 tomu^+ mu^-$ to occur at rates considerably over the standard model expectation. Our preliminary results significantly constrain this possibility.
Rare decay results from KTeV are reviewed, emphasizing modes that in principle provide information about the CKM matrix. Our recent results in lepton flavor violating modes are also presented.
