On the hierarchy of neutrino masses

We present a model of neutrino masses combining the seesaw mechanism and strong Dirac mass hierarchy and at the same time exhibiting a significantly reduced hierarchy at the level of active neutrino masses. The heavy Majorana masses are assumed to be degenerate. The suppression of the hierarchy is due to a symmetric and unitary operator R whose role is discussed. The model gives realistic mixing and mass spectrum. The mixing of atmospheric neutrinos is attributed to the charged lepton sector whereas the mixing of solar neutrinos is due to the neutrino sector. Small U_e3 is a consequence of the model. The masses of the active neutrinos are given by $\mu_3\approx\sqrt{\Delta m_{@}^2}$ and $\mu_1/\mu_2\approx \tan^2\theta_\odot$.Comment: 12 pages; Talk presented by M. Jezabek at 'Supersymmetry and Brane Worlds,' Fifth European Meeting Planck 02, Kazimierz, Poland, May 25-29, 2002, to appear in Acta Phys. Polon.

Precision W-boson and top-quark mass determinations at a muon collider

Precise determinations of the masses of the $W$ boson and of the top quark could stringently test the radiative structure of the Standard Model (SM) or provide evidence for new physics. We analyze the excellent prospects at a muon collider for measuring $M_W$ and $m_t$ in the $W^+W^-$ and $t\bar t$ threshold regions. With an integrated luminosity of 10 (100) fb$^{-1}$, the $W$-boson mass could be measured to a precision of 20 (6) MeV, and the top-quark mass to a precision of 200 (70) MeV, provided that theoretical and experimental systematics are understood. A measurement of $\Delta m_t=200$ MeV for fixed $M_W$ would constrain a 100 GeV SM Higgs mass within about $\pm 2$ GeV, while $\Delta M_W=6$ MeV for fixed $m_t$ would constrain $m_h$ to about $\pm 10$ GeV.Comment: 27 pages, 11 figures, postscript file available via anonymous ftp://ucdhep.ucdavis.edu/han/mumu/mwmt.p