A universal seesaw mechanism in five dimensions.pdf

a r X i v : h e p - p h / 0 6 0 8 2 5 7 v 1 2 2 A u g 2 0 0 6 TUM-HEP-643/06 A universal seesaw mechanism in five dimensions Naoyuki Haba ? Physik-Department, Technische Universita?t Mu?nchen, James-Franck-Strasse, D-85748 Garching, Germany Institute of Theoretical Physics, University of Tokushima, 770-8502 Tokushima, Japan Abstract We show the universal seesaw in the extra dimension setup, where three extra vector-like fields exist in the 5D bulk with heavy masses. We take the framework of the left-right symmetric model. The universal seesaw formula is easily obtained as a replacement of the vector-like mass in 4D case Mi to 2M? tan[πRMi] (M?: 5D Planck scale, Mi: vector-like bulk mass, and R: compactification radius). The smallness of Dirac neutrino mass can be naturally explained in the 5D setup. We also show the Majorana neutrino case. ?E-mail: haba@ph.tum.de 1 Introduction The origin of quark and lepton masses is still a mystery, which can be a very important key for searching a fundamental theory beyond the standard model (SM). Why are fermion masses except for top quark mass much smaller than electroweak breaking scale, and why are neutrino masses much smaller than other fermion masses? The ordinal seesaw mechanism was suggested to explain the smallness of neutrino masses[1]. The smallness of quark and lepton masses (except for top quark mass) comparing to the electroweak breaking scale can be explained in a different framework, where vector-like extra fermions are introduced with heavy masses. This mechanism is so-called universal seesaw mechanism[2]-[8]. The extra fermions have vector-like masses so that they have no effects to the electroweak precision measurements. The left-right symmetric model with a gauge group, SU(3)c×SU(2)L×SU(2)R×U(1)B?L, is one example which can have a universal seesaw structure. For the top quark mass which is the same order as electroweak symmetry breaking, we should take special setup[7]?. In this paper we show the universa