D-branes and the Standard Model

Abstract

We perform a systematic study of the Standard Model embedding in a D-brane configuration of type I string theory at the TeV scale. We end up with an attractive model and we study several phenomenological questions, such as gauge coupling unification, proton stability, fermion masses and neutrino oscillations. At the string scale, the gauge group is U(3) color × U(2) weak × U(1) 1 × U(1) bulk . The corresponding gauge bosons are localized on three collections of branes; two of them describe the strong and weak interactions, while the last abelian factor lives on a brane which is extended in two large extra dimensions with a size of a few microns. The hypercharge is a linear combination of the first three U(1)s. All remaining U(1)s get masses at the TeV scale due to anomalies, leaving the baryon and lepton numbers as (perturbatively) unbroken global symmetries at low energies. The conservation of baryon number assures proton stability, while lepton number symmetry guarantees light neutrino masses that involve a right-handed neutrino in the bulk. The model predicts the value of the weak angle which is compatible with the experiment when the string scale is in the TeV region. It also contains two Higgs doublets that provide tree-level masses to all fermions of the heaviest generation, with calculable Yukawa couplings; one obtains a naturally heavy top and the correct ratio m b /m τ . We also study neutrino masses and mixings in relation to recent solar and atmospheric neutrino data.

MSLED, neutrino oscillations and the cosmological constant

“…[11,12,13,14], but this earlier round of model building differs from the present picture in several important ways.…”

“…The possibility that large extra dimensions might have implications for neutrino masses has been explored with many variations [11,12,13,14,15]. The vast majority of these variations share the common feature that the neutrino mass eigenstates arise when the usual brane-bound neutrino flavours mix with various higher-dimensional neutral fermions which are assumed to reside within the bulk.…”

“…An efficient way to evade these bounds is to require all of the sterile neutrinos to thermally decouple at a temperature, T D , above the QCD phase transition so their abundance can be diluted by the entropy release which occurs during this transition. This requires T D > ∼ 1 GeV, where for each sterile neutrino species T D is related to the active-sterile mixing parameter ε by the condition 13) where G F ∼ 10 −5 GeV −2 is the Fermi coupling and M p ∼ 10 18 GeV is the…”

“…• There are two light non-degenerated eigenvalues given by 13) with v i = − 4(ǫg ′ − ǫ ′ g)S u i = −2 − 2(ǫ 2 + 2(ǫ ′2 + g 2 + 2g ′2 ))P − 4(ǫg + 2ǫ ′ g ′ ) 2 P 2 p i = −(ǫg + 2ǫ ′ g ′ ) 2 S 2 , (A.14)…”

Unification and fermion mass relations in low string scale D-brane models

“…A realization of this scenario can occur in type I string theory [4] where gauge interactions are mediated by open strings with their ends attached on some D-brane stack, while gravity is mediated by closed strings that propagate in the whole 10 dimensional space. In the context of Type I string theory using appropriate collections of parallel [5,6] or intersecting [7,8] D-branes, there has been considerable work in trying to derive the Standard Model theory or its Grand Unified extensions [9][10][11][12][13][14][15][16][17][18][19]. Some of these low energy models revealed rather interesting features: (i) The correct value of the weak mixing angle is obtained for a string scale of the order of a few TeV (ii) baryon and lepton numbers are conserved due to the existence of exact global symmetries which are remnants of additional anomalous U (1) factors broken by the Green-Schwarz mechanism (iii) supersymmetry is not necessary to solve the hierarchy problem.…”