TeV-Scale Z' Bosons from D-branes

TL;DR

This work analyzes how D-brane string models generically predict extra U(1) gauge bosons that can mix with the Standard Model Z, especially when the string scale $M_S$ lies near the TeV range. Masses arise from Green-Schwarz (B∧F) couplings via a Stückelberg mechanism, independent of the Higgs, with a hypercharge that stays massless; explicit D6- and D5-brane constructions are used to compute the mass matrices and mixing patterns. Including electroweak symmetry breaking, the Z boson acquires string-origin corrections to its mass, enabling bounds on $M_S$ from the $ ho$ parameter. The analysis finds that extra Z' bosons typically lie within an order of magnitude of the string scale, with ρ-parameter constraints often driving $M_S$ into the 1.5–40 TeV range, while still allowing some parameter choices that yield lighter or heavier spectra. These results support the phenomenological relevance of TeV-scale Z' searches as probes of string-scale physics in intersecting brane models.

Abstract

Generic D-brane string models of particle physics predict the existence of extra U(1) gauge symmetries beyond hypercharge. These symmetries are not of the E_6 class but rather include the gauging of Baryon and Lepton numbers as well as certain Peccei-Quinn-like symmetries. Some of the U(1)'s have triangle anomalies, but they are cancelled by a Green-Schwarz mechanism. The corresponding gauge bosons typically acquire a mass of order the string scale M_S by combining with two-index antisymmetric fields coming from the closed string sector of the theory. We argue that in string models with a low string scale M_S proportional to 1-10 TeV, the presence of these generic U(1)'s may be amenable to experimental test. Present constraints from electroweak precision data already set important bounds on the mass of these extra gauge bosons. In particular, for large classes of models, rho-parameter constraints imply M_S >= 1.5 TeV. In the present scheme some fraction of the experimentally measured Z^0 mass would be due not to the Higgs mechanism, but rather to the mixing with these closed string fields. We give explicit formulae for recently constructed classes of intersecting D6- and D5-brane models yielding the Standard Model (SM) fermion spectrum.