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Phenomenology of A Three-Family Standard-like String Model

Mirjam Cvetic, Paul Langacker, Gary Shiu

TL;DR

This work probes the phenomenology of a concrete Type II orientifold construction that yields a three-family Standard-like spectrum with extra U(1)'s and a quasi-hidden $Sp$ sector. The authors analyze the perturbative spectrum, gauge-coupling predictions, and the potential impact of a strongly coupled hidden sector, finding that moduli-dependent gauge couplings can reproduce strong sector behavior while electroweak couplings are too small due to the extended Higgs and exotic content. A distinctive feature is the emergence of fractionally charged exotic left-handed states that are argued to be confined and replaced by composite partners, consistent with anomaly matching. The study highlights the challenges of achieving realistic EW physics in such top-down models and contrasts the orientifold results with a heterotic CHL5 example, underscoring the role of moduli and strong dynamics in shaping low-energy phenomenology.

Abstract

We discuss the phenomenology of a three-family supersymmetric Standard-like Model derived from the orientifold construction, in which the ordinary chiral states are localized at the intersection of branes at angles. In addition to the Standard Model group, there are two additional U(1)' symmetries, one of which has family non-universal and therefore flavor changing couplings, and a quasi-hidden non-abelian sector which becomes strongly coupled above the electroweak scale. The perturbative spectrum contains a fourth family of exotic (SU(2)- singlet) quarks and leptons, in which, however, the left-chiral states have unphysical electric charges. It is argued that these decouple from the low energy spectrum due to hidden sector charge confinement, and that anomaly matching requires the physical left-chiral states to be composites. The model has multiple Higgs doublets and additional exotic states. The moduli-dependent predictions for the gauge couplings are discussed. The strong coupling agrees with experiment for reasonable moduli, but the electroweak couplings are too small.

Phenomenology of A Three-Family Standard-like String Model

TL;DR

This work probes the phenomenology of a concrete Type II orientifold construction that yields a three-family Standard-like spectrum with extra U(1)'s and a quasi-hidden sector. The authors analyze the perturbative spectrum, gauge-coupling predictions, and the potential impact of a strongly coupled hidden sector, finding that moduli-dependent gauge couplings can reproduce strong sector behavior while electroweak couplings are too small due to the extended Higgs and exotic content. A distinctive feature is the emergence of fractionally charged exotic left-handed states that are argued to be confined and replaced by composite partners, consistent with anomaly matching. The study highlights the challenges of achieving realistic EW physics in such top-down models and contrasts the orientifold results with a heterotic CHL5 example, underscoring the role of moduli and strong dynamics in shaping low-energy phenomenology.

Abstract

We discuss the phenomenology of a three-family supersymmetric Standard-like Model derived from the orientifold construction, in which the ordinary chiral states are localized at the intersection of branes at angles. In addition to the Standard Model group, there are two additional U(1)' symmetries, one of which has family non-universal and therefore flavor changing couplings, and a quasi-hidden non-abelian sector which becomes strongly coupled above the electroweak scale. The perturbative spectrum contains a fourth family of exotic (SU(2)- singlet) quarks and leptons, in which, however, the left-chiral states have unphysical electric charges. It is argued that these decouple from the low energy spectrum due to hidden sector charge confinement, and that anomaly matching requires the physical left-chiral states to be composites. The model has multiple Higgs doublets and additional exotic states. The moduli-dependent predictions for the gauge couplings are discussed. The strong coupling agrees with experiment for reasonable moduli, but the electroweak couplings are too small.

Paper Structure

This paper contains 7 sections, 18 equations, 2 figures, 6 tables.

Table of Contents

  1. Introduction
  2. Description of the Model
  3. The Perturbative Spectrum
  4. The Additional $U(1)$ Factors
  5. Gauge Couplings
  6. Implications of a Strongly Coupled Sector
  7. Discussion

Figures (2)

  • Figure 1: Predicted values of (a) $1/\alpha_3, (b) 1/\alpha = 1/\alpha_Y + 1/\alpha_2$, and (c) the weak angle $\sin^2 \theta_W = \alpha_Y/(\alpha_2 + \alpha_Y)$ at the electroweak scale as a function of $\chi$ for $M_P^{(4d)} = M_s$. Only the contributions $b_a({\rm int})$ of the states localized at the brane intersections (as well as the gauge bosons) to the running are included. The experimental values are $\sim$ 8.5, 128, and 0.23, respectively.
  • Figure 2: Scales $\tau = \frac{1}{2\pi} \ln \frac{M}{M_Z}$, where $M$ is the scale at which the $Sp$ group becomes strongly coupled for $M_P^{(4d)} = M_s$. The curves are $Sp(2)_A$ (solid), $Sp(2)_B$ (circles), $Sp(4)$ (crosses).