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Effects of mixing with quark singlets

J. A. Aguilar-Saavedra

TL;DR

The paper examines SM extensions with vector-like quark singlets that mix with SM quarks, focusing on how this mixing breaks CKM unitarity and induces tree-level FCNCs. It analyzes two realizations (up-type and down-type singlets) across direct searches, precision electroweak data, and a wide array of FCNC processes in K and B systems, using a detailed operator formalism for ΔF=2 and rare decays. The results show that substantial deviations in top-quark couplings (e.g., $|V_{tb}|$ and $X_{tt}$) and in rare kaon/B-meson processes (notably $K_L\to \pi^0\nu\bar{\nu}$ and $a_{D_s^+D_s^-}$) can occur while remaining consistent with current measurements, especially in Model I; down-type singlets yield more conservative but still observable effects. Mixing with two singlets can even improve the CKM fit and electroweak consistency, suggesting that future collider measurements and rare-decay experiments could reveal or tightly constrain such new physics. The study provides a comprehensive mapping of how vector-like singlet mixing could manifest in both high-energy and flavor observables, guiding experimental searches for non-SM quark dynamics.

Abstract

The mixing of the known quarks with new heavy singlets can modify significantly some observables with respect to the Standard Model predictions. We analyse the range of deviations permitted by the constraints from precision electroweak data and flavour-changing neutral processes at low energies. We study top charged current and neutral current couplings, which will be directly tested at top factories, like LHC and TESLA. We discuss some examples of observables from K and B physics, as the branching ratio of K_L -> pi^0 nu nu, the B^0_s mass difference or the time-dependent CP asymmetry in B_s^0 -> D_s^+ D_s^-, which can also show large new effects.

Effects of mixing with quark singlets

TL;DR

The paper examines SM extensions with vector-like quark singlets that mix with SM quarks, focusing on how this mixing breaks CKM unitarity and induces tree-level FCNCs. It analyzes two realizations (up-type and down-type singlets) across direct searches, precision electroweak data, and a wide array of FCNC processes in K and B systems, using a detailed operator formalism for ΔF=2 and rare decays. The results show that substantial deviations in top-quark couplings (e.g., and ) and in rare kaon/B-meson processes (notably and ) can occur while remaining consistent with current measurements, especially in Model I; down-type singlets yield more conservative but still observable effects. Mixing with two singlets can even improve the CKM fit and electroweak consistency, suggesting that future collider measurements and rare-decay experiments could reveal or tightly constrain such new physics. The study provides a comprehensive mapping of how vector-like singlet mixing could manifest in both high-energy and flavor observables, guiding experimental searches for non-SM quark dynamics.

Abstract

The mixing of the known quarks with new heavy singlets can modify significantly some observables with respect to the Standard Model predictions. We analyse the range of deviations permitted by the constraints from precision electroweak data and flavour-changing neutral processes at low energies. We study top charged current and neutral current couplings, which will be directly tested at top factories, like LHC and TESLA. We discuss some examples of observables from K and B physics, as the branching ratio of K_L -> pi^0 nu nu, the B^0_s mass difference or the time-dependent CP asymmetry in B_s^0 -> D_s^+ D_s^-, which can also show large new effects.

Paper Structure

This paper contains 33 sections, 94 equations, 11 figures, 13 tables.

Table of Contents

  1. Introduction
  2. Brief description of the models
  3. Direct limits
  4. Limits from precision electroweak data
  5. $R_b$, $R_c$ and FB asymmetries
  6. Oblique parameters
  7. Limits from FCN processes at low energies
  8. Neutral meson oscillations
  9. The $\Delta F=2$ effective Lagrangians
  10. $K^0$ oscillations
  11. $B^0$ oscillations
  12. $B^0_s$ oscillations
  13. $D^0$ oscillations
  14. $K$ decays
  15. $K^+ \to \pi^+ \nu \bar{\nu}$
  16. ...and 18 more sections

Figures (11)

  • Figure 1: Allowed values of $|V_{tb}|$ (shaded area) in Model I, as a function of the mass of the new quark.
  • Figure 2: Allowed values of the coupling $|V_{Tb}|$ of the new quark (shaded area) in Model I, as a function of its mass.
  • Figure 3: Allowed values of $|V_{ts}|$ (shaded area) in Model I, as a function of the mass of the new quark.
  • Figure 4: Allowed values of $|V_{td}|$ (shaded area) in Model I, as a function of the mass of the new quark.
  • Figure 5: Allowed values of the ratio $|V_{td}/V_{ts}|$ (shaded area) in Model I, as a function of the mass of the new quark.
  • ...and 6 more figures