[Paper Review] Scalar diquark in t tbar production and constraints on Yukawa sector of grand unified theories

Nejc Košnik, Ilja Doršner+3 more|arXiv (Cornell University)|Nov 2, 2011

Particle physics theoretical and experimental studies11 references4 citations

TL;DR

This paper proposes a colored scalar diquark (Δ) with hypercharge 4/3 as a solution to the top quark forward-backward asymmetry and the muon anomalous magnetic moment. It shows that Δ can simultaneously explain both anomalies while satisfying stringent flavor-changing neutral current and lepton flavor violation constraints, with a preferred mass window below 560 GeV and specific coupling hierarchies in the Yukawa sector of grand unified theories.

ABSTRACT

A colored weak singlet scalar state with hypercharge 4/3 is one of the possible candidates for the explanation of the unexpectedly large forward-backward asymmetry in $t \bar t$ production as measured by the CDF and D0 experiments. We investigate the role of this state in a plethora of flavor changing neutral current processes and precision observables of down-quarks and charged leptons. Our analysis includes tree- and loop-level mediated observables in the K and B systems, the charged lepton sector, as well as the $Z o b \bar b$ width. We perform a fit of the relevant scalar couplings. This approach can explain the $(g-2)_μ$ anomaly while tensions among the CP violating observables in the quark sector, most notably the nonstandard CP phase (and width difference) in the $B_s$ system cannot be fully relaxed. The results are interpreted in a class of GUT models which allow for a light colored scalar with a mass below 1 TeV.

Motivation & Objective

• To explain the large forward-backward asymmetry in t\bar{t} production at the Tevatron using a colored scalar diquark with hypercharge 4/3.
• To investigate the impact of the diquark on flavor-changing neutral current processes involving down-quarks and charged leptons.
• To assess the viability of the diquark in resolving the muon anomalous magnetic moment anomaly while satisfying low-energy precision observables.
• To constrain the Yukawa couplings of the diquark in the context of SU(5) and SO(10) grand unified theories.
• To determine the allowed parameter space for the diquark mass and couplings using a global fit to experimental data.

Proposed method

• The scalar diquark Δ couples to right-handed up-type quarks via an antisymmetric Yukawa coupling matrix $ g_{ij} $, with $ g_{ut} $ responsible for t\bar{t} production enhancement.
• Tree-level and loop-level contributions from Δ are evaluated in $ D-\bar{D} $ mixing, $ B_s $ mixing, $ Z \to b\bar{b} $ width, and rare decays using effective field theory at scale $ \mu = m_\Delta $.
• The effective Hamiltonian for $ C=2 $ $ D-\bar{D} $ mixing is derived as $ \mathcal{H}^{\Delta C=2} \propto (g_{ut}g_{ct}^*)^2 h(m_\Delta^2/m_t^2) Q_6 $, with $ Q_6 $ a four-quark operator.
• The anomalous magnetic moment of the muon is computed via a one-loop diagram involving $ Y_{\mu i} $ couplings to muons and down-quarks, yielding $ a_\mu^{\rm new} \propto \sum_i |Y_{\mu i}|^2 m_\mu^2 / m_\Delta^2 $.
• A global $ \chi^2 $ fit is performed across 15 degrees of freedom, including $ a_\mu $, CKM matrix elements, $ \Delta m_s $, and $ \sin 2\beta $, to constrain the $ Y $ matrix.
• Constraints from dijet and single-top production at the Tevatron are used to bound $ |g_{uc}| $, while LHC data on second-generation leptoquarks set a lower mass limit of $ m_\Delta \gtrsim 380 $ GeV.

Experimental results

Research questions

• RQ1 Can a colored scalar diquark with hypercharge 4/3 simultaneously explain the large forward-backward asymmetry in t\bar{t} production and the muon anomalous magnetic moment?
• RQ2 What are the constraints on the diquark's couplings to charm and up quarks from $ D-\bar{D} $ mixing and Tevatron dijet/single-top data?
• RQ3 How do lepton flavor violation and flavor-changing neutral current processes constrain the Yukawa couplings of the diquark to charged leptons and down-quarks?
• RQ4 What is the viable mass window for the diquark in SU(5) and SO(10) grand unified theories, given the combined experimental constraints?
• RQ5 Can a symmetric Yukawa structure in the down-quark and charged lepton sectors be compatible with the presence of a light diquark?

Key findings

• The diquark mass is constrained to $ m_\Delta < 560 $ GeV by perturbativity and $ m_\Delta \gtrsim 380 $ GeV by LHC data, resulting in a narrow discovery window.
• A global $ \chi^2 $ fit yields $ \chi^2_{\rm min} = 2.5 $ for 15 degrees of freedom, indicating excellent agreement with data, with the $ a_\mu $ anomaly fully resolved.
• The $ 1\sigma $ allowed range for $ Y_{\mu i} $ includes values up to order 1 for $ Y_{\mu d} $, $ Y_{\mu s} $, or $ Y_{\mu b} $, depending on the regime, while $ Y_{\mu e} $ is strongly suppressed.
• The coupling $ |g_{ct}| $ is bounded at $ < 0.0038 $ at 2σ CL due to $ D-\bar{D} $ mixing constraints, independent of phase assumptions.
• The $ |g_{uc}| $ coupling is constrained by dijet and single-top data, with the Tevatron excluding large regions of the $ m_\Delta $–$ |g_{uc}| $ plane.
• The symmetric Yukawa structure in the down-quark and charged lepton sectors is incompatible with the light diquark scenario unless $ m_\Delta $ is sufficiently heavy to decouple from low-energy physics.