[Paper Review] Flavor changing neutral currents in top quark production and decay

E. Yazgan|arXiv (Cornell University)|Dec 19, 2013

Particle physics theoretical and experimental studies13 references3 citations

TL;DR

This paper presents model-independent searches for top quark flavor-changing neutral currents (FCNCs) in top quark decays and production at the Tevatron and LHC, using effective field theory to interpret results. No significant signals were observed; the study sets stringent 95% confidence level upper limits on branching ratios (e.g., B(t→Zq) < 0.05%) and coupling strengths, with future LHC runs expected to improve sensitivity by an order of magnitude.

ABSTRACT

Top quark flavor changing neutral current (FCNC) interactions are highly suppressed in the Standard Model. Therefore, any large signal of FCNCs will indicate the existence of new interactions. In this paper, searches for FCNC interactions in top quark production and decay at the Tevatron and LHC are presented. FCNC searches in $t ightarrow qZ$ and $t ightarrow Hq$ decays, and in top quark production in $pp ightarrow t+j$, $pp ightarrow t+Z$ are summarized. Effect of top quark FCNCs on single top quark cross-section, and the searches for same-sign top quark pair production through FCNCs are also described. None of the searches yielded positive results and exclusion limits on branching rations, coupling strengths and cross-sections are obtained. Future prospects of FCNC searches are also briefly discussed.

Motivation & Objective

• To search for flavor-changing neutral currents (FCNCs) in top quark decays and production processes at the Tevatron and LHC.
• To set model-independent limits on FCNC couplings using effective field theory, assuming new physics at scales above the top quark mass.
• To probe the existence of new physics beyond the Standard Model through rare top quark decays such as t→Zq, t→Hq, and t→γq.
• To evaluate the impact of FCNCs on single top quark production cross-sections and same-sign top quark pair production.
• To provide future projections for improved sensitivity at the 14 TeV LHC with increased luminosity.

Proposed method

• Utilizes an effective field theory Lagrangian with dimension-5 operators to describe FCNC interactions in top quark decays and production.
• Analyzes data from ATLAS, CDF, CMS, and D0 collaborations using proton-proton and proton-antiproton collisions at √s = 7–8 TeV and 1.96 TeV.
• Employs event selection based on final states such as dilepton, trilepton, and diphoton channels, with kinematic reconstruction of top quarks.
• Uses Monte Carlo simulations (MadGraph + Pythia) for signal and data-driven methods for background estimation.
• Applies multivariate analysis (BDT) to enhance signal discrimination in t→qZ and t→Hq channels.
• Performs maximum likelihood fits to extract signal significance and set upper limits on branching ratios and coupling strengths.

Experimental results

Research questions

• RQ1 What are the current 95% confidence level upper limits on the branching ratio B(t→Zq) from LHC and Tevatron data?
• RQ2 How do experimental results constrain the effective coupling strengths for t→Hq decays, particularly in the H→γγ and H→WW* decay modes?
• RQ3 To what extent do FCNC interactions affect the single top quark t-channel cross-section, and can existing data distinguish such effects from the Standard Model?
• RQ4 What are the exclusion limits on same-sign top quark pair production via a Z′ boson, and how do they constrain models explaining the top quark forward-backward asymmetry?
• RQ5 How will future LHC runs at √s = 14 TeV improve sensitivity to top quark FCNC processes?

Key findings

• The most stringent 95% CL upper limit on B(t→Zq) is 0.05%, obtained by combining CMS data at 7 and 8 TeV with 19.7 fb⁻¹ at 8 TeV.
• For t→cH decays with H→γγ, the observed 95% CL limit on the branching ratio is 0.83%, with an upper bound on the λtcH coupling of 0.17.
• The CMS collaboration sets a combined 95% CL limit of 1.28% on B(t→cH) across multiple Higgs decay modes (WW*, ττ, ZZ*).
• No evidence for same-sign top quark pair production was found; the ATLAS collaboration sets 95% CL limits of σ(pp→tt) < 0.21 pb for right-right chirality configurations.
• The coupling strength limits are κgut/Λ < 0.10 TeV⁻¹, κgct/Λ < 0.35 TeV⁻¹, κzut/Λ < 0.45 TeV⁻¹, and κZct/Λ < 2.27 TeV⁻¹ for gluon and Z-mediated FCNCs.
• Future projections show that with 300 fb⁻¹ of 14 TeV data, ATLAS expects to exclude B(t→qZ) > 2×10⁻⁴ and CMS expects to reach B(t→qZ) < 10⁻⁵ at 95% CL.