Same-Sign Taus Signatures of Maximally Flavor-Violating Scalars at the LHC

TL;DR

This work investigates flavons—spin-zero scalars with maximally flavor-violating, non-diagonal couplings—in an EFT framework, focusing on masses below the top-quark mass. It analyzes single and double flavon production at 13 and 14 TeV LHC, emphasizing same-sign tau signatures and $\tau\ell$ resonances as clean observables, and assesses constraints from top/W/Z widths and LFV decays. The study finds that at 13 TeV the LHC can exclude $c_{tq}/\Lambda$ around $0.1$ TeV$^{-1}$ for flavon masses near 50–80 GeV, while HL-LHC greatly extends sensitivity to $\sim(0.03$–$0.05)\,\text{TeV}^{-1}$; several parameter choices can explain the muon $g-2$ anomaly while evading LFV and unitarity constraints. Diagonal couplings generally weaken LHC sensitivity but the double production channel remains a powerful probe, and future LFV experiments (e.g., MEG II) provide complementary constraints, highlighting a synergistic path to test maximally FV flavon scenarios.

Abstract

We explore single and double flavor-violating scalar (flavon) production at the 13 and 14 TeV LHC in an effective field theory formulation where flavons always change the flavor of the Standard Model fermions. When those scalars couple to mass, their flavor-changing couplings to top quarks and tau leptons are favored. Focusing on the mass region below the top-quark mass, we find couplings that fit the muon $(g-2)$ discrepancy and avoid several current experimental constraints. We determine the potential of the LHC to exclude or discover such a new physics scenario with clean signatures consisting of same-sign tau leptons and the simultaneous observation of resonances in the tau plus electron or muon invariant mass. We found that in the double production mode, effective couplings down to order $10^{-2}$ TeV$^{-1}$ can be probed for flavon masses in the 10--170 GeV range at the 14 TeV HL-LHC, but couplings down to 0.1 TeV$^{-1}$ can already be excluded at 95\% confidence level with data collected from the 13 TeV LHC in the same mass interval. We also explore the impact of sizeable diagonal flavon couplings on the prospects of LHC for the signals we propose.

Figures (13)

• Figure 1: Feynman diagrams for single (left and central panels) and double (right panel) production of maximally flavor-violating scalars in hadron collisions. Single production is suppressed by $(m_t/\Lambda)^2$ while double production is suppressed by $(m_t/\Lambda)^4$.
• Figure 2: The production cross section of flavon pairs ($\sigma_{q\bar{q}}$) and associated flavon+top quark ($\sigma_{qg}$) at the 14 TeV LHC. The dashed lines depict the cross-section times selection efficiency as discussed in the text. The branching ratio BR$(t \to aq)$ for various values of flavon masses as a function of the cutoff scale, $\Lambda$. The shaded area represents the constraint from Eq. \ref{['eq:br']}.
• Figure 3: Left: the percentage of surviving events after successive final state selections in the associate $a+t$ production. Right: the selection efficiencies for double production. In both cases, requiring two taus with the same charge has the biggest impact on the overall identification efficiency.
• Figure 4: The $\tau^\pm$-$\ell^\mp=e,\mu$ invariant mass for the double production $q \bar{q}^\prime \to aa$ for flavon masses of 80, 100, and 120 GeV. The histogram is normalized by the number of events expected at the 13 TeV LHC after 139 fb$^{-1}$. We estimate a negligible number of background events in this channel.
• Figure 5: 95% CL exclusion region in the $m_a \times c_{tq}/ \Lambda$ parameter space. Red and black lines represent the $pp \to aa$ and $pp \to at$ processes, respectively. The solid and dashed lines represent the reach of these channels for $139 \, fb^{-1}$@13 TeV and $3000 \, fb^{-1}$@14 TeV limits, respectively. Each panel assumes a different level of systematic error. For systematic errors greater than $5\%$, the $pp \to aa$ process is the most competitive to exclude the model.