Probing top-quark electroweak couplings indirectly at the Electron-Ion Collider

Top quark electroweak interactions serve as a sensitive probe for beyond-Standard-Model physics, potentially exhibiting significant deviations from Standard Model predictions and offering crucial tests of fundamental symmetries in ultraviolet physics. This Letter investigates their measurement through deep inelastic scattering (DIS) processes involving top quark loops at the Electron-Ion Collider (EIC), within the Standard Model Effective Field Theory framework. We demonstrate that left-handed electron beam polarization can significantly enhance DIS cross sections for these interactions compared to right-handed scenario, highlighting the pivotal role of high polarization at the EIC in constraining these couplings. Compared to direct $pp \to t\bar{t}Z$ measurements at the Large Hadron Collider (LHC), DIS measurements could significantly improve these coupling constraints and effectively resolve parameter space degeneracies present in LHC data. Moreover, the expected precision from the EIC’s high-luminosity phase matches that of LEP electroweak precision measurements, underscoring the EIC’s significant potential to probe top quark properties.

💡 Research Summary

This paper presents a detailed theoretical investigation into the potential of the upcoming Electron-Ion Collider (EIC) to probe the electroweak couplings of the top quark indirectly, offering a complementary approach to direct searches at high-energy colliders like the LHC.

The study operates within the Standard Model Effective Field Theory (SMEFT) framework, parameterizing potential Beyond-the-Standard-Model (BSM) effects through a set of dimension-six operators that modify the top quark’s interactions with the Z boson and photon (e.g., O_Ht, O_HQ, O_tW, O_tB). The Wilson coefficients of these operators are the targets of the analysis.

The proposed method leverages the deep inelastic scattering (DIS) process, e-p → e-j, at the EIC’s planned center-of-mass energy of √s = 100 GeV. Although the top quark is too heavy to be produced directly at this energy, its influence manifests through quantum loop corrections at the Next-to-Leading Order (NLO). Specifically, anomalous top-quark couplings contribute primarily via self-energy corrections to the gauge bosons propagators in the DIS process. The authors perform a precise calculation of these NLO effects, incorporating both direct loop contributions and indirect shifts in renormalized parameters (α, m_Z, s_W) using an effective “star scheme.”

The core findings highlight the unique advantages of the EIC:

1. Critical Role of Beam Polarization: The analysis reveals a dramatic dependence on the polarization of the electron beam. Cross sections for left-handed polarized electrons (P_e = -70%) are approximately an order of magnitude larger than for right-handed polarization (P_e = +70%). This translates into significantly enhanced sensitivity to the Wilson coefficients for left-handed beams, underscoring the essential value of the EIC’s highly polarized electron source for such precision measurements.
2. Projected Sensitivity: Through a χ² analysis based on expected statistical uncertainties, the paper derives projected constraints on the Wilson coefficients for both the EIC baseline (integrated luminosity of 100 fb⁻¹) and its high-luminosity phase (1000 fb⁻¹). The constraints exhibit strong directional sensitivity; for instance, in the plane of C_Ht and C_HQ^(-), the EIC can impose very tight limits on the axial-vector combination but much weaker limits on the vector combination, a consequence of the low momentum transfer involved.
3. Complementarity with LHC: A key conclusion is the complementary nature of the EIC’s indirect DIS probes and the LHC’s direct measurements of processes like pp → ttZ. The EIC’s precision measurements are sensitive in different directions of the parameter space, which can help resolve degeneracies present in LHC data alone. The expected precision from the EIC’s high-luminosity phase is noted to be comparable to that achieved by LEP electroweak precision data.
4. Observables: The study considers not only the total cross section but also the polarized forward-backward asymmetry (A_FB). While A_FB provides constraints in directions somewhat complementary to the unpolarized cross section, its constraining power is generally weaker than that obtained from the left-handed polarized cross section measurement.

In summary, the research makes a compelling case that the EIC, primarily designed for nuclear physics, has substantial and unique potential as a precision tool for particle physics. By exploiting its high electron beam polarization and clean experimental environment, the EIC can provide stringent, complementary constraints on top-quark electroweak couplings, thereby enhancing the global search for new physics.