Perturbative QCD Prediction of the Hyperon EDM from CP-violating Dipole Interactions

Electric dipole moment (EDM) of baryons provides a sensitive probe of CP-violating interactions beyond the Standard Model. Motivated by the recent BESIII measurement on the $Λ$-hyperon EDM [1], we present the first perturbative QCD analysis of the $Λ$ EDM form factor to elucidate its origin in CP-violating quark dipole interactions. In particular, we derive a QCD factorization formula that relates the $Λ$ EDM form factor to quark EDMs and chromo-electric dipole moments (CEDMs) through convolutions with the light-cone distribution amplitudes of $Λ$. These connections allow us to extract constraints on CP-violating dipole couplings from current and future hyperon EDM measurements. Our numerical analysis demonstrates that the $Λ$ EDM exhibits unique sensitivity to the strange-quark CEDM, providing complementary information to that obtained from the neutron EDM.

💡 Research Summary

The paper presents the first perturbative QCD (pQCD) analysis of the Λ‑hyperon electric dipole moment (EDM) form factor in the high‑energy regime, motivated by the recent BESIII measurement of the Λ EDM in J/ψ→Λ Λ̄ decays. The authors start from a CP‑violating effective Lagrangian that contains quark electric dipole moments (EDMs) d_q and chromo‑electric dipole moments (CEDMs) \tilde d_q for the light quarks u, d, and s. They define the EDM form factor d_Λ(Q) through the matrix element of the electromagnetic current between a Λ‑Λ̄ pair, with Q^2 = M_{J/ψ}^2 being the relevant momentum transfer.

In the limit Q ≫ Λ_QCD, the authors employ the collinear factorization framework for hard exclusive processes. The Λ baryon is described by its leading‑twist (twist‑3) light‑cone distribution amplitudes (LCDAs) V_Λ, A_Λ, and T_Λ, which encode the momentum‑fraction distribution of the three constituent quarks. Because the dipole operators themselves flip quark helicity, the helicity‑flip Λ‑Λ̄ transition receives non‑zero contributions already at twist‑3, leading to the scaling d_Λ(Q) ∝ Q^{‑4}, consistent with power‑counting rules for exclusive processes.

The authors calculate all relevant one‑loop diagrams. For the quark EDM contributions there are 14 distinct color topologies for each quark flavor, amounting to 42 diagrams in total. For the CEDM contributions each flavor yields 56 diagrams. Color‑singlet constraints eliminate diagrams with three‑gluon vertices, and helicity conservation forces the CEDM of gluons or the CP‑violating Weinberg operator to vanish at leading twist. The resulting factorization formulas are:

d_Λ^{EDM}(Q) = C_B (4π α_s)^2 Q^{‑4} ∫