Bypassing spin-analyzing power dependence for quantum entanglement at colliders: a case study of $$ \Lambda \overline{\Lambda} $$
Junle Pei, Tianjun Li, Lina Wu, Xiqing Hao, Xiaochuan Wang · Journal of High Energy Physics · 2026
A bstract We study, as a concrete case study using the $$ \Lambda \left(\to p{\pi}^{-}\right)\overline{\Lambda}\left(\to \overline{p}{\pi}^{+}\right) $$ Λ → p π − Λ ¯ → p ¯ π + system, whether quantum entanglement in fermion pairs produced at colliders can be certified solely using angular information from final-state decays, while remaining independent of the parity-violating decay parameters α Λ and $$ {\alpha}_{\overline{\Lambda}} $$ α Λ ¯ . Building on a general decomposition of any angular observable in terms of Wigner d-functions, we show that the expectation value must take the form $$ {\mathcal{O}}_0+{\mathcal{O}}_1{\alpha}_{\Lambda}+{\mathcal{O}}_2{\alpha}_{\overline{\Lambda}}+{\mathcal{O}}_3{\alpha}_{\Lambda}{\alpha}_{\overline{\Lambda}} $$ O 0 + O 1 α Λ + O 2 α Λ ¯ + O 3 α Λ α Λ ¯ , with coefficients 𝒪 i ( i = 0 , 1 , 2 , 3) linear in the spin-density matrix elements $$ {\alpha}_{k,j}{\alpha}_{m,n}^{\ast } $$ α k , j α m , n ∗ . We obtain the value ranges of observables over the general and separable spaces of α k,j , and demonstrate a sufficient entanglement condition for pure states, extending it to mixed states by convexity. In constructing an α Λ - and $$ {\alpha}_{\overline{\Lambda}} $$ α Λ ¯ -independent witness from angular observables alone, we find that there are obstacles to probe quantum entanglement via the inequality-type and ratio-type ways. In particular, for the ratio-type criterion 〈 A 〉/〈 B 〉, the presence of zeros of 〈 B 〉 in both the general and separable spaces of $$ {\alpha}_{k,j}\left(k,j=\pm \frac{1}{2}\right) $$ α k , j k j = ± 1 2