We study the spin-entangled mixed state of a spin- 1 / 2 baryon–antibaryon pair produced in the process e + e − → J / ψ , ψ ′ → B B ¯ . We show that the spin entanglement of the system can increase following the decays B → b + M and B ¯ → b ¯ + M ¯ , where b , b ¯ are spin- 1 / 2 baryons and M , M ¯ are spin-0 mesons. This phenomenon, known as entanglement autodistillation, manifests as a probabilistic amplification of entanglement during the decay process. We analyze the underlying mechanism and show that this phenomenon depends only on the initial state of the B B ¯ system and the decay parameter α D , but not on the decay parameter ϕ D . This effect only arises when parity is violated in the decay, when α D ≠ 0 .
We present a comprehensive theoretical framework for probing quantum entanglement in the decay angular distributions of a spin-
S particle-antiparticle pair
AAˉ, where each particle decays sequentially into a two-body final state,
A→B+C and
Aˉ→Bˉ+Cˉ, with
B(Bˉ) carrying spin
b and
C(Cˉ) being spinless. Starting from the most general polarized initial state, we derive the fully differential angular distribution
W(θ1,θ2,ϕ1,ϕ2) and identify observables
⟨cos(2S(ϕ1∓ϕ2))⟩ whose expectation values directly depend on the entanglement-sensitive coefficients
Re(α−S,∓SαS,±S∗) of the initial state. The proportionality factor
C(S,b) in these relations is computed explicitly. For bosonic decays (
b=0,1,2,…),
C(S,b) is universal and independent of decay dynamics; in particular,
C(S,0)=1/2 for any
S, and
C(1,1)=1/8, matching known results for
W+W− decays. For fermionic decays (
b=21,23,25…),
C(S,b) depends explicitly on the spin analysis powers
αA/Aˉ, making entanglement extraction more decay-dependent. We further demonstrate, within the context of
e+e−→γ∗→AAˉ production, how
αA/Aˉ can be determined experimentally using specific angular observables restricted to the beam-axis region. Our results highlight the special role of bosonic decays in providing clean, model-independent tests of quantum entanglement at colliders, while outlining a pathway for entanglement measurement in fermionic cases through supplementary polarization information.
We consider a pair of unstable fermions in a spin-entangled state. After the decay of one fermion, a spin measurement is performed on the surviving partner, with a Stern-Gerlach experiment or similar. The measurement not only projects the spin of the surviving fermion, but is also physically equivalent to a spin projection for the decayed one -- even when it no longer exists. This post-selection effect would be experimentally accessible using muon pairs in a maximally-entangled state, produced either in the decay of a scalar particle, or in
e+e− collisions at wide angles.
We consider a pair of unstable fermions in a spin-entangled state. After the decay of one fermion, a spin measurement is performed on the surviving partner, with a Stern-Gerlach experiment or similar. The measurement not only projects the spin of the surviving fermion, but is also physically equivalent to a spin projection for the decayed one — even when it no longer exists. This post-selection effect would be experimentally accessible using muon pairs in a maximally-entangled state, produced either in the decay of a scalar particle, or in e+e− collisions at wide angles.
We investigate the role of time ordering in entanglement experiments involving unstable particles, focusing on
μ+μ− pairs produced in a maximally-entangled spin state. We analyse the correlations between measurements performed by two experimenters, Alice (who measures
μ− spin) and Bob (who measures
μ+ decay products). Remarkably, the observed correlations persist irrespective of whether Bob's muon decays before or after Alice's spin measurement. We further discuss different interpretations of the same empirical results depending on the observer's reference frame. The findings reinforce the viewpoint that the Copenhagen interpretation of measurement is a mathematical tool rather than a literal account of physical reality.
Abstract Hyperon-antihyperon pairs produced in high-energy electron-positron annihilation constitute a naturally spin-entangled system in the high-energy regime. Recently, a probabilistic amplification of entanglement, termed autodistillation, has been found in the daughter baryon-antibaryon pairs from hyperon decay and is constrained by an upper boundary. This work demonstrates that the quantum entanglement in this process may be accompanied by a decrease, constrained by a lower boundary, but it will not be completely lost. Thus, the entanglement of these systems undergoes redistribution within the phase space during the sequential decays of hyperons, highlighting the important role of hyperon polarization. By using the explicit spin density matrix of baryon pairs, it is also found that the quantumness of the system, characterized by quantum discord, always has the possibility to increase during decay processes, even when entanglement evaluated by concurrence and negativity does not increase.