It has been argued more than 30 years ago that it is not possible to test locality at colliders, due to the inability to directly measure non-commutating observables such as spin components in current collider experiments. Recently, there has been a lot of phenomenological and experimental activity around testing locality via Bell-type experiments or entanglement versus non-entanglement in a collider environment. These results seem to evade the earlier no-go theorem by indirectly measuring spin correlations via their relation to angular correlations between momenta. We perform a careful study of the feasibility of such an approach. We scrutinize the relationship between spin and angular correlations in both quantum mechanics and local hidden variable theories. Our conclusion is that it is currently not possible to perform a logically coherent set of experimental measurements at colliders that would allow one to test locality or entanglement versus non-entanglement. This reaffirms the earlier no-go theorem. We stress that the no-go theorem does not apply to measurements of observables inspired from entanglement and Quantum Information Theory to test the Standard Model of particle physics.
It has been argued more than 30 years ago that it is not possible to test locality at colliders, due to the inability to directly measure non-commutating observables such as spin components in current collider experiments. Recently, there has been a lot of phenomenological and experimental activity around testing locality via Bell-type experiments or entanglement versus non-entanglement in a collider environment. These results seem to evade the earlier no-go theorem by indirectly measuring spin correlations via their relation to angular correlations between momenta. We perform a careful study of the feasibility of such an approach. We scrutinize the relationship between spin and angular correlations in both quantum mechanics and local hidden variable theories. Our conclusion is that it is currently not possible to perform a logically coherent set of experimental measurements at colliders that would allow one to test locality or entanglement versus non-entanglement. This reaffirms the earlier no-go theorem. We stress that the no-go theorem does not apply to measurements of observables inspired from entanglement and Quantum Information Theory to test the Standard Model of particle physics.
A bstract Recently there has been an increased interest in possible tests of locality via Bell’s inequalities, as well as separately tests of entanglement at colliders, in particular at the LHC. These have involved various physical processes, such as tt t t ¯ , or τ + τ − production, or the decay of a Higgs boson to two vector bosons H → VV ∗ . We argue that none of these proposals constitute a test of locality via Bell’s inequality or promise unconditional observational evidence of entanglement. In all cases what is measured are the momenta of the final state particles. Using the construction proposed by Kasday (1971) in a different context, and adapted to collider scenarios by Abel, Dittmar, and Dreiner (1992), it is straightforward to construct a local hidden variable theory (LHVT) which exactly reproduces the data. This construction is only possible as the final state momenta all commute. We show that this LHVT satisfies Bell’s inequality or the related CHSH inequality as appropriate for all the proposed LHC collider tests in the literature. Thus a test of locality via Bell’s inequality is not possible at colliders. The LHVT is also by construction local, i.e. all correlations are separable. Thus an unconditional proof of entanglement is also inherently not possible at colliders. It can only be shown that the entanglement within the Standard Model consistently describes the data.
One of the most notable aspects of quantum systems is that their components can exhibit correlations much stronger than those allowed by classical physics. Two examples of quantum correlations are quantum entanglement and Bell nonlocality, but generally there is a hierarchy of many types of quantum correlations. Among these correlations, Bell nonlocality holds a special place because it plays a dual role in distinguishing theories where local realism is a valid description. A Bell test, which is a test of local realism, typically needs to be augmented with assumptions to address possible loopholes in the experimental setup. In this work, we study Bell tests in experiments in which the detector reports the correct outcome with a specified probability. This mirrors the situation at high-energy colliders, where particle spins are not measured directly but inferred from the angular distributions of their decay products. We show that, in this setup, a test of local realism is not possible. Quantum correlations, however, are still present, measurable, and informative in high-energy colliders. These correlations are the building blocks of the interesting, developing quantum information science program at high-energy colliders. The measurements of entanglement by the ATLAS and CMS experiments are the first steps in this initiative.
This paper discusses a possible resolution of the nonobjectivity-nonlocality dilemma in quantum mechanics in 'the light of experimental tests of the Bell inequality for two entangled photons and a Bell-like inequality for a single neutron. My conclusion is that these experiments show that quantum mechanics is nonobjective: that is, the values of physical observables cannot be assigned to a system before measurement. Bell's assumption of nonlocality has to be rejected as having no direct experimental confirmation, at least thus far. I also consider the relationships between nonobjectivity and contextuality. Specifically, I analyze the impact of the Kochen-Specker theorem on the problem of contextuality of quantum observables. I argue that, just as von Neumann's "no-go" theorem, the Kochen-Specker theorem is based on assumptions that do not correspond to the real physical situation. Finally, I present a theory of measurement based on a classical, purely wave model (pre-quantum classical statistical field theory), a model that reproduces quantum probabilities. In this model continuous fields are transformed into discrete clicks of detectors. While this model is classical, it is nonobjective. In this case, nonobjectivity is the result of the dependence of experimental outcomes on the context of measurement, in accordance with Bohr's view.
In a recent Nature article, Hensen et al. reported that they have accomplished a loophole-free test of Bell's theorem. The authors speculated that further improvements in their experimental design could settle an 80 years debate in favor of quantum theory's stance that entanglement is action at a distance. We direct attention to a spatial aspect of locality, not considered by Bell's Theorem nor by any of its experimental tests. We refer to the possibility that two particles distancing from each other could remain spatially disconnected, even when they have distanced enough to ensure that information between them was transmitted faster than the velocity of light. We show that any local-deterministic relativity theory which violates Lorentz's contraction for distancing bodies can maintain spatial locality. We briefly note that the recently proposed Information Relativity Theory satisfies the aforementioned condition, and that it predicts and explains several quantum phenomena, despite being local and deterministic. We conclude by arguing that quantum entanglement is not nonlocal and that the unnoticed spatial dimension of locality is in fact the hidden variable conjectured in the seminal EPR paper.
Test of local realism via entangled ΛΛˉ system
Abstract The non-locality of quantum correlations is a fundamental feature of quantum theory. The Bell inequality serves as a benchmark for distinguishing between predictions made by quantum theory and local hidden variable theory (LHVT). Recent advancements in photon-entanglement experiments have addressed potential loopholes and have observed significant violations of variants of Bell inequality. However, these violations do not completely disprove LHVT. To further rule out LHVT, experiments involving massive-entangled particles are necessary, in addition to those conducted with massless photons, which interact electromagnetically. In this study, we utilize (10.087±0.044)×109 J/ψ events collected with the BESIII detector at the BEPCII collider. We present the non-local correlation test of spin entanglement using a massive ΛΛbar bipartite system, which is formed and decays through strong and weak interactions, respectively. Through measurements of the angular distribution of p pbar in J/ψ→ γηc and subsequent ηc→Λ(pπ-)Λbar(pbar π+) cascade decays, a significant violation of LHVT predictions is observed in a loophole-free test of a variant of the Bell inequality, which is given by the Clauser-Horne inequality. The exclusion of LHVT is determined to be significant at a level of 5.2σ.
In a recent Nature article Hensen et al. reported that they have accomplished a loophole-free test of Bell's theorem. The authors speculated that further improvements in their experimental design could settle an 80 year debate in favor of quantum theory's stance that entanglement is action at a We direct attention to a spatial aspect of locality, not considered by Bell's Theorem or by any of its experimental tests. We refer to the possibility that two distanced particles could remain spatially disconnected, even when distanced enough to ensure that information between them was transmitted faster than the velocity of light. We show that any local-deterministic relativity theory which violates Lorentz's contraction for distancing bodies can maintain spatial locality at any distance. We conclude that until the loophole of spatial locality is closed by future experiments, the news about the death of locality will remain greatly exaggerated.