[Paper Review] Exploring the N Lambda-N Lambda coupled system with high precision correlation techniques at the LHC

S. Acharya, D. Adamová+3 more|arXiv (Cornell University)|Jan 1, 2022

Particle physics theoretical and experimental studies60 references56 citations

TL;DR

This paper presents the first direct experimental observation of NΣ ↔ NΛ coupled-channel dynamics in the pΛ system using high-precision two-particle correlation techniques in high-multiplicity pp collisions at √s = 13 TeV at the LHC. A cusp-like structure at k* = 289 MeV/c in the correlation function provides evidence for inelastic NΣ channel opening, constraining the NΣ–NΛ transition potential and implying a weaker coupling than previously assumed, with implications for hyperon interactions in neutron stars and the nuclear equation of state.

ABSTRACT

The interaction of and hyperons (Y) with nucleons (N) is strongly influenced by the coupled-channel
 dynamics. Due to the small mass difference of the N and N systems, the sizable coupling strength of
 the N ↔ N processes constitutes a crucial element in the determination of the N interaction. In
 this letter we present the most precise measurements on the interaction of p pairs, from zero relative
 momentum up to the opening of the N channel. The correlation function in the relative momentum
 space for p ⊕ p pairs measured in high-multiplicity triggered pp collisions at √s = 13 TeV at the
 LHC is reported. The opening of the inelastic N channels is visible in the extracted correlation function
 as a cusp-like structure occurring at relative momentum k∗ = 289 MeV/c. This represents the first direct
 experimental observation of the N ↔ N coupled channel in the p system. The correlation function
 is compared with recent chiral effective field theory calculations, based on different strengths of the
 N ↔ N transition potential. A weaker coupling, as possibly supported by the present measurement,
 would require a more repulsive three-body NN interaction for a proper description of the in-medium
 properties, which has implications on the nuclear equation of state and for the presence of hyperons
 inside neutron stars.

Motivation & Objective

• To probe the NΛ–NΣ coupled-channel dynamics in the pΛ system with high precision using two-particle correlations.
• To resolve the long-standing uncertainty in the NΣ ↔ NΛ transition potential strength from experimental data.
• To constrain the NΛ interaction and its implications for three-body forces and the nuclear equation of state in dense matter.
• To test chiral effective field theory (χEFT) predictions by comparing measured correlation functions with theoretical calculations.

Proposed method

• Measurement of two-particle correlation functions C(k*) in high-multiplicity pp collisions at √s = 13 TeV using the ALICE detector.
• Reconstruction of pΛ and pΛ pairs via invariant mass analysis of decay products (p + π⁻) and particle identification using ITS, TPC, and TOF detectors.
• Use of mixed-event normalization to extract uncorrelated background, enabling precise extraction of the correlation function C(k*) = N·N(k*)/M(k*).
• Application of the source function formalism C(k*) = ∫ S(r*)|Ψ(k*, r*)|² d³r* to relate correlation to the effective emission source and relative wave function.
• Comparison of experimental correlation functions with chiral effective field theory (χEFT) calculations using different strengths of the NΣ ↔ NΛ transition potential.
• Selection of high-multiplicity events to enhance strangeness production and improve statistics for low-k* region analysis.

Experimental results

Research questions

• RQ1 What is the strength of the NΣ ↔ NΛ transition potential in the pΛ system, as constrained by high-precision correlation data?
• RQ2 Does the experimental correlation function exhibit a cusp-like structure at the NΣ threshold, indicating inelastic channel opening?
• RQ3 How do the measured correlation functions compare with recent chiral effective field theory calculations with varying NΣ–NΛ coupling strengths?
• RQ4 What are the implications of a weaker NΣ–NΛ coupling for the three-body NNΛ interaction and the equation of state in neutron stars?
• RQ5 Can two-particle correlations in small systems like pp collisions provide a sensitive probe of coupled-channel dynamics in hyperon-nucleon interactions?

Key findings

• A cusp-like structure is observed in the pΛ correlation function at a relative momentum of k* = 289 MeV/c, providing the first direct experimental evidence for the opening of the inelastic NΣ channel in the NΛ system.
• The observed cusp structure is consistent with a weaker NΣ ↔ NΛ transition potential than assumed in earlier theoretical models, such as the original NLO13 χEFT potential.
• The measured correlation function shows good agreement with chiral effective field theory calculations that employ a weaker NΣ–NΛ coupling, suggesting a need for a more repulsive three-body NNΛ interaction to reproduce medium effects.
• The high-multiplicity trigger increases the number of reconstructed pΛ pairs below k* = 200 MeV/c by a factor of ten compared to minimum-bias triggers, enabling high-statistics, low-k* correlation measurements.
• The results imply that a weaker NΣ–NΛ coupling would require a more repulsive three-body force to correctly describe Λ hyperon properties in dense nuclear matter, affecting predictions for hyperon presence in neutron stars.
• The study demonstrates the sensitivity of two-particle correlations in small systems to coupled-channel dynamics, offering a new precision tool for studying hadron-hyperon interactions.