The three-nucleon parameter

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About the speaker

Professor Alejandro Kievsky is a senior researcher at the INFN Pisa Division and a member of the Pisa Theory Group. His research focuses on nuclear interactions, few-nucleon and N-boson systems, Efimov states, and universality in few-body physics. He applies advanced variational methods to study bound and scattering states, contributing significantly to both nuclear and atomic physics through realistic theoretical modeling.

Abstract

A remarkable property of a three-body system when the two-body scattering length of the constituents is large is the Efimov effect, an infinite number of excited states geometrically distributed with an accumulation point at zero energy. This happens when the two-body scattering length diverges or, equivalently, when the two-body bound state is at the two-body threshold (unitary limit). In nuclear physics the two-body scattering lengths in the spin singlet and triplet states are large placing the two-nucleon system close to the unitary point. The consequence is the appearence of universal behavior, many properties in the low-energy region are governed by the value of the scattering lengths and result relatively insensitive to specific details of the interaction. The two-nucleon sector shows a continuous scale symmetry whereas the three-nucleon sector is characterized by a discrete scale symmetry. In this context the three-nucleon parameter emerges as a quantity determining the tower of infinite excited states. Though very shallow, the deuteron has not zero energy and the three-nucleon systems, 3H and 3He, are not exactly placed at the unitary limit. During my lecture I will show how to link the physical point to the unitary point, encoded in the three-nucleon parameter, and how nuclei are formed as the value of the scattering length moves from infinite to its physical value.