Femtoscopy of the Origin of the Nucleon Mass
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G. Krein
· T. C. Peixoto
Femtoscopy of the Origin of the Nucleon Mass
Received: 6 September 2020 / Accepted: 9 November 2020 © Springer-Verlag GmbH Austria, part of Springer Nature 2020
Abstract We study the prospects of using femtoscopic low-momentum correlation measurements at the Large Hadron Collider to access properties of the J/ψ-nucleon interaction. The QCD multipole expansion in terms of the J/ψ chromopolarizability relates the forward scattering amplitude to a key matrix element to the origin of the nucleon mass problem, the average chromoelectric gluon distribution in the nucleon. We use information on the J/ψ-nucleon interaction provided by lattice QCD simulations and phenomenological models to compute J/ψ-nucleon correlation functions. The computed correlation functions show clear sensitivity to the interaction, in particular to the J/ψ chromopolarizability. 1 Introduction What is the origin of the mass of protons and neutrons (nucleons) and, therefore, most of the universe’s visible matter? Computer simulations of quantum chromodynamics (QCD) have given an answer to the question, namely: the mass comes mostly from the gluons and the nearly massless quarks. Yet, we are still unsatisfied and want more; we want to understand, to quote Wilczek [1]: “How did it happen?”. With this mindset, an ever-growing effort is underway to find that kind of answer, both theoretically [2,3] and experimentally [4–6]. The present work adds to this effort. We study the prospects of using femtoscopy in high-energy proton-proton and heavy-ion collisions for learning about the origin of the nucleon mass. Femtoscopy [7] is a technique that makes it possible to obtain spatio-temporal information on particle production sources at the femtometer scale. Two-hadron momentum correlation functions carry such information [8,9]. These correlation functions, remarkably, also carry information on low-energy hadron-hadron forces as final-state effects [10,11]. Relevant to the origin of the mass problem is the correlation function of a heavy quarkonium (such as J/ψ, ηc , ϒ,ηb ) and a nucleon, for it gives direct access to the quarkonium-nucleon forward scattering amplitude. The QCD multipole expansion relates this amplitude to a key matrix element to the mass problem: the average chromoelectric gluon distribution in the nucleon [12–22]. It is key to the problem because it relates to the trace of the QCD energy-momentum tensor in the nucleon, which defines the nucleon mass [23,24]. The quarkonium-nucleon scattering amplitude is also accessible with J/ψ and ϒ electro- and photoproduction experiments [5]. However, the kinematics of the production process forbids direct access to the forward amplitude. In femtoscopy there are no such kinematics constraints. In addition, the two-particle G. Krein (B) · T. C. Peixoto Instituto de Física Teórica, Universidade Estadual Paulista, Rua Dr. Bento Teobaldo Ferraz, 271 - Bloco II, São Paulo, SP 01140070, Brazil E-mail: [email protected] T. C. Peixoto Instituto Federal de Educação, Ciênciae Tecnol
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