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Properties of heavy quark hadrons in nuclear medium

Grant number: 23/07313-6
Support Opportunities:Regular Research Grants
Duration: November 01, 2023 - October 31, 2025
Field of knowledge:Physical Sciences and Mathematics - Physics - Nuclear Physics
Principal Investigator:Kazuo Tsushima
Grantee:Kazuo Tsushima
Host Institution: Pró-Reitoria de Pós-Graduação, Pesquisa e Extensão. Universidade Cidade de São Paulo (UNICID). São Paulo , SP, Brazil


This project aims to explore the properties of strongly interacting particles, hadrons, in free space and in a nuclear medium. In particular, the focus is on the heavy hadrons with at least one heavy quark, charm (c) or bottom (b) quark. (We "treat" the strange (s) quark as a "heavy quark" to be explained.)The light hadrons composed of light quarks q = u, d, e.g., proton and neutron, get more than 90% of the masses by dynamically (strong forces due to the gluons). The proton and neutron masses in free space are generated by dynamically described by a local gauge theory, quantum chromodynamics (QCD), which is a part of the Standard Model (SM). On the other hand, the current masses of the quarks are generated by the Higgs mechanism, thus, the Higgs mechanism yields only about 5% of their masses.The discovery of the Higgs boson by the large hadron collider (LHC) in CERN, has fully established the Standard Model (SM). In the SM, the mechanism of spontaneous symmetry breaking plays a crucial role. The spontaneously broken vacuum gets non-zero vacuum expectation value (Higgs-doublet field), and this gives thebare masses of all the massive particles in the SM, namely the quarks (current quark masses), leptons and the massive gauge bosons. The Standard Model describes three of the four fundamental forces of nature, the strong, electromagnetic and weak interactions. However, the mass of the visible Universe, mostly contributed by protons, neutrons and atomic nuclei, arethe result of the strong force acting between the light quarks and gluons (dynamical symmetry breaking), which is described by QCD. The dynamical symmetry breaking dictated by QCD (trace anomaly), gives more than 90% of proton and neutron masses. Although QCD is the theory of strong interaction describing the dynamics of quarks and gluons inside hadrons in free space, the hadrons show much richer, unexpected properties in nuclear medium, which emerge from QCD but cannot be easily understood in terms of it. In particular, this becomes very evident, when light hadrons(light quarks) are immersed in nuclear medium.As mentioned above, the ordinary matter in our universe gets most of its physical mass in free space through spontaneous chiral symmetry breaking and confinement (dynamical symmetry breaking in QCD). Furthermore, QCD dictates that the dynamically broken chiral symmetry would be partially restored (partial restoration of chiral symmetry) in a dense and/or hot nuclear medium.As a consequence, the masses and the properties of light hadrons are expected to be modified in the nuclear medium. This is supported by several experimental facts and evidence, such as the EMC effect, the change of bound proton electromagnetic form factors, and the formation of quark-gluon plasma.On the other hand, the masses of heavy quarks are mainly generated by the Higgs mechanism as also mentioned. Thus, the Higgs mechanism is responsible for the masses of heavy quark hadrons. Although the impact of partial restoration of dynamical chiral symmetry in medium may have less direct impact on heavy quarks and hadrons, explicit studies on the effect of partial restoration of chiral symmetry should be made.Furthermore, when the heavy hadrons contain at least one light quark q, the properties of such heavy quark hadrons are also expected to be modified in nuclear medium somewhat due to the light quark content. Thus, the role of light quarks in nuclear medium can be explored.Moreover, even for the cc and bb quarkonia, as well as different flavor heavy quark mesons such as Bc(bc, bc), their masses in nuclear medium are expected to be modified, by the intermediate state excitations of the Qq and Qq mesons in the self-energy in nuclear medium.The focus of the project is on the properties of heavy quark hadrons in nuclear medium, i.e., heavy-heavy quark mesons and baryons composed of heavy quarks (Q) with at least one light quark (q). (AU)

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