Abstract: In recent years, experiments in high-energy accelerators have discovered a large number of unusual, heavy “cousins” of the proton—i.e., strongly interacting particles collectively known as hadrons. Ordinary hadrons are either baryons, which contain three quarks, or mesons, which contain one quark and one antiquark. The new, unusual hadrons contain either two quarks and two antiquarks (known as tetraquarks) or four quarks and an antiquark (known as pentaquarks). These particles are often referred to as multiquark or exotic hadrons. Given the large number of such exotic hadrons discovered thus far, it is becoming increasingly important to discuss which additional multiquark states are likely to be observed and how quarks are organized within these exotic hadrons. In my talk, I will address these questions and provide some guidelines about the essential differences between the two types of exotics. I will also discuss a very recent experimental result from the LHCb Collaboration at CERN that shows how potent Coulomb repulsion between quarks can significantly contribute to hadron mass, in a down-to-earth demonstration of E=mc2, flipping the expected mass hierarchy between isospin partners.

Marek Karliner is an emeritus professor of physics at Tel Aviv University (TAU). He was born in Poland in 1955. He received his Ph.D. from TAU in 1984. As a graduate student, he spent one year at SLAC at Stanford University as a Fulbright Fellow. From 1984 to 1988, he was a research associate at SLAC. He has been a faculty member at TAU since 1988 and a professor since 1995. His research focuses on the theoretical physics of elementary particles, particularly exotic hadrons such as tetraquarks and pentaquarks. He has made several successful theoretical predictions of hadrons containing heavy quarks, including the first pentaquark, doubly heavy baryons, and a tetraquark containing two charmed quarks. He also made the first robust theoretical prediction for a heavy tetraquark that is stable under strong interactions. In March 2026, the LHCb Collaboration at CERN provided precise confirmation of his 2017 prediction regarding the surprising magnitude and sign of isospin splitting between two doubly heavy baryons. From 2003 to 2005, he was a visiting professor at the Cavendish Laboratory at the University of Cambridge in the UK. From 2015 to 2025, he served as director of the TAU Institute of Advanced Studies. Since 2017, he has been a foreign member of the Polish Academy of Arts and Sciences, and since 2021,he has been a member of the Warsaw University Council. Since 2021, he has served as chairman of the Israeli Committee for High Energy Physics and as the scientific representative of Israel in the CERN Council.