Persistent Challenges of Quantum Chromodynamics

Unlike some models whose relevance to Nature is still a big question mark, Quantum Chromodynamics will stay with us forever. Quantum Chromodynamics (QCD), born in 1973, is a very rich theory supposed to describe the widest range of strong interaction phenomena: from nuclear physics to Regge behavior at large E, from color confinement to quark-gluon matter at high densities/temperatures (neutron stars); the vast horizons of the hadronic world: chiral dynamics, glueballs, exotics, light and heavy quarkonia and mixtures thereof, exclusive and inclusive phenomena, interplay between strong forces and weak interactions, etc. Efforts aimed at solving the underlying theory, QCD, continue. In a remarkable entanglement, theoretical constructions of the 1970s and 1990s combine with today's ideas based on holographic description and strong-weak coupling duality, to provide new insights and a deeper understanding.Comment: Julius Edgar Lilienfeld Prize Lecture at the April Meeting of APS, Dallas, TX, April 22-25, 2006; v.2: reference added; v.3: reference adde

A Chiral SU(N) Gauge Theory Planar Equivalent to Super-Yang-Mills

We consider the dynamics of a strongly coupled SU(N) chiral gauge theory. By using its large-N equivalence with N=1 super-Yang-Mills theory we find the vacuum structure of the former. We also consider its finite-N dynamics.Comment: 10 pages, Latex. 1 eps figur

Quantum Tunneling

This article is a slightly expanded version of the talk I delivered at the Special Plenary Session of the 46-th Annual Meeting of the Israel Physical Society (Technion, Haifa, May 11, 2000) dedicated to Misha Marinov. In the first part I briefly discuss quantum tunneling, a topic which Misha cherished and to which he was repeatedly returning through his career. My task was to show that Misha's work had been deeply woven in the fabric of today's theory. The second part is an attempt to highlight one of many facets of Misha's human portrait. In the 1980's, being a refusenik in Moscow, he volunteered to teach physics under unusual circumstances. I present recollections of people who were involved in this activity.Comment: Dedicated to the memory of Professor Michael Marinov, a friend, a colleague and a man of firm principles who believed in his right to live in peace in Israel, the land of his ancestors (Latex, 5 eps figures, uses sprocl style), to be published in the Michael Marinov Memorial Volume, Eds. M. Olshanetsky and A. Vainshtein (World Scientific, 2002

Non-Perturbative Yang-Mills from Supersymmetry and Strings, Or, in the Jungles of Strong Coupling

I summarize some recent developments in the issue of planar equivalence between supersymmetric Yang-Mills theory and its orbifold/orientifold daughters. This talk is based on works carried out in collaboration with Adi Armoni, Sasha Gorsky and Gabriele Veneziano.Comment: Based on talks delivered at Planck-05/Mohapatra-Fest, ICTP, Trieste, May 23-28, 2005, and PASCOS-05, Gyeongju, Korea, May 30 - June 4, and the Cracow School of Theoretical Physics, Zakopane, Poland, June 3-12, 2005. 24 pages, 6 figure

Remarks on Adjoint QCD with k Flavors, k>= 2

I summarize what we know of adjoint QCD. Some observations (albeit very simple) are new.Comment: 12 p. 3 figs; v2: a comment and 2 references adde

A Remark on Supersymmetric Bubbles and Spectrum Crossover

Using an exact expression for the domain wall tension in a supersymmetric model we show that a spectrum crossover takes place in passing from weak to strong coupling. In the weak coupling regime elementary excitations are the lightest states, while in the strong coupling regime solitonic objects of a special type -- bubbles -- assume the role of the lightest states. The crossover occurs at \lambda^2/(4\pi) \sim 0.4.Comment: 6 p., 1 fi