Stable Chromomagnetic QCD Vacuum and Confinement

The stable chromomagnetic vacuum for SU(2) Yang-Mills theory found earlier is shown to give a model for confinement in QCD, using Wilson loop, and a linear potential (in the leading order) for quark-antiquark interaction. The coefficient $k$ in this potential is found to be $\sim 0.25 GeV^2$, in satisfactory agreement with non-relativistic potential model calculations for charmonium. At finite temperature, the real effective energy density found earlier is used to obtain estimates of the deconfining temperature agreeing reasonably with lattice study for SU(2).Comment: Talk delivered at the conference on 'Strong Interactions in the 21st Century', held at the Tata Institute of Fundamental Research, Mumbai, Feb. 10 - 12, 201

Deconfinement Phase Transition in Hot and Dense QCD at Large N

We conjecture that the confinement- deconfinement phase transition in QCD at large number of colors $N$ and $N_f\ll N$ at $T\neq 0$ and $\mu\neq 0$ is triggered by the drastic change in $\theta$ behavior. The conjecture is motivated by the holographic model of QCD where confinement -deconfinement phase transition indeed happens precisely at $T=T_c$ where $\theta$ dependence experiences a sudden change in behavior. The conjecture is also supported by quantum field theory arguments when the instanton calculations (which trigger the $\theta$ dependence) are under complete theoretical control for $T>T_c$, suddenly break down immediately below $T<T_c$ with sharp changes in the $\theta$ dependence. Finally, the conjecture is supported by a number of numerical lattice results. We employ this conjecture to study confinement -deconfinement phase transition of hot and dense QCD in large $N$ limit by analyzing the $\theta$ dependence. We estimate the critical values for $T_c$ and $\mu_c$ where the phase transition happens by approaching the critical values from the hot and/or dense regions where the instanton calculations are under complete theoretical control. We also describe some defects of various codimensions within a holographic model of QCD by focusing on their role around the phase transition point.Comment: Talk at the Workshop honoring 60th anniversary of Misha Shifma

SU(N) gauge theories in 2+1 dimensions -- further results

We calculate the string tension and part of the mass spectrum of SU(4) and SU(6) gauge theories in 2+1 dimensions using lattice techniques. We combine these new results with older results for N=2,...,5 so as to obtain more accurate extrapolations to N=infinity. The qualitative conclusions of the earlier work are unchanged: SU(N) theories in 2+1 dimensions are linearly confining as N->infinity; the limit is achieved by keeping g.g.N fixed; SU(3), and even SU(2), are `close' to SU(infinity). We obtain more convincing evidence than before that the leading large-N correction is O(1/N.N). We look for the multiplication of states that one expects in simple flux loop models of glueballs, but find no evidence for this.Comment: 15 page

SU(N) gauge theories in four dimensions: exploring the approach to N = infinity

We calculate the string tension, K, and some of the lightest glueball masses, M, in 3+1 dimensional SU(N) lattice gauge theories for N=2,3,4,5 . From the continuum extrapolation of the lattice values, we find that the mass ratios, M/sqrt(K), appear to show a rapid approach to the large-N limit, and, indeed, can be described all the way down to SU(2) using just a leading O(1/NxN) correction. We confirm that the smooth large-N limit we find, is obtained by keeping a constant 't Hooft coupling. We also calculate the topological charge of the gauge fields. We observe that, as expected, the density of small-size instantons vanishes rapidly as N increases, while the topological susceptibility appears to have a non-zero N=infinity limit.Comment: Discussion on the correlation time of the topological charge improved and 1 figure added; other minor changes; conclusions unchanged. To appear on JHE

Glueball masses in the large N limit

The lowest-lying glueball masses are computed in SU($N$) gauge theory on a spacetime lattice for constant value of the lattice spacing $a$ and for $N$ ranging from 3 to 8. The lattice spacing is fixed using the deconfinement temperature at temporal extension of the lattice $N_T = 6$. The calculation is conducted employing in each channel a variational ansatz performed on a large basis of operators that includes also torelon and (for the lightest states) scattering trial functions. This basis is constructed using an automatic algorithm that allows us to build operators of any size and shape in any irreducible representation of the cubic group. A good signal is extracted for the ground state and the first excitation in several symmetry channels. It is shown that all the observed states are well described by their large $N$ values, with modest ${\cal O}(1/N^2)$ corrections. In addition spurious states are identified that couple to torelon and scattering operators. As a byproduct of our calculation, the critical couplings for the deconfinement phase transition for N=5 and N=7 and temporal extension of the lattice $N_T=6$ are determined.Comment: 1+36 pages, 22 tables, 21 figures. Typos corrected, conclusions unchanged, matches the published versio

What we do understand of Colour Confinement

A review is presented of what we understand of colour confinement in QCD. Lattice formulation provides evidence that QCD vacuum is a dual superconductor: the chromoelectric field of a $q\bar q$ pair is constrained by dual Meissner effect into a dual Abrikosov flux tube and the static potential energy is proportional to the distance.Comment: 10 pages, 5 figures, plenary talk at "Quark Matter 99", Torino, Italy, May 10-15, 199

Is Confinement a Phase of Broken Dual Gauge Symmetry?

We study whether broken dual gauge symmetry, as detected by a monopole order parameter introduced by the Pisa group, is necessarily associated with the confinement phase of a lattice gauge theory. We find a number of examples, including SU(2) gauge-Higgs theory, mixed fundamental-adjoint SU(2) gauge theory, and pure SU(5) gauge theory, which appear to indicate a dual gauge symmetry transition in the absence of a transition to or from a confined phase. While these results are not necessarily fatal to the dual superconductor hypothesis, they may pose some problems of interpretation for the present formulation of the Pisa monopole criterion.Comment: 6 pages, 5 figure

k-String tensions and their large-N dependence

We consider whether the 1/N corrections to k-string tensions must begin at order 1/N^2, as in the Sine Law, or whether odd powers of 1/N, as in Casimir Scaling, are also acceptable. The issue is important because different models of confinement differ in their predictions for the representation-dependence of k-string tensions, and corrections involving odd powers of 1/N would seem to be ruled out by the large-N expansion. We show, however, that k-string tensions may, in fact, have leading 1/N corrections, and consistency with the large-N expansion, in the open string sector, is achieved by an exact pairwise cancellation among terms involving odd powers of 1/N in particular combinations of Wilson loops. It is shown how these cancellations come about in a concrete example, namely, strong coupling lattice gauge theory with the heat-kernel action, in which k-string tensions follow the Casimir scaling rule.Comment: Talk presented at the XXIX International Symposium on Lattice Field Theory - Lattice 2011, July 10-16, 2011, Squaw Valley, Lake Tahoe, Californi

The deconfining phase transition in SU(N) gauge theories

We report on our ongoing investigation of the deconfining phase transition in SU(4) and SU(6) gauge theories. We calculate the critical couplings while taking care to avoid the influence of a nearby bulk phase transition. We determine the latent heat of the phase transition and investigate the order and the strength of the transition at large N. We also report on our determination of the critical temperature expressed in units of the string tension in the large N limit.Comment: Lattice 2002 (nonzerot), 3 pages, 2 figure

Features of SU(N) Gauge Theories

We review recent lattice results for the large $N$ limit of SU(N) gauge theories. In particular, we focus on glueball masses, topology and its relation to chiral symmetry breaking (relevant for phenomenology), on the tension of strings connecting sources in higher representations of the gauge group (relevant for models of confinement and as a comparative ground for theories beyond the Standard Model) and on the finite temperature deconfinement phase transition (relevant for RHIC-like experiments). In the final part we present open challenges for the future.Comment: 6 pages, 3 figures; summary of the talk given by B. Lucini and the poster presented by U. Wenger at the conference "Confinement 2003