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We study the dual Higgs mechanism induced by monopole condensation based on the dual gauge formalism in the maximally abelian (MA) gauge in the lattice QCD. To examine ``monopole condensation'' in QCD, we study the monopole part or the monopole-current system appearing in the MA gauge by extracting the dual gluon field $B_\mu$. First, we investigate the inter-monopole potential using the dual Wilson loop in the lattice QCD simulation. In the monopole part in the MA gauge, the inter-monopole potential is found to be flat, and can be fitted as the Yukawa potential in the infrared region. From more detailed analysis of the inter-monopole potential considering the monopole size, we estimate the effective dual-gluon mass $m_B \simeq 0.5$GeV and the effective monopole size $R_{^{_{\rm M}}} \simeq 0.2$fm. Second, we study the dual gluon propagator $G^D_{\mu\nu}(x-y) \equiv < B_\mu(x) B_\nu(y) >_{\rm MA}$ in the MA gauge, and find that $G^D_{\mu\mu}$ behaves as the massive vector-boson propagator with $m_B \simeq 0.4$ GeV in the infrared region. The effective-mass acquirement of the dual gluon field $B_\mu$ at the long distance can be regarded as the lattice QCD evidence of ``infrared monopole condensation'' in the MA gauge.

Using the SU(2) lattice QCD, we formulate the dual Wilson loop and study the dual Higgs mechanism induced by monopole condensation in the maximally abelian (MA) gauge, where QCD is reduced into an abelian gauge theory including the electric current $j_\mu$ and the monopole current $k_\mu$. After the abelian projection in the MA gauge, the system can be separated into the photon part and the monopole part corresponding to the separation of $j_\mu$ and $k_\mu$, respectively. We study here the monopole part (the monopole-current system), which is responsible to the electric confinement. Owing to the absence of electric currents, the monopole part is naturally described using the dual gluon field $B_\mu$ without the Dirac-string singularity. Defining the dual Wilson loop from the dual gluon $B_\mu$, we find the perimeter law of the dual Wilson loop in the lattice QCD simulation. In the monopole part in the MA gauge, the inter-monopole potential is found to be flat, and can be fitted as the Yukawa potential in the infrared region after the subtraction of the artificial finite-size effect on the dual Wilson loop. From more detailed analysis of the inter-monopole potential considering the monopole size, we estimate the effective dual-gluon mass $m_B \simeq 0.5$GeV and the effective monopole size $R \simeq 0.2$fm. The effective mass of the dual gluon field at the long distance can be regarded as an evidence of ``infrared monopole condensation''.

We study the confinement physics in QCD in the maximally abelian (MA) gauge using the SU(2) lattice QCD, based on the dual-superconductor picture. In the MA gauge, off-diagonal gluon components are forced to be small, and the off-diagonal angle variable $\chi_\mu(s)$ tends to be random. Within the random-variable approximation for $\chi_\mu(s)$, we analytically prove the perimeter law of the off-diagonal gluon contribution to the Wilson loop in the MA gauge, which leads to abelian dominance on the string tension. To clarify the origin of abelian dominance for the long-range physics, we study the charged-gluon propagator in the MA gauge using the lattice QCD, and find that the effective mass $m_{ch} \simeq 0.9 {\rm GeV}$ of the charged gluon is induced by the MA gauge fixing. In the MA gauge, there appears the macroscopic network of the monopole world-line covering the whole system, which would be identified as monopole condensation at a large scale. To prove monopole condensation in the field-theoretical manner, we derive the inter-monopole potential from the dual Wilson loop in the monopole part of QCD, which carries the nonperturbative QCD aspects, in the MA gauge. The dual gluon mass is evaluated as $m_B \simeq $0.5GeV in the monopole part in the infrared region, which is the evidence of the dual Higgs mechanism by monopole condensation., Comment: Talk given at Int. Workshop on ``Future Directions in Quark Nuclear Physics''(QNP98), Adelaide, Australia, Mar. 1998, and to be published in the Proceedings (World Scientific)

We study the confinement physics in QCD in the maximally abelian (MA) gauge using the SU(2) lattice QCD. To clarify the origin of abelian dominance for the long-range physics, we study the charged-gluon propagator in the lattice QCD, and find that the effective mass $m_{ch} \simeq 0.9 {\rm GeV}$ of the charged gluon is induced by the MA gauge fixing. In the MA gauge, there appears the global network of the monopole world-line covering the whole system, which would be identified as monopole condensation at a large scale. To prove monopole condensation, we apply the dual gauge formalism to the monopole part, and derive the inter-monopole potential from the dual Wilson loop in the MA gauge. In the monopole part, which carries the nonperturbative aspects of QCD, the dual gluon mass is evaluated as $m_B \simeq $0.5GeV, which is the evidence of the dual Higgs mechanism by monopole condensation. As for the monopole structure, the large fluctuation of off-diagonal gluons remains around the monopole in the MA gauge, and large cancellation occurs between the diagonal and off-diagonal action densities to keep the total QCD action finite. The charged-gluon rich region around the QCD-monopole would provide the effective monopole size as the critical scale of the abelian projected QCD. Instantons are expected to appear in the charged-gluon rich region around the monopole world-line in the MA gauge, which leads to the local correlation between monopoles and instantons., Comment: Invited Lecture presented by H. Suganuma at 1997 Yukawa International Seminar (YKIS'97) on `` Non-Perturbative QCD -Structure of the QCD Vacuum-'', 2-12 December 1997, in Kyoto, Japan, 12 pages, Plain Latex

We study the dual Wilson loop and the inter-monopole potential(the static potential between the color magnetic monopoles) in the maximally abelian gauge to clarify the dual Higgs mechanism induced by monopole condensation. There is no (color-)electric current in the monopole part, which includes the essence of the nonperturbative QCD, and hence the system can be described by the dual gauge field $B_\mu$ without the singularity like the Dirac string. We find that the dual Wilson loop seems to obey the perimeter law, and the inter-monopole potential becomes Yukawa-type in the infrared region. From the inter-monopole potential, we estimate the dual gluon mass $m_B$ and the effective size $R$ of the monopole: $m_B \simeq 0.5$GeV, $R \simeq 0.35$fm., Comment: Talk presented by A. Tanaka at ``INNOCOM '97'', XVII RCNP the International Symposium on Innovative Computational Methods in Nuclear Many-Body Problems, 10 - 15 November 1997, in Osaka, Japan, 3 pages, Plain Latex

The confinement mechanism in the nonperturbative QCD is studied in terms of topological excitation as QCD-monopoles and instantons. In the 't Hooft abelian gauge, QCD is reduced into an abelian gauge theory with monopoles, and the QCD vacuum can be regarded as the dual superconductor with monopole condensation, which leads to the dual Higgs mechanism. The monopole-current theory extracted from QCD is found to have essential features of confinement. We find also close relation between monopoles and instantons using the lattice QCD. In this framework, the lowest $0^{++}$ glueball (1.5 $\sim$ 1.7GeV) can be identified as the QCD-monopole or the dual Higgs particle., Comment: Talk presented by H.Suganuma at the 5th Topical Seminar on The Irresistible Rise of the Standard Model, San Miniato al Todesco, Italy, 21-25 April 1997 5 pages, Plain Latex

We study the role of the monopole for color confinement by using the monopole current system. For the self-energy of the monopole current less than ln$(2d-1)$, long and complicated monopole world-lines appear and the Wilson loop obeys the area law, and therefore the monopole current system almost reproduces essential features of confinement properties in the long-distance physics. In the short-distance physics, however, the monopole-current theory would become nonlocal due to the monopole size effect. This monopole size would provide a critical scale of QCD in terms of the dual Higgs mechanism., Comment: 3 pages LaTeX, 3 figures, uses espcrc2.sty, Talk presented at lattice97, Edinburgh, Scotland, July. 1997

We investigate the confinement properties in the multi-instanton system, where the size distribution is assumed to be $ \rho^{-5} $ for the large instanton size $ \rho $. We find that the instanton vacuum gives the area law behavior of the Wilson loop, which indicates existence of the linear confining potential. In the multi-instanton system, the string tension increases monotonously with the instanton density, and takes the standard value $ \sigma \simeq 1 GeV/fm $ for the density $ (N/V)^{{1/4}} = 200 MeV $. Thus, instantons directly relate to color confinement properties., Comment: Talk presented by M. Fukushima at ``Lattice '97'', the International Symposium on Lattice Field Theory, 22 - 26 July 1997, in Edinburgh, Scotland, 3 pages, Plain Latex

Color confinement can be understood by the dual Higgs theory, where monopole condensation leads to the exclusion of the electric flux from the QCD vacuum. We study the role of the monopole for color confinement by investigating the monopole current system. When the self-energy of the monopole current is small enough, long and complicated monopole world-lines appear, which is a signal of monopole condensation. In the dense monopole system, the Wilson loop obeys the area-law, and the string tension and the monopole density have similar behavior as the function of the self-energy, which seems that monopole condensation leads to color confinement. On the long-distance physics, the monopole current system almost reproduces essential features of confinement properties in lattice QCD. In the short-distance physics, however, the monopole-current theory would become nonlocal and complicated due to the monopole size effect. This monopole size would provide a critical scale of QCD in terms of the dual Higgs mechanism., Comment: 6 pages LaTeX, 5 figures, uses espcrc1.sty, Talk presented at International Conference on Quark Lepton Nuclear Physics, Osaka, May. 1997

We study the relation between the instanton distribution and the monopole loop length in the SU(2) gauge theory with the abelian gauge fixing. We measure the monopole current from the multi-instanton ensemble on the $16^4$ lattice using the maximally abelian gauge. When the instanton density is dilute, there appear only small monopole loops. On the other hand, in the dense case, there appears one very long monopole loop, which is responsible for the confinement property, in each gauge configuration. We find a clear monopole clustering in the histogram of the monopole loop length from 240 gauge configurations., Comment: Talk presented by M. Fukushima at ``Lattice '96'', the International Symposium on Lattice Field Theory, 4 - 8 June 1996, in St. Louis, Missouri, USA, 3 pages, Plain Latex

We study the correlation between instantons and QCD-monopoles both in the lattice gauge theory and in the multi-instanton system using the maximally abelian gauge. First, we find the existence of an almost linear correlation between the total length of monopole trajectories and the total number of pseudoparticles (instantons and anti-instantons) in the $16^{3}\times4$ SU(2) lattice. Second, we study the features of QCD-monopole in the SU(2) multi-instanton vacuum on the $16^{4}$ lattice as a random ensemble of pseudoparticles. A signal of monopole condensation is found as the clustering of monopole trajectories, when the topological pseudoparticles is sufficiently dense., Comment: Talk presented by S. Sasaki at International Conference on Quark Confinement and the Hadron Spectrum II, Como, Italy, 26-29 Jun 1996. Latex, uses epsf.tex and sprocl.sty, 4 pages, 3 postscript figures included

We generate a random instanton vacuum with various densities and size distributions. We perform numerically the maximally abelian gauge fixing of these configurations in order to find monopole trajectories induced by instantons. We find that instanton-induced monopole loops form enormous clusters occupying the whole physical volume, provided instantons are sufficiently dense. It indicates that confinement might be caused by instantons., Comment: 7 pages, Plain Latex, (3 figures - available on request from ssasaki@rcnp.osaka-u.ac.jp)

The correlation between instantons and QCD-monopoles is studied both in the lattice gauge theory and in the continuum theory. An analytical study in the Polyakov-like gauge, where $A_4(x)$ is diagonalized, shows that the QCD-monopole trajectory penetrates the center of each instanton, and becomes complicated in the multi-instanton system. Using the SU(2) lattice with $16^4$, the instanton number is measured in the singular (monopole-dominating) and regular (photon-dominating) parts, respectively. The monopole dominance for the topological charge is found both in the maximally abelian gauge and in the Polyakov gauge., Comment: 4 pages, Latex, 3 figures. Talk presented by H. Suganuma at International Symposium on 'Lattice Field Theory', July 11 - 15, 1995, Melbourne, Australia