Gluinos on the lattice: quenched calculations

As a preparation for the numerical study of the SU(2) gauge theory with gluinos, the spectral properties of the fermion matrix and the masses of pseudoscalar and scalar states are investigated in the quenched approximation. The behaviour of the disconnected fermion diagrams on small lattices is also studied.Comment: latex2e, 12 pages, 5 figures with epsfi

Three-dimensional gonihedric spin system

We perform Monte Carlo simulations of a three-dimensional spin system with a Hamiltonian which contains only four-spin interaction term. This system describes random surfaces with extrinsic curvature - gonihedric action. We study the anisotropic model when the coupling constants $\beta_S$ for the space-like plaquettes and $\beta_T$ for the transverse-like plaquettes are different. In the two limits $\beta_S=0$ and $\beta_T=0$ the system has been solved exactly and the main interest is to see what happens when we move away from these points towards the isotropic point, where we recover the original model. We find that the phase transition is of first order for $\beta_T = \beta_S \approx 0.25,$ while away from this point it becomes weaker and eventually turns to a crossover. The conclusion which can be drown from this result is that the exact solution at the point $\beta_S =0$ in terms of 2d-Ising model should be considered as a good zero order approximation in the description of the system also at the isotropic point $\beta_S =\beta_T$ and clearly confirms the earlier findings that at the isotropic point the original model shows a first order phase transition.Comment: 11 pages, 10 figures, shortened versio

Phase structure of four-dimensional gonihedric spin system

We perform Monte Carlo simulations of a gauge invariant spin system which describes random surfaces with gonihedric action in four dimensions. The Hamiltonian is a mixture of one-plaquette and additional two- and three-plaquette interaction terms with specially adjusted coupling constants. For the system with the large self-intersection coupling constant $k$ we observe the second-order phase transition at temperature $\beta_{c}\simeq 1.75$. The string tension is generated by quantum fluctuations as it was expected theoretically. This result suggests the existence of a noncritical string in four dimensions. For smaller values of $k$ the system undergoes the first order phase transition and for $k$ close to zero exhibits a smooth crossover.Comment: 14 pages, Latex, 10 figure

Four-dimensional gonihedric gauge spin system

We perform Monte Carlo simulations of a four-dimensional gauge invariant spin system which describes random surfaces with gonihedric action. We develop the analogy between the flat-crumpled phase transition of the lattice surface model and the liquid-gas phase transition of non-ideal gases, and identify the self-intersection coupling constant $k$ of the surface model with the pressure $P$. As $k$ increases the system moves to a critical point in complete analogy with the situation for non-ideal gases, where the liquid and the gas phases approach each other with increasing $P$. We measure vacuum expectation values of various operators and the corresponding critical indices.Comment: 12 pages, Late

Three-dimensional lattice U(1) gauge-Higgs model at low mHm_H

We study the non-compact version of the U(1) gauge-Higgs model in three dimensions for $m_H = 30 GeV.$ We found that, using this formulation, rather modest lattices approach quite well the infinite volume behaviour.The phase transition is first order, as expected for this Higgs mass. The latent heat (in units of $T_{cr}^4$) is compatible with the predictions of the two-loop effective potential; it is an order of magnitude less than the corresponding SU(2) value. The transition temperature and $$ in units of the critical temperature are also compatible with the perturbative results.Comment: 15 pages, latex, 9 figures, changes in the comparison with perturbation theor

The Abelian Higgs Model in Three Dimensions with Improved Action

We study the Abelian Higgs Model using an improved form of the action in the scalar sector. The subleading corrections are carefully analysed and the connection between lattice and continuous parameters is worked out. The simulation shows a remarkable improvement of the numerical performance.Comment: Minor correction to one-loop relations;reference adde

Catalysis of chiral symmetry breaking by external magnetic fields in three-dimensional lattice QED

The enhancement of the fermionic condensate due to the presence of both homogeneous and non-homogeneous external magnetic fields is studied for three-dimensional QED.Comment: 5 pages, contribution to "Strong and electroweak matter 98", Copenhagen, December 199

Exact Gravity Dual of a Gapless Superconductor

A model of an exact gravity dual of a gapless superconductor is presented in which the condensate is provided by a charged scalar field coupled to a bulk black hole of hyperbolic horizon in asymptotically AdS spacetime. Below a critical temperature, the black hole acquires its hair through a phase transition while an electromagnetic perturbation of the background Maxwell field determines the conductivity of the boundary theory.Comment: 21 pages, 8 figures, a section on the stability of the MTZ black hole is added, references are added, version to be published in JHE