Massless Excitations Research Articles

A description of multicharged black holes in terms of branes and antibranes

We describe multicharged black holes in terms of branes and antibranes together with multiple copies of gas of massless excitations. Assuming that energies of these copies of gas are all equal, we find that the entropy of the brane–antibrane configuration agrees with that of the multicharged black hole in supergravity approximation, upto a factor X. We find that X=1 for a suitable normalisation which admits a simple empirical interpretation that the available gas energy is all taken by one single gas which is, in a sense, a certain superposition of the multiple copies; and that the brane tensions are decreased by a factor of 4. This interpretation renders superfluous the assumption of equal energies, which is unnatural from a physical point of view.

A description of Schwarzschild black holes in terms of intersecting M-branes and antibranes

Intersecting M-branes are known to describe multi-charged black holes. Using a configuration of such intersecting branes and antibranes, together with massless excitations living on them, we give a description of Schwarzschild black holes following Danielsson, Guijosa, and Kruczenski. We calculate the entropy of these black holes and find that it agrees, upto a numerical factor, with the entropy of the corresponding Schwarzschild black holes in supergravity approximation. We give an empirical interpretation of this factor.

Role of spinon in the presence of spinon singlet pair excitations on phase transitions ind-wave superconductors

We examine the roles of massless Dirac spinon and spin singlet pair excitations on the phase transition in $d-wave$ superconductors. Although the massless spinon excitations in the presence of the spin singlet pair excitations do not alter the nature of the phase transition at $T = 0$, that is, the XY universality class, they are seen to induce an additional attractive interaction potential between vortices, further stabilizing vortex-antivortex pairs at low temperature for lightly doped high $T_c$ samples.

Finite-temperature supersymmetry: the Wess–Zumino model

We investigate the breakdown of supersymmetry at finite temperature. While it has been proven that temperature always breaks supersymmetry, the nature of this breaking is less clear. On the one hand, a study of the Ward–Takahashi identities suggests a spontaneous breakdown of supersymmetry without the existence of a Goldstino, while on the other hand it has been shown that in any supersymmetric plasma there should exist a massless fermionic collective excitation, the phonino. Aim of this work is to unify these two approaches. For the Wess–Zumino model, it is shown that the phonino exists and contributes to the supersymmetric Ward–Takahashi identities in the right way displaying that supersymmetry is broken spontaneously with the phonino as the Goldstone fermion.

Spontaneous symmetry breaking in noncommutative field theories

The spontaneous symmetry breaking of rotational O(N) symmetry in noncommutative field theory is investigated in a 2+1 dimensional model of scalar fields coupled through a combination of quartic and sextuple self-interactions. There are five possible orderings of the fields in the sextuple interaction and two for the quartic interaction. At one loop, we prove that for some choices of these orderings there is the absence of IR/UV mixing and the appearance of massless excitations. A supersymmetric extension of the model is also studied. Supersymmetry puts additional constraints on the couplings but for any given N there is a Moyal ordering of the superfields for which the requirement for the existence of Goldstone bosons is satisfied. For some ordering and when N goes to infinity we find evidence that the model is renormalizable to all orders in perturbation theory. We also consider a generic chiral model in 3+1 dimensions whose superpotential is invariant under local gauge transformations. We find that for any value of N there is no one loop correction to the pion mass and that, at two loops, there are no pion mass corrections for slowly varying superfields so that Goldstone theorem holds true. We also find a new purely noncommutative coupling which gives contributions starting at order N-2 loops.

Little string theories in heterotic backgrounds

We study Little String Theories (LST) with N=(1,0) supersymmetry arising, in a suitable double scaling limit, from 5-branes in heterotic string theory or in the heterotic-like type II/(−) F L ×shift. The limit in question, previously studied in the type II case, is such that the resulting holographically dual pairs, i.e., bulk string theory and LST are at a finite effective coupling. In particular, the internal (2,2) SCFT on the string theory side is non-singular and given by SL(2)/ U(1)× SU(2)/ U(1) coset. In the type II orbifold case, we determine the orbifold action on the internal SCFT and construct the boundary states describing the non-BPS massive states of a completely broken SO gauge theory, in agreement with the dual picture of D5-branes in type II /Ω× shift . We also describe a different orbifold action which gives rise to a Sp gauge theory with (1,1) supersymmetry. In both the heterotic SO(32) and E 8× E 8 cases, we determine the gauge bundles which correspond to the above SCFT and break down the gauge groups to SU(2)× SO(28) and E 7× E 8, respectively. The double scaling limit in this case involves taking small instanton together with small string coupling constant limit. We determine the spectrum of chiral gauge invariant operators with the corresponding global symmetry charges on the LST side and compare with the massless excitations on the string theory side, finding agreement for multiplicities and global charges.

At the Dawn of a Unifying Theory

I present a brief summary of the recent advancements on the unification of abelian quantum Hall states that covers the effects of disorder as well as the Coulomb interactions. I also present some exact results on the CPN−1 model at large N. In contrast to the previous expectations, this theory shows all the basic features of the quantum Hall effect, including the massless bulk excitations at θ=π.

Generalized mesons in dense QCD

QCD superconductors in the color–flavor-locked (CFL) phase support excitations (generalized mesons) that can be described as pairs of particles or holes (rather than particle–hole) around a gapped Fermi surface. In weak coupling and to leading logarithm accuracy the scalar and pseudoscalar excitations are massless and the vector and axial-vector excitations are massive and degenerate. The massless scalar excitations are combined with the longitudinal gluons leading to the Meissner effect in the CFL phase. The mass of the composite vector excitations is close to twice the gap in weak coupling, but goes asymptotically to zero with increasing coupling thereby realizing Georgi's vector limit in cold and dense matter. We discuss the possible mixing of the composite scalar and vector excitations with the gluons, their possible coupling to the modified photons and their decay into light pseudoscalars in the CFL phase. The issue of hidden gauge-symmetry in the QCD superconductor is critically examined. The physical implications of our results on soft dilepton and neutrino emission in cold and dense matter are briefly discussed.

States and curves of five-dimensional gauged supergravity

We consider the sector of N=8 five-dimensional gauged supergravity with nontrivial scalar fields in the coset space SL(6, R)/SO(6) , plus the metric. We find that the most general supersymmetric solution is parametrized by six real moduli and analyze its properties using the theory of algebraic curves. In the generic case, where no continuous subgroup of the original SO(6) symmetry remains unbroken, the algebraic curve of the corresponding solution is a Riemann surface of genus seven. When some cycles shrink to zero size the symmetry group is enhanced, whereas the genus of the Riemann surface is lowered accordingly. The uniformization of the curves is carried out explicitly and yields various supersymmetric configurations in terms of elliptic functions. We also analyze the ten-dimensional type-IIB supergravity origin of our solutions and show that they represent the gravitational field of a large number of D3-branes continuously distributed on hyper-surfaces embedded in the six-dimensional space transverse to the branes. The spectra of massless scalar and graviton excitations are also studied on these backgrounds by casting the associated differential equations into Schrödinger equations with nontrivial potentials. The potentials are found to be of Calogero type, rational or elliptic, depending on the background configuration that is used.

Topologically Massive Gauge Theories

Gauge vector and gravity models are studied in three-dimensional space-time, where novel, gauge invariant, P and T odd terms of topological origin give rise to masses for the gauge fields. In the vector case, the massless Maxwell excitation, which is spinless, becomes massive with spin 1. When interacting with fermions, the quantum theory is infrared and ultraviolet finite in perturbation theory. For non-Abelian models, topological considerations lead to a quantization condition on the dimensionless coupling constant–mass ratio. Ordinary Einstein gravity is trivial, but when augmented by our mass term, it acquires a propagating, massive, spin 2 mode. This theory is ghost-free and causal, although of third-derivative order. Quantum calculations are presented in both the Abelian and non-Abelian vector models, to exhibit some of the delicate aspects of infrared behavior, and regularization dependence.

ZERO-BRANE MATRIX MECHANICS, MONOPOLES AND MEMBRANE APPROACH IN QCD

We conjecture that a T-dual form of pure QCD describes dynamics of point-like monopoles. T-duality transforms the QCD Lagrangian into a matrix quantum mechanics of zero-branes which we identify with monopoles. At generic points of the monopole moduli space, the SU (N) gauge group is broken down to U (1)N-1 reproducing the key feature of 't Hooft's Abelian projection. There are certain points in the moduli space where monopole positions coincide, gauge symmetry is enhanced and gluons emerge as massless excitations. We show that there is a linearly rising potential between zero-branes. This indicates the presence of a stretched flux tube between monopoles. The lowest energy state is achieved when monopoles are sitting on top of each other and gauge symmetry is enhanced. In this case they behave as free massive particles and can be condensed. In fact, we find a constant eigenfunction of the corresponding Hamiltonian which describes condensation of monopoles. Using the monopole quantum mechanics, we argue that large-N QCD in this T-dual picture is a theory of a closed bosonic membrane propagating in five-dimensional space–time. QCD point-like monopoles can be regarded in this approach as constituents of the membrane.

(Mis-)handling gauge invariance in the theory of the quantum Hall effect. III. The instanton vacuum and chiral-edge physics

The concepts of an instanton vacuum and F invariance are used to derive a complete effective theory of massless edge excitations in the quantum Hall effect. Our theory includes the effects of disorder and Coulomb interactions, as well as the coupling to electromagnetic fields and statistical gauge fields. The results are obtained by studying the strong-coupling limit of a Finkelstein action, previously introduced for the purpose of unifying both integral and fractional quantum Hall regimes. We establish the fundamental relation between the instanton vacuum approach and the completely equivalent theory of chiral edge bosons. In this paper we limit the analysis to the integral regime. We show that our complete theory of edge dynamics can be used as an important tool to investigate long-standing problems such as long-range, smooth disorder, and Coulomb interaction effects. We introduce a two-dimensional network of chiral-edge states and tunneling centers (saddle points) as a model for smooth disorder. This network is then used to derive a mean-field theory of the conductances, and we work out the characteristic temperature $(T)$ scale at which the transport crosses over from mean-field behavior at high $T$ to the critical behavior plateau transitions at much lower $T.$ The results explain the apparent lack of scaling which is usually seen in the transport data taken from arbitrary samples at finite $T.$ Second, we address the problem of electron tunneling into the quantum Hall edge. We show that the tunneling density of states near the edge is affected by the combined effects of the Coulomb interactions and the smooth disorder in the bulk. We express the problem in terms of an effective Luttinger liquid with conductance parameter $(g)$ equal to the filling fraction (\ensuremath{\nu}) of the Landau band. Hence, even in the integral regime, our results for tunneling are completely non-Fermi-liquid-like, in sharp contrast to the predictions of single-edge theories.

Soliton lattices in the incommensurate spin-Peierls phase: Local distortions and magnetizations

It is shown that nonadiabatic fluctuations of the soliton lattice in the spin-Peierls system ${\mathrm{CuGeO}}_{3}$ lead to an important reduction of the NMR line widths. These fluctuations are the zero-point motion of the massless phasonic excitations. Furthermore, we show that the discrepancy of x-ray and NMR soliton widths can be understood as the difference between a distortive and a magnetic width. Their ratio is controlled by the frustration of the spin system. By this work, theoretical and experimental results can be reconciled in two important points.

Non-supersymmetric type I strings with zero vacuum energy

We study open descendants of non-supersymmetric type IIB asymmetric (freely acting) obifolds with zero cosmological constant. A generic feature of these models is that supersymmetry remains unbroken on the brane at all mass levels, while it is broken in the bulk in a way that preserves Fermi-Bose degeneracy in both the massless and massive (closed string) spectrum. This property remains valid in the heterotic dual of the type 11 model but only for the massless excitations. A possible application of these constructions concerns scenarios of low-energy supersymmetry breaking with large dimensions.

Path-integral formulation of the Dirichlet string in general backgrounds

We investigate the dynamics of an arbitrary Dirichlet (D-) string in presence of general curved backgrounds following a path-integral formalism. In particular, we consider the interaction of a D-string with the massless excitations of closed string in open bosonic string theory. The background fields induce invariant curvatures on the D-string manifold and the extrinsic curvature can be seen to contain a divergence at the disk boundary. The renormalization of D-string coordinates, next to the leading order in its derivative expansion, is performed to handle the divergence. Then we obtain the generalized Dirac-Born-Infeld action representing the effective dynamics of a D-string in the presence of the non-trivial backgrounds. On the other hand, the D-string acts as a source for the Ramond-Ramond two-form which induces an additional (lower) form due to its coupling to the U(1) gauge invariant fields on the D-string. These forms are reviewed in this formalism for an arbitrary D-string and are encoded in the Wess-Zumino action. Quantization of the D-string collective coordinates, in the U(1) gauge sector, is performed by taking into account the coupling to the lower form and the relevant features of the D-string are analyzed in the presence of background fields.

Critical Properties in Dynamical Charge Correlation Function for the One-Dimensional Mott Insulator

Critical properties in the dynamical charge correlation function for the one-dimensional Mott insulator are studied. By properly taking into account {\it the final-state interaction} between the charge and spin degrees of freedom, we find that the edge singularity in the charge correlation function is governed by massless spinon excitations, although it is naively expected that spinons do not directly contribute to the charge excitation over the Hubbard gap. We obtain the momentum-dependent anomalous critical exponent by applying the finite-size scaling analysis to the Bethe ansatz solution of the half-filled Hubbard model.

Supersymmetry breaking, open strings and M-theory

We study supersymmetry breaking by Scherk-Schwarz compactifications in type I string theory. While in the gravitational sector all mass splittings are proportional to a (large) compactification radius, supersymmetry remains unbroken for the massless excitations of D-branes orthogonal to the large dimension. In this sector, supersymmetry breaking can then be mediated by gravitational interactions alone, that are expected to be suppressed by powers of the Planck mass. The mechanism is non-perturbative from the heterotic viewpoint and requires a compactification radius at intermediate energies of order 10 12−10 14 GeV This can also explain the value of Newton's constant if the string scale is close to the unification scale, of order 10 16 GeV.

Massive and Massless Behavior in Dimerized Spin Ladders

We investigate the conditions under which a gap vanishes in the spectrum of dimerized coupled spin-1/2 chains by means of Abelian bosonization and Lanczos diagonalization techniques. Although both interchain ($J'$) and dimerization ($\delta$) couplings favor a gapful phase, it is shown that a suitable choice of these interactions yields massless spin excitations. We also discuss the influence of different arrays of relative dimerization on the appearance of non-trivial magnetization plateaus.

Born-Infeld corrections to D3 brane action inAdS5×S5andN=4,d=4primary superfields

We consider certain supersymmetric Born-Infeld couplings to the D3 brane action and show that they give rise to massless and massive KK excitations of type IIB on $AdS_5\times S_5$, in terms of singleton Yang-Mills superfields.

Gauge fields as composite boundary excitations

We investigate representations of the conformal group that describe "massless" particles in the interior and at the boundary of anti-de Sitter space. It turns out that massless gauge excitations in anti-de Sitter are gauge "current" operators at the boundary. Conversely, massless excitations at the boundary are topological singletons in the interior. These representations lie at the threshold of two "unitary bounds" that apply to any conformally invariant field theory. Gravity and Yang-Mills gauge symmetry in anti-De Sitter is translated to global translational symmetry and continuous $R$-symmetry of the boundary superconformal field theory.