Probe Brane Research Articles

Geometrical tachyon kinks andNS5 branes

We further investigate the $NS$5 ring background using the tachyon map. Mapping the radion fields to the rolling tachyon helps to explain the motion of a probe $Dp$-brane in this background. It turns out that the radion field becomes tachyonic when the brane is confined to one dimensional motion inside the ring. We find explicit solutions for the geometrical tachyon field that describe stable kink solutions which are similar to those of the open string tachyon. Interestingly in the case of the geometric tachyon, the dynamics is controlled by a cosine potential. In addition, we couple a constant electric field to the probe-brane, but find that the only stable kink solutions occur when there is zero electric field or a critical field value. We also investigate the behaviour of Non-BPS branes in this background, and find that the end state of any probe brane is that of tachyonic matter 'trapped' around the interior of the ring. We conclude by considering compactification of the ring solution in one of the transverse directions.

Mirage cosmology with an unstable probe D3-brane

We consider the mirage cosmology by an unstable probe brane whose action is represented by Dirac-Born-Infeld action with tachyon. We study how the presence of tachyon affects the evolution of the brane inflation. At the early stage of the brane inflation, the tachyon kinetic term can play an important role in curing the superluminal expansion in mirage cosmology.

Regge trajectories for mesons in the holographic dual of large-NcQCD

We discuss Regge trajectories of dynamical mesons in large-N_c QCD, using the supergravity background describing N_c D4-branes compactified on a thermal circle. The flavor degrees of freedom arise from the addition of N_f<<N_c D6 probe branes. Our work provides a string theoretical derivation, via the gauge/string correspondence, of a phenomenological model describing the meson as rotating point-like massive particles connected by a flux string. The massive endpoints induce nonlinearities for the Regge trajectory. For light quarks the Regge trajectories of mesons are essentially linear. For massive quarks our trajectories qualitatively capture the nonlinearity detected in lattice calculations.

Folding branes

We study classical dynamics of a probe Dp-brane moving in a background sourced by a stack of Dp-branes. In this context the physics is similar to that of the effective action for open-string tachyon condensation, but with a power-law runaway potential. We show that small inhomogeneous ripples of the probe brane embedding grow with time, leading to folding of the brane as it moves. We give a full nonlinear analytical treatment of inhomogeneous brane dynamics, suitable for the Dirac-Born-Infeld + Wess-Zumino theory with arbitrary runaway potential, in the case where the source branes are BPS. In the near-horizon geometry, the inhomogeneous brane motion has a dual description in terms of free streaming of massive relativistic test particles originating from the initial hypersurface of the probe brane. We discuss limitations of the effective action description around loci of self-crossing of the probe brane (caustics). We also discuss the effect of brane folding in application to the theory of cosmological fluctuations in string theory inflation.

One dimensional M5-brane intersections

We study one dimensional intersections of M5 branes with M5 and M2 branes. On the worldvolume of the M5-brane, such an intersection appears as a string soliton. We study this worldvolume theory in two different regimes: 1) Where the worldvolume theory is formulated in flat space and 2) where the worldvolume theory is studied in the supergravity background produced by a stack of M5 (or M2) branes. In both cases, we study the corresponding string solitons, and find the most general BPS configuration consistent with the fraction of supersymmetries preserved. We argue that M5 and M2 brane intersections leave different imprints on the worldvolume theory of the intersecting probe brane, although geometrically they appear to be similar.

Meson spectroscopy from holomorphic probes on the warped deformed conifold

We study D7 brane probes holomorphically embedded in the Klebanov-Strassler model. Analyzing the $\kappa$-symmetry condition for D7 branes wrapping a 4-cycle of a deformed conifold we find configurations that do not break N=1 supersymmetry of the background. We compute the fluctuations of the probe around one of these configurations and obtain the spectrum of vector and scalar flavored mesons in the dual gauge theory. The spectrum is discrete and exhibits a mass gap.

D7 Brane Embeddings and Chiral Symmetry Breaking

We study the embedding of D7 brane probes in five geometries that are deformations of AdS_5 x S^5. Each case corresponds to the inclusion of quark fields in a dual gauge theory where we are interested in investigating whether chiral symmetry breaking occurs. We use a supersymmetric geometry describing an N=2 theory on its moduli space and a dilaton driven non-supersymmetric flow to establish criteria for a chiral symmetry breaking embedding. We develop a simple spherical D7 embedding that tests the repulsion of the core of the geometry and signals dynamical symmetry breaking. We then use this tool in more complicated geometries to show that an N=2* theory and a non-supersymmetric theory with scalar masses do not induce a chiral condensate. Finally we provide evidence that the Yang Mills* geometry does.

D-Brane Dynamics and NS5 Rings

We consider the classical motion of a probe D-brane moving in the background geometry of a ring of NS5 branes, assuming that the latter are non-dynamical. We analyse the solutions to the Dirac-Born-Infield (DBI) action governing the approximate dynamics of the system. In the near horizon (throat) approximation we find several exact solutions for the probe brane motion. These are compared to numerical solutions obtained in more general cases. One solution of particular interest is when the probe undergoes oscillatory motion through the centre of the ring (and perpendicular to it). By taking the ring radius sufficiently large, this solution should remain stable to any stringy corrections coming from open-strings stretching between the probe and the NS5-branes along the ring.

Low Energy Hadron Physics in Holographic QCD

We present a holographic dual of four-dimensional, large N_c QCD with massless flavors. This model is constructed by placing N_f probe D8-branes into a D4 background, where supersymmetry is completely broken. The chiral symmetry breaking in QCD is manifested as a smooth interpolation of D8 - anti-D8 pairs in the supergravity background. The meson spectrum is examined by analyzing a five-dimensional Yang-Mills theory that originates from the non-Abelian DBI action of the probe D8-brane. It is found that our model yields massless pions, which are identified with Nambu-Goldstone bosons associated with the chiral symmetry breaking. We obtain the low-energy effective action of the pion field and show that it contains the usual kinetic term of the chiral Lagrangian and the Skyrme term. A brane configuration that defines a dynamical baryon is identified with the Skyrmion. We also derive the effective action including the lightest vector meson. Our model is closely related to that in the hidden local symmetry approach, and we obtain a Kawarabayashi-Suzuki-Riazuddin-Fayyazuddin-type relation among the couplings. Furthermore, we investigate the Chern-Simons term on the probe brane and show that it leads to the Wess-Zumino-Witten term. The mass of the \eta' meson is also considered, and we formulate a simple derivation of the \eta' mass term satisfying the Witten-Veneziano formula from supergravity.

Braneworld inflation

We discuss various realizations of the four dimensional braneworld inflation in warped geometries of string theory. In all models the inflaton field is represented by a Dp probe brane scalar specifying its position in the warped throat of the compactification manifold. We study existing inflationary throat local geometries, and construct a new example. The inflationary brane is either a D3- or a D5-brane of type IIB string theory. In the latter case the inflationary brane is wrapping a two-cycle of the compactification manifold. We discuss some phenomenological aspects of the model where slow-roll conditions are under computational control.

Universal Correlators from Geometry

Matrix model correlators show universal behaviour at short distances. We provide a derivation for these universal correlators by inserting probe branes in the underlying effective geometry. We generalize these results to study correlators of branes and their universal behaviour in the Calabi-Yau crystals, where we find a role for a generalized brane insertion.

Chiral symmetry breaking driven by dilaton

Dynamical properties of gauge theories with light flavor quarks are studied in a dual supergravity by adding a D7 brane probe into the AdS background deformed by the dilaton. By estimating the vev of flavor quark-bilinear in both the supersymmetric and non-supersymmetric gravity duals, we find spontaneous chiral symmetry breaking in the case of the non-supersymmetric background. We also study quark–antiquark potential for light quarks to see the quark confinement in the models considered here.

AdS/CFT Correspondence and the Reheating of the Brane-Universe

We present a mechanism for exit from inflation and reheating using the AdS/CFT correspondence. A cosmological evolution is induced on a probe D3-brane as it moves in a black D-brane background of type-0 string theory. If the tachyon field is non zero, inflation is induced on the brane-universe, with the equation of state parameter in the range -1<w<-1/3 depending on the position of the probe brane in the bulk. As the probe brane approaches the horizon of the background black hole, the inflation rate decreases and the value of w gets larger. At some critical distance away from the horizon, inflation ends. When the brane-universe reaches the horizon, the conformal invariance is restored, the background geometry becomes AdS{5}X S{5}, and the brane-universe feels the CFT thermal radiation and reheats.

PROBING MALDACENA–NUNEZ IN IR WITH p$\overline{\rm D3}$-BRANES

We probed Maldacena–Nunez solution in IR with p coincident anti-D3-branes and found that these probe branes become a fuzzy NS5-brane. Doing the dual analysis, i.e. from the NS5-brane point of view with the charge of p anti-D3-brane on the world-volume of NS5-brane, we showed that to leading order this potential matches with that of p anti-D3-branes and the potential on the NS5-brane has a stable minima and have also calculated the potential, from the NS5-brane point of view, for a small fluctuation along the radial direction.

Generalizations of the AdS/CFT correspondence

AbstractWe consider generalizations of the AdS/CFT correspondence in which probe branes are embedded in gravity backgrounds dual to either conformal or confining gauge theories. These correspond to defect conformal field theories (dCFT) or QCD‐like theories with fundamental matter, respectively. Moreover, starting from the dCFT we discuss the deconstruction of intersecting M5‐branes and M‐theory. We obtain the following results:i) Holography of defect conformal field theories. We consider holography for a general D3‐Dp brane intersection in type IIB string theory (p ∈ {3,5,7}). The corresponding near‐horizon geometry is given by a probe AdS‐brane in AdS5 × S5. The dual defect conformal field theory describes 𝒩 = 4 super Yang‐Mills degrees of freedom coupled to fundamental matter on a lower‐dimensional space‐time defect. We derive the spectrum of fluctuations about the brane embedding and determine the behaviour of correlation functions involving defect operators. We also study the dual conformal field theory in the case of intersecting D3‐branes. To this end, we develop a convenient superspace approach in which both two‐ and four‐dimensional fields are described in a two‐dimensional (2,2) superspace. We show that quantum corrections vanish to all orders in perturbation theory, such that the theory remains a (defect) conformal field theory when quantized.ii) Flavour in generalized AdS/CFT dualities. We present a holographic non‐perturbative description of QCD‐like theories with a large number of colours by embedding D7‐brane probes into two non‐supersymmetric gravity backgrounds. Both backgrounds exhibit confinement of fundamental matter and a discrete glueball and meson spectrum. We numerically compute the $\bar{\psi}\psi$ quark condensate and meson spectrum associated with these backgrounds. In the first background, we find some numerical evidence for a first order phase transition at a critical quark mass where the D7 embedding undergoes a geometric transition. In the second, we find a chiral symmetry breaking condensate as well as the associated Goldstone boson.iii) Deconstruction of extra dimensions. We apply the deconstruction method to the dCFT of intersecting D3‐branes to obtain a field theory description for intersecting M5‐branes. The resulting theory corresponds to two six‐dimensional (2,0) superconformal field theories which we show to have tensionless strings on their four‐dimensional intersection. Moreover, we argue that the SU(2)L R‐symmetry of the dCFT matches the manifest SU(2) R‐symmetry of the M5‐M5 intersection. We finally explore the fascinating idea of deconstructing M‐theory itself. We give arguments for an equivalence of M‐theory on a certain background with the Higgs branch of a four‐dimensional non‐supersymmetric (quiver) gauge theory: in addition to a string theoretical motivation, we find wrapped M2‐branes in the mass spectrum of the quiver theory at low energies.

Cosmology and two-body problem of D-branes

In this paper, we investigate the dynamics and the evolution of the scale factor of a probe $\mathrm{D}p\ensuremath{-}\mathrm{brane}$ which moves in the background of source $\mathrm{D}p\ensuremath{-}\mathrm{branes}.$ The action of the probe brane is described by the Born-Infeld action and the interaction with the background Ramond-Ramond field. When the probe brane moves away from the source branes, it expands as a power law whose index depends on the dimension of the brane. If the energy density of the gauge field on the brane is subdominant, the expansion decelerates irrespective of the dimension of the brane. On the other hand, when the probe brane is a Nambu-Goto brane, the energy density of the gauge field can be dominant, in which case accelerating expansion occurs for $p&lt;~4.$ The accelerating expansion stops when the brane has expanded sufficiently that the energy density of the gauge field becomes subdominant.

D-branes of covariant AdS superstrings

We consider D-branes of open superstrings in the AdS 5× S 5 background. The possible configurations of D-branes preserving half of supersymmetries are classified by analyzing the κ-invariance of an open superstring in a covariant manner. We also revisit the classification of D-branes in the pp-wave background. It is shown that Penrose limits of the possible D-branes in the AdS 5× S 5 give all of the D-branes in the pp-wave. In addition, a 1/4 supersymmetric D-string, which is related to the D-string preserving 8 dynamical supersymmetries in the pp-wave, is presented. We also discuss the relation between our result and the AdS branes in a brane probe analysis.

On the effective action of a space-like brane

Starting from the non-BPS D(p+1)-brane action, we derive an effective action in (p+1) space dimensions by studying the fluctuations of various bosonic fields around the time-like tachyonic kink solution (obtained by Wick rotation of the space-like tachyonic kink solution) of the non-BPS brane. In real time this describes the dynamics of a space-like or Euclidean brane in (p+1) dimensions containing a Dirac–Born–Infeld (DBI) part and an Wess–Zumino (WZ) part. The WZ part is purely imaginary and so the action is complex if it represents the source of the time-dependent background of type II string theory, i.e., S-brane. On the other hand, the WZ part as well as the action is real if it represents the source in type II∗ string theory, i.e., E-brane. The DBI part is the same as obtained before using different method. This is then further illustrated by considering brane probe in space-like brane background.

On bouncing brane worlds, S-branes and branonium cosmology

We present several higher-dimensional spacetimes for whichobservers living on 3-branes experience an induced metric whichbounces. The classes of examples include boundary branes on generalized S-brane backgrounds and probe branes inD-brane/anti-D-brane systems. The bounces we consider normallywould be expected to require an energy density which violatesthe weak energy condition, and for our codimension-one examplesthis is attributable to bulk curvature terms in the effectiveFriedmann equation. We examine the features of the accelerationwhich provides the bounce, including in some cases the existenceof positive acceleration without event horizons, and we give ageometrical interpretation for it. We discuss the stability ofthe solutions from the point of view of both the brane and thebulk. Some of our examples appear to be stable from the bulkpoint of view, suggesting the possible existence of stablebouncing cosmologies within the brane-world framework.

D-Brane Chemistry

We study several different kinds of bound states built from D-branes and orientifolds. These states are to atoms what branonium - the bound state of a brane and its anti-brane - is to positronium, inasmuch as they typically involve a light brane bound to a much heavier object with conserved charges which forbid the system's decay. We find the fully relativistic motion of a probe Dp'-brane in the presence of source Dp-branes is integrable by quadratures. Keplerian conic sections are obtained for special choices for p and p' and the systems are shown to be equivalent to nonrelativistic systems. Their quantum behaviour is also equivalent to the corresponding non-relativistic limit. In particular the p=6, p'=0 case is equivalent to a non-relativistic dyon in a magnetic monopole background, with the trajectories in the surface of a cone. We also show that the motion of the probe branes about D6-branes in IIA theory is equivalent to the motion of the corresponding probes in the uplift to M-theory in 11 dimensions, for which there are no D6-branes but their fields are replaced by a particular Taub-NUT geometry. We further discuss the interactions of D-branes and orientifold planes having the same dimension. this system behaves at large distances as a brane-brane system but at shorter distances it does not have the tachyon instability.