arXiv subjects

Shibaji Roy

Publications and source records attributed to Shibaji Roy.

At least 55 records · Page 3Linked to original sources

arXiv2009

Proper acceleration, geometric tachyon and dynamics of a fundamental string near D$p$ branes

We present a detailed analysis of our recent observation that the origin of the geometric tachyon, which arises when a D$p$-brane propagates in the vicinity of a stack of coincident NS5-branes, is due to the proper acceleration generated by the background dilaton field. We show that when a fundamental string (F-string), described by the Nambu-Goto action, is moving in the background of a stack of coincident D$p$-branes, the geometric tachyon mode can also appear since the overall conformal mode of the induced metric for the string can act as a source for proper acceleration. We also studied the detailed dynamics of the F-string as well as the instability by mapping the Nambu-Goto action of the F-string to the tachyon effective action of the non-BPS D-string. We qualitatively argue that the condensation of the geometric tachyon is responsible for the (F,D$p$) bound state formation.

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arXiv2008

Origin of the geometric tachyon

The motion of a Dp-brane in the background of a stack of coincident NS5-branes is analysed as the motion of a relativistic point particle in the transverse space of the five-branes. In this system, the particle experiences a proper acceleration orthogonal to its proper velocity due to the background dilaton field which changes the dynamics from that of a simple geodesic motion. In particular, we show that in the vicinity of the five-branes, it is this acceleration which is responsible for modifying the motion of the radial mode to that of an inverted simple harmonic oscillator leading to the tachyonic instability.

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arXiv2007

On Brane-Antibrane Forces

In this note, we will discuss two aspects of brane-antibrane forces. In one aspect, we generalize the force calculation of D0-${\bar {\rm D}}$0 of Banks and Susskind to Dp-${\bar {\rm D}}p$ for $1\le p \leq 8$. In particular, we find that the force is also divergent for p = 1 while for the other cases ($p \ge 2$) the forces are in general finite when $Z \to 0^+$, where $Z = \frac{Y^2}{2π^2α'} - 1$ with Y, the brane-antibrane separation. However, the forces are divergent for all cases when Z < 0, signalling the occurrence of open string tachyon condensation in this regime. The other deals with the puzzling static nature of the supergravity brane-antibrane configurations. We will show that the force on a brane probe due to a brane-antibrane background vanishes when the probe is placed at the location of the coincident brane-antibranes, thereby providing a direct evidence for the existence of general static brane-antibrane configuration in the supergravity approximation.

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arXiv2007

Alpha-prime corrections to space-like branes

Space-like branes or S-branes are certain class of time dependent solutions of M/string theories having isometries ISO(p+1) $\times$ SO(d-p-2,1), where $d=11,10$ respectively and have singularities at t=0. In \cite{Roy:2006du}, we found that the asymptotically flat S-branes have the structure of generalized Kasner geometry near t=0. In this work we evaluate higher order $α'$ corrections perturbatively to the heterotic string Kasner backgrounds to probe the singularity at t=0. We generally find that the perturbative corrections do not permit us to reach the singular point, as the supergravity framework fails near $t \sim {\sqrt α'}$ blurring the origin of space-like singularities. This is analogous to the concept of stretched horizons in the case of black holes.

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arXiv2006

Intersecting non-SUSY $p$-brane with chargeless 0-brane as black $p$-brane

Unlike BPS $p$-brane, non-supersymmetric (non-susy) $p$-brane could be either charged or chargeless. As envisaged in [hep-th/0503007], we construct an intersecting non-susy $p$-brane with chargeless non-susy $q$-brane by taking T-dualities along the delocalized directions of the non-susy $q$-brane solution delocalized in $(p-q)$ transverse directions (where $p\geq q$). In general these solutions are characterized by four independent parameters. We show that when $q=0$ the intersecting charged as well as chargeless non-susy $p$-brane with chargeless 0-brane can be mapped by a coordinate transformation to black $p$-brane when two of the four parameters characterizing the solution take some special values. For definiteness we restrict our discussion to space-time dimensions $d=10$. We observe that parameters characterizing the black brane and the related dynamics are in general in a different branch of the parameter space from those describing the brane-antibrane annihilation process. We demonstrate this in the two examples, namely, the non-susy D0-brane and the intersecting non-susy D4 and D0-branes, where the solutions with the explicit microscopic descriptions are known.

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arXiv2006

Space-like branes, accelerating cosmologies and the near `horizon' limit

It is known that there exist two different classes of time dependent solutions in the form of space-like (or S)-branes in the low energy M/string theory. Accelerating cosmologies are known to arise from S-branes in one class, but not in the other where the time-like holography in the dS/CFT type correspondence may be more transparent. We show how the accelerating cosmologies arise from S-branes in the other class. Although we do not get the de Sitter structure in the lowest order supergravity, the near `horizon' ($t\to 0$) limits of these S-branes are the generalized Kasner metric.

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arXiv2004

On the Construction of SL(2,Z) Type IIB 5-Branes

This talk reviews our recent work on the construction of SL(2,Z) multiplets of type IIB superfivebranes. We here pay particular attention to the methods employed and some salient features of the solutions.

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arXiv2004

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 part and a Wess-Zumino 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. the S-brane. On the other hand, the WZ part as well as the action is real if it represents the source in type II$^\ast$ string theory i.e. the 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.

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arXiv2003

Time dependent supergravity solutions in arbitrary dimensions

By directly solving the equations of motion we obtain the time dependent solutions of supergravities with dilaton and a $q$-form field-strength in arbitrary dimensions. The metrics are assumed to have the symmetries ISO($p+1$) $\times$ SO($d-p-2,1$) and can be regarded as those of the magnetically charged Euclidean or space-like branes. When we impose the extremality condition, we find that the magnetic charges of the branes become imaginary and the corresponding real solutions then represent the E$p$-branes of type II$^\ast$ theories (for the field-strengths belonging to the RR sector). On the other hand, when the extremality condition is relaxed we find real solutions in type II theories which resemble the solutions found by Kruczenski-Myers-Peet. In $d=10$ they match exactly. We point out the relations between the solutions found in this paper and those of Chen-Gal'tsov-Gutperle in arbitrary dimensions. Although there is no extremal limit for these solutions, we find another class of solutions, which resemble the solutions in the extremal case with imaginary magnetic charges and the corresponding real solutions can be regarded as the non-BPS E$p$-brane solutions of type II$^\ast$ theories (for the field-strengths in RR sector).

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arXiv2003

Dimensional reductions of M-theory S-branes to string theory S-branes

We study both the direct and the double dimensional reduction of space-like branes of M-theory and point out some peculiarities in the process unlike their time-like counterpart. In particular, we show how starting from SM2 and SM5-brane solutions we can obtain SD2 and SNS5-brane as well as SNS1 and SD4-brane solutions of string theory by direct and double dimensional reductions respectively. In the former case we need to use delocalized SM-brane solutions, whereas in the latter case we need to use anisotropic SM-brane solutions in the directions which are compactified.

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arXiv2002

On supergravity solutions of space-like Dp-branes

Recently the time dependent solutions of type II supergravities in $d = 10$, with the metric having the symmetry $ISO(p+1) \times SO(8-p, 1)$ have been given by two groups (Chen-Gal'tsov-Gutperle (CGG), [hep-th/0204071] and Kruczenski-Myers-Peet (KMP), [hep-th/0204144]). The supergravity solutions correspond to space-like D$p$-branes in type II string theory. While the CGG solution is a four parameter solution, the KMP solution is a three parameter solution and so in general they are different. This difference can be attributed to the fact that unlike the CGG solution, KMP uses a specific boundary condition for the metric and the dilaton field. It is shown that when we impose the boundary conditions used in the KMP solution to the CGG solution then both become three parameter solutions and they map to each other under a coordinate transformation along with a Hodge duality of the field strength. We also give the relations between the parameters characterizing the two solutions.

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arXiv2002

Correlators of open D-string theory from supergravity

We consider a minimally coupled massless scalar propagating in the background of NS5-branes in the presence of a 2-form RR electric field. The supergravity solution also called the (NS5,D1) bound state in the appropriate decoupling limit is the holographic dual of Open D-string (OD1) theory. Using this information, we compute the two-point function of the operators in OD1 theory which couples to the massless string states such as the dilaton. We will indicate how to obtain the absorption cross-section of a scalar on NS5-branes (i.e. Little String Theory (LST)) obtained earlier, from our results in the low-energy limit.

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arXiv2002

(NS5,D5,D3) bound state, OD3, OD5 limits and SL(2,Z) duality

We generalize the non-threshold bound state in type IIB supergravity of the form (NS5, D5, D3) constructed by the present authors (in hep-th/0011236) to non-zero asymptotic value of the axion $(χ_0$). We identify the decoupling limits corresponding to both the open D3-brane theory and open D5-brane theory for this supergravity solution as expected. However, we do not find any non-commutative Yang-Mills theory (NCYM) limit for this solution in the presence of NS5 branes. We then study the $SL(2, Z)$ duality symmetry of type IIB theory for both OD3-limit and OD5-limit. We find that for OD3 theory, a generic $SL(2, Z)$ duality always gives another OD3-theory irrespective of the value of $χ_0$ being rational or not. This indicates that OD3-theory is self-dual. But, under a special set of $SL(2, Z)$ transformations for which $χ_0$ is rational OD3-theory goes over to a 5+1 dimensional NCYM theory and these two theories in this case are related to each other by strong-weak duality symmetry. On the other hand, for OD5-theory, a generic $SL(2, Z)$ duality gives another OD5-theory if $χ_0$ is irrational, but when $χ_0$ is rational it gives the little string theory limit indicating that OD5-theory is S-dual to the type IIB little string theory.

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arXiv2000

(NS5,Dp) and (NS5,D(p+2),Dp) bound states of type IIB and type IIA string theories

Starting from the (q,p) 5-brane solution of type IIB string theory, we here construct the low energy configuration corresponding to (NS5,Dp)-brane bound states (for $0\leq p\leq 4$) using the T-duality map between type IIB and type IIA string theories. We use the SL(2,Z) symmetry on the type IIB bound state (NS5,D3) to construct (NS5,D5,D3) bound state. We then apply T-duality transformation again on this state to construct the bound states of the form (NS5,D(p+2),Dp) (for $0\leq p\leq 2$) of both type IIB and type IIA string theories. We give the tension formula for these states and show that they form non-threshold bound states. All these states preserve half of the space-time supersymmetries of string theories. We also briefly discuss the ODp-limits corresponding to (NS5,Dp) bound state solutions.

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arXiv1999

(F, D5) Bound State, SL(2, Z) Invariance and The Descendant States in Type IIB/A String Theory

Recently the space-time configurations of a set of non-threshold bound states, called the (F, Dp) bound states, have been constructed explicitly for every $p$ with $2 \le p \le 7$ in both type IIA (for $p$ even) and type IIB (for $p$ odd) string theories by the present authors. By making use of the SL(2, Z) symmetry of type IIB theory we construct a more general SL(2, Z) invariant bound state of the type ((F, D1), (NS5, D5)) in this theory from the (F, D5) bound state. There are actually an infinite number of $(m,n)$ strings forming bound states with $(m',n')$ 5-branes, where strings lie along one of the spatial directions of the 5-branes. By applying T-duality along one of the transverse directions we also construct the bound state ((F, D2), (KK, D6)) in type IIA string theory. Then we give a list of possible bound states which can be obtained from these newly constructed bound states by applying T-dualities along the longitudinal directions as well as S-dualities to those in type IIB theory.

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arXiv1999

((F, D1), D3) Bound State and Its T-dual Daughters

In the previous paper [hep-th/9904129], we constructed a general explicit BPS solution for (F, D3) non-threshold bound state. By using the SL(2,Z) symmetry of type IIB string theory, we here construct from (F, D3) a more general BPS configuration for a D3 brane with certain units of quantized 5-form flux and an infinite number of parallel (F, D1)-strings. We study its decoupling limit and find that given Maldacena's $AdS_5/CFT_4$ correspondence with respect to simple D3 branes and with the usual string coupling, we should have a similar correspondence with respect to this bound state but now with an effective string coupling. We discuss possible descendants of this bound state by T-dualities along its longitudinal or transverse directions. In particular, we present explicit configurations for ((F, Dp), D(p + 2)) bound states for $2 \le p \le 5$. All these configurations preserve one half of the spacetime supersymmetries.

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arXiv1999

Kaluza-Klein and H-Dyons in String Theory

Kaluza-Klein monopole and H-monopole solutions, which are T-dual to each other, are the well-known solutions of string theory compactified on $T^6$. Since string theory in this case has an S-duality symmetry, we explicitly construct the corresponding dyonic solutions by expressing the $D = 4$ string effective action in a manifestly $SL(2, R)$ invariant form with an $SL(2, R)$ invariant constraint. The Schwarz-Sen charge spectrum, the BPS saturated mass formula as well as the stability of these states are discussed briefly.

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arXiv1999

Non-threshold (F, Dp) bound states

In the previous paper [hep-th/9904112], we argued that there exist BPS bound states of Dp branes carrying certain units of quantized constant electric field for every $p$ (with $1 \le p \le 8$). Each of these bound states preserves one half of the spacetime supersymmetries. In this paper, we construct these bound state configurations explicitly for $2 \le p \le 7$ from Schwarz's $(m,n)$-string or (F, D1) bound state in type IIB string theory by T-dualities along the transverse directions. We calculate the charge per $(2π)^{p -1} α'^{(p - 1)/2}$ of $(p -1)$-dimensional area for F-strings in (F, Dp) and the tension for each of these bound states. The results agree precisely with those obtained previously from the worldvolume study. We study the decoupling limit for the (F, D3) bound state and find that Maldacena's $AdS_5/CFT_4$ correspondence may hold true even with respect to thisbound state but now with an effective string coupling rather than the usual string coupling. This coupling is quantized and can be independent of the usual string coupling in certain limit.

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