Wall Crossing Formula for N=4 Dyons: A Macroscopic Derivation

We derive the wall crossing formula for the decay of a quarter BPS dyon into a pair of half-BPS dyons by analyzing the quantum dynamics of multi-centered black holes in N=4 supersymmetric string theories. Our analysis encompasses the cases where the final decay products are non-primitive dyons. The results are in agreement with the microscopic formula for the dyon spectrum in the special case of heterotic string theory on T^6.Comment: LaTeX file, 12 pages; v2: minor change

Stable Non-BPS Bound States of BPS D-branes

S-duality symmetry of type IIB string theory predicts the existence of a stable non-BPS state on an orbifold five plane of the type IIB theory if the orbifold group is generated by the simultaneous action of (-1)^{F_L} and the reversal of sign of the four coordinates transverse to the orbifold plane. We calculate the mass of this state by starting from a pair of D-strings carrying the same charge as this state, and then identifying the point in the moduli space where this pair develops a tachyonic mode, signalling the appearance of a bound state of this configuration into the non-BPS state.Comment: LaTeX file, 8 figures, 30 pages, minor changes, references adde

Stretching the Horizon of a Higher Dimensional Small Black Hole

There is a general scaling argument that shows that the entropy of a small black hole, representing a half-BPS excitation of an elementary heterotic string in any dimension, agrees with the statistical entropy up to an overall numerical factor. We propose that for suitable choice of field variables the near horizon geometry of the black hole in D space-time dimensions takes the form of AdS_2\times S^{D-2} and demonstrate how this ansatz can be used to calculate the numerical factor in the expression for the black hole entropy if we know the higher derivative corrections to the action. We illustrate this by computing the entropy of these black holes in a theory where we modify the supergravity action by adding the Gauss-Bonnet term. The black hole entropy computed this way is finite and has the right dependence on the charges in accordance with the general scaling argument, but the overall numerical factor does not agree with that computed from the statistical entropy except for D=4 and D=5. This is not surprising in view of the fact that we do not use a fully supersymmetric action in our analysis; however this analysis demonstrates that higher derivative corrections are capable of stretching the horizon of a small black hole in arbitrary dimensions.Comment: LaTeX file, 26 pages; v2: a simple expression for entropy is given and references adde

SO(32) Spinors of Type I and Other Solitons on Brane-Antibrane Pair

We construct the SO(32) spinor state in weakly coupled type I string theory as a kink solution of the tachyon field on the D-string - anti-D-string pair and calculate its mass. We also give a description of this system in terms of an exact boundary conformal field theory and show that in this description this state can be regarded as a non-supersymmetric D0-brane in type I string theory. This construction can be generalised to represent the D0-brane in type IIA string theory as a vortex solution of the tachyon field on the membrane anti-membrane pair, and the D-string of type I string theory as a topological soliton of the tachyon field on the D5-brane anti- D5-brane pair.Comment: LaTeX file, 31 page

Universality of the Tachyon Potential

Using string field theory, we argue that the tachyon potential on a D-brane anti-D-brane system in type II string theory in arbitrary background has a universal form, independent of the boundary conformal field theory describing the brane. This implies that if at the minimum of the tachyon potential the total energy of the brane antibrane system vanishes in a particular background, then it vanishes in any other background. Similar result holds for the tachyon potential of the non-BPS D-branes of type II string theory, and the D-branes of bosonic string theory.Comment: LaTeX file, epsf, 1 figure, 21 pages, references adde

SL(2, Z) Duality and Magnetically Charged Strings

Postulate of SL(2,\Z) invariance of toroidally compactified heterotic string theory in four dimensions implies the existence of new string (dual string) states carrying both, electric and magnetic charges. In this paper we study the interaction between these dual strings. In particular, we consider scattering of two such strings in the limit where one string passes through the other string without touching it, and show that such a scattering leads to the exchange of a fixed amount of electric and magnetic charges between the two strings.Comment: 20 pages, phyzzx.tex, TIFR-TH-93-0

EXTREMAL BLACK HOLES AND ELEMENTARY STRING STATES

Some of the extremal black hole solutions in string theory have the same quantum numbers as the Bogomol'nyi saturated elementary string states. We explore the possibility that these black holes can be identified to elementary string excitations. It is shown that stringy effects could correct the Bekenstein-Hawking formula for the black hole entropy in such a way that it correctly reproduces the logarithm of the density of elementary string states. In particular, this entropy has the correct dependence on three independent parameters, the mass and the left-handed charge of the black hole, and the string coupling constant.Comment: 16 pages, LaTeX file, Example of a specific scenario modified, footnote and references adde