Collective Coordinates for D-Branes 1 Introduction
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View metadata, citation and similar papers at core.ac.uk brought to you by CORE provided by CERN Document Server UTTG-01-96 Collective Co ordinates for D-branes Willy Fischler, Sonia Paban and Moshe Rozali Theory Group, Department of Physics University of Texas, Austin, Texas 78712 Abstract We develop a formalism for the scattering o D-branes that includes collective co- ordinates. This allows a systematic expansion in the string coupling constant for such pro cesses, including a worldsheet calculation for the D-brane's mass. 1 Intro duction In the recent developments of string theory, solitons play a central role. For example, in the case of duality among various string theories, solitons of a weakly coupled string theory b ecome elementary excitations of the dual theory [1]. Another example, among many others, is the role of solitons in resolving singularities in compacti ed geometries, as they b ecome light[2]. In order to gain more insight in the various asp ects of these recent developments, it is imp ortant to understand the dynamics of solitons in the context of string theory.In this quest one of the to ols at our disp osal is the use of scattering involving these solitons. This includes the scattering of elementary string states o these solitons as well as the scattering among solitons. An imp ortant class of solitons that have emerged are D-branes [4]. In an in uential pap er [3]itwas shown that these D-branes carry R R charges, and are therefore a central ingredient in the aforementioned dualities. These ob jects are describ ed by simple conformal eld theories, which makes them particularly suited for explicit calculations. In this pap er we use some of the preliminary ideas ab out collective co ordinates de- velop ed in a previous pap er [5], to describ e the scattering of elementary string states o D-branes. We clarify and expand these ideas, providing a complete analysis of this pro cess. Research supp orted in part by the Rob ert A. WelchFoundation and NSF Grant PHY 9511632 The pap er b egins by reviewing the presence of non-lo cal violations of conformal in- variance that are a consequence of the existence of translational zero mo des. It is then shown how the world sheet theory removes these anomalies through the intro duction of collective co ordinates (which app ear as \wormhole parameters"), and the inclusion of the contribution of disconnected worldsheets to the S-matrix elements. As a bypro duct of this analysis we calculate the mass of the soliton by a purely worldsheet approach (that is amenable to higher order corrections). We thereby recover a result obtained in [4] using e ectivelow energy considerations. These ideas are then applied to the example of a closed string state scattering o a 0-brane. We showhow to include the recoil of the brane, and discover the interpretation of the wormhole parameters as collective co ordinates. The conclusion contains some comments on higher order corrections to scattering and to mass renormalization as well as p ossible extensions of this work in di erent directions. 2 Conformal anomalies. D-branes placed at di erent lo cations in space have the same energy. This implies the existence of spacetime zero mo des, one for each spatial direction transverse to the D-brane (i.e. one for each Dirichlet direction). Because of these zero mo des, the attempts to calculate scattering matrix elements in a weak coupling expansion encounter spacetime infra-red divergences. In p oint-like eld theory these divergences arise from zero mo des propagating in Feynman diagrams (for example diagrams as in Fig. 1 , where is a zero 0 mo de). Fig. 1: Diagram containing the propagation of a zero-mo de. 2 Such b ehavior can also b e found in the canonical formalism, where it manifests itself by the presence of vanishing energy denominators, characteristic of degenerate p erturbation theory. This is resolved in quantum eld theory by the intro duction of collective co ordi- nates. The right basis of states, in which the degeneracies are lifted, are the eigenstates of momenta conjugate to these co ordinates. In string theory,however, we do not havea workable second quantized formalism. Instead, wehave to rely on a rst quantized description. The weak coupling expansion consists of histories spanning worldsheets of successively higher top ologies. Yet, the afore- mentioned divergences are still present, and are found in a sp eci c corner of mo duli space. The divergences app ear as violations of conformal invariance, as describ ed b elow. To the lowest order, these singularities are found when one consideres worldsheets as depicted in Fig. 2. P Fig. 2: The surface . P This surface, , is obtained by attaching a very long and thin strip to the b oundary of the disc. This is equivalent to removing two small segments of the b oundary of the disc and identifying them, thus creating a surface with the top ology of an annulus. In the degeneration limit, when the strip b ecomes in nitely long and thin, the only contribution comes from massless op en string states propagating along the strip. In the case of Dirichlet b oundary conditions, the only such states are the translational zero mo des. In the case of closed string solitons, the divergence app ears for degenerate surfaces containing long thin handles, along which the zero mo de, a closed string state in that 3 case, propagates. In b oth cases these limiting world-histories are reminiscent of the eld theory Feynman diagrams, as in Fig. 1 . We are now ready to prop erly extract the divergences. As discussed in [7] for the closed b osonic string theory, the matrix element in the degeneration limit can b e obtained by a conformal eld theory calculation on the lower genus surface, with op erators inserted on the degenerating segments (see Fig. 3 ). P Fig. 3: The degenerations limit of the surface . This can b e expressed formally as follows: Z X dq 2h a q h (s ) (s ) V :::V i (2.1) hV :::V i = a 1 a 2 1 n Disc 1 n q a where f g is a complete set of eigenstates of L (which generates translations along the a 0 strip), with weights h . The limit q ! 0, therefore, pro jects the sum into the lowest a weight states. As an illustration, it is useful to rst consider the same limit for the usual op en b osonic string (with Neumann b oundary conditions). The lowest weight states (excluding the tachyon) are: ! i p dX i ik X = N g e (2.2) k k ds 2 k with h = and N a normalization factor. Using the ab ove formula (2.1), one gets: k k 2 Z Z d D E X d k 2 (1+k ) i i hV :::V i = dq q (s ) (s ) V :::V : (2.3) 1 n 1 2 1 n k k d Disc (2 ) i This expression exhibits the exchange of a photon, where log q is the time parametrizing the photon worldline. The requirement of factorization xes the normalization of the op erators . a 4 Wenow turn to the analysis for the D-branes, which is rather similar. The Dirichlet N 0 p p-brane is de ned by imp osing Neumann b oundary conditions on X = X :::X , and D Dirichlet b oundary conditions on the remaining co ordinates, X . The lightest states (excluding the tachyon) are the translational zero mo des of the D-brane: D i! X 0 = N@ X e i = p +1;:::;d 1 : (2.4) i n i where X is target time, @ is the derivative normal to the b oundary, and N is a normal- 0 n ization constant. The existence of these zero mo des re ects the degeneracy of di erent ~ D-branes con gurations, obtained by rigid translations (k = 0) in the Dirichlet direc- N tions. In order to allow a general p-brane (p> 0) to recoil one needs to compactify the spatial Neumann directions and wrap the p-brane around them. In that case the mass of the p- brane is nite and prop ortional to the spatial Neumann volume. For the sake of clarity we fo cus on the scattering o a 0-brane, where no such compacti cation is necessary. The normalization constant N that app ears in (2.4) is determined by factorization; consider the S-matrix elementinvolving two closed string tachyons whose vertex op erators are: Z D D ~ ~ 2 i! X (z )ik X (z ) 1 0 1 1 1 V = N d z e 1 t 1 Z D D ~ ~ 2 i! X (z )ik X (z ) 2 0 2 2 2 V = N d z e : (2.5) 2 t 2 The factorization on op en string p oles is obtained by letting one of the tachyons approach the b oundary (see Fig. 4). Fig. 4: Factorization on the Disc. In the factorization one recognizes the various op en string p oles, in particular the 1 2 2 tachyon at ! = , and the zero mo de at ! = 0. This yields the normalization factor 2 5 1 for the zero mo de: p g : (2.6) N = 4 Since these states don't carry Dirichlet momenta, they are discrete quantum mechan- ical states. We are now ready to extract the violations of conformal invariance for S-matrix ele- ments calculated on the annulus. As mentioned ab ove, these divergences app ear in the limit of mo duli space describ ed in Fig.