Jokela, Niko

Helsingin yliopiston digitaalinen arkisto

Faculty of Science Department of Physical SciencesNiko JokelaStable and Unstable D-branesTheoretical PhysicsMaster’s Thesis November 2004 74String Theory, D-brane, TachyonKumpula Science Library, University of HelsinkiIn string theory there are also higher dimensional objects, such as Dirichlet p-branes, where pcounts the spatial dimensions. A D-brane is defined as a hypersurface where an open string canend. Type IIA (IIB) string theory admits Dp-branes for even (odd) p. They are charged non-perturbative objects and play a key role in various string dualities. Type IIA (IIB) containsalso Dp-branes for odd (even) p which are uncharged and expected to be unstable againstdecay to the vacuum. In addition, a system of D-brane anti-D-brane pair and any dimensionalbosonic D-brane are uncharged unstable configurations.In this thesis we start by studying basic properties of stable BPS Dp-branes in Type II stringtheories. D-branes are found to be dynamical rather than rigid objects, when the open stringexcitations are taken into account. It has been proposed that the low-energy dynamical behav-ior of D-branes is correctly described by the fully supersymmetric Dirac-Born-Infeld action.D-branes are heavy objects, hence they curve spacetime around them. In this thesis we alsostudy the corresponding supergravity solutions.Having learned some essential properties of stable D-branes, we proceed to unstable configu-rations. The instability is manifested as a state with m2 < 0 in the spectrum of open strings,a tachyon. An unstable D-brane configuration is supposed to decay as the tachyon rolls downfrom the maximum of its potential, the unstable vacuum, to the minimum, the stable vacu-um. This process is called the tachyon condensation. We review Sen’s approach to tachyoncondensation and outline the static properties of unstable D-brane configurations.Tachyon condensation as a time-dependent process has attracted attention only recently. SinceD-branes act as sources for various closed string fields, a time-dependent open string fieldconfiguration such as the rolling tachyon solution acts as a time-dependent source for closedstring fields, and produces closed string radiation. We review the tree level computation byusing standard techniques of conformal field theory. For unstable bosonic Dp-branes with p ≤ 2we observe that the total energy carried by the closed string radiation is infinite. However sincethe initial Dp-brane has finite energy it is appropriate to regulate this divergence by puttingan upper cutoff on the energy of the emitted closed string. A natural choice of this cutoff isthe initial energy of the D-brane. In that case all the energy of the D-brane is radiated awayinto closed strings.Tiedekunta/Osasto — Fakultet/Sektion — Faculty Laitos — Institution — DepartmentTekija¨ — Fo¨rfattare — AuthorTyo¨n nimi — Arbetets titel — TitleOppiaine — La¨roa¨mne — SubjectTyo¨n laji — Arbetets art — Level Aika — Datum — Month and year Sivuma¨a¨ra¨ — Sidoantal — Number of pagesTiivistelma¨ — Referat — AbstractAvainsanat — Nyckelord — KeywordsSa¨ilytyspaikka — Fo¨rvaringssta¨lle — Where depositedMuita tietoja — O¨vriga uppgifter — Additional informationHELSINGIN YLIOPISTO — HELSINGFORS UNIVERSITET — UNIVERSITY OF HELSINKI

Stable and Unstable D-branes

Niko Jokela

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http://ethesis.helsinki.fi/julkaisut/mat/fysik/pg/jokela/stablean.pdf

Stable and Unstable D-branes

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Helsingin yliopiston digitaalinen arkisto

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