27,432 research outputs found
D-brane Bound States and the Generalised ADHM Construction
We discuss the sigma model description of a D-string bound to k D-fivebranes
in type I string theory. The effective theory is an (0,4) supersymmetric
hyper-Kahler with torsion sigma model on the moduli space of Sp(k) instantons
on R^4. Upon toroidal compactification to five dimensions the model is related
to the type II picture where the target space is a symmetric product of K3's.Comment: 17 pages Phyzzx. Minor corrections, to appear in Nucl. Phys.
Non-Abelian Field Theory of stable non-BPS Branes
We derive the action for the non-abelian field theory living on parallel
non-BPS D3-branes in type IIA theory on the orbifold T^4/I_4(-1)^F_L. The
classical moduli space for the massless scalars originating in the ``would be''
tachyonic sector shows an interesting structure. In particular, it contains
non-abelian flat directions. At a generic point in this branch of the moduli
space the scalars corresponding to the the separations of the branes acquire
masses and the branes condense. Although these tree level flat directions are
removed by quantum corrections we argue that within the loop approximation the
branes still condense.Comment: 1 reference adde
Dyonic Instantons in Five Dimensional Gauge Theories
We show that there exist finite energy, non-singular instanton solutions for
five-dimensional theories with broken gauge symmetry. The soliton is supported
against collapse by a non-zero electric charge. The low-energy dynamics of
these solutions is described by motion on the ADHM moduli space with potential.Comment: 9 pages. Latex. Minor typos, including correction to final equatio
Non-Abelian Born-Infeld Action and Solitons for Critical Non-BPS Branes
The non-abelian flat directions in the tachyon potential of stable non-BPS
branes recently found are shown to persist to all orders in alpha' at tree
level in the string coupling. We also obtain the non-abelian Born-Infeld action
including the tachyon potential for a stack of stable non-BPS branes on a
critical orbifold. Finally we discuss stable soliton states on the non-BPS
brane.Comment: 12+1 pages, Latex. Typos, a reference and the discussion of the
monopole solution are correcte
Lepton asymmetry and primordial nucleosynthesis in the era of precision cosmology
We calculate and display the primordial light-element abundances as a
function of a neutrino degeneracy parameter \xi common to all flavors. It is
the only unknown parameter characterizing the thermal medium at the primordial
nucleosynthesis epoch. The observed primordial helium abundance Y_p is the most
sensitive cosmic ``leptometer.'' Adopting the conservative Y_p error analysis
of Olive and Skillman implies -0.04 \alt \xi \alt 0.07 whereas the errors
stated by Izotov and Thuan imply \xi=0.0245+-0.0092 (1 sigma). Improved
determinations of the baryon abundance have no significant impact on this
situation. A determination of Y_p that reliably distinguishes between a
vanishing or nonvanishing \xi is a crucial test of the cosmological standard
assumption that sphaleron effects equilibrate the cosmic lepton and baryon
asymmetries.Comment: 5 pages, 2 figures; minor changes, references added, replaced to
match the published version in PRD (Brief Reports
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