594 research outputs found
On D-branes in the Nappi-Witten and GMM gauged WZW models
We construct D-branes in the Nappi-Witten (NW) and
Guadagnini-Martellini-Mintchev (GMM) gauged WZW models. For the NW and GMM models we present
the explicit equations describing the D-brane hypersurfaces in their target
spaces. In the latter case we show that the D-branes are classified according
to the Cardy theorem. We also present the semiclassical mass computation and
find its agreement with the CFT predictions.Comment: 16 pages, harvma
Pion Structure Function in the Nambu and Jona-Lasinio model
The pion structure function is studied in the Nambu and Jona-Lasinio (NJL)
model. We calculate the forward scattering amplitude of a virtual photon from a
pion target in the Bjorken limit, and obtain valence quark distributions of the
pion at the low energy hadronic scale, where the NJL model is supposed to work.
The calculated distribution functions are evolved to the experimental momentum
scale using the Altarelli-Parisi equation. The resulting distributions are in a
reasonable agreement with experiment. We calculate also the kaon structure
function and compare the ratio of kaon to pion valence u-quark distributions
with experiment.Comment: 15 pages with 5 figures as uuencoded postscript files, TMU-NT-930301
(plain LaTeX
Space-time Structure of Initial Parton Production in Ultrarelativistic Heavy Ion Collisions
The space and time evolution of initial parton production in
ultrarelativistic heavy ion collisions is investigated within the framework of
perturbative QCD which includes both initial and final state radiations.
Uncertainty principle is used to relate the life time of a radiating parton to
its virtuality and momentum. The interaction time of each hard or semihard
parton scattering is also taken into account. For central collisions at
GeV, most of the partons are found to be produced within 0.5
fm/c after the total overlap of the two colliding nuclei. The local momentum
distribution is approximately isotropical at that time. The implication on how
to treat correctly the the secondary scattering in an ultimate parton cascading
model is also discussed.Comment: 19 pages in REVTEX with 12 figures in separate uuencoded postscript
files, LBL-3415
The effects of meson mixing on dilepton spectra
The effect of scalar and vector meson mixing on the dilepton radiation from
hot and dense hadronic matter is estimated in different isospin channels. In
particular, we study the effect of - and mixing and
calculate the corresponding rates. Effects are found to be significant compared
to standard - and - annihilations. While the mixing in
the isoscalar channel mostly gives a contribution in the invariant mass range
between the two-pion threshold and the peak, the isovector channel
mixing induces an additional peak just below that of the .
Experimentally, the dilepton signals from - mixing seem to be more
tractable than those from - mixing.Comment: 10 pages, 9 figure
Characterising competitive equilibrium in terms of opportunity
This paper analyses alternative profiles of opportunity sets for individuals in an exchange economy, without assuming that individuals’ choices reveal coherent preferences. It introduces the concept of a ‘market-clearing single-price regime’, representing a profile of opportunity sets consistent with competitive equilibrium. It also proposes an opportunity-based normative criterion, the Strong Opportunity Criterion, which is analogous with the core in preference-based analysis. It shows that every market-clearing single-price regime satisfies the Strong Opportunity Criterion and that, in the limit as an economy is replicated, only such regimes have this property
Loop Operators and the Kondo Problem
We analyse the renormalisation group flow for D-branes in WZW models from the
point of view of the boundary states. To this end we consider loop operators
that perturb the boundary states away from their ultraviolet fixed points, and
show how to regularise and renormalise them consistently with the global
symmetries of the problem. We pay particular attention to the chiral operators
that only depend on left-moving currents, and which are attractors of the
renormalisation group flow. We check (to lowest non-trivial order in the
coupling constant) that at their stable infrared fixed points these operators
measure quantum monodromies, in agreement with previous semiclassical studies.
Our results help clarify the general relationship between boundary transfer
matrices and defect lines, which parallels the relation between
(non-commutative) fields on (a stack of) D-branes and their push-forwards to
the target-space bulk.Comment: 22 pages, 2 figure
Generalised permutation branes
We propose a new class of non-factorising D-branes in the product group GxG
where the fluxes and metrics on the two factors do not necessarily coincide.
They generalise the maximally symmetric permutation branes which are known to
exist when the fluxes agree, but break the symmetry down to the diagonal
current algebra in the generic case. Evidence for the existence of these branes
comes from a Lagrangian description for the open string world-sheet and from
effective Dirac-Born-Infeld theory. We state the geometry, gauge fields and, in
the case of SU(2)xSU(2), tensions and partial results on the open string
spectrum. In the latter case the generalised permutation branes provide a
natural and complete explanation for the charges predicted by K-theory
including their torsion.Comment: 33 pages, 6 figures, v2: Extended discussion of K-theory
interpretation of our branes for products of higher rank groups in the
conclusions; v3: Correction of formula (35) and adjustment of the discussion
below equation (45) (no change of result). Footnote 9 points out a previously
unnoticed subtlety and provides a reference to a more detailed discussio
Signatures of Thermal Dilepton Radiation at RHIC
The properties of thermal dilepton production from heavy-ion collisions in
the RHIC energy regime are evaluated for invariant masses ranging from 0.5 to 3
GeV. Using an expanding thermal fireball to model the evolution through both
quark-gluon and hadronic phases various features of the spectra are addressed.
In the low-mass region, due to an expected large background, the focus is on
possible medium modifications of the narrow resonance structures from
and mesons, whereas in the intermediate-mass region the old idea of
identifying QGP radiation is reiterated including effects of chemical
under-saturation in the early stages of central Au+Au collisions.Comment: 17 pages ReVTeX including 16 figure
Relativistic Heavy-Ion Physics: Experimental Overview
The field of relativistic heavy-ion physics is reviewed with emphasis on new
results and highlights from the first run of the Relativistic Heavy-Ion
Collider at BNL and the 15 year research programme at the SPS at CERN and the
AGS at BNL.Comment: 15 pages, 9 figures. Invited Talk at the Fourth International
Conference on Physics and Astrophysics of Quark Gluon Plasma, (ICPAQGP-2001)
Jaipur, India, November 26-30, 200
Geometric K-Homology of Flat D-Branes
We use the Baum-Douglas construction of K-homology to explicitly describe
various aspects of D-branes in Type II superstring theory in the absence of
background supergravity form fields. We rigorously derive various stability
criteria for states of D-branes and show how standard bound state constructions
are naturally realized directly in terms of topological K-cycles. We formulate
the mechanism of flux stabilization in terms of the K-homology of non-trivial
fibre bundles. Along the way we derive a number of new mathematical results in
topological K-homology of independent interest.Comment: 45 pages; v2: References added; v3: Some substantial revision and
corrections, main results unchanged but presentation improved, references
added; to be published in Communications in Mathematical Physic
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