6,902 research outputs found
Particle number fluctuations in nuclear collisions within excluded volume hadron gas model
The multiplicity fluctuations are studied in the van der Waals excluded
volume hadron-resonance gas model. The calculations are done in the grand
canonical ensemble within the Boltzmann statistics approximation. The scaled
variances for positive, negative and all charged hadrons are calculated along
the chemical freeze-out line of nucleus-nucleus collisions at different
collision energies. The multiplicity fluctuations are found to be suppressed in
the van der Waals gas. The numerical calculations are presented for two values
of hard-core hadron radius, fm and 0.5 fm, as well as for the upper
limit of the excluded volume suppression effects.Comment: 19 pages, 4 figure
Black Hole Feedback On The First Galaxies
We study how the first galaxies were assembled under feedback from the accretion onto a central black hole (BH) that is left behind by the first generation of metal-free stars through self-consistent, cosmological simulations. X-ray radiation from the accretion of gas onto BH remnants of Population III (Pop III) stars, or from high-mass X-ray binaries (HMXBs), again involving Pop III stars, influences the mode of second generation star formation. We track the evolution of the black hole accretion rate and the associated X-ray feedback starting with the death of the Pop III progenitor star inside a minihalo and following the subsequent evolution of the black hole as the minihalo grows to become an atomically cooling galaxy. We find that X-ray photoionization heating from a stellar-mass BH is able to quench further star formation in the host halo at all times before the halo enters the atomic cooling phase. X-ray radiation from a HMXB, assuming a luminosity close to the Eddington value, exerts an even stronger, and more diverse, feedback on star formation. It photoheats the gas inside the host halo, but also promotes the formation of molecular hydrogen and cooling of gas in the intergalactic medium and in nearby minihalos, leading to a net increase in the number of stars formed at early times. Our simulations further show that the radiative feedback from the first BHs may strongly suppress early BH growth, thus constraining models for the formation of supermassive BHs.Astronom
Ramond-Ramond Cohomology and O(D,D) T-duality
In the name of supersymmetric double field theory, superstring effective
actions can be reformulated into simple forms. They feature a pair of vielbeins
corresponding to the same spacetime metric, and hence enjoy double local
Lorentz symmetries. In a manifestly covariant manner --with regard to O(D,D)
T-duality, diffeomorphism, B-field gauge symmetry and the pair of local Lorentz
symmetries-- we incorporate R-R potentials into double field theory. We take
them as a single object which is in a bi-fundamental spinorial representation
of the double Lorentz groups. We identify cohomological structure relevant to
the field strength. A priori, the R-R sector as well as all the fermions are
O(D,D) singlet. Yet, gauge fixing the two vielbeins equal to each other
modifies the O(D,D) transformation rule to call for a compensating local
Lorentz rotation, such that the R-R potential may turn into an O(D,D) spinor
and T-duality can flip the chirality exchanging type IIA and IIB
supergravities.Comment: 1+37 pages, no figure; Structure reorganized, References added, To
appear in JHEP. cf. Gong Show of Strings 2012
(http://wwwth.mpp.mpg.de/members/strings/strings2012/strings_files/program/Talks/Thursday/Gongshow/Lee.pdf
Particle Number Fluctuations in the Microcanonical Ensemble
Particle number fluctuations are studied in the microcanonical ensemble. For
the Boltzmann statistics we deduce exact analytical formulae for the
microcanonical partition functions in the case of non-interacting massless
neutral particles and charged particles with zero net charge. The particle
number fluctuations are calculated and we find that in the microcanonical
ensemble they are suppressed in comparison to the fluctuations in the canonical
and grand canonical ensembles. This remains valid in the thermodynamic limit
too, so that the well-known equivalence of all statistical ensembles refers to
average quantities, but does not apply to fluctuations. In the thermodynamic
limit we are able to calculate the particle number fluctuations in the system
of massive bosons and fermions when the exact conservation laws of both the
energy and charge are taken into account.Comment: REVTeX, 17 pages, 9 figures, v3: misprints a correcte
A Double Sigma Model for Double Field Theory
We define a sigma model with doubled target space and calculate its
background field equations. These coincide with generalised metric equation of
motion of double field theory, thus the double field theory is the effective
field theory for the sigma model.Comment: 26 pages, v1: 37 pages, v2: references added, v3: updated to match
published version - background and detail of calculations substantially
condensed, motivation expanded, refs added, results unchange
The local symmetries of M-theory and their formulation in generalised geometry
In the doubled field theory approach to string theory, the T-duality group is
promoted to a manifest symmetry at the expense of replacing ordinary Riemannian
geometry with generalised geometry on a doubled space. The local symmetries are
then given by a generalised Lie derivative and its associated algebra. This
paper constructs an analogous structure for M-theory. A crucial by-product of
this is the derivation of the physical section condition for M-theory
formulated in an extended space.Comment: 20 pages, v2: Author Name corrected, v3: typos correcte
Quantum and frustration effects on fluctuations of the inverse compressibility in two-dimensional Coulomb glasses
We consider interacting electrons in a two-dimensional quantum Coulomb glass
and investigate by means of the Hartree-Fock approximation the combined effects
of the electron-electron interaction and the transverse magnetic field on
fluctuations of the inverse compressibility. Preceding systematic study of the
system in the absence of the magnetic field identifies the source of the
fluctuations, interplay of disorder and interaction, and effects of hopping.
Revealed in sufficiently clean samples with strong interactions is an unusual
right-biased distribution of the inverse compressibility, which is neither of
the Gaussian nor of the Wigner-Dyson type. While in most cases weak magnetic
fields tend to suppress fluctuations, in relatively clean samples with weak
interactions fluctuations are found to grow with the magnetic field. This is
attributed to the localization properties of the electron states, which may be
measured by the participation ratio and the inverse participation number. It is
also observed that at the frustration where the Fermi level is degenerate,
localization or modulation of electrons is enhanced, raising fluctuations.
Strong frustration in general suppresses effects of the interaction on the
inverse compressibility and on the configuration of electrons.Comment: 15 pages, 18 figures, To appear in Phys. Rev.
Multiplicity Fluctuations in Hadron-Resonance Gas
The charged hadron multiplicity fluctuations are considered in the canonical
ensemble. The microscopic correlator method is extended to include three
conserved charges: baryon number, electric charge and strangeness. The
analytical formulae are presented that allow to include resonance decay
contributions to correlations and fluctuations. We make the predictions for the
scaled variances of negative, positive and all charged hadrons in the most
central Pb+Pb (Au+Au) collisions for different collision energies from SIS and
AGS to SPS and RHIC.Comment: 19 pages, 4 figure
Multiplicity Fluctuations in the Pion-Fireball Gas
The pion number fluctuations are considered in the system of pions and large
mass fireballs decaying finally into pions. A formulation which gives an
extension of the model of independent sources is suggested. The grand canonical
and micro-canonical ensemble formulations of the pion-fireball gas are
considered as particular examples.Comment: 13 pages, 4 figure
Duality Invariant M-theory: Gauged supergravities and Scherk-Schwarz reductions
We consider the reduction of the duality invariant approach to M-theory by a
U-duality group valued Scherk-Schwarz twist. The result is to produce
potentials for gauged supergravities that are normally associated with
non-geometric compactifications. The local symmetry reduces to gauge
transformations with the gaugings exactly matching those of the embedding
tensor approach to gauged supergravity. Importantly, this approach now includes
a nontrivial dependence of the fields on the extra coordinates of the extended
space.Comment: 22 pages Latex; v2: typos corrected and references adde
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