1,228 research outputs found
Hot String Soup
Above the Hagedorn energy density closed fundamental strings form a long
string phase. The dynamics of weakly interacting long strings is described by a
simple Boltzmann equation which can be solved explicitly for equilibrium
distributions. The average total number of long strings grows logarithmically
with total energy in the microcanonical ensemble. This is consistent with
calculations of the free single string density of states provided the
thermodynamic limit is carefully defined. If the theory contains open strings
the long string phase is suppressed.Comment: 13 pages, no figures, uses LaTex, some errors in equations have been
corrected, NSF-ITP-94-83, UCSBTH-94-3
Photoproduction of the f2(1270) resonance
We have performed a calculation of the ÎłpâĎ+Ďâp reaction, where the two pions have been separated in D-wave producing the f2(1270) resonance. We use elements of the local hidden gauge approach that provides the interaction of vector mesons in which the f2(1270) resonance appears as a Ď-Ď molecular state in L=0 and spin 2. The vector meson dominance, incorporated in the local hidden gauge approach converts a photon into a Ď0 meson and the other meson connects the photon with the proton. The picture is simple and has no free parameters, since the parameters of the theory have been constrained in the previous study of the vector-vector states. In a second step we introduce new elements, not present in the local hidden gauge approach, adapting the Ď propagator to Regge phenomenology and introducing the ĎNN tensor coupling. We find that both the differential cross section as well as the t dependence of the cross section are in good agreement with the experimental results and provide support for the molecular picture of the f2(1270) in the first baryonic reaction where it has been tested
Random Field Models for Relaxor Ferroelectric Behavior
Heat bath Monte Carlo simulations have been used to study a four-state clock
model with a type of random field on simple cubic lattices. The model has the
standard nonrandom two-spin exchange term with coupling energy and a random
field which consists of adding an energy to one of the four spin states,
chosen randomly at each site. This Ashkin-Teller-like model does not separate;
the two random-field Ising model components are coupled. When , the
ground states of the model remain fully aligned. When , a
different type of ground state is found, in which the occupation of two of the
four spin states is close to 50%, and the other two are nearly absent. This
means that one of the Ising components is almost completely ordered, while the
other one has only short-range correlations. A large peak in the structure
factor appears at small for temperatures well above the transition
to long-range order, and the appearance of this peak is associated with slow,
"glassy" dynamics. The phase transition into the state where one Ising
component is long-range ordered appears to be first order, but the latent heat
is very small.Comment: 7 pages + 12 eps figures, to appear in Phys Rev
Thin accretion disc with a corona in a central magnetic field
We study the steady-state structure of an accretion disc with a corona
surrounding a central, rotating, magnetized star. We assume that the
magneto-rotational instability is the dominant mechanism of angular momentum
transport inside the disc and is responsible for producing magnetic tubes above
the disc. In our model, a fraction of the dissipated energy inside the disc is
transported to the corona via these magnetic tubes. This energy exchange from
the disc to the corona which depends on the disc physical properties is
modified because of the magnetic interaction between the stellar magnetic field
and the accretion disc. According to our fully analytical solutions for such a
system, the existence of a corona not only increases the surface density but
reduces the temperature of the accretion disc. Also, the presence of a corona
enhances the ratio of gas pressure to the total pressure. Our solutions show
that when the strength of the magnetic field of the central neutron star is
large or the star is rotating fast enough, profiles of the physical variables
of the disc significantly modify due to the existence of a corona.Comment: Accepted for publication in Astrophysics & Space Scienc
Atomic Dark Matter
We propose that dark matter is dominantly comprised of atomic bound states.
We build a simple model and map the parameter space that results in the early
universe formation of hydrogen-like dark atoms. We find that atomic dark matter
has interesting implications for cosmology as well as direct detection:
Protohalo formation can be suppressed below for weak scale dark matter due to Ion-Radiation interactions in the
dark sector. Moreover, weak-scale dark atoms can accommodate hyperfine
splittings of order 100 \kev, consistent with the inelastic dark matter
interpretation of the DAMA data while naturally evading direct detection
bounds.Comment: 17 pages, 3 figure
Utilizing Difluorinated Thiophene Units to Improve the Performance of Polymer Solar Cells
While there are numerous approaches to functionalize conjugated polymers for organic solar cells (OSCs), one widely adopted approach is fluorination. Of the many different locations for fluorination, one of the least studied is the conjugated linker which connects the donor and acceptor moieties; further, all existing reports primarily explore monofluorinated thiophene units. Herein, we synthesize and compare two conjugated polymers, HTAZ and DFT-HTAZ, which have different thiophene linkers. In HTAZ, a bare thiophene unit connects the donor and acceptor moieties, while DFT-HTAZ utilizes difluorinated thiophene (DFT) linkers. These polymers serve as the model system to explore the impact of DFT units in OSCs; additionally, this is the first publication to investigate polymers containing DFT units paired with non-fullerene acceptors. Compared to HTAZ, the incorporation of the DFT units maintained the optical properties while lowering the energy levels by â0.4 eV, which allowed for a much improved Voc value of â1 V. Importantly, when compared with the appropriate non-fullerene acceptor, DFT-HTAZ:ITIC-Th1 blends reached an efficiency of â10%, which is nearly 3à that of the nonfluorinated HTAZ. As most OSC polymers have thiophene linkers, using DFT units could serve as a proficient method to increase OSC performance in many polymer systems, especially those that do not have locations for functionalization on the acceptor moiety
Drum vortons in high density QCD
Recently it was shown that high density QCD supports of number of topological
defects. In particular, there are U(1)_Y strings that arise due to K^0
condensation that occurs when the strange quark mass is relatively large. The
unique feature of these strings is that they possess a nonzero K^+ condensate
that is trapped on the core. In the following we will show that these strings
(with nontrivial core structure) can form closed loops with conserved charge
and currents trapped on the string worldsheet. The presence of conserved
charges allows these topological defects, called vortons, to carry angular
momentum, which makes them classically stable objects. We also give arguments
demonstrating that vortons carry angular momentum very efficiently (in terms of
energy per unit angular momentum) such that they might be the important degrees
of freedom in the cores of neutron stars.Comment: 11 pages, accepted for publication in Physical Review
A radium assay technique using hydrous titanium oxide adsorbent for the Sudbury Neutrino Observatory
As photodisintegration of deuterons mimics the disintegration of deuterons by
neutrinos, the accurate measurement of the radioactivity from thorium and
uranium decay chains in the heavy water in the Sudbury Neutrino Observatory
(SNO) is essential for the determination of the total solar neutrino flux. A
radium assay technique of the required sensitivity is described that uses
hydrous titanium oxide adsorbent on a filtration membrane together with a
beta-alpha delayed coincidence counting system. For a 200 tonne assay the
detection limit for 232Th is a concentration of 3 x 10^(-16) g Th/g water and
for 238U of 3 x 10^(-16) g U/g water. Results of assays of both the heavy and
light water carried out during the first two years of data collection of SNO
are presented.Comment: 12 pages, 4 figure
Mass measurements of neutron-deficient Y, Zr, and Nb isotopes and their impact on rp and νp nucleosynthesis processes
Š 2018 The Authors. Published by Elsevier B.V. This manuscript is made available under the terms of the Creative Commons Attribution-NonCommercial-NoDerivatives 4.0 International licence (CC BY-NC-ND 4.0). For further details please see: https://creativecommons.org/licenses/by-nc-nd/4.0/Using isochronous mass spectrometry at the experimental storage ring CSRe in Lanzhou, the masses of 82Zr and 84Nb were measured for the first time with an uncertainty of âź10 keV, and the masses of 79Y, 81Zr, and 83Nb were re-determined with a higher precision. The latter are significantly less bound than their literature values. Our new and accurate masses remove the irregularities of the mass surface in this region of the nuclear chart. Our results do not support the predicted island of pronounced low Îą separation energies for neutron-deficient Mo and Tc isotopes, making the formation of ZrâNb cycle in the rp-process unlikely. The new proton separation energy of 83Nb was determined to be 490(400) keV smaller than that in the Atomic Mass Evaluation 2012. This partly removes the overproduction of the p-nucleus 84Sr relative to the neutron-deficient molybdenum isotopes in the previous νp-process simulations.Peer reviewe
B_s --> mu+ mu- decay in the R-parity violating minimal supergravity
We study B_s --> mu+ mu- in the context of the R-parity violating minimal
supergravity in the high tan beta regime. We find that the lowest value of the
branching ratio can go well below the present LHCb sensitivity and hence B_s
--> mu+ mu- can even be invisible to the LHC. We also find that the present
upper bound on Br(B_s --> mu+ mu-) puts strong constraint on the minimal
supergravity parameter space. The constraints become more severe if the upper
bound is close to its standard model prediction.Comment: 18 pages, 10 figures; version to be published in European Physical
Journal
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