422 research outputs found
Mapping Monte Carlo to Langevin dynamics: A Fokker-Planck approach
We propose a general method of using the Fokker-Planck equation (FPE) to link
the Monte-Carlo (MC) and the Langevin micromagnetic schemes. We derive the
drift and disusion FPE terms corresponding to the MC method and show that it is
analytically equivalent to the stochastic Landau-Lifshitz-Gilbert (LLG)
equation of Langevin-based micromagnetics. Subsequent results such as the time
quantification factor for the Metropolis MC method can be rigorously derived
from this mapping equivalence. The validity of the mapping is shown by the
close numerical convergence between the MC method and the LLG equation for the
case of a single magnetic particle as well as interacting arrays of particles.
We also found that our Metropolis MC is accurate for a large range of damping
factors , unlike previous time-quantified MC methods which break down
at low , where precessional motion dominates.Comment: 4 pages, 4 figures. Accepted for publication in Phys. Rev. Let
Treg-Therapy Allows Mixed Chimerism and Transplantation Tolerance Without Cytoreductive Conditioning
Establishment of mixed chimerism through transplantation of allogeneic donor bone marrow (BM) into sufficiently conditioned recipients is an effective experimental approach for the induction of transplantation tolerance. Clinical translation, however, is impeded by the lack of feasible protocols devoid of cytoreductive conditioning (i.e. irradiation and cytotoxic drugs/mAbs). The therapeutic application of regulatory T cells (Tregs) prolongs allograft survival in experimental models, but appears insufficient to induce robust tolerance on its own. We thus investigated whether mixed chimerism and tolerance could be realized without the need for cytoreductive treatment by combining Treg therapy with BM transplantation (BMT). Polyclonal recipient Tregs were cotransplanted with a moderate dose of fully mismatched allogeneic donor BM into recipients conditioned solely with short-course costimulation blockade and rapamycin. This combination treatment led to long-term multilineage chimerism and donor-specific skin graft tolerance. Chimeras also developed humoral and in vitro tolerance. Both deletional and nondeletional mechanisms contributed to maintenance of tolerance. All tested populations of polyclonal Tregs (FoxP3-transduced Tregs, natural Tregs and TGF-β induced Tregs) were effective in this setting. Thus, Treg therapy achieves mixed chimerism and tolerance without cytoreductive recipient treatment, thereby eliminating a major toxic element impeding clinical translation of this approach
Positronium Portal into Hidden Sector: A new Experiment to Search for Mirror Dark Matter
The understanding of the origin of dark matter has great importance for
cosmology and particle physics. Several interesting extensions of the standard
model dealing with solution of this problem motivate the concept of hidden
sectors consisting of SU(3)xSU(2)_LxU(1)_Y singlet fields. Among these models,
the mirror matter model is certainly one of the most interesting. The model
explains the origin of parity violation in weak interactions, it could also
explain the baryon asymmetry of the Universe and provide a natural ground for
the explanation of dark matter. The mirror matter could have a portal to our
world through photon-mirror photon mixing (epsilon). This mixing would lead to
orthopositronium (o-Ps) to mirror orthopositronium oscillations, the
experimental signature of which is the apparently invisible decay of o-Ps. In
this paper, we describe an experiment to search for the decay o-Ps -> invisible
in vacuum by using a pulsed slow positron beam and a massive 4pi BGO crystal
calorimeter. The developed high efficiency positron tagging system, the low
calorimeter energy threshold and high hermiticity allow the expected
sensitivity in mixing strength to be epsilon about 10^-9, which is more than
one order of magnitude below the current Big Bang Nucleosynthesis limit and in
a region of parameter space of great theoretical and phenomenological interest.
The vacuum experiment with such sensitivity is particularly timely in light of
the recent DAMA/LIBRA observations of the annual modulation signal consistent
with a mirror type dark matter interpretation.Comment: 40 pages, 29 Figures 2 Tables v2: Ref. added, Fig. 29 and some text
added to explain idea for backscattering e+ background suppression, corrected
typos v3: minor corrections: Eq 2.1 corrected (6 lines-> 5 lines), Eq.2.17:
two extra "-" signs remove
Do Instantons Like a Colorful Background?
We investigate chiral symmetry breaking and color symmetry breaking in QCD.
The effective potential of the corresponding scalar condensates is discussed in
the presence of non-perturbative contributions from the semiclassical
one-instanton sector. We concentrate on a color singlet scalar background which
can describe chiral condensation, as well as a color octet scalar background
which can generate mass for the gluons. Whereas a non-vanishing singlet chiral
field is favored by the instantons, we have found no indication for a
preference of color octet backgrounds.Comment: 25 pages, 7 figure
Removing the Big Bang Singularity: The role of the generalized uncertainty principle in quantum gravity
The possibility of avoiding the big bang singularity by means of a
generalized uncertainty principle is investigated. In relation with this
matter, the statistical mechanics of a free-particle system obeying the
generalized uncertainty principle is studied and it is shown that the entropy
of the system has a finite value in the infinite temperature limit. It is then
argued that negative temperatures and negative pressures are possible in this
system. Finally, it is shown that this model can remove the big bang
singularity.Comment: 8 pages, Accepted for publication in Astrophysics & Space Scienc
The generalized second law of thermodynamics in generalized gravity theories
We investigate the generalized second law of thermodynamics (GSL) in
generalized theories of gravity. We examine the total entropy evolution with
time including the horizon entropy, the non-equilibrium entropy production, and
the entropy of all matter, field and energy components. We derive a universal
condition to protect the generalized second law and study its validity in
different gravity theories. In Einstein gravity, (even in the phantom-dominated
universe with a Schwarzschild black hole), Lovelock gravity, and braneworld
gravity, we show that the condition to keep the GSL can always be satisfied. In
gravity and scalar-tensor gravity, the condition to protect the GSL can
also hold because the gravity is always attractive and the effective Newton
constant should be approximate constant satisfying the experimental bounds.Comment: 19 pages, no figure, mistakes corrected, references added, to appear
in Class. Quantum Gra
A High Spectral Resolution Study of the Soft X-ray Background with the X-ray Quantum Calorimeter
We present here a combined analysis of four high spectral resolution
observations of the Diffuse X-ray Background (DXRB), made using the University
of Wisconsin-Madison/Goddard Space Flight Center X-ray Quantum Calorimeter
(XQC) sounding rocket payload. The observed spectra support the existence of a
keV Local Hot Bubble and a keV Hot Halo, with discrepancies
between repeated observations compatible with expected contributions of
time-variable emission from Solar Wind Charge Exchange (SWCX). An additional
component of keV emission observed only at low galactic latitudes can
be consistently explained by unresolved dM stars.Comment: 21 pages, 6 figures, accepted for publication in Ap
External Fields as a Probe for Fundamental Physics
Quantum vacuum experiments are becoming a flexible tool for investigating
fundamental physics. They are particularly powerful for searching for new light
but weakly interacting degrees of freedom and are thus complementary to
accelerator-driven experiments. I review recent developments in this field,
focusing on optical experiments in strong electromagnetic fields. In order to
characterize potential optical signatures, I discuss various low-energy
effective actions which parameterize the interaction of particle-physics
candidates with optical photons and external electromagnetic fields.
Experiments with an electromagnetized quantum vacuum and optical probes do not
only have the potential to collect evidence for new physics, but
special-purpose setups can also distinguish between different particle-physics
scenarios and extract information about underlying microscopic properties.Comment: 12 pages, plenary talk at QFEXT07, Leipzig, September 200
Hamiltonian walks on Sierpinski and n-simplex fractals
We study Hamiltonian walks (HWs) on Sierpinski and --simplex fractals. Via
numerical analysis of exact recursion relations for the number of HWs we
calculate the connectivity constant and find the asymptotic behaviour
of the number of HWs. Depending on whether or not the polymer collapse
transition is possible on a studied lattice, different scaling relations for
the number of HWs are obtained. These relations are in general different from
the well-known form characteristic of homogeneous lattices which has thus far
been assumed to hold for fractal lattices too.Comment: 22 pages, 6 figures; final versio
Chiral phase boundary of QCD at finite temperature
We analyze the approach to chiral symmetry breaking in QCD at finite
temperature, using the functional renormalization group. We compute the running
gauge coupling in QCD for all temperatures and scales within a simple truncated
renormalization flow. At finite temperature, the coupling is governed by a
fixed point of the 3-dimensional theory for scales smaller than the
corresponding temperature. Chiral symmetry breaking is approached if the
running coupling drives the quark sector to criticality. We quantitatively
determine the phase boundary in the plane of temperature and number of flavors
and find good agreement with lattice results. As a generic and testable
prediction, we observe that our underlying IR fixed-point scenario leaves its
imprint in the shape of the phase boundary near the critical flavor number:
here, the scaling of the critical temperature is determined by the
zero-temperature IR critical exponent of the running coupling.Comment: 39 pages, 8 figure
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