21,015 research outputs found
Chiral order and fluctuations in multi-flavour QCD
Multi-flavour (N_f>=3) Chiral Perturbation Theory (ChPT) may exhibit
instabilities due to vacuum fluctuations of sea q-bar q pairs. Keeping the
fluctuations small would require a very precise fine-tuning of the low-energy
constants L_4 and L_6 to L_4[crit](M_rho) = - 0.51 * 10^(-3), and
L_6[crit](M_rho) = - 0.26 * 10^(-3). A small deviation from these critical
values -- like the one suggested by the phenomenology of OZI-rule violation in
the scalar channel -- is amplified by huge numerical factors inducing large
effects of vacuum fluctuations. This would lead in particular to a strong
N_f-dependence of chiral symmetry breaking and a suppression of multi-flavour
chiral order parameters. A simple resummation is shown to cure the instability
of N_f>=3 ChPT, but it modifies the standard expressions of some O(p^2) and
O(p^4) low-energy parameters in terms of observables. On the other hand, for
r=m_s/m > 15, the two-flavour condensate is not suppressed, due to the
contribution induced by massive vacuum s-bar s pairs. Thanks to the latter, the
standard two-flavour ChPT is protected from multi-flavour instabilities and
could provide a well-defined expansion scheme in powers of non-strange quark
masses.Comment: Published versio
Analysis and interpretation of new low-energy Pi-Pi scattering data
The recently published E865 data on charged K_e4 decays and Pi-Pi phases are
reanalyzed to extract values of the two S-wave scattering lengths, of the
subthreshold parameters alpha and beta, of the low-energy constants l3-bar and
l4-bar as well as of the main two-flavour order parameters: and F_pi
in the limit m_u = m_d = 0 taken at the physical value of the strange quark
mass. Our analysis is exclusively based on direct experimental information on
Pi-Pi phases below 800 MeV and on the new solutions of the Roy equations by
Ananthanarayan et al. The result is compared with the theoretical prediction
relating 2 a_0^0 - 5 a_0^2 and the scalar radius of the pion, which was
obtained in two-loop Chiral Perturbation Theory. A discrepancy at the 1-sigma
level is found and commented upon.Comment: Published version, to appear in Eur. Phys. J.
Capacitance of Gated GaAs/AlGaAs Heterostructures Subject to In-plane Magnetic Fields
A detailed analysis of the capacitance of gated GaAs/AlGaAs heterostructures
is presented. The nonlinear dependence of the capacitance on the gate voltage
and in-plane magnetic field is discussed together with the capacitance quantum
steps connected with a population of higher 2D gas subbands. The results of
full self-consistent numerical calculations are compared to recent experimental
data.Comment: 4 pages, Revtex. 4 PostScript figures in an uuencoded compressed file
available upon request. Phys. Rev.B, in pres
Electronic dummy for acoustical testing
Electronic Dummy /ED/ used for acoustical testing represents the average male torso from the Xiphoid process upward and includes an acoustic replica of the human head. This head simulates natural flesh, and has an artificial voice and artificial ears that measure sound pressures at the eardrum or the entrance to the ear canal
IR Kuiper Belt Constraints
We compute the temperature and IR signal of particles of radius and
albedo at heliocentric distance , taking into account the
emissivity effect, and give an interpolating formula for the result. We compare
with analyses of COBE DIRBE data by others (including recent detection of the
cosmic IR background) for various values of heliocentric distance, ,
particle radius, , and particle albedo, . We then apply these
results to a recently-developed picture of the Kuiper belt as a two-sector disk
with a nearby, low-density sector (40<R<50-90 AU) and a more distant sector
with a higher density. We consider the case in which passage through a
molecular cloud essentially cleans the Solar System of dust. We apply a simple
model of dust production by comet collisions and removal by the
Poynting-Robertson effect to find limits on total and dust masses in the near
and far sectors as a function of time since such a passage. Finally we compare
Kuiper belt IR spectra for various parameter values.Comment: 34 pages, LaTeX, uses aasms4.sty, 11 PostScript figures not embedded.
A number of substantive comments by a particularly thoughtful referee have
been addresse
Decoherence and Recoherence in Model Quantum Systems
We discuss the various manifestations of quantum decoherence in the forms of
dephasing, entanglement with the environment, and revelation of "which-path"
information. As a specific example, we consider an electron interference
experiment. The coupling of the coherent electrons to the quantized
electromagnetic field illustrates all of these versions of decoherence. This
decoherence has two equivalent interpretations, in terms of photon emission or
in terms of Aharonov-Bohm phase fluctuations. We consider the case when the
coherent electrons are coupled to photons in a squeezed vacuum state. The
time-averaged result is increased decoherence. However, if only electrons which
are emitted during selected periods are counted, the decoherence can be
suppressed below the level for the photon vacuum. This is the phenomenon of
recoherence. This effect is closely related to the quantum violations of the
weak energy condition, and is restricted by similar inequalities. We give some
estimates of the magnitude of the recoherence effect and discuss prospects for
observing it in an electron interferometry experiment.Comment: 8 pages, 3 figures, talk presented at the 7th Friedmann Seminar, Joao
Pessoa, Brazil, July 200
On the Absence of Continuous Symmetries for Noncommutative 3-Spheres
A large class of noncommutative spherical manifolds was obtained recently
from cohomology considerations. A one-parameter family of twisted 3-spheres was
discovered by Connes and Landi, and later generalized to a three-parameter
family by Connes and Dubois-Violette. The spheres of Connes and Landi were
shown to be homogeneous spaces for certain compact quantum groups. Here we
investigate whether or not this property can be extended to the noncommutative
three-spheres of Connes and Dubois-Violette. Upon restricting to quantum groups
which are continuous deformations of Spin(4) and SO(4) with standard
co-actions, our results suggest that this is not the case.Comment: 15 pages, no figure
Cancer therapeutic potential of combinatorial immuno- and vaso-modulatory interventions
Currently, most of the basic mechanisms governing tumor-immune system
interactions, in combination with modulations of tumor-associated vasculature,
are far from being completely understood. Here, we propose a mathematical model
of vascularized tumor growth, where the main novelty is the modeling of the
interplay between functional tumor vasculature and effector cell recruitment
dynamics. Parameters are calibrated on the basis of different in vivo
immunocompromised Rag1-/- and wild-type (WT) BALB/c murine tumor growth
experiments. The model analysis supports that tumor vasculature normalization
can be a plausible and effective strategy to treat cancer when combined with
appropriate immuno-stimulations. We find that improved levels of functional
tumor vasculature, potentially mediated by normalization or stress alleviation
strategies, can provide beneficial outcomes in terms of tumor burden reduction
and growth control. Normalization of tumor blood vessels opens a therapeutic
window of opportunity to augment the antitumor immune responses, as well as to
reduce the intratumoral immunosuppression and induced-hypoxia due to vascular
abnormalities. The potential success of normalizing tumor-associated
vasculature closely depends on the effector cell recruitment dynamics and tumor
sizes. Furthermore, an arbitrary increase of initial effector cell
concentration does not necessarily imply a better tumor control. We evidence
the existence of an optimal concentration range of effector cells for tumor
shrinkage. Based on these findings, we suggest a theory-driven therapeutic
proposal that optimally combines immuno- and vaso-modulatory interventions
The class II MHC protein HLA-DR1 in complex with an endogenous peptide: implications for the structural basis of the specificity of peptide binding
AbstractBackground: Class II major histocompatibility complex (MHC) proteins are cell surface glycoproteins that bind peptides and present them to T cells as part of the mechanism for detecting and responding to foreign material in the body. The peptide-binding activity exhibits allele-specific preferences for particular sidechains at some positions, although the structural basis of these preferences is not understood in detail. We have determined the 2.45 Ă
crystal structure of the human class II MHC protein HLA-DR1 in complex with the tight binding endogenous peptide A2(103â117) in order to discover peptideâMHC interactions that are important in determining the binding motif and to investigate conformational constraints on the bound peptide.Results: The bound peptide adopts a polyproline II-like conformation and places several sidechains within pockets in the binding site. Bound water molecules mediate MHCâpeptide contacts at several sites. A tryptophan residue from the ÎČ2 âlowerâ domain of HLA-DR1 was found to project into a pocket underneath the peptide-binding domain and may be important in modulating interdomain interactions in MHC proteins.Conclusions: The peptide-binding motif of HLA-DR1 includes an aromatic residue at position +1, an arginine residue at position +2, and a small residue at position +6 (where the numbering refers to the normal MHC class II convention); these preferences can be understood in light of interactions observed in the peptideâMHC complex. Comparison of the structure with that of another MHCâpeptide complex shows that completely different peptide sequences bind in essentially the same conformation and are accommodated with only minimal rearrangement of HLA-DR1 residues. Small conformational differences that are observed appear to be important in interactions with other proteins
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