2,800 research outputs found
Breakdown of staggered fermions at nonzero chemical potential
The staggered fermion determinant is complex when the quark chemical
potential mu is nonzero. Its fourth root, used in simulations with dynamical
fermions, will have phase ambiguities that become acute when Re mu is
sufficiently large. We show how to resolve these ambiguities, but our
prescription only works very close to the continuum limit. We argue that this
regime is far from current capabilities. Other procedures require being even
closer to the continuum limit, or fail altogether, because of unphysical
discontinuities in the measure. At zero temperature the breakdown is expected
when Re mu is greater than approximately half the pion mass. Estimates of the
location of the breakdown at nonzero temperature are less certain.Comment: 6 pages RevTeX, 2 figures. Returning to v5 after erroneous
replacement. Apologie
Running couplings in equivariantly gauge-fixed SU(N) Yang--Mills theories
In equivariantly gauge-fixed SU(N) Yang--Mills theories, the gauge symmetry
is only partially fixed, leaving a subgroup unfixed. Such
theories avoid Neuberger's nogo theorem if the subgroup contains at least
the Cartan subgroup , and they are thus non-perturbatively well
defined if regulated on a finite lattice. We calculate the one-loop beta
function for the coupling , where is the gauge
coupling and is the gauge parameter, for a class of subgroups including
the cases that or . The
coupling represents the strength of the interaction of the gauge
degrees of freedom associated with the coset . We find that
, like , is asymptotically free. We solve the
renormalization-group equations for the running of the couplings and
, and find that dimensional transmutation takes place also for the
coupling , generating a scale which can be larger
than or equal to the scale associated with the gauge coupling ,
but not smaller. We speculate on the possible implications of these results.Comment: 14 pages, late
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Application of temporal streamflow descriptors in hydrologic model parameter estimation
This paper presents a parameter estimation approach based on hydrograph descriptors that capture dominant streamflow characteristics at three timescales (monthly, yearly, and record extent). The scheme, entitled hydrograph descriptors multitemporal sensitivity analyses (HYDMUS), yields an ensemble of model simulations generated from a reduced parameter space, based on a set of streamflow descriptors that emphasize the timescale dynamics of streamflow record. In this procedure the posterior distributions of model parameters derived at coarser timescales are used to sample model parameters for the next finer timescale. The procedure was used to estimate the parameters of the Sacramento soil moisture accounting model (SAC-SMA) for the Leaf River, Mississippi. The results indicated that in addition to a significant reduction in the range of parameter uncertainty, HYDMUS improved parameter identifiability for all 13 of the model parameters. The performance of the procedure was compared to four previous calibration studies on the same watershed. Although our application of HYDMUS did not explicitly consider the error at each simulation time step during the calibration process, the model performance was, in some important respects, found to be better than in previous deterministic studies. Copyright 2005 by the American Geophysical Union
Observations on staggered fermions at non-zero lattice spacing
We show that the use of the fourth-root trick in lattice QCD with staggered
fermions corresponds to a non-local theory at non-zero lattice spacing, but
argue that the non-local behavior is likely to go away in the continuum limit.
We give examples of this non-local behavior in the free theory, and for the
case of a fixed topologically non-trivial background gauge field. In both
special cases, the non-local behavior indeed disappears in the continuum limit.
Our results invalidate a recent claim that at non-zero lattice spacing an
additive mass renormalization is needed because of taste-symmetry breaking.Comment: 17 pages, two refs. and a note adde
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The role of hydrograph indices in parameter estimation of rainfall-runoff models
A reliable prediction of hydrologic models, among other things, requires a set of plausible parameters that correspond with physiographic properties of the basin. This study proposes a parameter estimation approach, which is based on extracting, through hydrograph diagnoses, information in the form of indices that carry intrinsic properties of a basin. This concept is demonstrated by introducing two indices that describe the shape of a streamflow hydrograph in an integrated manner. Nineteen mid-size (223-4790 km2) perennial headwater basins with a long record of streamflow data were selected to evaluate the ability of these indices to capture basin response characteristics. An examination of the utility of the proposed indices in parameter estimation is conducted for a five-parameter hydrologic model using data from the Leaf River, located in Fort Collins, Mississippi. It is shown that constraining the parameter estimation by selecting only those parameters that result in model output which maintains the indices as found in the historical data can improve the reliability of model predictions. These improvements were manifested in (a) improvement of the prediction of low and high flow, (b) improvement of the overall total biases, and (c) maintenance of the hydrograph's shape for both long-term and short-term predictions. Copyright © 2005 John Wiley & Sons, Ltd
The Tunneling Hybrid Monte-Carlo algorithm
The hermitian Wilson kernel used in the construction of the domain-wall and
overlap Dirac operators has exceptionally small eigenvalues that make it
expensive to reach high-quality chiral symmetry for domain-wall fermions, or
high precision in the case of the overlap operator. An efficient way of
suppressing such eigenmodes consists of including a positive power of the
determinant of the Wilson kernel in the Boltzmann weight, but doing this also
suppresses tunneling between topological sectors. Here we propose a
modification of the Hybrid Monte-Carlo algorithm which aims to restore
tunneling between topological sectors by excluding the lowest eigenmodes of the
Wilson kernel from the molecular-dynamics evolution, and correcting for this at
the accept/reject step. We discuss the implications of this modification for
the acceptance rate.Comment: improved discussion in appendix B, RevTeX, 19 page
Algebraic renormalization of supersymmetric gauge theories with dimensionful parameters
It is usually believed that there are no perturbative anomalies in
supersymmetric gauge theories beyond the well-known chiral anomaly. In this
paper we revisit this issue, because previously given arguments are incomplete.
Specifically, we rule out the existence of soft anomalies, i.e., quantum
violations of supersymmetric Ward identities proportional to a mass parameter
in a classically supersymmetric theory. We do this by combining a previously
proven theorem on the absence of hard anomalies with a spurion analysis, using
the methods of Algebraic Renormalization. We work in the on-shell component
formalism throughout. In order to deal with the nonlinearity of on-shell
supersymmetry transformations, we take the spurions to be dynamical, and show
how they nevertheless can be decoupled.Comment: Final version, typoes fixed. Revtex, 48 page
One loop renormalization for the axial Ward-Takahashi identity in Domain-wall QCD
We calculate one-loop correction to the axial Ward-Takahashi identity given
by Furman and Shamir in domain-wall QCD. It is shown perturbatively that the
renormalized axial Ward-Takahashi identity is satisfied without fine tuning and
the ``conserved'' axial current receives no renormalization, giving .
This fact will simplify the calculation of the pion decay constant in numerical
simulations since the decay constant defined by this current needs no lattice
renormalization factor.Comment: 16 pages, 3 axodraw.sty figure
Pathway redundancy and protein essentiality revealed in the Saccharomyces cerevisiae interaction networks
The biological interpretation of genetic interactions is a major challenge. Recently, Kelley and Ideker proposed a method to analyze together genetic and physical networks, which explains many of the known genetic interactions as linking different pathways in the physical network. Here, we extend this method and devise novel analytic tools for interpreting genetic interactions in a physical context. Applying these tools on a large-scale Saccharomyces cerevisiae data set, our analysis reveals 140 between-pathway models that explain 3765 genetic interactions, roughly doubling those that were previously explained. Model genes tend to have short mRNA half-lives and many phosphorylation sites, suggesting that their stringent regulation is linked to pathway redundancy. We also identify ‘pivot' proteins that have many physical interactions with both pathways in our models, and show that pivots tend to be essential and highly conserved. Our analysis of models and pivots sheds light on the organization of the cellular machinery as well as on the roles of individual proteins
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