arXiv subjects

Stephen D. H. Hsu

Publications and source records attributed to Stephen D. H. Hsu.

At least 109 records · Page 6Linked to original sources

arXiv1998

Pion Breather States in QCD

We describe a class of pionic breather solutions (PBS) which appear in the chiral lagrangian description of low-energy QCD. These configurations are long-lived, with lifetimes greater than $10^3$ fm/c, and could arise as remnants of disoriented chiral condensate (DCC) formation at RHIC. We show that the chiral lagrangian equations of motion for a uniformly isospin-polarized domain reduce to those of the sine-gordon model. Consequently, our solutions are directly related to the breather solutions of sine-gordon theory in 3+1 dimensions. We investigate the possibility of PBS formation from multiple domains of DCC, and show that the probability of formation is non-negligible.

hep-ph↗

arXiv1998

On the QCD Phase Transition at Finite Baryon Density

We investigate the QCD chiral phase transition at finite chemical potential $μ$, using the renormalization group (RG) to characterize the infrared behavior of sigma models constrained by the flavor and spacetime symmetries. The results are similar to those obtained from RG analysis of the finite temperature transition at zero baryon density. When there are more than two massless flavors of quarks, a first order transition is predicted for the entire phase boundary. In the two flavor case, a boundary with first and second order regions separated by a tricritical point seems most likely. We discuss the real-world case with two light quarks and an intermediate mass strange quark. Improved lattice data on the temperature transition will strongly constrain the possibilities for the phase boundary.

hep-ph↗

arXiv1998

Isospin Fluctuations from Multiple Domains of Disoriented Chiral Condensate

We describe some detailed numerical simulations of Disoriented Chiral Condensates (DCCs), using the chiral lagrangian as a controlled long-wavelength description. We focus on the possibility of multiple, independently coherent domains, and investigate the degree to which the DCC signal is attenuated. As an intermediate step in our analysis we compute the expected number of detector events in each isospin and momentum channel for a given asymptotic classical field configuration. We find that for sufficiently large initial field strengths, the non-linear interactions between domains become important and can lead to a randomization of isospin orientations. Nevertheless, we argue that viable signals exist for DCC detection, even in the case of multiple domains and strong domain-domain interactions. We briefly discuss some long-lived `pseudo-bound state' configurations which arise at large field strengths and might be observable in HBT correlations.

hep-ph↗

arXiv1998

Gaugino Determinant in Supersymmetric Yang-Mills Theory

We resolve an ambiguity in the sign of the gaugino determinant in supersymmetric models. The result, that the gaugino determinant can be taken positive for all background gauge configurations, is necessary for application of QCD inequalities and lattice Monte Carlo methods to supersymmetric Yang-Mills models.

hep-th↗

arXiv1998

Zero Energy Configurations in General Relativity

We investigate the ratio of gravitational binding energy to rest mass in general relativity. For N pointlike masses, an upper bound on the magnitude of this ratio can be derived using the second law of black hole dynamics. Only as N approaches infinity can it approach one. A configuration that saturates the $N = \infty$ bound is a thin spherical shell as its radius is taken to zero. This system provides, in principle, a perfectly efficient mechanism for converting rest mass into energy. We mention possible implications for the black hole information problem.

gr-qc↗

arXiv1998

A Note on Supersymmetry Breaking

Using a simple observation based on holomorphy, we argue that any model which spontaneously breaks supersymmetry for some range of a parameter will do so generically for all values of that parameter, modulo some isolated exceptional points. Conversely, a model which preserves supersymmetry for some range of a parameter will also do so everywhere except at isolated exceptional points. We discuss how these observations can be useful in the construction of new models which break supersymmetry and discuss some simple examples. We also comment on the relation of these results to the Witten index.

hep-th↗

arXiv1996

Regularization of Chiral Gauge Theories

We propose a nonperturbative formulation of chiral gauge theories. The method involves a `pre-regulation' of the gauge fields, which may be implemented on a lattice, followed by a computation of the chiral fermion determinant in the form of a functional integral which is regularized in the continuum. Our result for the chiral determinant is expressed in terms of the vector-like Dirac operator and hence can be realized in lattice simulations. We investigate the local and global anomalies within our regularization scheme. We also compare our result for the chiral determinant to previous exact $ζ$-function results. Finally, we use a symmetry property of the chiral determinant to show that the partition function for a chiral gauge theory is real.

hep-th↗

arXiv1995

Exact Results And Soft Breaking Masses In Supersymmetric Gauge Theory

We give an explicit formalism connecting softly broken supersymmetric gauge theories (with QCD as one limit) to $N=2$ and $N=1$ supersymmetric theories possessing exact solutions, using spurion fields to embed these models in an enlarged $N=1$ model. The functional forms of effective Lagrangian terms resulting from soft supersymmetry breaking are constrained by the symmetries of the enlarged model, although not well enough to fully determine the vacuum structure of generic softly broken models. Nevertheless by perturbing the exact $N=1$ model results with sufficiently small soft breaking masses, we show that there exist nonsupersymmetric models that exhibit monopole condensation and confinement in the same modes as the $N=1$ case.

hep-th↗

arXiv1995

Exact Results in Softly Broken Supersymmetric Models

We show how recent exact results in supersymmetric theories can be extended to models which include {\it explicit} soft supersymmetry breaking terms. We thus derive new exact results for non-supersymmetric models.

hep-th↗

arXiv1994

Anomalous Violation of Conservation Laws in Minkowski Space: Spontaneously Broken Gauge Theories

We extend our previous results on the evolution of quantum fermi fields in Minkowski gauge field backgrounds to the case of spontaneously broken gauge theories. We obtain a selection rule which relates the amount of fermion number violation to the change in Higgs winding number of the configuration. This selection rule is applicable to any classical solution which dissipates at both early and late times.

hep-ph↗

arXiv1994

Anomalous Violation of Conservation Laws in Minkowski Space

We consider the evolution of quantum fermi fields in Minkowski gauge field backgrounds. Our motivation is the study of anomalous fermion number violating processes. We derive selection rules for fermion scattering amplitudes which relate the violation of fermionic charge to the change in winding between early and late times of the vacuum part of the gauge field. We find that the amount of fermion number violation is always integer, even when the topological charge of the gauge field is fractional. As an explicit example, we apply our results to spherically symmetric Minkowski solutions.

hep-ph↗

arXiv1994

Quantum Scattering and Classical Solutions

I discuss a formalism for computing quantum scattering amplitudes using a semiclassical expansion of a functional integral representation for the S-matrix. The classical background for the expansion is determined by solving the equations of motion subject to nontrivial boundary conditions determined by the initial and final quantum states. The formalism is designed to accommodate intrinsically nonperturbative processes such as baryon number violation, quantum tunneling and multiparticle scattering. It appears to yield a controlled small coupling expansion even at asymptotically high energies where instanton methods fail. (Contribution to proceedings for Sintra '94 NATO Workshop on Electroweak Physics and the Early Universe)

hep-ph↗

arXiv1994

Quantum Scattering from Classical Field Theory

We show that scattering amplitudes between initial wave packet states and certain coherent final states can be computed in a systematic weak coupling expansion about classical solutions satisfying initial value conditions. The initial value conditions are such as to make the solution of the classical field equations amenable to numerical methods. We propose a practical procedure for computing classical solutions which contribute to high energy two particle scattering amplitudes. We consider in this regard the implications of a recent numerical simulation in classical SU(2) Yang-Mills theory for multiparticle scattering in quantum gauge theories and speculate on its generalization to electroweak theory. The generalization of our results to complex trajectories allows its application to it any wave packet to coherent state transition. Finally, we discuss the relevance of these results to the issues of baryon number violation and multiparticle scattering at high energies.

hep-ph↗

arXiv1993

Spacetime Symmetries and Semiclassical Amplitudes

We examine the spacetime symmetries of forward $2 \rightarrow 2$ scattering. These symmetries have non-trivial consequences for any class of configurations which might dominate the amplitude in the semiclassical approximation. We derive some dynamical results regarding the stability of configurations which arise solely from reflection symmetry and positivity of the (Euclidean) path integral action. We show that in the case of on-shell scattering, any semiclassical configuration must have $O(3)$ symmetric asymptotic behavior. We argue that non--$O(3)$ symmetric configurations are irrelevant to the semiclassical exponent.

hep-ph↗

arXiv1993

Nonperturbative Decoupling and Effective Field Theory

We examine recent claims that nonperturbative effects can prevent the decoupling of a heavy fermion whose mass arises from a Yukawa coupling to a scalar field. We show that in weakly coupled, four dimensional models such as the standard model with heavy mirror fermions the effects of the heavy fermions can always be accounted for by {\it local} operators involving light fields. We contrast this with the case of the 1+1 dimensional Abelian Higgs model, in which there does not appear to be a local effective field theory describing the low energy physics when heavy fermions are integrated out. (Mailer corruption hopefully fixed!)

hep-ph↗

arXiv1993

Resummation Methods at Finite Temperature: The Tadpole Way

We examine several resummation methods for computing higher order corrections to the finite temperature effective potential, in the context of a scalar $ϕ^4$ theory. We show by explicit calculation to four loops that dressing the propagator, not the vertex, of the one-loop tadpole correctly counts ``daisy'' and ``super-daisy'' diagrams.

hep-ph↗

arXiv1993

Complementarity and Chiral Fermions in SU(2) gauge Theories

Complementarity - the absence of a phase boundary separating the Higgs and confinement phases of a gauge theory - can be violated by the addition of chiral fermions. We utilize chiral symmetry violating fermion correlators such as $ \langle \bps ψ\rangle $ as order parameters to investigate this issue. Using inequalities similar to those of Vafa-Witten and Weingarten, we show that SU(2) gauge theories with Higgs and fermion fields in the fundamental representation exhibit chiral symmetry breaking in the confined phase and therefore do {\it not} lead to massless composite fermions. We discuss the implications for the Abbott-Farhi strongly interacting standard model.

hep-ph↗

arXiv1992

A New Anomaly Matching Condition?

We formulate ``Witten'' matching conditions for confining gauge theories. The conditions are analogous to 't Hooft's, but involve Witten's global SU(2) anomaly. Using a group theoretic result of Geng, Marshak, Zhao and Okubo, we show that if the fourth homotopy group of the flavor group $H$ is trivial ($Π_4(H) = 0$) then realizations of massless composite fermions that satisfy the 't Hooft conditions also satisfy the Witten conditions. If $Π_4 (H)$ is nontrivial, the new matching conditions can yield additional information about the low energy spectrum of the theory. We give a simple physical proof of Geng, et. al.'s result.

hep-ph↗