arXiv subjects

Stephen D. H. Hsu

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

At least 91 records · Page 5Linked to original sources

arXiv2004

Entropy Bounds and Dark Energy

Entropy bounds render quantum corrections to the cosmological constant $Λ$ finite. Under certain assumptions, the natural value of $Λ$ is of order the observed dark energy density $\sim 10^{-10} {\rm eV}^4$, thereby resolving the cosmological constant problem. We note that the dark energy equation of state in these scenarios is $w \equiv p / ρ= 0$ over cosmological distances, and is strongly disfavored by observational data. Alternatively, $Λ$ in these scenarios might account for the diffuse dark matter component of the cosmological energy density.

hep-th↗

arXiv2004

Holography, Entropy and Extra Dimensions

We show that higher dimensional models (brane worlds) in which the scale of quantum gravity $M_*$ is much smaller than the apparent scale $M_P \sim 10^{19}$ GeV are in conflict with bounds arising from holography and black hole entropy. The thermodynamic entropy of astrophysical black holes and sub-horizon volumes during big bang nucleosynthesis exceed the relevant bounds unless $M_* > 10^{(4-6)}$ TeV, so a hierarchy relative to the weak scale is unavoidable. We discuss the implications for extra dimensions as well as holography.

hep-ph↗

arXiv2004

Brane World Confronts Holography

Holography principle imposes a stringent constraint on the scale of quantum gravity $M_*$ in brane-world scenarios, where all matter is confined on the brane. The thermodynamic entropy of astrophysical black holes and sub-horizon volumes during big bang nucleosynthesis exceed the relevant bounds unless $M_* > 10^{(4-6)}$ TeV, so a hierarchy relative to the weak scale is unavoidable. We discuss the implications for extra dimensions as well as holography.

hep-th↗

arXiv2003

Positivity and Fermionic Dense Matter

Euclidean dense matter generically suffers from the fermion sign problem. However, we argue that the sign problem is absent if one considers only low-energy degrees of freedom. Specifically, the low energy effective theory of dense QCD has positive Euclidean path integral measure, which allows one to establish rigorous inequalities showing that the color-flavor locked (CFL) phase is the true vacuum of three flavor, massless QCD. We then describe a method for simulating dense QCD on the lattice. We also discuss applications to electronic systems in condensed matter, such as generalized Hubbard models.

hep-lat↗

arXiv2003

Positivity and Dense Matter

We elaborate on previous results concerning the positivity of the Euclidean path integral measure for low-energy modes in dense fermionic matter. We show that the sign problem usually associated with fermions is absent if one considers only low-energy degrees of freedom. We describe a method for simulating dense QCD on the lattice and give a proof using rigorous inequalities that the color-flavor locked (CFL) phase is the true vacuum of three flavor, massless QCD. We also discuss applications to electronic systems in condensed matter, such as generalized Hubbard models.

hep-ph↗

arXiv2003

Cosmology of Nonlinear Oscillations

The nonlinear oscillations of a scalar field are shown to have cosmological equations of state with $w = p / ρ$ ranging from $-1 < w < 1$. We investigate the possibility that the dark energy is due to such oscillations.

astro-ph↗

arXiv2003

High Temperature Superfluid and Feshbach Resonance

We study an effective field theory describing cold fermionic atoms near a Feshbach resonance. The theory gives a unique description of the dynamics in the limit that the energy of the Feshbach resonance is tuned to be twice that of the Fermi surface. We show that in this limit the zero temperature superfluid condensate is of order the Fermi energy, and obtain a critical temperature $T_C \simeq 0.43 T_F$

cond-mat↗

arXiv2003

Particle Multiplicities and Thermalization in High Energy Collisions

We investigate the conditions under which particle multiplicities in high energy collisions are Boltzmann distributed, as is the case for hadron production in e^+e^-, pp, p-pbar and heavy ion collisions. We show that the apparent temperature governing this distribution does not necessarily imply equilibrium (thermal or chemical) in the usual sense, as we explain. We discuss an explicit example using tree level amplitudes for N photon production in which a Boltzmann-like distribution is obtained without any equilibration. We argue that the failure of statistical techniques based on free particle ensembles may provide a signal for collective phenomena (such as large shifts in masses and widths of resonances) related to the QCD phase transition.

nucl-th↗

arXiv2002

On the Intrinsic Parity of Black Holes

We investigate the intrinsic parity of black holes. It appears that discrete symmetries require the black hole Hilbert space to be larger than suggested by the usual quantum numbers M (mass), Q (charge) and J (angular momentum). Recent results on black hole production in trans-Planckian scattering lead to gravitational effects which do not decouple from low-energy physics. Dispersion relations incorporating these effects imply that the semi-classical black hole spectrum is similar in parity even and odd channels. This result can be generalized to other discrete and continuous symmetries.

gr-qc↗

arXiv2002

Positivity of High Density Effective Theory

We show that the effective field theory of low energy modes in dense QCD has positive Euclidean path integral measure. The complexity of the measure of QCD at finite chemical potential can be ascribed to modes which are irrelevant to the dynamics at sufficiently high density. Rigorous inequalities follow at asymptotic density. Lattice simulation of dense QCD should be possible using the quark determinant calculated in the effective theory.

hep-ph↗

arXiv2002

Quantum Production of Black Holes

We give a path integral expression for the quantum amplitude to produce a black hole from particle collisions. When expanded about an appropriate classical solution it yields the leading order contribution to the production amplitude in a curvature expansion. Classical solutions describing black hole production resulting from two particle scattering at non-zero impact parameter, combined with our formalism, indicate a geometric cross section for the quantum process. In TeV gravity scenarios these solutions may exhibit large curvatures, but (modulo a mild assumption about quantum gravity) corrections to the semi-classical cross section are small.

hep-ph↗

arXiv2001

The QCD Phase Diagram and Explosive Astrophysics

I give a brief overview of our current understanding of the QCD phase diagram at finite temperature and baryon density. I emphasize phase transitions which occur at high density and low temperature (relative to the QCD scale), such as transition(s) to the color superconducting and chirally symmetric phases. These transitions might occur during violent astrophysical activity such as type II supernovae core bounce or neutron star mergers, and I discuss the possible consequences.

hep-ph↗

arXiv2001

Supernovae, Hypernovae and Color Superconductivity

We argue that Color Superconductivity (CSC, Cooper pairing in quark matter leading to the breaking of SU(3) color symmetry) may play a role in triggering the explosive endpoint of stellar evolution in massive stars (M > 8 M_{\odot}). We show that the binding energy release in the transition of a sub-core region to the CSC phase can be of the same order of magnitude as the gravitational binding energy release from core collapse. The core temperature during collapse is likely below the critical temperature for CSC, and the transition is first order, proceeding on Fermi timescales when the pressure reaches a critical value of several times nuclear density. We also discuss the implications for hypernova events with total ejecta energy of 10-100 times that of type II supernova.

hep-ph↗

arXiv2000

Color Superconductivity in High Density Quark Matter

We review recent progress on the phenomena of color superconductivity in high density quark matter. We begin with a brief overview of the unique aspects of physics near a Fermi surface and the implications for renormalization group (RG) techniques. We next discuss the qualitative differences between asymptotic densities, where the effective coupling constant can be made arbitrarily small, and intermediate densities where quark matter is still strongly coupled. It is in the latter regime where RG techniques are particularly useful, in that they yield a generic description of possible behaviors without relying on an expansion in the strong coupling constant. Finally, we discuss aspects of the QCD groundstate at asymptotic densities, which can be determined in a systematic weak coupling expansion.

hep-ph↗

arXiv1999

On the QCD Ground State at High Density

We investigate the possible ground states of QCD at asymptotic densities, where the theory is expected to exhibit color superconductivity. We characterize the color-flavor structure of possible diquark condensates, and find those that are energy extrema by solving the weak-coupling Dyson-Schwinger equations, including Landau damping and the Meissner effect. We show that, as previously anticipated, in the two flavor case the vacuum breaks SU(3) color to SU(2) and in the three flavor case the vacua with color-flavor locking (CFL) have the lowest energy. We identify a number of relatively flat directions in the potential along which the pattern of gauge symmetry breaking changes and parity is violated. We discuss possible phenomenological consequences of our results.

hep-ph↗

arXiv1999

Magnetic Interactions, the Renormalization Group and Color Superconductivity in High Density QCD

We investigate the effect of long range magnetic interactions on the renormalization group (RG) evolution of local Cooper pairing interactions near the Fermi surface in high density QCD. We use an explicit cut-off on momentum modes, with special emphasis on screening effects such as Landau damping, to derive the RG equations in a gauge invariant, weak coupling expansion. We obtain the Landau pole $Δ\sim μg^{-5} \exp(- \frac{3 π^2}{\sqrt{2} g})$, although the structure of our equations differs from previous results. We also investigate the gap equation, including condensates of higher angular momentum. We show that rotational invariance is unbroken at asymptotically high density, and verify that $Δ$ is the correct value of the gap when higher modes are included in the analysis.

hep-ph↗

arXiv1999

On Spherically Symmetric Breathers in Scalar Theories

We develop an algorithm which can be used to exclude the existence of classical breathers (periodic finite energy solutions) in scalar field theories, and apply it to several cases of interest. In particular, the technique is used to show that a pair of potentially periodic solutions of the 3+1 Sine-Gordon Lagrangian, found numerically in earlier work, are not breathers. These ``pseudo-breather states'' do have a signature in our method, which we suggest can be used to find similar quasi-bound state configurations in other theories. We also discuss the results of our algorithm when applied to the 1+1 Sine-Gordon model (which exhibits a well-known set of breathers), and $ϕ^4$ theory.

hep-th↗

arXiv1999

Neutron Star Vortex Dynamics and Magnetic Field Decay: Implications for High Density Nuclear Matter

We investigate the effect of the density-dependent proton and neutron gaps on vortex dynamics in neutron stars. We argue that the persistence of neutron star magnetic fields on timescales of $10^9$ y suggests a superconducting gap curve with local maximum at intermediate density. We discuss the implications for exotic core phenomena such as pion/kaon condensation or a transition to quark matter.

nucl-th↗