Negative-parity Baryon Masses using an O(a)-improved Fermion Action

We present a calculation of the mass of the lowest-lying negative-parity J=1/2- state in quenched QCD. Results are obtained using a non-perturbatively O(a)-improved clover fermion action, and a splitting is found between the mass of the nucleon and its parity partner. The calculation is performed on two lattice volumes and at three lattice spacings, enabling a study of both finite-volume and finite lattice-spacing uncertainties. A comparison is made with results obtained using the unimproved Wilson fermion action.Comment: 13 pages, 8 figures, revtex Version accepted for publication in Physics Letters B. Some changes to discussion of chiral extrapolations; primary results unchange

Experimental Proposal for Achieving Superadditive Communication Capacities with a Binary Quantum Alphabet

We demonstrate superadditivity in the communication capacity of a binary alphabet consisting of two nonorthogonal quantum states. For this scheme, collective decoding is performed two transmissions at a time. This improves upon the previous schemes of Sasaki et al. [Phys. Rev. A 58, 146 (1998)] where superadditivity was not achieved until a decoding of three or more transmissions at a time. This places superadditivity within the regime of a near-term laboratory demonstration. We propose an experimental test based upon an alphabet of low photon-number coherent states where the signal decoding is done with atomic state measurements on a single atom in a high-finesse optical cavity.Comment: 7 pages, 5 figure

Excited nucleon spectrum using a non-perturbatively improved clover fermion action

We discuss the extraction of negative-parity baryon masses from lattice QCD calculations. The mass of the lowest-lying negative-parity $J = 1/2^{-}$ state is computed in quenched lattice QCD using an ${\cal O}(a)$-improved clover fermion action, and a splitting found with the nucleon mass. The calculation is performed on two lattice volumes, and three lattice spacings enabling a study of both finite-volume and finite-lattice-spacing uncertainties. A measurement of the first excited radial excitation of the nucleon finds a mass considerably larger than that of the negative-parity ground state, in accord with other lattice determinations but in disagreement with experiment. Results are also presented for the lightest negative-parity $I=3/2$ state.Comment: 7 pages, 5 figures, uses espcrc2. Talk presented at Workshop on Lattice Hadron Physics, Colonial Club Resort, Cairns, Australia, July 9-18, 2001. Corrected error in determination of mass of excited, positive-parity nucleon resonanc

Spatial averaging and apparent acceleration in inhomogeneous spaces

As an alternative to dark energy that explains the observed acceleration of the universe, it has been suggested that we may be at the center of an inhomogeneous isotropic universe described by a Lemaitre-Tolman-Bondi (LTB) solution of Einstein's field equations. To test this possibility, it is necessary to solve the null geodesics. In this paper we first give a detailed derivation of a fully analytical set of differential equations for the radial null geodesics as functions of the redshift in LTB models. As an application we use these equaions to show that a positive averaged acceleration $a_D$ obtained in LTB models through spatial averaging can be incompatible with cosmological observations. We provide examples of LTB models with positive $a_D$ which fail to reproduce the observed luminosity distance $D_L(z)$. Since the apparent cosmic acceleration $a^{FLRW}$ is obtained from fitting the observed luminosity distance to a FLRW model we conclude that in general a positive $a_D$ in LTB models does not imply a positive $a^{FLRW}$.Comment: 16 pages, 12 figures. Explicit derivation of the fully analytical null geodesic equations has been added. Published in GR

Pentaquark baryon production from photon-neuteron reactions

Extending the hadronic Lagrangians that we recently introduced for studying pentaquark $\Theta^+$ baryon production from meson-proton, proton-proton, and photon-proton reactions near threshold to include the anomalous interaction between $\gamma$ and $K^*K$, we evaluate the cross section for $\Theta^+$ production from photon-neutron reactions, in which the $\Theta^+$ was first detected in the SPring-8 experiment in Japan and the CLAS experiment at Thomas Jefferson National Laboratory. With empirical coupling constants and form factors, and assuming that the decay width of $\Theta^+$ is 20 MeV, the predicted cross section is found to have a peak value of about 280 nb, which is substantially larger than that for $\Theta^+$ production from photon-proton reactions.Comment: 13 pages, 6 figure

Curvature perturbation in multi-field inflation with non-minimal coupling

In this paper we discuss a multi-field model of inflation in which generally all fields are non-minimally coupled to the Ricci scalar and have non-canonical kinetic terms. The background evolution and first-order perturbations for the model are evaluated in both the Jordan and Einstein frames, and the respective curvature perturbations compared. We confirm that they are indeed not the same - unlike in the single-field case - and also that the difference is a direct consequence of the isocurvature perturbations inherent to multi-field models. This result leads us to conclude that the notion of adiabaticity is not invariant under conformal transformations. Using a two-field example we show that even if in one frame the evolution is adiabatic, meaning that the curvature perturbation is conserved on super-horizon scales, in general in the other frame isocurvature perturbations continue to source the curvature perturbation. We also find that it is possible to realise a particular model in which curvature perturbations in both frames are conserved but with each being of different magnitude. These examples highlight that the curvature perturbation itself, despite being gauge-invariant, does not correspond directly to an observable. The non-equivalence of the two curvature perturbations would also be important when considering the addition of Standard Model matter into the system.Comment: 21 pages, 2 figures, references added, typos corrected, version to appear in JCA

Gravitational radiation from a particle in circular orbit around a black hole. VI. Accuracy of the post-Newtonian expansion

A particle of mass $\mu$ moves on a circular orbit around a nonrotating black hole of mass $M$. Under the assumption $\mu \ll M$ the gravitational waves emitted by such a binary system can be calculated exactly numerically using black-hole perturbation theory. If, further, the particle is slowly moving, then the waves can be calculated approximately analytically, and expressed in the form of a post-Newtonian expansion. We determine the accuracy of this expansion in a quantitative way by calculating the reduction in signal-to-noise ratio incurred when matched filtering the exact signal with a nonoptimal, post-Newtonian filter.Comment: 5 pages, ReVTeX, 1 figure. A typographical error was discovered in the computer code used to generate the results presented in the paper. The corrected results are presented in an Erratum, which also incorporates new results, obtained using the recently improved post-Newtonian calculations of Tanaka, Tagoshi, and Sasak

Ocean acidification boosts reproduction in fish via indirect effects

Ocean acidification affects species populations and biodiversity through direct negative effects on physiology and behaviour. The indirect effects of elevated CO₂ are less well known and can sometimes be counterintuitive. Reproduction lies at the crux of species population replenishment, but we do not know how ocean acidification affects reproduction in the wild. Here, we use natural CO₂ vents at a temperate rocky reef and show that even though ocean acidification acts as a direct stressor, it can indirectly increase energy budgets of fish to stimulate reproduction at no cost to physiological homeostasis. Female fish maintained energy levels by compensation: They reduced activity (foraging and aggression) to increase reproduction. In male fish, increased reproductive investment was linked to increased energy intake as mediated by intensified foraging on more abundant prey. Greater biomass of prey at the vents was linked to greater biomass of algae, as mediated by a fertilisation effect of elevated CO₂ on primary production. Additionally, the abundance and aggression of paternal carers were elevated at the CO₂ vents, which may further boost reproductive success. These positive indirect effects of elevated CO₂ were only observed for the species of fish that was generalistic and competitively dominant, but not for 3 species of subordinate and more specialised fishes. Hence, species that capitalise on future resource enrichment can accelerate their reproduction and increase their populations, thereby altering species communities in a future ocean.Ivan Nagelkerken, Tiphaine Alemany, Julie M. Anquetin, Camilo M. Ferreira, Kim E. Ludwig, Minami Sasaki, Sean D. Connel

Pentaquark baryon production at the Relativistic Heavy Ion Collider

Production of pentaquark $\Theta^+$ baryons in central relativistic heavy ion collisions is studied in a kinetic model. Assuming that a quark-gluon plasma is produced in the collisions, we first determine the number of $\Theta^+$ produced from the quark-gluon plasma using a parton coalescence model, and then take into consideration its production and absorption in subsequent hadronic matter via the reactions $KN\leftrightarrow\Theta$, $KN\leftrightarrow\pi\Theta$, and \piN\leftrightarrow\bar K\Theta. We find that although the final $\Theta^+$ number is affected by hadronic interactions, it remains sensitive to the initial number of $\Theta^+$ produced from the quark-gluon plasma, particularly in the case of a small $\Theta^+$ width as imposed by the $K^+N$ and $K^+d$ scattering data. Because of small baryon chemical potential in the hot dense matter produced in these collisions, the number of produced anti-$\Theta$ is only slightly smaller than that of $\Theta^+$.Comment: 6 pages, 2 figures, revised version, to appear in PL

Simple Dynamics on the Brane

We apply methods of dynamical systems to study the behaviour of the Randall-Sundrum models. We determine evolutionary paths for all possible initial conditions in a 2-dimensional phase space and we investigate the set of accelerated models. The simplicity of our formulation in comparison to some earlier studies is expressed in the following: our dynamical system is a 2-dimensional Hamiltonian system, and what is more advantageous, it is free from the degeneracy of critical points so that the system is structurally stable. The phase plane analysis of Randall-Sundrum models with isotropic Friedmann geometry clearly shows that qualitatively we deal with the same types of evolution as in general relativity, although quantitatively there are important differences.Comment: an improved version, 34 pages, 9 eps figure