QED and Fundamental Symmetries in Positronium Decays
We discuss positronium decays with emphasis on tests of fundamental symmetries and the constraints from measurements of other precision observables involving electrons and photons.
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arXiv subjects
Steven D. Bass
Publications and source records attributed to Steven D. Bass.
Etaprime and Eta Mesons with Connection to Anomalous Glue
We review the present understanding of etaprime and eta meson physics and these mesons as a probe of gluon dynamics in low-energy QCD. Recent highlights include the production mechanism of eta and etaprime mesons in proton-nucleon collisions from threshold to high-energy, the etaprime effective mass shift in the nuclear medium, searches for possible eta and etaprime bound states in nuclei as well as precision measurements of eta decays as a probe of light-quark masses. We discuss recent experimental data, theoretical interpretation of the different measurements and the open questions and challenges for future investigation.
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Updating spin-dependent Regge intercepts
We use new high statistics data from CLAS and COMPASS on the nucleon's spin structure function at low Bjorken x and low virtuality, Q^2 < 0.5 GeV^2, together with earlier measurements from the SLAC E-143, HERMES and GDH experiments to estimate the effective intercept(s) for spin dependent Regge theory. We find alpha_{a_1} = 0.31 \pm 0.04 for the intercept describing the high-energy behaviour of spin dependent photoabsorption together with a new estimate for the high-energy part of the Gerasimov-Drell-Hearn sum-rule, -15 \pm 2 mu b from photon-proton centre-of-mass energy greater than 2.5 GeV. Our value of alpha_[a_1} suggests QCD physics beyond a simple straight-line a_1 trajectory.
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Etaprime interactions with nucleons and nuclei
We summarise recent progress in theory and experiment towards understanding etaprime meson interactions with nucleons and nuclei. Highlights include the production mechanism of etaprime mesons in proton-proton collisions close to threshold, the etaprime effective mass shift in nuclei and the determination of the etaprime-nucleon scattering length in free space.
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The cosmological constant and stability of the Higgs vacuum
We discuss the cosmological constant puzzle and possible connections to the (meta-)stability of the Higgs vacuum suggested by recent LHC results. A possible explanation involves new critical phenomena in the ultraviolet, close to the Planck scale.
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Vacuum energy and the cosmological constant
The accelerating expansion of the Universe points to a small positive value for the cosmological constant or vacuum energy density. We discuss recent ideas that the cosmological constant plus LHC results might hint at critical phenomena near the Planck scale.
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QCD symmetries and the eta and etaprime in nuclei
We discuss the role of QCD symmetries in understanding the eta and etaprime mesons in nuclear media. Recent results on the etaprime mass in nuclei from the CBELSA/TAPS collaboration are very similar to the prediction of the Quark Meson Coupling model.
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Aspects of anomalous glue
Non-perturbative glue associated with gluon topology plays a vital role in determining the mass of the etaprime meson. We give an introduction to axial U(1) physics and explain how this non-perturbative glue also contributes to etaprime interactions with other hadrons. We concentrate on resonant etaprime pi production in partial waves with exotic quantum numbers not compatible with a quark-antiquark state and on the etaprime in nuclear media where there are new results from the COMPASS and CBELSA/TAPS experiments respectively.
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QCD symmetries in eta and etaprime mesic nuclei
We discuss the role of QCD symmetries and confinement in understanding eta and etaprime mesic nuclei. Eta and etaprime bound states in nuclei are sensitive to the flavour-singlet component in the meson. The bigger the singlet component, the more attraction and the greater the binding. Recent results on the etaprime mass in nuclei from the CBELSA/TAPS collaboration are very similar to the prediction of the Quark Meson Coupling model. In the model eta-etaprime mixing induces a factor of two enhancement of the eta-nucleon scattering length relative to the prediction with a pure octet eta, with real part about 0.8 fm.
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The Spin Structure of the Nucleon
This article reviews our present understanding of QCD spin physics: the proton spin puzzle and new developments aimed at understanding the transverse structure of the nucleon. We discuss present experimental investigations of the nucleon's internal spin structure, the theoretical interpretation of the different measurements and the open questions and challenges for future investigation.
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The cosmological constant puzzle: Vacuum energies from QCD to dark energy
The accelerating expansion of the Universe points to a small positive vacuum energy density and negative vacuum pressure. A strong candidate is the cosmological constant in Einstein's equations of General Relativity. The vacuum dark energy density extracted from astrophysics is 10^56 times smaller than the value expected from the Higgs potential in Standard Model particle physics. The dark energy scale is however close to the range of possible values expected for the light neutrino mass. We investigate this physics in a simple toy model where the chirality of the neutrino is treated by analogy as an Ising-like "spin" degree of freedom.
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Gluon polarization in the proton
We combine heavy-quark renormalization group arguments with our understanding of the nucleon's wavefunction to deduce a bound on the gluon polarization Delta g in the proton. The bound is consistent with the values extracted from spin experiments at COMPASS and RHIC.
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Eta-etaprime mixing in eta-mesic nuclei
Eta bound states in nuclei are sensitive to the flavour-singlet component in the eta. The bigger the singlet component, the more attraction and the greater the binding. eta-etaprime mixing plays an important role in understanding the value of the eta-nucleon scattering length a_{ηN}. Working with the Quark Meson Coupling model, we find a factor of two enhancement from mixing relative to the prediction with a pure octet eta.
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Quark spin in the proton
The proton spin puzzle has challenged our understanding of QCD for the last 20 years. We survey new developments in theory and experiment. The proton spin puzzle seems to be telling us about the interplay of valence quarks with chiral dynamics and the complex vacuum structure of QCD.
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The nucleon's octet axial-charge g_A^8 with chiral corrections
The value of the nucleon's flavour-singlet axial-charge extracted from polarised deep inelastic scattering is sensitive to the value of the octet axial-charge g_A^8 which is usually taken from an analysis of hyperon beta-decays within the framework of SU(3) symmetry, namely 0.58 \pm 0.03. Using the Cloudy Bag model we find that the value of g_A^8 is reduced by as much as 20% below the usual phenomenological value. This increases the value of the flavour singlet axial charge (g_A^0|_inv) derived from deep inelastic data and significantly reduces the difference between it and g_A^8.
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The proton spin puzzle: where are we today ?
The proton spin puzzle has challenged our understanding of QCD for the last 20 years. New measurements of polarized glue, valence and sea quark polarization, including strange quark polarization, are available. What is new and exciting in the data, and what might this tell us about the structure of the proton ? The proton spin puzzle seems to be telling us about the interplay of valence quarks with the complex vacuum structure of QCD.
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Anomalous glue, eta and eta-prime mesons
Axial U(1) dynamics are characterised by large OZI violations. Here we review the phenomenology of eta and eta-prime production and decay processes, and its connection to the anomalous glue that generates a large part of the masses of these pseudoscalar mesons.
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Neutrinos from spin dynamics
We conjecture that neutrino physics might correspond to the spontaneous magnetisation phase of an Ising-like spin model interaction coupled to neutrino chirality which operates at scales close to the Planck mass. We argue that this scenario leads to a simple extension of the Standard Model with no additional parameters that dynamically generates parity violation and spontaneous symmetry breaking for the gauge bosons which couple to the neutrino. The neutrino mass in the model ~ (the electroweak scale)^2 / 2M where M is the scale of the ``spin-spin'' interaction. For the ground state of the model the free energy density corresponding to the cosmological constant is ~ the neutrino mass, consistent with observation.
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