arXiv subjects

A. W. Thomas

Publications and source records attributed to A. W. Thomas.

At least 235 records · Page 13Linked to original sources

arXiv2005

Liquid-gas phase transition and Coulomb instability of asymmetric nuclear systems

We use a chiral SU(3) quark mean field model to study the properties of nuclear systems at finite temperature. The liquid-gas phase transition of symmetric and asymmetric nuclear matter is discussed. For two formulations of the model the critical temperature, $T_c$, for symmetric nuclear matter is found to be 15.8 MeV and 17.9 MeV. These values are consistent with those derived from recent experiments. The limiting temperatures for finite nuclei are in good agreement with the experimental points.

nucl-th↗

arXiv2005

Liquid-gas phase transition in nuclear matter including strangeness

We apply the chiral SU(3) quark mean field model to study the properties of strange hadronic matter at finite temperature. The liquid-gas phase transition is studied as a function of the strangeness fraction. The pressure of the system cannot remain constant during the phase transition, since there are two independent conserved charges (baryon and strangeness number). In a range of temperatures around 15 MeV (precise values depending on the model used) the equation of state exhibits multiple bifurcates. The difference in the strangeness fraction $f_s$ between the liquid and gas phases is small when they coexist. The critical temperature of strange matter turns out to be a non-trivial function of the strangeness fraction.

nucl-th↗

arXiv2004

Near threshold enhancement of the ppbar mass spectrum in J/Psi decay

We investigate the nature of the near-threshold enhancement in the ppbar invariant mass spectrum of the reaction J/Psi -> gamma ppbar reported recently by the BES Collaboration. Using the Juelich NNbar model we show that the mass dependence of the ppbar spectrum close to the threshold can be reproduced by the S-wave ppbar final state interaction in the isospin I=1 state within the Watson-Migdal approach. However, because of our poor knowledge of the NNbar interaction near threshold and of the J/Psi -> gamma ppbar reaction mechanism and in view of the controversal situation in the decay J/Psi -> pi0 ppbar, where no obvious signs of a ppbar final state interaction are seen, explanations other than final state interactions cannot be ruled out at the present stage.

hep-ph↗

arXiv2004

Self-Organized Criticality and Stock Market Dynamics: an Empirical Study

The Stock Market is a complex self-interacting system, characterized by an intermittent behaviour. Periods of high activity alternate with periods of relative calm. In the present work we investigate empirically about the possibility that the market is in a self-organized critical state (SOC). A wavelet transform method is used in order to separate high activity periods, related to the avalanches of sandpile models, from quiescent. A statistical analysis of the filtered data show a power law behaviour in the avalanche size, duration and laminar times. The memory process, implied by the power law distribution, of the laminar times is not consistent with classical conservative models for self-organized criticality. We argue that a ``near-SOC'' state or a time dependence in the driver, which may be chaotic, can explain this behaviour.

cond-mat.other↗

arXiv2004

Direct CP Violation in B decays with rho0-omega Mixing

A complete study of the process B to pi+ pi- V(V=1--) is performed both in the framework of the helicity formalism and the effective lagrangian approach. Emphasis is put on the factorization hypothesis and the importance of the rho0- omega mixing in enhancing the direct CP violation. New results involving some branching ratios and the ratio of the Penguin/Tree amplitude are given in details.

hep-ph↗

arXiv2004

Towards a Connection Between Nuclear Structure and QCD

As we search for an ever deeper understanding of the structure of hadronic matter one of the most fundamental questions is whether or not one can make a connection to the underlying theory of the strong interaction, QCD. We build on recent advances in the chiral extrapolation problem linking lattice QCD at relatively large ``light quark'' masses to the physical world to estimate the scalar polarizability of the nucleon. The latter plays a key role in modern relativistic mean-field descriptions of nuclei and nuclear matter (such as QMC) and, in particular, leads to a very natural saturation mechanism. We demonstrate that the value of the scalar polarizability extracted from the lattice data is consistent with that needed for a successful description of nuclei within the framework of QMC. In a very real sense this is the first hint of a direct connection between QCD and the properties of finite nuclei.

nucl-th↗

arXiv2004

New treatment of the chiral SU(3) quark mean field model

We perform a study of infinite hadronic matter, finite nuclei and hypernuclei with an improved method of calculating the effective baryon mass. A detailed study of the predictions of the model is made in comparison with the available data and the level of agreement is generally very good. Comparison with an earlier treatment shows relatively minor differences at or below normal nuclear matter density, while at high density the improved calculation is quite different. In particular, we find no phase transition corresponding to chiral symmetry restoration in high density nuclear matter.

nucl-th↗

arXiv2004

Neutron Star Properties from an NJL Model Modified to Simulate Confinement

The NJL model has recently been extended with a method to simulate confinement. This leads in mean field approximation to a natural mechanism for the saturation of nuclear matter. We use the model to investigate the equation of state of asymmetric nuclear matter and then use it to compute the properties of neutron stars.

nucl-th↗

arXiv2004

Constraints on Parton Charge Symmetry and Implications for Neutrino Reactions

For the first time, charge symmetry breaking terms in parton distribution functions have been included in a global fit to high energy data. We review the results obtained for both valence and sea quark charge symmetry violation, and we compare these results with the most stringent experimental upper limits on charge symmetry violation for parton distribution functions, and with theoretical estimates of charge symmetry violation. The limits allowed in the global fit would tolerate a rather large violation of charge symmetry. We discuss the implications of this for the extraction of the Weinberg angle in neutrino DIS by the NuTeV collaboration.

hep-ph↗

arXiv2004

Precise Determination of Electroweak Parameters in Neutrino-Nucleon Scattering

A systematic error in the extraction of $\sin^2 θ_W$ from nuclear deep inelastic scattering of neutrinos and antineutrinos arises from higher-twist effects arising from nuclear shadowing. We explain that these effects cause a correction to the results of the recently reported significant deviation from the Standard Model that is potentially as large as the deviation claimed, and of a sign that cannot be determined without an extremely careful study of the data set used to model the input parton distribution functions.

hep-ex↗

arXiv2004

Leading Quenching Effects in the Proton Magnetic Moment

We present the first investigation of the extrapolation of quenched nucleon magnetic moments in quenched chiral effective field theory. We utilize established techniques in finite-range regularisation and compare with standard dimensional regularisation methods. Finite-volume corrections to the relevant loop integrals are also addressed. Finally, the contributions of dynamical sea quarks to the proton moment are estimated using a recently discovered phenomenological link between quenched and physical QCD.

hep-lat↗

arXiv2004

The strangeness magnetic moment of the nucleon from FLIC fermions

By imposing the constraints of charge symmetry we show that the strangeness magnetic moment of the nucleon can be expressed in terms of empirical magnetic moments and ratios of valence quark magnetic moments. The latter are determined using modern chiral extrapolation techniques and recent low mass lattice QCD simulations of the individual quark contributions to the magnetic moments of the nucleon octet. The result is a precise determination of G_M^s, namely -0.043 +/- 0.026 mu_N, which is consistent with the latest experimental measurements.

hep-lat↗

arXiv2004

Quark Distributions in a Medium

We derive the formal expressions needed to discuss the change of the twist-two parton distribution functions when a hadron is placed in a medium with relativistic scalar and vector mean fields

hep-ph↗

arXiv2004

Quark structure and nuclear effective forces

We formulate the quark meson coupling model as a many-body effective Hamiltonian. This leads naturally to the appearance of many-body forces. We investigate the zero range limit of the model and compare its Hartree-Fock Hamiltonian to that corresponding to the Skyrme effective force. By fixing the three parameters of the model to reproduce the binding and symmetry energy of nuclear matter, we find that it allows a very satisfactory interpretation of the Skyrme force.

nucl-th↗

arXiv2004

Limits on the temporal variation of the fine structure constant, quark masses and strong interaction from quasar absorption spectra and atomic clock experiments

We perform calculations of the dependence of nuclear magnetic moments on quark masses and obtain limits on the variation of $(m_q/Λ_{QCD})$ from recent measurements of hydrogen hyperfine (21 cm) and molecular rotational transitions in quasar absorption systems, atomic clock experiments with hyperfine transitions in H, Rb, Cs, Yb$^+$, Hg$^+$ and optical transition in Hg$^+$. Experiments with Cd$^+$, deuterium/hydrogen, molecular SF$_6$ and Zeeman transitions in $^3$He/Xe are also discussed.

hep-ph↗

arXiv2003

Quark distributions in nuclear matter and the EMC effect

Quark light cone momentum distributions in nuclear matter and the structure function of a bound nucleon are investigated in the framework of the Nambu-Jona-Lasinio model. This framework describes the nucleon as a relativistic quark-diquark state, and the nuclear matter equation of state by using the mean field approximation. The scalar and vector mean fields in the nuclear medium couple to the quarks in the nucleon and their effect on the spin independent nuclear structure function is investigated in detail. Special emphasis is placed on the important effect of the vector mean field and on a formulation which guarantees the validity of the number and momentum sum rules from the outset.

nucl-th↗

arXiv2003

A Method to Unambiguously Determine the Parity of the Theta+ Pentaquark

With the recent discovery of the $Θ(1540)$ pentaquark, the question of its parity is paramount since this will constrain the correct description of its internal structure. We show that the measurement of the spin singlet and triplet cross sections for the reaction $\vec{p}\vec{p} \to Σ^+ Θ^+$ will unambiguously determine the parity of the $Θ^+$.

hep-ph↗

arXiv2003

Chiral extrapolation and physical insights

It has recently been established that finite-range regularisation in chiral effective field theory enables the accurate extrapolation of modern lattice QCD results to the chiral regime. We review some of the highlights of extrapolations of quenched lattice QCD results, including spectroscopy and magnetic moments. The $Δ$ resonance displays peculiar chiral features in the quenched theory which can be exploited to demonstrate the presence of significant chiral corrections.

hep-lat↗