arXiv subjects

Anthony W. Thomas

Publications and source records attributed to Anthony W. Thomas.

At least 55 records · Page 3Linked to original sources

arXiv2021

Hamiltonian effective field theory in elongated or moving finite volume

We extend previous work concerning rest-frame partial-wave mixing in Hamiltonian effective field theory to both elongated and moving systems, where two particles are in a periodic elongated cube or have nonzero total momentum, respectively. We also consider the combination of the two systems when directions of the elongation and the moving momentum are aligned. This extension should also be applicable in any Hamiltonian formalism. As a demonstration, we analyze lattice QCD results for the spectrum of an isospin-2 $ππ$ scattering system and determine the $s$, $d$, and $g$ partial-wave scattering information. The inclusion of lattice simulation results from moving frames significantly improves the uncertainty in the scattering information.

hep-lat↗

arXiv2020

Censored EM algorithm for Weibull mixtures: application to arrival times of market orders

In a previous analysis the problem of "zero-inflated" time data (caused by high frequency trading in the electronic order book) was handled by left-truncating the inter-arrival times. We demonstrated, using rigorous statistical methods, that the Weibull distribution describes the corresponding stochastic dynamics for all inter-arrival time differences except in the region near zero. However, since the truncated Weibull distribution was not able to describe the huge "zero-inflated" probability mass in the neighbourhood of zero (making up approximately 50\% of the data for limit orders), it became clear that the entire probability distribution is a mixture distribution of which the Weibull distribution is a significant part. Here we use a censored EM algorithm to analyse data for the difference of the arrival times of market orders, which usually have a much lower percentage of zero inflation, for four selected stocks trading on the London Stock Exchange.

q-fin.ST↗

arXiv2020

Hybrid Stars with Hyperons and Strange Quark Matter

We consider the possibility of having hybrid stars with a phase transition from hadrons into strange matter at the core of a neutron star in b equilibrium. For the hadron phase equation of state (EoS) the quark-meson coupling model is used, while the MIT bag model is used to describe the strange matter phase. The phase transition is treated using the Gibbs construction method and results are calculated and checked against the observational constaints imposed on the EoS. The results are also compared with the hadronic EoS including hyperons, F-QMC700

hep-ph↗

arXiv2020

Kaonic Hydrogen and Deuterium in Hamiltonian Effective Field Theory

The anti-kaon nucleon scattering lengths resulting from a Hamiltonian effective field theory analysis of experimental data and lattice QCD studies are presented. The same Hamiltonian is then used to compute the scattering length for the $K^- d$ system, taking careful account of the effects of recoil on the energy at which the $\bar{K}N$ T-matrices are evaluated. These results are then used to estimate the shift and width of the $1S$ levels of anti-kaonic hydrogen and deuterium. The $K^- p$ result is in excellent agreement with the SIDDHARTA measurement. In the $K^- d$ case the imaginary part of the scattering length and consequently the width of the $1S$ state are considerably larger than found in earlier work. This is a consequence of the effect of recoil on the energy of the $\bar{K}N$ energy, which enhances the role of the $Λ(1405)$ resonance.

hep-ph↗

arXiv2020

Partial Wave Mixing in Hamiltonian Effective Field Theory

The spectrum of excited states observed in the finite volume of lattice QCD is governed by the discrete symmetries of the cubic group. This finite group permits the mixing of orbital angular momentum quanta in the finite volume. As experimental results refer to specific angular momentum in a partial-wave decomposition, a formalism mapping the partial-wave scattering potentials to the finite volume is required. This formalism is developed herein for Hamiltonian effective field theory, an extension of chiral effective field theory incorporating the Lüscher relation linking the energy levels observed in finite volume to the scattering phase shift. The formalism provides an optimal set of rest-frame basis states maximally reducing the dimension of the Hamiltonian, and it should work in any Hamiltonian formalism. As a first example of the formalism's implementation, lattice QCD results for the spectrum of an isospin-2 $ππ$ scattering system are analyzed to determine the $s$, $d$, and $g$ partial-wave scattering information.

hep-lat↗

arXiv2020

An Improved Hadronic Model for Pion Electroproduction

Current measurements of the high energy behavior of the pion form factor are obtained from pion electroproduction data. These values are model dependent, utilizing the Vanderhaeghen, Guidal and Laget Regge (VGL) Model for their extraction. Recent work which examined the implementation of gauge invariance in that model suggested that it might lead to extracted pion form factors larger than the true values. Here we introduce a new model which preserves the successes of the VGL Model but implements gauge invariance in a new way. To demonstrate the validity of this new approach, we first use it to extract the pion form factor in a simple toy model. When compared with the previous extraction method, the improved model leads to a more reliable extraction. The success in this simple model leads us to reanalyse the electroproduction cross section data, where we obtain comparable values for the pion form factor to those obtained using the VGL procedure.

hep-ph↗

arXiv2020

Neutron stars from crust to core within the Quark-meson coupling model

Recent years continue to be an exciting time for the neutron star physics, providing many new observations and insights to these natural laboratories of cold dense matter. To describe them, there are many models on the market but still none that would reproduce all observed and experimental data. The quark-meson coupling model stands out with its natural inclusion of hyperons as dense matter building blocks, and fewer parameters necessary to obtain the nuclear matter equation of state. The latest advances of the QMC model and its application to the neutron star physics will be presented, within which we build the neutron star's outer crust from finite nuclei up to the neutron drip line. The appearance of different elements and their position in the crust of a neutron star is explored and compared to the predictions of various models, giving the same quality of the results for the QMC model as for the models when the nucleon structure is not taken into account.

nucl-th↗

arXiv2019

Partial Wave Mixing in Hamiltonian Effective Field Theory

Within general partial-wave mixing, a method for reducing the high dimension of the finite-volume Hamiltonian from Hamiltonian effective field theory is proposed. This method provides a new viewpoint on partial-wave mixing, and a set of matrices that can reflect the degree of partial-wave mixing. An example of isospin-2 $ππ$ scattering is used to examine the consistency between this method and Lüscher's method.

hep-lat↗

arXiv2019

Partial Wave Mixing in Hamiltonian Effective Field Theory

We explore partial-wave mixing in the finite volume based on HEFT, and provide the P-Matrix to show the degree of partial-wave mixing. An example of isospin-2 $ππ$ scattering is used to check the consistency between HEFT and Lüscher's method.

hep-lat↗

arXiv2019

New way to access the quark fragmentation functions in electron-positron annihilation

The description of the polarized quark hadronization process is one of the most challenging problems in the physics of strong interactions. The various single hadron and dihadron fragmentation functions, that quantify this process, are determined by analyzing the inclusive production of hadrons in electron-positron semi-inclusive annihilation process (SIA). These, in turn, are used to extract the transverse momentum dependent parton distribution functions from the experiments on semi-inclusive deep inelastic scattering process (SIDIS), elucidating the spin-orbit correlations in nucleon. Here we present a framework for new measurements of polarized quark fragmentation functions in electron-positron annihilation and deep inelastic semi-inclusive processes. These measurements offer a number of exciting opportunities to improve our understanding of the polarized quark hadronization and test the universality of the fragmentation functions. Such measurements can be performed at the upcoming BELLE II, JLab-12 and the future EIC experiments

hep-ph↗

arXiv2018

Model Dependence of the Pion Form Factor Extracted from Pion Electro-production

In 2008 the Jefferson Laboratory $F_π$ Collaboration released results for the pion electromagnetic form factor, which they extracted from pion electro-production data. The measured values for the pion form factor are model dependent, and require the use of the Vanderhaeghen, Guidal and Laget Regge Model for their extraction. While agreement between this model and data is impressive, the theoretical implementation of gauge invariance is less satisfying. We would like to establish how well the extracted form factor corresponds to the true form factor. To do this, we use a simple toy model, which imposes gauge invariance in a more theoretically satisfying way. The model form factor is extracted from our model cross section using the method employed by the $F_π$ Collaboration to extract the experimental pion form factor. We conclude that the reconstructed model form factor is a reasonable representation of the true model form factor for the kinematics chosen, although we note that the extracted form factor is smaller than the true form factor. This suggests that current extracted values of pion form factor may be overestimated.

nucl-th↗

arXiv2018

Semi-inclusive back-to-back production of a hadron pair and a single hadron in $e^+e^-$ annihilation

Inclusive hadron production in $e^+e^-$ annihilation has long been used to study both single hadron fragmentation functions (FF) and dihadron fragmentation functions (DiFF). In particular, the polarized DiFFs can be accessed in electron-positron annihilation by measuring azimuthal correlations between two back-to-back pairs of hadrons in the center of mass system, where the relevant structure functions can be expressed as convolutions of two (polarized) DiFFs. Here we explore the advantages of measuring the inclusive back-to-back production of a single hadron on one side against a hadron pair on the opposite side of the detector in two jet events. The leading twist cross section for this process contains convolutions of the corresponding single hadron FFs on one side and the DiFFs for the hadron pair on the other side, which furnishes several interesting new opportunities. A measurement of the unpolarized cross section with a number of different types of observed hadrons will help in untangling the quark flavor dependence of the unpolarized DiFFs, when the results are analyzed together with the inclusive measurements of dihadron pairs, such as those recently performed by the $\texttt{BELLE}$ collaboration. Even more interesting, with a polarized hyperon on one side we can study the quark spin-dependent DiFFs of an unpolarized hadron pair on the other side. This, in turn, will allow us to test the universality of the spin-dependent DiFFs entering the cross sections of electron-positron annihilation and semi-inclusive deep inelastic scattering processes.

hep-ph↗

arXiv2018

Reflections on the Origin of the EMC Effect

In the 35 years since the European Muon Collaboration announced the astonishing result that the valence structure of a nucleus was very different from that of a free nucleon, many explanations have been suggested. The first of the two most promising explanations is based upon the different effects of the strong Lorentz scalar and vector mean fields known to exist in a nucleus on the internal structure of the nucleon-like clusters which occupy shell model states. The second links the effect to the modification of the structure of nucleons involved in short-range correlations, which are far off their mass shell. We explore some of the methods which have been proposed to give complementary information on this puzzle, especially the spin-dependent EMC effect and the isovector EMC effect, both proposed by Cloet, Bentz and Thomas. It is shown that the predictions for the spin dependent EMC effect, in particular, differ substantially within the mean-field and short-range correlation approaches. Hence the measurement of the spin dependent EMC effect at Jefferson Lab should give us a deeper understanding of the origin of the EMC effect and, indeed, of the structure of atomic nuclei.

hep-ph↗

arXiv2018

Semi-inclusive production of two back-to-back hadron pairs in $e^+e^-$ annihilation revisited

The cross section for back-to-back hadron pair production in $e^+e^-$ annihilation provides access to the dihadron fragmentation functions (DiFF) needed to extract nucleon parton distribution functions from the semi-inclusive deep inelastic scattering (SIDIS) experiments with two detected final state hadrons. Particular attention is given to the so-called interference DiFF (IFF), which makes it possible to extract the transversity parton distribution of the nucleon in the collinear framework. However, previously unnoticed discrepancies were recently highlighted between the definitions of the IFFs appearing in the collinear kinematics when reconstructed from DiFFs entering the unintegrated fully differential cross sections of SIDIS and $e^+e^-$ annihilation processes. In this work, to clarify this problem we rederive the fully differential cross section for $e^+e^-$ annihilation at the leading-twist approximation. We find a mistake in the definition of the kinematics in the original expression that systematically affects a subset of terms and that leads to two significant consequences. First, the discrepancy between the IFF definitions in the cross sections for SIDIS and $e^+e^-$ annihilation is resolved. Second, the previously derived azimuthal asymmetry for accessing the helicity dependent DiFF $G_1^\perp$ in $e^+e^-$ annihilation vanishes, which explains the nonobservation of this asymmetry in the recent experimental searches by the ${\tt BELLE}$ Collaboration. We discuss the recently proposed alternative option to extract $G_1^\perp$.

hep-ph↗

arXiv2018

Accessing Quark Helicity through Dihadron Studies

We present a new proposal to study the helicity-dependent dihadron fragmentation functions (DiFF), which describe the correlations of the longitudinal polarization of a fragmenting quark with the transverse momenta of the produced hadron pair. Recent experimental searches for this DiFF via azimuthal asymmetries in back-to-back hadron pair production in $e^+e^-$ annihilation by the ${\tt BELLE}$ Collaboration did not yield a signal. Here we propose a new way to access this DiFF in $e^+e^-$ annihilation, motivated by the recently recalculated cross section of this reaction, which explains why there was in fact no signal for the ${\tt BELLE}$ Collaboration to see. In this new approach the azimuthal asymmetry is weighted by the virtual photon's transverse momentum square multiplying sine and cosine functions of difference of azimuthal angles of relative and total momentum for each pair. The integration over the virtual photon's transverse momentum has the effect of separating the convolution between the helicity-dependent DiFFs in the quark and antiquark jets and results in a nonzero collinear expression containing Fourier moments of helicity-dependent DiFFs. A second new measurement is also proposed for two-hadron production in semi-inclusive deep inelastic scattering, where the asymmetry is weighted in a similar way for a single pair. This results in a collinear factorized form of the asymmetry, which includes the quark helicity parton distribution function and the same helicity-dependent DiFF as in $e^+e^-$ production and will allow us to check the universality of this DiFF.

hep-ph↗

arXiv2018

Polarized EMC Effect in the QMC Model

The ratios of the in-medium to free nucleon structure functions for the unpolarized and polarized cases are obtained using the MIT bag model for the free case, along with the QMC model to incorporate the in-medium modifications of the structure functions. As discussed in earlier work, the observed nuclear EMC effect is reasonably well described. This gives us confidence to investigate the predictions of the model for the polarized EMC effect, the ratio $g_1^*(x)/g_1(x)$ for a bound proton to that of a free proton. This ratio is found to be substantially different from unity and very similar in shape and size to that found in the unpolarized case. This prediction of such a fundamental change in the valence structure of a bound nucleon needs to be tested experimentally at the earliest opportunity.

nucl-th↗

arXiv2018

Charge Symmetry Breaking Effects in Pion and Kaon Structure

Charge symmetry breaking (CSB) effects associated with the $u$ and $d$ quark mass difference are investigated in the quark distribution functions and spacelike electromagnetic form factors of the pion and kaon. We use a confining version of the Nambu--Jona-Lasinio model, where CSB effects at the infrared scale associated with the model are driven by the dressed $u$ and $d$ quark mass ratio, which because of dynamical chiral symmetry breaking is much closer to unity than the associated current quark mass ratio. The pion and kaon are given as bound states of a dressed quark and a dressed antiquark governed by the Bethe-Salpeter equation, and exhibit the properties of Goldstone bosons, with a pion mass difference given by $m_{π^+}^2 - m_{π^0}^2 \propto (m_u - m_d)^2$ as demanded by dynamical chiral symmetry breaking. We find significant CSB effects for realistic current quark mass ratios ($m_u/m_d \sim 0.5$) in the quark flavor-sector electromagnetic form factors of both the pion and kaon. For example, the difference between the $u$ and $d$ quark contributions to the $π^+$ electromagnetic form factors is about 8\% at a momentum transfer of $Q^2 \simeq 10\,$GeV$^2$, while the analogous effect for the light quark sector form factors in the $K^+$ and $K^0$ is about twice as large. For the Parton distribution functions, we find CSB effects which are considerably smaller than those found in the electromagnetic form factors.

nucl-th↗

arXiv2018

IUPAP Report 41 Introduction

IUPAP Report 41 is a document that gives summary information about the major nuclear physics facilities around the world. It is updated approximately every five years by Working Group 9 of the International Union of Pure and Applied Physics. The introduction to the report gives an overview of the field of nuclear physics and outlines the major questions facing the field. This most recent version of Report 41 has been updated to reflect the state of the field in 2018. The full report can be found at http://www.triumf.info/hosted/iupap/icnp/report41.html

nucl-ex↗