arXiv subjects

Anthony G. Williams

Publications and source records attributed to Anthony G. Williams.

At least 55 records · Page 3Linked to original sources

arXiv2016

Gamma rays from muons from WIMPs: Implementation of radiative muon decays for dark matter analyses

Dark matter searches in gamma ray final states often make use of the fact that photons can be produced from final state muons. Modern Monte Carlo generators and DM codes include the effects of final state radiation from muons produced in the dark matter annihilation process itself, but neglect the O(1%) radiative correction that arises from the subsequent muon decay. After implementing this correction we demonstrate the effect that it can have on dark matter phenomenology by considering the case of dark matter annihilation to four muons via scalar mediator production. We first show that the AMS-02 positron excess can no longer easily be made consistent with this final state once the Fermi-LAT dwarf limits are calculated with the inclusion of radiative muon decays, and we next show that the Fermi-LAT galactic centre gamma excess can be improved with this final state after inclusion of the same effect. We provide code and tables for the implementation of this effect in the popular dark matter code micrOMEGAs, providing a solution for any model producing final state muons.

hep-ph↗

arXiv2016

Tau reconstruction methods at an electron-positron collider in the search for new physics

By exploiting the relatively long lifetime of the tau lepton, we propose several novel methods for searching for new physics at an electron-positron collider. We consider processes that involve final states consisting of a tau lepton pair plus two missing particles. The mass and spin of the new physics particles can be measured in 3-prong tau decays. The tau polarization, which reflects the coupling to new physics, can be measured from the $τ\to πν$ decay channel using the impact parameter distribution of the charged pion. We also discuss the corresponding backgrounds for these measurements, the next-to-leading order (NLO) effects, and the implications of finite detector resolution.

hep-ph↗

arXiv2015

Spin glass behavior of the antiferromagnetic Heisenberg model on scale free network

Randomness and frustration are considered to be the key ingredients for the existence of spin glass (SG) phase. In a canonical system, these ingredients are realized by the random mixture of ferromagnetic (FM) and antiferromagnetic (AF) couplings. The study by Bartolozzi {\it et al.} [Phys. Rev. B{\bf 73}, 224419 (2006)] who observed the presence of SG phase on the AF Ising model on scale free network (SFN) is stimulating. It is a new type of SG system where randomness and frustration are not caused by the presence of FM and AF couplings. To further elaborate this type of system, here we study Heisenberg model on AF SFN and search for the SG phase. The canonical SG Heisenberg model is not observed in $d$-dimensional regular lattices for ($d \leq 3$). We can make an analogy for the connectivity density ($m$) of SFN with the dimensionality of the regular lattice. It should be plausible to find the critical value of $m$ for the existence of SG behaviour, analogous to the lower critical dimension ($d_l$) for the canonical SG systems. Here we study system with $m=2,3,4$ and $5$. We used Replica Exchange algorithm of Monte Carlo Method and calculated the SG order parameter. We observed SG phase for each value of $m$ and estimated its corersponding critical temperature.

cond-mat.dis-nn↗

arXiv2015

LUX likelihood and limits on spin-independent and spin-dependent WIMP couplings with LUXCalc

We present LUXCalc, a new utility for calculating likelihoods and deriving WIMP-nucleon coupling limits from the recent results of the LUX direct search dark matter experiment. After a brief review of WIMP-nucleon scattering, we derive LUX limits on the spin-dependent WIMP-nucleon couplings over a broad range of WIMP masses, under standard assumptions on the relevant astrophysical parameters. We find that, under these and other common assumptions, LUX excludes the entire spin-dependent parameter space consistent with a dark matter interpretation of DAMA's anomalous signal, the first time a single experiment has been able to do so. We also revisit the case of spin-independent couplings, and demonstrate good agreement between our results and the published LUX results. Finally, we derive constraints on the parameters of an effective dark matter theory in which a spin-1 mediator interacts with a fermionic WIMP and Standard Model fermions via axial-vector couplings. A detailed appendix describes the use of LUXCalc with standard codes to place constraints on generic dark matter theories.

hep-ph↗

arXiv2015

NMSSM explanations of the Galactic center gamma ray excess and promising LHC searches

The Galactic Center Excess (GCE) is explained in the framework of the Next-to-Minimal Supersymmetric Standard Model (NMSSM) with a $Z_3$ discrete symmetry. We show that a resonant CP-odd Higgs boson with mass twice that of the Dark Matter (DM) candidate is favoured. Meanwhile, the DM candidate is required to have relatively large coupling with the $Z$ boson through its Higgsino component in order to obtain correct DM relic density. Its LHC discovery potential via four signatures is discussed in detail. We find that the most sensitive signals are provided by the Higgsino-like chargino and neutralino pair production with their subsequent decays into $W$ bosons, $Z$ bosons, and DM. The majority of the relevant parameter space can be probed at the Large Hadron Collider with a centre-of-mass energy of 14 TeV and an integrated luminosity 1000 fb$^{-1}$.

hep-ph↗

arXiv2015

Two-color QCD at high density

QCD at high chemical potential has interesting properties such as deconfinement of quarks. Two-color QCD, which enables numerical simulations on the lattice, constitutes a laboratory to study QCD at high chemical potential. Among the interesting properties of two-color QCD at high density is the diquark condensation, for which we present recent results obtained on a finer lattice compared to previous studies. The quark propagator in two-color QCD at non-zero chemical potential is referred to as the Gor'kov propagator. We express the Gor'kov propagator in terms of form factors and present recent lattice simulation results.

hep-lat↗

arXiv2014

Dynamical mass generation in unquenched QED using the Dyson--Schwinger equations

We present a comprehensive numerical study of dynamical mass generation for unquenched QED in four dimensions, in the absence of four-fermion interactions, using the Dyson--Schwinger approach. We begin with an overview of previous investigations of criticality in the quenched approximation. To this we add an analysis using a new fermion-antifermion-boson interaction ansatz, the Kizilersu-Pennington (KP) vertex, developed for an unquenched treatment. After surveying criticality in previous unquenched studies, we investigate the performance of the KP vertex in dynamical mass generation using a renormalized fully unquenched system of equations. This we compare with the results for two hybrid vertices incorporating the Curtis--Pennington vertex in the fermion equation. We conclude that the KP vertex is as yet incomplete, and its relative gauge-variance is due to its lack of massive transverse components in its design.

hep-ph↗

arXiv2014

Searching for low-lying multi-particle thresholds in lattice spectroscopy

We explore the Euclidean-time tails of odd-parity nucleon correlation functions in a search for the S-wave pion-nucleon scattering-state threshold contribution. The analysis is performed using 2+1 flavor 32^3 x 64 PACS-CS gauge configurations available via the ILDG. Correlation matrices composed with various levels of fermion source/sink smearing are used to project low-lying states. The consideration of 25,600 fermion propagators reveals the presence of more than one state in what would normally be regarded as an eigenstate-projected correlation function. This observation is in accord with the scenario where the eigenstates contain a strong mixing of single and multi-particle states but only the single particle component has a strong coupling to the interpolating field. Employing a two-exponential fit to the eigenvector-projected correlation function, we are able to confirm the presence of two eigenstates. The lower-lying eigenstate is consistent with a N-pi scattering threshold and has a relatively small coupling to the three-quark interpolating field. We discuss the impact of this small scattering-state contamination in the eigenvector projected correlation function on previous results presented in the literature.

hep-lat↗

arXiv2013

Strongly-Coupled Unquenched QED4 Propagators Using Schwinger-Dyson Equations

We study unquenched QED in four dimensions using renormalised Schwinger-Dyson equations and focus on the behaviour of the fermion and photon propagators. For this purpose we use an improved Kizilersu-Pennington (KP) vertex which respects gauge invariance, multiplicative renormalizability for the massless case, agrees with perturbation theory in the weak coupling regime and is free of kinematic singularities. We find that the KP vertex performs very well as expected specially in comparison with other vertex choices. We find that the Landau pole problem familiar from perturbative QED persists in the nonperturbative case with the renormalised inverse photon propagator having zero crossing.

hep-ph↗

arXiv2013

Structure and Flow of the Nucleon Eigenstates in Lattice QCD

A determination of the excited energy eigenstates of the nucleon, $s=c{1}{2}$, $I={1}{2}$, $N^{\pm}$, is presented in full QCD using 2+1 flavor PACS-CS gauge configurations. The correlation-matrix method is used and is built using standard nucleon interpolators employing smearings at the fermion sources and sinks. We develop and demonstrate a new technique that allows the eigenvectors obtained to be utilized to track the propagation of the intrinsic nature of energy-states from one quark mass to the next. This approach is particularly useful for larger dimension correlation matrices where more near-degenerate energy-states can appear in the spectrum.

hep-lat↗

arXiv2012

Low-lying Odd-parity States of the Nucleon in Lattice QCD

The world's first examination of the odd-parity nucleon spectrum at light quark masses in 2+1 flavor lattice QCD is presented. Configurations generated by the PACS-CS collaboration and made available through the ILDG are used, with the lightest pion mass at 156 MeV. A novel method for tracking the individual energy eigenstates as the quark mass changes is introduced. The success of this approach reveals the flow of the states towards the physical masses. Using the correlation matrix method, the two lowest-energy states revealed are found to be in accord with the physical spectrum of Nature.

hep-lat↗

arXiv2012

Bosonic stringlike behavior and the Ultraviolet filtering of QCD

The gluonic action density is calculated in static mesons at finite temperature just below the deconfinement point. Our focus is to elucidate the role of vacuum ultraviolet fluctuations which are filtered using an improved smearing algorithm. In the intermediate source separation distance, where the free string picture poorly describes the flux tube width profile, we find upon reducing the vacuum action towards the classical instanton vacuum, the characteristics of the flux tube converge and compare favorably with the predictions of the free bosonic string. This result establishes a connection between the free string action and vacuum gauge fields and reveals the important role of ultraviolet physics in understanding the lattice data at this temperature scale. As a by-product of these calculations, we find the broadening of the QCD flux tube to be independent of the ultraviolet filtering at large distances. Our results exhibit a linearly divergent pattern in agreement with the string picture predictions.

hep-lat↗

arXiv2012

Roper Resonance in 2+1 Flavor QCD

The low-lying even-parity states of the nucleon are explored in lattice QCD using the PACS-CS collaboration 2+1-flavor dynamical-QCD gauge-field configurations made available through the International Lattice Datagrid (ILDG). The established correlation-matrix approach is used, in which various fermion source and sink smearings are utilized to provide an effective basis of interpolating fields to span the space of low-lying energy eigenstates. Of particular interest is the nature of the first excited state of the nucleon, the $N{1/2}^{+}$ Roper resonance of $P_{11}$ pion-nucleon scattering. The Roper state of the present analysis approaches the physical mass, displaying significant chiral curvature at the lightest quark mass. These full QCD results, providing the world's first insight into the nucleon mass spectrum in the light-quark regime, are significantly different from those of quenched QCD and provide interesting insights into the dynamics of QCD.

hep-lat↗

arXiv2011

Gluonic profile of the static baryon at finite temperature

The gluon flux distribution of a static three quark system has been revealed at finite temperature in the pure SU(3) Yang-Mills theory. An action density operator is correlated with three Polyakov loops representing the baryonic state at a temperatures near the end of the QCD plateau, T/T_c = 0.8, and another just before the deconfinement point, T/T_c = 0.9. The flux distributions at short distance separations between the quarks display an action-density profile consistent with a filled Delta shape iso-surface. However the Delta-shaped action iso-surface distributions are found to persist even at large inter-quark separations for both temperatures. The action density distribution in the quark plane exhibits a nonuniform pattern for all quark separations considered. This result contrasts the well-known Y-shaped uniform action density gluonic-flux profile obtained using the Wilson-loop as a quark source operator at zero temperature. We systematically measure and compare the main aspects of the profile of the flux distribution at the two considered temperature scales for three sets of isosceles triangle quark configurations. The radii, amplitudes and rate of change of the width of the flux distribution are found to reverse their behavior as the temperature increase from the end of the QCD plateau region towards the deconfinement point. Remarkably, we find the mean square width of the flux distribution shrinks and localizes for quark separations larger than 1.0 fm at T/T_c = 0.8 which results in an identifiable Y-shaped radius profile. Near the deconfinement point, the action-density delocalizes and the width broadens linearly at large quark separations.

hep-lat↗

arXiv2011

On the ground state of Yang-Mills theory

We investigate the overlap of the ground state meson potential with sets of mesonic-trial wave functions corresponding to different gluonic distributions. We probe the transverse structure of the flux tube through the creation of non-uniform smearing profiles for the string of glue connecting two color sources in Wilson loop operator. The non-uniformly UV-regulated flux-tube operators are found to optimize the overlap with the ground state and display interesting features in the ground state overlap.

hep-lat↗

arXiv2010

Positive-parity Excited-states of the Nucleon in Quenched Lattice QCD

Positive-parity spin-1/2 excitations of the nucleon are explored in lattice QCD. The variational method is used in this investigation and several correlation matrices are employed. As our focus is on the utility and methodology of the variational approach, we work in the quenched approximation to QCD. Various sweeps of Gaussian fermion-field smearing is applied at the source and at the sink of $χ_{1}\barχ_{1}$ and $χ_{1}χ_{2}$ correlation functions to obtain a large basis of operators. Using several different approaches for constructing basis interpolators, we demonstrate how improving the basis can split what otherwise might be interpreted as a single state into multiple eigenstates. Consistency of the extracted excited energy states are explored over various dimensions of the correlation matrices. The use of large correlation matrices is emphasized for the reliable extraction of the excited eigenstates of QCD.

hep-lat↗

arXiv2010

Role of center vortices in chiral symmetry breaking in SU(3) gauge theory

We study the behavior of the AsqTad quark propagator in Landau gauge on SU(3) Yang-Mills gauge configurations under the removal of center vortices. In SU(2) gauge theory, center vortices have been observed to generate chiral symmetry breaking and dominate the infrared behavior of the quark propagator. In contrast, we report a weak dependence on the vortex content of the gauge configurations, including the survival of dynamical mass generation on configurations with vanishing string tension.

hep-lat↗

arXiv2010

Ordering of Spin-$\frac{1}{2}$ Excitations of the Nucleon in Lattice QCD

We present results for the negative parity low-lying state of the nucleon, $N{\frac{1}{2}}^{-}$ (1535 MeV) ${\rm S}_{11}$, from a variational analysis method. The analysis is performed in quenched QCD with the FLIC fermion action. The principal focus of this paper is to explore the level ordering between the Roper (${\rm{P}}_{11}$) and the negative parity ground (${\rm{S}}_{11}$) states of the nucleon. Evidence of the physical level ordering is observed at light quark masses. A wide variety of smeared-smeared correlation functions are used to construct correlation matrices. A comprehensive correlation matrix analysis is performed to ensure an accurate isolation of the $N{\frac{1}{2}}^{-}$ state.

hep-lat↗