arXiv subjects

Huey-Wen Lin

Publications and source records attributed to Huey-Wen Lin.

At least 127 records · Page 7Linked to original sources

arXiv2014

Probing TeV scale physics in precision UCN decays

We present the calculation of matrix elements of iso-vector scalar, axial and tensor charges between a neutron and a proton state on dynamical $N_f=2+1+1$ HISQ configurations generated by the MILC Collaboration using valence clover fermions. These matrix elements are needed to probe novel scalar and tensor interactions in neutron beta-decay that can arise in extensions to the Standard Model at the TeV scale. Results are presented at one value of the lattice spacing, $a=0.12$ fm, and two values of light quarks corresponding to $M_π=310$ and $220$ MeV. We discuss two sources of systematic errors, contribution of excited states to these matrix elements and the renormalization constants, and the efficacy of methods used to control them.

hep-lat↗

arXiv2013

Studies of Nucleon Resonance Structure in Exclusive Meson Electroproduction

Studies of the structure of excited baryons are key to the N* program at Jefferson Lab. Within the first year of data taking with the Hall B CLAS12 detector following the 12 GeV upgrade, a dedicated experiment will aim to extract the N* electrocouplings at high photon virtualities Q2. This experiment will allow exploration of the structure of N* resonances at the highest photon virtualities ever yet achieved, with a kinematic reach up to Q2 = 12 GeV2. This high-Q2 reach will make it possible to probe the excited nucleon structures at distance scales ranging from where effective degrees of freedom, such as constituent quarks, are dominant through the transition to where nearly massless bare-quark degrees of freedom are relevant. In this document, we present a detailed description of the physics that can be addressed through N* structure studies in exclusive meson electroproduction. The discussion includes recent advances in reaction theory for extracting N* electrocouplings from meson electroproduction off protons, along with QCD-based approaches to the theoretical interpretation of these fundamental quantities. This program will afford access to the dynamics of the non-perturbative strong interaction responsible for resonance formation, and will be crucial in understanding the nature of confinement and dynamical chiral symmetry breaking in baryons, and how excited nucleons emerge from QCD.

nucl-th↗

arXiv2013

Neutral Meson Decays into Two Photons from Lattice QCD

A precision determination of the neutral-pion width would improve determinations of the splitting between the up- and down-quark masses, and matrix elements for the decay of neutral mesons into two photons could play a role in the attempt to probe beyond-the-Standard Model physics in muon $g-2$ experiments. The theoretical error is dominated by hadronic light-by-light diagrams, and since direct measurements are extremely difficult, model calculations factorize it into two-photon diagrams connected by the lightest hadrons. We employ perturbative techniques to express the photon as a superposition of QCD eigenstates accessible in lattice-QCD calculations and found that vector-meson dominance is a poor description of the two-photon decay process when both photons are off shell.

hep-lat↗

arXiv2012

Lattice Hadron Structure: Applications within and beyond QCD

Study of the hadronic matrix elements can provide not only tests of the QCD sector of the Standard Model (in comparing with existing experiments) but also reliable low-energy hadronic quantities applicable to a wide range of beyond-the-Standard Model scenarios where experiments or theoretical calculations are limited or difficult. On the QCD side, progress has been made in the notoriously difficult problem of addressing gluonic structure inside the nucleon, reaching higher-$Q^2$ region of the form factors, and providing a complete picture of the proton spin. However, even further study and improvement of systematic uncertainties are needed. There are also proposed calculations of higher-order operators in the neutron electric dipole moment Lagrangian, which would be useful when combined with effective theory to probe BSM. Lattice isovector tensor and scalar charges can be combined with upcoming neutron beta-decay measurements of the Fierz interference term and neutrino asymmetry parameter to probe new interactions in the effective theory, revealing the scale of potential new TeV particles. Finally, I revisit the systematic uncertainties in recent calculations of $g_A$ and review prospects for future calculations.

hep-lat↗

arXiv2012

Probing novel TeV physics through precision calculations of scalar and tensor charges of the nucleon

We present an update on the calculation of matrix elements of iso-vector scalar, axial and tensor charges between a neutron and a proton state. These matrix elements are needed to probe novel scalar and tensor interactions in neutron beta-decay that can arise in extensions of the Standard Model at the TeV scale. Our calculations are being done using valence clover fermions on dynamical N_f=2+1+1 HISQ configurations generated by the MILC Collaboration. We provide preliminary estimates of the dependence of these matrix elements on the light quark masses, lattice spacing, and the time separation between the source and sink of the nucleons. We also find that the renormalization constants calculated using the RI-sMOM scheme are close to unity for the HYP smeared HISQ lattices.

hep-lat↗

arXiv2012

Charmed-Baryon Spectroscopy from Lattice QCD with N_f=2+1+1 Flavors

We present the results of a calculation of the positive-parity ground-state charmed-baryon spectrum using 2+1+1 flavors of dynamical quarks. The calculation uses a relativistic heavy-quark action for the valence charm quark, clover-Wilson fermions for the valence light and strange quarks, and HISQ sea quarks. The spectrum is calculated with a lightest pion mass around 220 MeV, and three lattice spacings (a \approx 0.12 fm, 0.09 fm, and 0.06 fm) are used to extrapolate to the continuum. The light-quark mass extrapolation is performed using heavy-hadron chiral perturbation theory up to O(m_pi^3) and at next-to-leading order in the heavy-quark mass. For the well-measured charmed baryons, our results show consistency with the experimental values. For the controversial J=1/2 Xi_{cc}, we obtain the isospin-averaged value M_{Xi_{cc}}=3595(39)(20)(6) MeV (the three uncertainties are statistics, fitting-window systematic, and systematics from other lattice artifacts, such as lattice scale setting and pion-mass determination), which shows a 1.7 sigma deviation from the experimental value. We predict the yet-to-be-discovered doubly and triply charmed baryons Xi_{cc}^*, Omega_{cc}, Omega_{cc}^* and Omega_{ccc} to have masses 3648(42)(18)(7) MeV, 3679(40)(17)(5) MeV, 3765(43)(17)(5) MeV and 4761(52)(21)(6) MeV, respectively.

hep-lat↗

arXiv2012

Theoretical Bounds on New Four-Fermion Interactions and TeV Scale Physics

The standard model weak interactions can be described by four-fermion V-A operators at low energies. New physics at the TeV scale can, however, generate the other Lorentz structures. In this talk, we review the constraints on such interactions from nuclear and hadronic decays, as well as from collider searches. Currently the most stringent bounds come from the analysis of the 0+ to 0+ nuclear and the pi to e nu gamma radiative pion decays. In the near future, the ultracold neutron beta decay experiments and the direct LHC measurements will compete in setting the most stringent bounds, provided, however, that the neutron-to-proton non-perturbative transition matrix elements can be calculated to a level of 10-20% accuracy.

hep-lat↗

arXiv2012

Probing TeV scale physics via ultra cold neutron decays and calculating non-standard baryon matrix elements

We motivate undertaking precision analyses of neutron decays to look for signatures of new scalar and tensor interactions that can arise in extensions of the Standard Model at the TeV scale. The key ingrediant needed to connect experimental data with theoretical analysis are high-precision calculations of matrix elements of isovector bilinear operators between the decaying neutron and final state proton. We describe the status of our Lattice QCD program of using valence clover fermions on dynamical N_f=2+1+1 HISQ configurations generated by the MILC Collaboration. On the theoretical side we use the effective field theory method and provide both model independent and dependent analyses to obtain bounds on possible scalar and tensor interactions, both from low energy experiments and LHC data.

hep-lat↗

arXiv2011

Medium Effects in Parton Distributions

Understanding the effects of a background hadronic medium on hadronic observables is important in the context of hadron structure. Many experiments probing nucleon structure make use of nuclear targets and unraveling the modifications that ensue is a complex task. Using lattice QCD, we investigate the ab initio computation of hadron structure in a medium, focusing on the structure of the pion in a Bose-condensed gas of pions.

hep-lat↗

arXiv2011

Lattice QCD for Precision Nucleon Matrix Elements

Precision measurements on nucleons provide constraints on the Standard Model and can also discern the signatures predicted for particles beyond the Standard Model. Knowing the Standard Model inputs to nucleon matrix elements will be necessary to constrain the couplings of dark matter candidates such as the neutralino, to relate the neutron electric dipole moment to the CP-violating theta parameter, or to search for new TeV-scale particles though non-$V-A$ interactions in neutron beta decay. However, these matrix elements derive from the properties of quantum chromodynamics (QCD) at low energies, where the coupling is strong and thus perturbative treatments fail. Using lattice gauge theory, we can nonperturbatively calculate the QCD path integral on a supercomputer. In this proceeding, I will review a few representative areas in which lattice QCD can contribute to understanding the structure inside nucleon and how they can contribute to the search for beyond-the-Standard Model physics, with discussions of the difficulties and prospects for future development.

hep-lat↗

arXiv2011

Charmed Baryon Spectroscopy from Lattice QCD with $N_f=2+1+1$ flavors

We present preliminary results of the calculation of the positive-parity ground state charm baryon spectrum using of N_f=2+1+1$ dynamical quarks. The calculation uses a relativistic heavy-quark action for the valence charm quark, clover-Wilson fermions for the valence light quarks and HISQ sea quarks. The spectrum is calculated with a lightest pion mass around 220 MeV and two lattice spacings ($a\sim0.12$ fm and 0.09 fm) are used to extrapolate to continuum limit. Our preliminary results are consistent with the currently measured baryon spectrum, except for the isospin-averaged $J=1/2$ $Ξ_{cc}$ which is approximately $2 σ$ above the SELEX observed value. We predict the yet-to-be-discovered double and triple-charm baryons $Ξ_{cc}^*$, $Ω_{cc}$, $Ω_{cc}^*$, $Ω_{ccc}$ to have masses 3665(42)(29) MeV, 3694(40)(45) MeV, 3739(35)(21) MeV and 4782(24)(28) MeV, respectively.

hep-lat↗

arXiv2011

Comment on `Controversy concerning the definition of quark and gluon angular momentum' by Elliot Leader (arXiv:1101.5956, PRD 83, 096012 (2011))

It is argued by the author that the canonical form of the quark energy-momentum tensor with a partial derivative instead of the covariant derivative is the correct definition for the quark momentum and angular momentum fraction of the nucleon in covariant quantization. Although it is not manifestly gauge invariant, its matrix elements in the nucleon will be non-vanishing and are gauge invariant. We test this idea in the path-integral quantization by calculating correlation functions on the lattice with a gauge-invariant nucleon interpolation field and replacing the gauge link in the quark lattice momentum operator with unity, which corresponds to the partial derivative in the continuum. We find that the ratios of three-point to two-point functions are zero within errors for both the u and d quarks, contrary to the case without setting the gauge links to unity.

hep-ph↗

arXiv2011

Probing Novel Scalar and Tensor Interactions from (Ultra)Cold Neutrons to the LHC

Scalar and tensor interactions were once competitors to the now well-established V-A structure of the Standard Model weak interactions. We revisit these interactions and survey constraints from low-energy probes (neutron, nuclear, and pion decays) as well as collider searches. Currently, the most stringent limit on scalar and tensor interactions arise from 0+ -> 0+ nuclear decays and the radiative pion decay pi -> e nu gamma, respectively. For the future, we find that upcoming neutron beta decay and LHC measurements will compete in setting the most stringent bounds. For neutron beta decay, we demonstrate the importance of lattice computations of the neutron-to-proton matrix elements to setting limits on these interactions, and provide the first lattice estimate of the scalar charge and a new average of existing results for the tensor charge. Data taken at the LHC is currently probing these interactions at the 10^-2 level (relative to the standard weak interactions), with the potential to reach the 10^-3 level. We show that, with some theoretical assumptions, the discovery of a charged spin-0 resonance decaying to an electron and missing energy implies a lower limit on the strength of scalar interactions probed at low energy.

hep-ph↗

arXiv2011

Beyond the Standard Model with Precision Nucleon Matrix Elements on the Lattice

Precision measurements of nucleons provide constraints on the Standard Model and can discern the signatures predicted for particles beyond the Standard Model (BSM). Knowing the Standard Model inputs to nucleon matrix elements will be necessary to constrain the couplings of dark matter candidates such as the neutralino, to relate the neutron electric dipole moment to the CP-violating theta parameter, or to search for new TeV-scale particles though non-$V-A$ interactions in neutron beta decay. However, these matrix elements derive from the properties of quantum chromodynamics (QCD) at low energies, where perturbative treatments fail. Using lattice gauge theory, we can nonperturbatively calculate the QCD path integral on a supercomputer. In this proceeding, I will discuss a few representative areas in which lattice QCD (LQCD) can contribute to the search for BSM physics, emphasizing suppressed operators in neutron decay, and outline prospects for future development.

hep-lat↗

arXiv2011

Roper Properties on the Lattice: An Update

In this proceeding, we give an overview of the latest progress in lattice-QCD calculations of the Roper resonance, and Roper-nucleon electromagnetic and axial transition form factors using dynamical gauge ensembles.

hep-lat↗

arXiv2011

Review of Baryon Spectroscopy in Lattice QCD

The complex patterns of the hadronic spectrum have puzzled physicists since the early discovery of the "particle zoo" in the 1960s. Today, the properties of these myriad particles are understood to be the result of quantum chromodynamics (QCD) with some modification by the electroweak interactions. Despite the discovery of this fundamental theory, the description of the hadronic spectrum has long been dominated by phenomenological models, due to the difficulties of addressing QCD in the strong-coupling regime, where nonperturbative effects are essential. By making numerical calculations in discretized spacetime, lattice gauge theory enables the ab initio study of many low-energy properties of QCD. Significant efforts are underway internationally to use lattice QCD to directly compute properties of ground and excited-state baryons. Detailed knowledge of the hadronic spectrum will provide insight into the character of these states beyond what can be extracted from models. In this review, I will focus on the latest progress in lattice calculations of the $P_{11}(1440)$, the poorly known hyperon spectrum and the energies of highly-excited states of the nucleon, Delta and other light-flavor baryons. In the heavy-flavor sector, I will concentrate on recent lattice-QCD calculations of baryon masses, particularly those that make predictions concerning yet-to-be-discovered baryons, such as $Ω_{cc}$, $Ξ^\prime_b$ or triply-heavy baryons.

hep-lat↗

arXiv2011

Nucleon and Pion Form Factors from $N_f=2+1$ Anisotropic Lattices

We report a recent lattice-QCD calculation of nucleon and pion electromagnetic form factors and nucleon axial form factors, with special emphasis on large $Q^2$. Conventional lattice form-factor calculations can only reach about 2.5 GeV$^2$, but in this work the transfer momentum is pushed as large as 6 GeV$^2$. Here, we demonstrate the results on 2+1-flavor anisotropic clover lattices for the nucleon and pion, comparing with low-$Q^2$ quantities, such as Dirac and Pauli radii, anomalous magnetic moments, $g_A$ and $M_A$. Our approach can be applied to isotropic lattices and lattices with smaller lattice spacing to achieve even larger-$Q^2$ form factors. The form factors are processed to obtain transverse charge and magnetization densities across 2-dimensional impact-parameter space. These measurements could give important theoretical input to experiments, such as those of JLab's 12-GeV program, and provide insight into hadronic structure.

hep-lat↗

arXiv2010

Nucleon, Delta and Omega excited state spectra at three pion mass values

The energies of the excited states of the Nucleon, Delta and Omega are computed in lattice QCD, using two light quarks and one strange quark on anisotropic lattices. The calculations are performed at three values of the pion mass: 392(4), 438(3) and 521(3) MeV. We employ the variational method with a basis of about ten interpolating operators enabling six energies to be distinguished clearly in each irreducible representation of the octahedral group. We compare our calculations of nucleon excited states with the low-lying experimental spectrum. There is reasonable agreement for the pattern of states.

hep-lat↗