Axial Current Operators Research Articles

Renormalization of proton-proton fusion in chiral effective field theory

Renormalization of proton-proton fusion is studied in the framework of chiral effective field theory. Strict perturbative treatment of subleading corrections is applied in the analysis. Possible enhancement of two-nucleon contact axial current operators is the focus of the study. We find evidence that supports a previous proposal in the literature to promote one of the contact axial current operators.

The [formula omitted] renormalization constant of the singlet axial current operator at [formula omitted] in QCD

We provide the MS‾ factor of the renormalization constant of the singlet axial-current operator in dimensional regularization at O(αs5) in perturbative Quantum Chromodynamics (QCD). The result is obtained from a formula derived using the Adler-Bell-Jackiw equation in terms of renormalized operators. The required input consists of the complete four-loop results for operator renormalization constants obtained by explicit diagrammatic computation in a previous publication.

Renormalization of the axial current operator in dimensional regularization at four-loop in QCD

We provide the renormalization constants of the axial current operators, both singlet and non-singlet, in dimensional regularization up to four-loop order in (QCD), determined using the off-shell Ward-Takahashi identity for an axial current with a non-anticommuting γ5. A possible application of the result for the singlet axial current operator is the extraction of the non-decoupling mass logarithms in the axial quark form factors.

Microscopic Theory of Electrically Induced Spin Torques in Magnetic Weyl Semimetals

We theoretically study current- and charge-induced spin torques in magnetic Weyl semimetals (WSMs). In magnetic WSMs, a topologically nontrivial band structure mediates the anomalous coupling between magnetization and transport, making WSMs preferable for spintronics systems. In this study, we determine all current-induced spin torques including spin–orbit torque and spin-transfer torque, up to the first order with respect to spatial and temporal derivatives and electrical currents. We also calculate the charge-induced spin torque microscopically. We find that the charge-induced spin torque originates from the chiral anomaly due to the correspondence between spin operators and axial current operators in our model.

Box diagram contribution to the axial two-nucleon current

Recently, we have worked out the axial two-nucleon current operator to leading one-loop order in chiral effective field theory using the method of unitary transformation. Our final expressions, however, differ from the ones derived by the JLab-Pisa group using time-ordered perturbation theory (Phys. Rev. C 93, no. 1, 015501 (2016) Erratum: [Phys. Rev. C 93, no. 4, 049902 (2016)] Erratum: [Phys. Rev. C 95, no. 5, 059901 (2017)]). In this paper we consider the box diagram contribution to the axial current and demonstrate that the results obtained using the two methods are unitary equivalent at the Fock-space level. We adjust the unitary phases by matching the corresponding two-pion exchange nucleon-nucleon potentials and rederive the box diagram contribution to the axial current operator following the approach of the JLab-Pisa group, thereby reproducing our original result. We provide a detailed information on the calculation including the relevant intermediate steps in order to facilitate a clarification of this disagreement.

Nuclear Electromagnetic Currents to Fourth Order in Chiral Effective Field Theory

Recently, we have shown that the continuity equation for the nuclear vector and axial current operators acquires additional terms if the latter depend on the energy transfer. We analyze in detail the electromagnetic single-nucleon four-current operators and verify the validity of the modified continuity equation for all one- and two-nucleon contributions up to fourth order in the chiral expansion. We also derive, for the first time, the leading contribution to the three-nucleon charge operator which appears at this order. Our study completes the derivation of the electroweak nuclear currents to fourth order in the chiral expansion.

Quasinormal modes of charged magnetic black branes & chiral magnetic transport

We compute quasinormal modes (QNMs) of the metric and gauge field perturbations about black branes electrically and magnetically charged in the Einstein-Maxwell-Chern-Simons theory. By the gauge/gravity correspondence, this theory is dual to a particular class of field theories with a chiral anomaly, in a thermal charged plasma state subjected to a constant external magnetic field, B. The QNMs are dual to the poles of the two-point functions of the energy-momentum and axial current operators, and they encode information about the dissipation and transport of charges in the plasma. Complementary to the gravity calculation, we work out the hydrodynamic description of the dual field theory in the presence of a chiral anomaly, and a constant external B. We find good agreement with the weak field hydrodynamics, which can extend beyond the weak B regime into intermediate regimes. Furthermore, we provide results that can be tested against thermodynamics and hydrodynamics in the strong B regime. We find QNMs exhibiting Landau level behavior, which become long-lived at large B if the anomaly coefficient exceeds a critical magnitude. Chiral transport is analyzed beyond the hydrodynamic approximation for the five (formerly) hydrodynamic modes, including a chiral magnetic wave.

Nuclear axial current operators to fourth order in chiral effective field theory

We present the complete derivation of the nuclear axial charge and current operators as well as the pseudoscalar operators to fourth order in the chiral expansion relative to the dominant one-body contribution using the method of unitary transformation. We demonstrate that the unitary ambiguity in the resulting operators can be eliminated by the requirement of renormalizability and by matching of the pion-pole contributions to the nuclear forces. We give expressions for the renormalized single-, two- and three-nucleon contributions to the charge and current operators and pseudoscalar operators including the relevant relativistic corrections. We also verify explicitly the validity of the continuity equation.

Electromagnetic and Axial Form Factors of Octet and Decuplet Baryons

We report from a study of the elastic electromagnetic and axial form factors of all lowest baryon states with flavors up, down, and strange along relativistic constituent-quark models. We consider the baryons as relativistic bound states of three constituent quarks and solve the eigenvalue problem of the invariant mass operator. The corresponding eigenstates are employed to calculate manifestly covariant form factors within the point form of Poincare-invariant quantum mechanics. The electromagnetic and axial current operators are constructed along the spectator model in point-form relativistic dynamics. We have thus obtained covariant predictions for the electroweak form factors, for momentum transfers up to Q2 ~ 4 GeV2, as well as the electric radii, magnetic moments, and axial charges. The theoretical results in general agree very well with existing phenomenological data. In cases, where no experimental information is yet available, the results are well compatible with data from lattice quantum chromodynamics.

Weak transitions inA=6and 7 nuclei

The ${}^{6}\mathrm{He}$ $\ensuremath{\beta}$ decay and ${}^{7}\mathrm{Be}$ electron capture processes are studied using variational Monte Carlo wave functions, derived from a realistic Hamiltonian consisting of the Argonne ${v}_{18}$ two-nucleon and Urbana-IX three-nucleon interactions. The model for the nuclear weak axial current includes one- and two-body operators with the strength of the leading two-body term---associated with $\ensuremath{\Delta}$-isobar excitation of the nucleon---adjusted to reproduce the Gamow-Teller matrix element in tritium $\ensuremath{\beta}$ decay. The measured half-life of ${}^{6}\mathrm{He}$ is underpredicted by theory by $\ensuremath{\simeq}8%,$ while that of ${}^{7}\mathrm{Be}$ for decay into the ground and first excited states of ${}^{7}\mathrm{Li}$ is overpredicted by $\ensuremath{\simeq}9%.$ However, the experimentally known branching ratio for these latter processes is in good agreement with the calculated value. Two-body axial current contributions lead to a $\ensuremath{\simeq}1.7%$ (4.4%) increase in the value of the Gamow-Teller matrix element of ${}^{6}\mathrm{He}$ ${(}^{7}\mathrm{Be}),$ obtained with one-body currents only, and slightly worsen (appreciably improve) the agreement between the calculated and measured half-life. Corrections due to retardation effects associated with the finite lepton momentum transfers involved in the decays, as well as contributions of suppressed transitions induced by the weak vector charge and axial current operators, have also been calculated and found to be negligible.

Hard-soft renormalization and the exact renormalization group

The Wilsonian exact renormalization group gives a natural framework in which ultraviolet and infrared divergences can be treated separately. In massless QED we introduce, as the only mass parameter, a renormalization scale Λ R 0. We prove, using the flow equation technique, that infrared convergence is a necessary consequence of any zero-momentum renormalization condition at Λ R compatible with the effective Ward identities and axial symmetry. The same formalism is applied to renormalize gauge-invariant composite operators and to prove their infrared finiteness; in particular we consider the case of the axial current operator and its anomaly.

Axial current conservation in the Bethe-Salpeter approach to the nuclear two-body problem.

We investigate the structure of the one- and two-nucleon axial-current operators necessary for the partial conservation of the nuclear axial-current elastic matrix element in the one-boson-exchange approximation to the two-nucleon Bethe-Salpeter equation. We use three models for this purpose: (a) the linear sigma model, (b) the nonlinear sigma model, (c) a hybrid model, which is, roughly speaking, a linear combination of (a) and (b). We construct a partially conserved nuclear axial-current elastic matrix element in models (a) and (c) provided that the associated nuclear wave functions are solutions to the Bethe-Salpeter equation with a potential made of one-boson-exchange diagrams. In the nonlinear sigma model the nuclear one-body axial current is partially conserved by itself, without reference to the nuclear wave function, whereas the two-body axial current partial conservation is violated by terms of order 1/${\mathit{f}}_{\mathrm{\ensuremath{\pi}}}^{2}$. The complete axial current in models (a) and (c) and the one-body axial current in model (b) are applicable to the construction of the deuteron electroweak process amplitudes, for example. The divergence of the axial-current matrix element is proportional to the pion absorption nuclear matrix element, which leads to another potential application in the foundation of chiral perturbation theory for pion-two-nucleon processes. Consistency between the nuclear axial-currents and the underlying nuclear dynamics in models (a) and (c) is a new condition imposed by the partial conservation of the nuclear axial-current matrix element. We also examine conditions imposed on the form of the nucleon self-energy by the nucleon and meson axial Ward-Takahashi identities, as well as the approximations that satisfy the said conditions. We show that besides the first Born approximation, the so-called Hartree+random-phase approximation satisfies chiral Ward-Takahashi idenitities in models (a) and (c). \textcopyright{} 1996 The American Physical Society.

The Contribution to Δs and Δg from the Hybrid Components of a proton11The project partially supported by National Natural Science Foundation of China

The hybrid components of a pro ton can give rise to direct contribution to Δg due to the gluon operator and Δs due to the axial current operator as well. We evaluate such contribution precisely within the framework of the bag model, but the result shows that the contribution is not significant.

Muon capture in deuterium and the meson exchange current effect

One-meson exchange current effect in reaction μ − + d → 2 n + v μ is calculated by using the weak axial current operator which satisfies the nuclear continuity equation up to the order 1 M 2 ( M is the nucleon mass). In the vector part of the weak nuclear interaction the vector-isovector pion exchange currents are also included. Their contribution is found to be non-negligible. The nuclear wave functions are generated from the Paris, Reid soft-core and Bonn potential models. Our result is compatible with the previous one, obtained with only the static axial exchange currents included.

Partial conservation of axial-vector current and nuclear beta decay

The disagreement pointed out by Armstrong and Kim between the elementary particle approach and the impulse-approximation predictions for $\ensuremath{\Delta}{J}^{P}={0}^{\ensuremath{-}}$ nuclear $\ensuremath{\beta}$ decay is examined from the point of view of the axial current operator. Naive theoretical predictions are found to be in disagreement with present data.[RADIOACTIVITY Discuss PCAC in $\ensuremath{\beta}$ decay; calculated spectra for ${0}^{+}$-${0}^{\ensuremath{-}}$ transition.]