Quark Magnetic Moment Research Articles

Quark and proton anomalous magnetic moments in confining models

We compute the one-loop quark-photon vertex and the quark magnetic moment in three different models with an infrared-modified gluon propagator, namely: the massive (Curci-Ferrari-like) model, the Gribov-Zwanziger model, and the refined Gribov-Zwanziger model. We show results for the F2 form factor and analyze the role played by the mass parameters associated with the confined gluon in these models. Using the framework of the constituent quark model, we further construct the observable proton magnetic moment including the effects of quantum chromodynamics (QCD) interactions generated by the different confining models. Our results show that there is no qualitative observable difference stemming from the presence of complex-conjugated poles. On the other hand, quantitative differences between the various confining models can be sizable, so that observable constraints would in principle be possible, provided one has sufficient information about the constituent quark mass and the QCD running coupling. Published by the American Physical Society 2024

Decuplet baryons in nuclear and hyperonic medium

Masses and the magnetic moments of baryon decuplet are evaluated in the symmetric nuclear and hyperonic matter at finite temperature using a chiral SU(3) quark mean field model approach. The decuplet baryon masses considerably decrease with the rise in the baryonic density of the medium. With the increase in strangeness (more number of hyperons as compared to nucleons) fraction, the non-strange baryons show an increase in their masses, whereas the strange baryons show a decrease. The contributions coming from the valence quarks, quark sea and the orbital angular momentum of the quark sea have been considered to calculate net magnetic moment. The magnetic moments of decuplet baryons are found to show a considerable increase with the baryonic density of the medium since constituent quark magnetic moment and the quark spin polarizations show significant variation in the nuclear medium especially in the low temperature and baryonic density regime. The increase is however quantitatively less as compared to the case of octet baryon members. With the inclusion of hyperons along with nucleons, the non-strange baryons show a decrease in the magnitude of their effective magnetic moments, whereas the strange baryons show an increase. The strangeness fraction of the medium is found to largely affect the valence quark magnetic moment and quark sea magnetic moment of the baryons.

Dynamically generated magnetic moment in the Wigner-function formalism

We study how the mass and magnetic moment of the quarks are dynamically generated in nonequilibrium quark matter. We derive the equal-time transport and constraint equations for the quark Wigner function in a magnetized quark model and solve them in the semi-classical expansion. The quark mass and magnetic moment are self-consistently coupled to the Wigner function and controlled by the kinetic equations. While the quark mass is dynamically generated at the classical level, the quark magnetic moment is a pure quantum effect, induced by the quark spin interaction with the external magnetic field.

Strange Quark Magnetic Moment of the Nucleon at the Physical Point.

We report a lattice QCD calculation of the strange quark contribution to the nucleon's magnetic moment and charge radius. This analysis presents the first direct determination of strange electromagnetic form factors including at the physical pion mass. We perform a model-independent extraction of the strange magnetic moment and the strange charge radius from the electromagnetic form factors in the momentum transfer range of 0.051 GeV^{2}≲Q^{2}≲1.31 GeV^{2}. The finite lattice spacing and finite volume corrections are included in a global fit with 24 valence quark masses on four lattices with different lattice spacings, different volumes, and four sea quark masses including one at the physical pion mass. We obtain the strange magnetic moment G_{M}^{s}(0)=-0.064(14)(09)μ_{N}. The four-sigma precision in statistics is achieved partly due to low-mode averaging of the quark loop and low-mode substitution to improve the statistics of the nucleon propagator. We also obtain the strange charge radius ⟨r_{s}^{2}⟩_{E}=-0.0043(16)(14) fm^{2}.

Radiative decays of light vector mesons in Poincare invariant quantum mechanics

This work dedicated to describing two-body compounded system of quarks (meson) in point form of Poincare invariant quantum mechanics with potential, which was offered by so-called Mock-meson model. Authors shows process of calculation the basis parameters of the model by the variational method and followed applications for radiative decay processes. For such calculation authors use the simplest radiative decay scheme. Comparing results with experimental data was shown how to calculate the anomalous part of quark magnetic moment.

Hindered M1 radiative decay ofϒ(2S)from lattice NRQCD

We present a calculation of the hindered M$1$ $\Upsilon(2S) \to \eta_b(1S) \gamma$ decay rate using lattice non-relativistic QCD. The calculation includes spin-dependent relativistic corrections to the NRQCD action through $\mathcal{O}(v^6)$ in the quark's relative velocity, relativistic corrections to the leading order current which mediates the transition through the quark's magnetic moment, radiative corrections to the leading spin-magnetic coupling and for the first time a full error budget. We also use gluon field ensembles at multiple lattice spacing values, all of which include $u$, $d$, $s$ and $c$ quark vacuum polarisation. Our result for the branching fraction is $\mathcal{B}(\Upsilon(2S)\to\eta_b(1S)\gamma) = 5.4(1.8)\times 10^{-4} $, which agrees with the current experimental value.

Magnetic field, chiral transition and Pauli interaction

The possibility that the Pauli interaction could influence the critical temperature of chiral transition is investigated. We work within the Nambu-Jona–Lasinio model at the mean field level, with quark anomalous magnetic moment as a parameter.

Higher-order multipole amplitudes in charmonium radiative transitions

Using 24×106 ψ′≡ψ(2S) decays in CLEO-c, we have searched for higher multipole admixtures in electric-dipole-dominated radiative transitions in charmonia. We find good agreement between our data and theoretical predictions for magnetic quadrupole (M2) amplitudes in the transitions ψ′→γχc1,c2 and χc1,c2→γJ/ψ, in striking contrast to some previous measurements. Let b2J and a2J denote the normalized M2 amplitudes in the respective aforementioned decays, where the superscript J refers to the angular momentum of the χcJ. By performing unbinned maximum likelihood fits to full five-parameter angular distributions, we found the following values of M2 admixtures for Jχ=1: a2J=1=(−6.26±0.63±0.24)×10−2 and b2J=1=(2.76±0.73±0.23)×10−2, which agree well with theoretical expectations for a vanishing anomalous magnetic moment of the charm quark. For Jχ=2, if we fix the electric octupole (E3) amplitudes to zero as theory predicts for transitions between charmonium S states and P states, we find a2J=2=(−9.3±1.6±0.3)×10−2 and b2J=2=(1.0±1.3±0.3)×10−2. If we allow for E3 amplitudes we find, with a four-parameter fit, a2J=2=(−7.9±1.9±0.3)×10−2, b2J=2=(0.2±1.4±0.4)×10−2, a3J=2=(1.7±1.4±0.3)×10−2, and b3J=2=(−0.8±1.2±0.2)×10−2. We determine the ratios a2J=1/a2J=2=0.67−0.13+0.19 and a2J=1/b2J=1=−2.27−0.99+0.57, where the theoretical predictions are independent of the charmed quark magnetic moment and are a2J=1/a2J=2=0.676±0.071 and a2J=1/b2J=1=−2.27±0.16.Received 1 October 2009DOI:https://doi.org/10.1103/PhysRevD.80.112003©2009 American Physical Society

The confining interaction and radiative decays of heavy quarkonia

The radiative spin-flip transition rates of heavy quarkonium states depend sensitively on the matrix elements of the effective confining interaction through the associated two-quark exchange current operator. The Hamiltonian model based on a scalar linear confining interaction with a single gluon exchange hyperfine term is shown to provide an adequate description of the J ψ → η cγ and ψ(2 S) → η c γ decay widths once the relativistic single quark magnetic moment operator is treated without approximation and the exchange current is taken into account. Predictions are given for the radiative spin-flip decay widths of the 1 S, 2 S and 3 S states of the c c , b b and B c + systems. In the B c + system the gluon exchange current also contributes to the spin-flip transition rates.

Extraction of the [formula omitted] coupling from [formula omitted] decays

The decays D ∗ → Dπ and D ∗ → Dγ are well described by heavy meson chiral perturbation theory. With the recent measurement of B (D ∗+ → D +γ), the D ∗0, D ∗+ and D s ∗ branching fractions can be used to extract the D ∗Dπ and D ∗Dγ couplings g and β. The D ∗ → Dγ decays receive important corrections at order √ m q and, from the heavy quark magnetic moment, at order 1/ m c . Here all the decay rates are computed to one loop, to first order in m q and 1/ m c , including the effect of heavy meson mass splittings, and the counterterms at order m q . A fit to the experimental data gives two possible solutions, g = 0.27 −0.02−0.02 +0.04+0.05, β= 0.85 −0.1 − 0.1 +0.2 + 0.3 GeV − or g = 0.76 −0.03 −0.1 +0.03+0.2, β = 4.90 −0.3−0.7 +0.3+5.0 GeV −1. The first errors are experimental, while the second are estimates of the uncertainty induced by the counterterms. (The experimental limit Γ D ∗+ < 0.13 MeV excludes the g = 0.76 solution.) Predictions for the D ∗ and B ∗ widths are given.

Baryon Magnetic Moments in Quark Models with Anomalous Quark Moments

The magnetic moments of the baryon octet are investigated within a class of nonrelativistic quark models. The class of models considered have baryon wave functions which are dominated by the usual pure S-wave, 56-dimensional representation of SU(6), but small anomalous quark moments and admixtures of nonzero orbital angular momentum are allowed. The effect of two-body exchange moments is also considered. It is found that models within this class can account for the observed moments only if the quarks have anomalous magnetic moments which are not proportional to their charges. In particular, the magnitude of the ratio of the up quark magnetic moment to the down quark magnetic moment, |μ u /μ d |, is less than 2. The allowed range of values for μ u /μ d is shown to be consistent with an estimate calculated using the chiral quark model. If exchange moments are neglected, then the baryon wave functions must all contain some admixtures of nonzero relative orbital angular momentum and they must break SU(3) flavor symmetry. The strange quark magnetic moment is more negative than the λ 0 moment. Finally, it is argued that the inclusion of small exchange moments of a particular type can significantly reduce the amount of configuration mixing needed.

The shell model approach—Key to hadron structure?

A shell model approach leads to a simple constituent quark model for hadron structure in which mesons and baryons consist only of constituent quarks. Hadron masses are the sums of the constituents quark effective masses and a hyperfine interaction inversely proportional to the product of these same masses. Hadron masses and magnetic moments are related by the assumption that the same effective mass parameter appears in the additive mass term, the hyperfine interaction, and the quark magnetic moment, both in mesons and baryons. The analysis pinpoints the physical assumptions needed for each relation and gives two new mass relations. Application to weak decays and recent polarized EMC data confirms conclusions previously obtained that the current quark contribution to the spin structure of the proton vanishes, but without need for the questionable assumption of SU(3) symmetry relating hyperon decays and proton structure. SU(3) symmetry breaking is clarified.

The constituent quark model revisited. Masses and magnetic moments

Eight relations between masses and magnetic moments of nonstrange and singly strange hadrons are btained from a simple constituent quark model in which masses are the sums of the constituent quark effective masses and a hyperfine interaction inversely proportional to the product of these same masses. The analysis pinpoints the physical assumptions needed for each relation and offers a provocative challenge for the search for a more fundamental description based on QCD with two new surprisingly successful mass relations following from the extreme assumption that the same effective mass parameter appears in the additive mass term, the hyperfine interaction, and the quark magnetic moment, both in mesons and baryons.

Quark-Moment Contributions to Baryon Magnetic Moments

Recent baryon magnetic moment measurements are used to isolate individual quark magnetic moment contributions, including nonstatic effects. The pattern of symmetry breaking observed is that quarks in spin-triplet states preserve SU(3), while quarks in spin-singlet states break SU(3). Relativistic effects are suggested as the dominant symmetry breaking mechanism. The present ..sigma../sup -/ moment determination is incompatible with this analysis.