Quark Magnetic Moments Research Articles

From Ds → γ in lattice QCD to Bs → μμγ at high q2

We use a recent lattice determination of the vector and axial Ds → γ form factors at high squared momentum transfer q2 to infer their Bs → γ counterparts. To this end, we introduce a phenomenological approach summarized as follows. First, we describe the lattice data with different fit templates motivated by vector-meson dominance, that is expected to hold in the high-q2 region considered. We identify reference fit ansaetze with one or two physical poles, that we validate against alternative templates. Then, the pole residues can be unambiguously related to the appropriate couplings involving the pseudoscalar, the vector mesons concerned, and the photon — or tri-couplings — and the latter can be expressed as sums over quark magnetic moments, weighed by their e.m. charges. This description obeys a well-defined heavy-quark scaling, that allows to parametrically scale up the form factors to the Bs → γ case. We discuss a number of cross-checks of the whole approach, whose validation rests ultimately in a first-principle determination, e.g. in lattice QCD. Finally, we use our obtained form factors to reassess the SM prediction of ℬ(Bs → μ+μ−γ) in the range sqrt{q^2}in [4.2, 5.0] GeV, where an experimental measurement is awaited.

Neutral pseudoscalar and vector meson masses under strong magnetic fields in an extended NJL model: Mixing effects

Mixing effects on the mass spectrum of light neutral pseudoscalar and vector mesons in the presence of an external uniform magnetic field $\vec B$ are studied in the framework of a two-flavor NJL-like model. The model includes isoscalar and isovector couplings both in the scalar-pseudoscalar and vector sectors, and also incorporates flavor mixing through a 't Hooft-like term. Numerical results for the $B$ dependence of meson masses are compared with present lattice QCD results. In particular, it is shown that the mixing between pseudoscalar and vector meson states leads to a significant reduction of the mass of the lightest state. The role of chiral symmetry and the effect of the alignment of quark magnetic moments in the presence of the magnetic field are discussed.

New probes for axionlike particles at hadron colliders

Axion-like particles (ALPs) appear from spontaneous global symmetry breaking in many extensions of the Standard Model (SM). In this paper, we find bounds on ALP ($a$) model parameters at the LHC from the ALP production associated with a photon and a jet ($j+\gamma+a$) as well as single top and top quark pairs ($t+j+a$, $t\bar{t}+a$) in a model independent approach. In particular, it is shown that the ALP production associated with a photon plus a jet at the LHC is a promising channel with significant sensitivity to probe the ALP couplings to gluons and electroweak gauge bosons. The prospects are presented at the High Luminosity LHC including a realistic detector simulation and pile up effects. Furthermore, the ALP model is examined through its contributions to the top quark (chromo)magnetic dipole moments. It is shown that the top quark magnetic and chromomagnetic dipole moments enable us to probe the ALP couplings to top quark and gauge bosons at a time. The constraints are complementary to those obtained from direct searches, as they are sensitive to light ALPs.

Modification of meson properties in the vicinty of nuclei

We suggest that modification of meson properties (lifetimes and branching ratios) can occur due to the interaction of constituent quark magnetic moments with strong magnetic fields present in the close vicinity of nuclei. A superposition of (J =0) and (J =1, mz =0) particle-antiparticle quantum states (as observed for ortho-Positronium) may occur also in the case of quarkonium states J/Ψ, ηc ϒ, ηb in heavy ion collisions. We speculate on possible modification of η(548) meson properties (related to C parity and CP violation) in strong magnetic fields which are present in the vicinity of nuclei.

Magnetic moments of octet baryons, angular momenta of quarks, and sea antiquark polarizations

One can determine antiquark polarizations in proton using the information from deep inelastic scattering, $\beta$ decays of baryons, orbital angular momenta of quarks, as well as their integrated magnetic distributions. The last quantities were determined previously by us performing a fit to magnetic moments of baryon octet. However, because of the SU(3) symmetry our results depend on two parameters. The quantity $\Gamma_{V}$, measured recently in a COMPASS experiment, gives the relation between these parameters. We can fix the last unknown parameter using the ratio of up and down quark magnetic moments which one can get from the fit to radiative vector meson decays. We calculate antiquark polarizations with the orbital momenta of valence quarks that follow from lattice calculations. The value of difference for up and down antiquark polarizations obtained in our calculations is consistent with the result obtained in a HERMES experiment.

New quark relations for hadron masses and magnetic moments: A challenge for explanation from QCD

Prompted by the recent surprising results in QCD spectroscopy, we extend the treatment of the constituent quark model showing that mass differences and ratios have the same values when obtained from mesons and baryons. We obtain several new successful relations involving hadrons containing two and three strange quarks and hadrons containing heavy quarks and give a new prediction regarding spin splitting between doubly charmed baryons. We provide numerical evidence for an effective supersymmetry between mesons and baryons related by replacing a light antiquark by a light diquark. We also obtain new relations between quark magnetic moments and hadron masses. Limits of validity of this approach and disagreements with experiment in properties of the Σ and Ξ baryons are discussed as possible clues to a derivation from QCD.

Magnetic moments of octet baryons and sea antiquark polarizations

Using generalized Sehgal equations for magnetic moments of baryon octet and taking into account ${\ensuremath{\Sigma}}^{0}\ensuremath{-}\ensuremath{\Lambda}$ mixing and two particle corrections to independent quark contributions we obtain very good fit using experimental values for errors of such moments. We present sum rules for quark magnetic moments ratios and for integrated spin densities ratios. Because of the $SU(3)$ structure of our equations the results for magnetic moments of quarks and their densities depend on two additional parameters. Using information from deep inelastic scattering and baryon $\ensuremath{\beta}$-decays we discuss the dependence of antiquark polarizations on introduced parameters. For some plausible values of these parameters we show that these polarizations are small if we neglect angular momenta of quarks. Our very good fit to magnetic moments of baryon octet can still be improved by using specific model for angular momentum of quarks.

The strangeness magnetic moment of the nucleon from FLIC fermions

By imposing the constraints of charge symmetry we show that the strangeness magnetic moment of the nucleon can be expressed in terms of empirical magnetic moments and ratios of valence quark magnetic moments. The latter are determined using modern chiral extrapolation techniques and recent low mass lattice QCD simulations of the individual quark contributions to the magnetic moments of the nucleon octet. The result is a precise determination of G_M^s, namely -0.043 +/- 0.026 mu_N, which is consistent with the latest experimental measurements.

On calculation of the neutron charge radius

We show that the anomalous quark magnetic moments and relativistic effects in the nucleon wave function result in the correct value of the neutron charge radius.

Magnetic moments of the 3/2 resonances and their quark spin structure.

We discuss magnetic moments of the $J=\frac{3}{2}$ baryons based on an earlier model for the baryon magnetic moments, allowing for flavor symmetry breaking in the quark magnetic moments as well as a general quark spin structure. From our earlier analysis of the nucleon-hyperon magnetic moments and the measured values of the magnetic moments of ${\ensuremath{\Delta}}^{++}$ and ${\ensuremath{\Omega}}^{\ensuremath{-}}$ we predict the other magnetic moments and deduce the spin structure of the resonance particles. We find from experiment that the total spin polarization of the decuplet baryons, $\ensuremath{\Delta}\ensuremath{\Sigma}(\frac{3}{2})$, is considerably smaller than the nonrelativistic quark model value of 3, although the data are still not good enough to give a precise determination.

Essential strangeness in nucleon magnetic moments

Effective quark magnetic moments are extracted from experimental measurements as a function of the strangeness magnetic moment of the nucleon. Assumptions made in even the most general quark model analyses are ruled out by this investigation. Ab initio QCD calculations demand a non-trivial role for strange quarks in the nucleon. The effective moments from QCD calculations are reproduced for a strangeness magnetic moment contribution to the proton of 0.11 μ N , which corresponds to F 2 5(0) = −0.33 μ N .

Flavor symmetry breaking in quark magnetic moments

We discuss the magnetic moments of the baryons allowing for flavor symmetry breaking in the quark magnetic moments. We show that there is a correlation between isospin symmetry breaking and data for the nucleon spin structure obtained from deep inelastic scattering. For small values of the isospin symmetry breaking, of the order of $5\%$, the magnetic moments and weak axial-vector form factors alone indicate a value of the spin polarization $\Delta\Sigma \leq 0.20$. Larger values of the spin polarization are compatible only with large isospin symmetry breaking. We also calculate weak axial-vector form factors, which are independent of the symmetry breakings, from magnetic moment data and find good agreement with experiment.

Decuplet baryon structure from lattice QCD.

The electromagnetic properties of the SU(3)-flavor baryon decuplet are examined within a lattice simulation of quenched QCD. Electric charge radii, magnetic moments, and magnetic radii are extracted from the $E0$ and $M1$ form factors. Preliminary results for the $E2$ and $M3$ moments are presented, giving the first model-independent insight to the shape of the quark distribution in the baryon ground state. As in our octet-baryon analysis, the lattice results give evidence of spin-dependent forces and mass effects in the electromagnetic properties. The quark charge distribution radii indicate these effects act in opposing directions. Some baryon dependence of the effective quark magnetic moments is seen. However, this dependence in decuplet baryons is more subtle than that for octet baryons. Of particular interest are the lattice predictions for the magnetic moments of ${\ensuremath{\Omega}}^{\ensuremath{-}}$ and ${\ensuremath{\Delta}}^{++}$ for which new recent experimental measurements are available. The lattice prediction of the $\frac{{\ensuremath{\Delta}}^{++}}{p}$ ratio appears larger than the experimental ratio, while the lattice prediction for the $\frac{{\ensuremath{\Omega}}^{\ensuremath{-}}}{p}$ magnetic moment ratio is in good agreement with the experimental ratio.

Generalized SU(3) Nambu-Jona-Lasinio model: (II). From current to constituent quarks

We investigate the properties of constituent quarks, i.e. quarks dressed by their strong interactions, in a generalized Nambu-Jona-Lasinio model with N f = 3 flavours. In the Hartree-Fock approximation, the step from structureless current quarks to massive constituent quarks is made through dynamical mass generation which implies spontaneous chiral symmetry breaking. We study, in particular, the quark scalar, vector and axial vector currents within this framework. We demonstrate that, at low energy and momentum transfers, single valence quarks are strongly screened by quark-antiquark polarization effects. For the electromagnetic currents, we recover relationships familiar from the vector meson dominance model. For the axial current, screening by J π = 1 + quark-antiquark modes leads to an effective quark axial vector constant g A < 1 which satisfies the Goldberger-Treiman relation at the quark level. The spin content of the proton is also discussed in this framework. We calculate quark magnetic moments and electromagnetic form factors and discuss their related sizes.

The magnetic moments of baryons and the spin of the proton

Under reasonable assumptions we derive from the proton electromagnetic form factors those of the quark vector currents separately. Then we deduce the quark radii and magnetic moments in the proton and find a surprisingly large strangeness contribution. Using a constituent quark model, we relate the quark magnetic moments to the spin of the proton and confirm previous conclusions that it is not carried completely by quarks. If we extend the role of s-quarks in the proton to the SU(3) F octet then we predict the magnetic moments of the other baryons.

The magnetic moments of baryons and the spin of the proton

Under reasonable assumptions we derive from the proton electromagnetic form factors those of the quark vector currents separately. Then we deduce the quark radii and magnetic moments in the proton and find a surprisingly large strangeness contribution. Using a constituent quark model, we relate the quark magnetic moments to the spin of the proton and confirm previous conclusions that it is not carried completely by quarks. If we extend the role of s-quarks in the proton to the SU(3) F octet then we predict the magnetic moments of the other baryons.

Baryon magnetic moments with confined quarks.

Within a rather general framework for confined quarks in a baryon, we derive an approximate expression for the effective quark masses and quark magnetic moments. Use of the additive quark model with the spin-flavor SU(6) wave functions may be justified for the relativistic quarks and lead to a good agreement between theory and experiment for the baryon magnetic moments.

Magnetic moments of quarks and leptons as three spin-1/2 composites

Magnetic moments of quarks and leptons built out of three spin-1/2 preons are calculated in the nonrelativistic approximation. It is shown that the Harariet al. model predicts the proton-neutron magnetic-moment ratio to be the same as in the staticSU(6) quark model for baryons,i.e. μp/μn=-3/2. The Terezawaet al. model gives the proton-neutron magnetic-moment ratio in terms of masses and charges of the preons. The masses and charges of the preons can be chosen to give μp/μn=-3/2.

Angular Distributions in Z Decays via Resonances of Quarks, Leptons and Gauge Bosons

Properties of resonances of quarks, leptons and gauge bosons are studied. An effective coupling of resonances to a quark or lepton is constrained by naturalness argument on the smallness of masses and anomalous magnetic moments of quarks and leptons induced by resonances. Special emphasis is laid on angular distributions in qq (or e+ e-)-> Z-> I lr via resonances. Our results are compared with those from the recently found llr events in the CERN SPfJS coIlide.r experiments.

Magnetic moments of top hadrons

Using the wave functions and the expressions for the magnetic moments of top hadrons, given in tables I–IV, as well as the estimates of the quark magnetic moments, the numerical values of the magnetic moments of top hadrons can be calculated.