arXiv subjects
Publications and source records attributed to M. Wakamatsu.
A model independent prediction is given for the nucleon spin contents, based only upon a reasonable theoretical postulate on the isosinglet anomalous gravitomagnetic moment of the nucleon.
With a special intention of clarifying the underlying spin contents of the nucleon, we investigate the generalized form factors of the nucleon, which are defined as the $n$-th $x$-moments of the generalized parton distribution functions, within the framework of the chiral quark soliton model. A particular emphasis is put on the pion mass dependence of final predictions, which we shall compare with the predictions of lattice QCD simulations carried out in the so-called heavy pion region around $m_π\simeq (700 \sim 900) {MeV}$. We find that some observables are very sensitive to the variation of the pion mass. It will be argued that the negligible importance of the quark orbital angular momentum indicated by the LHPC and QCDSF lattice collaborations might be true in the unrealistic heavy pion world, but it is not necessarily the case in our real world close to the chiral limit.
It is shown that, based only upon two empirically known facts besides two reasonable theoretical postulates, we are inevitably led to a model-independent conclusion that the quark orbital angular momentum carries nearly half of the total nucleon spin at the low energy scale of nonperturbative QCD. Also shown are explicit model predictions for the forward limit of the unpolarized spin-flip GPDs, which are believed to give valuable information on the distributions of quark angular momentum inside the nucleon.
It is shown that, based only on two empirically known facts besides two reasonable theoretical postulates, we are inevitably led to a conclusion that the quark orbital angular momentum carries nearly half of the total nucleon spin. We also perform a model analysis to find that the quark spin fraction $ΔΣ$ is extremely sensitive to the pion mass, which may resolve the discrepancy between the observation and the prediction of the recent lattice QCD simulation carried out in the heavy pion region.
Our recent theoretical analysis based on the flavor SU(3) chiral quark soliton model predicts fairly large particle-antiparticle asymmetry of the strange sea in the nucleon. We point out that the predicted magnitude of asymmetry is large enough to solely resolve the so-called NuTeV anomaly on the fundamental parameter $\sin^2 θ_W$ in the standard model.
The theoretical predictions are given for the forward limit of the unpolarized spin-flip isovector generalized parton distribution function $(E^u - E^d)(x, ξ, t)$ within the framework of the chiral quark soliton model, with full inclusion of the polarization of Dirac sea quarks. We observe that $[(H^u - H^d) + (E^u - E^d)](x,0,0)$ has a sharp peak around $x=0$, which we interpret as a signal of the importance of the pionic $q \bar{q}$ excitation with large spatial extension in the transverse direction. Another interesting indication given by the predicted distribution in combination with Ji's angular momentum sum rule is that the $\bar{d}$-quark carries more angular momentum than the $\bar{u}$-quark in the proton, which may have some relation with the physics of the violation of the Gottfried sum rule.
We clarify the nonperturbative origin of the $δ$-function singularity at $x = 0$ in the chiral-odd twist-3 distribution function $e(x)$ of the nucleon. We also compare a theoretical prediction for $e(x)$ based on the chiral quark soliton model with the empirical information extracted from the CLAS semi-inclusive DIS measurement.
The isosinglet combination of the chiral-odd twist-3 distribution function $e^u(x)+e^d(x)$ of the nucleon has outstanding properties that its first moment is proportional to the well-known $πN$ sigma-term and that it contains a $δ$-function singularity at $x=0$. These two features are inseparably connected in that the above sum rule would be violated, if there is no such a singularity in $e^u(x)+e^d(x)$. Very recently, we found that the physical origin of this $δ$-function singularity can be traced back to the long-range quark-quark correlation of scalar type, which signals the spontaneous chiral symmetry breaking of the QCD vacuum. The main purpose of the present paper is to give complete theoretical predictions for the chiral-odd twist-3 distribution function $e^a(x)$ of each flavor $a$ on the basis of the chiral quark soliton model, without recourse to the derivative expansion type approximation. These theoretical predictions are then compared with the empirical information extracted from the CLAS data of the semi-inclusive DIS processes by assuming the Collins mechanism only. A good agreement with the CLAS data is indicative of a sizable violation of the $πN$ sigma-term sum rule, or equivalently, the existence of a $δ$-function singularity in $e^u(x) + e^d(x)$.
We analytically prove that the existence of the delta-function singularity in the chirally-odd twist-3 distribution $e(x)$ of the nucleon is inseparably connected with the nonvanishing quark condensate as a signal of the spontaneous chiral symmetry breaking of the QCD vacuum. This singularity in $e(x)$, which would be observed as a sizable violation of the 1st moment sum rule, is then interpreted as giving a very rare case that the nontrivial vacuum structure of QCD manifests in an observable of a localized QCD excitation, i.e. the nucleon.
The path integral formulation is given to obtain quark and antiquark distribution functions in the nucleon within the flavor SU(3) version of the chiral quark soliton model. The basic model action is a straightforward generalization of the corresponding SU(2) one, except for one distinguishable feature, i.e. the presence of the SU(3) symmetry breaking term arising from the sizably large mass difference $Δm_s$ between the strange and nonstrange quarks. We treat this SU(3) symmetry breaking effect by relying upon the first order perturbation theory in the mass parameter $Δm_s$. We also address to the problem of ordering ambiguity of the relevant collective space operators, which arises in the evaluation of the parton distribution functions at the subleading order of $1/N_c$ expansion.
Theoretical predictions are given for the light-flavor sea-quark distributions including the strange quark ones on the basis of the flavor SU(3) version of the chiral quark soliton model. Careful account is taken here of the SU(3) symmetry breaking effects due to the mass difference between the strange and nonstrange quarks. This effective mass difference $Δm_s$ between the strange and nonstrange quarks is the only one parameter necessary for the flavor SU(3) generalization of the model. A particular emphasis of study is put on the {\it light-flavor sea-quark asymmetry} as exemplified by the observables $\bar{d} (x) - \bar{u} (x), \bar{d} (x) / \bar{u} (x), Δ\bar{u} (x) - Δ\bar{d} (x)$ as well as on the {\it particle-antiparticle asymmetry} of the strange quark distributions represented by $s (x) - \bar{s} (x), s (x) / \bar{s} (x), Δs (x) - Δ\bar{s} (x)$ etc. As for the unpolarized sea-quark distributions, the predictions of the model seem qualitatively consistent with the available phenomenological information provided by the NMC data for $\bar{d} (x) - \bar{u} (x)$, the E866 data for $\bar{d} (x) / \bar{u} (x)$, the CCFR data and Barone et al.'s fit for $s (x) / \bar{s} (x)$ etc. The model is shown to give several unique predictions also for the spin-dependent sea-quark distribution, such that $Δs (x) \ll Δ\bar{s}(x) \lesssim 0$ and $Δ\bar{d}(x) < 0 < Δ\bar{u}(x)$, although the verification of these predictions must await more elaborate experimental investigations in the near future.
Theoretical predictions are given for the strange sea distributions in the nucleon based on the flavor SU(3) chiral quark soliton model, with emphasis upon the asymmetry of quark and antiquark distributions. We find that the quark-antiquark asymmetry of the strange sea is much larger for the longitudinally polarized distribution functions than for the unpolarized ones. A preliminary comparison with the CCFR data for the unpolarized $s$-quark distribution and with the LSS fits of the longitudinally polarized distribution functions is encouraging.
The recent measurements of azimuthal single spin asymmetries by the HERMES collaboration has opened up new possibility to measure chiral-odd distribution functions through semi-inclusive deep-inelastic scatterings. Here, predictions are given for the twist-2 and twist-3 chiral-odd distribution functions of the nucleon within the framework of the chiral quark soliton model, with full inclusion of the vacuum polarization effects as well as the subleading $1/N_c$ corrections. The importance of the vacuum polarization effects is demonstrated by showing that the so-called Soffer inequality holds not only for the quark distributions but also for the antiquark ones.
An incomparable feature of the chiral quark soliton model as compared with many other effective models like the MIT bag model is that it can give reasonable predictions not only for quark distributions but also for antiquark distributions. This will be exemplified by the argument on the positivity constraint for $\bar{u} (x) + \bar{d} (x)$ as well as the Soffer inequality for quark and antiquark distributions. We also explain how the model can resolve the so-called nucleon spin puzzle without assuming large gluon polarization at the low energy scale.
The theoretical predictions of the chiral quark soliton model for the unpolarized and longitudinally polarized structure functions of the nucleon are compared with recent high energy data. The theory is shown to explain all the qualitatively noticeable features of the existing experiments, including the light flavor sea-quark asymmetry for the unpolarized distribution functions established by the NMC measurement as well as very small quark spin fraction of the nucleon indicated by the EMC measurement. Another unique feature of the model is that it predicts sizably large isospin asymmetry also for the spin-dependent sea-quark distribution functions.
The spin-dependent structure functions g_2 (x, Q^2) are investigated within the framework of the chiral quark soliton model. It turns out that the twist-3 part of g_2 (x, Q^2) gives nonnegligible contributions to the total distributions at the energy scale of Q^2 = 5 {GeV}^2 but mainly in the smaller x region only, so that the corresponding third moments \int_0^1 x^2 \bar{g}_2 (x, Q^2) dx are pretty small for both of the proton and neutron in conformity with the recent E155 data.
A theoretical prediction is given for the spin and orbital angular momentum distribution functions of the nucleon within the framework of an effective quark model of QCD, i.e. the chiral quark soliton model. An outstanding feature of the model is that it predicts fairly small quark spin fraction of the nucleon $ΔΣ\simeq 0.35$, which in turn dictates that the remaining 65% of the nucleon spin is carried by the orbital angular momentum of quarks and antiquarks at the model energy scale of $Q^2 \simeq 0.3 {GeV}^2$. This large orbital angular momentum necessarily affects the scenario of scale dependence of the nucleon spin contents in a drastic way.
After taking account of the scale dependence by means of the standard DGLAP evolution equation, the theoretical predictions of the chiral quark soliton model for the unpolarized and longitudinally polarized structure functions of the nucleon are compared with the recent high energy data. The theory is shown to explain all the qualitative features of the experiments, including the NMC data for $F_2^p (x) - F_2^n (x)$, $F_2^n (x) / F_2^p (x)$, the Hermes and NuSea data for $\bar{d}(x) - \bar{u}(x)$, the EMC and SMC data for $g_1^p(x)$, $g_1^n(x)$ and $g_1^d(x)$. Among others, flavor asymmetry of the longitudinally polarized sea-quark distributions is a remarkable prediction of this model, i.e., it predicts that $Δ\bar{d}(x) - Δ\bar{u}(x) = C x^α [ \bar{d}(x) - \bar{u}(x)]$ with a sizable negative coefficient $C \simeq -2.0$ (and $α\simeq 0.12$) in qualitative consistency with the recent semi-phenomenological analysis by Morii and Yamanishi.