Spin Content Research Articles

Structure of the nucleon in the unquenched quark model

We discuss the flavor asymmetry of the nucleon sea and the spin content of the proton in an unquenched quark model. It is shown that the inclusion of hadron loops leads automatically to an excess of d̄ over baru and introduces a sizeable contribution of orbital angular momentum to the spin of the proton and the Λ hyperon. Special attention is paid to the symmetries of the unquenched quark model.

Higher spin particles in the discrete string model approach

A new method is proposed to describe higher spin composite systems. Our goal is to introduce a discrete string model representing bound states in an N-relativistic particle system. The spectrum of the physical states is obtained for an arbitrary number N of particles in an arbitrary spacetime dimension. Particular illustrative examples of the new method for N = 2, 3, 4 and 5 are given, and the conditions on the existence of physical states with a precise spin content in any particular number of spacetime dimensions are clearly established.

Scalar-tensor theories with pseudoscalar couplings

We consider the scalar-tensor theories of gravity extended by the pseudoscalar couplings to matter and gauge fields and derive constraints on the CP-odd combinations of scalar and pseudoscalar couplings from laboratory spin precession experiments and from the evolution of photon polarization over cosmological distances. We show the complimentary character of local and cosmological constraints, and derive novel bounds on the pseudoscalar couplings to photons from the laboratory experiments. It is also shown that the more accurate treatment of the spin content of nuclei used in the spin precession experiments allows to tighten bounds on Lorentz-violating backgrounds coupled to the proton spin.

Refined, Motivic, and Quantum

It is well known that in string compactifications on toric Calabi–Yau manifolds one can introduce refined BPS invariants that carry information not only about the charge of the BPS state but also about the spin content. In this paper we study how these invariants behave under wall crossing. In particular, by applying a refined wall crossing formula, we obtain the refined BPS degeneracies for the conifold in different chambers. The result can be interpreted in terms of a new statistical model that counts “refined” pyramid partitions; the model provides a combinatorial realization of wall crossing and clarifies the relation between refined pyramid partitions and the refined topological vertex. We also compare the wall crossing behavior of the refined BPS invariants with that of the motivic Donaldson–Thomas invariants introduced by Kontsevich–Soibelman. In particular, we argue that, in the context of BPS state counting, the three adjectives in the title of this paper are essentially synonymous.

The Chiral and Angular Momentum Content of the ρ-Meson

It is possible to define and calculate in a gauge-invariant manner the chiral as well as the partial wave content of the quark-antiquark Fock component of a meson in the infrared, where mass is generated. Using the variational method and a set of interpolators that span a complete chiral basis we extract in a lattice QCD Monte Carlo simulation with two dynamical light quarks the orbital angular momentum and spin content of the rho-meson. We obtain in the infrared a simple 3S1 component as a leading component of the rho-meson with a small admixture of the 3D1 partial wave, in agreement with the SU(6) flavor-spin symmetry.

Publisher’s Note: Spin content ofΛin QCD sum rules [Phys. Rev. D79, 114028 (2009)

Publisher’s Note: Spin content of<mml:math xmlns:mml="http://www.w3.org/1998/Math/MathML" display="inline"><mml:mi>Λ</mml:mi></mml:math>in QCD sum rules [Phys. Rev. D<b>79</b>, 114028 (2009)

Spin content ofΛin QCD sum rules

We calculate the isoscalar axial-vector coupling constants of the {lambda} hyperon using the method of QCD sum rules. A determination of these coupling constants reveals the individual contributions of the u, d, and the s quarks to the spin content of {lambda}. Our results for the light-quark contributions are in agreement with those from experiment assuming flavor SU(3). We also find that the flavor-SU(3)-breaking effects are small and the contributions from the u and the d quarks to the {lambda} polarization are negatively polarized as in the flavor-SU(3) limit.

SPIN AND ORBITAL ANGULAR MOMENTUM IN THE PROTON

Since the announcement of the proton spin crisis by the European Muon Collaboration there has been considerable progress in unravelling the distribution of spin and orbital angular momentum within the proton. We review the current status of the problem, showing that not only have strong upper limits have been placed on the amount of polarized glue in the proton but that the experimental determination of the spin content has become much more precise. It is now clear that the origin of the discrepancy between experiment and the naive expectation of the fraction of spin carried by the quarks and anti-quarks in the proton lies in the non-perturbative structure of the proton. We explain how the features expected in a modern, relativistic and chirally symmetric description of nucleon structure naturally explain the current data. The consequences of this explanation for the presence of orbital angular momentum on quarks and gluons is reviewed and comparison made with recent results from lattice QCD and experimental data.

Contributions of qqqqq̄ components to nucleon spin structure

The spin structure of nucleons is presented in the framework of an extended quark model which in addition to the conventional qqq structure also takes into account qqqqq̄ admixtures in the nucleon wave functions, where the qqqqq̄ components are in colored quark cluster configurations. The axial vector weak coupling constant and spin distributions for polarized nucleons as well as spin content are obtained for the lowest positive parity qqqqq̄ configurations in flavor-spin dependent interaction. In particular, the contributions of the down and strange quarks to the proton spin and the sum rule for polarized neutron are negative, in agreement with recent experiments.

Gravitons, induced geometry and expectation value formalism at finite temperature

AbstractAfter establishing the positivity constraint and spin content of the theory for gravitons interacting with a necessarily, and a priori, non‐conserved external energy‐momentum tensor, the expectation value formalism of the theory is developed at finite temperature in the functional differential treatment of quantum field theory. The necessity of having, a priori, a non‐conserved external energy‐momentum tensor is an obvious technical requirement so that its respective ten components may be varied independently in order to generate expectation values and non‐linearities in the theory. The covariance of the induced Riemann curvature tensor, in the initial vacuum, is established even for the quantization in a gauge corresponding only to two physical states of the gravitons as established above. As an application, the induced correction to the metric and the underlying geometry is investigated due to a closed string arising from the Nambu action as a solution of a circularly oscillating string as, perhaps, the simplest generalization of a limiting point‐like object. Finally it is discussed on why the geometry of spacetime may, in general, depend on temperature due to radiative corrections and its physical significance is emphasized.

Phenomenological analysis of the nucleon spin contents and their scale dependence

In the past few years, a great deal of evidence has accumulated which indicates that the gluon polarization inside the nucleon is likely to be small at least at the low renormalization scales. On the other hand, the recent lattice QCD analyses suggest that the net orbital angular momentum carried by the quarks is nearly zero. There is also some indication, noted by Brodsky and Gardner based on the COMPASS observation of small single-spin asymmetry on the isoscalar deuteron target, that the gluon orbital angular momentum inside the nucleon is likely to be small. Naively combining all these observations, we are led to a rather embarrassing conclusion that the nucleon constituents altogether do not carry an adequate amount of angular momentum saturating the total nucleon spin. We show that this somewhat confused state of affairs can be cleared up only by paying careful attention to the scale dependencies of the nucleon spin decomposition.

Hadronic uncertainties in the elastic scattering of supersymmetric dark matter

We review the uncertainties in the spin-independent and spin-dependent elastic scattering cross sections of supersymmetric dark matter particles on protons and neutrons. We propagate the uncertainties in quark masses and hadronic matrix elements that are related to the $\ensuremath{\pi}$-nucleon $\ensuremath{\sigma}$ term and the spin content of the nucleon. By far the largest single uncertainty is that in spin-independent scattering induced by our ignorance of the $⟨N|\overline{q}q|N⟩$ matrix elements linked to the $\ensuremath{\pi}$-nucleon $\ensuremath{\sigma}$ term, which affects the ratio of cross sections on proton and neutron targets as well as their absolute values. This uncertainty is already impacting the interpretations of experimental searches for cold dark matter. We plead for an experimental campaign to determine better the $\ensuremath{\pi}$-nucleon $\ensuremath{\sigma}$ term. Uncertainties in the spin content of the proton affect significantly, but less strongly, the calculation of rates used in indirect searches.

The spin of the proton

The twenty years since the announcement of the proton spin crisis by the European Muon Collaboration has seen tremendous progress in our knowledge of the distribution of spin within the proton. The problem is reviewed, beginning with the original data and the suggestion that polarized gluons may play a crucial role in resolving the problem through the U ( 1 ) axial anomaly. The discussion continues to the present day where not only have strong limits been placed on the amount of polarized glue in the proton but the experimental determination of the spin content has also become much more precise. It is now clear that the origin of the discrepancy between the experiment and the naive expectation of the fraction of spin carried by the quarks and anti-quarks in the proton lies in the non-perturabtive structure of the proton. We explain how the features expected in a modern, relativistic and chirally symmetric description of nucleon structure naturally explain the current data.

Polarized structure of nucleon in the valon representation

We have utilized the concept of valon model to calculate the spin structure functions of proton, neutron, and deuteron. The valon structure itself is universal and arises from the perturbative dressing of the valence quark in QCD. Our results agree rather well with all of the relevant experimental data on g1p,n,d and gA/gV, and suggests that the sea quark contribution to the spin of proton is consistent with zero. It also reveals that while the total quark contribution to the spin of a valon, ΔΣvalon, is almost constant at Q2 ≥ 1 the gluon contribution grows with the increase of Q2 and hence requiring a sizable negative orbital angular momentum component Lz. This component along with the singlet and non-singlet parts are calculated in the Next-to-Leading order in QCD . We speculate that gluon contribution to the spin content of the proton is about 60% for all Q2 values. Finally, we show that the size of gluon polarization and hence, Lz, is sensitive to the initial scale Q02.

FandDvalues with explicit flavor symmetry breaking andΔscontents of nucleons

We propose a new model for describing baryon semileptonic decays for estimating $F$ and $D$ values with explicit breaking effects of both SU(3) and SU(2) flavor symmetry, where all possible SU(3) and SU(2) breaking effects are induced from an effective interaction. An overall fit including the weak magnetism form factor yields $F=0.477\ifmmode\pm\else\textpm\fi{}0.001$, $D=0.835\ifmmode\pm\else\textpm\fi{}0.001$, ${V}_{ud}=0.975\ifmmode\pm\else\textpm\fi{}0.002$, and ${V}_{us}=0.221\ifmmode\pm\else\textpm\fi{}0.002$ with ${\ensuremath{\chi}}^{2}=4.43/5\text{ }\text{ }\mathrm{d}.\mathrm{o}.\mathrm{f}.$. The spin content of strange quarks $\ensuremath{\Delta}s$ is estimated from the obtained values $F$ and $D$, and the nucleon spin problem is reexamined. Furthermore, the unmeasured values of $({g}_{1}/{f}_{1})$ and $({g}_{1})$ for other hyperon semileptonic decays are predicted from this new formula.

An interaction Lagrangian for two spin-1/2 elementary Dirac particles

The kinematical formalism for describing spinning particles developed by the author is based upon the idea that an elementary particle is a physical system with no excited states. It can be annihilated by the interaction with its antiparticle but, if not destroyed, its internal structure can never be modified. All possible states of the particle are just kinematical modifications of any one of them. The kinematical state space of the variational formalism of an elementary particle is necessarily a homogeneous space of the kinematical group of spacetime symmetries. By assuming Poincaré invariance we have already described a model of a classical spinning particle which satisfies Dirac's equation when quantized. We have recently shown that the spacetime symmetry group of this Dirac particle is larger than the Poincaré group. It also contains spacetime dilations and local rotations. In this work we obtain an interaction Lagrangian for two Dirac particles, which is invariant under this enlarged spacetime group. It describes a short- and long-range interaction such that when averaged, to suppress the spin content of the particles, describes the instantaneous Coulomb interaction between them. As an application, we analyse the interaction between two spinning particles, and show that the existence is possible of metastable bound states for two particles of the same charge, when the spins are parallel and provided some initial conditions are fulfilled. The possibility of formation of bound pairs is due to the zitterbewegung spin structure of the particles because when the spin is neglected, the bound states vanish.

RHIC’s future looks bright

Bertram Schwarzschild’s Issues and Events piece on the National Research Council’s report Revealing the Hidden Nature of Space and Time (Physics Today, June 2006, page 26) states, “Fermilab’s Tevatron is unlikely to outlive the decade. Neither is the PEP-II asymmetric electron–positron collider at SLAC nor the Relativistic Heavy Ion Collider at Brookhaven National Laboratory.”Placing RHIC in this context is odd since the NRC report nowhere mentions it. RHIC is funded by the Office of Nuclear Physics in the US Department of Energy’s Office of Science, not by the Office of High Energy Physics, for which the NRC committee was charged with recommending priorities for the next 15 years.More important, the notion that RHIC is “unlikely to outlive the decade” is misbegotten. The scientific impact of RHIC has been outstanding; its discovery of the “perfect liquid” of quarks and gluons was named the number-one physics story of 2005 by the American Institute of Physics publication Physics News Update and garnered media coverage around the world.Brookhaven National Laboratory is currently working with the Office of Nuclear Physics to implement for RHIC a strategy for the period 2006–11 aimed at a 10-fold luminosity upgrade and detector upgrades. This strategy will place RHIC at the forefront of research in high-temperature quantum chromodynamics (QCD) for at least another 10 years. Furthermore, RHIC is the first and only hadron collider with the ability to accelerate, store, and collide polarized protons at energies up to 500 GeV in the center-of-mass frame. It therefore provides unique opportunities to study the spin content of the nucleon—a program that also will extend into the next decade.Beyond that is the prospect of using RHIC as the basis for a polarized electron-ion collider, an option for an international next-generation facility for the study of QCD. That option will be discussed by the Nuclear Science Advisory Committee in 2007 as it develops its long-range plan for the field. If longevity is based on compelling science to be done, such a QCD facility—with ion–ion, proton–ion, polarized proton–proton, polarized electron–proton, and electron–ion collisions at high energy—would likely outlive the next decade.© 2006 American Institute of Physics.

Generalized form factors, generalized parton distributions, and the spin contents 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 ${m}_{\ensuremath{\pi}}\ensuremath{\simeq}(700--900)\text{ }\text{ }\mathrm{MeV}$. 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.

Possible0−+glueball candidateX(1835)

The possibility of $X(1835)$ discovered by BESII as a ${0}^{\ensuremath{-}+}$ glueball is studied in this paper. The decay rates of $X(1835)\ensuremath{\rightarrow}p\overline{p}$, $VV$ are associated with the gluon spin contents of proton and vector mesons, respectively. Estimations of the decay rates of $X(1835)\ensuremath{\rightarrow}VV$, $\ensuremath{\gamma}V$, $\ensuremath{\gamma}\ensuremath{\gamma}$ and the cross sections of $\ensuremath{\gamma}\ensuremath{\gamma}\ensuremath{\rightarrow}X(1835)\ensuremath{\rightarrow}f$ and ${h}_{1}+{h}_{2}\ensuremath{\rightarrow}X(1835)+\dots{}$ are presented. It predicts that as a glueball $X(1835)$ is unlikely to be produced in $J/\ensuremath{\psi}$ hadronic decays but as a $p\overline{p}$ it is likely.

Moments of generalized parton distribution functions and the nucleon spin contents

It is shown that, based only upon empirically and semiempirically known facts together with some reasonable theoretical postulates, we are inevitably led to a novel conclusion that the quark orbital angular momentum carries nearly half of the total nucleon spin at the low energy scale around ${Q}^{2}\ensuremath{\simeq}(600\text{ }\mathrm{MeV}{)}^{2}$. We also perform an explicit model analysis to find that the quark spin fraction $\ensuremath{\Delta}\ensuremath{\Sigma}$ 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.