The generalized Gerasimov-Drell-Hearn (GDH) sum rule is understood based on the polarized parton distributions of the proton with the higher twist contributions. A simple parameterized formula is proposed to clearly present the contributions of different components in proton to $Γ_1^p(Q^2)$. We find that the contribution of quark helicity to $Γ_1^p(Q^2)$ is almost constant ($\sim 0.123$); the twist-4 effect dominates the suppression of $Γ_1^p(Q^2)$ at $Q^2<3GeV^2$, while the twist-6 effect arises a dramatic change of $Γ_1^p(Q^2)$ at $Q^2<0.3 GeV^2$, it implies a possible extended objects with size $0.2-0.3~ fm$ inside the proton.
The status of the Gerasimov-Drell-Hearn sum rules for polarized inclusive photo-production on nucleons is reviewed. It is shown that results from currently available data compare favorably with an estimate based on an extended current algebra. Implications for integrals of spin-dependent structure functions are also briefly discussed.
We study the current-algebra approach to the Gerasimov-Drell-Hearn sum rule, paying particular attention to the infinite-momentum limit. Employing the order-alpha^2 Weinberg-Salam model of weak interactions as a testing ground, we find that the legitimacy of the infinite-momentum limit is intimately connected with the validity of the naive equal-times algebra of electric charge densities. Our results considerably reduce the reliability of a recently proposed modification of the Gerasimov-Drell-Hearn sum rule, originating from an anomalous charge-density algebra.
The Drell-Hearn-Gerasimov sum-rule is really two sum-rules: one for each of the valence and the sea/glue contributions to the nucleon wavefunction. The convergence of these sum-rules follows from the Froissart bound for spin dependent processes in QCD and is necessary for the consistency of the constituent quark model of low energy QCD. Some challenges for future polarised photoproduction experiments, for example at ELFE, are discussed.
Photoproduction spin sum-rules offer a new window on the spin structure of the nucleon that complements the information we can learn from polarised deep inelastic scattering experiments. We review the theory and present status of the Drell-Hearn-Gerasimov sum-rule in QCD, emphasising the possible relation between the present ``discrepancy'' in this sum-rule and the nucleon's strangeness magnetic moment. In the case of an elementary electron or photon target (say at the NLC) the Drell-Hearn-Gerasimov sum-rule provides a test for physics beyond the minimal Standard Model.
We calculate the forward spin-dependent photon-nucleon Compton amplitudes $S_1$ and $S_2$ as functions of photon energy $ν$ and mass $Q^2$ at the next-to-leading (${\cal O}(p^4)$) order in chiral perturbation theory, from which we extract the contribution to a generalized Drell-Hearn-Gerasimov sum rule at low $Q^2$. The result indicates a much rapid $Q^2$ variation of the sum rule than a simple dimensional analysis would yield.
One-loop diagrams containing a graviton provide a finite contribution to the anomalous magnetic moment $\al$ of a lepton, whether or not the graviton propagates in $n$ large extra compact dimensions. In the present work, the tree graph photoproduction of a graviton, integrated up to an arbitrary cutoff, is shown to violate the Drell-Hearn-Gerasimov sum rule for $\al^2$. The possibility of resurrecting the sum rule from high energy contributions originating in string excitations is discussed in a qualitative manner, and various problems associated with such a program are pointed out.
The elastic contribution to the first moment of $g_2(x,Q^2)$ is analysed using a Drell-Yan-West type of relation and is shown to be negative. For a qualitative estimate the one-loop contributions to the polarized DIS sum rules in QED are studied. The behaviour of the generalized Gerasimov-Drell-Hearn sum rule is sensitive to infrared regularization. With a lower threshold for the gluon virtuality the relation of the generalized GDH sum rule to the Burkhardt-Cottingham sum rule is studied. We conclude that the elastic part has to be included for long range interactions but can be consistently d…
We show that the Drell-Hearn-Gerasimov sum rule for the nucleon is entirely saturated by the Δresonance in the limit of a large number of colors, N_c \to \infty. Corrections are at relative order 1/N_c^2.
We investigate the momentum dependence of the extended Drell-Hearn-Gerasimov sum rule. An economical formalism is developed which allows to express the extended DHG sum rule in terms of a single virtual Compton amplitude in forward direction. Rigorous results for the small momentum evolution are derived from chiral perturbation theory within the one-loop approximation. Furthermore, we evaluate some higher order contributions arising from $Δ(1232)$ intermediate states and relativistic corrections. (2 figures available upon request).
The spin structure functions g1 and g2 have been calculated in the resonance region and for small and intermediate momentum transfer. The calculation is based on a gauge-invariant and unitary model for one-pion photo- and electroproduction. The predictions of the model agree with the asymmetries and the spin sturcture functions recently measured at SLAC, and the first moments of the calculated spin structure functions fullfil the Gerasimov-Drell-Hearn and Burkhardt-Cottingham sum rules within an error of typically 5-10 %.
The Gerasimov-Drell-Hearn sum rule is evaluated for the deuteron by explicit integration up to a photon energy of 550 MeV including contributions from the photodisintegration channel and from coherent and incoherent single pion production as well. The photodisintegration channel converges fast enough in this energy range and gives a large negative contribution, essentially from the $^1S_0$ resonant state near threshold. Its absolute value is about the same size than the sum of proton and neutron GDH values. It is only partially cancelled by the single pion production contribution. But the inco…
We study the chiral loop corrections to the generalized Gerasimov-Drell-Hearn sum rule of the nucleon for finite photon virtuality in the framework of a Lorentz-invariant formulation of baryon chiral perturbation theory. We perform a complete one-loop calculation and obtain significant differences to previously found results based on the heavy baryon approach for the proton and neutron spin-dependent forward Compton amplitudes.
The Drell-Hearn-Gerasimov-Iddings (DHGI) sum rule for electrons is evaluated at order $α^3$ and shown to agree with the Schwinger contribution to the anomalous magnetic moment.
The long-standing discrepancy between the Gerasimov-Drell-Hearn sum rule and the analysis of pion photoproduction multipoles is greatly diminished by use of s-wave multipoles that are in accord with the predictions of chiral perturbation theory and describe the experimental data in the threshold region. The remaining difference may be due to contributions of channels with more pions and/or heavier mesons whose contributions to the sum rule remain to be investigated by a direct measurement of the photoabsorption cross sections.
A description of the generalized Gerasimov-Drell-Hearn sum rules for proton, neutron and deuteron is suggested. The proton consideration confirms the earlier conjecture, that the structure function $g_T$ features a smooth $Q^2-$dependence, while the structure function $g_2$ is changing rapidly, due to the elastic contribution to the Burkhardt-Cottingham sum rule. The neutron case features the close connection to the Bjorken sum rule. The deuteron case reveals the crucial role of photodesintegration channel, resulting in the strong nuclear correction at very low $Q^2$. The new approximate sum r…
Studying nuclear medium modification using the Gerasimov-Drell-Hearn sum rule
Thomas Jefferson National Accelerator Facility（托马斯·杰斐逊国家加速器设施）；Faculty of Mathematics and Physics, University of Ljubljana（卢布尔雅那大学数学与物理学院）；Jožef Stefan Institute（约热·斯特凡研究所）
A modification of the Gerasimov--Drell--Hearn sum rule suggested by the current experimental data is presented. Within the conventional theoretical framework, we find it necessary to consider the possibility of the presence of a localized region inside a nucleon, in which the electromagnetic (EM) gauge symmetry is spontaneous broken down, if the constraints of the gauge invariance, Lorentz invariance and the assumption of the commutativity of the EM charge density operator at equal-time are considered. We also discuss the propagation of a virtual photon inside a nucleon under such a scenario. …
Applications of the chiral expansion to generalize the Gerasimov-Drell-Hearn sum rule for finite $Q^2$ are discussed. The observation of several authors that the corrections to the leading order contributions are large and limit the applicability to a very small range in $Q^2$, are valid when considering the sum rule for protons and neutrons, separately. When using the proton-neutron difference, the chiral expansion may be valid to considerably higher momentum transfers where hadronic degrees of freedom at large distances may connect up with quark and gluon degrees of freedom. This could mark …
We derive a sum rule which establishes a linear relation between a particle's anomalous magnetic moment and a quantity connected to the photoabsorption cross-section. This quantity cannot be measured directly. However, it can be computed within a given theory. As an example, we demonstrate validity of the sum rule in QED at tree level--the renowned Schwinger's correction to the anomalous magnetic moment is readily reproduced. In the case of the strong interactions, we also consider the calculation of the nucleon magnetic moment within chiral theories.
All derivations of the GDH sum rule are presented and discussed in detail, focussing particularly on the validity of the underlying assumptions. Several tests of the sum rule within perturbative models are reviewed. Various possible sources of modifications of the sum rule are examined. With respect to the current-algebra derivation, the crucial role of the infinite-momentum limit is pointed out. A derivation is presented that exhibits the infinite-momentum limit as its last step, opening the prospect of studying its legitimacy within perturbative models. Adopting the Weinberg-Salam model as a…
The Gerasimov-Drell-Hearn sum rule is one of several dispersive sum rules that connect the Compton scattering amplitudes to the inclusive photoproduction cross sections of the target under investigation. Being based on such universal principles as causality, unitarity, and gauge invariance, these sum rules provide a unique testing ground to study the internal degrees of freedom that hold the system together. The present article reviews these sum rules for the spin-dependent cross sections of the nucleon by presenting an overview of recent experiments and theoretical approaches. The generalizat…
The Drell-Hearn-Gerasimov sum rule for the nucleon is investigated within a relativistic constituent quark model formulated on the light-front. The contribution of the N - Delta(1232) transition is explicitly evaluated using different forms for the baryon wave functions and adopting a one-body relativistic current for the constituent quarks. It is shown that the N - Delta(1232) contribution to the sum rule is sharply sensitive to the introduction of anomalous magnetic moments for the constituent quarks, at variance with the findings of non-relativistic and relativized quark models. The experim…
Phenomenological amplitudes obtained in partial-wave analyses (PWA) of single-pion photoproduction are used to evaluate the contribution of this process to the Gerasimov-Drell-Hearn (GDH), Baldin and Gell-Mann-Goldberger-Thirring (GGT) sum rules, by integrating up to 2 GeV in photon energy. Our study confirms that the single-pion contribution to all these sum rules converges even before the highest considered photon energy, but the levels of saturation are very different in the three cases. Single-pion production almost saturates the GDH sum rule for the proton, while a large fraction is missi…
The spin structure of the nucleon resonance region is analyzed on the basis of our phenomenological model MAID. Predictions are given for the Gerasimov-Drell-Hearn sum rule as well as generalized integrals over spin structure functions. The dependence of these integrals on momentum transfer is studied and rigorous relationships between various definitions of generalized Gerasimov-Drell-Hearn integrals and spin polarizabilities are derived. These results are compared to the predictions of chiral perturbation theory and phenomenological models.
The Gerasimov-Drell-Hearn integral $I_{GDH}(Q^2)$, and its relation to polarized nucleon structure functions, is discussed from the lattice perspective. Of particular interest is the variation of $I_{GDH}(Q^2)$ with $Q^2$, and what it may teach us about the origin and magnitude of higher-twist contributions.
The history and present status of several sum rules for deep-inelastic lepton scattering are reviewed, with particular attention to the discovery of scaling, partons, quarks and QCD. Two outstanding issues are then discussed in more detail: the singlet (Ellis-Jaffe) nucleon spin sum rule and the Drell-Hearn-Gerasimov-Iddings sum rule.
The contribution of the $G_2$ structure function to polarized deep inelastic scattering is slightly redefined in order to avoid kinematical zeros. Its strong $Q^2$-dependence implied by the Burkhardt-Cottingham (BC) sum rule naturally explains the sign change of the generalized Gerasimov-Drell-Hearn (GDH) sum rule. The status of the BC sum rule and implications for other spin processes are discussed.
A description of the generalized Gerasimov-Drell-Hearn sum rules for proton and neutron is suggested, using their relation to the Bjorken sum rule. The results support an earlier conjecture, that the structure function g_T features a smooth Q^2-dependence, while the structure function g_2 is changing rapidly, due to the elastic contribution to the Burkhardt-Cottingham sum rule. A possible violation of this later sum rule is briefly discussed.
The "kinematical" resolution of the $U_A(1)$ problem is proposed. The cancellation of photonic and gluonic anomalies suggested by the t'Hooft consistency principle naturally explains the zero singlet contributions to the first moments of proton and photon spin structure functions. The new sum rule for the photon spin structure function is derived. The strong strong $Q^2$-dependence of the Gerasimov-Drell-Hearn sum rule is related to the $G_2$ structure function and to the gluonic anomaly.
Contents: 1. The sum rules or $Γ_{p,n}$.Theoretical status. 2. Calculations of matrix elements over the polarized nucleon by the QCD sum rule approach. 3. Twist-4 corrections to $Γ_{p,n}$ from QCD sum rules. 4. Gerasimov, Drell-Hearn (GDH) sum rules. The estimate of higher twist corrections to $Γ_{p,n}$ using interpolation between $Γ_{p,n}$ and GDH sum rules. 5. Comparison with experiment. 6. The calculations of polarized structure functions by the QCD sum rule method. 7. Calculation of the chirality violating structure function $h_1(x)$ by QCD sum rules. 8. Conclusions.
We show that the matrix element of a local quark-gluon operator in the photon state, $<γ(kλ)|\hat O| γ(kλ)>$, can be calculated in lattice QCD. The result is generalized to other quantities involving space-like photons, including the transition form factor $γγ^*\to π^0$ and the virtual-photon-nucleon Compton amplitude $<γ^*N |γ^*N>$ which can be used to define the generalized Drell-Hearn-Gerasimov and Bjorken sum rules.
Gamow-Teller transitions in nuclei tell us that the nucleon's axial charge $g_A^{(3)}$ is quenched in large nuclei by about 20%. This result tells us that the spin structure of the nucleon is modified in nuclei and disfavours models of the medium dependence of parton structure based only on nucleon short range correlations in nuclei. For polarized photoproduction the Gerasimov-Drell-Hearn integral is expected to be strongly enhanced in medium.
Sum rules of superconvergence type for parity violating amplitudes (p.v. analogue of Gerasimov-Drell-Hearn sum rule) are considered. Elementary processes initiated by polarized photons in the lowest order of electroweak theory are calculated as examples illustrating the validity of the p.v.sum rules. The parity violating polarized photon-induced processes for proton target are considered in the frame of effective low energy theories and phenomenological models based on p.v. nucleon-meson effective interactions. Assuming the saturation of p.v. sum rule the possibility to limit the range of the …
We present a new extraction of the effective strong coupling constant α_{s,g_1}(Q^2). The result agrees with a previous determination and extends the measurement of the low and high Q^2 behavior of α_{s,g_1}(Q^2) that was previously deduced from sum rules. In particular, it experimentally verifies the lack of Q^2-dependence of α_{s,g_1}(Q^2) in the low Q^2 limit. This fact is necessary for application of the AdS/CFT correspondence to QCD calculations. We provide a parameterization of α_{s,g_1}(Q^2) thatcan equivalently be used to parameterize the Q^2-dependence of the generalized Gerasimov-Dre…
Using the Gerasimov-Drell-Hearn sum rule and experimental total cross sections for photoproduction of $η$, $K$ and $ϕ$ mesons on the proton, we obtain upper bounds for the contribution of strange quarks to the anomalous magnetic moment of the proton, $|κ_{s}| \le 0.20 κ_{p}$. The proposed experiments to measure the spin-dependence of the absorption cross section are expected to lower these bounds considerably. The existing data on $η$ production and phenomenological models for $K$ production agree with a negative sign for $κ_{s}$.
The generalized Gerasimov-Drell-Hearn (GDH) sum rule is known to be very sensitive to QCD radiative and power corrections. We improve the previously developed QCD-inspired model for the $Q^2$-dependence of the GDH sum rule. We take into account higher order radiative and higher twist power corrections extracted from precise Jefferson Lab data on the lowest moment of the spin-dependent proton structure function $Γ_1^{p}(Q^2)$ and on the Bjorken sum rule $Γ_1^{p-n}(Q^2)$. By using the singularity-free analytic perturbation theory we demonstrate that the matching point between chiral-like positiv…
We review the present data on high-energy, spin-dependent photoabsorption. We find a strong isotriplet term in $(σ_A - σ_P)$ which persists from $Q^2 \sim 0.25$GeV$^2$ to high $Q^2$ polarised deep inelastic scattering. For $Q^2 \sim 4$GeV$^2$ and $x$ between 0.01 and 0.12 the isotriplet part of $g_1$ behaves as $g_1^{(p-n)} \sim x^{-{1 \over 2}}$, in contrast to soft Regge theory which predicts that $g_1^{(p-n)}$ should converge as $x \to 0$. The isotriplet, polarised structure function $2x g_1^{(p-n)}$ is significantly greater than the isotriplet, unpolarised structure function $F_2^{(p-n)}$ …
The determination of twist-4 corrections to the structure functions of polarized $e(μ)N$ scattering by QCD sum rules is reviewed and critically analyzed. It is found that in the case of the Bjorken sum rule the twist-4 correction is small at $Q^2 > 5 GeV^2$. However, the accuracy of the today experimental data is insufficient to reliably determine $α_s$ from the Bjorken sum rule. For the singlet sum rule -- $p+n$ -- the QCD sum rule gives only the order of magnitude of twist-4 correction. At low and intermediate $Q^2$ the model is presented which realizes a smooth connection of the Gerasimov-D…
Theoretical predictions for polarized nucleon structure function $g_1(x,Q^2)$ at low $Q^2$ are obtained in the framework of the Generalized Vector Meson Dominance model. Contributions from both light and heavy vector mesons are evaluated. In the photoproduction limit the first moment of $g_1$ is related to the static properties of nucleon via the Drell-Hearn-Gerasimov-Hosoda-Yamamoto sum rule. This property is employed to fix the magnitude of the light vector meson contribution to $g_1$, using the recent measurements in the region of baryonic resonances. Results are compared to the data on $g_…
By using the Gerasimov-Drell-Hearn (GDH) sum rule and an isobaric model of kaon photoproduction, we calculate contributions of kaon-hyperon final states to the magnetic moment of the proton and the neutron. We find that the contributions are small. The approximation of sigma_{TT'} by sigma_{T} clearly overestimates the value of the GDH integral. We find a smaller upper bound for the contributions of kaon-hyperon final states to the proton's anomalous magnetic moment in kaon photoproduction, and a positive contribution for the square of the neutron's magnetic moment.
The sum rules for the spin dependent structure function $g_1^{ab}$ in the null-plane formalism corresponding to the moment at $n=0$ has been transformed to the sum rule which relates the $g_1^{ab}$ with the cross section of the isovector photon or the real photon. Based on these sum rules, we argue that there is a deep connection among the elastic, the resonance, and the non-resonant contributions, and that it explains why the sign of the generalized Gerasimov-Drell-Hearn sum changes at very small $Q^2$.
We calculate the proton's magnetic moment $μ_p$ and its axial-vector coupling $g_A$ as a function of its Dirac radius $R_1$ using a relativisitic three-quark model formulated on the light-cone. The relationship between $μ_p$ and $g_A$ is found to be independent of the assumed form of the light-cone wavefunction. At the physical radius $R_1=0.76$ fm, one obtains the experimental values for both $μ_p$ and $g_A$, and the helicity carried by the valence $u$ and $d$ quarks are each reduced by a factor $\simeq 0.75$ relative to their non-relativistic values. At large proton radius, $μ_p$ and $g_A$ a…
A description of the virtual photon absorption cross section difference σ_{1/2}-σ_{3/2} for the proton and the neutron is obtained with a parameterisation based on a Regge type approach. The parameterisation is obtained from global fits to the cross section data derived from the spin asymmetries measured in DIS of longitudinally polarised leptons from polarised ^1H, ^3He and ^2H targets in the range 0.3 GeV^2 < Q^2 < 70 GeV^2 and 4 GeV^2 < W^2 < 300 GeV^2. The fits give a reliable description of the data and provide predictions for the photo-production through a smooth Q^2-transition. The cont…
We extend the earlier suggested QCD-motivated model for the $Q^2$-dependence of the generalized Gerasimov-Drell-Hearn (GDH) sum rule which assumes the smooth dependence of the structure function $g_T$, while the sharp dependence is due to the $g_2$ contribution and is described by the elastic part of the Burkhardt-Cottingham sum rule. The model successfully predicts the low crossing point for the proton GDH integral, but is at variance with the recent very accurate JLAB data. We show that, at this level of accuracy, one should include the previously neglected radiative and power QCD correction…
Phenomenological parameterizations of proton and neutron polarized structure functions, g1p and g1n, are developed for x > 0.02 using deep inelastic data up to ~ 50 (GeV/c)**2 as well as available experimental results on photo- and electro-production of nucleon resonances. The generalized Drell-Hearn-Gerasimov sum rules are predicted from low to high values of Q**2 and compared with proton and neutron data. Furthermore, the main results of the power correction analysis carried out on the Q**2-behavior of the polarized proton Nachtmann moments, evaluated using our parameterization of g1p, are b…
Recent results from Jefferson Lab on measurement of inclusive double polarization asymmetries in the nucleon resonance region are discussed. Preliminary results on the first moment of the structure function g_1(x,Q^2) for protons, and on the generalized Gerasimov-Drell-Hearn integral for neutrons are presented as well. The first moment for protons shows a strong Q^2 dependence below Q^2 = 1 GeV^2, and changes sign near Q^2 = 0.3 GeV^2. Strong double polarization asymmetries are observed in exclusive electroproduction of pions from polarized protons.
The spin structure of the nucleon is analyzed in the framework of a Lorentz-invariant formulation of baryon chiral perturbation theory. The structure functions of doubly virtual Compton scattering are calculated to one-loop accuracy (fourth order in the chiral expansion). We discuss the generalization of the Gerasimov-Drell-Hearn sum rule, the Burkhardt-Cottingham sum rule and moments of these. We give predictions for the forward and the longitudinal-transverse spin polarizabilities of the proton and the neutron at zero and finite photon virtuality. A detailed comparison to results obtained in…
We examine the energy dependence of single-meson photoproduction as it contributes to the Gerasimov-Drell-Hearn (GDH) sum rule. For photon energies above approximately 1 GeV, through the full resonance region, this contribution dominates the proton sum rule integral. Over the same energy region, our single-pion contribution to the neutron sum rule also qualitatively follows a recent set of GDH data. The predicted neutral-pion contribution to the neutron sum rule is nearly zero above 1 GeV in this result. The SAID and Mainz (MAID) results are very different for a number of observables over this…
We compute spin-flip cross section for graviton photoproduction on a spin-1/2 target of finite mass. Using this tree-level result, we find one-loop graviton correction to the spin-flip low-energy forward Compton scattering amplitude by using Gerasimov-Drell-Hearn sum rule. We show that this result agrees with the corresponding perturbative computations, implying the validity of the sum rule at one-loop level, contrary to the previous claims. We discuss possible effects from the black hole production and string Regge trajectory exchange at very high energies. These effects seem to soften the UV…
The Gerasimov-Drell-Hearn sum rule and related dispersive integrals connect real and virtual Compton scattering to inclusive photo- and electroproduction. Being based on universal principles as causality, unitarity, and gauge invariance, these relations provide a unique testing ground to study the internal degrees of freedom that hold a system together. The present contribution reviews the spin-dependent sum rules and cross sections of the nucleon. At small momentum transfer, the data sample information on the long range phenomena (Goldstone bosons and collective resonances), whereas the prima…
A unified presentation is given on the use of dispersion relations in the real and virtual Compton scattering processes off the nucleon. The way in which dispersion relations for Compton scattering amplitudes establish connections between low energy nucleon structure quantities, such as polarizabilities or anomalous magnetic moments, and the nucleon excitation spectrum is reviewed. We discuss various sum rules for forward real and virtual Compton scattering, such as the Gerasimov-Drell-Hearn sum rule and its generalizations, the Burkhardt-Cottingham sum rule, as well as sum rules for forward n…
The nonperturbative $Q^2$- dependence of the sum rules for the structure functions of polarized $e(μ)N$ scattering is discussed. The determination of twist-4 corrections to the structure functions at high $Q^2$ by QCD sum rules is reviewed and critically analyzed. It is found that in the case of the Bjorken sum rule the twist-4 correction is small at $Q^2 > 5 GeV^2$ and does not influence the value of $α_s$ determined from this sum rule. However, the accuracy of the today experimental data is insufficient to reliably determine $α_s$ from the Bjorken sum rule. For the singlet sum rule -- $p+n$ …
We use new high statistics data from CLAS and COMPASS on the nucleon's spin structure function at low Bjorken x and low virtuality, Q^2 < 0.5 GeV^2, together with earlier measurements from the SLAC E-143, HERMES and GDH experiments to estimate the effective intercept(s) for spin dependent Regge theory. We find alpha_{a_1} = 0.31 \pm 0.04 for the intercept describing the high-energy behaviour of spin dependent photoabsorption together with a new estimate for the high-energy part of the Gerasimov-Drell-Hearn sum-rule, -15 \pm 2 mu b from photon-proton centre-of-mass energy greater than 2.5 GeV. …
Consequences of QCD current algebra formulated on a light-like hyperplane are derived for the forward scattering of vector and axial-vector currents on an arbitrary hadronic target. It is shown that current algebra gives rise to a special class of sum rules that are direct consequences of the independent chiral symmetry that exists at every point on the two-dimensional transverse plane orthogonal to the lightlike direction. These sum rules are obtained by exploiting the closed, infinite-dimensional algebra satisfied by the transverse moments of null-plane axial-vector and vector charge distrib…
Nuclear effects in the spin-dependent structure function $g_1$ of the deuteron are studied in the kinematics of future experiments at CEBAF, ($ν\leq 3~GeV, ~Q^2 \leq 2~GeV^2$). The magnitude of nuclear effects is found to be significantly larger than the one occurring in deep inelastic scattering ($ν\to \infty, ~Q^2\to \infty$). A possibility to measure the neutron structure functions in the CEBAF experiments with deuterium is analysed. It is found that disregarding or improperly treating nuclear effects in the region of nucleon resonances would lead to the ``extraction" of an unreliable funct…
This paper contains three parts relating to the nucleon spin structure in a simple picture of the nucleon: (i) The polarized gluon distribution in the proton is dynamically predicted starting from a low scale by using a nonlinear QCD evolution equation-the DGLAP equation with the parton recombination corrections, where the nucleon is almost only consisted of valence quarks. We find that the contribution of the gluon polarization to the nucleon spin structure is much larger than the predictions of most other theories. This result suggests a significant orbital angular momentum of the gluons is …
