arXiv subjects

S. Kumano

Publications and source records attributed to S. Kumano.

At least 145 records · Page 8Linked to original sources

arXiv1999

Polarized proton-deuteron Drell-Yan processes and parton distributions

We show in general that there are 108 structure functions in the proton-deuteron Drell-Yan processes. However, there exist only 22 finite ones after integrating the cross section over the virtual-photon transverse momentum \vec Q_T or after taking the limit Q_T->0. There are 11 new structure functions in comparison with the ones of the proton-proton reactions, and they are related to the tensor structure of the deuteron. Parton-model analyses indicate an important tensor structure function V_T^{UQ_0}, which can be measured by a quadrupole spin asymmetry. The Drell-Yan process has an advantage over lepton reactions in finding tensor polarized antiquark distributions. We hope that our studies will be realized in the next-generation RHIC-Spin project and other ones.

hep-ph↗

arXiv1999

Structure functions in the polarized Drell-Yan processes with spin-1/2 and spin-1 hadrons: I. general formalism

We discuss general formalism for the structure functions which can be investigated in the polarized Drell-Yan processes with spin-1/2 and spin-1 hadrons. To be specific, the formalism can be applied to the proton-deuteron Drell-Yan processes. Because of the spin-1 nature, there are new structure functions which cannot be studied in the proton-proton reactions. Imposing Hermiticity, parity conservation, and time-reversal invariance, we find that 108 structure functions exist in the Drell-Yan processes. However, the number reduces to 22 after integrating the cross section over the virtual-photon transverse momentum Q_T or after taking the limit Q_T->0. There are 11 new structure functions in addition to the 11 ones in the Drell-Yan processes of spin-1/2 hadrons. The additional structure functions are associated with the tensor structure of the spin-1 hadron, and they could be measured by quadrupole spin asymmetries. For example, the structure functions exist for "intermediate" polarization although their contributions vanish in the longitudinal and transverse polarization reactions. We show a number of spin asymmetries for extracting the polarized structure functions. The proton-deuteron reaction may be realized in the RHIC-Spin project and other future ones, and it could be a new direction of next generation high-energy spin physics.

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arXiv1998

Transversity distributions and spin asymmetries

We discuss transversity distributions and transverse spin asymmetries. First, $Q^2$ evolution results are shown for transversity and longitudinally polarized distributions. Second, the antiquark flavor asymmetry $Δ_{_T} \bar u/Δ_{_T} \bar d$ is discussed in two different descriptions, a meson-cloud model and an exclusion model. Both calculations produce a significant $Δ_T \bar d$ excess over $Δ_{_T} \bar u$. Third, we explain general formalism of the polarized proton-deuteron Drell-Yan processes. This study is partly intended to find the asymmetry $Δ_{_T} \bar u/Δ_{_T} \bar d$ by the p-d asymmetry. However, the existence of tensor structure in the deuteron produces a number of new structure functions. There exist 108 structure functions and 22 ones even after integrating the cross section over the virtual-photon transverse momentum $\vec Q_T$.

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arXiv1998

Studies of structure functions at a low-energy facility

We discuss the studies on structure functions at the possible future RCNP facility. At this stage, an electron-proton or proton-proton collider with sqrt{s}=5 - 10 GeV is considered. We explain large-x physics, nuclear modification of sea-quark and gluon distributions, and tensor spin structure function as the interesting topics at the facility. The large-x parton distributions are important for finding new physics beyond QCD in anomalous events such as the CDF jet data. The nuclear parton distributions are valuable in detecting a quark-gluon signature in heavy-ion reactions. The tensor structure function b_1 is a new field of high-energy spin physics. Considering these physics possibilities, we believe that the possible RCNP facility is important for the hadron-structure community.

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arXiv1998

Spin asymmetries at RHIC and polarized parton distributions

We discuss polarized parton distributions and their effects on spin asymmetries at RHIC. In particular, transversity distributions and transverse spin asymmetry are studied. First, we show the $Q^2$ evolution difference between a transversity distribution and a corresponding longitudinally polarized distribution. The difference could be an important test of perturbative QCD in high-energy spin physics. Then, the transverse spin asymmetry $A_{TT}$ is calculated with possible transversity distributions. Next, we study antiquark flavor asymmetry $Δ_{_T} \bar u/ Δ_{_T} \bar d$ in the transversity distributions by using a simple model. Its effects on the transverse spin asymmetry are also discussed.

hep-ph↗

arXiv1998

Modification of parton distributions in nuclei

Nuclear parton distributions are studied in a parton model with rescaling and recombination mechanisms. Parton x distributions are first calculated at Q^2=1 GeV^2 by the model, and they are evolved to larger Q^2 in order to be compared with various F_2 data. The experimental shadowing is explained by the parton-recombination effects. Then, the modification of sea-quark and gluon distributions is investigated. We show x and Q^2 dependent results of the ratios S_A/S_D and G_A/G_D. They indicate significant shadowing at small x and large ratios in the medium-x region. Our theoretical results should be tested by future experiments, for example, at RHIC (Relativistic Heavy Ion Collider). Furthermore, our studies are important for finding a signature of the quark-gluon plasma in heavy-ion reactions. We also indicate that nuclear effects in the deuteron should be taken into account for finding the accurate F_2 structure function of the neutron.

hep-ph↗

arXiv1998

Numerical solution of Q^2 evolution equations for polarized structure functions

We investigate numerical solution of Dokshitzer-Gribov-Lipatov-Altarelli-Parisi (DGLAP) Q^2 evolution equations for longitudinally polarized structure functions. Flavor nonsinglet and singlet equations with next-to-leading-order $α_s$ corrections are studied. A brute-force method is employed. Dividing the variables x and Q^2 into small steps, we simply solve the integrodifferential equations. Numerical results indicate that accuracy is better than 1% in the region 10^{-5}<x<0.8 if more than two-hundred Q^2 steps and more than one-thousand x steps are taken. Our evolution results are compared with polarized experimental data of the spin asymmetry A_1 by the SLAC-E130, SLAC-E143, EMC, and SMC collaborations. The comparison indicates that we cannot assume A_1 is independent of Q^2. We provide a FORTRAN program for the Q^2 evolution and devolution of polarized nonsinglet-quark, singlet-quark, Delta q_i+ Delta q-bar_i, and gluon distributions (and corresponding structure functions).

hep-ph↗

arXiv1998

Studies of polarized parton distributions at RHIC

We discuss our studies of polarized parton distributions which are related to the RHIC-Spin project. First, the parametrization of unpolarized parton distributions is explained as an introduction to general audience. Second, activities of the RHIC-Spin-J working group are reported. In particular, preliminary results are shown on the parametrization of polarized parton distributions. Third, we discuss numerical solution of the DGLAP Q^2 evolution equations for longitudinally polarized and transversity distributions. An efficient and accurate computer program is important for analyzing future RHIC experimental data.

hep-ph↗

arXiv1998

Flavor Asymmetry of Antiquark Distributions in the Nucleon

1. Introduction 2. Possible violation of the Gottfried sum rule 2.1 Gottfried sum rule 2.2 Early experimental results 2.3 NMC finding and recent progress 2.4 Small $x$ contribution 2.5 Nuclear correction: shadowing in the deuteron 2.6 Parametrization of antiquark distributions 3 Expectations in perturbative QCD 3.1 Operator product expansion 3.2 Perturbative correction to the Gottfried sum 4 Theoretical ideas for the sum-rule violation 4.1 Lattice QCD 4.2 Pauli exclusion principle 4.3 Mesonic models 4.3.1 Meson-cloud contribution 4.3.2 Chiral models 4.3.3 Anomalous $Q^2$ evolution 4.4 Diquark model 4.5 Isospin symmetry violation 4.6 Flavor asymmetry ubar-dbar in nuclei 4.7 Relation to nucleon spin 4.8 Comment on effects of quark mass and transverse motion 5 Finding the flavor asymmetry ubar-dbar in various processes 5.1 Drell-Yan process 5.1.1 Fermilab-E866 results 5.2 W and Z production 5.3 Quarkonium production at large $x_{_F}$ 5.4 Charged hadron production 5.5 Neutrino scattering 5.6 Experiments to find isospin symmetry violation 6 Related topics on antiquark distributions 7 Summary and outlook

hep-ph↗

arXiv1997

Numerical solution of Q^2 evolution equation for the transversity distribution Delta_T q

We investigate numerical solution of the Dokshitzer-Gribov-Lipatov-Altarelli- Parisi (DGLAP) Q^2 evolution equation for the transversity distribution Delta_T q or the structure function h_1. The leading-order (LO) and next-to- leading-order (NLO) evolution equations are studied. The renormalization scheme is MS or overline{MS} in the NLO case. Dividing the variables x and Q^2 into small steps, we solve the integrodifferential equations by the Euler method in the variable Q^2 and by the Simpson method in the variable x. Numerical results indicate that accuracy is better than 1% in the region 10^{-5}<x<0.8 if more than fifty Q^2 steps and more than five hundred x steps are taken. We provide a FORTRAN program for the Q^2 evolution and devolution of the transversity distribution Delta_T q or h_1. Using the program, we show the LO and NLO evolution results of the valence-quark distribution Delta_T u_v + Delta_T d_v, the singlet distribution sum_i (Delta_T q_i + Delta_T qbar_i), and the flavor asymmetric distribution Delta_T ubar - Delta_T dbar.They are also compared with the longitudinal evolution results.

hep-ph↗

arXiv1997

One and two loop anomalous dimensions for the chiral-odd structure function h1

Because the chiral-odd structure function h_1 will be measured in the polarized Drell-Yan process, it is important to predict the behavior of h_1 before the measurement. In order to study the Q^2 evolution of h_1, we discuss one and two loop anomalous dimensions which are calculated in the Feynman gauge and minimal subtraction scheme.

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arXiv1997

Antiquark Flavor Asymmetry with New Accelerator Facilities

Flavor asymmetry in light antiquark distributions is discussed. In particular, recent progress on the u-bar/d-bar asymmetry is explained. Then, we discuss possible future experimental studies. 1. Introduction 2. Present situation 3. Future u-bar/d-bar asymmetry studies 3.1 Drell-Yan process 3.2 Charged-hadron production 3.3 W charge asymmetry 3.4 Deuteron acceleration at HERA

hep-ph↗

arXiv1997

Two-loop anomalous dimensions for the structure function h1

Chiral-odd structure function h1 is expected to be measured in polarized Drell-Yan process. We calculate two-loop anomalous dimensions for h1 in the minimal subtraction scheme. Dimensional regularization and Feynman gauge are used for calculating the two-loop contributions. Our results are important in studying Q^2 dependence of h1.

hep-ph↗

arXiv1996

Flavor asymmetry $\bar u-\bar d$ in the nucleon

We give a brief summary of the Gottfried sum rule and a flavor asymmetric distribution $\bar u-\bar d$ in the nucleon. First, experimental history of the sum-rule studies is discussed. Second, future prospects for studying the asymmetry at Fermilab and RHIC are discussed. We also comment on possible nuclear modification.

hep-ph↗

arXiv1996

Numerical solution of NLO Q^2 evolution equations for spin-dependent structure functions

Numerical solution of DGLAP $Q^2$ evolution equations is studied for polarized parton distributions by using a ``brute-force" method. NLO contributions to splitting functions are recently calculated,and they are included in our analysis. Numerical results in polarized parton distributions and in the structure function $g_1$ are shown. In particular, we discuss how numerical accuracy depends on number of steps in the variable $x$ and in $Q^2$.

hep-ph↗

arXiv1996

Q^2 evolution studies of nuclear structure function F_2 at HERA

Q**2 dependence of nuclear structure functions is important for understanding perturbative QCD in nuclear environment. However, current experimental data are not sufficient for studying nuclear dependence in the Q**2 evolution because the data are limited in the small Q**2 region at small x. The future HERA program can study the evolution in the large Q**2 region, and it provides us crucial information on recombination effects and on higher-order alpha_s effects in the nuclear structure functions.

hep-ph↗

arXiv1996

Nuclear modification of the flavor asymmetry ubar - dbar

In order to test the NMC finding of flavor asymmetry $\overline u-\overline d$ in the nucleon, existing Drell-Yan data for the tungsten target are often used. However, we have to be careful in comparing nuclear data with the nucleon ones. We investigate whether there exists significant nuclear modification of the $\overline u - \overline d$ distribution in a parton-recombination model. It should be noted that a finite $\overline u - \overline d$ distribution is theoretically possible in nuclei even if the sea is symmetric in the nucleon. In neutron-excess nuclei such as the tungsten, there exist more $d$-valence quarks than $u$-valence quarks, so that more $\overline d$-quarks are lost than $\overline u$-quarks due to parton recombinations. Our results suggest that the nuclear modification in the tungsten is a 2--10 \% effect on the NMC $\overline u - \overline d$ distribution. The nuclear modification of the flavor asymmetry should be an interesting topic in connection with ongoing Drell-Yan experiments.

hep-ph↗

arXiv1996

Parton Distributions in Nuclei: Overview and Prospect

We give a brief overview of nuclear parton distributions. First, the EMC effect is discussed together with possible interpretations such as nuclear binding and $Q^2$ rescaling. Next, we explain shadowing descriptions: vector-meson-dominance-type and parton-recombination models. Nuclear dependence of $Q^2$ evolution should be interesting in testing whether or not DGLAP equations could be applied to nuclear structure functions. Status of nuclear sea-quark and gluon distributions is discussed. The structure function $b_1$, which will be measured at HERMES and possibly at ELFE, could shed light on a new aspect of high-energy spin physics.

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