arXiv subjects

S. Kumano

Publications and source records attributed to S. Kumano.

161 records · Page 9Linked to original sources

arXiv1996

Shadowing Aspects of Nuclear Parton Distributions

Various shadowing aspects of nuclear parton distributions are discussed in a parton-model framework. We explain existing $x$ dependent data of $F_2^A/F_2^D$, then the model is applied to a flavor asymmetry $\bar u-\bar d$ in a nucleus, to shadowing in valence-quark distributions, and to nuclear dependence of $Q^2$ evolution. First, we find that a finite nuclear $\bar u-\bar d$ distribution could be possible due to nuclear interactions even in the flavor symmetric case in the nucleon ($\bar u -\bar d$=0). Second, it is indicated that valence-quark shadowing could be used for discriminating among various shadowing models. Third, we find that nuclear dependence of $Q^2$ evolution $\partial [F_2^{Sn}/F_2^C]/ \partial [\ln Q^2]$, which was found by NMC, is essentially understood by modification of parton distributions in nuclei. However, higher-twist effects in nuclear interactions could be tested by studying the details of the nuclear $Q^2$ evolution.

hep-ph↗

arXiv1995

Nuclear dependence of Q^2 evolution in the structure function F_2

$Q^2$ evolution of the structure functions $F_2$ in tin and carbon nuclei is investigated in order to understand recent NMC measurements. $F_2$ is evolved by using leading-order DGLAP, next-to-leading-order DGLAP, and parton-recombination equations. NMC experimental result $\partial [F_2^{Sn}/F_2^C]/ \partial [\ln Q^2]\ne 0$ could be essentially understood by the difference of parton distributions in the tin and carbon nuclei. However, we find an interesting indication that large higher-twist effects on the $Q^2$ evolution could be ruled out. Nuclear dependence of the $Q^2$ evolution could be interesting for further detailed studies. 1. Introduction 2. Q^2 evolution of F_2 in tin and carbon nuclei 3. Conclusions

hep-ph↗

arXiv1995

Studies of valence-quark shadowing at HERA

We propose to measure valence-quark shadowing by observing charged pion productions in electron-nucleus scattering at HERA. Studies of shadowing phenomena in the valence-quark distribution could be useful in discriminating among various models, which produce similar results in the $F_2$ shadowing.

nucl-th↗

arXiv1995

Numerical solution of $Q^2$ evolution equations in a brute-force method

We investigate numerical solution of $Q^2$ evolution equations for structure functions in the nucleon and in nuclei. (Dokshitzer-Gribov-Lipatov-)Altarelli-Parisi and Mueller-Qiu evolution equations are solved in a brute-force method. Spin-independent flavor-nonsinglet and singlet equations with next-to-leading-order $α_s$ corrections are studied. Dividing the variables $x$ and $Q^2$ into small steps, we simply solve the integrodifferential equations. Numerical results indicate that accuracy is better than 2\% in the region $10^{-4}<x<0.8$ if more than two-hundred $Q^2$ steps and more than one-thousand $x$ steps are taken. The numerical solution is discussed in detail, and evolution results are compared with $Q^2$ dependent data in CDHSW, SLAC, BCDMS, EMC, NMC, Fermilab-E665, ZEUS, and H1 experiments. We provide a FORTRAN program for Q$^2$ evolution (and ``devolution'') of nonsinglet-quark, singlet-quark, $q_i+\bar q_i$, and gluon distributions (and corresponding structure functions) in the nucleon and in nuclei. This is a very useful program for studying spin-independent structure functions.

hep-ph↗

arXiv1995

Nuclear modification in the structure function $F_3$

Nuclear shadowing in the structure function $F_3$ is investigated as a possible test of shadowing models. The $F_3$ shadowing is studied in two different theoretical models: a parton-recombination model with $Q^2$ rescaling and an aligned-jet model. We find that predictions in these models differ completely in the small $x$ region. It suggests that the $F_3$ shadowing could be used for discriminating among different models, which produce similar shadowing behavior in the structure function $F_2$.

hep-ph↗

arXiv1995

Nuclear Shadowing in the Structure Function $F_3(x)$

Nuclear modification of the structure function $F_3$ is investigated. Although it could be estimated in the medium and large $x$ regions from the nuclear structure function $F_2^A$, it is essentially unknown at small $x$. The nuclear structure function $F_3^A$ at small $x$ is investigated in two different theoretical models: a parton-recombination model with $Q^2$ rescaling and an aligned-jet model. We find that these models predict completely different behavior at small $x$: {\it antishadowing} in the first parton model and {\it shadowing} in the aligned-jet model. Therefore, studies of the ratio $F_3^A/F_3^D$ at small $x$ could be useful in discriminating among different models, which produce similar shadowing behavior in the structure function $F_2$. We also estimate currently acceptable nuclear modification of $F_3$ at small $x$ by using $F_2^A/F_2^D$ experimental data and baryon-number conservation.

hep-ph↗

arXiv1994

Numerical Solution of Altarelli-Parisi Equations

We discuss numerical solution of Altarelli-Parisi equations in a Laguerre-polynomial method and in a brute-force method. In the Laguerre method, we get good accuracy by taking about twenty Laguerre polynomials in the flavor-nonsinglet case. Excellent evolution results are obtained in the singlet case by taking only ten Laguerre polynomials. The accuracy becomes slightly worse in the small and large $x$ regions, especially in the nonsinglet case. These problems could be implemented by using the brute-force method; however, running CPU time could be significantly longer than the one in the Laguerre method.

hep-ph↗

arXiv1994

Parton Distributions in Nuclei

We discuss two topics in this talk. One is a brief overview of nuclear parton distributions, and the other is SU(2)-flavor-symmetry breaking in {\it nuclear} antiquark distributions. First, we show that nuclear structure functions $F_2^A(x)$ could be explained in a $Q^2$ rescaling model with parton recombination effects. Then, nuclear gluon distributions are discussed in this parton model. We point out other interesting distributions for future investigations. Second, $\bar u-\bar d$ distributions in nuclei are discussed in order to shed light on nuclear modification of the $\bar u-\bar d$ distribution. -------------------- * This talk was supported by the Yamada Science Foundation.

hep-ph↗

arXiv1994

FORTRAN program for a numerical solution of the nonsinglet Altarelli-Parisi equation

We investigate a numerical solution of the flavor-nonsinglet Altarelli-Parisi equation with next-to-leading-order $α_s$ corrections by using Laguerre polynomials. Expanding a structure function (or a quark distribution) and a splitting function by the Laguerre polynomials, we reduce an integrodifferential equation to a summation of finite number of Laguerre coefficients. We provide a FORTRAN program for Q$^2$ evolution of nonsinglet structure functions (F$_1$, F$_2$, and F$_3$) and nonsinglet quark distributions. This is a very effective program with typical running time of a few seconds on SUN-IPX or on VAX-4000/500. Accurate evolution results are obtained by taking approximately twenty Laguerre polynomials.

hep-ph↗

arXiv1994

SU(2)-Flavor-Symmetry Breaking in Nuclear Antiquark Distributions

SU(2)-flavor-symmetry breaking in antiquark distributions of the nucleon was suggested by the New Muon Collaboration (NMC) in deep inelastic muon scattering. As an independent test, Drell-Yan data for the tungsten target have been used for examining the asymmetry. We investigate whether there exists significant modification of the $\bar u -\bar d$ distribution in nuclei in a parton recombination model. It should be noted that a finite $\bar u-\bar d$ distribution is theoretically possible in nuclei even if the sea is $SU(2)_f$ 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 $\bar d$-quarks are lost than $\bar u$-quarks are due to parton recombinations in the small $x$ region. Our results suggest that the nuclear modification in the tungsten is a 2--10 % effect on the $\bar u-\bar d$ distribution suggested by the NMC data. Nuclear effects on the flavor asymmetric distribution could be an interesting topic for future theoretical and experimental investigations.

hep-ph↗

arXiv1994

Nuclear Shadowing in a Parton Recombination Model: Q$^2$ Variation

Q$^2$ variation of the nuclear-structure-function ratio $F_2^A(x,Q^2)/F_2^D(x,Q^2)$ is investigated in a parton model with Q$^2$-rescaling and parton-recombination effects. Calculated results are compared with the NMC (New Muon Collaboration) and the Fermilab-E665 experimental data. We find that our theoretical results show small Q$^2$ variations and that they are consistent with the data within present experimental accuracy.

hep-ph↗

arXiv1993

1) Spin and flavor sum rules based on a parton model, 2) SU(2) flavor breaking in antiquark distributions

1) We discuss a sum rule of the tensor structure function $b_1(x)$ for spin-one hadrons along with the Gottfried sum rule. Both sum rules are similar in the sense that they are phenomenological ones based on a naive parton model. As the Gottfried sum rule became useful for studying SU(2)$_{flavor}$ breaking in antiquark distributions, the $b_1$ sum rule could provide important information on sea-quark tensor polarization. 2) Pionic contributions to the antiquark distribution $\bar u-\bar d$ and violation of the Gottfried sum rule are discussed. We find that the contributions account for about half of the discrepancy found by the New Muon Collaboration between their result and the sum rule. In order to distinguish various theoretical explanations, we need independent experimental tests, for example, by using Drell-Yan processes. As an application of mesonic calculations, the difference between strange and antistrange quark distributions in the nucleon is briefly discussed.

hep-ph↗

arXiv1993

Nuclear Shadowing in a Parton Recombination Model

Deep inelastic structure functions $F_2^A(x)$ are investigated in a $Q^2$ rescaling model with parton recombination effects. We find that the model can explain experimentally measured $F_2^A(x)$ structure functions reasonably well in the wide Bjorken$-x$ range ($0.005<x<0.8$). In the very small $x$ region ($x<0.02$), recombination results are very sensitive to input sea-quark and gluon distributions.

hep-ph↗

arXiv1993

Tensor Structure Function b$_1$(x) For Spin-One Hadrons

High-energy spin physics became a popular topic recently after the EMC finding for the proton's spin content. There exist unmeasured spin-dependent structure functions ($b_1$, $b_2$, $b_3$, and $b_4$) for spin-one hadrons such as the deuteron. The tensor structure function $b_1(x)$ could be measured by the proposed 15 GeV European Electron Facility. The measurement provides important clues to physics of non-nucleonic components in spin-one nuclei and to tensor structures on the quark-parton level.

hep-ph↗

arXiv1993

Nuclear Gluon Distributions in a Parton Model

Gluon distributions in the carbon and tin nuclei are investigated by using a $Q^2$ rescaling model with parton recombination effects. We obtain strong shadowings in the small $x$ region due to the recombinations. The ratio $G_A(x)/G_N(x)$ in the medium $x$ region is typically 0.9 for medium size nuclei. At large $x$, the ratio becomes large due to gluon fusions from different nucleons. Comparisons with recent New Muon Collaboration data for $G_{Sn} (x)/ G_C (x)$ indicate that more accurate experimental data are needed for testing the model.

hep-ph↗

arXiv1993

Scalar Mesons in $ϕ$ Radiative Decay: their implications for spectroscopy and for studies of CP-violation at $ϕ$ factories

Existing predictions for the branching ratio for $ϕ\rightarrow K\bar{K}γ$ {\it via} $ϕ\rightarrow Sγ$ (where $S$ denotes one of the scalar mesons $f_0$(975) and $a_0$(980)) vary by several orders of magnitude. Given the importance of these processes for both hadron spectroscopy and CP-violation studies at $ϕ$ factories (where $ϕ\rightarrow K^0 \bar K^0 γ$ poses a possible background problem), this state of affairs is very undesirable. We show that the variety of predictions is due in part to errors and in part to differences in modelling. The latter variation leads us to argue that the radiative decays of these scalar states are interesting in their own right and may offer unique insights into the nature of the scalar mesons. As a byproduct we find that the branching ratio for $ϕ\rightarrow K^0\bar{K}^0γ$ is $\lapproxeq 0(10^{-7})$ and will pose no significant background to proposed studies of CP-violation.

hep-ph↗

arXiv1992

Test of Scalar Meson Structure in $ϕ$ Radiative Decays

We show that $ϕ$ radiative decays into scalar mesons [$f_0$(975), $a_0$(980)$\equiv S$] can provide important clues on the internal structures of these mesons. Radiative decay widths vary widely: $B.R.=10^{-4}-10^{-6}$ depending on the substructures ($q\bar q$, $qq\bar q\bar q$, $K\bar K$, glueball). Hence, we could discriminate among various models by measuring these widths at future $ϕ$ factories. The understanding of these meson structures is valuable not only in hadron spectroscopy but also in nuclear physics in connection with the widely-used but little-understood $σ$ meson. We also find that the decay $ϕ\rightarrow Sγ\rightarrow K^0 \bar K^0γ$ is not strong enough to pose a significant background problem for studying CP violation via $ϕ\rightarrow K^0 \bar K^0$ at the $ϕ$ factories.

nucl-th↗