arXiv subjects

S. Kumano

Publications and source records attributed to S. Kumano.

At least 127 records · Page 7Linked to original sources

arXiv2002

Summary of Working Group 3 of the Neutrino Factory '02 Workshop

The parallel sessions of the working group 3 were devoted to the discussions of short-baseline neutrino physics program at neutrino factories. First, possible studies of parton distribution functions (PDFs), in particular nuclear and polarized PDFs, are discussed in this summary. Second, the extractions of alpha_s from sum rules and structure functions of deep-inelastic neutrino-nucleon scattering, higher-order perturbative QCD corrections and estimates of high-twist effects are summarized. Third, the situation of the observed NuTeV sin^2 theta_W anomaly is discussed.

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arXiv2002

Possible Studies of Parton Distribution Functions at JHF

We discuss possible studies of parton distribution functions (PDFs) in the nucleon and nuclei at the Japan Hadron Facility (JHF). First, the PDFs could be investigated by the 50 GeV primary proton facility. The distributions at medium x are determined, for example, by Drell-Yan measurements. Second, there are feasibility studies to propose a neutrino factory within the 50 GeV proton ring. If such an intensive high-energy neutrino facility is built, neutrino reactions should be able to provide valuable information on the PDFs, whereas the current structure functions have been measured mainly for neutrino-iron reactions.

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arXiv2002

Studies of parton distributions at a neutrino factory

The determination of parton distribution functions in the nucleon and nuclei is important for obtaining precise hadron-reaction cross sections, from which any new exotic signature could be found. We show that a future neutrino factory could provide important information on the parton distributions. First, a recent effort concerning the parametrization of nuclear parton distributions is explained. It suggests that the factory should be important for determining unknown behavior of valence-quark distributions in nuclei at small $x$. Second, the facility could be used for understanding nucleon spin structure and isospin violation in the parton distributions.

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arXiv2002

Structure functions of the nucleon and nuclei in neutrino reactions

Determination of parton distribution functions in nuclei is important for calculating nuclear corrections in oscillation experiments, from which detailed information should be extracted on neutrino properties. First, nuclear parton distributions are discussed for explaining high-energy nuclear reaction data. Possible nuclear modification is explained for valence-quark and antiquark distributions. It is rather difficult to determine gluon distributions in nuclei. Next, reversing the topic, we discuss structure functions which could be investigated by neutrino reactions. Determination of polarized parton distributions in the nucleon is discussed in polarized neutrino reactions. In addition, neutrino reactions should be important for finding nuclear modification of valence-quark distributions at small x if structure function ratios F_3^A/F_3^D are measured for various nuclei.

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arXiv2001

A model prediction for polarized antiquark flavor asymmetry

Polarized flavor asymmetry Delta u-bar/Delta d-bar is investigated in a meson-cloud model. A polarized nucleon splits into a rho meson and a baryon, then the polarized rho meson interacts with the virtual photon. Because of the difference between the longitudinally polarized distributions Delta u-bar and Delta d-bar in rho, the polarized flavor asymmetry is produced in the nucleon. In addition, we show that the g_2 part of rho contributes to the asymmetry especially at medium x with small Q^2.

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arXiv2001

Polarized light-antiquark distributions in a meson-cloud model

Flavor asymmetry is investigated in polarized light-antiquark distributions by a meson-cloud model. In particular, rho meson contributions to Delta u-bar - Delta d-bar are calculated. We point out that the g_2 part of rho contributes to the structure function g_1 of the proton in addition to the ordinary longitudinally polarized distributions in rho. This kind of contribution becomes important at medium x (>0.2) with small Q^2 (~1 GeV^2). Including N->rho N and N->rho Delta splitting processes, we obtain the polarized rho effects on the light-antiquark flavor asymmetry in the proton. The results show Delta d-bar excess over Delta u-bar, which is very different from some theoretical predictions. Our model could be tested by experiments in the near future.

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arXiv2001

Determination of Parton Distribution Functions in Nuclei

Nuclear parton distribution functions are obtained by a chi^2 analysis of lepton deep inelastic experimental data. It is possible to determine valence-quark distributions at medium x and antiquark distributions at small x; however, the distributions in other x regions and gluon distributions cannot be fixed. We need a variety of experimental data and also further analysis refinements.

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arXiv2001

Parton distribution functions in nuclei

Optimum nuclear parton distributions are determined by an analysis of muon and electron deep inelastic scattering data. Assuming simple A dependence and polynomial functions of x and 1-x for nuclear modification of parton distributions, we determine the initial distributions by a chi^2 analysis. Although valence-quark distributions are relatively well determined except for the small-x region, antiquark distributions cannot be fixed at medium and large x. It is also difficult to fix gluon distributions.

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arXiv2001

Parametrization of nuclear parton distributions

Optimum nuclear parton distributions are obtained by analyzing available experimental data on electron and muon deep inelastic scattering (DIS). The distributions are given at Q^2=1 GeV^2 with a number of parameters, which are determined by a chi^2 analysis of the data. Valence-quark distributions are relatively well determined at medium x, but they are slightly dependent on the assumed parametrization form particularly at small x. Although antiquark distributions are shadowed at small x, their behavior is not obvious at medium x from the F_2 data. The gluon distributions could not be restricted well by the inclusive DIS data; however, the analysis tends to support the gluon shadowing at small x. We provide analytical expressions and computer subroutines for calculating the nuclear parton distributions, so that other researchers could use them for applications to other high-energy nuclear reactions.

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arXiv2001

Determination of nuclear parton distributions

Parametrization of nuclear parton distributions is investigated in the leading order of alpha_s. The parton distributions are provided at Q^2=1 GeV^2 with a number of parameters, which are determined by a chi^2 analysis of the data on nuclear structure functions. Quadratic or cubic functional form is assumed for the initial distributions. Although valence quark distributions in the medium x region are relatively well determined, the small x distributions depend slightly on the assumed functional form. It is difficult to determine the antiquark distributions at medium x and gluon distributions. From the analysis, we propose parton distributions at Q^2=1 GeV^2 for nuclei from deuteron to heavy ones with the mass number A~208. They are provided either analytical expressions or computer subroutines for practical usage. Our studies should be important for understanding the physics mechanism of the nuclear modification and also for applications to heavy-ion reactions. This kind of nuclear parametrization should also affect existing parametrization studies in the nucleon because "nuclear" data are partially used for obtaining the optimum distributions in the "nucleon".

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arXiv2000

Polarized structure functions of the deuteron

Physics of spin-1 hadron is an unexplored topic in the high-energy region although spin-1/2 physics has been well investigated in the last decade. It is important to test our knowledge of hadron spin structure in a quite different field of spin physics. We discuss tensor structure functions, which do not exist for the spin-1/2 nucleon, in lepton scattering and in hadron reactions such as the polarized proton-deuteron Drell-Yan process.

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arXiv2000

Parametrization of polarized parton distributions

We report the polarized parton distributions proposed by the Asymmetry Analysis Collaboration (AAC). Using parametrized distributions at Q^2=1 GeV^2 and measured A_1 data, we determine optimum polarized distributions in the leading order (LO) and next-to-leading order (NLO). We find that the polarized antiquark distribution is not well determined particularly at small x. It could lead to a rather small quark spin content in comparison with usually-quoted values of 10-30%. In our analysis, it varies from 5% to 28% depending on the small-x extrapolation. It is necessary to have small-x (<10^{-3}) data for precise determination. In addition, the large-x region should be also studied for Δq-bar, which cannot be determined solely by g_1. We propose three sets of distributions as the longitudinally polarized parton distribution functions.

hep-ph↗

arXiv2000

Proton-deuteron asymmetry in Drell-Yan processes and polarized light-antiquark distributions

We discuss the relation between the ratio of the proton-deuteron (pd) Drell-Yan cross section to the proton-proton (pp) one Δ_{(T)} σ_{pd}/2 Δ_{(T)} σ_{pp} and the flavor asymmetry in polarized light-antiquark distributions. Using a recent formalism of the polarized pd Drell-Yan process, we show that the difference between the pp and pd cross sections is valuable for finding not only the flavor asymmetry in longitudinally polarized antiquark distributions but also the one in transversity distributions. It is especially important that we point out the possibility of measuring the flavor asymmetry in the transversity distributions because it cannot be found in W production processes and inclusive lepton scattering due to the chiral-odd property.

hep-ph↗

arXiv2000

Flavor asymmetry in polarized proton-deuteron Drell-Yan process

We discuss the possibility of finding polarized antiquark flavor asymmetry in Drell-Yan processes. We find that the difference between polarized proton-proton and proton-deuteron Drell-Yan cross sections should provide valuable information on the polarized flavor asymmetry. Numerical results indicate that the asymmetry effects are conspicuous especially in the large-x_F region. Our analysis is important for the transversity distributions because the flavor asymmetry cannot be found by inclusive lepton scattering and W-production processes.

hep-ph↗

arXiv2000

Possible Studies of Parton Distributions at JHF: polarized pd Drell-Yan process

We discuss possible studies of parton distributions at the proposed 50-GeV PS facility. First, comments are given in general on the importance of large-x physics for finding new physics. Second, polarized deuteron reactions are discussed in connection with new structure functions which are specific for a spin-1 hadron. Third, if the polarized proton-deuteron Drell-Yan process is studied at the facility, it could provide information on the light-antiquark flavor asymmetry in the polarized distributions.

hep-ph↗

arXiv2000

Polarized Parton Distribution Functions in the Nucleon

Polarized parton distribution functions are determined by using world data from the longitudinally polarized deep inelastic scattering experiments. A new parametrization of the parton distribution functions is adopted by taking into account the positivity and the counting rule. From the fit to the asymmetry data A_1, the polarized distribution functions of u and d valence quarks, sea quarks, and gluon are obtained. The results indicate that the quark spin content is ΔΣ=0.20 and 0.05 in the leading order (LO) and the next-to-leading-order (NLO) MS-bar scheme, respectively. However, if x dependence of the sea-quark distribution is fixed at small x by "perturbative QCD" and Regge theory, it becomes ΔΣ=0.24 ~ 0.28 in the NLO. The small-x behavior cannot be uniquely determined by the existing data, which indicates the importance of future experiments. From our analysis, we propose one set of LO distributions and two sets of NLO ones as the longitudinally-polarized parton distribution functions.

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arXiv1999

Structure functions in polarized proton-deuteron Drell-Yan processes

Polarized proton-deuteron (pd) Drell-Yan processes are investigated for studying new spin structure of a spin-1 hadron. Our formalism indicates that there exist many structure functions: 108 in general and 22 in the \vec Q_T-integrated case. A naive parton model suggests that at least one of them should be related to the tensor polarized distributions, which have not been measured at all. There are some experimental possibilities at FNAL, HERA, and RHIC to study the polarized pd reactions.

hep-ph↗

arXiv1999

Structure functions in the polarized Drell-Yan processes with spin-1/2 and spin-1 hadrons: II. parton model

We analyze the polarized Drell-Yan processes with spin-1/2 and spin-1 hadrons in a parton model. Quark and antiquark correlation functions are expressed in terms of possible combinations of Lorentz vectors and pseudovectors with the constrains of Hermiticity, parity conservation, and time-reversal invariance. Then, we find tensor polarized distributions for a spin-1 hadron. The naive parton model predicts that there exist 19 structure functions. However, there are only four or five non-vanishing structure functions, depending on whether the cross section is integrated over the virtual-photon transverse momentum \vec Q_T or the limit Q_T->0 is taken. One of the finite structure functions is related to the tensor polarized distribution b_1, and it does not exist in the proton-proton reactions. The vanishing structure functions should be associated with higher-twist physics. The tensor distributions can be measured by the quadrupole polarization measurements. The Drell-Yan process has an advantage over the lepton reaction in the sense that the antiquark tensor polarization could be extracted rather easily.

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