arXiv subjects

S. Kumano

Publications and source records attributed to S. Kumano.

At least 109 records · Page 6Linked to original sources

arXiv2006

Determination of fragmentation functions and their uncertainties from e+ + e- -> h + X data

Fragmentation functions are determined for pions, kaons, and nucleons by a global analysis of charged-hadron production data in electron-positron annihilation. The optimum functions are obtained in both leading order (LO) and next-to-leading order (NLO) of alpha_s. It is important that uncertainties of the fragmentation functions are estimated in this work by the Hessian method. We found that the uncertainties are large at small Q^2 and that they are generally reduced in the NLO in comparison with the LO ones. We supply a code for calculating the fragmentation functions and their uncertainties for the pions, kaons, and nucleons at given z and Q^2.

hep-ph↗

arXiv2006

Hadron Physics at J-PARC

An outline is explained for hadron-physics projects at J-PARC, which is considered to be one of the flagship facilities in hadron physics from 2008. The facility provides an intensity frontier with 50 GeV proton beam for nuclear and particle physics. It could cover a wide range of hadron physics from strongly interacting many-body systems with an extended hadronic degree of freedom, strangeness, to new forms of hadrons and hadronic matters. These studies lead not only to create innovative fields of hadron physics but also, possibly, to understand fundamental interactions because of recent progress on AdS/CFT correspondence. At the first stage of the J-PARC operation, hadron topics are mainly on strangeness nuclear physics such as hypernuclei, kaonic nuclei, and possible pentaquark hadrons. Then, the studies could be extended to exotic hadron searches, chiral dynamics in nuclear medium, structure functions, hard exclusive processes, hadron physics in neutrino scattering, and spin structure of the nucleon. With major upgrades of the facility, extensive studies could be done for the nucleon spin, heavy-ion physics, and hadron physics at a high-energy neutrino factory.

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arXiv2006

Constraint on $Δg(x)$ at large $x$

We investigate the polarized gluon distribution Δg(x) by a global analysis of current DIS data and the π^0 data from RHIC-Spin experiments. The π^0 data provide a strong constraint on Δg(x), so that its uncertainty is reduced. Furthermore, new DIS data of COMPASS and HERMES play an important role in determining Δg(x) at large x.

hep-ph↗

arXiv2006

Determination of polarized parton distribution functions with recent data on polarization asymmetries

Global analysis has been performed within the next-to-leading order in Quantum Chromodynamics (QCD) to determine polarized parton distributions with new experimental data in spin asymmetries. The new data set includes JLab, HERMES, and COMPASS measurements on spin asymmetry A_1 for the neutron and deuteron in lepton scattering. Our new analysis also utilizes the double-spin asymmetry for pi^0 production in polarized pp collisions, A_{LL}^{pi^0}, measured by the PHENIX collaboration. Because of these new data, uncertainties of the polarized PDFs are reduced. In particular, the JLab, HERMES, and COMPASS measurements are valuable for determining Delta d_v(x) at large x and Delta qbar(x) at x~0.1. The PHENIX pi^0 data significantly reduce the uncertainty of Delta g(x). Furthermore, we discuss a possible constraint on Delta g(x) at large x by using the HERMES data on g_1^d in comparison with the COMPASS ones at x~0.05.

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arXiv2006

Global analysis of AAC for determining polarized parton distribution functions

We report global analysis results for polarized parton distribution functions in the nucleon. The optimum distributions are determined by using spin asymmetry data on polarized lepton scattering on proton, neutron, and deuteron. Their uncertainties are estimated by the Hessian method. As a result, polarized quark distributions are relatively well determined, whereas the polarized gluon distribution has a large uncertainty band. We find that the obtained gluon distribution is compatible with recent Δg/g measurements in high-p_T hadron productions.

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arXiv2006

A possible nuclear effect on the NuTeV sin^2 theta_W anomaly

We investigate a possible explanation for the NuTeV anomaly in terms of a nuclear correction difference between u_v and d_v distributions. Analyzing deep elastic scattering and Drell-Yan data for nuclear targets, we try to determine the correction difference and its effect on the anomaly. We find that the difference cannot be precisely determined at this stage due to the lack of data which are sensitive to the difference. Therefore, it is difficult to draw a solid conclusion about its effect although the anomaly could be explained, at least partially, by this kind of nuclear correction.

hep-ph↗

arXiv2005

Nuclear modification difference between u_v and d_v distributions and its relation to NuTeV sin^2 theta_W anomaly

We investigate a possible nuclear correction to the NuTeV measurement of the weak-mixing angle sin^2 theta_W. In particular, a nuclear modification difference between u_v and d_v distributions contributes to the NuTeV measurement with the iron target. First, the modification differenceis determined by a chi^2 analysis so as to reproduce nuclear data on the structure function F_2 and Drell-Yan processes. Then, taking the NuTeV kinematics into account, we calculate a contribution to the sin^2 theta_W determination. In addition, its uncertainty is estimated by the Hessian method. Although the uncertainty becomes comparable to the NuTeV deviation, the effect is not large enough to explain the whole NuTeV sin^2 theta_W anomaly at this stage. However, it is difficult to determine such a nuclear modification difference, so that we need further investigations on the difference and its effect on the NuTeV anomaly.

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arXiv2004

Nuclear modification of valence-quark distributions and its effects on NuTeV sin^2 theta_W anomaly

We investigated a nuclear modification difference between up- and down-valence quark distributions by analyzing structure function F_2 and Drell-Yan cross-section ratios. Although nuclear modifications of the valence-quark distributions themselves are rather well determined, it is difficult to find their difference from the present data. We estimated such an effect on the NuTeV sin^2 theta_W value and its uncertainty by the Hessian method. At this stage, it is not large enough to explain the whole NuTeV anomaly. However, the modification difference cannot be precisely determined, so that further studies are needed.

hep-ph↗

arXiv2004

Nuclear corrections of parton distribution functions

We report global analysis results of experimental data for nuclear structure-function ratios F_2^A/F_2^{A'} and proton-nucleus Drell-Yan cross-section ratios sigma_{DY}^{pA}/sigma_{DY}^{pA'} in order to determine optimum parton distribution functions (PDFs) in nuclei. An important point of this analysis is to show uncertainties of the distributions by the Hessian method. The results indicate that the uncertainties are large for gluon distributions in the whole x region and for antiquark distributions at x>0.2. We provide a code for calculating any nuclear PDFs at given x and Q^2 for general users. They can be used for calculating high-energy nuclear reactions including neutrino-nucleus interactions, which are discussed at this workshop.

hep-ph↗

arXiv2004

Nuclear parton distribution functions and their effects on sin^2 theta_W anomaly

Nuclear parton distribution functions (NPDFs) are investigated by analyzing the data on structure functions F_2^A and Drell-Yan cross sections sigma_{DY}^{pA}. An important point of this analysis is to show uncertainties of the NPDFs by the Hessian method. The analysis indicates that the uncertainties are large for antiquark distributions at x>0.2 and gluon distributions in the whole x region. We also discuss a nuclear effect on the NuTeV sin^2 theta_W anomaly as an application.

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arXiv2004

AAC analysis of polarized parton distributions with uncertainties

We report recent studies of the Asymmetry Analysis Collaboration (AAC) on polarized parton distribution functions (PDFs). Using the data on the spin symmetry A_1 in deep inelastic lepton scattering, we investigate optimum polarized PDFs. Their uncertainties are estimated by the Hessian method. The uncertainties are large for the polarized antiquark and gluon distributions. We discuss the role of accurate SLAC-E155 proton data on the determination of the PDFs. The obtained distributions are compared with other parametrization results.

hep-ph↗

arXiv2004

Nuclear parton distribution functions and their uncertainties

We analyze experimental data of nuclear structure-function ratios F_2^A/F_2^{A'} and Drell-Yan cross section ratios for obtaining optimum parton distribution functions (PDFs) in nuclei. Then, uncertainties of the nuclear PDFs are estimated by the Hessian method. Valence-quark distributions are determined by the F_2 data at large x; however, the small-x part is not obvious from the data. On the other hand, the antiquark distributions are determined well at x~0.01 from the F_2 data and at x~0.1 by the Drell-Yan data; however, the large-x behavior is not clear. Gluon distributions cannot be fixed by the present data and they have large uncertainties in the whole x region. Parametrization results are shown in comparison with the data. We provide a useful code for calculating nuclear PDFs at given x and Q^2.

hep-ph↗

arXiv2004

Comparison of numerical solutions for Q^2 evolution equations

Q^2 evolution equations are important not only for describing hadron reactions in accelerator experiments but also for investigating ultrahigh-energy cosmic rays. The standard ones are called DGLAP evolution equations, which are integrodifferential equations. There are methods for solving the Q^2 evolution equations for parton-distribution and fragmentation functions. Because the equations cannot be solved analytically, various methods have been developed for the numerical solution. We compare brute-force, Laguerre-polynomial, and Mellin-transformation methods particularly by focusing on the numerical accuracy and computational efficiency. An efficient solution could be used, for example, in the studies of a top-down scenario for the ultrahigh-energy cosmic rays.

hep-ph↗

arXiv2003

Determination of polarized parton distribution functions and their uncertainties

We investigate the polarized parton distribution functions (PDFs) and their uncertainties by using the world data on the spin asymmetry A_1. The uncertainties of the polarized PDFs are estimated by the Hessian method. The up and down valence-quark distributions are determined well. However, the antiquark distributions have large uncertainties at this stage, and it is particularly difficult to fix the gluon distribution. The χ^2 analysis produces a positively polarized gluon distribution, but even Δg(x)=0 could be allowed according to our uncertainty estimation. In comparison with the previous AAC (Asymmetry Analysis Collaboration) parameterization in 2000, accurate SLAC-E155 proton data are added to the analysis. We find that the E155 data improve the determination of the polarized PDFs, especially the polarized antiquark distributions. In addition, the gluon-distribution uncertainties are reduced due to the correlation with the antiquark distributions. We also show the global analysis results with the condition Δg(x)=0 at the initial scale, Q^2=1 GeV^2, for clarifying the error correlation effects with the gluon distribution.

hep-ph↗

arXiv2003

Neutrino Scattering Physics at Superbeams and Neutrino Factories

Neutrino scattering physics is discussed for investigating internal structure of the nucleon and nuclei at future neutrino facilities. We explain structure functions in neutrino scattering. In particular, there are new polarized functions g_3, g_4, and g_5, and they should provide us important information for determining internal nucleon spin structure. Next, nuclear structure functions are discussed. From F_3 structure function measurements, valence-quark shadowing should be clarified. Nuclear effects on the NuTeV sin^2θ_W anomaly are explained. We also comment on low-energy neutrino scattering, which is relevant to current long-baseline neutrino oscillation experiments.

hep-ph↗

arXiv2003

Nuclear modification of structure functions in lepton scattering

We discuss nuclear structure functions in lepton scattering including neutrino reactions. First, the determination of nuclear parton distribution functions is explained by using the data of electron and muon deep inelastic scattering and those of Drell-Yan processes. Second, NuTeV sin^2 theta_W anomaly is discussed by focusing on nuclear corrections in the iron target. Third, we show that the HERMES effect, which indicates nuclear modification of the longitudinal-transverse structure function ratio, should exist at large x with small Q^2 in spite of recent experimental denials at small x.

hep-ph↗

arXiv2002

Nuclear modification of transverse-longitudinal structure function ratio

We investigate the nuclear effects on the transverse and longitudinal responses in the deep inelastic region due to the nuclear binding and nucleon Fermi motion. We display the role of the transverse-longitudinal admixture due to the transverse nucleon momentum. The mixing effect is appreciable at small Q^2 values, and gradually disappears at large Q^2 values. The nuclear modification is then dominated by the binding and Fermi-motion effects which are contained in the spectral function.

hep-ph↗

arXiv2002

Modified Paschos-Wolfenstein relation and extraction of weak mixing angle sin^2 theta_W

The NuTeV collaboration reported anomalously large weak mixing angle sin^2 theta_W in comparison with the standard model prediction. Neutrino and antineutrino charged- and neutral-current events are analyzed for extracting sin^2 theta_W. Although the Paschos-Wolfenstein relation is not directly used in the analysis, it plays an important role in the determination. Noting that the target nucleus, iron, is not an isoscalar nucleus, we derive a leading-order expression for a modified Paschos-Wolfenstein relation for nuclei, which may have neutron excess. Then, using charge and baryon-number conservations for nuclei, we discuss a nuclear correction in the sin^2 theta_W determination. It is noteworthy that nuclear modifications are different between valence up- and down-quark distributions. We show this difference effect on the NuTeV sin^2 theta_W deviation.

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