arXiv subjects

M. Miyama

Publications and source records attributed to M. Miyama.

At least 19 recordsLinked to original sources

arXiv2013

Characterization of the gaseous companion κ Andromedae b: New Keck and LBTI high-contrast observations

We previously reported the direct detection of a low mass companion at a projected separation of 55+-2 AU around the B9 type star κ Andromedae. The properties of the system (mass ratio, separation) make it a benchmark for the understanding of the formation and evolution of gas giant planets and brown dwarfs on wide-orbits. We present new angular differential imaging (ADI) images of the Kappa Andromedae system at 2.146 (Ks), 3.776 (L'), 4.052 (NB 4.05) and 4.78 μm (M') obtained with Keck/NIRC2 and LBTI/LMIRCam, as well as more accurate near-infrared photometry of the star with the MIMIR instrument. We derive a more accurate J = 15.86 +- 0.21, H = 14.95 +- 0.13, Ks = 14.32 +- 0.09 mag for κ And b. We redetect the companion in all our high contrast observations. We confirm previous contrasts obtained at Ks and L' band. We derive NB 4.05 = 13.0 +- 0.2 and M' = 13.3 +- 0.3 mag and estimate Log10(L/Lsun) = -3.76 +- 0.06. We build the 1-5 microns spectral energy distribution of the companion and compare it to seven PHOENIX-based atmospheric models in order to derive Teff = 1900+100-200 K. Models do not set constrains on the surface gravity. ``Hot-start" evolutionary models predict masses of 14+25-2 MJup based on the luminosity and temperature estimates, and considering a conservative age range for the system (30+120-10 Myr). ``warm-start" evolutionary tracks constrain the mass to M >= 11 MJup. Therefore, the mass of κ Andromedae b mostly falls in the brown-dwarf regime, due to remaining uncertainties in age and mass-luminosity models. According to the formation models, disk instability in a primordial disk could account for the position and a wide range of plausible masses of κ And b.

astro-ph.EP↗

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.

hep-ph↗

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.

hep-ph↗

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.

hep-ph↗

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".

hep-ph↗

arXiv2000

Determination of polarized parton distribution functions

We study parametrization of polarized parton distribution functions in the α_s leading order (LO) and in the next-to-leading order (NLO). From χ^2 fitting to the experimental data on A_1, optimum polarized distribution functions are determined. The quark spin content ΔΣis very sensitive to the small-x behavior of antiquark distributions which suggests that small-x data are needed for precise determination of ΔΣ. We propose three sets of distributions and also provide FORTRAN library for our distributions.

hep-ph↗

arXiv2000

Drell-Yan proton-deuteron asymmetry and polarized light-antiquark distributions

We discuss the Drell-Yan proton-deuteron (p-d) asymmetry R_{pd}, which is defined by the proton-proton and proton-deuteron cross-section ratio Δ_{(T)}σ^{pd}/2Δ_{(T)}σ^{pp}, and its relation to the polarized light-antiquark flavor asymmetry. Using a formalism of the polarized {\it pd} Drell-Yan process, we show that the R_{pd} especially in the large-x_F region is very useful for finding the flavor asymmetry in the longitudinally-polarized and transversity distributions. Our results are particularly important to study the flavor asymmetry in the transversity distributions because they cannot be measured by inclusive deep inelastic scattering and W-production process.

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

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.

hep-ph↗

arXiv1999

Transversity distributions and Drell-Yan spin asymmetries

We discuss transversity distributions and Drell-Yan transverse double spin asymmetries. First, the antiquark flavor asymmetry $Δ_{_T} \bar u/Δ_{_T} \bar d$ is discussed by using two different descriptions, a meson-cloud model and a Pauli exclusion model. We find that both calculations produce a significant $Δ_{_T} \bar d$ excess over $Δ_{_T} \bar u$. Next, we study its effects on the transverse spin asymmetry $A_{_{TT}}$ and on the Drell-Yan proton-deuteron asymmetry $Δ_{_T}σ^{pd}/2 Δ_{_T}σ^{pp}$. We find that the ratio $Δ_{_T}σ^{pd}/2 Δ_{_T}σ^{pp}$ is very useful for investigating the flavor asymmetry effect.

hep-ph↗

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 $\vec Q_T$.

hep-ph↗

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

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↗

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.

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

$Q^2$ evolution of structure functions in the nucleon and nuclei

$Q^2$ evolution of structure functions in the nucleon and nuclei is investigated by using usual DGLAP equations and parton-recombination equations. Calculated results for proton's $F_2$ and for the ratio $F_2^{Ca}/F_2^D$ are compared with various experimental data. Furthermore, we study nuclear dependence of $Q^2$ evolution in tin and carbon nuclei: $\partial [F_2^{Sn}/F_2^C]/ \partial [ln Q^2]\ne 0$, which was found by NMC.

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↗