arXiv subjects

A. Hayashigaki

Publications and source records attributed to A. Hayashigaki.

12 recordsLinked to original sources

arXiv2005

Low-energy J/psi-Hadron Interactions from Quenched Lattice QCD

The J/psi-hadron interaction is a key ingredient in analyzing the J/psi suppression in hot hadronic matter as well as the propagation of J/psi in nuclei. As a first step to clarify the J/psi-hadron interactions at low energies, we have calculated J/psi-pi, J/psi-rho and J/psi-nucleon scattering lengths by the quenched lattice QCD simulations with Wilson fermions for beta=6.2 on 24^3*48 and 32^3*48 lattices. Using the Luscher's method to extract the scattering length from the simulations in a finite box, we find an attractive interaction in the S-wave channel for all three systems: Among others, the J/psi-nucleon interaction is most attractive. Possibility of the J/psi-nucleon bound state is also discussed.

hep-lat↗

arXiv2004

Charmed-meson spectroscopy in QCD sum rule

We elaborate masses of open-charm mesons (c\bar{d} and c\bar{s}) with J^P=0^-,1^-,0^+,1^+ from a viewpoint of ordinary light-heavy systems in the analysis of standard Borel-transformed QCD sum rule. This analysis is implemented with the operator product expansion up to dimension 6, permitting corrections to the order alpha_s and to the order m_s, and without relying on 1/m_c-expansion. The obtained results following our stringent criteria for the continuum-threshold dependence, the Borel window and the Borel stability, indicate that the charmed-meson masses in the 0^+ channel are overestimated by 100 \sim 200 MeV in comparison with the experimental data, which were recently reported as the rather broad state of D^{*+}(2351) and the extremely narrow state of D_s^{*+}(2317), respectively. Such large mass-discrepancies from the data cannot be seen in other channels, where conversely our results of the c\bar{s}-meson masses are even underestimated somewhat in comparison with data, independent of the value of the strange-quark mass adopted in our calculations. From these results, it might be expected that the measured low mass of D_s^{*+}(2317) is a manifestation of an exotic state with the structures of a four-quark or a molecule, while at present the D^{*+} is not in conflict with the existing data due to its large width.

hep-ph↗

arXiv2003

Nuclear Hadronization: Within or Without?

Nuclei are unique analyzers for the early stage of the space-time development of hadronization. DIS at medium energies is especially suitable for this task being sensitive to hadronization dynamics, since the production length is comparable with the nuclear size. This was the driving motivation to propose measurements at HERMES using nuclear targets, and to provide predictions based on a pQCD model of hadronization [1]. Now when the first results of the experiment are released [2,3], one can compare the predictions with the data. The model successfully describes with no adjustment the nuclear effects for various energies, zh, pT, and Q2, for different flavors and different nuclei. It turns out that the main source of nuclear suppression of the hadron production rate is attenuation of colorless pre-hadrons in the medium. An alternative model [4] is based upon an ad hoc assumption that the colorless pre-hadron is produced outside the nucleus. This model has apparent problems attempting to explain certain features of the results from HERMES. A good understanding of the hadronization dynamics is important for proper interpretation of the strong suppression of high-pT hadrons observed in heavy ion collisions at RHIC. We demonstrate that the production length is even shorter in this case and keeps contracting with rising pT.

hep-ph↗

arXiv2002

Diffractive heavy pseudoscalar-meson productions by weak neutral currents

A first theoretical study for neutrino-induced diffractive productions of heavy pseudoscalar-mesons, η_c and η_b, off a nucleon is performed based on factorization formalism in QCD. We evaluate the forward diffractive production cross section in perturbative QCD in terms of the light-cone wave functions of Z boson and η_{c,b} mesons, and the gluon distribution of the nucleon. The diffractive production of η_c is governed by the axial vector coupling of the longitudinally polarized Z boson to Q\bar{Q} pair, and the resulting η_c production cross section is larger than the J/ψone by one order of magnitude. The bottomonium η_b production, which shows up for higher beam energy, is also discussed.

hep-ph↗

arXiv2002

Diffractive η_c and η_b productions by neutrinos via neutral currents

We report a first theoretical study for neutrino-induced diffractive productions of heavy pseudoscalar mesons, η_c and η_b, off a nucleon. Based on factorization formalism for exclusive processes, we evaluate the forward diffractive production cross section in perturbative QCD in terms of the light-cone Q\bar{Q} wave functions (WFs) of η_{c,b} mesons and the gluon distribution of the nucleon. The light-cone WFs of the η_c (η_b) meson are constructed to satisfy the spin symmetry relations with those of the J/ψ(Υ) meson. The diffractive η_c production is governed by the axial-vector coupling of the longitudinally polarized Z boson to Q\bar{Q} pair, and the resulting η_c production rate is larger than the J/ψone by one order of magnitude. We also discuss the production of bottomonium η_b, which shows up for higher beam energy.

hep-ph↗

arXiv2002

Simultaneous Softening of sigma and rho Mesons associated with Chiral Restoration

Complex poles of the unitarized pi-pi scattering amplitude in nuclear matter are studied. Partial restoration of chiral symmetry is modeled by the decrease of in-medium pion decay constant f*_{pi}. For large chiral restoration (f*_{pi}/f_{pi} << 1), 2nd sheet poles in the scalar (sigma) and the vector (rho) mesons are both dictated by the Lambert W function and show universal softening as f*_{pi} decreases. In-medium pi-pi cross section receives substantial contribution from the soft mode and exhibits a large enhancement in low-energy region. Fate of this universality for small chiral restoration (f*_{pi}/f_{pi} ~ 1) is also discussed.

hep-ph↗

arXiv2001

In-medium Σ^0-ΛMixing in QCD Sum Rules

The Σ^0-Λmixing angle in isospin-asymmetric nuclear medium is investigated by using QCD sum rules. From the general consideration of the in-medium baryonic correlations, in-medium baryon mixings are shown to have several Lorentz structures such as the scalar mixing angle θ^S and the vector mixing angle θ^V. This causes a difference between the particle mixing θ(= θ^S + θ^V) and the anti-particle mixing \barθ (= θ^S - θ^V). From the finite energy sum rules for the Σ^0-Λmixing, we find that the in-medium part of the mixing angle has a relation θ^S_{Med} \simeq - θ^V_{Med} in the isospin-asymmetric medium. This implies that the medium affects mainly the anti-particle mixing. From the Borel sum rules, we obtain |\barθ - θ_0| \simeq 0.39 |(ρ_n - ρ_p)| / ρ_0 with θ_0, ρ_n, ρ_p and ρ_0 being the vacuum mixing angle, the neutron density, the proton density and the normal nuclear matter density respectively.

hep-ph↗

arXiv2000

Diffractive $ϕ$ Production in a Perturbative QCD Model

The elastic leptoproduction of $ϕ$ measured by the H1 collaboration at HERA is described by a perturbative QCD model, based on open $s\bar{s}$ production and parton hadron duality, proposed by Martin et al. We observe that both the total cross section and the ratio of the longitudinal and transverse cross sections are well reproduced with an effective strange-quark mass $m_s \sim 320- 380 $ MeV for various gluon distribution functions. Possible connection of the effective mass and the momentum dependent dynamical mass associated with dynamical breaking of chiral symmetry is also discussed.

hep-ph↗

arXiv2000

Validity of the Color Dipole Approximation for Diffractive Production of Heavy Quarkonium

We study the diffractive photo- and leptoproductions of $J / ψ$ and $ψ'$ on the proton, and examine the validity of the small-size color dipole approximation to the production of radially excited heavy quarkonium. The dipole model predicts a small ratio of $ψ'$ to $J / ψ$ photoproduction cross sections, which does not agree with experimental data. We show that this discrepancy originates from a large transverse size of $ψ'$ which makes the convergence of the transverse size expansion questionable, and the calculation without the dipole approximation turns out to be consistent with the data. Productions of $Υ'(2S,3S)$ are also discussed, and the dipole approximation is found to be reasonable for the $Υ$-family.

hep-ph↗

arXiv1998

NLO $Q^2$-evolution of the nucleon's transversity distribution $h_1(x, Q^2)$

We present a calculation of the two-loop anomalous dimension for the transversity distribution $h_1(x,Q^2)$, $γ^{h(1)}_n$, in the MS scheme of the dimensional regularization. Because of the chiral-odd nature, $h_1$ does not mix with the gluon distributions, and thus our result is the same for the flavor-singlet and nonsinglet distributions. At small $n$ (moment of $h_1$), $γ^{h(1)}_n$ is significantly larger than $γ^{f,g(1)}_n$ (the anomalous dimension for the nonsinglet $f_1$ and $g_1$), but approaches $γ^{f,g(1)}_n$ very quickly at large $n$, keeping the relation $γ^{h(1)}_n > γ^{f,g(1)}_n$. This feature is in parallel to the relation between the one-loop anomalous dimension for $f_1$ $(g_1)$ and $h_1$. We also show that this difference in the anomalous dimension between $h_1$ and $g_1$ leads to a drastic difference in the $Q^2$-evolution of those distributions in the small $x$ region.

hep-ph↗

arXiv1997

QCD sum rules for ρ, ω, ϕmeson-nucleon scattering lengths and the mass shifts in nuclear medium

A new QCD sum rule analysis on the spin-isospin averaged $ρ$, $ω$ and $ϕ$ meson-nucleon scattering lengths is presented. By introducing the constraint relation on the the low energy limit of the vector-current nucleon forward scattering amplitude, we get $a_ρ=-0.47\pm 0.05$ fm, $a_ω=-0.41\pm 0.05$ fm and $a_ϕ=-0.15\pm 0.02$ fm, which suggests that these V-N interactions are attractive. It is also proved that the previous studies on the mass shift of these vector mesons in the nuclear medium are essentially the ones obtained from these scattering lengths in the linear density approximation.

nucl-th↗

arXiv1997

Next-to-leading order Q^2-evolution of the transversity distribution h_1(x, Q^2)

We present a calculation of the two-loop anomalous dimension for the transversity distribution h_1(x,Q^2), $γ^{h(1)}_n$, in the MS scheme of the dimensional regularization. Due to the chiral-odd nature, h_1 does not mix with the gluon distributions, and thus our result is the same for the flavor-singlet and nonsinglet distributions. At small n (moment of h_1), $γ^{h(1)}_n$ is significantly larger than $γ^{f(1)}_n$ (the anomalous dimension for the nonsinglet f_1), but approaches $γ^{f(1)}_n$ very quickly at large n, keeping the relation $γ^{h(1)}_n > γ^{f(1)}_n$. This feature is in parallel to the relation between the one-loop anomalous dimension for f_1 and h_1.

hep-ph↗