arXiv subjects

Arata Hayashigaki

Publications and source records attributed to Arata Hayashigaki.

13 recordsLinked to original sources

arXiv2007

Exotic Hadron in Pole-dominated QCD Sum Rules

We study pentaquark $Θ^{+} (I=0,J=1/2)$ in the QCD sum rules emphasizing that we can not extract any properties of the pentaquark outside of the Borel window. To find the appropriate Borel window, we prepare a favorable set up of the correlators and carry out the operator product expansion up to dimension 15 within factorization hypothesis. Our procedures reduce the unwanted high energy contaminations and enhance the low energy correlation. In the Borel window, independent analyses for the chiral-even and odd sum rules give the consistent values of the $Θ^+$ mass, $1.68\pm0.22$ GeV, and the residue. The parity is found to be {\it positive}.

hep-ph↗

arXiv2006

The eccentricity in heavy-ion collisions from Color Glass Condensate initial conditions

The eccentricity in coordinate-space at midrapidity of the overlap zone in high-energy heavy-ion collisions predicted by the $k_\perp$-factorization formalism is generically larger than expected from scaling with the number of participants. We provide a simple qualitative explanation of the effect which shows that it is not caused predominantly by edge effects. We also show that it is quite insensitive to ``details'' of the unintegrated gluon distribution functions such as the presence of leading-twist shadowing and of an extended geometric scaling window. The larger eccentricity increases the azimuthal asymmetry of high transverse momentum particles. Finally, we point out that the longitudinal structure of the Color Glass Condensate initial condition for hydrodynamics away from midrapidity is non-trivial but requires understanding of large-$x$ effects.

nucl-th↗

arXiv2006

Single transverse-spin asymmetries in forward pion production at high energy: incorporating small-x effects in the target

We consider single-inclusive forward pion production in high-energy proton-proton collisions at RHIC energies. A good baseline description of the transverse momentum distributions at high rapidity is obtained within Mueller's dipole formalism with an anomalous dimension incorporating an ``extended geometric scaling'' window between the saturation and DGLAP regimes. We then formulate pion production for transversely polarized projectiles within the same approach. We assume that an azimuthal, spin-dependent asymmetry arises from the so-called Sivers effect and investigate the single transverse-spin asymmetry A_N at 200 GeV and 500 GeV center-of-mass energy. A simple parameterization of the Sivers functions from the literature compares reasonably well with the high-energy STAR data if the overall normalization is scaled up by at least a factor of two. The STAR data might therefore indicate that the Sivers effect is significantly stronger than thought so far. We also analyze higher-twist contributions to A_N and find that they largely cancel.

hep-ph↗

arXiv2006

Pentaquark state in pole-dominated QCD sum rules

We propose a new approach in QCD sum rules applied for exotic hadrons with a number of quarks, exemplifying the pentaquark Theta^{+} (I=0,J=1/2) in the Borel sum rule. Our approach enables reliable extraction of the pentaquark properties from the sum rule with good stability in a remarkably wide Borel window. The appearance of its valid window originates from a favorable setup of the correlation functions with the aid of it chirality of the interpolating fields on the analogy of the Weinberg sum rule for the vector currents. Our setup leads to large suppression of the continuum contributions which have spoiled the Borel stability in the previous analyses, and consequently enhances importance of the higher-dimensional contributions of the OPE, which are indispensable for investigating the pentaquark properties. Implementing the OPE analysis up to dimension 15, we find that the sum rules for the chiral-even and odd parts independently give the Theta^{+} mass of 1.68 pm 0.22 GeV with uncertainties of the condensate values. Our sum rule indeed gives rather flat Borel curves almost independent of the continuum thresholds both for the mass and pole residue. Finally, we also discuss possible isolation of the observed states from the KN scattering state on view of chiral symmetry.

hep-ph↗

arXiv2006

First lattice study of low-energy charmonium-hadron interaction

We study the scattering lengths of charmonia (J/psi and eta_c) with light hadrons (pi, rho and N) by the quenched lattice QCD simulations on 24x24x24x48, 32x32x32x48 and 48x48x48x48 lattices with the lattice spacing a = 0.068 fm. The scattering length is extracted by using the Luscher's phase-shift formula together with the measurement of the energy shift Delta E of two hadrons on the lattice. We find that there exist attractive interactions in all channels, J/psi(eta_c)-pi, J/psi(eta_c)-rho and J/psi(eta_c)-N: The s-wave J/psi-pi (eta_c-pi) scattering length is determined as 0.0119+-0.0039 fm (0.0113+-0.0035 fm) and the corresponding elastic cross section at the threshold becomes 0.018+0.013-0.010 mb (0.016+0.011-0.008 mb). Also, the J/psi-N (eta_c-N) spin-averaged scattering length is 0.71+-0.48 fm (0.70+-0.66 fm), which is at least an order of magnitude larger than the charmonium-pion scattering length. The volume dependence of the energy shifts is also investigated to check the expected 1/L^3 behavior of Delta E at a large spatial size L.

hep-lat↗

arXiv2006

Proton and anti-proton production in the forward region of d+Au collisions at RHIC from the color glass condensate

The power-law tail of high-pt pi^\pm spectra observed in forward d+Au collisions at RHIC can be attributed to the power-law decrease of the dipole forward scattering amplitude appearing in the color glass condensate (CGC) approach. Forward particle production probes the small-x gluon distribution of the target nucleus where its anomalous dimension is rather flat (gamma = 0.6 \sim 0.8) for moderately high pt (\lsim 5 GeV), and where the leading-twist DGLAP approximation is not valid. In the same framework, we examine p and \bar{p} production using baryon fragmentation functions parameterized in the Lund fragmentation scheme. This provides a good description of the forward \bar{p} spectrum while it underestimates the p data by as much as a factor of 2 \sim 3 at pt \lsim 4 GeV. Part of this anomalous baryon excess can be attributed to surviving constituent diquarks from the deuteron projectile. Thus, the contribution from diquark scattering may play an essential role for forward baryon formation.

hep-ph↗

arXiv2006

Geometric scaling violations in the central rapidity region of d+Au collisions at RHIC

We show that geometric scaling is satisfied to good accuracy in the forward region of d+Au collisions at RHIC. Scaling violations do show up, however, at mid-rapidity, and the anomalous dimension of the small-x gluon distribution evolves to near its DGLAP limit for transverse momenta of a few GeV. This represents a first consistency check of RHIC deuteron-nucleus and HERA DIS phenomenology, and of the universality of the underlying Color Glass Condensate (CGC) theory, which describes both phenomena. It also reconciles successful leading-twist LO and NLO perturbative QCD computations of mid-rapidity particle production with small-x evolution. Finally, we introduce a new parameterization for the anomalous dimension of the small-x gluon distribution which properly reproduces known theoretical limits at large rapidity, at large virtuality, and on the saturation boundary, and still fits the available data from d+Au collisions at RHIC. We find indications that sub-asymptotic terms in the rapidity-evolution of the anomalous dimension are large.

hep-ph↗

arXiv2005

Isospin quantum number of D_{s0}^+(2317)

The D_s^+ pi^0 and D_s^{*+} gamma decays of the D_{s0}^+(2317) are studied by assigning it to various charmed strange scalar mesons. As a result, it is found that its assignment to an iso-triplet four-quark meson is favored by the severest experimental constraint on the ratio of the rates for these decays, while assigning it to an I=0 state (a four-quark or a conventional {c\bar{s}}) is inconsistent with this constraint.

hep-ph↗

arXiv2005

The Color Glass Condensate and hadron production in the forward region

We consider one loop corrections to single inclusive particle production in parton-nucleus scattering at high energies, treating the target nucleus as a Color Glass Condensate. We prove by explicit computation that in the leading log Q^2 approximation, these corrections lead to collinear factorization and DGLAP evolution of the projectile parton distribution and hadron fragmentation functions. At leading logarithmic level, and in single-inclusive cross sections, only two-point functions of Wilson lines in the adjoint and fundamental representations (Mueller's dipoles) arise, which can be obtained from the solution of the JIMWLK equations. The application of our results to forward-rapidity production at RHIC energy shows that, in general, recoil effects are large and that the forward rapidity region at RHIC is rather different from the central region at LHC, despite comparable gluon densities in the target. We show that both the quantum x-evolution of the high-density target as well as the DGLAP Q^2-evolution of the parton distribution and fragmentation functions are clearly seen in the BRAHMS data. This provides additional strong evidence for the Color Glass Condensate at RHIC.

hep-ph↗

arXiv2004

Transverse quark motion inside charmonia in diffractive photo- and electroproductions

We reexamine the Fermi motion effects on the diffractive photo- and electroproductions of heavy vector-mesons, J/psi and psi', off a nucleon in the leading ln(Q^2/Lambda_QCD^2) approximation (LLA) of QCD. We take into account all the Fermi motion corrections arising from the relative motion of quarks inside the charmonium, which is treated as a nonrelativistic bound state of c and \bar{c}. Our key ingredients are the correct spin structure for the ^3S_1 c\bar{c} bound state, the off-shellness in the c\bar{c} \to J/psi (psi') hadronization vertex, and the modification of the gluon's longitudinal momentum fraction probed by the process, due to the relative motion between c and \bar{c}. We demonstrate that these three contributions produce the new Fermi motion effects in the LLA diffractive amplitude in QCD. It is found that our new effects moderate the strong suppression of the diffractive J/psi (psi') production cross sections, which was reported in the previous works on the Fermi motion effects. We emphasize the role of the transverse quark motion for the heavy meson production, and also discuss the strong helicity dependence of the Fermi motion effects and its implication in the longitudinal to transverse production ratio, sigma_L/sigma_T.

hep-ph↗

arXiv2003

Investigation of the color-dipole structure in diffractive t-slopes of charmonia photo- and electroproductions

The diffractive t-slope, B_V, of elastic charmonia (V=J/psi, psi') photo- and leptoproductions off a nucleon is studied at low |t| (< 1 GeV^2) in the leading logarithmic approximation of perturbative QCD, with a special emphasis on the space-time evolution of the c\bar{c}-dipole. We obey a framework based on QCD factorization, which describes a certain Fermi motion effect due to the $c(\bar{c})$-quarks in the proper manner and includes appropriately kinematical corrections of the momentum transfer Delta_perp in the t-channel. Assuming the universal two-gluon form factor of the nucleon, we show that the difference of t-slopes for J/psi and psi' is dominated by the contribution from the dipole-charmonium transition process. The calculated difference is found to be B_J/psi-B_psi' \sim 0.53 GeV^-2 for the photoproduction, in agreement with HERA data. We also calculate the t-dependence of the total cross sections for several center-of-mass energies W. A good agreement of the results with the available data demands that the mass scale appearing in the gluon form factor should significantly decrease with increasing W.

hep-ph↗

arXiv2000

Mass modification of D-meson at finite density in QCD sum rule

We evaluate the mass shift of isospin-averaged D-meson in the nuclear medium. Borel-transformed QCD sum rules are used to describe an interaction between the D-meson and a nucleon by taking into account all the lowest dimension-4 operators in the operator product expansion (OPE). We find at normal matter density the D-meson mass shift is about 10 times (\sim 50 MeV) larger than that of J/ψ. This originates from the fact that the dominant contribution in the OPE for the D-meson is the nucleon matrix element of m_c\bar{q}q, where m_c is the charm-quark mass and q denotes light quarks. We also discuss that the mass shift of the D-meson in nuclear matter may cause the level crossings of the charmonium states and the D\bar{D} threshold. This suggests an additional mechanism of the J/ψsuppression in high energy heavy-ion collisions.

nucl-th↗

arXiv1998

$J/ψ$-Nucleon Scattering Length and In-medium Mass Shift of $J/ψ$ in QCD Sum Rule Analysis

We calculate the spin-averaged $J/ψ$-nucleon scattering length $a_{J/ψ}$ by directly applying QCD sum rule to $J/ψ$-$N$ forward scattering amplitude. Our result $a_{J/ψ} = - 0.10 \pm 0.02$ fm, predicts the possibility of bound states with nuclei, though the force is weaker than that of the light vector mesons ($ρ,ω,ϕ$)-$N$ cases. Up to dimension-4 gluonic operators, we evaluate the scattering length with twist-2 contribution, which increases the absolute value of the scattering length about 30%. If we apply $a_{J/ψ}$ to the effective mass of $J/ψ$ in nuclear matter on the basis of the linear density approximation, it shows very slight decrease ($4 \sim 7$ MeV) at normal matter density.

nucl-th↗