arXiv subjects

Shigehiro Yasui

Publications and source records attributed to Shigehiro Yasui.

At least 73 records · Page 4Linked to original sources

arXiv2015

Compositeness of the Delta(1232) resonance in pi N scattering

We evaluate the $πN$ compositeness of the $Δ(1232)$ resonance so as to clarify the internal structure of $Δ(1232)$ in terms of the $πN$ component. Here the compositeness is defined as contributions from two-body wave functions to the normalization of the total wave function and is extracted from the $πN$ scattering amplitude. In this study we employ the chiral unitary approach with the interaction up to the next-to-leading order plus a bare $Δ$ term in chiral perturbation theory and describe $Δ(1232)$ in an elastic $πN$ scattering. Fitting the $πN$ scattering amplitude to the solution of the partial wave analysis, we obtain a large real part of the $πN$ compositeness for $Δ(1232)$ comparable to unity and non-negligible imaginary part as well, with which we reconfirm the result in the previous study on the $πN$ compositeness for $Δ(1232)$.

hep-ph↗

arXiv2015

QCD Kondo effect: quark matter with heavy-flavor impurities

We show that the Kondo effect occurs in light quark matter which contains heavy quarks as impurities. We consider a scattering between a heavy-flavor impurity and a light quark near a Fermi surface which is mediated by gluon-exchange interactions. We find that the scattering amplitude has a logarithmic infrared divergence originating from imperfect cancellation between quark-impurity and hole-impurity scatterings in a loop integral, implying the presence of a strongly coupled regime near the Fermi surface. Renormalization group method is used to find the Kondo scale where a running coupling constant hits a Landau pole. Following an illustration by a simple contact-interaction model, we examine gluon-exchange interactions on the basis of high density QCD.

hep-ph↗

arXiv2014

Exotic dibaryons with a heavy antiquark

We discuss the possible existence of exotic dibaryons with a heavy antiquark, being realized three-body systems, Dbar^(*)NN and B^(*)NN. These are genuinely exotic states with no quark-antiquark annihilation. We consider the heavy quark spin and chiral symmetries, and introduce the one pion exchange potential between a Dbar^(*) (B^(*)) meson and a nucleon N. As for the NN interaction, we employ the Argonne v^\prime_8 potential. By solving the coupled-channel equations for PNN and P^* NN (P^(*)=Dbar^(*) and B^(*)), we find bound states for (I,J^P)=(1/2,0^-) as well as resonant states for (I,J^P)=(1/2,1^-) both in Dbar^(*)NN and B^(*)NN systems. We also discuss the heavy quark limit, and find that the spin degeneracy is realized in the bound states with (I,J^P)=(1/2,0^-) and (1/2,1^-).

nucl-th↗

arXiv2014

Vortices and Other Topological Solitons in Dense Quark Matter

In this review, we discuss various properties of topological solitons in dense QCD matter, with a particular emphasis on the CFL phase exhibiting superfluidity and superconductivity, and their phenomenological implications in terms of the effective field theories such as the Ginzburg-Landau theory, the chiral Lagrangian, or the Bogoliubov--de Gennes equation. The most fundamental topological excitations are non-Abelian vortices, which are 1/3 quantized superfluid vortices and color magnetic flux tubes. They are created at a phase transition or a rotation such compact stars. The intervortex-interaction is repulsive and consequently a vortex lattice is formed. Bosonic and fermionic zero-energy modes are trapped in the vortex core and propagate along it as gapless excitations. The former consists of translational zero modes (a Kelvin mode) with a quadratic dispersion and CP(2) Nambu-Goldstone gapless modes with a linear dispersion, while the latter is the triplet Majorana fermion zero modes. The low-energy effective theory of the bosonic zero modes is a non-relativistic free complex scalar field and a CP(2) model in 1+1 dimensions. The effects of strange quark mass, electromagnetic interactions and non-perturbative quantum corrections are taken into account. Colorful boojums at the CFL interface, quantum color magnetic monopole confined by vortices, which supports the notion of quark-hadron duality, and Yang-Mills instantons inside a vortex as lumps are discussed. The interactions between a vortex and quasi-particles such as phonons, gluons, mesons, and photons are studied. A vortex lattice is shown to behave as a cosmic polarizer. Non-Abelian vortices are shown to behave as a novel kind of non-Abelian anyons. For the chiral symmetry breaking, we discuss fractional and integer axial domain walls, Abelian and non-Abelian axial vortices, axial wall-vortex composites, and Skyrmions.

hep-ph↗

arXiv2013

Decays of Zb -> Upsilon + pi via triangle diagrams in heavy meson molecules

Bottomonium-like resonances Zb(10610) and Zb'(10650) are good candidates of hadronic molecules composed of BBbar* (or B*Bbar) and B*Bbar*, respectively. Considering Zb (Zb') as heavy meson molecules, we investigate the decays of Zb(')^+ -> Upsilon (nS) pi^+$ in terms of the heavy meson effective theory. We find that the intermediate B (B*) and Bbar (Bbar*) meson loops and the form factors play a significant role to reproduce the experimental values of the decay widths. We also predict the decay widths of Zc^+ -> J/ψpi^+ and psi(2S) pi^+ for a charmonium-like resonance Zc which has been reported recently in experiments.

hep-ph↗

arXiv2013

Non-Abelian statistics of vortices with non-Abelian Dirac fermions

We extend our previous analysis on the exchange statistics of vortices having a single Dirac fermion trapped in each core, to the case where vortices trap two Dirac fermions with U(2) symmetry. Such a system of vortices with non-Abelian Dirac fermions appears in color superconductors at extremely high densities, and in supersymmetric QCD. We show that the exchange of two vortices having doublet Dirac fermions in each core is expressed by non-Abelian representations of a braid group, which is explicitly verified in the matrix representation of the exchange operators when the number of vortices is up to four. We find that the result contains the matrices previously obtained for the vortices with a single Dirac fermion in each core as a special case. The whole braid group does not immediately imply non-Abelian statistics of identical particles because it also contains exchanges between vortices with different numbers of Dirac fermions. However, we find that it does contain, as its subgroup, a genuine non-Abelian statistics for the exchange of the identical particles, that is, vortices with the same number of Dirac fermions. This result is surprising compared with conventional understanding because all Dirac fermions are defined locally at each vortex, unlike the case of Majorana fermions for which Dirac fermions are defined non-locally by Majorana fermions located at two spatially separated vortices.

cond-mat.supr-con↗

arXiv2013

Production of doubly charmed tetraquarks with exotic color configurations in electron-positron collisions

Structure and production of doubly charmed tetraquarks T_cc (cc ubar dbar) are studied from the viewpoint of color configurations. Based on the diquark correlation, the tetraquark T_cc with I(JP)=0(1+) is considered to be stable against strong decay. We discuss that the mixing probability of color antitriplet and sextet cc components in T_cc is suppressed by 1/m_c^2, so the two configurations are separately realized in the heavy quark limit. Utilizing the nonrelativistic QCD framework, we evaluate the production cross sections of T_cc in electron-positron collisions. The momentum dependence of the cross section of color antitriplet is found to be different from that of sextet, which can be used to discriminate the color structure of the T_cc states in experimental measurements.

hep-ph↗

arXiv2013

Hadronic molecules for charmed and bottom baryons near thresholds

We study hadronic molecules formed by a heavy meson and a nucleon, DN and D*N (BbarN and Bbar*N) systems. Respecting the heavy quark symmetry and chiral symmetry, we consider the DN-D*N (BbarN-Bbar*N$) mixing induced by the one boson exchange potential including the tensor force. We find many bound and resonant states with J^P=1/2^\pm, 3/2^\pm, 5/2^\pm and 7/2^- in isospin singlet channels, while only a few resonant states with J^P=1/2^- in isospin triplet channels. The analysis of DN and D*N (BbarN and Bbar*N) molecules will be useful to study mass spectra of excited charmed (bottom) baryons with large angular momenta, when their masses are close to the DN and D*N (BbarN and Bbar*N) thresholds.

hep-ph↗

arXiv2012

Decays and productions via bottomonium for Z_b resonances and other B anti-B molecules

We discuss decays and productions for the possible molecular states formed by bottom mesons B (B*) and anti-B (anti-B*). The twin resonances found by Belle, Zb(10610) and Zb(10650), are such candidates. The spin wave functions of the molecular states are rearranged into those of heavy and light spin degrees of freedom by using the re-coupling formulae of angular momentum. By applying the heavy quark symmetry we derive model independent relations among various decay and production rates, which can be tested in experiments.

hep-ph↗

arXiv2012

Exotic baryons from a heavy meson and a nucleon

We evaluate a hadronic molecule formed by a heavy meson and a nucleon respecting heavy quark symmetry. The tensor force of pion exchange potential plays a dominate role to produce an strong attraction in this system. Solving coupled channel Schrödinger equations for PN and P* N, we find many bound and resonant states with isospin I=0 while there are few resonances in I=1 state. The rich structures with I=0 indicate that the spectrum of heavy baryons near the threshold is influenced by the contributions from such hadron composite structures.

hep-ph↗

arXiv2012

Exotic mesons with hidden bottom near thresholds

We study heavy hadron spectroscopy near open bottom thresholds. We employ B and B* mesons as effective degrees of freedom near the thresholds, and consider meson exchange potentials between them. All possible composite states which can be constructed from the B and B* mesons are studied up to the total angular momentum J = 0, 1, 2. We consider, as exotic states, isosinglet states with exotic (J,PC) quantum numbers and isotriplet states. We solve numerically the Schrodinger equation with channel-couplings for each state. The masses of twin resonances Zb(10608) and Zb(10653) recently found by Belle are reproduced. We predict several possible bound and/or resonant states in other channels for future experiments.

hep-ph↗

arXiv2012

Exotic mesons with double charm and bottom flavor

We study exotic mesons with double charm and bottom flavor, whose quark configuration is \bar{Q}\bar{Q}qq. This quark configuration has no annihilation process of quark and antiquark, and hence is a genuinely exotic states. We take a hadronic picture by considering the molecular states composed of a pair of heavy mesons, such as DD, DD* and D*D* for charm flavor, and BB, BB* and B*B* for bottom flavor. The interactions between heavy mesons are derived from the heavy quark effective theory. All molecular states are classified by I(J^P) quantum numbers, and are systematically studied up to the total angular momentum J \leq 2. By solving the coupled channel Schrodinger equations, due to the strong tensor force of one pion exchanging, we find bound and/or resonant states of various quantum numbers.

hep-ph↗

arXiv2012

Non-Abelian statistics of vortices with multiple Majorana fermions

We consider the exchange statistics of vortices, each of which traps an odd number ($N$) of Majorana fermions. We assume that the fermions in a vortex transform in the vector representation of the SO(N) group. Exchange of two vortices turns out to be non-Abelian, and the corresponding operator is further decomposed into two parts: a part that is essentially equivalent to the exchange operator of vortices having a single Majorana fermion in each vortex, and a part representing the Coxeter group. Similar decomposition was already found in the case with N=3, and the result shown here is a generalization to the case with an arbitrary odd $N$. We can obtain the matrix representation of the exchange operators in the Hilbert space that is constructed by using Dirac fermions non-locally defined by Majorana fermions trapped in separated vortices. We also show that the decomposition of the exchange operator implies tensor product structure in its matrix representation.

cond-mat.supr-con↗

arXiv2012

Dirac returns: non-Abelian statistics of vortices with Dirac fermions

Topological superconductors classified as type D admit zero-energy Majorana fermions inside vortex cores, and consequently the exchange statistics of vortices becomes non-Abelian, giving a promising example of non-Abelian anyons. On the other hand, types C and DIII admit zero-energy Dirac fermions inside vortex cores. It has been long believed that an essential condition for the realization of non-Abelian statistics is non-locality of Dirac fermions made of two Majorana fermions trapped inside two well-separated vortices as in the case of type D. Contrary to this conventional wisdom, however, we show that vortices with local Dirac fermions also obey non-Abelian statistics.

cond-mat.supr-con↗

arXiv2011

Exotic baryons from a heavy meson and a nucleon - Positive parity states -

We study heavy baryons with exotic flavor quantum numbers formed by a heavy meson and a nucleon (DbarN and BN) with positive parity. One pion exchange interaction, providing a tensor force, dominates as a long range force to bind the DbarN and BN ystems. In the heavy quark mass limit, pseudoscalar meson and vector meson are degenerate and the binding mechanism by the tensor force analogous to that in the nuclear systems becomes important. As a result, we obtain the DbarN and BN resonant states in the J^P=1/2^+, 3/2^+ and 5/2^+ channels with I=0.

hep-ph↗

arXiv2011

Hadron resonances with coexistence of different natures

We discuss coexistence/mixing of different natures of hadronic composite (molecule) and elementary (quark-intrinsic) ones in hadron resonances. The discussions here are based on our previous publications on the origin of hadron resonances \cite{Hyodo:2008xr}, exotic $\bar D$ meson-nucleons as hadronic composites containing one anti-heavy quark \cite{Yamaguchi:2011xb}, and the study of $a_1$ as a typical example to show explicitly the mixing of the two different natures \cite{Nagahiro:2011jn}. In all cases, interactions are derived from the chiral dynamics of the light flavor sector. These interactions generate in various cases hadronic composite/molecule states, serving varieties of structure beyond the conventional quark model.

hep-ph↗

arXiv2011

Studying Exotic Hadrons in Heavy Ion Collisions

We investigate the possibilities of using measurements in present and future experiments on heavy ion collisions to answer some longstanding problems in hadronic physics, namely identifying hadronic molecular states and exotic hadrons with multiquark components. The yields of a selected set of exotic hadron candidates in relativistic heavy ion collisions are discussed in the coalescence model in comparison with the statistical model. We find that the yield of a hadron is typically an order of magnitude smaller when it is a compact multiquark state, compared to that of an excited hadronic state with normal quark numbers. We also find that some loosely bound hadronic molecules are formed more abundantly than the statistical model prediction by a factor of two or more. Moreover, due to the significant numbers of charm and bottom quarks produced at RHIC and even larger numbers expected at LHC, some of the proposed heavy exotic hadrons could be produced with sufficient abundance for detection, making it possible to study these new exotic hadrons in heavy ion collisions.

nucl-th↗

arXiv2011

Exotic baryons from a heavy meson and a nucleon -- Negative parity states --

We study heavy baryons with an exotic flavor quantum number formed by a heavy meson and a nucleon (Dbar-N and B-N) through a long range one pion exchange interaction. The bound state found previously in the (I,JP) =(0,1/2-) channel survives when short range interaction is included. In addition, we find a resonant state with (I,JP)=(0,3/2-) as a Feshbach resonance predominated by a heavy vector meson and a nucleon (Dbar*-N and B*-N). We find that these exotic states exist for the charm and heavier flavor region.

hep-ph↗