arXiv subjects

Masayasu Harada

Publications and source records attributed to Masayasu Harada.

At least 73 records · Page 4Linked to original sources

arXiv2016

Degeneracy of doubly heavy baryons from heavy quark symmetry

The spectroscopy of the doubly heavy baryons including different heavy quarks is studied based on the heavy quark symmetry of QCD. We point out that, when the two heavy quarks are in $S$-wave, these baryons with a certain spin $j_l$ of the light cloud can be classified into two sets: a heavy quark singlet with total spin of $j=j_l$ and a heavy quark multiplet with $j= (j_l+1) , j_l ,\ldots \vert j_l-1\vert$, all the baryons in these two sets have the same mass and, the baryons with the same quantum numbers in these two sets do not mix with each other. We finally point out that the strong decay of the first excited baryon with light spin $j_l = 1/2$ to the ground state and one-pion is determined by the mass splitting through the generalized Goldberger--Treiman relation.

hep-ph↗

arXiv2016

The mass of heavy-light mesons in a constituent quark picture with partially restored chiral symmetry

We probe effects of the partial chiral symmetry restoration to the mass of heavy-light mesons in a constituent quark model by changing the constituent quark mass of the light quark. Due to the competing effect between the quark mass and the linearly rising potential, whose contribution to the energy increases as the quark mass decreases, the heavy-light meson mass has a minimum value near the constituent quark mass typically used in the vacuum. Hence, the meson mass increases as one decreases the constituent quark mass consistent with recent QCD sum rule analyses, which show an increasing $D$ meson mass as the chiral order parameter decreases.

nucl-th↗

arXiv2015

Effects of scalar mesons in a Skyrme model with hidden local symmetry

We study the effects of light scalar mesons on the skyrmion properties by constructing and examining a mesonic model including pion, rho meson, and omega meson fields as well as two-quark and four-quark scalar meson fields. In our model, the physical scalar mesons are defined as mixing states of the two- and four-quark fields. We first omit the four-quark scalar meson field from the model and find that when there is no direct coupling between the two-quark scalar meson and the vector mesons, the soliton mass is smaller and the soliton size is larger for lighter scalar mesons; when direct coupling is switched on, as the coupling strength increases, the soliton becomes heavy, and the radius of the baryon number density becomes large, as the repulsive force arising from the $ω$ meson becomes strong. We then include the four-quark scalar meson field in the model and find that mixing between the two-quark and four-quark components of the scalar meson fields also affects the properties of the soliton. When the two-quark component of the lighter scalar meson is increased, the soliton mass decreases and the soliton size increases.

hep-ph↗

arXiv2015

Extended Goldberger-Treiman relation obtained in a three-flavor parity doublet model

We study masses and decay widths of positive and negative parity nucleons using a three-flavor parity doublet model, in which we introduce three representations, $\left[({\bf 3} , \bar{\bf 3})\oplus (\bar{\bf 3} , {\bf 3})\right]$, $\left[({\bf 3} , {\bf 6}) \oplus ({\bf 6}, {\bf 3})\right]$, and $\left[({\bf 8} , {\bf 1}) \oplus ({\bf 1} , {\bf 8})\right]$ of the chiral U$(3)_{\rm L}\times$U$(3)_{\rm R}$ symmetry. We find an extended version of the Goldberger-Treiman relation among the mass differences and the coupling constants for pionic transitions. This relation leads to an upper bound for the decay width of $N(1440) \rightarrow N(939) + π$ independently of the model parameters. We perform the numerical fitting of the model parameters and derive several predictions, which can be tested in future experiments or lattice QCD analysis. Furthermore, when we use the axial coupling of the excited nucleons obtained from lattice QCD analyses as inputs, we find that the ground state nucleon $N(939)$ consists of about 80% of $\left[({\bf 3} , {\bf 6}) \oplus ({\bf 6}, {\bf 3})\right]$ component, and that the chiral invariant mass of $N(939)$ is roughly $500$ -- $800$MeV.

hep-ph↗

arXiv2015

Inhomogeneous quark condensate in compressed skyrmion matter

The inhomogeneous quark condensate, responsible for dynamical chiral symmetry breaking in cold nuclear matter, is studied by putting skyrmions onto the face-centered cubic crystal and treating the skyrmion matter as nuclear matter. By varying the crystal size, we explore the effect of density on the local structure of the quark-antiquark condensate. By endowing the light vector mesons $ρ$ and $ω$ with hidden local symmetry and incorporating a scalar meson as a dilaton of spontaneously broken scale symmetry, we uncover the intricate interplay of heavy mesons in the local structure of the quark condensate in dense baryonic matter described in terms of skyrmion crystal. It is found that the inhomogeneous quark density persists to as high a density as $\sim 4$ times nuclear matter density. The difference between the result from the present approach and that from the chiral density wave ansatz is also discussed.

hep-ph↗

arXiv2015

Asymmetric nuclear matter in a parity doublet model with hidden local symmetry

We construct a model to describe dense hadronic matter at zero and finite temperature, based on the parity doublet model of DeTar and Kunihiro, with including the iso-singlet scalar meson $σ$ as well as $ρ$ and $ω$ mesons. We show that, by including a six-point interaction of $σ$ meson, the model reasonably reproduces the properties of the normal nuclear matter with the chiral invariant nucleon mass $m_0$ in the range from $500~{\rm MeV}$ to $900~{\rm MeV}$. Furthermore, we study the phase diagram based on the model, which shows that the value of the chiral condensate drops at the liquid-gas phase transition point and at the chiral phase transition point. We also study asymmetric nuclear matter and find that the first order phase transition for the liquid-gas phase transition disappears in asymmetric matter and that the critical density for the chiral phase transition at non-zero density becomes smaller for larger asymmetry.

nucl-th↗

arXiv2015

Doubly heavy baryons with chiral partner structure

The spectrum and dominant strong decay properties of the doubly heavy baryons are revisited by using a chiral effective model with chiral partner structure. By regarding the doubly heavy baryons in the ground states and light angular momentum $j_l = 1/2$ sector of the first orbitally excited states as chiral partners to each other, we estimate the mass splitting of the chiral partners which arises from the spontaneous breaking of chiral symmetry to be about $430$ MeV for baryons including an unflavored light quark and about $350$ MeV for that including a strange quark. We point out that, similar to the heavy-light meson sector, the intermultiplet decay from a baryon with negative parity to its chiral partner and a pion is determined by the mass splitting throught the generalized Goldberger-Treiman relation. Furthermore, the isospin violating decay of $Ω_{cc}$ baryon, $((1/2)^-, (3/2)^-)_s \to ((1/2)^+, (3/2)^+)_s + π^0$ through the $η$-$π^0$ mixing is the dominant decay channel of the doubly heavy baryons including a strange quark.

hep-ph↗

arXiv2014

Mass degeneracy of the heavy-light mesons with chiral partner structure in the half-skyrmion phase

We explore the mass splitting of the heavy-light mesons with chiral partner structure in nuclear matter. In our calculation, we employed the heavy hadron chiral perturbation theory with chiral partner structure and the nuclear matter is constructed by putting skyrmions from the standard Skyrme model onto the face-centered cubic crystal and regarding the skyrmion matter as nuclear matter. We find that, although the masses of the heavy-light mesons with chiral partner structure are splitted in the matter-free space and skyrmion phase, they are degenerated in the half-skyrmion phase in which the chiral symmetry is restored globally. This observation suggests that the magnitude of the mass splitting of the heavy-light mesons with chiral partner structure can be used as a probe of the phase structure of the nuclear matter.

hep-ph↗

arXiv2014

Equation of state in the pion condensation phase in the asymmetric nuclear matter using a holographic QCD model

We study the asymmetric nuclear matter using a holographic QCD model by introducing a baryonic charge in the infrared boundary. We first show that, in the normal hadron phase, the predicted values of the symmetry energy and it's slope parameter are comparable with the empirical values. We find that the phase transition from the normal phase to the pion condensation phase is delayed compared with the pure mesonic matter: The critical chemical potential is larger than the pion mass which is obtained for the pure mesonic matter. We also show that, in the pion condensation phase, the pion contribution to the isospin number density increases with the chemical potential, while the baryonic contribution is almost constant. Furthermore, the value of chiral condensation implies that the enhancement of the chiral symmetry breaking occurs in the asymmetric nuclear matter as in the pure mesonic matter. We also give a discussion on how to understand the delay in terms of the 4-dimensional chiral Lagrangian including the rho and omega mesons based on the hidden local symmetry.

hep-ph↗

arXiv2014

Dense baryonic matter in conformally-compensated hidden local symmetry: Vector manifestation and chiral symmetry restoration

We find that, when the dilaton is implemented as a (pseudo-)Nambu-Goldstone boson using a conformal compensator or "conformon" in a hidden gauge symmetric Lagrangian written to $O(p^4)$ from which baryons arise as solitons, namely, skyrmions, the vector manifestation and chiral symmetry restoration at high density predicted in hidden local symmetry theory --- which is consistent with Brown-Rho scaling --- are lost or sent to infinite density. It is shown that they can be restored if in medium the behavior of the $ω$ field is taken to deviate from that of the $ρ$ meson in such a way that the flavor $U(2)$ symmetry is strongly broken at increasing density. The hitherto unexposed crucial role of the $ω$ meson in the structure of elementary baryon and multibaryon systems is uncovered in this work. In the state of half-skyrmions to which the skyrmions transform at a density $n_{1/2}^{} \gtrsim n_0^{}$ (where $n_0^{}$ is the normal nuclear matter density), characterized by the vanishing (space averaged) quark condensate but nonzero pion decay constant, the nucleon mass remains more or less constant at a value $\gtrsim$ 60 \% of the vacuum value indicating a large component of the nucleon mass that is not associated with the spontaneous breaking of chiral symmetry. We discuss its connection to the chiral-invariant mass $m_0^{}$ that figures in the parity-doublet baryon model.

hep-ph↗

arXiv2014

$D$ and $D^{\ast}$ meson mixing in spin-isospin correlated cold nuclear matter

We propose to study the mass spectrum of the heavy-light mesons to probe the structure of the spin-isospin correlation in the nuclear medium. We point out that the spin-isospin correlation in the nuclear medium generates a mixing among the heavy-light mesons carrying different spins and isospins such as $D^+$, $D^0$, $D^{\ast +}$, and $D^{\ast 0}$ mesons. We use two types of correlations motivated by the skyrmion crystal and the chiral density wave as typical examples to obtain the mass splitting caused by the mixing. Our result shows that the structure of the mixing reflects the pattern of the correlation, i.e., the remaining symmetry. Furthermore, the magnitude of the mass modification provides information of the strength of the correlation.

hep-ph↗

arXiv2014

Chiral effective theories from holographic QCD with scalars

We develop a method for integrating out the heavy Kaluza-Klein modes of scalar type as well as those of vector and axial-vector types, in a class of hard-wall bottom-up approaches of holographic QCD models, including the Dirac-Born-Infeld and Chern-Simons parts. By keeping only the lowest-lying vector mesons, we first obtain an effective chiral Lagrangian of the vector mesons based on the hidden local symmetry, and all the low-energy constants in the HLS Lagrangian are expressed in terms of holographic integrals and, consequently, are fully determined by the holographic geometry and a few constants of mesons. We find that the Gell-Mann--Oakes--Renner relation is manifestly reproduced at the lowest order of derivative expansion. We also explicitly show that a naive inclusion of the Chern-Simons term cannot reproduce the desired chiral anomaly in QCD, and hence, some counterterms should be provided: This implies that the holographic QCD models of hard-wall type cannot give definite predictions for the intrinsic parity-odd vertices involving vector and axial-vector mesons. After integrating out the vector mesons from the HLS Lagrangian, we further obtain the Lagrangian of chiral perturbation theory for pseudoscalar mesons with all the low-energy constants fully determined.

hep-ph↗

arXiv2014

Pion condensation phase at finite isospin chemical potential in a Holographic QCD model

We summarize main points of our recent work where we studied the pion condensation for non-zero isospin chemical potential within a holographic QCD model. We confirmed that the second order phase transition with the mean field exponent occurs when the isospin chemical potential $μ_I$ exceed the pion mass, which is consistent with the result obtained by the chiral effective Lagrangian at $O(p^2)$. We find that a deviation for large $μ_I$ region can be understood as a higher order effects in the chiral effective Lagrangian. Our result shows that the chiral condensate defined by $\tildeσ \equiv \sqrt{ \langle σ\rangle^2 + \langle π^a \rangle^2 }$ is almost constant in the small $μ_I$ region, while it grows with $μ_I$ in the large $μ_I$ region showing an enhancement of the chiral symmetry breaking.

hep-ph↗

arXiv2014

Enhancement of Chiral Symmetry Breaking from the Pion condensation at finite isospin chemical potential in a holographic QCD model

We study the pion condensation at finite isospin chemical potential using a holographic QCD model. By solving the equations of motion for the pion fields together with those for the iso-singlet scalar and iso-triplet vector meson fields, we show that the phase transition from the normal phase to the pion condensation phase is second order with the mean field exponent, and that the critical value of the isospin chemical potential $μ_I$ is equal to the pion mass, consistently with the result obtained by the chiral effective Lagrangian at $O(p^2)$. For higher chemical potential, we find a deviation, which can be understood as a higher order effect in the chiral effective Lagrangian. We investigate the $μ_I$-dependence of the chiral condensate defined by $\tildeσ \equiv \sqrt{ \langle σ\rangle^2 + \langle π^a \rangle^2 }$. We find that $\tildeσ$ is almost constant in the small $μ_I$ region, while it grows with $μ_I$ in the large $μ_I$ region. This implies that the strength of the chiral symmetry breaking is not changed for small $μ_I$: The isospin chemical potential plays a role to rotate the "vacuum angle" of the chiral circle $\tan^{-1} \sqrt{ \langle π^a \rangle^2 / \langle σ\rangle^2 } $ with keeping the "radius" $\tildeσ$ unchanged for small $μ_I$. For large $μ_I$ region, on the other hand, the chiral symmetry breaking is enhanced by the existence of $μ_I$.

hep-ph↗

arXiv2013

Skyrmions, half-skyrmions and nucleon mass in dense baryonic matter

We explore the hadron properties in dense baryonic matter in a unified way by using a Skyrme model constructed with an effective Lagrangian which includes the $ρ$ and $ω$ vector mesons as hidden gauge bosons and is valid up to $O(p^4)$ in chiral expansion including the homogeneous Wess-Zumino terms. With the two input values of pion decay constant and the lowest lying vector meson mass which can be fixed in free space, all the other low energy constants in the effective Lagrangian are determined by their master formulas derived from holographic QCD models, which allows us to study the baryonic matter properties with no additional free parameters and thus without ambiguities. We find that the $ω$ field that figures in the homogeneous Wess-Zumino term plays a crucial role in the skyrmion structure and its matter properties. The most striking and intriguing observation is that the pion decay constant that smoothly drops with increasing density in the Skyrmion phase stops decreasing at $n_{1/2}^{}$ at which the skyrmions in medium fractionize into half-skyrmions and remains nearly constant in the half-skyrmion phase. In accordance with the large $N_c$ consideration, the baryon mass also stays non-scaling in the half-skyrmion phase. This feature is supported by the nuclear effective field theory with the parameters of the Lagrangian scaling modified at the skyrmion--half-skyrmion phase transition. Our exploration also uncovers the crucial role of the $ω$ meson in multi-baryon systems as well as in the structure of a single skyrmion.

hep-ph↗

arXiv2013

Dense Baryonic Matter in Hidden Local Symmetry Approach: Half-Skyrmions and Nucleon Mass

Hadron properties in dense medium are treated in a unified way in a skyrmion model constructed with an effective Lagrangian, in which the rho and omega vector mesons are introduced as hidden gauge bosons, valid up to O(p^4) terms in chiral expansion including the homogeneous Wess-Zumino terms. All the low energy constants - apart from f_pi and m_rho - are fixed by the master formula derived from the relation between 5-D hQCD and 4-D HLS. This allows one to pin down the density n_1/2 at which the skyrmions in medium fractionize into half-skyrmions, bringing in a drastic change in the EoS of dense matter. We find that the U(1) field that figures in the CS term in the hQCD action or equivalently the omega field in the hWZ term in the dimensionally reduced HLS action plays a crucial role in the half-skyrmion phase. The importance of the omega degree of freedom may be connected to what happens in the instanton structure of elementary baryon noticed in hQCD. The most striking and intriguing in what is found in the model is that f_pi that smoothly drops with increasing density in the skyrmion phase stops decreasing at n_1/2 and remains nearly constant in the half-skyrmion phase. In accordance with the large N_c consideration, the baryon mass also stays non-scaling in the half-skyrmion phase. This feature which is reminiscent of the parity-doublet baryon model with a chirally invariant mass m_0 is supported by the nuclear EFT with the parameters of the Lagrangian scaling modified at the skyrmion-half-skyrmion phase transition. It also matches with one-loop RG analysis based on HLS. A link between a non-vanishing m_0 and the origin of nucleon mass distinctive from dynamically generated mass is suggested. We briefly discuss the possible consequences of the topology change found in this paper on the forthcoming experiments at the RIB machines under construction.

hep-ph↗

arXiv2013

Parity doubling structure of nucleon at non-zero density in the holographic mean field theory

We develope the holographic mean field theory approach in a bottom-up holographic QCD model including baryons and scalar mesons in addition to vector mesons and pions. We study the effect of parity doubling structure of baryons at non-zero density to the equation of state between the chemical potential and the baryon number density. We first show that we can adjust the amount of nucleon mass coming from the chiral symmetry breaking by changing the boundary value of the five-dimensional baryon fields. Then, introducing the mean field for the baryon fields, we calculate the equation of state between the baryon number density and its corresponding chemical potential. Then, comparing the predicted equation of state with the one obtained in a Walecka type model, we extract the density dependence of the effective nucleon mass. The result shows that the effective mass decreases with increasing density, and that the rate of decreasing is more rapid for the larger percentage of the mass coming from the chiral symmetry breaking.

hep-ph↗

arXiv2013

Baryonic Matter in the Hidden Local Symmetry Induced from Holographic QCD Models

Baryonic matter is studied in the Skyrme model by taking into account the roles of $π,$ $ρ$, and $ω$ mesons through the hidden local symmetry up to $\mathcal{O}(p^4)$ terms including the homogeneous Wess-Zumino (hWZ) terms. Using the master formulas for the low energy constants derived from holographic QCD models the skyrmion matter properties can be quantitatively calculated with the input values of the pion decay constant $f_π$ and the vector meson mass $m_ρ^{}$. We find that the hWZ terms are responsible for the repulsive interactions of the $ω$ meson. In addition, the self-consistently included $\mathcal{O}(p^4)$ terms with the hWZ terms is found to increase the half skyrmion phase transition point above the normal nucleon density.

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