arXiv subjects

Chihiro Sasaki

Publications and source records attributed to Chihiro Sasaki.

104 records · Page 6Linked to original sources

arXiv2007

Thermal Dilepton Production Rate from Dropping rho in the Vector Manifestation

In this write-up we summarize main points of our recent analysis on the thermal dilepton production rate from the dropping rho based on the vector manifestation (VM). In the analysis, we studied the effect of the strong violation of the vector dominance (VD), which is predicted by the VM, and showed that the effect of the violation of the VD substantially suppresses the dilepton production rate compared with the one predicted by assuming the VD together with the dropping rho.

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arXiv2007

Dropping rho and A1 Meson Masses at the Chiral Phase Transition in the Generalized Hidden Local Symmetry

In this contribution to the proceedings, we present our recent analysis on the chiral symmetry restoration in the generalized hidden local symmetry (GHLS) which incorporates the rho meson, $A_1$ meson and the pion consistently with the chiral symmetry of QCD. It is shown that a set of parameter relations, which ensures the first and second Weinberg sum rules, is invariant under the renormalization group evolution. Then, it is found that the Weinberg sum rules together with the matching of the vector and axial-vector current correlators to the OPE inevitably leads to {\it the dropping masses of both rho and $A_1$ mesons} at the symmetry restoration point, and that the mass ratio flows into one of three fixed points.

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arXiv2005

Pion Velocity near the Chiral Phase Transition Point in the Vector Manifestation

We study the pion velocity near the critical temperature $T_c$ of chiral symmetry restoration in QCD. Using chiral perturbation theory based on hidden local symmetry (HLS) as an effective field theory, where the chiral symmetry restoration is realized as the vector manifestation (VM), we show that the pion velocity for $T \to T_c$ is close to the speed of light, $v_π(T) = 0.83 - 0.99$ which is at variance with the sigma model result which predicts $v_π(T_c) = 0$. Our prediction on $v_π(T_c)$ is compared with the value extracted from the recent RHIC data.

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arXiv2005

Chiral Phase Transition in QCD and Vector Manifestation

Spontaneous chiral symmetry breaking is one of the most important features in low-energy QCD. The chiral symmetry is expected to be restored at very high temperature and/or density. Accompanied by the chiral phase transition, properties of hadrons will be changed especially near the critical point. The study of the phenomena associated with the chiral phase transition will give us some clues on the connection between the chiral symmetry and the low-energy hadron dynamics. We develop the theory based on the hidden local symmetry (HLS) at finite temperature, which is an effective field theory of QCD and includes pions and vector mesons as the dynamical degrees of freedom, and study the chiral phase transition in hot matter. We show that the chiral symmetry is restored as the vector manifestation (VM), in which the massless degenerate pion (and its flavor partners) and the longitudinal $ρ$ meson (and its flavor partners) as the chiral partner. We also present several predictions based on the VM. We estimate the critical temperature $T_c$ and show the following phenomena near $T_c$: the vector charge susceptibility becomes equal to the axial-vector charge susceptibility; the vector dominance of the electromagnetic form factor of the pion is largely violated; the pion velocity is close to the speed of light. Furthermore, we show that the remnant of the VM can be clearly seen in the system of heavy mesons. We expect that the VM and its predictions are testable by current and future experiments and the lattice analysis.

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arXiv2004

An Effective Field Theory at Finite Density

An effective theory to treat the dense nuclear medium by the perturbative expansion method is proposed as a natural extension of the Heavy Baryon Chiral Perturbation Theory (HBChPT). Treating the Fermi momentum scale as a separate scale of the system, we get an improved convergence and the conceptually clear interpretation. We compute the pion decay constant and the pion velocity in the nuclear medium, and find their characters different from what the usual HBChPT predicts. We also obtain the Debye screening scale at the normal nuclear matter density, and the damping scale of the pion wave. Those results indicate that the present theory, albeit its improvement over the HBChPT, has the limitation yet to go over to the medium of about 1.3 times of normal matter density due to the absence of the intrinsic density dependence of the coupling constants. We discuss how we overcome this limitation in terms of the renormalization method.

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arXiv2004

Chiral Doubling of Heavy-Light Hadrons and the Vector Manifestation of Hidden Local Symmetry

Starting with a hidden local symmetry Lagrangian at the vector manifestation (VM) fixed point that incorporates heavy-quark symmetry and matching the bare theory to QCD, we calculate the splitting of chiral doublers of heavy-light mesons proposed by Nowak, Rho and Zahed, and Bardeen and Hill. We show, in the three-flavor chiral limit, that the splitting is directly proportional to the light-quark condensate <\bar{q}q> and comes out to be \sim (1/3) m_N where m_N is the nucleon mass, implying that the splitting vanishes in the chiral limit at the chiral restoration point -- temperature T_c, density n_c or number of flavors N_F^c. The result turns out to be surprisingly simple with the vector (ρ) meson playing the crucial role in quantum corrections, pointing to the relevance of the VM to QCD in the way chiral symmetry is manifested in hadronic matter. We also make predictions on the hadronic decay processes of the excited heavy (charm) -light mesons \tilde{D}.

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arXiv2004

Non-renormalization Theorem Originating in a New Fixed Point of the Vector Manifestation

We study the pion velocity at the critical temperature of chiral symmetry restoration in QCD. Starting from the premise that the bare effective field theory is to be defined from the underlying QCD, we incorporate the effects of Lorentz non-invariance into the bare theory by matching an effective field theory to QCD at a suitable matching scale, and investigate how the Lorentz non-invariance existing in the bare theory influences physical quantities. Using the hidden local symmetry model as the effective field theory, where the chiral symmetry restoration is realized as the vector manifestation (VM), we find that the pion velocity at the critical temperature receives neither quantum nor (thermal) hadronic corrections at the critical temperature even when we start from the bare theory with Lorentz symmetry breaking. This is likely the manifestation of a new fixed point in the Lorentz non-invariant formulation of the VM.

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arXiv2004

Vector Manifestation and Violation of Vector Dominance in Hot Matter

We show the details of the calculation of the hadronic thermal corrections to the two-point functions in the effective field theory of QCD for pions and vector mesons based on the hidden local symmetry (HLS) in hot matter using the background field gauge. We study the temperature dependence of the pion velocity in the low temperature region determined from the hadronic thermal corrections, and show that, due to the presence of the dynamical vector meson, the pion velocity is smaller than the speed of the light already at one-loop level, in contrast to the result obtained in the ordinary chiral perturbation theory including only the pion at one-loop. Including the intrinsic temperature dependences of the parameters of the HLS Lagrangian determined from the underlying QCD through the Wilsonian matching, we show how the vector manifestation (VM), in which the massless vector meson becomes the chiral partner of pion, is realized at the critical temperature. We present a new prediction of the VM on the direct photon-$π$-$π$ coupling which measures the validity of the vector dominance (VD) of the electromagnetic form factor of the pion: We find that the VD is largely violated at the critical temperature, which indicates that the assumption of the VD made in several analyses on the dilepton spectra in hot matter may need to be weakened for consistently including the effect of the dropping mass of the vector meson.

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arXiv2004

Pion Velocity near the Chiral Phase Transition

We study the pion velocity near the critical temperature $T_c$ of chiral symmetry restoration in QCD. Using the hidden local symmetry (HLS) model as the effective field theory, where the chiral symmetry restoration is realized as the vector manifestation (VM), we show that the pion velocity for $T \to T_c$ receives neither quantum nor (thermal) hadronic corrections at the critical temperature even when we start from the bare theory with Lorentz symmetry breaking. We show that this is related to a new fixed point structure originated in the VM. Further we match at a matching scale the axial-vector current correlator in the HLS with the one in the operator product expansion for QCD, and present the matching condition to determine the bare pion velocity. We find that the pion velocity is close to the speed of light, $v_π(T) = 0.83 - 0.99$.

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arXiv2003

The Pion Velocity at Chiral Restoration and the Vector Manifestation

We study the effects of Lorentz non-invariance on the physical pion velocity at the critical temperature $T_c$ in an effective theory of hidden local symmetry (HLS) with the "vector manifestation" fixed point. We match at a "matching scale" $Λ_M$ the axial-vector current correlator in the HLS with the one in the operator product expansion for QCD, and present the matching condition to determine the bare pion velocity. We find that the physical pion velocity, which is found to be one at $T=T_c$ when starting from the Lorentz invariant bare HLS, remains close to one with the Lorentz non-invariance, $v_π(T_c) = 0.83 - 0.99$. This result is quite similar to the pion velocity in dense matter.

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arXiv2003

Vector Manifestation in Hot Matter and Violation of Vector Dominance

We summarize main mechanisms to realize the vector manifestation (VM), in which the massless vector meson becomes the chiral partner of pion, at the critical temperature in hot QCD within the framework of the model based on the hidden local symmetry. Then, we show a recent analysis on the direct photon-pi-pi coupling which measures the validity of the vector dominance (VD) of the electromagnetic form factor of the pion: The VM predicts that the VD is largely violated at the critical temperature.

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arXiv2003

Violation of Vector Dominance in the Vector Manifestation

The vector manifestation (VM) is a new pattern for realizing the chiral symmetry in QCD. In the VM, the massless vector meson becomes the chiral partner of pion at the critical point, in contrast with the restoration based on the linear sigma model. Including the intrinsic temperature dependences of the parameters of the HLS Lagrangian determined from the underlying QCD through the Wilsonian matching together with the hadronic thermal corrections, we present a new prediction of the VM on the direct photon-$π$-$π$ coupling which measures the validity of the vector dominance (VD) of the electromagnetic form factor of the pion. We find that the VD is largely violated at the critical temperature, which indicates that the assumption of the VD made in several analysis on the dilepton spectra in hot matter may need to be weakened for consistently including the effect of the dropping mass of the vector meson.

hep-ph↗

arXiv2003

Effective Degrees of Freedom at Chiral Restoration and the Vector Manifestation in HLS theory

The question as to what the relevant effective degrees of freedom at the chiral phase transition are remains largely unanswered and must be addressed in confronting both terrestrial and space laboratory observations purporting to probe matter under extreme conditions. We address this question in terms of the vector susceptibility χ_V (VSUS in short) and the axial-vector susceptibility χ_A (ASUS in short) at the temperature-induced chiral transition. We consider two possible, albeit simplified, cases that are contrasting, one that is given by the standard chiral theory where only the pions figure in the vicinity of the transition and the other that is described by hidden local symmetry (HLS) theory with the Harada-Yamawaki vector manifestation (VM) where nearly massless vector mesons also enter. We find that while in the standard chiral theory, the pion velocity v_πproportional to the ratio of the space component f_π^s of the pion decay constant over the time component f_π^t tends to zero near chiral restoration with f_π^t\neq 0, in the presence of the vector mesons with vanishing mass, the result is drastically different: HLS with VM {\it predicts} that χ_V automatically equals χ_A in consistency with chiral invariance and that v_π\sim 1$ with $f_π^t\approx f_π^s\to 0 as T\to T_c. These results are obtained in the leading order in power counting but we expect their qualitative features to remain valid more generally in the chiral limit thanks to the VM point.

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arXiv2002

Vector Manifestation in Hot Matter

Based on the hidden local symmetry (HLS) Lagrangian as an effective field theory of QCD, we find that the chiral symmetry restoration for hot QCD can be realized through the Vector Manifestation where the rho meson becomes massless degenerate with pi as the chiral partner. This is done by including, in addition to the hadronic thermal effects due to the pi- and rho-loops, the intrinsic temperature dependences of the parameters of the HLS Lagrangian through the matching of the HLS with the underlying QCD.

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