Kaon Condensation Research Articles

HESS J1731-347 and the existence of exotic matter: Kaon condensation in neutron stars

The recent observation of a compact star with the lowest observable mass of 0.77+0.20 –0.17 M⊙ within the supernova remnant HESS J1731-347 has redefined our understanding for the dense nuclear matter equation of state and enhances the existence of exotic matter. In the present work, we investigate the possible existence of kaon condensation in hadronic neutron stars through the framework of the Momentum-Dependent Interaction nuclear model describing the dense nuclear matter. We concentrate on the strangeness content of the proton and its implications on both the microscopic and macroscopic properties of neutron stars. The aforementioned quantity is of interest as it is also directly related to the location of the phase transition to kaons. The simultaneous description of the compact object within the HESS J1731-347 and the maximum observable neutron star mass imposes severe constraints on the strangeness content of the proton and the critical density for kaon condensation. The present study along with observations of neutron stars, may help to provide tighter constraints on the equation of state of dense nuclear matter and, even more, to shed light to the existence of an exotic core in neutron stars.

Kaon condensation in skyrmion matter and compact stars

We address the possibility of the appearance of a charged kaon condensate in neutron star cores described within a generalized Skyrme model. Our treatment of strange degrees of freedom is based on the Bound State Approach by Callan and Klebanov, which allows to obtain an in-medium effective potential for the $s$-wave kaon condensate. We predict the onset of kaon condensation at a certain threshold density - whose value depends on the parameters of the model, and ranges between 1.5 and 2.5 times saturation density -, and obtain both the particle fractions and equation of state for dense matter in the kaon condensed phase. Finally, we discuss the effect of such condensates on the mass-radius curves and other observable properties of neutron stars with kaon condensed cores.

(Anti)kaon condensation in strongly magnetized dense matter

Recent observations of several massive pulsars, with masses near and above $2~M_\odot$, point towards the existence of matter at very high densities, compared to normal matter that we are familiar with in our terrestrial world. This leads to the possibility of appearance of exotic degrees of freedom other than nucleons inside the core of the neutrons stars (NS). Another significant property of NSs is the presence of high surface magnetic field, with highest range of the order of $\sim~10^{16}$ G. We study the properties of highly dense matter with the possibility of appearance of heavier strange and non-strange baryons, and kaons in presence of strong magnetic field. We find that the presence of a strong magnetic field stiffens the matter at high density, delaying the kaon appearance and, hence, increasing the maximum attainable mass of NS family.

Kaon–baryon coupling schemes and kaon condensation in hyperon-mixed matter

Abstract The possible coexistence of kaon condensation and hyperons in highly dense matter [the (Y + K) phase] is investigated on the basis of the relativistic mean-field theory combined with the effective chiral Lagrangian. Two coupling schemes for the s-wave kaon–baryon interaction are compared regarding the onset density of kaon condensation in hyperon-mixed matter and the equation of state for the developed (Y + K) phase. One is the contact interaction scheme related to the nonlinear effective chiral Lagrangian. The other is the meson exchange scheme, where the interaction vertices between the kaon field and baryons are described by an exchange of mesons (σ, σ* mesons for scalar coupling, and ω, ρ, ϕ mesons for vector coupling). It is shown that in the meson exchange scheme, the contribution from the nonlinear scalar self-interaction gives rise to a repulsive effect for the kaon effective energy, pushing up the onset density of kaon condensation as compared with the contact interaction scheme. In general, the difference in kaon–baryon dynamics between the contact interaction scheme and the meson exchange scheme relies on the specific forms of the nonlinear self-interacting meson terms. They generate many-baryon forces through the equations of motion for the meson mean fields. However, they should have a definite effect on the ground state properties of nuclear matter only around the saturation density. It is shown that the nonlinear self-interacting term is not relevant to repulsive energy leading to stiffening of the equation of state at high densities, and that it cannot be compensated with a large attractive energy due to the appearance of the (Y + K) phase in the case of the contact interaction scheme. We also discuss what effects are necessary in the contact interaction scheme to make the equation of state with (Y + K) phase stiff enough to be consistent with recent observations of massive neutron stars.

Kaon meson condensate in neutron star matter including hyperons

The recent measurement of the mass of neutron stars (PSR J1614 - 2230, PSR J0348 + 0432, MSP J0740 + 6620) restricts the lower limit $\sim 2M_{\odot}$ of the maximum mass of such compact stars, making it possible for dense matter to exist in massive stars. The relativistic mean field theory with parameter sets FSUGold including Kaon condensation is used to describe the properties of neutron stars in $\beta$ equilibrium. Through careful choice of the parameter of the $\sigma$-cut $c_{\sigma}$, we are able to produce a maximum mass neutron star with Kaon condensation heavier than $2M_{\odot}$, and we find that the parameter $\Lambda_{\nu}$ of the $\rho-\omega$ interaction term in this model has a significant effect on $K^{-}$ condensation. In the case of using $\sigma$-cut scheme, $K^{-}$ condensation occurs only when the $\rho-\omega$ interaction $\Lambda_{\nu}$ is switched off.

Phases and condensates in zero-temperature QCD at finite μI and μS

I discuss pion and kaon condensation and the the properties of the phases of QCD at finite isospin chemical potential μI and strangeness chemical potential μS at zero temperature using three-flavor chiral perturbation theory. Electromagnetic effects are included in the calculation of the phase diagram, which implies that the charged meson condensed phases become superconducting phases of QCD with a massive photon via the Higgs mechanism. Without electromagnetic effects, we show results for the light quark condensate and the pion condensate as functions of μI at next-to-leading (NLO) order in the lowenergy expansion. The results are compared with recent lattice simulations and by including the NLO corrections, one obtains very good agreement.

Strange mesons in nuclei and neutron stars

The present status in the field of strange mesons in nuclei and neutron stars is reviewed. In particular, the K̅N interaction, that is governed by the presence of the Λ(1405), is analyzed and the formation of the K̅NN bound state is discussed. Moreover, the properties of K̅ in dense nuclear matter are studied, in connection with strangeness production in nuclear collisions and kaon condensation in neutron stars.

Binary neutron star mergers within kaon condensation:GW170817

The equation of state (EoS) of neutron star (NS) matter is investigated considering kaon condensation. Moreover, the tidal parameters related to the components of binary neutron star mergers are determined and compared to the constraints of GW170817 imposed on these quantities. In this study, we employ the lowest-order constrained variational (LOCV) approach and utilize AV6’, AV8’, and AV18 potentials accompanied by the three nucleon interaction (TNI) in order to consider the nucleon-nucleon interaction. It is known that the existence of kaons in the core of NSs softens the EoS and thus lowers the value of maximum mass which is expected to be greater than 2 M⊙. Our results demonstrate that considering kaon condensation with a strangeness value, a3 m s = −134 MeV satisfies the maximum mass constraint except for AV8’+TNI potential. However, the calculation of dimensionless tidal deformability shows that with the decrease of strangeness value, the neutron star gets less deformed.

Effects of three-baryon forces on kaon condensation in hyperon-mixed matter

Possibility of kaon-condensed phase in hyperon-mixed matter is considered on the basis of chiral symmetry for kaon-baryon and kaon-kaon interactions, being combined with the relativistic mean-field theory for two-body baryon interaction. In addition, universal three-baryon repulsive force in the string-junction model and phenomenological three-nucleon attractive force are introduced. It is shown that softening of the equation of state stemming from both kaon condensation and mixing of hyperons is compensated with the repulsive effect of the three-baryon force and the relativistic effect for two-body baryon-baryon interaction. The latter effect reflects the density-dependence of scalar and vector meson mean-fields, which is constrained by the contribution of the attractive three-nucleon force to the binding energy at saturation density. The kaon-condensed phase in hyperon-mixed matter becomes stiff enough to be consistent with recent observations of massive neutron stars.

Massive Δ -resonance admixed hypernuclear stars with antikaon condensations

In this work, we study the effect of (anti)kaon condensation on the properties of compact stars that develop hypernuclear cores with and without an admixture of $\Delta$-resonances. We work within the covariant density functional theory with the parameters adjusted to $K$-atomic and kaon-nucleon scattering data in the kaonic sector. The density-dependent parameters in the hyperonic sector are adjusted to the data on $\Lambda$ and $\Xi^-$ hypernuclei data. The $\Delta$-resonance couplings are tuned to the data obtained from their scattering off nuclei and heavy-ion collision experiments. We find that (anti)kaon condensate leads to a softening of the equation of state and lower maximum masses of compact stars than in the absence of the condensate. Both the $K^-$ and $\bar K^0$-condensations occur through a second-order phase transition, which implies no mixed-phase formation. For large values of (anti)kaon and $\Delta$-resonance potentials in symmetric nuclear matter, we observe that condensation leads to an extinction of $ \Xi^{-,0}$ hyperons. We also investigate the influence of inclusion of additional hidden-strangeness $\sigma^{*}$ meson in the functional and find that it leads to a substantial softening of the equation of state and delay in the onset of (anti)kaons.

Dense matter equation of state of a massive neutron star with antikaon condensation

Recent measurements of neutron star mass from several candidates (PSR J$1614-2230$, PSR J$0348+0432$, MSP J$0740+6620$) set the lower bound on the maximum possible mass for this class of compact objects $\sim 2$ M$_\odot$. Existence of stars with high mass brings the possibility of existence of exotic matter (hyperons, meson condensates) at the core region of the objects. In this work, we investigate the (anti)kaon ($K^-, \bar{K}^0$) condensation in $\beta-$equilibrated nuclear matter within the framework of covariant density functional theory. The functionals in the kaonic sector are constrained by the experimental studies on $K^-$ atomic, kaon-nucleon scattering data fits. We find that the equation of state softens with the inclusion of (anti)kaon condensates, which lowers the maximum mass of neutron star. In one of the density-independent coupling cases, the $K^-$ condensation is through a first-order phase transition type, which produces a $2$ M$_\odot$ neutron star. The first-order phase transition results in mixed phase region in the inner core of the stars. While $\bar{K}^0$ condensation appears via second-order phase transition for all the models we consider here.

The QCD axion at finite density

We show how the properties of the QCD axion change in systems at finite baryonic density, such as neutron stars. At nuclear saturation densities, where corrections can be reliably computed, we find a mild reduction of the axion mass and up to an order of magnitude enhancement in the model-independent axion coupling to neutrons. At moderately higher densities, if realized, meson (kaon) condensation can trigger axion condensation. We also study the axion potential at asymptotically large densities, where the color-superconducting phase of QCD potentially leads to axion condensation, and the mass of the axion is generically several orders of magnitude smaller than in vacuum due to the suppressed instantons. Several phenomenological consequences of the axion being sourced by neutron stars are discussed, such as its contribution to their total mass, the presence of an axionic brane, or axion-photon conversion in the magnetosphere.

Pion and kaon condensation at zero temperature in three-flavor \u03c7PT at nonzero isospin and strange chemical potentials at next-to-leading order

We consider three-flavor chiral perturbation theory (χPT) at zero temperature and nonzero isospin (μI) and strange (μS) chemical potentials. The effective potential is calculated to next-to-leading order (NLO) in the π±-condensed phase, the K±-condensed phase, and the {K}^0/{overline{K}}^0 -condensed phase. It is shown that the transitions from the vacuum phase to these phases are second order and take place when, left|{mu}_Iright|={m}_{pi },left|frac{1}{2}{mu}_I+{mu}_Sright|={m}_K , and left|-frac{1}{2}{mu}_I+{mu}_Sright|={m}_K , respectively at tree level and remains unchanged at NLO. The transition between the two condensed phases is first order. The effective potential in the pion-condensed phase is independent of μS and in the kaon-condensed phases, it only depends on the combinations pm frac{1}{2}{mu}_I+{mu}_S and not separately on μI and μS. We calculate the pressure, isospin density and the equation of state in the pion-condensed phase and compare our results with recent (2 + 1)-flavor lattice QCD data. We find that the three-flavor χPT results are in good agreement with lattice QCD for μI< 200 MeV, however for larger values χPT produces values for observables that are consistently above lattice results. For μI> 200 MeV, the two-flavor results are in better agreement with lattice data. Finally, we consider the observables in the limit of very heavy s-quark, where they reduce to their two-flavor counterparts with renormalized couplings. The disagreement between the predictions of two and three flavor χPT can largely be explained by the differences in the experimental values of the low-energy constants.

Fluctuations in cool quark matter and the phase diagram of quantum chromodynamics

We consider the phase diagram of hadronic matter as a function of temperature, T , and baryon chemical potential, mu. Currently the dominant paradigm is a line of first order transitions which ends at a critical endpoint. In this work we suggest that spatially inhomogenous phases are a generic feature of the hadronic phase diagram at nonzero mu and low T . Familiar examples are pion and kaon condensates. At higher densities, we argue that these condensates connect onto chiral spirals in a quarkyonic regime. Both of these phases exhibit the spontaneous breaking of a global U(1) symmetry and quasi-long range order, analogous to smectic liquid crystals. We argue that there is a continuous line of first order transitions which separate spatially inhomogenous from homogenous phases, where the latter can be either a hadronic phase or a quark-gluon plasma. While mean field theory predicts that there is a Lifshitz point along this line of first order transitions, in three spatial dimensions strong infrared fluctuations wash out any Lifshitz point. Using known results from inhomogenous polymers, we suggest that instead there is a Lifshitz regime. Non-perturbative effects are large in this regime, where the momentum dependent terms for the propagators of pions and associated modes are dominated not by terms quadratic in momenta, but quartic. Fluctuations in a Lifshitz regime may be directly relevant to the collisions of heavy ions at (relatively) low energies, sqrt(s)/A : 1 to 20 GeV.

Nuclear Matter Under Extreme Conditions: from Quark-Gluon Plasma to Neutron Stars

Most of the mass in the Universe is dynamically generated by the strong interaction that can be described by the Quantum Chromodynamics (QCD) in the present Standard Model. However, QCD works only in the limit of asymptotic freedom with small coupling constant at high energies. When the two heavy ions collide each other at high energies, very dense quark-gluon matter (the so-called quark-gluon plasma or QGP) is formed, and QCD cannot properly describe the dynamics in QGP at present. Another major trend in the contemporary QCD research is to explore the neutron-rich nuclear matter produced by the radioactive ion beams. Such an abnormal state of the strongly interacting matter is conjectured to be the core of the neutron star. The dense neutron-rich matter is expected to be the ground of the various exotic phenomena like kaon condensation. Understanding the structure and the dynamics of the neutron matter is one of the major research topics in not only nuclear physics but also astrophysics. In this review, the status and future perspectives of the research on nuclear matter under extreme conditions are briefly summarized.

3a-A-2 K中間子凝縮のモデル依存性

3a-A-2 K中間子凝縮のモデル依存性

Meson properties in asymmetric matter

In this work we study dynamic and thermodynamic (at T = 0) properties of mesons in asymmetric matter in the framework of Chiral Perturbation Theory. We consider a system at vanishing temperature with nonzero isospin chemical potential and strangeness chemical potential; meson masses and mixing in the normal phase, the pion condensation phase and the kaon condensation phase are described. We find differences with previous works, but the results presented here are supported by both theory group analysis and by direct calculations. Some pion decay channels in the normal and the pion condensation phases are studied, finding a nonmonotonic behavior of the decay width as a function of µI. Furthermore, pressure, density and equation of state of the system at T = 0 are studied, finding remarkable agreement with analogue studies performed by lattice calculations.

Instability of baryonic matter in relativistic mean-field models

In this article we evaluate some of the properties of strongly interacting hadronic matter using non-linear Walecka model in relativistic mean-field approximation. We find that due to the coupling of baryons and bosons, baryonic matter develops instability that is characterised by the appearance of baryonic state with negative entropy density. In the region of instability of baryonic matter we find no evidence of negative in-medium mass of baryons or any signal of kaon condensation. Further we find that this instability of baryonic matter can halt the process of first order phase transition from strongly interacting hadronic matter consisting of coupled baryons and bosons to a weakly interacting quark–gluon plasma phase.

Meson physics in asymmetric matter

This paper describes dynamic and thermodynamic (at T = 0) properties of mesons in asymmetric matter in the framework of Chiral Perturbation Theory. We consider the effect of nonzero isospin and strangeness chemical potentials on a mesonic system and report on the corresponding phase diagram. We also study meson masses and mixing in the resulting normal phase, pion condensation phase and kaon condensation phase. We find differences with previous papers regarding meson masses and mixing in the condensed phases; the results presented here are supported by theory group analysis and direct calculations. Pressure, density and equation of state of the system at T = 0 and nonzero μI are calculated, finding remarkable agreement with analogue studies performed by lattice calculations.

Renormalisation group flow and kaon condensation

The functional renormalisation group approach is applied to a system of kaons with finite chemical potential. A set of approximate flow equations for the effective couplings are derived and solved. At high scale, the system is found to be at the normal phase whereas at some critical values of the running scale it undergoes phase transition to the phase with spontaneously broken symmetry with the kaon condensate as an order parameter. The value of the condensate turns out to be quite sensitive to the kaon–kaon scattering length.