Nonleptonic Weak Interactions Research Articles

Hyperon bulk viscosity and r-modes of neutron stars

ABSTRACT We propose and apply a new parametrization of the modified chiral effective model to study rotating neutron stars with hyperon cores in the framework of the relativistic mean-field theory. The inclusion of mesonic cross couplings in the model has improved the density content of the symmetry energy slope parameters, which are in agreement with the findings from recent terrestrial experiments. The bulk viscosity of the hyperonic medium is analyzed to investigate its role in the suppression of gravitationally driven r-modes. The hyperonic bulk viscosity coefficient caused by non-leptonic weak interactions and the corresponding damping time-scales are calculated and the r-mode instability windows are obtained. The present model predicts a significant reduction of the unstable region due to a more effective damping of oscillations. We find that from ∼108 K to ∼109 K, hyperonic bulk viscosity completely suppresses the r-modes leading to a stable region between the instability windows. Our analysis indicates that the instability can reduce the angular velocity of the star up to ∼0.3 ΩK, where ΩK is the Kepler frequency of the star.

THE MOST LUMINOUS SUPERNOVA ASASSN-15LH: SIGNATURE OF A NEWBORN RAPIDLY ROTATING STRANGE QUARK STAR

ABSTRACT In this paper we show that the most luminous supernova discovered very recently, ASASSN-15lh, could have been powered by a newborn ultra-strongly magnetized pulsar, which initially rotates near the Kepler limit. We find that if this pulsar is a neutron star, its rotational energy could be quickly lost as a result of gravitational-radiation-driven r-mode instability; if it is a strange quark star (SQS), however, this instability is highly suppressed due to a large bulk viscosity associated with the nonleptonic weak interaction among quarks and thus most of its rotational energy could be extracted to drive ASASSN-15lh. Therefore, we conclude that such an ultra-energetic supernova provides a possible signature for the birth of an SQS.

K ->2 Decay in the Nambu-Jona-Lasinio Model

We study the K →2π decays using the UL(3) × UR(3) version of the Nambu-Jona-Lasinio model with the effective ΔS = l nonleptonic weak interaction. The ΔI=3/2 amplitude is in reasonable agreement with experimental data. On the other hand, the calculated ΔI=1/2 amplitudes strongly depend on the mass of the low-lying scalar-isoscalar σ meson, and therefore give a strong constraint on the parameters of the model.

Hadronic parity violation at next-to-leading order

The flavor-conserving non-leptonic weak interaction can be studied experimentally through the observation of parity violation in nuclear and few-body systems. At hadronic scales, matrix elements of parity-violating four-quark operators ultimately give rise to the parity violating couplings between hadrons, and such matrix elements can be calculated non-perturbatively using lattice QCD. In this work, we investigate the running of isovector parity-violating operators from the weak scale down to hadronic scales using the renormalization group. We work at next-to-leading order in the QCD coupling, and include both neutral-current and charged-current interactions. At this order, results are renormalization scheme dependent, and we utilize 't Hooft-Veltman dimensional regularization. The evolution of Wilson coefficients at leading and next-to-leading order is compared. Next-to-leading order effects are shown to be non-negligible at hadronic scales.

Holographic approach to low-energy weak interactions of mesons

We apply the double-trace formalism to incorporate nonleptonic weak interactions of mesons into holographic models of the strong interactions. We focus our attention upon $\ensuremath{\Delta}S=1$ nonleptonic kaon decays. By working with a Yang-Mills--Chern-Simons 5-dimensional action, we explicitly show how, at low energies, one recovers the $\ensuremath{\Delta}S=1$ weak chiral Lagrangian for both the anomalous and nonanomalous sectors. We provide definite predictions for the low-energy coefficients in terms of the AdS metric and argue that the double-trace formalism is a 5-dimensional avatar of the Weak Deformation Model introduced long ago by Ecker et al. As a significant phenomenological application, we reassess the $K\ensuremath{\rightarrow}3\ensuremath{\pi}$ decays in the light of the holographic model. Previous models found a fine-tuned cancellation of resonance exchange in these decays, which was both conceptually puzzling and quantitatively in disagreement with experimental results. The holographic model we build is an illustrative counterexample showing that the cancellation encountered in the literature is not generic but a model-dependent statement and that agreement with experiment can be obtained.

Large amplitude behavior of the bulk viscosity of dense matter

We study the bulk viscosity of dense matter, taking into account nonlinear effects that arise in the large amplitude ‘supra-thermal’ region where the deviation of the chemical potentials from chemical equilibrium fulfills . This regime is relevant to unstable modes, such as r-modes, which grow in amplitude until saturated by nonlinear effects. We study the damping due to direct and modified Urca processes in hadronic matter, and due to non-leptonic weak interactions in strange quark matter. We give general results valid for an arbitrary equation of state of dense matter and find that the viscosity can be strongly enhanced by supra-thermal effects. Our study confirms previous results on quark matter and shows that the nonlinear enhancement is even stronger in the case of hadronic matter. Our results can be applied to calculations of the r-mode-induced spin-down of fast-rotating neutron stars, where the spin-down time will depend on the saturation amplitude of the r-mode.

Bulk viscosity in a hyperonic star andr-mode instability

We consider a rotating neutron star with the presence of hyperons in its core, using an equation of state in an effective chiral model within the relativistic mean field approximation. We calculate the hyperonic bulk viscosity coefficient due to nonleptonic weak interactions. By estimating the damping timescales of the dissipative processes, we investigate its role in the suppression of gravitationally driven instabilities in the $r$-mode. We observe that $r$-mode instability remains very much significant for hyperon core temperature of around $10^8 $K, resulting in a comparatively larger instability window. We find that such instability can reduce the angular velocity of the rapidly rotating star considerably upto $\sim0.04 \Omega_K$, with $\Omega_K$ as the Keplerian angular velocity.

Parity violation in neutron spin rotation

Nonleptonic weak interactions remain one of the most poorly understood sectors of the Standard Model. A quantitative description of the weak nucleon–nucleon (NN) interaction is needed to understand weak interaction phenomena in atomic, nuclear, and hadronic systems. Measurements with low energy neutrons can lead to significant experimental progress and have the potential to illuminate poorly understood features of QCD. I describe the phenomenon of parity-odd neutron spin rotation and outline experimental strategies to isolate this phenomenon in the NN system and in light nuclei.

Parity-violating nuclear force as derived from QCD sum rules

Parity-violating nuclear force, as may be accessed from parity-violation studies in nuclear systems, represents an area of nonleptonic weak interactions that has been the subject of experimental investigations for several decades. In the simple meson-exchange picture, a parity-violating nuclear force may be parametrized as arising from the exchange of $\ensuremath{\pi},\ensuremath{\rho},\ensuremath{\omega}$, or other mesons with strong meson-nucleon coupling at one vertex and weak parity-violating meson-nucleon coupling at the other vertex. The QCD sum rule method allows for a fairly complicated, but nevertheless straightforward, leading-order loop-contribution determination of the various parity-violating $\mathrm{MNN}$ couplings starting from QCD (with the nontrivial vacuum) and Glashow-Salam-Weinberg electroweak theory. We continue our earlier investigation of the parity-violating $\ensuremath{\pi}\mathit{NN}$ coupling (by Henley, Hwang, and Kisslinger) to other parity-violating couplings. Our predictions are in reasonable overall agreement with the results estimated on phenomenological grounds, such as in the now classic paper of Desplanques, Donoghue, and Holstein, in the global experimental fit of Adelberger and Haxton, or the effective field theory thinking of Ramsey-Musolf and Page.

Bulk viscosity of mixed nucleon���hyperon���quark matter in neutron stars

We calculate the coefficient of bulk viscosity by considering the non-leptonic weak interactions in the cores of hybrid stars with both hyperons and quarks. We first determine the dependence of the production rate of neutrons on the reaction rate of quarks in the non-leptonic processes, that is, Γn=KsΓs+ΓΛ+ 2Γ Σ −. The conversion rate, Ks, in our scenario is a complicated function of baryon number density. We also consider the medium effect of quark matter on bulk viscosity. Using these results, we estimate the limiting rotation of the hybrid stars, which may suppress the r-mode instability more effectively. Hybrid stars should be the candidates for the extremely rapid rotators.

Generating functional for strong and nonleptonic weak interactions

The generating functional for Green functions of quark currents is given in closed form to next-to-leading order in the low-energy expansion for chiral SU(3), including one-loop amplitudes with up to three meson propagators. Matrix elements and form factors for strong and nonleptonic weak processes with at most six external states can be extracted from this functional by performing three-dimensional flavour traces. To implement this procedure, a Mathematica program is provided that evaluates amplitudes with at most six external mesons, photons (real or virtual) and virtual W (semileptonic form factors). The program is illustrated with several examples that can be compared with existing calculations.

Effect of hyperon bulk viscosity on neutron-starr-modes

Neutron stars are expected to contain a significant number of hyperons in addition to protons and neutrons in the highest density portions of their cores. Following the work of Jones, we calculate the coefficient of bulk viscosity due to nonleptonic weak interactions involving hyperons in neutron-star cores, including new relativistic and superfluid effects. We evaluate the influence of this new bulk viscosity on the gravitational radiation driven instability in the r-modes. We find that the instability is completely suppressed in stars with cores cooler than a few times 10 9 K, but that stars rotating more rapidly than 10‐30% of maximum are unstable for temperatures around 10 10 K. Since neutron-star cores are expected to cool to a few times 10 9 K within seconds ~much shorter than the r-mode instability growth time! due to direct Urca processes, we conclude that the gravitational radiation instability will be suppressed in young neutron stars before it can significantly change the angular momentum of the star.

Electromagnetism in nonleptonic weak interactions

We construct a low-energy effective field theory that permits the complete treatment of isospin-breaking effects in nonleptonic weak interactions to next-to-leading order. To this end, we enlarge the chiral Lagrangian describing strong and Delta S=1 weak interactions by including electromagnetic terms with the photon as additional dynamical degree of freedom. The complete and minimal list of local terms at next-to-leading order is given. We perform the one-loop renormalization at the level of the generating functional and specialize to K -> pi pi decays.

Weak Hyperon-Nucleon Interaction in a Quark Model and Application to thepn→ΛpScattering

The standard theory of elementary particles has achieved remarkable success in describing weak-electromagnetic properties and reactions of leptons. It is successful for hadrons although a care must be taken for corrections due to strong interaction. At high energy perturbative calculations are reliable thanks to the asymptotic free nature of the strong interaction, while low energy phenomena can be studied with the help of low energy effective theories based on the low energy symmetries, such as gauge invariance and chiral symmetry. Also the renormalizability guaranteed by the gauge invariance allows us to relate phenomena at different energy-momentum scale with each other. For instance, employing the renormalization group equation we study energy-scale dependence of physical matrix elements. Yet, a few unsolved problems remain for the hadronic weak interactions. One such problem is the mechanism for nonleptonic weak interactions of hadrons. For instance, the ∆I =1 /2 dominance in the flavor changing weak decays has not convincingly been explained. Chiral perturbation theory for the weak decays of hyperons cannot even fit experimental data of the decay amplitudes. One of the reasons of the difficulty seems that nonperturbative corrections of the strong interaction are fully intermingled with the weak interaction. There is no current conservation which protects the weak vertex from strong renormalization contrary to leptonic processes, where the vector and the axial vector currents are (almost) conserved. This has been a challenging problem in the hadron physics. Recent experimental studies of weak decays of hypernuclei have added a new difficulty to the nonleptonic weak interaction of hadrons. In hypernuclei, the Λ decays from the ground state into nonstrange hadrons. The standard decay modes of Λ are Λ → pπ − and Λ → nπ 0 , but these modes are known to be suppressed in nuclear medium as the momentum of the final nucleon is not large enough to go above the Fermi surface. Instead, the main decay modes of medium-to heavy

Axion emissivity from the conversion of a neutron star into a strange star

The conversion of neutron matter into strange matter in a neutron star occurs through the non-leptonic weak-interaction process. We study the energy loss of the neutron star by the emission of axions in that process. Owing to that process, the neutron star will liberate the energy which can in no way be negligible as an axion burst.

K→2π Decay in the Nambu-Jona-Lasinio Model

We study the $K \to 2 \pi$ decays using the $U_L(3) \times U_R(3)$ version of the Nambu-Jona-Lasinio model with the effective $\Delta S = 1$ nonleptonic weak interaction. The $\Delta I = {3/2}$ amplitude is in reasonable agreement with experimental data. On the other hand, the calculated $\Delta I = {1/2}$ amplitudes strongly depend on the mass of the low-lying scalar-isoscalar $\sigma$ meson, and therefore give a strong constraint on the parameters of the model.

Chiral perturbation theory

The main elements and methods of chiral perturbation theory, the effective field theory of the Standard Model below the scale of spontaneous chiral symmetry breaking, are summarized. Applications to the interactions of mesons and baryons at low energies are reviewed, with special emphasis on developments of the last three years. Among the topics covered are the strong, electromagnetic and semileptonic weak interactions of mesons at and beyond next-to-leading order in the chiral expansion, nonleptonic weak interactions of mesons, virtual photon corrections and the meson-baryon system. The discussion is limited to processes at zero temperature, for infinite volume and with at most one baryon.

Strong and Weak Interactions of Strange Hadrons

I review hadronic processes involving strange hadrons, especially hyperons from the quark structure point of view. The strong interaction of quarks expects several important new features when the strangeness is introduced upon the non-strange degrees of freedom. I concentrate on the instanton induced interaction, effects of the Pauli principle for strangeness and the nonleptonic weak interaction of strangeness. The single hadron as well as the two baryon systems including the $H$ dibaryon and other exotic hadronic systems with strangeness are studied in this context.

Chiral pole model and nonleptonic weak interactions

We employ chiral symmetry in quark and in hadron language to show why theCP-conserving KL-KS mass difference ΔmLS is about one-half of the KS total decay rate Γs

Chiral pole model and nonleptonic weak interactions

The linear σ-model for chiral-limiting strong interactions is extended via a chiral pole model to CP-conserving nonleptonic weak decays. Then the eight measured modes of K S → ππ, K L → πππ, K S → ππγ, K L → γγ, K S → γγ, K L → πγγ are mapped out in terms of only reduced matrix element scale | | ≃ ≃ 3.4 . 10 −8 GeV 2 . Again employing chiral symmetry, this scale is correctly predicted by a ΔI = 1/2 s-d self-energy graph in the quark model