28063362
Convergence to a selfsimilar solution in general relativistic gravitational collapse
We study the spherical collapse of a perfect fluid with an equation of state $P=k\rho$ by full general relativistic numerical simulations. For $0<k\alt 0.036$, it has been known that there exists a general relativistic counterpart of the Larson-Penston self-similar Newtonian solution. The numerical simulations strongly suggest that, in the neighborhood of the center, generic collapse converges to this solution in an approach to a singularity and that self-similar solutions other than this solution, including a ``critical solution'' in the black hole critical behavior, are relevant only when the parameters which parametrize initial data are fine-tuned. This result is supported by a mode analysis on the pertinent self-similar solutions. Since a naked singularity forms in the general relativistic Larson-Penston solution for $0<k\alt0.0105$, this will be the most serious known counterexample against cosmic censorship. It also provides strong evidence for the self-similarity hypothesis in general relativistic gravitational collapse. The direct consequence is that critical phenomena will be observed in the collapse of isothermal gas in Newton gravity, and the critical exponent $\gamma$ will be given by $\gamma\approx 0.11$, though the order parameter cannot be the black hole mass.
119202614
Upper limits of particle emission from high-energy collision and reaction near a maximally rotating Kerr black hole
The center-of-mass energy of two particles colliding near the horizon of a maximally rotating black hole can be arbitrarily high if the angular momentum of either of the incident particles is fine-tuned, which we call a critical particle. We study particle emission from such high-energy collision and reaction in the equatorial plane fully analytically. We show that the unconditional upper limit of the energy of the emitted particle is given by 218.6% of that of the injected critical particle, irrespective of the details of the reaction and this upper limit can be realized for massless particle emission. The upper limit of the energy extraction efficiency for this emission as a collisional Penrose process is given by 146.6%, which can be realized in the collision of two massive particles with optimized mass ratio. Moreover, we analyze perfectly elastic collision, Compton scattering, and pair annihilation and show that net positive energy extraction is really possible for these three reactions. The Compton scattering is most efficient among them and the efficiency can reach 137.2%. On the other hand, our result is qualitatively consistent with the earlier claim that the mass and energy of the emitted particle are at most of order the total energy of the injected particles and hence we can observe neither super-heavy nor super-energetic particles.
119490437
Have we already detected astrophysical symptoms of space-time noncommutativity?
We discuss astrophysical implications of $\kappa$-Minkowski space-time, in which there appears space-time noncommutativity. We first derive a velocity formula for particles based on the motion of a wave packet. The result is that a massless particle moves at a constant speed as in the usual Minkowski space-time, which implies that an arrival time analysis by $\gamma$-rays from Markarian (Mk) 421 does not exclude space-time noncommutativity. Based on this observation, we analyze reaction processes in $\kappa$-Minkowski space-time which are related to the puzzling detections of extremely high-energy cosmic rays above the Greisen-Zatsepin-Kuzmin cutoff and of high-energy ($\sim$20 TeV) $\gamma$-rays from Mk 501.
83177
Particle velocity in noncommutative space-time
We investigate particle velocity in $\ensuremath{\kappa}$-Minkowski space-time, which is one of the realizations of noncommutative space-time. We emphasize that arrival time analyses by high-energy $\ensuremath{\gamma}$ rays or neutrinos, which have been considered as powerful tools to restrict the violation of Lorentz invariance, are not effective in detecting space-time noncommutativity. In contrast with these examples, we point out the possibility that low-energy massive particles play an important role in detecting it.
207379213
Kink instability and stabilization of the Friedmann universe with scalar fields
The evolution of weak discontinuity is investigated in the flat FRW universe with a single scalar field and with multiple scalar fields. We consider both massless scalar fields and scalar fields with exponential potentials. Then we find that a new type of instability, i.e., kink instability develops in the flat FRW universe with massless scalar fields. The kink instability develops with scalar fields with considerably steep exponential potentials, while less steep exponential potentials do not suffer from kink instability. In particular, assisted inflation with multiple scalar fields does not suffer from kink instability. The stability of general spherically symmetric self-similar solutions is also discussed.
119742208
Horizon Instability: a Local Analysis
The evolution of weak discontinuity is investigated on horizons in the n-dimensional static solutions in the Einstein-Maxwell-scalar-\Lambda system, including 
the Reissner-Nordstr"om-(anti) de Sitter black hole. The analysis is essentially local and nonlinear. We find that the Cauchy horizon is unstable, whereas both 
the black-hole event horizon and the cosmological event horizon are stable. This new instability, the so-called kink instability, of the Cauchy horizon is completely different from the well-known "infinite-blueshift" instability. The kink instability makes the analytic continuation beyond the Cauchy horizon unstable.
123245410
Neutron-rich Hypernuclei
Abstract Production and spectroscopy of neutron-rich Λ hypernuclei by double-charge exchange reactions ( π − , K + ) on nuclear targets are investigated in a theoretical viewpoint. The Λ–Σ coupling in nuclei induces a Σ admixture, which is enhanced in a neutron-excess environment, and plays an important role in production by a one-step mechanism π − p → K + Σ − via Σ − doorways in these reactions. A feasibility of extracting the Σ − admixture probability in the neutron-rich 10ΛLi hypernucleus by the 10B( π − , K + ) reaction is examined within a distorted-wave impulse approximation.
119341375
Calculation of the 3He (in-flight K−,n) reaction for searching the deeply-bound K−pp state
Abstract The formation of a deeply-bound K − p p state by the 3 He (in-flight K − , n ) reaction is investigated theoretically in the distorted-wave impulse approximation using the Green's function method. The expected inclusive and semi-exclusive spectra at p K − = 1.0 GeV / c and θ n = 0 ° are calculated for the forthcoming J-PARC E15 experiment. We employ optical potentials between the K − and “ pp ” core-nucleus, and demonstrate systematically the dependence of the spectral shape on V 0 and W 0 , which are the real and imaginary parts of the strength for the optical potential, respectively. The necessary condition to observe a distinct peak of the K − p p bound state with I = 1 / 2 , J π = 0 − in the spectrum turns out to be that the value of V 0 is deeper than ∼ − 100 MeV and W 0 shallower than ∼ − 100  MeV , of which the strength parameters come up to recent theoretical predictions.
118332512
Isospin Properties of (K-, N) Reactions for the Formation of Deeply-bound Antikaonic Nuclei
Abstract The formation of deeply-bound antikaonic K − / K ¯ 0 nuclear states in nuclear ( K − , N ) reactions is investigated theoretically within a distorted-wave impulse approximation (DWIA), considering the isospin properties of the Fermi-averaged K − + N → N + K ¯ elementary amplitudes. We calculate the formation cross sections of the deeply-bound K ¯ states by the ( K − , N ) reactions on the nuclear targets, 12C and 28Si, at incident K − lab momentum p K − = 1.0 GeV / c and θ lab = 0 ° , introducing a complex effective nucleon number N eff for unstable bound states in the DWIA. The results show that the deeply-bound K ¯ states can be populated dominantly by the ( K − , n ) reaction via the total isoscalar Δ T = 0 transition owing to the isospin nature of the K − + N → N + K ¯ amplitudes, and that the cross sections described by Re N eff and Arg N eff enable to deduce the structure of the K ¯ nuclear states; the calculated inclusive nucleon spectra for a deep K ¯ -nucleus potential do not show distinct peak structure in the bound region. The few-body K ¯ ⊗ [ N N ] and K ¯ ⊗ [ N N N ] states formed in ( K − , N ) reactions on s-shell nuclear targets, 3He, 3H and 4He, are also discussed.
121881632
Production and structure of light Sigma-hypernuclei