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Register a new userNonlinear flavor development of a two-dimensional neutrino gas
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We present a numerical survey of the nonlinear flavor development of dense neutrino gases. This study is based on the stationary, two-dimensional ($x$ and $z$), two-beam, monochromatic neutrino line model with a periodic boundary condition along the $x$ direction. Similar to a previous work, we find that small-scale flavor structures can develop in a neutrino gas even if the physical conditions are nearly homogeneous along the $x$ axis initially. The power diffusion from the large-scale to small-scale structures increases with the neutrino density and helps to establish a semi-exponential dependence of the magnitudes of the Fourier moments on the corresponding wave numbers. The overall flavor conversion probabilities in the neutrino gases with small initial sinusoidal perturbations reach certain equilibrium values at large distances which are mainly determined by the neutrino-antineutrino asymmetry. Similar phenomena also exist in a neutrino gas with a localized initial perturbation, albeit only inside an expanding flavor conversion region. Our work suggests that a statistical treatment may be possible for the collective flavor oscillations of a dense neutrino gas in a multi-dimensional environment.
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We investigate the importance of going beyond the mean-field approximation in the dynamics of collective neutrino oscillations. To expand our understanding of the coherent neutrino oscillation problem, we apply concepts from many-body physics and quantum information theory. Specifically, we use measures of nontrivial correlations (otherwise known as entanglement) between the constituent neutrinos of the many-body system, such as the entanglement entropy and the Bloch vector of the reduced density matrix. The relevance of going beyond the mean field is demonstrated by comparisons between the evolution of the neutrino state in the many-body picture vs the mean-field limit, for different initial conditions.
Neutrinos emitted deep within a supernova explosion experience a self-induced index of refraction. In the stationary, one-dimensional (1D) supernova bulb model, this self-induced refraction can lead to a collective flavor transformation which is coherent among different neutrino momentum modes. Such collective oscillations can produce partial swaps of the energy spectra of different neutrino species as the neutrinos stream away from the proto-neutron star. However, it has been demonstrated that the spatial symmetries (such as the spherical symmetry in the bulb model) can be broken spontaneously by collective neutrino oscillations in multi-dimensional models. Using a stationary, 2D neutrino ring model we demonstrate that there exist two limiting scenarios where collective oscillations may occur. In one limit, the collective flavor transformation begins at a radius with relatively high neutrino densities and develops small-scale flavor structures. The loss of the spatial correlation in the neutrino flavor field results in similar (average) energy spectra for the anti-neutrinos of almost all energies and the neutrinos of relatively high energies. In the other limit, the flavor transformation starts at a radius where the neutrino densities are smaller (e.g., due to the suppression of the high matter density near the proto-neutron star). Although the spatial symmetry is broken initially, it is restored as the neutrino densities decrease, and the neutrinos of different flavors partially swap their energy spectra as in the 1D bulb model. This finding may have interesting ramifications in other aspects of supernova physics.
The flavor transformation in a dense neutrino gas can have a significant impact on the physical and chemical evolution of its surroundings. In this work we demonstrate that a dynamic, fast flavor oscillation wave can develop spontaneously in a one-dimensional (1D) neutrino gas when the angular distributions of the electron neutrino and antineutrino cross each other. Unlike the 2D stationary models which are plagued with small-scale flavor structures, the fast flavor oscillation waves remain coherent in the dynamic 1D model in both the position and momentum spaces of the neutrino. The electron lepton number is redistributed and transported in space as the flavor oscillation wave propagates, although the total lepton number remains constant. This result may have interesting implications in the neutrino emission in and the evolution of the compact objects such as core-collapse supernovae.
Neutrino flavor oscillations in the presence of ambient neutrinos is nonlinear in nature which leads to interesting phenomenology that has not been well understood. It was recently shown that, in the two-dimensional, two-beam neutrino Line model, the inhomogeneous neutrino oscillation modes on small distance scales can become unstable at larger neutrino densities than the homogeneous mode does. We develop a numerical code to solve neutrino oscillations in the multi-angle/beam Line model with a continuous neutrino angular distribution. We show that the inhomogeneous oscillation modes can occur at even higher neutrino densities in the multi-angle model than in the two-beam model. We also find that the inhomogeneous modes on sufficiently small scales can be unstable at smaller neutrino densities with ambient matter than without, although a larger matter density does shift the instability region of the homogeneous mode to higher neutrino densities in the Line model as it does in the one-dimensional supernova Bulb model. Our results suggest that the inhomogeneous neutrino oscillation modes can be difficult to treat numerically because the problem of spurious oscillations becomes more severe for oscillations on smaller scales.
We investigate the impact of the nonzero neutrino splitting and elastic neutrino-nucleon collisions on fast neutrino oscillations. Our calculations confirm that a small neutrino mass splitting and the neutrino mass hierarchy have very little effect on fast oscillation waves. We also demonstrate explicitly that fast oscillations remain largely unaffected for the time/distance scales that are much smaller than the neutrino mean free path but are damped on larger scales. This damping originates from both the direct modification of the dispersion relation of the oscillation waves in the neutrino medium and the flattening of the neutrino angular distributions over time. Our work suggests that fast neutrino oscillation waves produced near the neutrino sphere can propagate essentially unimpeded which may have ramifications in various aspects of the supernova physics.
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