Occurrence of fast neutrino flavor conversions in QCD phase-transition supernovae

Abstract

Core-collapse supernovae undergoing a first-order quantum chromodynamics (QCD) phase transition experience the collapse of the central proto-neutron star that leads to a second bounce. This event is accompanied by the release of a second neutrino burst. Unlike the first stellar core bounce neutrino burst which consists exclusively of electron neutrinos, the second burst is dominated by electron antineutrinos. Such a condition makes QCD supernovae an ideal site for the occurrence of fast neutrino flavor conversion (FFC), which can lead to rapid flavor equilibration and significantly impact the related neutrino signal. In this work, we perform a detailed analysis of the conditions for fast flavor instability (FFI) around and after the second neutrino burst in QCD phase transition supernova models launched from 25~$M_\odot$ and 40~$M_\odot$ progenitor models. We evaluate the relevant instability criteria and find two major phases of FFC. The first phase is closely associated with the collapse and the rapidly expanding shock wave, which is a direct consequence of the proto-neutron star collapse due to the phase transition. The second phase takes place a few milliseconds later when electron degeneracy is restored near the proto-neutron star surface. We also characterize the growth rate of FFI and estimate its impact on the evolution of the neutrino flavor content. The potential observational consequences on neutrino signals are evaluated by comparing a scenario assuming complete flavor equipartition with other scenarios without FFC. Finally, we investigate how FFC may influences $r$-process nucleosynthesis associated with QCD phase transition driven supernova explosions.

Figures (12)

• Figure 1: Radial evolution of selected quantities for the reference SN model s25a28 RDF-1.9, showing the rest-mass density $\rho$ (a--b), the entropy per particle $s$ (c), and the electron fraction $Y_e$ (d), with respect to the post bounce time $t_{\rm pb}$ (top panels) and post-second-bounce time $t_{\rm p2b}$ (bottom panels). Black, blue, red, and cyan dashed curves in (a) and (b) show the radii of PNS, $\nu_e$-, $\bar{\nu}_e$-, and $\nu_x$-spheres, respectively.
• Figure 2: Radial profiles of rest-mass density, $\rho$, temperature, $T$ , radial velocity, $v$, and electron fraction, $Y_e$, at different post-second-bounce times, $t_{\rm p2b}$, showing the early (a--d) and the later evolution (e--h).
• Figure 3: Evolution of the neutrino number luminosities $L_\nu^{\rm num}$ in (a) and mean energies $\langle E_\nu \rangle$ in (b) as a function of the post-second-bounce time, $t_{\rm p2b}$, sampled in the comoving frame of reference at 500 km.
• Figure 4: Evolution of radial profiles of the ratio of neutrino number densities $n_{\bar{\nu}_e}/n_{\nu_e}$ (a) and regions associated with FFI (b) as functions of $t_{\rm p2b}$. Black, blue, red, and cyan dashed curves (a) show the radii of PNS, $\nu_e$-, $\bar{\nu}_e$-, and $\nu_x$-spheres, respectively. Black solid curves mark four contour lines of $n_{\bar{\nu}_e}/n_{\nu_e}=1$. Both black and gray domains (b) show the existence of ELN angular crossing. Considering more variations may be achieved in multi-dimensional SN simulations, the green and blue domains show where FFI exists when the angular distribution of $\bar{\nu}_e$ is amplified by 12% or suppressed by 10%, respectively. Below the pink curves (b) is the region prohibiting any collective flavor instability due to the violation of $\nu$ELN.
• Figure 5: Neutrino ELN angular distributions showing radial profiles of the radial velocity $v$, for different times $t_{\rm p2b}$.