Abstract
The oscillation of neutron n into mirror neutron n', its mass degenerate partner from dark mirror sector, can gradually transform the neutron stars into the mixed stars consisting in part of mirror dark matter. In quark stars n-n' transitions are suppressed. We study the structure of mixed stars and derive the mass-radius scaling relations between the configurations of purely neutron star and maximally mixed star (MMS) containing equal amounts of ordinary and mirror components. In particular, we show that the MMS masses can be at most M^{mathrm{max}}_{NS}/sqrt{2}, where M^mathrm{max}_{NS} is a maximum mass of a pure neutron star allowed by a given equation of state. We evaluate n-n' transition rate in neutron stars, and show that various astrophysical limits on pulsar properties exclude the transition times in a wide range 10^{5},text {year}< tau _varepsilon < 10^{15},text {year}. For short transition times, tau _varepsilon < 10^5 year, the different mixed stars of the same mass can have different radii, depending on their age, which possibility can be tested by the NICER measurements. We also discuss subtleties related with the possible existence of mixed quark stars, and possible implications for the gravitational waves from the neutron star mergers and associated electromagnetic signals.
Highlights
The idea that there may exist a hidden particle sector consisting of mirror duplicates of the observed standard particles was introduced long time ago for restoring parity as a fundamental symmetry [1,2,3]
We have discussed the possibility that the ordinary neutron stars, via n − n conversion, can develop the mirror matter cores which gradually increase in time
The stars with masses less than some critical value can survive asymptotically in time reaching the maximally mixed configuration: the heavier stars should collapse into black holes or quark stars
Summary
The idea that there may exist a hidden particle sector consisting of mirror duplicates of the observed standard particles was introduced long time ago for restoring parity as a fundamental symmetry [1,2,3]. The case of spontaneously broken Z2 symmetry has been considered in the literature [8,9] in which case the weak scales in two sectors can be different, and M particles and their O partners are no more mass degenerate This occurs in the mirror twin Higgs scenarios [5,10]. N − n mixing can be as large as ε ∼ 10−15 eV, corresponding to the characteristic time τnn = ε−1 ∼ 1 s or even smaller This possibility is not excluded by existing astrophysical and cosmological limits [33], and it can have observable effects for the ultra-high energy cosmic rays [43,44], for neutrons from the solar flares [45] and for primordial nucleosynthesis [46].
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