The scale of the seesaw mechanism is typically much larger than the electroweak scale. This hierarchy can be naturally explained by U(1)B−L symmetry, which after spontaneous symmetry breaking, simultaneously generates Majorana masses for neutrinos and produces a network of cosmic strings. Such strings generate a gravitational wave (GW) spectrum which is expected to be almost uniform in frequency unless there is a departure from the usual early radiation domination. We explore this possibility in Type I, II and III seesaw frameworks, finding that only for Type-I, long-lived right-handed neutrinos (RHN) may provide a period of early matter domination for parts of the parameter space, even if they are thermally produced. Such a period leaves distinctive imprints in the GW spectrum in the form of characteristic breaks and a knee feature, arising due to the end and start of the periods of RHN domination. These features, if detected, directly determine the right-handed neutrino mass M, and associated left-handed effective neutrino mass <InlineEquation ID="IEq1"> <EquationSource Format="MATHML"><math display="inline"> <mover accent="true"> <mi>m</mi> <mo stretchy="true">~</mo> </mover> </math></EquationSource> <EquationSource Format="TEX">$ \overset{\sim }{m} $</EquationSource> </InlineEquation> of the dominating RHN. We find that GW detectors like LISA and ET could probe RHN masses in the range M ∈ [0.1, 109] GeV and effective neutrino masses in the <InlineEquation ID="IEq2"> <EquationSource Format="MATHML"><math display="inline"> <mover accent="true"> <mi>m</mi> <mo stretchy="true">~</mo> </mover> </math></EquationSource> <EquationSource Format="TEX">$ \overset{\sim }{m} $</EquationSource> </InlineEquation> ∈ [10−10, 10−8] eV range. We investigate the phenomenological implications of long-lived right-handed neutrinos for both local and global U(1)B−L strings, focusing on dark matter production and leptogenesis. We map the viable and detectable parameter space for successful baryogenesis and asymmetric dark matter production from right-handed neutrino decays. We derive analytical and semi-analytical relations correlating the characteristic gravitational-wave frequencies to the neutrino parameters <InlineEquation ID="IEq3"> <EquationSource Format="MATHML"><math display="inline"> <mover accent="true"> <mi>m</mi> <mo stretchy="true">~</mo> </mover> </math></EquationSource> <EquationSource Format="TEX">$ \overset{\sim }{m} $</EquationSource> </InlineEquation> and M, as well as to the relic abundances of dark matter and baryons. We find that the detectable parameter space reaches the boundary of hierarchical leptogenesis and encompasses a substantial portion of the near-resonant regime.

Gravitational wave spectral shapes as a probe of long lived right-handed neutrinos, leptogenesis and dark matter. Global versus local B − L cosmic strings

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2026年3月25日

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Satyabrata Datta,
Anish Ghoshal,
Angus Spalding,
Graham White

摘要

The scale of the seesaw mechanism is typically much larger than the electroweak scale. This hierarchy can be naturally explained by U(1)_B−L symmetry, which after spontaneous symmetry breaking, simultaneously generates Majorana masses for neutrinos and produces a network of cosmic strings. Such strings generate a gravitational wave (GW) spectrum which is expected to be almost uniform in frequency unless there is a departure from the usual early radiation domination. We explore this possibility in Type I, II and III seesaw frameworks, finding that only for Type-I, long-lived right-handed neutrinos (RHN) may provide a period of early matter domination for parts of the parameter space, even if they are thermally produced. Such a period leaves distinctive imprints in the GW spectrum in the form of characteristic breaks and a knee feature, arising due to the end and start of the periods of RHN domination. These features, if detected, directly determine the right-handed neutrino mass M, and associated left-handed effective neutrino mass $\tilde{m}$ $ \overset{\sim }{m} $ of the dominating RHN. We find that GW detectors like LISA and ET could probe RHN masses in the range M ∈ [0.1, 10⁹] GeV and effective neutrino masses in the $\tilde{m}$ $ \overset{\sim }{m} $ ∈ [10⁻¹⁰, 10⁻⁸] eV range. We investigate the phenomenological implications of long-lived right-handed neutrinos for both local and global U(1)_B−L strings, focusing on dark matter production and leptogenesis. We map the viable and detectable parameter space for successful baryogenesis and asymmetric dark matter production from right-handed neutrino decays. We derive analytical and semi-analytical relations correlating the characteristic gravitational-wave frequencies to the neutrino parameters $\tilde{m}$ $ \overset{\sim }{m} $ and M, as well as to the relic abundances of dark matter and baryons. We find that the detectable parameter space reaches the boundary of hierarchical leptogenesis and encompasses a substantial portion of the near-resonant regime.