Towards distinguishing Dirac from Majorana neutrino mass with gravitational waves

Abstract

We propose a new method towards distinguishing the Dirac versus Majorana nature of neutrino masses from the spectrum of gravitational waves (GWs) associated with neutrino mass genesis. Motivated by the principle of generating small neutrino masses without tiny Yukawa couplings, we assume generic seesaw mechanisms for both Majorana and Dirac neutrino masses. For Majorana neutrinos, we further assume a spontaneously broken gauged $U(1{)}_{B-L}$ symmetry, independently of the type of Majorana seesaw mechanism, which gives a cosmic string induced GW signal flat over a wide range of frequencies. For Dirac neutrinos, we assume the spontaneous breaking of a $Z_2$ symmetry, the minimal symmetry choice associated with all Dirac seesaw mechanisms, which is softly broken, generating a peaked GW spectrum from the annihilation of the resulting domain walls. In fact, the GW spectra for all types of Dirac seesaws with such a broken $Z_2$ symmetry are identical, subject to a mild caveat. As an illustrative example, we study the simplest respective type-I seesaw mechanisms, and show that the striking difference in the shapes of the GW spectra can help differentiate between these Dirac and Majorana seesaws, complementing results of neutrinoless double beta decay experiments. We also discuss detailed implications of the recent NANOGrav data for Majorana and Dirac seesaw models.

26 pages, 7 figures, 1 table. Version to appear in PRD

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• Did we hear the sound of the Universe boiling? Analysis using the full fluid velocity profiles and NANOGrav 15-year data
• Simultaneously probing the sound speed and equation of state of the early Universe with pulsar timing arrays
• Exploring the NANOGrav Signal and Planet-mass Primordial Black Holes through Higgs Inflation
• Search for Non-Tensorial Gravitational-Wave Backgrounds in the NANOGrav 15-Year Data Set
• Revised bounds on local cosmic strings from NANOGrav observations
• Prospects for Taiji to detect a gravitational-wave background from cosmic strings
• Can we distinguish the adiabatic fluctuations and isocurvature fluctuations with pulsar timing arrays?
• Constraining the history of reheating with the NANOGrav 15-year data
• Signals of merging supermassive primordial black holes in pulsar timing arrays
• Dark Matter Spike surrounding Supermassive Black Holes Binary and the Nanohertz Stochastic Gravitational Wave Background
• Signatures of Bulk Neutrinos in the Early Universe
• From $U(1)\times U(1)$ Symmetry Breaking to Majoron Cosmology: Insights from NANOGrav 15-year Data
• Probing the Neutrino Seesaw Scale with Gravitational Waves
• Cogenesis of visible and dark matter in type-I Dirac seesaw