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  • Open Access

Naturally small Dirac neutrino mass and B−L dark matter

Ernest Ma1, Partha Kumar Paul2,*, and Narendra Sahu2,†

  • *Contact author: ph22resch11012@iith.ac.in
  • †Contact author: nsahu@phy.iith.ac.in
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Phys. Rev. D 114, 055047 – Published 23 September, 2026

DOI: https://doi.org/10.1103/386s-sj3r

Abstract

In the conventional gauged B−L extension of the standard model, the B−L charge of the singlet scalar χ, responsible for the breaking of U(1)B−L symmetry, is taken to be 2, such that it can anchor type-I seesaw by giving Majorana masses to the right-handed neutrinos, νR. In this paper, we consider instead the cases χ∼3 or 4 under B−L, so that νR may not acquire any Majorana mass and neutrinos are Dirac fermions. We then consider a vectorlike fermion S with two units of B−L charge, which becomes a good candidate for dark matter, either Dirac for χ∼3 or Majorana for χ∼4. In both cases, spontaneous B−L breaking can induce a strong first-order phase transition, producing stochastic gravitational waves (GW), which can be tested at GW experiments. Moreover, the presence of light νRs gives rise to an additional contribution to the effective number of relativistic degrees of freedom, ΔNeff, providing complementary constraints from current and upcoming CMB observations.

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Phys. Rev. D 114, 055047– Published 23 September, 2026
  • Received 16 January 2026
  • Accepted 4 September 2026
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