Connecting light dirac neutrinos to a multi-component dark matter scenario in gauged $$B-L$$ model
Dibyendu Nanda, Debasish Borah
Abstract
Abstract We propose a new gauged $$B-L$$ <mml:math xmlns:mml="http://www.w3.org/1998/Math/MathML"><mml:mrow><mml:mi>B</mml:mi><mml:mo>-</mml:mo><mml:mi>L</mml:mi></mml:mrow></mml:math> extension of the standard model where light neutrinos are of Dirac type, naturally acquiring sub-eV mass after electroweak symmetry breaking, without any additional global symmetries. This is realised by choosing a different $$B-L$$ <mml:math xmlns:mml="http://www.w3.org/1998/Math/MathML"><mml:mrow><mml:mi>B</mml:mi><mml:mo>-</mml:mo><mml:mi>L</mml:mi></mml:mrow></mml:math> charge for right handed neutrinos than the usual $$-1$$ <mml:math xmlns:mml="http://www.w3.org/1998/Math/MathML"><mml:mrow><mml:mo>-</mml:mo><mml:mn>1</mml:mn></mml:mrow></mml:math> so that the Dirac Yukawa coupling involves an additional neutrinophilic scalar doublet instead of the usual Higgs doublet. The model can be made anomaly free by considering four additional chiral fermions which give rise to two massive Dirac fermions by appropriate choice of singlet scalars. The choice of scalars not only helps in achieving the desired particle mass spectra via spontaneous symmetry breaking, but also leaves a remnant $$Z_2 \times Z'_2$$ <mml:math xmlns:mml="http://www.w3.org/1998/Math/MathML"><mml:mrow><mml:msub><mml:mi>Z</mml:mi><mml:mn>2</mml:mn></mml:msub><mml:mo>×</mml:mo><mml:msubsup><mml:mi>Z</mml:mi><mml:mn>2</mml:mn><mml:mo>′</mml:mo></mml:msubsup></mml:mrow></mml:math> symmetry to stabilise the two dark matter candidates. Apart from this interesting link between Dirac nature of light neutrinos and multi-component dark matter sector, we also find that the dark matter parameter space is constrained mostly by the cosmological upper limit on effective relativistic degrees of freedom $$\Delta N_{\mathrm{eff}}$$ <mml:math xmlns:mml="http://www.w3.org/1998/Math/MathML"><mml:mrow><mml:mi>Δ</mml:mi><mml:msub><mml:mi>N</mml:mi><mml:mi>eff</mml:mi></mml:msub></mml:mrow></mml:math> which gets enhanced in this model due to the thermalisation of the light right handed neutrinos by virtue of their sizeable $$B-L$$ <mml:math xmlns:mml="http://www.w3.org/1998/Math/MathML"><mml:mrow><mml:mi>B</mml:mi><mml:mo>-</mml:mo><mml:mi>L</mml:mi></mml:mrow></mml:math> gauge interactions.