The EW-<math><msub><mrow><mi>ν</mi></mrow><mrow><mi>R</mi></mrow></msub></math> model was constructed in order to provide a seesaw scenario operating at the Electroweak scale <math><msub><mrow><mi>Λ</mi></mrow><mrow><mi>E</mi><mi>W</mi></mrow></msub><mo>∼</mo><mn>246</mn></math> GeV, keeping the same SM gauge structure. In this model, right-handed neutrinos are non-sterile and have masses of the order <math><msub><mrow><mi>Λ</mi></mrow><mrow><mi>E</mi><mi>W</mi></mrow></msub></math>. They can be searched for at the LHC along with heavy mirror quarks and leptons, the lightest of which has large decay lengths. The model also incorporates a rich scalar sector, consistent with various experimental constraints, predicts a ∼125 GeV scalar with the SM Higgs characteristics satisfying the current LHC Higgs boson data. The seesaw mechanism requires the existence of a complex scalar which is singlet under the SM gauge group. The imaginary part of this complex scalar denoted by <math><msubsup><mrow><mi>A</mi></mrow><mrow><mi>s</mi></mrow><mrow><mn>0</mn></mrow></msubsup></math> is proposed to be the sub-MeV dark matter candidate in this manuscript. We find that the sub-MeV scalar can serve as a viable non-thermal feebly interacting massive particle (FIMP)-DM candidate. This <math><msubsup><mrow><mi>A</mi></mrow><mrow><mi>s</mi></mrow><mrow><mn>0</mn></mrow></msubsup></math> can be a naturally light sub-MeV DM candidate due to its nature as a pseudo-Nambu-Goldstone (PNG) boson in the model. We show that the well-studied freeze out mechanism falls short in this particular framework producing DM overabundance. We identify that the freeze in mechanism produces the correct order of relic density for the sub-MeV DM candidate satisfying all applicable constraints. We also discuss the allowed parameter space arising from the current indirect search bounds for this sub-MeV DM.

United States

Shreyashi Chakdar

India

Dilip Kumar Ghosh

P.Q. Hung

Najimuddin Khan

Dibyendu Nanda

A model of light pseudoscalar dark matter

We study the phenomenology of a vector dark matter (VDM) in a <math><mi>U</mi><mo>(</mo><mn>1</mn><msub><mo>)</mo><mi>X</mi></msub></math> gauged extension of the Standard Model (SM), which is set in a type II seesaw framework. When this <math><mi>U</mi><mo>(</mo><mn>1</mn><msub><mo>)</mo><mi>X</mi></msub></math> symmetry is spontaneously broken by the vacuum expectation value (VEV) of a newly introduced complex scalar singlet, the gauge boson <math><msup><mi>Z</mi><mo>′</mo></msup></math> becomes massive. The stability of the dark matter (DM) is ensured by the presence of an exact charge conjugation symmetry resulting from the structure of the Lagrangian. On the other hand, the <math><mi>S</mi><mi>U</mi><mo>(</mo><mn>2</mn><msub><mo>)</mo><mi>L</mi></msub></math> triplet scalar facilitates light neutrino masses via the type II seesaw mechanism. We have studied the phenomenology of the usual WIMP DM, considering all possible theoretical and experimental constraints that are applicable. Due to the presence of a triplet scalar, the present framework can also accommodate the observed <math><mn>2</mn><mi>σ</mi></math> deviation in <math><mi>h</mi><mo>→</mo><mi>Z</mi><mi>γ</mi></math> decay rate, recently measured at the LHC. The possibility of nonthermal production of DM from the decay of the singlet scalar has also been briefly discussed.

Nandini Das

Finland

Tapoja Jha

South Korea

Vector dark matter with Higgs portal in type II seesaw framework

We study the possibility of generating light Dirac neutrino mass via scotogenic mechanism where singlet-doublet fermion Dark Matter (DM) plays non-trivial role in generating one-loop neutrino mass, anomalous magnetic moment of muon: (g − 2) μ  as well as additional relativistic degrees of freedom ∆N eff within reach of cosmic microwave background (CMB) experiments. We show that the Dirac nature of neutrinos can bring interesting correlations within the parameter space satisfying the (g − 2) μ , DM relic density and the effective relativistic degrees of freedom ∆N eff. While we stick to thermal singlet-doublet DM with promising detection prospects, both thermal and non-thermal origin of ∆N eff have been explored. In addition to detection prospects of the model at DM, (g − 2) μ  and other particle physics experiments, it remains verifiable at future CMB experiments like CMB-S4 and SPT-3G.

Debasish Borah

Satyabrata Mahapatra

Sujit Sahoo

Narendra Sahu

Singlet-doublet fermion Dark Matter with Dirac neutrino mass, (g − 2) μ  and ∆N eff

The addition of a Higgs quadruplet to the standard model (SM) of quarks and leptons would shift the <math><mi>W</mi></math>-boson mass upward. It could also facilitate the production of dark matter through the conventional thermal freeze-out scenario via Yukawa interaction with the Higgs quadruplet or freeze-in production from the decay of SM Higgs. We investigate the same-sign lepton smoking gun signature of the double-charged scalar component of the quadruplet Higgs at the LHC.

Bangladesh

Talal Ahmed Chowdhury

Egypt

Kareem Ezzat

Shaaban Khalil

Ernest Ma

Higgs quadruplet impact on <math><mi>W</mi></math> mass shift, dark matter, and LHC signatures

We study the possibility of probing the radiative Dirac seesaw model with dark sector particles going inside the loop, popularly referred to as the Dirac scotogenic model via measurements of effective relativistic degrees of freedom  $$\Delta {\textrm{N}}_{\textrm{eff}}$$  <math> <mrow> <mi>Δ</mi> <msub> <mtext>N</mtext> <mtext>eff</mtext> </msub> </mrow> </math>   at cosmic microwave background (CMB) experiments. The loop suppression and additional free parameters involved in neutrino mass generation allow large  $$(\sim {\mathcal {O}}(1))$$  <math> <mrow> <mo>(</mo> <mo>∼</mo> <mi>O</mi> <mo>(</mo> <mn>1</mn> <mo>)</mo> <mo>)</mo> </mrow> </math>   coupling of light Dirac neutrinos with the dark sector particles. Such large Yukawa coupling not only dictates the relic abundance of heavy fermion singlet dark matter but also can lead to the thermalisation of the right chiral part of Dirac neutrinos, generating additional relativistic degrees of freedom  $$\Delta {\textrm{N}}_{\textrm{eff}}.$$  <math> <mrow> <mi>Δ</mi> <msub> <mtext>N</mtext> <mtext>eff</mtext> </msub> <mo>.</mo> </mrow> </math>   We find that the parameter space consistent with dark matter phenomenology and neutrino mass bounds can also be probed at future cosmic microwave background experiments like CMB-S4 via precision measurements of  $$\Delta {\textrm{N}}_{\textrm{eff}}.$$  <math> <mrow> <mi>Δ</mi> <msub> <mtext>N</mtext> <mtext>eff</mtext> </msub> <mo>.</mo> </mrow> </math>   The same parameter space, while leading to loop-suppressed direct detection cross-section of dark matter outside future sensitivities, can also have other interesting and complementary observational prospects via charged lepton flavour violation and collider signatures.

Pritam Das

Observable  $$\Delta {\textrm{N}}_{\textrm{eff}}$$  <math> <mrow> <mi>Δ</mi> <msub> <mtext>N</mtext> <mtext>eff</mtext> </msub> </mrow> </math>   in Dirac scotogenic model

We study the imprints of high scale non-thermal leptogenesis on cosmic microwave background (CMB) from the measurements of inflationary spectral index (<math><msub><mrow><mi>n</mi></mrow><mrow><mi>s</mi></mrow></msub></math>) and tensor-to-scalar ratio (r), which otherwise is inaccessible to the conventional laboratory experiments. We argue that non-thermal production of baryon (lepton) asymmetry from subsequent decays of inflaton to heavy right-handed neutrinos (RHN) and RHN to SM leptons is sensitive to the reheating dynamics in the early Universe after the end of inflation. Such dependence provides detectable imprints on the <math><msub><mrow><mi>n</mi></mrow><mrow><mi>s</mi></mrow></msub><mo>−</mo><mi>r</mi></math> plane which is well constrained by the Planck experiment. We investigate two separate cases, (I) inflaton decays to radiation dominantly and (II) inflaton decays to RHN dominantly which further decays to the SM particles to reheat the Universe adequately. Considering a class of α− attractor inflation models, we obtain the allowed mass ranges for RHN for both cases and thereafter furnish the estimates for <math><msub><mrow><mi>n</mi></mrow><mrow><mi>s</mi></mrow></msub></math> and r. The prescription proposed here is general and can be implemented in various kinds of single-field inflationary models given the conditions for non-thermal leptogenesis are satisfied.

Poland

Anish Ghoshal

Abhijit Kumar Saha

CMB imprints of high scale non-thermal leptogenesis

The dark sector is postulated to be invariant under an <math><mi>S</mi><mi>U</mi><mo>(</mo><mn>2</mn><mo>)</mo></math> gauge symmetry, spontaneously broken by a Higgs quadruplet to a conserved residual <math><msub><mrow><mi>Z</mi></mrow><mrow><mn>3</mn></mrow></msub><mo>×</mo><msub><mrow><mi>Z</mi></mrow><mrow><mn>2</mn></mrow></msub></math> symmetry. The resulting dark matter phenomenology is studied.

Dark (2)→ × Gauge Symmetry

We propose a scenario where dark matter (DM) can be generated nonthermally due to the presence of a light Dirac neutrino portal between the standard model (SM) and dark sector particles. The SM is minimally extended by three right-handed neutrinos (<math><msub><mi>ν</mi><mi>R</mi></msub></math>), a Dirac fermion DM candidate (<math><mi>ψ</mi></math>) and a complex scalar (<math><mi>ϕ</mi></math>), transforming nontrivially under an unbroken <math><msub><mi>Z</mi><mn>4</mn></msub></math> symmetry while being singlets under the SM gauge group. While DM and <math><msub><mi>ν</mi><mi>R</mi></msub></math> couplings are considered to be tiny in order to be in the nonthermal or freeze-in regime, <math><mi>ϕ</mi></math> can be produced either thermally or nonthermally depending upon the strength of its Higgs portal coupling. We consider both these possibilities and find out the resulting DM abundance via freeze-in mechanism to constrain the model parameters in the light of Planck 2018 data. Since the interactions producing DM also produce relativistic <math><msub><mi>ν</mi><mi>R</mi></msub></math>, we check the enhanced contribution to the effective relativistic degrees of freedom <math><mi>Δ</mi><msub><mi>N</mi><mi>eff</mi></msub></math> in view of existing bounds as well as future sensitivities. We also check the stringent constraints on free-streaming length of such freeze-in DM from structure formation requirements. Such constraints can rule out DM mass all the way up to <math><mi>O</mi><mo>(</mo><mn>100</mn><mtext> </mtext><mtext> </mtext><mi>keV</mi><mo>)</mo></math> keeping the <math><mi>Δ</mi><msub><mi>N</mi><mi>eff</mi></msub><mo>≤</mo><mi>O</mi><mo>(</mo><msup><mn>10</mn><mrow><mo>−</mo><mn>3</mn></mrow></msup><mo>)</mo></math> out of reach from near future experiments. Possible extensions of this minimal model can lead to observable <math><mi>Δ</mi><msub><mi>N</mi><mi>eff</mi></msub></math> which can be probed at next generation experiments.

Anirban Biswas

Nayan Das

Freeze-in dark matter via a light Dirac neutrino portal

The presence of any extra radiation energy density at the time of cosmic microwave background formation can significantly impact the measurement of the effective relativistic neutrino degrees of freedom or <math><mrow><mi>Δ</mi><msub><mrow><mi>N</mi></mrow><mrow><mi>eff</mi></mrow></msub></mrow></math>, which is very precisely measured by the Planck Collaboration. Here, we propose a scenario where a long-lived inert scalar, which is very weakly coupled to the dark sector, decays to a fermion dark matter via a “freeze-in” mechanism plus standard model neutrinos at very low temperature <math><mo>(</mo><mi>T</mi><mo>&lt;</mo><msub><mi>T</mi><mrow><mi>BBN</mi></mrow></msub><mo>)</mo></math>. We explore this model in the fast expanding Universe, where it is assumed that the early epoch <math><mo>(</mo><mi>T</mi><mo>&gt;</mo><msub><mi>T</mi><mrow><mi>BBN</mi></mrow></msub><mo>)</mo></math> of the Universe is dominated by a nonstandard species <math><mi>Φ</mi></math> instead of the standard radiation. In this nonstandard cosmological picture, such late-time decay of the inert scalar can inject some entropy to the neutrino sector after it decouples from the thermal bath and this will make substantial contribution to <math><mrow><mi>Δ</mi><msub><mrow><mi>N</mi></mrow><mrow><mi>eff</mi></mrow></msub></mrow></math>. Additionally, in this scenario, the new contribution to <math><mrow><mi>Δ</mi><msub><mrow><mi>N</mi></mrow><mrow><mi>eff</mi></mrow></msub></mrow></math> is highly correlated with the dark matter sector. Thus, one can explore such feebly interacting dark matter particles by the precise measurement of <math><mrow><mi>Δ</mi><msub><mrow><mi>N</mi></mrow><mrow><mi>eff</mi></mrow></msub></mrow></math> using the current (Planck 2018) and forthcoming (CMB-S4 and SPT3G) experiments.

Sk Jeesun

Long-lived inert Higgs boson in a fast expanding universe and its imprint on the cosmic microwave background

Dark matter is postulated to transform under the non-Abelian <math><mi>S</mi><mi>U</mi><msub><mrow><mo>(</mo><mn>2</mn><mo>)</mo></mrow><mrow><mi>D</mi></mrow></msub></math> gauge symmetry. Its connection to Dirac neutrino masses is explored.