New light species can contribute to the number of effective relativistic degrees of freedom ( <math altimg='si1.svg'><msub><mrow><mi>N</mi></mrow><mrow><mi mathvariant='normal'>eff</mi></mrow></msub></math>) at Cosmic Microwave Background (CMB) which is precisely measured by Planck. In this work, we consider an MeV scale thermally decoupled non-minimal dark sector and study the imprint of the dark sector dynamics on the measurement of <math altimg='si1.svg'><msub><mrow><mi>N</mi></mrow><mrow><mi mathvariant='normal'>eff</mi></mrow></msub></math> at the time of CMB formation. We have predicted the allowed region of model parameter space in the light of constraints arising from the measurements of both <math altimg='si1.svg'><msub><mrow><mi>N</mi></mrow><mrow><mi mathvariant='normal'>eff</mi></mrow></msub></math> and dark matter relic density by Planck. It turns out that the impact of the dark sector dynamics on <math altimg='si1.svg'><msub><mrow><mi>N</mi></mrow><mrow><mi mathvariant='normal'>eff</mi></mrow></msub></math> is significant in case of a non-hierarchical mass spectrum of the dark sector particles.

India

Sougata Ganguly

Sourov Roy

Abhijit Kumar Saha

Imprints of MeV scale hidden dark sector at Planck data

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

South Korea

Dibyendu Nanda

CMB imprints of high scale non-thermal leptogenesis

In a pure inverse seesaw framework, achieving a substantial lepton asymmetry that can be converted into the observed baryon asymmetry of the Universe is extremely challenging. The difficulty arises primarily due to two reasons, (i) partial cancellation of the lepton asymmetries associated with the components of a pseudo-Dirac pair, and (ii) strong wash out caused by the inverse decays. In this work we offer two possible resolutions to overcome the above mentioned challenges considering a (3,3) ISS framework. Our first proposal is based on the assumption of a non-standard cosmological era in the pre-BBN epoch, that triggers a faster expansion of the Universe, thereby reducing the washout by several orders of magnitude. The second proposition is an alternative of first which considers a quasi-degenerate mass spectrum for the singlet heavy neutrinos, resulting into a larger order of lepton asymmetry that survives the impact of strong washout to account for the observed BAU. The viable parameters space, as obtained can be tested at present and future Lepton Flavour Violation experiments e.g. MEG and MEG II.

Ananya Mukherjee

Rescuing leptogenesis parameter space of inverse seesaw

We investigate the possibility of keV scale thermal dark matter (DM) in a gauged <math><mi>B</mi><mo>−</mo><mi>L</mi></math> extension of the standard model with three right-handed neutrinos (RHNs) and one vector like fermion in the context of cosmic inflation. The complex singlet scalar field responsible for the spontaneous breaking of <math><mi>B</mi><mo>−</mo><mi>L</mi></math> gauge symmetry is nonminimally coupled to gravity and serves the role of inflaton. The keV scale vector like fermion DM gives rise to the possibility of warm dark matter, but it gets overproduced thermally. The subsequent entropy dilution due to one of the RHN decay can bring the thermal abundance of DM within the observed limit. The dynamics of both the DM and the diluter are regulated by the <math><mi>B</mi><mo>−</mo><mi>L</mi></math> model parameters which are also restricted by the requirement of successful inflationary dynamics. We constrain the model parameter space from the requirement of producing sufficient entropy dilution to obtain correct DM relic, inflationary observables along with other phenomenological constraints. Interestingly, we obtain unique predictions for the order of lightest active neutrino mass (<math><msub><mi>m</mi><msub><mi>ν</mi><mi>l</mi></msub></msub><mo>≲</mo><msup><mn>10</mn><mrow><mo>−</mo><mn>14</mn></mrow></msup><mtext> </mtext><mtext> </mtext><mi>eV</mi></math>) as a function of the <math><mi>B</mi><mo>−</mo><mi>L</mi></math> gauge coupling for keV scale DM. The proposed framework also explains the origin of observed baryon asymmetry from the decay of other two heavier RHNs by overcoming the entropy dilution effect.

Debasish Borah

Suruj Jyoti Das

Thermal keV dark matter in a gauged <math><mi>B</mi><mo>−</mo><mi>L</mi></math> model with cosmic inflation

We propose a novel probe of weakly interacting massive particle (WIMP) dark matter (DM) candidates of a wide mass range which fall short of the required annihilation rates to satisfy correct thermal relic abundance, dubbed as miracle-less WIMP. If the DM interactions are mediated by an Abelian gauge boson like B-L, its annihilation rates typically remain smaller than the WIMP ballpark for very high scale B-L symmetry breaking, leading to overproduction. The thermally overproduced relic is brought within observed limits via late entropy dilution from one of the three right-handed neutrinos (RHN) present for keeping the model anomaly free and generating light neutrino masses. Such late entropy injection leads to peculiar spectral shapes of gravitational waves (GWs) generated by cosmic strings, formed as a result of B-L symmetry breaking. We find an interesting correlation between DM mass and turning frequency of the GW spectrum with the latter being within reach of future experiments. The two other RHNs play a major role in generating light neutrino masses and baryon asymmetry of the universe via leptogenesis. Successful leptogenesis with miracle-less WIMP together restrict the turning frequencies to lie within the sensitivity limits of near future GW experiments.

Czechia

Rome Samanta

Probing WIMP dark matter via gravitational waves’ spectral shapes

The formation and decay of metastable bound states can significantly decrease the thermal-relic dark matter density, particularly for dark matter masses around and above the TeV scale. Incorporating bound-state effects in the dark matter thermal decoupling requires in principle a set of coupled Boltzmann equations for the bound and unbound species. However, decaying bound states attain and remain in a quasi-steady state. Here we prove in generality that this reduces the coupled system into a single Boltzmann equation of the standard form, with an effective cross-section that describes the interplay among bound-state formation, ionisation, transitions and decays. We derive a closed-form expression for the effective cross-section for an arbitrary number of bound states, and show that bound-to-bound transitions can only increase it. Excited bound levels may thus decrease the dark matter density more significantly than otherwise estimated. Our results generalise the Saha ionisation equilibrium to metastable bound states, potentially with applications beyond the dark matter thermal decoupling.

Japan

Tobias Binder

Netherlands

Anastasiia Filimonova

France

Kalliopi Petraki

Graham White

Saha equilibrium for metastable bound states and dark matter freeze-out

We examine the impact of a faster expanding Universe on the phenomenology of scalar dark matter (DM) associated with <math><mrow><mi>S</mi><mi>U</mi><mo>(</mo><mn>2</mn><msub><mrow><mo>)</mo></mrow><mrow><mi>L</mi></mrow></msub></mrow></math> multiplets. Earlier works with a radiation dominated Universe have reported the presence of desert region for both inert <math><mi>S</mi><mi>U</mi><mo>(</mo><mn>2</mn><msub><mo>)</mo><mi>L</mi></msub></math> doublet and triplet DM candidates where the DM is underabundant. We find that the existence of a faster expanding component before big bang nucleosynthesis can revive a substantial part of the desert parameter space consistent with relic density requirements and other direct and indirect search bounds. We also review the possible collider search prospects of the newly obtained parameter space and predict that such region might be probed at the future colliders with improved sensitivity via a disappearing/stable charged track.

Basabendu Barman

Purusottam Ghosh

Brazil

Farinaldo S. Queiroz

Scalar multiplet dark matter in a fast expanding Universe: Resurrection of the desert region

We examine a simple extension of the standard model with a pair of fermions, one singlet and a doublet, in a common thread linking the dark matter problem with the smallness of neutrino masses associated with several exciting features. In the presence of a small bare Majorana mass term, the singlet fermion brings in a pseudo-Dirac dark matter capable of evading the strong spin-independent direct detection bound by suppressing the dark matter annihilation processes mediated by the neutral current. In consequence, the allowed range of a mixing angle between the doublet and the singlet fermions gets enhanced substantially. The presence of the same mass term in an association with singlet scalars also elevates tiny but nonzero masses radiatively for light Majorana neutrino satisfying observed oscillation data.

Partha Konar

Sudipta Show

Linking pseudo-Dirac dark matter to radiative neutrino masses in a singlet-doublet scenario

We propose a minimal extension of the standard model of particle physics to accommodate cosmic inflation, dark matter, and light neutrino masses. While the inflationary phase is obtained from a modified chaotic inflation scenario, consistent with the latest cosmology data, the dark matter particle is a fermion singlet that remains out of equilibrium in the early universe. The scalar field that revives the chaotic inflation scenario by suitable modification also assists in generating tiny couplings of dark matter with its mother particle, naturally realizing the nonthermal or freeze-in type dark matter scenario. Interestingly, the same assisting scalar field also helps in realizing tiny Yukawa couplings required to generate the sub–electron volt Dirac neutrino mass from neutrino couplings to the standard model–like Higgs field. The minimality as well as providing a unified solution to all three problems keeps the model predictive at experiments spanning out to all frontiers.

Common origin of modified chaotic inflation, nonthermal dark matter, and Dirac neutrino mass

The production of light (anti-)(hyper-)nuclei in heavy-ion collisions at the LHC is considered in the framework of the Saha equation, making use of the analogy between the evolution of the early universe after the Big Bang and that of “Little Bangs” created in the lab. Assuming that disintegration and regeneration reactions involving light nuclei proceed in relative chemical equilibrium after the chemical freeze-out of hadrons, their abundances are determined through the famous cosmological Saha equation of primordial nucleosynthesis and show no exponential dependence on the temperature typical for the thermal model. A quantitative analysis, performed using the hadron resonance gas model in partial chemical equilibrium, shows agreement with experimental data of the ALICE collaboration on d, $^{3}$He, <math><mmultiscripts><mrow><mtext>H</mtext></mrow><mprescripts></mprescripts><mrow><mi>Λ</mi></mrow><mrow><mn>3</mn></mrow></mmultiscripts></math>, and $^{4}$He yields for a very broad range of temperatures at <math><mi>T</mi><mo>≲</mo><mn>155</mn></math> MeV. The presented picture is supported by the observed suppression of resonance yields in central Pb–Pb collisions at the LHC.