Arnab Dasgupta

We study the cosmology and observational probes of majoron dark matter in a high-scale seesaw framework with spontaneously broken lepton number. Right-handed neutrinos naturally generate light neutrino masses and can realize thermal leptogenesis, while the associated majoron is a light pseudo-Nambu-Goldstone boson that can be cosmologically stable and serve as a viable dark matter candidate for sub-MeV masses. We analyze both preinflationary and postinflationary histories of lepton number breaking. In the preinflationary scenario, majoron dark matter is produced by misalignment and constrained by CMB isocurvature. In the postinflationary scenario, the majoron abundance receives nonthermal contributions from spatially averaged misalignment, majoron radiation from global cosmic strings, and the collapse of the string-domain wall network, as well as a thermally produced component. This scenario can also be probed by future searches for the stochastic gravitational wave background produced by cosmic strings. We map the viable majoron dark matter parameter space and examine complementary probes from x-ray and soft γ -ray searches for majoron decays to photons, black hole superradiance, and Lyman- α forest observations. These results demonstrate that majoron dark matter offers a distinctive cosmological probe of high-scale lepton number breaking and thermal leptogenesis.

A bstract: We propose a novel collider probe of baryogenesis at TeV scale by measuring decay asymmetries into particle and anti-particle final states. Motivated by the idea of Dirac leptogenesis, we consider an extension of the standard model with new colored and SU(2) _L singlet particles in such a way that the out-of-equilibrium decay of heavy colored fermions creates equal and opposite CP asymmetries in two sectors, prevented from equilibrating with each other. While the TeV scale viability of this mechanism requires a resonantly enhanced CP asymmetry, the latter also plays a crucial role leading to observable decay asymmetries in colliders. In addition to discussing conventional signatures of such heavy colored particles, namely, mono-jet plus missing transverse energy, displaced vertex, colored track at hadron colliders, we also show the unique possibility of measuring decay asymmetries via forward-backward and charge asymmetries at future muon colliders. In addition to being a verifiable TeV-scale baryogenesis scenario, the model also predicts a singlet scalar dark matter candidate consistent with the required thermal dark matter properties near the Higgs resonance.

The persistent null results at dark matter (DM) direct-detection experiments have pushed the popular weakly interacting massive particle (WIMP) DM to tight corners. Generic WIMP models with direct-detection rates below the current upper limits often lead to a thermally overproduced relic abundance after freeze-out. To resolve this conundrum, we propose a novel scenario where DM has temperature-dependent couplings with the standard model (SM) bath. A scalar field having a large vacuum expectation value (VEV) at high temperatures generates sizeable DM-SM interactions, leading to efficient DM annihilations responsible for generating the desired thermal relic. At lower temperatures, the scalar field VEV settles down to a small value as a result of a phase transition which can generically be of first order, effectively leading to a suppressed DM-SM interaction rate at low temperature, consistent with null results at direct-detection experiments. The upper bound on thermal DM mass forces the first-order phase transition to occur at scales such that the corresponding gravitational wave signal remains within reach of future experiments such as the Laser Interferometer Space Antenna.

We propose a minimal extension of the type-I seesaw model to realise leptogenesis from the coannihilation of dark sector particles. The type-I seesaw model is extended with a singlet fermion and two singlet scalars charged under a Z ₂ symmetry. The Z ₂ -odd singlet scalar is the dark matter candidate. Here the usual type-I seesaw mechanism generates neutrino mass, and a net lepton asymmetry is generated from the coannihilation of the dark matter and the Z ₂ -odd singlet fermion. The Z ₂ -even singlet scalar is important in dark matter phenomenology. Successful leptogenesis is possible at TeV-scale, unlike the vanilla case. This minimal extension provides an elegant explanation of successful leptogenesis with direct connection to the dark matter abundance in the Universe.

We propose a novel cogenesis scenario by utilizing the two-body decay of heavy right-handed neutrinos (RHNs) via an effective operator involving an axionlike particle (ALP), dark matter (DM), and a light chiral fermion ν R . This allows the two-body decay of heavy RHNs into ν R and ALP, thereby generating a lepton asymmetry in ν R which later gets transferred to left-handed leptons via sizeable Yukawa coupling with a neutrinophilic Higgs doublet. The asymmetry in left-handed leptons is then converted into baryon asymmetry via electroweak sphalerons. The lepton number violation by heavy RHNs also induces a one-loop Majorana mass of ν R rendering the light neutrinos to be Majorana fermions. Successful leptogenesis constrains the parameter space in terms of RHN mass and axion decay constant. This has interesting consequences for both ALP and QCD axion DM parameter space within reach of several ongoing and near future experiments. We also propose a Dirac version of this scenario without any total lepton number violation. This leads to a long-lived asymmetric Dirac fermion contributing partially to DM, thereby opening up more parameter space for ALP. In addition to axion search experiments, the proposed scenarios can have observable signatures at cosmic microwave background and DM search as well as terrestrial particle physics experiments.

The left-right model with the simplest Higgs structure has a S U ( 2 ) L doublet H L , a S U ( 2 ) R doublet H R , and a singlet σ that couples to the doublets as σ ( | H L | 2 − | H R | 2 ) . The left-right symmetry has H L ↔ H R and σ ↔ − σ . We study gravitational wave production and baryogenesis in the electroweak phase transitions in the model. For two benchmark points, σ first gets a vev, followed by H R and then H L . An interesting feature is that the vev of H L is initially much higher than its zero temperature value, leading to a more strongly first-order transition and higher frequency gravitational waves. An unusual benchmark point has the σ vev at zero when the electroweak symmetry breaks. This results in both H L and H R vevs being equal and in the multi-TeV range. At a lower temperature, the σ vev turns on, breaking the left-right symmetry and H L drops to its standard model value. Since the electroweak symmetry is broken at the multi-TeV scale, the frequency of gravitational waves will be much higher than usual. Baryogenesis is also discussed.

The persistent null results at dark matter (DM) direct detection experiments have pushed the popular weakly interacting massive particle (WIMP) DM to tight corners. Generic WIMP models with direct detection rate below the current upper limits often lead to thermally overproduced relic abundance after freeze-out. To resolve this conundrum, we propose a novel scenario where DM has temperature-dependent couplings with the standard model (SM) bath. A scalar field having a large vacuum expectation value (VEV) at high temperatures generates sizeable DM-SM interactions leading to efficient DM annihilations responsible for generating the desired thermal relic. At lower temperatures, the scalar field VEV settles down to a small value as a result of a first-order phase transition (FOPT), effectively leading to suppressed DM-SM interaction rate at low temperature, consistent with null results at direct detection experiments. Upper bound on thermal DM mass forces the FOPT to occur at scales such that the corresponding gravitational wave signal remains within reach of future experiments like LISA.

We study the possibility of constraining a scenario with high scale first order phase transition (FOPT) responsible for the cogenesis of baryon and dark matter using gravitational wave (GW) (non)observations. While the FOPT at high scale is responsible for generating baryon asymmetry through leptogenesis and dark matter via the mass-gain mechanism, the resulting GW spectrum falls within the ongoing LIGO-Virgo experimental sensitivity. The dark matter is preferred to be in the nonthermal ballpark with sub-GeV masses and the criteria of successful dark matter relic rules out a large portion of the parameter space consistent with high scale FOPT and successful leptogenesis. Some part of the parameter space allowed from dark matter and leptogenesis criteria also gives rise to a large signal-to-noise ratio at ongoing experiments and hence can be disfavored in a conservative way from the nonobservation of such stochastic GW background. Future data from ongoing and planned experiments will offer a complementary and indirect probe of the remaining parameter space which is typically outside the reach of any direct experimental probe.