Search

We revisit the generalized two-Higgs-doublet model (2HDM) with a minimally perturbed lepton specific (Type-X) Yukawa sector in context of the muon collider. The model offers a large region of parameter space where it can provide a solution to the observed muon ($g-2$) excess. Specifically the low mass CP odd scalar of this model plays an important role to fit this anomaly. Interestingly, the coupling of the pseudo scalar with the muons is proportional to $\tan \beta$ (the ratio of VEVs of the Higgs doublets) which dominantly controls the one loop and two loop contributions of the muon magnetic moment, therefore it can be probed directly at a muon collider with greater sensitivity in comparison to Large Hadron Collider (LHC). Primarily focusing on a parameter space where muon $g-2$ anomaly is satisfied, we explore the available theoretical and experimental constraints arising from vacuum stability, unitarity, lepton falvour violation (LFV), electroweak precision constraint, B-meson decay and collider experiments on the model. In the parameter space where all these constraints are satisfied, we propose to study the production of the light pseudo-scalar $A$ in association with a photon at a $3$ TeV muon collider. Subsequent decay of $A$ into a $\tau$ pair will produce a striking $l^+ l^- \gamma$ plus missing energy signature., Comment: 20 pages, 3 figures, 2 tables, matches the version accepted for publication in PRD

We study the phenomenology of a vector dark matter (VDM) in a $U(1)_X$ gauged extension of the Standard Model (SM) which is connected to the type II seesaw framework via the Higgs portal. When this $U(1)_X$ symmetry is spontaneously broken by the vacuum expectation value (VEV) of a complex scalar singlet, the gauge boson $Z^\prime$ becomes massive. The stability of the dark matter (DM) is ensured by the introduction of an exact charge conjugation symmetry. On the other hand, the $SU(2)_L$ triplet scalar generates light neutrino masses through 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 triplet scalar, our scenario can accommodate the observed $2\sigma$ deviation in $h \to Z \gamma $ decay. We have also briefly discussed the possibility of non-thermal production of DM from the decay of the same complex scalar that is responsible for the breaking of this $U(1)_X$ symmetry., Comment: 33 pages, 11 figures, 1 table, matches the version accepted for publication in PRD

We investigate the phenomenology of a non-thermal dark matter (DM) candidate in the context of flavor models that explain the hierarchy in the masses and mixings of quarks and leptons via the Froggatt-Nielsen (FN) mechanism. A flavor-dependent $U(1)_{\rm FN}$ symmetry explains the fermion mass and mixing hierarchy, and also provides a mechanism for suppressed interactions of the DM, assumed to be a Majorana fermion, with the Standard Model (SM) particles, resulting in its FIMP (feebly interacting massive particle) character. Such feeble interactions are mediated by a flavon field through higher dimensional operators governed by the $U(1)_{\rm FN}$ charges. We point out a natural stabilizing mechanism for the DM within this framework with the choice of half-integer $U(1)_{\rm FN}$ charge $n$ for the DM fermion, along with integer charges for the SM fermions and the flavon field. In this flavon portal scenario, the DM is non-thermally produced from the decay of the flavon in the early universe which becomes a relic through the freeze-in mechanism. We explore the allowed parameter space for this DM candidate from relic abundance by solving the relevant Boltzmann equations. We find that reproducing the correct relic density requires the DM mass to be in the range $(100-300)$ keV for $n=7.5$ and $(3-10)$ MeV for $n=8.5$ where $n$ is the $U(1)_{\rm FN}$ charge of the DM fermion., Comment: Accepted for publication in JHEP ; Added a discussion on free streaming length in Section-5

The latent heat (L) associated with the orbital order-disorder transition at T_JT is found to depend significantly on the average particle size (d) of LaMnO3. It rises slowly with the decrease in d down to ~100 nm and then jumps by more than an order of magnitude in between d ~ 100 nm and ~30 nm. Finally, L falls sharply to zero at a critical particle size d_c ~ 19 nm. The transition temperature T_JT also exhibits an almost similar trend of variation with the particle size, near d ~ 30 nm and below, even though the extent of variation is relatively small. The zero-field-cooled (ZFC) and field-cooled (FC) magnetization versus temperature study over a temperature range 10-300 K reveals that the antiferromagnetic transition temperature decreases with d while the temperature range, over which the ZFC and FC data diverge, increases with the drop in d. The FC magnetization also is found to increase sharply with the drop in particle size. A conjecture of nonmonotonic variation in orbital domain structure with decrease in particle size - from smaller domains with large number of boundaries to larger domains with small number of boundaries due to lesser lattice defects and, finally, down to even finer domain structures with higher degree of metastability - along with increase in surface area in core-shell structure, could possibly rationalize the observed L versus d and T_JT versus d patterns. Transmission electron microscopy data provide evidence for presence of core-shell structure as well as for increase in lattice defects in finer particles., Comment: 26 pages including 5 figures; pdf only; accepted for publication in Phys. Rev. B