Search

Discrepancies of the $\Lambda {\rm CDM}$ model with small-scale cosmological observations and stringent constraints from direct search experiments cast doubts over typical weak scale cold dark matter candidates e.g. WIMPs. Self-interacting dark matter (SIDM) is a very promising alternative to WIMP, which not only alleviates the small-scale anomalies of the $\Lambda {\rm CDM}$ model, but also matches with the highly accurate large-scale $\Lambda {\rm CDM}$ predictions. The small-scale anomalies can be resolved with a self-scattering cross-section $\sigma/m_{DM} \sim 1 cm^2/gm$. Such large cross-sections can be realised in models of DM with a light MeV scale mediator and a sizeable coupling. We assume the DM to be Dirac fermion and the mediator to be either a light scalar or vector boson. In a standard cosmological history, one major issue with such models is to realise the correct relic density of dark matter via thermal freeze-out as the DM annihilates efficiently to the light mediators and ends up with an under-abundant relic. However, we show that, if the expansion rate of the universe is not radiation-dominated (RD) during the epoch of SIDM freeze-out, its relic abundance is enhanced significantly. We assume a non-standard expansion history of the universe by introducing a non-radiation like component in the early universe. In such a scenario, DM freezes out at an earlier epoch resulting in enhanced DM abundance, which can be matched with the correct relic density with suitable model parameters. The light mediator can also couple to an SM mediator and pave a way to detect SIDM at terrestrial laboratories. The mixing between the mediators can be constrained by data from direct search experiments. We find out the viable parameter space for a generic SIDM model taking into account the relevant phenomenological and experimental constraints., Comment: 11 pages, 11 figures

The nature of neutrinos, whether Dirac or Majorana, is hitherto not known. Assuming that the neutrinos are Dirac, which needs $B-L$ to be an exact symmetry, we make an attempt to explain the observed proportionality between the relic densities of dark matter (DM) and baryonic matter in the present Universe ${\it i.e.,}\,\, \Omega_{\rm DM} \approx 5\, \Omega_{\rm B}$. Assuming the existence of heavy $SU(2)_L$ scalar doublet $(X= (X^0, X^-)^T)$ in the early Universe, an equal and opposite $B-L$ asymmetry can be generated in left and right-handed sectors by the CP-violating out-of-equilibrium decay $X^0 \to \nu_L \nu_R$ since $B-L$ is an exact symmetry. We ensure that $\nu_L-\nu_R$ equilibration does not occur until below the electroweak (EW) phase transition during which a part of the lepton asymmetry gets converted to dark matter asymmetry through a dimension eight operator, which conserves $B-L$ symmetry and is in thermal equilibrium. The remaining $B-L$ asymmetry then gets converted to a net B-asymmetry through EW-sphalerons which are active at a temperature above 100 GeV. To alleviate the small-scale anomalies of $\Lambda$CDM, we assume the DM to be self-interacting via a light mediator, which not only depletes the symmetric component of the DM, but also paves a way to detect the DM at terrestrial laboratories through scalar portal mixing., Comment: 17 pages, 10 captioned figures, Version accepted for publication in PRD

Self-interacting dark matter (SIDM) with a light mediator is a promising scenario to alleviate the small-scale problems of the cold dark matter paradigm while being consistent with the latter at large scales, as suggested by astrophysical observations. This, however, leads to an under-abundant SIDM relic due to large annihilation rates into mediator particles, often requiring an extension of the simplest thermal or non-thermal relic generation mechanism. In this work, we consider a singlet-doublet fermion dark matter scenario where the singlet fermion with a light scalar mediator gives rise to the velocity-dependent dark matter self-interaction through a Yukawa type attractive potential. The doublet fermion, by virtue of its tiny mixing with the singlet, can be long-lived and can provide a non-thermal contribution to the singlet relic at late epochs, filling the deficit in the thermal relic of singlet SIDM. The light scalar mediator, due to its mixing with the standard model Higgs, paves the path for detecting such SIDM at terrestrial laboratories leading to constraints on model parameters from CRESST-III and XENON1T experiments. Enlarging the dark sector particles by two more singlet fermions and one scalar doublet, all odd under an unbroken $\mathcal{Z}_2$ symmetry can also explain non-zero neutrino mass in scotogenic fashion., Comment: 17 pages, 16 captioned figures, Version accepted for publication in PRD

We propose a self-interacting dark matter (DM) scenario with right handed neutrino (RHN) portal to the standard model (SM). The dark sector consists of a particle DM, assumed to be a Dirac fermion, and a light mediator in terms of a dark Abelian vector boson to give rise to the required velocity dependent self-interactions in agreement with astrophysical observations. Irrespective of thermal or non-thermal production of such a DM, its final relic remains under-abundant due to efficient annihilation rates of DM into light mediators by virtue of large self-interaction coupling. We then show that a feeble portal of DM-SM interaction via RHN offers a possibility to fill the relic deficit of DM via the late decay of RHN. As RHN also arises naturally in seesaw models explaining the origin of light neutrino masses, we outline two UV complete realizations of the minimal setup in terms of scotogenic and gauged B-L frameworks where connection to neutrino mass and other phenomenology like complementary discovery prospects are discussed., Comment: 48 pages, 21 captioned figures, version accepted for publication in PRD

We study an extension of the minimal gauged $L_{\mu}-L_{\tau}$ model in order to explain the anomalous magnetic moments of muon and electron simultaneously. Presence of an additional scalar doublet $\eta$ and an in-built $Z_2$ symmetry under which the right handed singlet fermions and $\eta$ are odd, leads to light neutrino mass in scotogenic fashion along with a stable dark matter candidate. In spite of the possibility of having positive and negative contributions to $(g-2)$ from vector boson and charged scalar loops respectively, the minimal scotogenic $L_{\mu}-L_{\tau}$ model can not explain muon and electron $(g-2)$ simultaneously while being consistent with other experimental bounds. We then extend the model with a vector like lepton doublet which not only leads to a chirally enhanced negative contribution to electron $(g-2)$ but also leads to the popular singlet-doublet fermion dark matter scenario. With this extension, the model can explain both electron and muon $(g-2)$ while being consistent with neutrino mass, dark matter and other direct search bounds. The model remains predictive at high energy experiments like collider as well as low energy experiments looking for charged lepton flavour violation, dark photon searches, in addition to future $(g-2)$ measurements., Comment: One typo in the previous version has been corrected

We present a self-interacting boosted dark matter (DM) scenario as a possible explanation of the recently reported excess of electron recoil events by the XENON1T experiment. The Standard Model (SM) has been extended with two vector-like fermion singlets charged under a dark $U(1)_D$ gauge symmetry to describe the dark sector. While the presence of light vector boson mediator leads to sufficient DM self-interactions to address the small scale issues of cold dark matter, the model with sub-GeV scale DM can explain the XENON1T excess via elastic scattering of boosted DM component with electrons at the detector. Strong annihilation of DM into the light mediator leads to a suppressed thermal relic. A hybrid setup of dark freeze-out and non-thermal contribution from the late decay of a scalar can lead to correct relic abundance. We fit our model with XENON1T data and also find the final parameter space consistent with self-interaction of DM, DM-electron scattering rate, as well as astrophysical and cosmological observations. A tiny parameter space consistent with all these constraints and requirements can be further scrutinized in near-future experiments., Comment: 30 Pages, 13 captioned figures, Version accepted for publication in Nucl. Phys. B

A minimal extension of the Standard Model (SM) by a vector-like fermion doublet and three right handed (RH) singlet neutrinos is proposed in order to explain dark matter and tiny neutrino mass simultaneously. The DM arises as a mixture of the neutral component of the fermion doublet and one of the RH neutrinos, both assumed to be odd under an imposed $\mathcal{Z}_2$ symmetry. Being Majorana in nature, the DM escapes from $Z$-mediated direct search constraints to mark a significant difference from singlet-doublet Dirac DM. The other two $\mathcal{Z}_2$ even heavy RH neutrinos give rise masses and mixing of light neutrinos via Type-I Seesaw mechanism. Relic density and direct search allowed parameter space for the model is investigated through detailed numerical scan., Comment: DAE-BRNS-HEP symposium 2020 Proccedings

Motivated by the growing evidence for lepton flavour universality violation after the first results from Fermilab's muon $(g-2)$ measurement, we revisit one of the most widely studied anomaly free extensions of the standard model namely, gauged $L_{\mu}-L_{\tau}$ model, known to be providing a natural explanation for muon $(g-2)$. We also incorporate the presence of dark matter (DM) in this model in order to explain the recently reported electron recoil excess by the XENON1T collaboration. We show that the same neutral gauge boson responsible for generating the required muon $(g-2)$ can also mediate interactions between electron and dark fermions boosted by dark matter annihilation. The required DM annihilation rate into dark fermion require a hybrid setup of thermal and non-thermal mechanisms to generate DM relic density. The tightly constrained parameter space from all requirements remain sensitive to ongoing and near future experiments, keeping the scenario very predictive., Comment: 9 pages, 6 figures, References Added, Accepted for publication in Phys. Lett. B

We propose a self-interacting inelastic dark matter (DM) scenario as a possible origin of the recently reported excess of electron recoil events by the XENON1T experiment. Two quasi-degenerate Majorana fermion DM interact within themselves via a light hidden sector massive gauge boson and with the standard model particles via gauge kinetic mixing. We also consider an additional long-lived singlet scalar which helps in realising correct dark matter relic abundance via a hybrid setup comprising of both freeze-in and freeze-out mechanisms. While being consistent with the required DM phenomenology along with sufficient self-interactions to address the small scale issues of cold dark matter, the model with GeV scale DM can explain the XENON1T excess via inelastic down scattering of heavier DM component into the lighter one. All these requirements leave a very tiny parameter space keeping the model very predictive for near future experiments., Comment: 34 pages, 14 figures, version accepted for publication in PRD

In a bid to simultaneous explanation of dark matter (DM) and tiny but non-zero masses of left-handed neutrinos, we propose a minimal extension of the Standard Model (SM) by a vector-like fermion doublet and three right handed (RH) singlet neutrinos. The DM arises as a mixture of the neutral component of the fermion doublet and one of the RH neutrinos, both assumed to be odd under an additional $\mathcal{Z}_2$ symmetry. As a result, the DM emerges to be a dominantly Majorana particle and escapes from $Z$-mediated direct search constraints to mark a significant difference from singlet-doublet Dirac DM. The other two $\mathcal{Z}_2$ even heavy RH neutrinos give rise masses and mixing of light neutrinos via Type-I Seesaw mechanism. The particle content automatically allows us to extend the model by a gauged $U(1)_{B-L}$ symmetry, which is anomaly free and brings an additional portal between DM and SM particles. Relic density and direct search allowed parameter space for both the cases are investigated through detailed numerical scan, while collider search strategies are also indicated., Comment: 49 pages, 25 figures, 4 tables, new references and figures added. Version accepted for publication in JCAP

The Stanford Workshop made a comprehensive review of the relevant issues for analyzing the employment profile of the Asian/Pacific Americans and also of the available data base for initiating a scientific economic study. The recommendations and conclusions were: (1) an independent survey should be undertaken; (2) Asian/Pacific Americans constitute the smallest minority, and a major segment of them constitutes the major portion of new Americans, yet there exists an absence of adequate information for proper manpower planning for them; (3) the Asian/Pacific American labor market should be studied as a composite group, since studies covering sub-groups (e.g., Japanese, Chinese, etc.) remain too fragmented; (4) the proposed study should not be postponed until the 1980 census data are available; (5) the proposed study would be completed by 1982, thus providing a set of findings much sooner for possible formulation of the nation's manpower needs; (6) the manpower planning of the nation has been related to the immigration policy; and (7) the address-banks of alternate sources (for identifying Asian Americans) should be used as the "universe" for the survey of the proposed study. (Author/RLV)