• Debasish Majumdar

Articles written in Pramana – Journal of Physics

• Solar neutrino oscillation diagnostics at SuperKamiokande and Sudbury Neutrino Observatory

Results for solar neutrino detection from the SuperKamiokande collaboration have been presented recently while those from the Sudbury Neutrino Observatory are expected in the near future. These experiments are sensitive to the8B neutrinos from the sun, the shape of whose spectrum is well-known but the normalization is less certain. We propose several variables, insensitive to the absolute flux of the incident beam, which probe the shape of the observed spectrum and can sensitively signal neutrino oscillations. They provide methods to extract the neutrino mixing angle and mass splitting from the data and also to distinguish oscillation to sequential neutrinos from those to a sterile neutrino.

• Variables for probing neutrino oscillation at super-Kamiokande and the Sudbury Neutrino Observatory

We propose several new variables, insensitive to the absolute flux of the incident solar or supernova neutrino beam, which probe the shape of the observed spectrum at super-Kamiokande and Sudbury Neutrino Observatory experiments and can sensitively signal neutrino oscillations. One class of such variables involve moments of the distributions recorded at the two facilities while another variable, specific to SNO, utilises the integrated charged and neutral current signals. The utility of these variables in the context of supernova neutrinos both from the collapse epoch and the post-bounce era is also discussed.

• Discussion on a possible neutrino detector located in India

We have identified some important and worthwhile physics opportunities with a possible neutrino detector located in India. Particular emphasis is placed on the geographical advantage with a stress on the complimentary aspects with respect to other neutrino detectors already in operation.

• SNO results and neutrino magnetic moment solution to the solar neutrino problem

We have analysed the solar neutrino data obtained from chlorine, gallium and Super-Kamiokande (SK) experiments (1258 days) and also the new results that came from Sudbury Neutrino Observatory (SNO) charge current (CC) and elastic scattering (ES) experiments considering that the solar neutrino deﬁcit is due to the interaction of neutrino transition magnetic moment with the solar magnetic ﬁeld. We have also analysed the moments of the spectrum of scattered electrons at SK. Another new feature in the analysis is that for the global analysis, we have replaced the spectrum by its centroid.

• Beta decay rates of nuclei with $65 &lt; A &lt; 75$ for pre-supernova and supernova evolution

The half-lives are calculated for the $\beta^{-}$ decay process for nuclei in the mass range $\sim 65-75$ relevant for the core of a massive star at the late burning stage of stellar evolution and the collapse that leads to supernova explosion. These half-lives and rates are calculated by expressing the $\beta^{-}$ Gamow-Teller decay strengths in terms of smoothed bivariate strength densities. These strength densities are constructed in the framework of spectral averaging theory for two-body nuclear Hamiltonian in a large nuclear shell model space. The method has a natural extension to electron captures as well as weak interaction rates for 𝑟 and $rp$-processes.

• Probing pseudo-Dirac neutrino through detection of neutrino-induced muons from gamma ray burst neutrinos

The possibility to verify the pseudo-Dirac nature of neutrinos is investigated here via the detection of ultra-high energy neutrinos from distant cosmological objects like 𝛾-ray bursts (GRBs). The very long baseline and the energy range from ∼TeV to ∼EeV for such neutrinos invoke the likelihood to probe very small pseudo-Dirac splittings. The expected secondary muons from such neutrinos that can be detected by a kilometer scale detector such as ICECUBE is calculated and compared with the same in the case of mass-flavour oscillations and for no oscillation cases. The calculated muon yields indicate that to probe such small pseudo-Dirac splittings one needs to look for a nearby GRB (red shift $z \sim 0:03$ or less) whereas for a distant GRB ($z \sim 1$) the flux will be much depleted and such phenomenon cannot be distinguished. Also calculated are the muon-to-shower ratios.

• Real gauge singlet scalar extension of the Standard Model: A possible candidate for cold dark matter

The simplest extension of Standard Model (SM) is considered in which a real SM gauge singlet scalar with an additional discrete symmetry $Z_{2}$ is introduced to SM. This additional scalar can be a viable candidate of cold dark matter (CDM) since the stability of 𝑆 is achieved by the application of $Z_{2}$ symmetry on 𝑆. Considering 𝑆 as a possible candidate of CDM, Boltzmann’s equation is solved to find the freeze-out temperature and relic density of 𝑆 for Higgs mass 120 GeV in the scalar mass range 5 GeV to 1 TeV. As HHSS coupling $\delta_{2}$ appearing in Lagrangian depends upon the value of scalar mass $m_{S}$ and Higgs mass $m_{h}$, the $m_{S} − \delta_{2}$ parameter space has been constrained by using the Wilkinson microwave anisotropy probe (WMAP) limit on the relic density of DM in the Universe and the results of recent ongoing DM direct search experiments, namely CDMS-II, CoGeNT, DAMA, EDELWEISS-II, XENON-10 and XENON-100. From such analyses, two distinct mass regions are found (a lower and higher mass domain) for such a DM candidate that satisfy both the WMAP limit and the experimental results considered here. The possible differential direct detection rates and annual variation of total detection rates have been estimated for this scalar DM candidate 𝑆 for two detector materials, namely Ge and Xe. Finally, the 𝛾-ray flux has been calculated from the galactic centre due to annihilation of two 130 GeV scalar DM into two monoenergetic 𝛾-rays.

• A two-component dark matter model with real singlet scalars confronting GeV 𝛾-ray excess from galactic centre and Fermi bubble

We propose a two-component dark matter (DM) model, each component of which is a real singlet scalar, to explain results from both direct and indirect detection experiments. We put the constraints on the model parameters from theoretical bounds, PLANCK relic density results and direct DM experiments. The 𝛾-ray flux is computed from DM annihilation in this framework and is then compared with the Fermi-LAT observations from galactic centre region and Fermi bubble.

• Physics potential of the ICAL detector at the India-based Neutrino Observatory (INO)

The upcoming 50 kt magnetized iron calorimeter (ICAL) detector at the India-based Neutrino Observatory (INO) is designed to study the atmospheric neutrinos and antineutrinos separately over a wide range of energies andpath lengths. The primary focus of this experiment is to explore the Earth matter effects by observing the energy and zenith angle dependence of the atmospheric neutrinos in the multi-GeV range. This study will be crucial toaddress some of the outstanding issues in neutrino oscillation physics, including the fundamental issue of neutrino mass hierarchy. In this document, we present the physics potential of the detector as obtained from realistic detector simulations.We describe the simulation framework, the neutrino interactions in the detector, and the expected responseof the detector to particles traversing it. The ICAL detector can determine the energy and direction of the muons to a high precision, and in addition, its sensitivity to multi-GeV hadrons increases its physics reach substantially. Itscharge identification capability, and hence its ability to distinguish neutrinos from antineutrinos, makes it an efficient detector for determining the neutrino mass hierarchy. In this report, we outline the analyses carried out for the determination of neutrino mass hierarchy and precision measurements of atmospheric neutrino mixing parameters at ICAL, and give the expected physics reach of the detector with 10 years of runtime. We also explore the potential of ICAL for probing new physics scenarios like CPT violation and the presence of magnetic monopoles.

• Exploring a hidden fermionic dark sector

We propose a hidden sector fermion dark matter model which follows a dark $SU(2)_H$ symmetry. Fermions in the dark sector also carry a global $U(1)_H$ charge while the gauge bosons and dark scalar do not have any global $U(1)_H$ charge. The lightest fermion in dark sector can serve as a potential dark matter candidate. We investigate whether the proposed dark matter candidate can explain indirect detection results from galactic centre.

• Neutron star cooling via axion emission by nucleon–nucleon axion bremsstrahlung

Neutron stars generally cool off by the emission of $\gamma$-rays and neutrinos. But axions can also be produced inside a neutron star by the process of nucleon–nucleon axion bremsstrahlung. The escape of these axions adds to the cooling process of the neutron star.We explore the nature of cooling of neutron stars including the axion emission and compare our result with the scenario when the neutron star is cooled by only the emission of $\gamma$-rays and neutrinos. In our calculations we consider both the degenerate and non-degenerate limits for such axion energy loss rate and the resulting variation of luminosity with time and variation of surface temperature with time of the neutron star. In short, the thermal evolution of a neutron star is studied with three neutron star masses (1.0, 1.4 and 1.8 solar masses) and by including the effect of axion emission for different axion masses ($m_{a} = 10^{−5}, 10^{−3}$ and $10^{−2} eV$) and compared with the same when the axion emission is not considered. We compared theoretical cooling curve with the observational data of three pulsars PSR B0656+14, Geminga and PSR B1055-52 and finally gave an upper bound on axion mass limits $m_{a} \leq 10^{−3}$ eV which implies that the axion decay constant $f_{a} \geq 0.6×10^{10}$ GeV.

• # Pramana – Journal of Physics

Volume 96, 2022
All articles
Continuous Article Publishing mode

• # Editorial Note on Continuous Article Publication

Posted on July 25, 2019

Click here for Editorial Note on CAP Mode

© 2021-2022 Indian Academy of Sciences, Bengaluru.