arXiv subjects

Shahariar Sarkar

Publications and source records attributed to Shahariar Sarkar.

6 recordsLinked to original sources

arXiv2025

Low-energy neutrino responses for 71Ga by electron capture rates, charge exchange reactions and shell model calculations

Weak Gamow-Teller (GT) responses for low-lying states in ${}^{71}\mathrm{Ga}$ are crucial for studying low-energy solar neutrinos and the Ga anomaly, i.e., the possible transition to the sterile state. The responses for the ground state, the first excited state, and the second excited state are evaluated for the first time using the experimental electron capture rates, the experimental charge exchange reaction (CER) rates corrected for the tensor-interaction effect and the theoretical interacting shell model (ISM) calculations. The contributions from the two excited states to the solar and ${}^{51}\mathrm{Cr}$ neutrinos are found to be $4.2 \pm 1.2\%$ of that for the ground state. This is slightly larger than the ISM values but little smaller than the CER values without corrections for the tensor interaction effect. The Ga anomaly is far beyond the uncertainty of the obtained nuclear responses.

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arXiv2023

Nuclear matrix elements calculation for $0νββ$ decay of $^{124}$Sn using nonclosure approach in nuclear shell model

In this study, we calculate the nuclear matrix elements (NMEs) for the light neutrino-exchange mechanism of neutrinoless double beta $0νββ$) decay of $^{124}$Sn within the framework of the interacting nuclear shell model using the effective shell model Hamiltonian GCN5082. A novel method based on a nonclosure approach is employed, wherein for the intermediate nucleus $^{124}$Sb, effects of energy of 100 states for each $J_{k}^π$=$0^{+}$ to $11^{+}$ and $2^{-}$ to $9^{-}$ ($ΔJ_{k}$=1) are explicitly included in the NMEs calculation. Other common effects such as the finite size of nucleons, higher-order effects of nucleon currents, and short-range correlations (SRC) of nucleons are also taken into account. The extracted optimal closure energy is 2.9 MeV for a total NME of $^{124}$Sn $0νββ$ decay, which is independent of different forms of SRC parametrizations. A comparison of NMEs and half-lives with some of the recent calculations is presented. Further, to gain a comprehensive understanding of the role of nuclear structure on the $0νββ$ decay, the dependence of NMEs on spin-parity of the intermediate states, coupled spin-parity of neutrons and protons, and the number of intermediate states, is explored. It is observed that the inclusion of the effects of excitation energies of the intermediate nucleus yields more reliable NMEs. The present findings provide valuable insights for experimental investigations of $0νββ$ decay of $^{124}$Sn in India and elsewhere.

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arXiv2020

Modification of tensor force in \textit{p}-shell effective interaction

In many shell model interactions, the tensor force monopole matrix elements often retain systematic trends originating in the bare tensor force. However, in the present work, we find that Isospin T = 0 tensor force monopole matrix elements of \textit{p}-shell effective interaction CK(8-16) do not share these systematic. We correct these discrepancies by modifying T = 0 tensor force two-body matrix elements (TBMEs) of CK(8-16) by the analytically calculated tensor force TBMEs. With some additional modification of single-particle energies and TBMEs, the revised effective interaction is named as CKN. The effective interaction CKN has been tested for the calculations of \textit{p}-shell nuclei of normal parity states from various physics viewpoints such as excitation spectra, electromagnetic moments, and electromagnetic and \textit{G}amow-\textit{T}eller (\textit{GT}) transitions. The obtained results are found to be satisfactory.

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arXiv2020

A way forward towards improvement of tensor force in \textit{pf}-shell

In many shell model interactions, the tensor force monopole matrix elements often retain systematic trends originating in the bare tensor force. In this work, however, we note for GX-interactions of \textit{pf}-shell that the seven out of ten T = 1 tensor force monopole matrix elements do not share these systematic. We ameliorate this disparity making use of Yukawa-type tensor force and spin-tensor decomposition. Furthermore, we modify the single-particle energy of $1p_{3/2}$ orbit and two TBMEs of $0f$-orbit,and test the revised interaction from Ca to Ge isotopes with various physics viewpoints. The results are found to be satisfactory with respect to the experimental data.

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arXiv2020

Role of individual components of two-nucleon interaction in nuclear matrix elements of $2νββ$ and $0νββ$ of $^\textbf{48}$Ca: Beyond the closure approximation

In the present work, we examine the role of central (C), spin-orbit (SO) and tensor (T) components of two-nucleon interaction in the nuclear matrix elements (NMEs) of the two-neutrino double beta decay ($2νββ$) and the light neutrino-exchange mechanism of neutrinoless double beta decay ($0νββ$) of $^{48}$Ca in closure approximation and nonclosure approach. The NMEs are calculated in the nuclear shell-model framework using two-nucleon effective interaction GXPF1A used for $pf$ shell. The decomposition of the shell model two-nucleon interaction into its individual components is performed using the spin-tensor decomposition (STD). The NMEs for $2νββ$ are calculated in running nonclosure method. The NMEs for $0νββ$ are calculated with four different methods, namely, closure, running closure, running nonclosure, and mixed method. Results show that the magnitude of NMEs for $2νββ$ decreases about 7\% with the C+SO component of the interaction as compared to the C component. The magnitude of NMEs is further decreased about 9\% by adding T component to the C+SO component. The NMEs of $0νββ$ calculated in running nonclosure method are enhanced by about 8-10\%, 8-10\%, and 9-12\%, respectively, as compared to corresponding NMEs calculated in running closure method with C, C+SO components and total (C+SO+T) GXPF1A interaction for different SRC parametrization. For both $2νββ$ and $0νββ$, the NMEs calculated with C+SO component is in opposite phase with the NMEs calculated with C component and the total GXPF1A interaction.

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arXiv2020

Nuclear matrix elements for $λ$ mechanism of $0νββ$ of $^{48}$Ca in nuclear shell-model: Closure versus nonclosure approach

The $λ$ and $m_{ββ}$ mechanisms of neutrinoless double beta decay ($0νββ$) occur with light neutrino exchange via $W_L-W_R$, and $W_L-W_L$ mediation, respectively. In the present study, we calculate the nuclear matrix elements (NMEs) for the $m_{ββ}$ and $λ$ mechanisms of $0νββ$, which has origin in the left-right symmetric model with right-handed gauge boson at TeV scale. The NMEs are calculated for one of the $0νββ$ decaying isotope $^{48}$Ca in the interacting nuclear shell-model using the GXPF1A effective interaction of $pf$-shell. The NMEs are calculated in both closure and nonclosure approaches using four different methods: closure, running closure, running nonclosure, and mixed methods. All the NMEs are calculated incorporating the effects of the finite size of nucleons and the revisited higher order terms such as isoscalar and weak magnetism terms of the nucleon currents. Inclusion of the short-range nature of nucleon-nucleon interaction in Miller-Spencer, CD-Bonn, and AV18 parametrizations is also taken care of. The comparative dependence of the running closure and running nonclosure NMEs with the spin-parity of the allowed states of intermediate nucleus $^{48}$Sc, the coupled spin-parity of the two initial decaying neutrons and the final two protons, the cutoff excitation energy of $^{48}$Sc, the cutoff number of states of $^{48}$Sc are also examined. Results show that there are about 2-20\% enhancements in different types of total NMEs, calculated in the nonclosure approach as compared to the closure approach. The significant enhancements are found in the $M_{qGT}$ and $M_{qT}$ type NMEs for the inclusion of the higher-order terms of the nucleon currents.

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