arXiv subjects

F. Simkovic

Publications and source records attributed to F. Simkovic.

71 records · Page 4Linked to original sources

arXiv2003

Two-neutrino double beta decay within Fully-Renormalized QRPA: Effect of the restoration of the Ikeda sum rule

The recently proposed Fully-Renormalized QRPA (FR-QRPA), which fullfils the Ikeda sum rule (ISR) exactly, is applied to the two-neutrino double beta decay of $^{76}$Ge, $^{82}$Se, $^{100}$Mo, $^{116}$Cd, $^{128}$Te and $^{130}$Xe. The results obtained are compared with those of other approaches, standard QRPA and self-consistent QRPA (SCQRPA). The similarities and the differences among the methods are discussed. The influence of the restoration of the Ikeda sum rule on the $2νββ$-decay amplitude is analyzed.

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arXiv2003

Gamow-Teller strength distributions in 76Ge and 76Se from deformed QRPA

We study Gamow-Teller strength distributions of 76Ge and 76Se within a deformed QRPA formalism, which includes residual spin-isospin forces in the particle-hole and particle-particle channels. We consider two different methods to construct the quasiparticle basis, a selfconsistent approach based on a deformed Hartree-Fock calculation with density-dependent Skyrme forces and a more phenomenological approach based on a deformed Woods-Saxon potential. Both methods contain pairing correlations in the BCS approach. We discuss the sensitivity of Gamow-Teller strength distributions to the deformed mean field and residual interactions.

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arXiv2002

Analysis of solar neutrino induced double beta processes for several nuclei

We investigate neutrino flux induced double beta transitions in targets built of Cd112, Cd114, Cd116 and O18 isotopes. In addition to known beta^- beta^- channel we consider new beta^- beta^+ and beta^- beta^+ gamma modes of the neutrino induced double beta process. A possibility of detection of the solar neutrinos via the induced double beta transitions of interest is discussed. We note that the beta^- part of the solar neutrino induced beta^- beta^+ process in O18 was already discussed in Ref. [9,10] in connection with a possible influence of high energy electron production of this origin on the SuperKamiokande results.

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arXiv2002

Phenomenological Study of Solar-Neutrino Induced Double Beta Decay of Mo100

The detection of solar-neutrinos of different origin via induced beta beta process of Mo100 is investigated. The particular counting rates and energy distributions of emitted electrons are presented. A discussion in respect to solar-neutrino detector consisting of 10 tones of Mo100 is included. Both the cases of the standard solar model and neutrino oscillation scenarios are analyzed. Moreover, new beta^- beta^+ and beta^-/EC channels of the double-beta process are introduced and possibilities of their experimental observation are addressed.

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arXiv2001

Neutrino mediated muon--electron conversion in nuclei revisited

The non-photonic neutrino exchange mechanism of the lepton flavor violating muon-electron conversion in nuclei is revisited. First we determine the nucleon coupling constants for the neutrino exchange mechanism in a relativistic quark model taking into account quark confinement and chiral symmetry requirements. This includes a new, previously overlooked tree-level contribution from neutrino exchange between two quarks in the same nucleon. Then for the case of an additional sterile neutrino we reconsider the coherent mode of this process. The presence of a mixed sterile-active neutrino state heavier than the quark confinement scale Lambda_c (~1 GeV) may significantly improve the prospects for observation of this process in future experiments as compared to the conventional scenario with only light neutrinos. Turning the arguments around we derive new experimental constraints on |U_(mu h)| and |U_(e h)| mixing matrix elements from the non-observation of muon--electron conversion.

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arXiv2001

Neutrinoless Double Beta Decay of Ge76, Se82, Mo100 and Xe136 to excited 0^+ states

The neutrinoless double beta decay transition to the first excited 0^+ collective final state is examined for A=76, 82, 100 and 136 nuclei by assuming light and heavy Majorana neutrino exchange mechanisms as well as the trilinear R-parity violating contributions. Realistic calculations of nuclear matrix elements have been performed within the renormalized quasiparticle random phase approximation. Transitions to the first excited two-quadrupole phonon 0^+ state are described within a boson expansion formalism and alternatively by using the operator recoupling method. We present the sensitivity parameters to different lepton number violating signals, which can be used in planning the neutrinoless double beta decay experiments. The half-life of neutrinoless double beta decay to the first excited state 0^+_1 is by a factor of 10 to 100 larger than that of the transition to the ground state.

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arXiv2001

New results for two-neutrino double beta decay with large particle-particle two body proton-neutron interaction

A model many-body Hamiltonian describing an heterogenous system of paired protons and paired neutrons and interacting among themselves through monopole particle-hole and monopole particle-particle interactions is used to study the double beta decay of Fermi type. The states are described by time dependent approaches choosing as trial functions coherent states of the symmetry groups underlying the model Hamiltonian. One formalism, VP1, is fully equivalent with the standard pnQRPA and therefore fails at a critical value of the particle-particle interaction strength while another one, VP2, corresponds to a two step BCS treatment, i.e. the proton quasiparticles are paired with the neutron quasiparticles. In this way a harmonic description for the double beta transition amplitude is provided for any strength of the particle-particle interaction. The approximation quality is judged by comparing the actual results with the exact result as well as with those corresponding to various truncations of the boson expanded Hamiltonian and transition operator. Finally it is shown that the dynamic ground states provided by VP1 and VP2 are reasonable well approximated by solutions of a variational principle. This remark constitutes a step forward finding an approach where the RPA ground state is a solution of a variational principle equation.

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arXiv2001

Quasiparticle Random Phase Approximation with an optimal Ground State

A new Quasiparticle Random Phase Approximation approach is presented. The corresponding ground state is variationally determined and exhibits a minimum energy. New solutions for the ground state, some with spontaneously broken symmetry, of a solvable Hamiltonian are found. A non-iterative procedure to solve the non-linear QRPA equations is used and thus all possible solutions are found. These are compared with the exact results as well as with the solutions provided by other approaches.

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arXiv2000

Neutrinoless double beta decay within Self-consistent Renormalized Quasiparticle Random Phase Approximation and inclusion of induced nucleon currents

The first, to our knowledge, calculation of neutrinoless double beta decay ($0νββ$-decay) matrix elements within the self-consistent renormalised Quasiparticle Random Phase Approximation (SRQRPA) is presented. The contribution from the momentum-dependent induced nucleon currents to $0νββ$-decay amplitude is taken into account. A detailed nuclear structure study includes the discussion of the sensitivity of the obtained SRQRPA results for $0νββ$-decay of $^{76}$Ge to the parameters of nuclear Hamiltonian, two-nucleon short-range correlations and the truncation of the model space. A comparision with the standard and renormalized QRPA is presented. We have found a considerable reduction of the SRQRPA nuclear matrix elements, resulting in less stringent limits for the effective neutrino mass.

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arXiv2000

The Single State Dominance Hypothesis and the Two-Neutrino Double Beta Decay of Mo100

The hypothesis of the single state dominance (SSD) in the calculation of the two-neutrino double beta decay of Mo100 is tested by exact consideration of the energy denominators of the perturbation theory. Both transitions to the ground state as well as to the 0+ and 2+ excited states of the final nucleus Ru100 are considered. We demonstrate, that by experimental investigation of the single electron energy distribution and the angular correlation of the outgoing electrons, the SSD hypothesis can be confirmed or ruled out by a precise two-neutrino double beta decay measurement (e.g. by NEMO III collaboration).

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arXiv2000

Quasiparticle Random Phase Approximation with inclusion of the Pauli Exclusion Principle

Limitations of the Quasiparticle Random Phase Approximation (QRPA) are studied within an exactly solvable model, with a two body interaction of Fermi type. A special attention is paid to the violation of the Pauli exclusion principle (PEP) in solving the QRPA equation. A comparison of the exact solution, obtained by the diagonalization of a schematic nuclear Hamiltonian and those obtained within the standard QRPA, the renormalized QRPA, the QRPA with pertubative treatment of the PEP and the QRPA with exact consideration of the PEP, is presented. The agreement quality is judged in terms of the quasiparticle number operator matrix elements in the ground state and in the first excited states, of the beta transition amplitudes, of the Ikeda sum rule and of the nuclear matrix element for the double beta decay. We have found that restoring the PEP, the QRPA solutions are considerably stabilized and a better agreement with the exact solution is obtained.

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arXiv1999

Additional Nucleon Current Contributions to Neutrinoless Double Beta Decay

We have examined the importance of momentum dependent induced nucleon currents such as weak-magnetism and pseudoscalar couplings to the amplitude of neutrinoless double beta decay in the mechanisms of light and heavy Majorana neutrino as well as in that of Majoron emission. Such effects are expected to occur in all nuclear models in the direction of reducing the light neutrino matrix elements by about 30%. To test this we have performed a calculation of the nuclear matrix elements of the experimentally interesting nuclei A = 76, 82, 96, 100, 116, 128, 130, 136 and 150 within the pn-RQRPA. We have found that indeed such corrections vary somewhat from nucleus to nucleus, but in all cases they are greater than 25 percent. In the case of heavy neutrino the effect is much larger (a factor of 3). Combining out results with the best presently available experimental limits on the half-life of the neutrinoless double beta decay we have extracted new limits on the effective neutrino mass (light and heavy) and the effective Majoron coupling constant.

hep-ph↗

arXiv1997

Two-Neutrino Double Beta Decay: Critical Analysis

We have performed a critical analysis of different approximation schemes for the calculation of two-neutrino double beta decay (TNDBD) matrix elements. We have shown that within the single-particle approximation of nuclear Hamiltonian the TNDBD matrix element is equal to zero. The (renormalized) quasiboson approximation scheme imply for TNDBD transition operator to be a constant, if one requires the equivalence of initial and final (renormalized) QRPA Hamiltonians. It means that TNDBD is a higher order process in the boson expansion of the nuclear Hamiltonian. We have found that the mismatching of both Hamiltonians is getting worse with increasing strength of particle- particle interaction especially in the case of QRPA Hamiltonians. It is supposed to be one of the reasons of the extreme sensitivity of studied matrix element to the residual interaction appearing in explicit calculations involving the intermediate nucleus. Further, the Operator Expansion Method (OEM) has been reconsidered and new transition operators have been rederived in a consistent way. The validity of the OEM approximation has been discussed in respect to the other approximation schemes. The OEM combined with QRPA or RQRPA ground state wave functions reflects sensitively the instabilities incorporated in the considered ground states. Therefore, the predicting power of the OEM should be studied with help of other ground state wave functions. e.g. shell model ones.

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arXiv1997

A large Hilbert space QRPA and RQRPA calculation of neutrinoless double beta decay

A large Hilbert space is used for the calculation of the nuclear matrix elements governing the light neutrino mass mediated mode of neutrinoless double beta decay of Ge76, Mo100, Cd116, Te128 and Xe136 within the proton-neutron quasiparticle random phase approximation (pn-QRPA) and the renormalized QRPA with proton-neutron pairing (full-RQRPA) methods. We have found that the nuclear matrix elements obtained with the standard pn-QRPA for several nuclear transitions are extremely sensitive to the renormalization of the particle-particle component of the residual interaction of the nuclear hamiltonian. Therefore the standard pn-QRPA does not guarantee the necessary accuracy to allow us to extract a reliable limit on the effective neutrino mass. This behaviour, already known from the calculation of the two-neutrino double beta decay matrix elements, manifests itself in the neutrinoless double-beta decay but only if a large model space is used. The full-RQRPA, which takes into account proton-neutron pairing and considers the Pauli principle in an approximate way, offers a stable solution in the physically acceptable region of the particle-particle strength. In this way more accurate values on the effective neutrino mass have been deduced from the experimental lower limits of the half-lifes of neutrinoless double beta decay.

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arXiv1997

State-by-state calculations for all channels of the exotic $(μ^-,e^-)$ conversion process

The coherent and incoherent channels of the neutrinoless muon to electron conversion in nuclei, $μ^- (A,Z) \to e^- (A,Z)^*$, are studied throughout the periodic table. The relevant nuclear matrix elements are computed by explicitly constructing all possible final nuclear states in the context of the quasi-particle RPA. The obtained results are discussed in view of the existing at PSI and TRIUMF experimental data for $^{48}Ti$ and $^{208}Pb$ and compared with results obtained by: (i) shell model sum-rule techniques (ii) nuclear matter mapped into nuclei via a local density approximation and (iii) earlier similar calculations.

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arXiv1996

Non-collapsing renormalized QRPA with proton-neutron pairing for neutrinoless double beta decay

Using the renormalized quasiparticle random phase approximation (RQRPA), we calculate the light neutrino mass mediated mode of neutrinoless double beta decay of Ge76, Mo100, Te128 and Te130. Our results indicate that the simple quasiboson approximation is not good enough to study the neutrinoless double beta decay, because its solutions collapse for physical values of g_pp. We find that extension of the Hilbert space and inclusion of the Pauli Principle in the QRPA with proton-neutron pairing, allows us to extend our calculations beyond the point of collapse, for physical values of the nuclear force strength. As a consequence one might be able to extract more accurate values on the effective neutrino mass by using the best available experimental limits on the half-life of neutrinoless double beta decay.

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arXiv1996

Neutrinoless Double Beta Decay within QRPA with Proton-Neutron Pairing

We have investigated the role of proton-neutron pairing in the context of the Quasiparticle Random Phase approximation formalism. This way the neutrinoless double beta decay matrix elements of the experimentally interesting A= 48, 76, 82, 96, 100, 116, 128, 130 and 136 systems have been calculated. We have found that the inclusion of proton-neutron pairing influences the neutrinoless double beta decay rates significantly, in all cases allowing for larger values of the expectation value of light neutrino masses. Using the best presently available experimental limits on the half life-time of neutrinoless double beta decay we have extracted the limits on lepton number violating parameters.

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