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T. S. Kosmas

Publications and source records attributed to T. S. Kosmas.

At least 37 records · Page 2Linked to original sources

Standard and non-standard neutrino-nucleus reactions cross sections and event rates to neutrino detection experiments

Open neutrino physics issues require precision studies, both theoretical and experimental ones, and towards this aim coherent neutral current neutrino-nucleus scattering events are expected to be observed soon. In this work, we explore $ν$-nucleus processes from a nuclear theory point of view and obtain results with high confidence level based on accurate nuclear structure cross sections calculations. Besides cross sections, the present study includes simulated signals expected to be recorded by nuclear detectors, differential event rates as well as total number of events predicted to be measured. Our original cross sections calculations are focused on measurable rates for the Standard Model process, but we also perform calculations for various channels of the non-standard neutrino-nucleus reactions and come out with promising results within the current upper limits of the corresponding exotic parameters. We concentrate on the possibility of detecting (i) supernova neutrinos by using massive detectors like those of the GERDA and SuperCDMS dark matter experiments and (ii) laboratory neutrinos produced near the spallation neutron source facilities (at Oak Ridge National Lab) by the COHERENT experiment. Our nuclear calculations take advantage of the relevant experimental sensitivity and employ the severe bounds extracted for the exotic parameters entering the Lagrangians of various particle physics models and specifically those resulting from the charged lepton flavour violating $μ^{-} \rightarrow e^{-}$ experiments (Mu2e and COMET experiments).

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Nuclear aspects of neutral current non-standard $ν$-nucleus reactions and the role of the exotic $μ^-\to e^{-}$ transitions experimental limits

The nuclear aspects of flavour changing neutral current (FCNC) processes, predicted by various new-physics models to occur in the presence of nuclei, are examined by computing the relevant nuclear matrix elements within the context of the quasi-particle RPA using realistic strong two-body forces. One of our aims is to explore the role of the non-standard interactions (NSI) in the leptonic sector and specifically: (i) in lepton flavour violating (LFV) processes involving the neutral particles $ν_\ell$ and $\tildeν_\ell$, $\ell = e,μ,τ$ and (ii) in charged lepton flavour violating (cLFV) processes involving the charged leptons $\ell^-$ or $\ell^+$. As concrete nuclear systems we have chosen the stopping targets of $μ^-\rightarrow e^-$ conversion experiments, i.e. the $^{48}\mathrm{Ti}$ nucleus of the PRIME/PRISM experiment at J-PARC and the $^{27}\mathrm{Al}$ of the COMET at J-PARC as well as of the Mu2e at Fermilab. These experiments have been designed to reduce the single event sensitivity down to $10^{-16}$--$10^{-18}$ in searching for charged lepton mixing events. Our goal is, by taking advantage of our detailed nuclear structure calculations and using the present limits or the sensitivity of the aforementioned exotic $μ^- \rightarrow e^-$ experiments, to put stringent constraints on the parameters of NSI Lagrangians.

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Dynamical and radiative simulations of $γ$-ray jets in microquasars

The emission of $γ$-rays in jets emanating from the vicinity of collapsed stellar remnants, in binary systems known as microquasars, is investigated using a three dimensional relativistic hydrocode (PLUTO), in combination with two in-house radiative transfer codes. Even though a great number of stellar systems may be addressed by such models, we restrict ourselves to the concrete example of the SS433 X-ray binary, the only microquasar with a definite hadronic content in its jets, as verified from spectral line observations. A variety of system configurations have been examined, by employing a hadron-based emission mechanism. The dependence of the $γ$-ray emissions on certain dynamical source properties, such as the hydrodynamic parameters of the mass-flow density, the gas-pressure and the temperature of the ejected matter, is simulated. Radiative properties, especially the assumed high energy proton population inside the jet plasma, and its effect on the calculated emission, are also examined. Two sets of initial conditions of the chosen microquasar are employed, in order to cover different scenarios pertaining to the system under consideration.

astro-ph.HE↗

The role of the continuum and the spurious 1- transitions in incoherent mu - e conversion rate calculations

By using the Continuum RPA (CRPA) method, the incoherent transition strength of the exotic mu - e conversion in the 208Pb and 40Ca nuclei is investigated. The question whether excited nuclear states lying high in the continuum give an important contribution to the incoherent rate is addressed. The admixture of spurious components in the rate coming from 1- excitations is investigated in detail by using the self-consistent CRPA with Skyrme interactions as well as a less consistent version and by employing two ways to remove the spurious strength: the use of effective operators or simply the exclusion of the spurious state appearing close to zero energy. In all cases, the correction achieved is quite large.

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Enhanced mu-e conversion in nuclei in the inverse seesaw model

We investigate nuclear mu-e conversion in the framework of an effective Lagrangian arising from the inverse seesaw model of neutrino masses. We consider lepton flavour violation interactions that arise from short range (non-photonic) as well as long range (photonic) contributions. Upper bounds for the LFV parameters characterizing mu-e conversion are derived in the inverse seesaw model Lagrangian using the available limits on the mu-e conversion branching ratio, as well as the expected sensitivities of upcoming experiments. We comment on the relative importance of these two types of contributions and their relationship with the measured solar neutrino mixing angle theta_12 and the dependence on theta_13. Finally we show how the LFV mu-e conversion and the mu -> e gamma rates are strongly correlated in this model.

hep-ph↗

Continuum RPA study of the incoherent mu-e conversion rate and its spurious 1- admixture

The incoherent transition strength of the exotic mu-e conversion in the 208Pb nucleus is investigated by utilizing the Continuum RPA method, appropriate for the evaluation of the rate that goes to the continuum of the nuclear spectrum. We find that the contribution of resonances lying high in the continuum is not negligible. Special attention is paid to the detailed study of the pronounced 1- contribution which, according to previous calculations, dominates the overall incoherent rate in about all the nuclear targets. The spurious center of mass admixture to the partial rate originating from the 1- excitations is explored, and its elimination is performed by correcting properly the dipole operators. The results found this way show that the greatest portion of the total 1- contribution to the incoherent rate is spurious.

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The effects of deformation and pairing correlations on nuclear charge form factor

A set of moderately deformed $s-d$ shell nuclei is employed for testing the reliability of the nuclear ground state wave functions which are obtained in the context of a BCS approach and offer a simultaneous consideration of deformation and pairing correlations effects. In this method, the mean field is assumed to be an axially symmetric Woods-Saxon potential and the effective two-body interaction is a monopole pairing force. As quantities of main interest we have chosen the nuclear form factors, the occupancies of the active (surface) orbits and the Fermi sea depletion, which provide quite good tests for microscopic descriptions of nuclei within many body theories. For our comparisons with results emerging from other similar methods, an axially deformed harmonic oscillator field is also utilized.

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The Two-Body Momentum Distribution in Finite Nuclei

The two-body momentum distribution n2(p1,p2) of nuclei is studied. First, a compact analytical expression is derived for Z=N l-closed nuclei, within the context of the independent-particle shell model. Application to the light closed-shell nucleus 16O is included and discussed. Next, the effect of dynamical, short-range correlations is investigated in the case of 4He, by including Jastrow-type correlations in the formalism. The effect is significant for large values of p1 and p2 and for angles between the vectors p1 and p2 close to 180 deg. and 0.

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The exotic double-charge exchange muon positron conversion in nuclei

The formalism for the neutrinoless muon positron conversion is investigated in detail and the relevant nuclear matrix elements for light intermediate neutrinos in the case of the experimentally interesting A=27 system are calculated. The nucleus is going to be used as a stopping target in the MECO experiment at Brookhaven, one of the most sensitive probes expected to reach a sensitivity in the branching ratio within the next few years, which will be an improvement of three orders of magnitude. The relevant transition operators are constructed utilizing a variety of mechanisms present in current gauge theories, with emphasis on the intermediate neutrinos, both light and heavy, and heavy SUSY particles. The nuclear wave functions, both for the initial state and all excited final states are obtained in the framework of 1s-0d shell model employing the well-known and tested Wildenthal realistic interaction. In the case of the light intermediate neutrinos the transition rates to all excited final states up to 25 MeV in energy are calculated. We find that the imaginary part of the amplitude is dominant. The total rate is calculated by summing over all these partial transition strengths. We also find that the rate due to the real part of the amplitude is much smaller than the corresponding quantity found previously by the closure approximation.

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Nuclear matrix elements for exclusive neutrino-nucleus reactions

The formalism describing (anti)neutrino-nucleus reaction cross-sections in neutral- and charged-current processes is improved. Compact formulae for the single-particle transition matrix elements, based on the multipole expansion treatment of the relevant hadronic currents and the use of harmonic oscillator basis, are presented. As an application, the nucleus $^{127}I$, a promising target for detection of solar- and supernova-neutrinos as well as of cold dark matter candidates, is considered. Our matrix elements refer to exclusive processes of the charge-changing $^{127}I(ν_l,l^-)^{127}Xe^{\star}$ reaction and especially to the transition ${5/2}^+ \to {3/2}^+$ leading from the ground state of $^{127}$I to the lowest excitation of $^{127}$Xe nucleus.

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Closed expressions for the nuclear moments in semi-leptonic processes

We discuss a general formalism needed for a unified description of the weak and electromagnetic processes in nuclei which is based on the multipole decomposition of the hadronic currents. The use of harmonic oscillator single-particle basis, which is commonly employed in many-body nuclear calculations, simplifies the relevant expressions. We present analytic formulas for the corresponding radial integrals which enter the transition matrix elements of one-body operators in various semi-leptonic nuclear processes. As an example, we apply our formalism in order to simplify previous formalism giving the nuclear moments for the $0νββ$ - decay. Our results for the radial integrals refer to the 1s-0d-1p-0f model space.

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On the nuclear dependence of the mu-e conversion branching ratio

The variation of the coherent branching ratio $R_{μe}$ (ratio of the $μ^-\to e^-$ reaction rate divided by the total muon-capture rate) through the periodic table is studied by using exact muon wave functions. It was found that, by using very heavy nuclei (e.g. \nuc{197}Au, the SINDRUM II target) as $μ^-\to e^-$ conversion stopping-targets, the above ratio is favored by a factor of about four to five than by using light ones (e.g. \nuc{48}Ti, chosen as PRIME target).

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Muon-electron conversion in strange quark sea

We study nuclear muon-electron (mu-e) conversion in the general framework of effective Lagrangian approach without referring to any specific realization of the physics beyond the standard model (SM) responsible for lepton flavor violation (LFV). All the possible types of short range interactions (non-photonic mechanisms), i.e. (pseudo-)scalar, (axial-)vector and tensor, are included in our formalism. We show that the mu-e conversion in the strange nucleon sea via the scalar interactions is comparable with that in the valence quarks. This provides an insight into the strange quark couplings beyond the SM. From the available experimental data on mu-e conversion and expected sensitivities of planned experiments we derived upper bounds on the generic LFV - parameters of mu-e conversion sensitive to the relevant u-,d- and s-quark couplings.

hep-ph↗

B-quark mediated neutrinoless $μ^--e^-$ conversion in presence of R-parity violation

We found that in supersymmetric models with R-parity non-conservation the b-quarks may appreciably contribute to exotic neutrinoless muon-electron conversion in nuclei via the triangle diagram with two external gluons. This allowed us to extract previously overlooked constraints on the third generation trilinear R-parity violating parameters significantly more stringent than those existing in the literature.

hep-ph↗

Exotic muon-electron conversion in nuclei and R-parity violating supersymmetry

The flavor violating $μ^--e^-$ conversion in nuclei is studied within the minimal supersymmetric standard model. We focus on the R-parity violating contributions at tree level including the trilinear and the bilinear terms in the superpotential as well as in the soft supersymmetry breaking sector. The nucleon and nuclear structure have consistently been taken into account in the expression of the $μ^--e^-$ conversion branching ratio constructed in this framework. We have found that the contribution of the strange quark sea of the nucleon is comparable with that of the valence quarks. From the available experimental data on $μ^--e^-$ conversion in $^{48}$Ti and $^{208}$Pb and the expected sensitivity of the MECO experiment for $^{27}$Al we have extracted new stringent limits on the R-parity violating parameters.

hep-ph↗

Lepton Flavor Violation: Constraints from exotic muon to electron conversion

The exotic neutrinoless $μ^- - e^-$ nuclear conversion is studied within the conventional extensions of the standard model as well as in the minimal supersymmetric (SUSY) models with and without R-parity conservation. The dependence of the $μ^- - e^-$ conversion rates on the nucleon and nuclear structure is consistently taken into account. Using our calculated transition matrix elements and the available experimental data on the branching ratio $R_{μe^-}$ for $^{48}$Ti and $^{208}$Pb as well as the expected experimental sensitivity for $^{27}$Al employed as a target in the planned at Brookhaven $μ^--e^-$ conversion (MECO) experiment, we extract very severe constraints for the flavor violation parameters. We especially emphasize on the constraints resulting for SUSY R-parity violating parameters.

hep-ph↗

Searching for Cold Dark Matter. A case of coexistence of Supersymmetry and Nuclear Physics

The direct detection rate for supersymmetric cold dark matter (CDM) particles is calculated for a number of suitable nuclear targets. Both the coherent and spin contributions are considered. By considering representative phenomenologically acceptable input in the restricted SUSY parameter space, detectable rates are predicted for some choices of the parameters. The modulation effect, due to the Earth's annual motion, has also been considered and found to be $\le 4%$. Its precise value depends on the mass of CDM particles (LSP) and the structure of the target.

hep-ph↗

Muon number violating processes in nuclei

The flavour violating neutrinoless muon decays in the presence of nuclei, are discussed. We focus on the theoretical aspects of $μ^-(A,Z)\to e^-(A,Z)^*$ (muon-to-electron conversion), one of the most prominent flavour changing reactions, emphasizing its connection with the physics beyond the standard model. This process offers the most severe limits of the lepton flavour violation. By using the nuclear transition matrix elements calculated with several methods, and the recent experimental data of the branching ratio $R_{μe^-}$, we determine limits for the flavour changing parameters entering the elementary sector part of $R_{μe^-}$. These results are discussed in view of the ongoing experiment at PSI and the designed at Brookhaven, which are expected to push down by some orders of magnitude the experimental sensitivity the next few years with the hope to see ``new physics''.

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