arXiv subjects

H. Lenske

Publications and source records attributed to H. Lenske.

At least 37 records · Page 2Linked to original sources

arXiv2018

The NUMEN project: NUclear Matrix Elements for Neutrinoless double beta decay

The article describes the main achievements of the NUMEN project together with an updated and detailed overview of the related R&D activities and theoretical developments. NUMEN proposes an innovative technique to access the nuclear matrix elements entering the expression of the lifetime of the double beta decay by cross section measurements of heavy-ion induced Double Charge Exchange (DCE) reactions. Despite the two processes, namely neutrinoless double beta decay and DCE reactions, are triggered by the weak and strong interaction respectively, important analogies are suggested. The basic point is the coincidence of the initial and final state many-body wave-functions in the two types of processes and the formal similarity of the transition operators. First experimental results obtained at the INFN-LNS laboratory for the 40Ca(18O,18Ne)40Ar reaction at 270 MeV, give encouraging indication on the capability of the proposed technique to access relevant quantitative information. The two major aspects for this project are the K800 Superconducting Cyclotron and MAGNEX spectrometer. The former is used for the acceleration of the required high resolution and low emittance heavy ion beams and the latter is the large acceptance magnetic spectrometer for the detection of the ejectiles. The use of the high-order trajectory reconstruction technique, implemented in MAGNEX, allows to reach the experimental resolution and sensitivity required for the accurate measurement of the DCE cross sections at forward angles. However, the tiny values of such cross sections and the resolution requirements demand beam intensities much larger than manageable with the present facility. The on-going upgrade of the INFN-LNS facilities in this perspective is part of the NUMEN project and will be discussed in the article.

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arXiv2018

Fine structure of the pygmy quadrupole resonance in $^{112,114}$Sn isotopes

The electric quadrupole response in $^{112,114}$Sn isotopes is investigated by energy-density functional (EDF) and three-phonon quasiparticle-phonon model (QPM) theory with special emphasis on 2$^+$ excitations located above the first collective quadrupole state and below 5 MeV. Additional quadrupole strength clustering as a sequence of states similar to the recently observed pygmy quadrupole resonance in $^{124}$Sn is found. The spectral distributions and transition densities of these 2$^+$ states show special features being compatible with oscillations of a neutron skin against the isospin-symmetric nuclear core. Furthermore, two new ($p$, $p' γ$) Doppler-shift attenuation (DSA) coincidence experiments were performed at the SONIC@HORUS setup. Quadrupole states with excitation energies up to 4.2 MeV were populated in $^{112,114}$Sn. Lifetimes and branching ratios were measured allowing for the determination of the reduced quadrupole transition strengths to the ground state. A stringent comparison of the new data to EDF+QPM theory in $^{112}$Sn and $^{114}$Sn isotopes hints at the occurrence of a low-energy quadrupole mode of unique character which could be interpreted as pygmy quadrupole resonance.

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arXiv2017

Distillation of scalar exchange by coherent hypernucleus production in antiproton-nucleus collisions

The total and angular differential cross sections of the coherent process $\bar p + {}^AZ \to {}^A_Λ(Z-1) + \barΛ$ are evaluated at the beam momenta $1.5\div20$ GeV/c within the meson exchange model with bound proton and $Λ$-hyperon wave functions. It is shown that the shape of the beam momentum dependence of the hypernucleus production cross sections with various discrete $Λ$ states is strongly sensitive to the presence of the scalar $κ$-meson exchange in the $\bar p p \to \barΛΛ$ amplitude. This can be used as a clean test of the exchange by scalar $πK$ correlation in coherent $\bar p A$ reactions.

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arXiv2017

Pygmy and core polarization dipole modes in $^{206}$Pb: connecting nuclear structure to stellar nucleosynthesis

A high-resolution study of the electromagnetic response of $^{206}$Pb below the neutron separation energy is performed using a ($\vecγ$,$γ'$) experiment at the HI$\vecγ$S facility. Nuclear resonance fluorescence with 100% linearly polarized photon beams is used to measure spins, parities, branching ratios, and decay widths of excited states in $^{206}$Pb from $4.9$ to $8.1$MeV. The extracted $Σ$$B$(E1)$\uparrow$ and $Σ$$B$(M1)$\uparrow$ values for the total electric and magnetic dipole strength below the neutron separation energy are 0.9$\pm$0.2e$^2$fm$^2$ and 8.3$\pm$2.0$μ_{N}^2$, respectively. These measurements are found to be in very good agreement with the predictions from an energy-density functional (EDF) plus quasiparticle phonon model (QPM). Such a detailed theoretical analysis allows to separate the pygmy dipole resonance from both the tail of the giant dipole resonance and multi-phonon excitations. Combined with earlier photonuclear experiments above the neutron separation energy, one extracts a value for the electric dipole polarizability of $^{206}$Pb of $α_{D}\!=\!122\pm10$mb/MeV. When compared to predictions from both the EDF+QPM and accurately calibrated relativistic EDFs, one deduces a range for the neutron-skin thickness of $R_{\rm skin}^{206}\!=\!0.12$-$0.19$fm and a corresponding range for the slope of the symmetry energy of $L\!=\!48$-$60$MeV. This newly obtained information is also used to estimate the Maxwellian-averaged radiative cross section $^{205}$Pb(n,$γ$)$^{206}$Pb at 30keV to be $σ\!=\!130\!\pm\!25$mb. The astrophysical impact of this measurement--on both the s-process in stellar nucleosynthesis and on the equation of state of neutron-rich matter--is discussed.

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arXiv2017

Compton scattering off proton in the third resonance region

Compton scattering off the proton in the third resonance region is analyzed for the first time, owing to the full combined analysis of pion- and photo-induced reactions in a coupled-channel effective Lagrangian model with K-matrix approximation. Two isospin $I=3/2$ resonances $D_{33}(1700)$ and $F_{35}(1930)$ are found to be essential in the range of 1.6 - 1.8 GeV. The recent beam asymmetry data of Compton scattering from the GRAAL facility are used to determine the helicity couplings of these resonances, and strong constraints are coming also from $πN$ and $KΣ$ photoproduction data. The possible spin and parity of new narrow resonances is discussed.

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arXiv2017

Coherent Hypernucleus Production in Antiproton-Nucleus Annihilation Reactions as a Probe for $κ$ meson Exchange

The hypernucleus production reaction $\bar p + {}^AZ \to {}^A_Λ(Z-1) + \barΛ$ in the beam momentum range 1.5-20 GeV/c is addressed theoretically as a coherent process. The calculations are based on a covariant $t$-channel meson exchange model for the elementary $\bar p p \to \barΛΛ$ annihilation amplitude with parameters fixed by comparison with empirical data. Besides pseudo-scalar $K$ and vector $K^*$ mesons we also account for correlated $πK$ contributions, modelled by the scalar $K^*_0(800)$ or $κ$ meson. Initial and final state nuclear interactions are taken into account in eikonal approximation. The bound baryon wave functions are obtained self-consistently in a covariant mean-field approach. It is shown that the hypernucleus production cross sections populating discrete states are dominated by the vector and scalar interaction channels. The pronounced sensitivity on the scalar $κ$ meson exchange contributions indicates that these reactions are well suited as a probe for correlated $πK$ exchange in the scalar $κ/K^*_0$ interaction channel.

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arXiv2016

Elastic scattering, polarization and absorption of relativistic antiprotons on nuclei

We perform Glauber model calculations of the antiproton-nucleus elastic and quasielastic scattering and absorption in the beam momentum range $\sim 0.5\div10$ GeV/c. A good agreement of our calculations with available LEAR data and with earlier Glauber model studies of the $\bar p A$ elastic scattering allows us to make predictions at the beam momenta of $\sim 10$ GeV/c, i.e. at the regime of the PANDA experiment at FAIR. The comparison with the proton-nucleus elastic scattering cross sections shows that the diffractive minima are much deeper in the $\bar p A$ case due to smaller absolute value of the ratio of the real-to-imaginary part of the elementary elastic amplitude. Significant polarization signal for $\bar p A$ elastic scattering at 10 GeV/c is expected. We have also revealed a strong dependence of the $\bar p A$ absorption cross section on the slope parameter of the transverse momentum dependence of the elementary $\bar pN$ amplitude. The $\bar p A$ optical potential is discussed.

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arXiv2016

Multi-Strangeness Production in Hadron Induced Reactions

We discuss in detail the formation and propagation of multi-strangeness particles in reactions induced by hadron beams relevant for the forthcoming experiments at FAIR. We focus the discussion on the production of the decuplet-particle $Ω$ and study for the first time the production and propagation mechanism of this heavy hyperon inside hadronic environments. The transport calculations show the possibility of $Ω$-production in the forthcoming \panda-experiment, which can be achieved with measurable probabilities using high-energy secondary $Ξ$-beams. We predict cross sections for $Ω$-production. The theoretical results are important in understanding the hyperon-nucleon and, in particular, the hyperon-hyperon interactions also in the high-strangeness sector. We emphasize the importance of our studies for the research plans at FAIR.

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arXiv2016

$2π$ production in the Giessen coupled-channel model

We present a coupled-channel Lagrangian approach (GiM) to describe the $πN \to πN$, $2πN$ scattering in the resonance energy region. The $2πN$ production has been significantly improved by using the isobar approximation with $σN$ and $πΔ(1232)$ in the intermediate state. The three-body unitarity is maintained up to interference pattern between the isobar subchannels. The scattering amplitudes are obtained as a solution of the Bethe-Salpeter equation in the $K$ matrix approximation. As a first application we perform a partial wave analysis of the $πN \to πN$, $π^0π^0 N$ reactions in the Roper resonance region. We obtain $R_{σN}(1440)=27^{+4}_{-9}$\,\% and $R_{σN}(1440)=12^{+5}_{-3}$\,\% for the $σN$ and $πΔ$ decay branching ratios of $N^*(1440)$ respectively. The extracted $πN$ inelasticities and reaction amplitudes are consistent with the results from other groups.

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arXiv2016

SU(3) Approach to Hypernuclear Interactions and Spectroscopy

The $SU(3)$ meson exchange approach to interactions within the baryon octet and nuclear density functional theory are used to derive an \emph{ab initio} description of hypernuclear interactions. The density dependence of interactions is recast into a DFT with density dependent interaction vertices. The field-theoretical structure is retained by expressing the vertices as functionals of the matter field operators. Applications to infinite hypermatter and neutron star matter are discussed. A new approach is presented allowing to determine in-medium coupling constants out of the $NN$-vertex functionals, obtained e.g. by DBHF theory, for the full baryon octet by exploiting $SU(3)$ relations.

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arXiv2016

The decay of quadrupole-octupole $1^-$ states in $^{40}$Ca and $^{140}$Ce

Background: Two-phonon excitations originating from the coupling of two collective one-phonon states are of great interest in nuclear structure physics. One possibility to generate low-lying $E1$ excitations is the coupling of quadrupole and octupole phonons. Purpose: In this work, the $γ$-decay behavior of candidates for the $(2_1^+\otimes 3_1^-)_{1^-}$ state in the doubly-magic nucleus $^{40}$Ca and in the heavier and semi-magic nucleus $^{140}$Ce is investigated. Methods: $(\vecγ,γ')$ experiments have been carried out at the High Intensity $γ$-ray Source (HI$γ$S) facility in combination with the high-efficiency $γ$-ray spectroscopy setup $γ^3$ consisting of HPGe and LaBr$_3$ detectors. The setup enables the acquisition of $γ$-$γ$ coincidence data and, hence, the detection of direct decay paths. Results: In addition to the known ground-state decays, for $^{40}$Ca the decay into the $3^-_1$ state was observed, while for $^{140}$Ce the direct decays into the $2^+_1$ and the $0^+_2$ state were detected. The experimentally deduced transition strengths and excitation energies are compared to theoretical calculations in the framework of EDF theory plus QPM approach and systematically analyzed for $N=82$ isotones. In addition, negative parities for two $J=1$ states in $^{44}$Ca were deduced simultaneously. Conclusions: The experimental findings together with the theoretical calculations support the two-phonon character of the $1^-_1$ excitation in the light-to-medium-mass nucleus $^{40}$Ca as well as in the stable even-even $N=82$ nuclei.

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arXiv2016

${\bar p}p$ annihilation into ${\bar D}D$ meson pair within an effective Lagrangian model

We study the charmed meson pair (${\bar D}^0 D^0$ and $D^- D^+$) production in ${\bar p}p$ annihilation within an effective Lagrangian model that has only the baryon-meson degrees of freedom and involves the physical hadron masses. The reaction amplitudes include terms corresponding to the t-channel $Λ_c^+$, $Σ_c^+$ and $Σ_c^{++}$ baryon exchanges and the s-channel excitation, propagation and decay of the $Ψ$(3770) resonance into the charmed mesons. The initial- and final-state distortion effects have been accounted for by using a simple eikonal approximation-based procedure in the same way as was done in our previous study of the ${\bar p}p \to {\bar Λ}_c^-Λ_c^+$ reaction within a similar model. The ${\bar D}^0 D^0$ production reaction is dominated by the $Λ_c^+$ baryon exchange process, and the corresponding total cross sections are predicted to be in the range of 0.18 - 0.7 $μ$b for antiproton beam momenta varying between threshold and 20 $GeV/c$. The $Ψ$(3770) resonance contributions have a large influence on the differential cross sections of the $D^- D^+$ production reaction.

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arXiv2015

The nature and line shapes of charmonium in the $e^+e^- \to D\bar{D}$ reactions

We explore the nature of sharp resonances with asymmetric line shape observed in cross section data, a general physical phenomenon produced by the interference of continuum background and resonances. A Fano scheme and the coupled-channel T-matrix approach are employed to this aim and their close relationship is present. As a typical example, we point out that the $ψ(3770)$ state observed in the $e^+e^-$ reactions with an anomalous line shape can be explained naturally as a resonance embedded in the $D\bar{D}$ continuum. From a coupled-channels analysis the background of $ψ(3770)$ resonance is found to originate from a pole at $\sqrt{s}=3716.0 \pm$ 30.0~MeV. As a by-product, the broad structure $X(3900)/G(3900)$ seen in the Belle data, is found to be the tail of the $ψ(3770)$ state, distorted by the opening of the $D^*\bar{D} + c.c$ channel and the onset of the $ψ(4040)$ spectral distribution, thus making the assignment as a genuine charmonium state unlikely.

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arXiv2014

Production of multi-strangeness hypernuclei and the YN-interaction

We investigate for the first time the influence of hyperon-nucleon (YN) interaction models on the strangeness dynamics of antiproton- and $Ξ$-nucleus interactions. Of particular interest is the formation of bound multi-strangeness hypermatter in reactions relevant for \panda. The main features of two well-established microscopic approaches for YN-scattering are first discussed and their results are then analysed such that they can be applied in transport-theoretical simulations. The transport calculations for reactions induced by antiproton beams on a primary target including also the secondary cascade beams on a secondary target show a strong sensitivity on the underlying YN-interaction. In particular, we predict the formation of $Ξ$-hypernuclei with an observable sensitivity on the underlying $Ξ$N-interaction. We conclude the importance of our studies for the forthcoming research plans at FAIR.

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arXiv2014

Charmonium resonances and Fano line shapes

Anomalous line shapes of quarkonia are explained naturally as an interference effect of a $c\bar c$ confined closed channel with the surrounding continua, well established in other fields of physics as Fano-resonances. We discuss a quark model coupled-channel analysis describing quarkonium as a mixing of closed $Q\bar Q$ and molecular-like $D\bar D$ open channels. The asymmetric line shapes observed in $ψ(3770)$ production cross sections in $e^+e^-$ annihilation to $D^0\bar{D}^0$ and $D^+ D^-$, respectively, are described very well. The method allows to extract directly from the data the amount of $Q\bar Q \leftrightarrow D\bar D$ configuration mixing.

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arXiv2014

Low-energy dipole strength in 112,120Sn

The 112,120Sn(gamma,gamma') reactions below the neutron separation energies have been studied at the superconducting Darmstadt electron linear accelerator S-DALINAC for different endpoint energies of the incident bremsstrahlung spectrum. Dipole strength distributions are extracted for 112Sn up to 9.5 MeV and for 120Sn up to 9.1 MeV. A concentration of dipole excitations is observed between 5 and 8 MeV in both nuclei. Missing strength due to unobserved decays to excited states is estimated in a statistical model. A fluctuation analysis is applied to the photon scattering spectra to extract the amount of the unresolved strength hidden in background due to fragmentation. The strength distributions are discussed within different model approaches such as the quasiparticle-phonon model and the relativistic time blocking approximation allowing for an inclusion of complex configurations beyond the initial particle-hole states. While a satisfactory description of the fragmentation can be achieved for sufficently large model spaces, the predicted centroids and total electric dipole strengths for stable tin isotopes strongly depend on the assumptions about the underlying mean field.

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arXiv2014

Reaction ${\bar p}p \to {\bar Λ}_c^-Λ_c^+$ within an effective Lagrangian model

We study the charmed baryon production reaction ${\bar p} p \to {\bar Λ}_c^- Λ_c^+$ using the $t$-channel $D^0$ and $D^{*0}$ meson exchange diagrams within an effective Lagrangian model involving the physical hadron masses and the coupling constants determined from SU(4) flavor symmetry. The initial and final state distortion effects are accounted for by using a simple eikonal approximation based procedure. The vertex parameters of our model have been checked by employing them to calculate the cross sections for the ${\bar p} p \to {\bar Λ} Λ$ reaction within a similar model. We predict the ${\bar Λ}_c^- Λ_c^+$ production cross sections in the range of 1-30 $μb$ for antiproton beam momenta varying between threshold to 20 GeV/c. The respective roles of $D^0$ and $D^{*0}$ meson exchanges and also those of the vector and tensor components of the $D^{*0}$ coupling have been investigated

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arXiv2013

Non-yrast spectra of odd-A nuclei in a model of coherent quadrupole-octupole motion

The model of coherent quadrupole and octupole motion (CQOM) is applied to describe non-yrast split parity-doublet spectra in odd-mass nuclei. The yrast levels are described as low-energy rotation-vibration modes coupled to the ground single-particle (s.p.) state, while the non-yrast parity-doublet structures are obtained as higher-energy rotation-vibration modes coupled to excited s.p. states. It is shown that the extended model scheme describes both the yrast and non-yrast quasi parity-doublet spectra and the related B(E1) and B(E2) transition rates in different regions of heavy odd-A nuclei. The involvement of the reflection-asymmetric deformed shell model to describe the single-particle motion and the Coriolis interaction on a deeper level is discussed.

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