arXiv subjects

R. Krücken

Publications and source records attributed to R. Krücken.

At least 91 records · Page 5Linked to original sources

arXiv2012

Determination of the $\mathbf{Σ(1385)^{0}/Λ(1405)}$ ratio in p+p collisions at 3.5 GeV

The aim of the present analysis is to determine the relative production cross sections of the $Λ$(1405) and $Σ(1385)^{0}$ resonances in p+p collisions at E$_{kin}$=3.5 GeV measured with HADES. Upper and lower limits have been determined for the ratio $σ_{(Σ(1385)^{0}+p+K^{+})}/σ_{(Λ(1405)+p+K^{+})}=0.76_{-0.26}^{+0.54}$. The knowledge of this ratio is an essential input for the analysis of the decay $Λ(1405)\rightarrowΣ^{\pm}π^{\mp}$, where an unambiguous separation of the $Λ$(1405) and $Σ(1385)^{0}$ signals is not possible.

nucl-ex↗

arXiv2011

Inclusive dielectron spectra in p+p collisions at 3.5 GeV

We present the inclusive invariant-mass, transverse momentum and rapidity distributions of dielectrons (e$^{+}$e$^{-}$ pairs) in p+p interactions at 3.5 GeV beam kinetic energy. In the vector-meson mass region, a distinct peak corresponding to direct $ω$ decays is reconstructed with 2% mass resolution. The data is compared to predictions from three model calculations. Due to the large acceptance of the HADES apparatus for $e^{+}e^{-}$ invariant masses above 0.2 GeV/$c^{2}$ and for transverse pair momenta p$_{t}$ $ <$ 1 GeV/$c$, acceptance corrections are to a large extent model independent. This allows us to extract from dielectron data for the first time at this energy the inclusive production cross sections for light vector mesons. Inclusive production cross sections for $π^o$ and $η$ mesons are also reported. The obtained results will serve as an important reference for the study of vector meson production in proton-nucleus and heavy-ion collisions. Furthermore, using this data, an improved value for the upper bound of the branching ratio for direct $η$ decays into the electron-positron channel is obtained.

nucl-ex↗

arXiv2011

Dielectron production in Ar+KCl collisions at 1.76A GeV

We present results on dielectron production in Ar+KCl collisions at 1.76A GeV. For the first time $ω$ mesons could be reconstructed in a heavy-ion reaction at a bombarding energy which is well below the production threshold in free nucleon-nucleon collisions. The omega multiplicity has been extracted and compared to the yields of other particles, in particular of the phi meson. At intermediate e+e- invariant masses, we find a strong enhancement of the pair yield over a reference spectrum from elementary nucleon-nucleon reactions suggesting the onset of non-trivial effects of the nuclear medium. Transverse-mass spectra and angular distributions have been reconstructed in three invariant mass bins. In the former unexpectedly large slopes are found for high-mass pairs. The latter, in particular the helicity-angle distributions, are largely consistent with expectations for a pair cocktail dominated at intermediate masses by delta Dalitz decays.

nucl-ex↗

arXiv2011

Structural evolution in the neutron-rich nuclei 106Zr and 108Zr

The low-lying states in 106Zr and 108Zr have been investigated by means of β-γ and isomer spectroscopy at the RI beam factory, respectively. A new isomer with a half-life of 620\pm150 ns has been identified in 108Zr. For the sequence of even-even Zr isotopes, the excitation energies of the first 2+ states reach a minimum at N = 64 and gradually increase as the neutron number increases up to N = 68, suggesting a deformed sub-shell closure at N = 64. The deformed ground state of 108Zr indicates that a spherical sub-shell gap predicted at N = 70 is not large enough to change the ground state of 108Zr to the spherical shape. The possibility of a tetrahedral shape isomer in 108Zr is also discussed.

nucl-ex↗

arXiv2011

Cross section in deuteron-proton elastic scattering at 1.25 GeV/u

First results of the differential cross section in dp elastic scattering at 1.25 GeV/u measured with the HADES over a large angular range are reported. The obtained data corresponds to large transverse momenta, where a high sensitivity to the two-nucleon and three-nucleon short-range correlations is expected.

nucl-ex↗

arXiv2011

Single and double pion production in np collisions at 1.25 GeV with HADES

The preliminary results on charged pion production in np collisions at an incident beam energy of 1.25 GeV measured with HADES are presented. The np reactions were isolated in dp collisions at 1.25 GeV/u using the Forward Wall hodoscope, which allowed to register spectator protons. The results for np -> pppi-, np -> nppi+pi- and np -> dpi+pi- channels are compared with OPE calculations. A reasonable agreement between experimental results and the predictions of the OPE+OBE model is observed.

nucl-ex↗

arXiv2010

Coulomb excitation of 73Ga

The B(E2; Ii -> If) values for transitions in 71Ga and 73Ga were deduced from a Coulomb excitation experiment at the safe energy of 2.95 MeV/nucleon using post-accelerated beams of 71,73Ga at the REX-ISOLDE on-line isotope mass separator facility. The emitted gamma rays were detected by the MINIBALL-detector array and B(E2; Ii->If) values were obtained from the yields normalized to the known strength of the 2+ -> 0+ transition in the 120Sn target. The comparison of these new results with the data of less neutron-rich gallium isotopes shows a shift of the E2 collectivity towards lower excitation energy when adding neutrons beyond N = 40. This supports conclusions from previous studies of the gallium isotopes which indicated a structural change in this isotopical chain between N = 40 and N = 42. Combined with recent measurements from collinear laser spectroscopy showing a 1/2- spin and parity for the ground state, the extracted results revealed evidence for a 1/2-; 3/2- doublet near the ground state in 73 31Ga42 differing by at most 0.8 keV in energy.

nucl-ex↗

arXiv2010

Isospin Character of the Pygmy Dipole Resonance in 124Sn

The pygmy dipole resonance has been studied in the proton-magic nucleus 124Sn with the (a,a'g) coincidence method at E=136 MeV. The comparison with results of photon-scattering experiments reveals a splitting into two components with different structure: one group of states which is excited in (a,a'g) as well as in (g,g') reactions and a group of states at higher energies which is only excited in (g,g') reactions. Calculations with the self-consistent relativistic quasiparticle time-blocking approximation and the quasiparticle phonon model are in qualitative agreement with the experimental results and predict a low-lying isoscalar component dominated by neutron-skin oscillations and a higher-lying more isovector component on the tail of the giant dipole resonance.

nucl-ex↗

arXiv2010

Discovery of the shape coexisting 0+ state in 32Mg by a two neutron transfer reaction

The Island of Inversion nucleus 32Mg has been studied by a (t,p) two neutron transfer reaction in inverse kinematics at REX-ISOLDE. The shape coexistent excited 0+ state in 32Mg has been identified by the characteristic angular distribution of the protons of the DeltaL = 0 transfer. The excitation energy of 1058 keV is much lower than predicted by any theoretical model. The low gamma-ray intensity observed for the decay of this 0+ state indicates a lifetime of more than 10 ns. Deduced spectroscopic amplitudes are compared with occupation numbers from shell model calculations.

nucl-ex↗

arXiv2010

In-Medium Effects on K0 Mesons in Relativistic Heavy-Ion Collisions

We present the transverse momentum spectra and rapidity distributions of $π^{-}$ and K$^0_S$ in Ar+KCl reactions at a beam kinetic energy of 1.756 A GeV measured with the spectrometer HADES. The reconstructed K$^0_S$ sample is characterized by good event statistics for a wide range in momentum and rapidity. We compare the experimental $π^{-}$ and K$^0_S$ distributions to predictions by the IQMD model. The model calculations show that K$^0_S$ at low tranverse momenta constitute a particularly well suited tool to investigate the kaon in-medium potential. Our K$^0_S$ data suggest a strong repulsive in-medium K$^0$ potential of about 40 MeV strength.

nucl-ex↗

arXiv2010

Lambda-p femtoscopy in collisions of Ar+KCl at 1.76 AGeV

Results on $Λ$p femtoscopy are reported at the lowest energy so far. At a beam energy of 1.76A~GeV, the reaction Ar+KCl was studied with the High Acceptance Di-Electron Spectrometer (HADES) at SIS18/GSI. A high-statistics and high-purity $Λ$ sample was collected, allowing for the investigation of $Λ$ p correlations at small relative momenta. The experimental correlation function is compared to corresponding model calculations allowing the determination of the space-time extent of the $Λ$p emission source. The $Λ$p source radius is found slightly smaller than the pp correlation radius for a similar collis ion system. The present $Λ$p radius is significantly smaller than that found for Au+Au/Pb+Pb collisio ns in the AGS, SPS and RHIC energy domains, but larger than that observed for electroproduction from He. Taking into account all available data, we find the $Λ$p source radius to increase almost linearly with the number of participants to the power of one-third.

nucl-ex↗

arXiv2009

Recent results from HADES on electron pair production in relativistic heavy-ion collisions

Systematic investigations of dilepton production are performed at the SIS accelerator of GSI with the HADES spectrometer. The goal of this program is a detailed understanding of di-electron emission from hadronic systems at moderate temperatures and densities. New results obtained in HADES experiments focussing on electron pair production in elementary collisions are reported here. They pave the way to a better understanding of the origin of the so-called excess pairs earlier on observed in heavy-ion collisions by the DLS collaboration and lately confirmed in two measurements of the HADES collaboration using C+C and Ar+KCl collisions. Results of these studies are discussed.

nucl-ex↗

arXiv2009

Origin of the low-mass electron pair excess in light nucleus-nucleus collisions

We report measurements of electron pair production in elementary p+p and d+p reactions at 1.25 GeV/u with the HADES spectrometer. For the first time, the electron pairs were reconstructed for n+p reactions by detecting the proton spectator from the deuteron breakup. We find that the yield of electron pairs with invariant mass Me+e- > 0.15 GeV/c2 is about an order of magnitude larger in n+p reactions as compared to p+p. A comparison to model calculations demonstrates that the production mechanism is not sufficiently described yet. The electron pair spectra measured in C+C reactions are compatible with a superposition of elementary n+p and p+p collisions, leaving little room for additional electron pair sources in such light collision systems.

nucl-ex↗

arXiv2009

Deep sub-threshold $Ξ^-$ production in Ar+KCl reactions at 1.76A GeV

We report first results on a deep sub-threshold production of the doubly strange hyperon $Ξ^-$ in a heavy-ion reaction. At a beam energy of 1.76A GeV the reaction Ar+KCl was studied with the High Acceptance Di-Electron Spectrometer (HADES) at SIS18/GSI. A high-statistics and high-purity $Λ$ sample was collected, allowing for the investigation of the decay channel $Ξ^- \to Λπ^-$. The deduced $Ξ^-/(Λ+Σ^0)$ production ratio of $(5.6 \pm 1.2 ^{+1.8}_{-1.7})\cdot 10^{-3}$ is significantly larger than available model predictions.

nucl-ex↗

arXiv2009

Spectroscopy of 32Ne and the Island of Inversion

We report on the first spectroscopic study of the N=22 nucleus 32Ne at the newly completed RIKEN Radioactive Ion Beam Factory. A single gamma-ray line with an energy of 722(9) keV was observed in both inelastic scattering of a 226 MeV/u 32Ne beam on a Carbon target and proton removal from 33Na at 245 MeV/u. This transition is assigned to the de-excitation of the first J^pi = 2+ state in 32Ne to the 0+ ground state. Interpreted through comparison with state-of-the-art shell model calculations, the low excitation energy demonstrates that the Island of Inversion extends to at least N=22 for the Ne isotopes.

nucl-ex↗

arXiv2009

Structure of 55Ti from relativistic one-neutron knockout

Results are presented from a one-neutron knockout reaction at relativistic energies on 56Ti using the GSI FRS as a two-stage magnetic spectrometer and the Miniball array for gamma-ray detection. Inclusive and exclusive longitudinal momentum distributions and cross-sections were measured enabling the determination of the orbital angular momentum of the populated states. First-time observation of the 955(6) keV nu p3/2-hole state in 55Ti is reported. The measured data for the first time proves that the ground state of 55Ti is a 1/2- state, in agreement with shell-model calculations using the GXPF1A interaction that predict a sizable N=34 gap in 54Ca.

nucl-ex↗

arXiv2008

Coulomb excitation of $^{68}$Ni at safe energies

The $B(E2;0^+\to2^+)$ value in $^{68}$Ni has been measured using Coulomb excitation at safe energies. The $^{68}$Ni radioactive beam was post-accelerated at the ISOLDE facility (CERN) to 2.9 MeV/u. The emitted $γ$ rays were detected by the MINIBALL detector array. A kinematic particle reconstruction was performed in order to increase the measured c.m. angular range of the excitation cross section. The obtained value of 2.8$^{+1.2}_{-1.0}$ 10$^2$ e$^2$fm$^4$ is in good agreement with the value measured at intermediate energy Coulomb excitation, confirming the low $0^+\to2^+$ transition probability.

nucl-ex↗

arXiv2008

First identification of large electric monopole strength in well-deformed rare earth nuclei

Excited states in the well-deformed rare earth isotopes $^{154}$Sm and $^{166}$Er were populated via ``safe'' Coulomb excitation at the Munich MLL Tandem accelerator. Conversion electrons were registered in a cooled Si(Li) detector in conjunction with a magnetic transport and filter system, the Mini-Orange spectrometer. For the first excited $0^+$ state in $^{154}$Sm at 1099 keV a large value of the monopole strength for the transition to the ground state of $ρ^2(\text{E0}; 0^+_2 \to 0^+_\text{g}) = 96(42)\cdot 10^{-3}$ could be extracted. This confirms the interpretation of the lowest excited $0^+$ state in $^{154}$Sm as the collective $β$-vibrational excitation of the ground state. In $^{166}$Er the measured large electric monopole strength of $ρ^2(\text{E0}; 0^+_4 \to 0^+_1) = 127(60)\cdot 10^{-3}$ clearly identifies the $0_4^+$ state at 1934 keV to be the $β$-vibrational excitation of the ground state.

nucl-ex↗