arXiv subjects

P. Grabmayr

Publications and source records attributed to P. Grabmayr.

At least 55 records · Page 3Linked to original sources

arXiv2009

Investigation of the Exclusive ^{3}He(e,e'pn)p Reaction

Cross sections for the ^{3}He(e,e'pn)p reaction were measured for the first time at energy transfers of 220 and 270 MeV for several momentum transfers ranging from 300 to 450 MeV/c. Cross sections are presented as a function of the momentum of the recoil proton and the momentum transfer. Continuum Faddeev calculations using the Argonne V18 and Bonn-B nucleon-nucleon potentials overestimate the measured cross sections by a factor 5 at low recoil proton momentum with the discrepancy becoming much smaller at higher recoil momentum.

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arXiv2008

Nuclear Structure Relevant to Neutrinoless Double Beta Decay: the Valence Protons in 76Ge and 76Se

The possibility of observing neutrinoless double beta decay offers the opportunity of determining the effective neutrino mass if the nuclear matrix element were known. Theoretical calculations are uncertain and the occupations of valence orbits by nucleons active in the decay are likely to be important. The occupation of valence proton orbits in the ground states of 76Ge, a candidate for such decay, and 76Se, the corresponding daughter nucleus, were determined by precisely measuring cross sections for proton-removing transfer reactions. As in previous work on neutron occupations, we find that the Fermi surface for protons is much more diffuse than previously thought, and the occupancies of at least three orbits change significantly between the two 0+ ground states.

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arXiv2008

Experimental and theoretical evidences for an intermediate $σ$-dressed dibaryon in the NN interaction

Numerous theoretical and experimental arguments are presented in favor of the generation of intermediate $σ$-dressed dibaryon in $NN$ interaction at intermediate and short distances. We argue that this intermediate dibaryon can be responsible for the strong intermediate-range attraction and the short-range repulsion in the $NN$ interaction, and also for the short-range correlations in nuclei. The suggested mechanism for the $σ$-dressing of the dibaryon is identical to that which explains the Roper resonance structure, its dominant decay modes and its extraordinary low mass. A similar transformation mechanism from the glue to the scalar field was discovered in $J/Ψ$ decays. The new experimental data on 2$π$-production in the scalar-isoscalar channel produced in $pn$- and $pd$-collisions and in particular the very recent data on $γγ$ correlations in $p$C and $d$C scattering in the GeV region seems to corroborate the existence of the $σ$-dressed dibaryon in two- and three nucleon interactions.

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arXiv2007

Incoherent pion photoproduction on $^{12}$C

We present the first detailed measurement of incoherent photoproduction of neutral pions to a discrete state of a residual nucleus. The $^{12}$C$(γ,π^{0})^{12}$C$^{*}_{4.4MeV}$ reaction has been studied with the Glasgow photon tagger at MAMI employing a new technique which uses the large solid angle Crystal Ball detector both as a $π^{0}$ spectrometer and to detect decay photons from the excited residual nucleus. The technique has potential applications to a broad range of future nuclear measurements with the Crystal Ball and similar detector systems elsewhere. The data are sensitive to the propagation of the $Δ$ in the nuclear medium and will give the first information on matter transition form factors from measurements with an electromagnetic probe. The incoherent cross sections are compared to two theoretical predictions including a $Δ$-hole model.

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arXiv2007

First measurements of the ^16O(e,e'pn)^14N reaction

This paper reports on the first measurement of the ^16O(e,e'pn)^14N reaction. Data were measured in kinematics centred on a super-parallel geometry at energy and momentum transfers of 215 MeV and 316 MeV/c. The experimental resolution was sufficient to distinguish groups of states in the residual nucleus but not good enough to separate individual states. The data show a strong dependence on missing momentum and this dependence appears to be different for two groups of states in the residual nucleus. Theoretical calculations of the reaction using the Pavia code do not reproduce the shape or the magnitude of the data.

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arXiv2006

Isoscalar short-range current in the deuteron induced by an intermediate dibaryon

A new model for short-range isoscalar currents in the deuteron and in the NN system is developed; it is based on the generation of an intermediate dibaryon which is the basic ingredient for the medium- and short-range NN interaction which was proposed recently by the present authors.This new current model can very well describe the experimental data for the three basic deuteron observables of isoscalar magnetic type, viz. the magnetic moment, the circular polarization of the photon in the $np\to dγ$ process at thermal neutron energies and the structure function B up to Q$^2$=60 fm$^{-2}$.

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arXiv2005

Measurement of the asymmetries in 3He(e,e'p)d and 3He(e,e'p)np

The electron-target-asymmetries A_parallel and A_perpendicular with target spin parallel and perpendicular to the momentum transfer q were measured for both the two-- and three-body breakup of 3He in the 3He(e,e'p)-reaction. Polarized electrons were scattered off polarized 3He in the quasielastic regime in parallel kinematics with the scattered electron and the knocked-out proton detected using the Three-Spectrometer-Facility at MAMI. The results are compared to Faddeev calculations which take into account Final State Interactions as well as Meson Exchange Currents. The experiment confirms the prediction of a large effect of Final State Interactions in the asymmetry of the three-body breakup and of an almost negligible one for the two-body breakup.

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arXiv2005

Dependence of the $^{12}$C($\vecγ$,pd) reaction on photon linear polarisation

The sensitivity of the $^{12}$C$(\vecγ,pd)$ reaction to photon linear polarisation has been determined at MAMI, giving the first measurement of the reaction for a nucleus heavier than $^{3}$He. Photon asymmetries and cross sections were measured for $E_γ$=170 to 350 MeV. For $E_γ$ below the $Δ$ resonance, reactions leaving the residual $^{9}$Be near its ground state show a positive asymmetry of up to 0.3, similar to that observed for $^{3}$He suggesting a similar reaction mechanism for the two nuclei.

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arXiv2004

Measurement of the pi^+ meson polarizabilities via the gamma p->gamma pi^+ n reaction

An experiment on the radiative pi^+ meson photoproduction from the proton (gamma p->gamma pi^+ n) was carried out at the Mainz Microtron MAMI in the kinematic region 537 MeV <E_{gamma}<817 MeV, 140^o<theta_{gamma-gamma'}^cm<180^o. The pi^+ meson polarizabilities have been determined from a comparison of the data with the predictions of two different theoretical models, the first one being based on an effective pole model with pseudoscalar coupling while the second one is based on diagrams describing both resonant and nonresonant contributions. The validity of the models has been verified by comparing the predictions with the present experimental data in the kinematic region where the pion polarizability contribution is negligible (s_1<5 mu^2) and where the difference between the predictions of the two models does not exceed 3%. In the region, where the pion polarizability contribution is substantial (5<s_1/mu^2<15, -12<t/mu^2<-2), the difference (alpha-beta)_{pi^+} of the electric (alpha) and the magnetic (beta) polarizabilities has been determined. As a result we find: (alpha-beta)_{pi^+}=(11.6\pm 1.5_{stat}\pm 3.0_{syst}\pm 0.5_{mod})x10^-4fm^3. This result is at variance with recent calculations in the framework of chiral perturbation theory.

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arXiv2003

Quasi-free $π^0$ Photoproduction from the Bound Nucleon

Differential cross-sections for quasi-free $π^0$ photoproduction from the proton and neutron bound in the deuteron have been measured for $E_γ= 200 - 400$ MeV at $θ^{\rm lab}_γ= 136.2^\circ$ usind the Glasgow photon tagger at MAMI, the Mainz 48 cm $\varnothing$ $\times$ 64 cm NaI(Tl) photon detector and the Göttingen SENECA recoil detector. For the proton measurements made with both liquid deuterium and liquid hydrogen targets allow direct comparison of "free" $π^0$ photoproduction cross-sections as extracted from the bound proton data with experimental free cross sections which are found to be in reasonable agreement below 320 MeV. At higher energies the "free" cross sections extracted from quasifree data are significantly smaller than the experimental free cross sections and theoretical predictions based on multipole analysis. For the first time, "free" neutron cross sections have been extracted in the $Δ$-region. They are also in agreement with the predictions from multipole analysis up to 320 MeV and significantly smaller at higher photon energies.

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arXiv2003

Magnetization screening from gluonic currents and scaling law violation in the ratio of magnetic form factors for neutron and proton

The ratio ${μ_p}G_E^p/G_M^p$ exhibits a decrease for four-momentum transfer Q^2 increasing beyond 1 GeV^2 indicating different spatial distributions for charge and for magnetization inside the proton. One-gluon exchange currents can explain this behaviour. The SU(6) breaking induced by gluonic currents predicts furthermore that the ratio of neutron to proton magnetic form factors ${μ_p}G_M^n/{μ_n}G_M^p$ falls with increasing Q^2. We find that the experimental data are consistent with our expectations of an almost linear decrease of the ratio ${μ_p}G_M^n/{μ_n}G_M^p$ with increasing Q^2, supporting the statement that the spatial distributions of magnetization are different for protons and for neutrons.

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arXiv2003

A six-quark dressed-bag description of np -> d γradiative capture

The radiative capture process $n p \to d γ$ is considered within the framework of a recently developed six-quark dressed-bag model for the nucleon-nucleon interaction. The calculations presented here include both the nucleon current and the meson-exchange current contributions. The latter uses short-range hadronic form factors for the pion exchange currents consistent with the soft cut-off parameter $Λ_{πNN}$ from the $NN$-potential. Contributions of the pion exchange current and $Δ$-isobar current to the total cross section still cannot explain the discrepancy between the theoretical and experimental cross sections. Possibilities for new types of meson exchange currents associated with chiral fields inside multi-quark dressed-bag states in nuclei are discussed.

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arXiv2003

Effect of gluon-exchange pair-currents on the ratio G(E(P))/G(M(P))

The effect of one-gluon-exchange (OGE) pair-currents on the ratio $μ_p G_E^p/G_M^p$ for the proton is investigated within a nonrelativistic constituent quark model (CQM) starting from $SU(6) \times O(3)$ nucleon wave functions, but with relativistic corrections. We found that the OGE pair-currents are important to reproduce well the ratio $μ_p G_E^p/G_M^p$. With the assumption that the OGE pair-currents are the driving mechanism for the violation of the scaling law we give a prediction for the ratio $μ_n G_E^n/G_M^n$ of the neutron.

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arXiv2003

Quasi-free Compton Scattering and the Polarizabilities of the Neutron

Differential cross sections for quasi-free Compton scattering from the proton and neutron bound in the deuteron have been measured using the Glasgow/Mainz tagging spectrometer at the Mainz MAMI accelerator together with the Mainz 48 cm $\oslash$ $\times$ 64 cm NaI(Tl) photon detector and the Göttingen SENECA recoil detector. The data cover photon energies ranging from 200 MeV to 400 MeV at $θ^{LAB}_γ=136.2^\circ$. Liquid deuterium and hydrogen targets allowed direct comparison of free and quasi-free scattering from the proton. The neutron detection efficiency of the SENECA detector was measured via the reaction $p(γ,π^+ n)$. The "free" proton Compton scattering cross sections extracted from the bound proton data are in reasonable agreement with those for the free proton which gives confidence in the method to extract the differential cross section for free scattering from quasi-free data. Differential cross sections on the free neutron have been extracted and the difference of the electromagnetic polarizabilities of the neutron have been obtained to be $α-β= 9.8\pm 3.6(stat){}^{2.1}_1.1(syst)\pm 2.2(model)$ in units $10^{-4}fm^3$. In combination with the polarizability sum $α+β=15.2\pm 0.5$ deduced from photoabsorption data, the neutron electric and magnetic polarizabilities, $α_n=12.5\pm 1.8(stat){}^{+1.1}_{-0.6}\pm 1.1(model)$ and $β_n=2.7\mp 1.8(stat){}^{+0.6}_{-1.1}(syst)\mp 1.1(model)$ are obtained. The backward spin polarizability of the neutron was determined to be $γ^{(n)}_π=(58.6\pm 4.0)\times 10^{-4}fm^4$.

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arXiv2002

A dressed bag model study of the final-state $NΔ$ interaction in photoproduction processes off the deuteron

The impact of the short-range $NΔ$ interaction on the pion photoproduction processes off the deuteron in the $Δ$-resonance region is studied in the framework of recently proposed dressed-bag model. A common dressing procedure for bare three- and six-quark states is used to describe both the pion decay widths of baryon resonances and the effective $NN$ (or $NΔ$) interaction at short ranges related to the inner dressed-bag states. It is shown that the effect of short-range $NΔ$ interaction for the forward-angle $π^\mathrm{o}$ photoproduction off the deuteron cannot be neglected. The prospects for further development of the model to describe the short-range $NN$ (or $NΔ$) correlations in the lightest nuclei are discussed.

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arXiv2002

Three-nucleon mechanisms in photoreactions

The $^{12}$C$(γ,ppn)$ reaction has been measured for E$_γ$=150-800 MeV in the first study of this reaction in a target heavier than $^3$He. The experimental data are compared to a microscopic many body calculation. The model, which predicts that the largest contribution to the reaction arises from final state interactions following an initial pion production process, overestimates the measured cross sections and there are strong indications that the overestimate arises in this two-step process. The selection of suitable kinematic conditions strongly suppresses this two-step contribution leaving cross sections in which up to half the yield is predicted to arise from the absorption of the photon on three interacting nucleons and which agree with the model. The results indicate $(γ,3N)$ measurements on nuclei may be a valuable tool for obtaining information on the nuclear three-body interaction.

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arXiv2002

Neutron polarizabilities investigated by quasi-free Compton scattering from the deuteron

Measuring Compton scattered photons and recoil neutrons in coincidence, quasi-free Compton scattering by the neutron has been investigated at MAMI (Mainz) at $theta^{lab}_γ=136^o$ in an energy range from 200 to 400 MeV. From the data a polarizability difference of $α_n - β_n = 9.8 \pm 3.6(stat)^{+2.1}_{-1.1}(syst)\pm 2.2(model)$ in units of $10^{-4}fm^3$ has been determined. In combination with the polarizability sum $α_n+β_n= 15.2\pm 0.5$ deduced from photo absorption data, the neutron electric and magnetic polarizabilities, $α_n=12.5\pm 1.8(stat)^{+1.1}_{-0.6}(syst)\pm 1.1(model)$ and $β_n = 2.7\mp 1.8(stat)^{+0.6}_{-1.1}(syst)\mp 1.1(model)$, are obtained.

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arXiv2002

Search for Delta^++ component in 12^C ground state using 12^C(gamma, pi^+ p) reaction

The differential cross section for the $^{12}$C$(γ,π^{+}p)$ reaction has been measured in the $Δ$(1232) resonance region at high recoil momenta of the residual nuclear system. The data are analyzed under the assumption that the formation of the $π^+p$ pairs may be interpreted as a $γΔ^{++}\toπ^+p$ process which takes place on a $Δ^{++}$ preexisting in the target nucleus. Estimates of the $Δ^{++}$ momentum distribution $ρ_{Δ^{++}}(\bar{p})$=0.17 fm$^3$ for a mean momentum $\bar{p}=300\pm49$ MeV/c as well as the number of $Δ$ isobars per nucleon $N_Δ=0.017$ were obtained for $^{12}$C.

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