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 photon tagging spectrometer at the Mainz MAMI accelerator together with the Mainz [48]cm ;SPMOslash; × [64]cm NaI(Tl) photon detector and the G?ttingen SENECA recoil detector. The data cover photon energies ranging from [200]MeV to [400]MeV at θ^LAB _γ = 136.2^°. 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 has been determined to be α_n - β_n = 9.8±3.6(stat)^+2.1 _-1.1(syst)±2.2(model) in units of [10^-4]fm ³. In combination with the polarizability sum α_n + β_n = 15.2±0.5 deduced from photoabsorption data, the neutron electric and magnetic polarizabilities, α_n = 12.5±1.8(stat)^+1.1 _-0.6(syst)±1.1(model) and β_n = 2.7±1.8(stat)^+0.6 _-1.1(syst)±1.1(model) are obtained. The backward spin polarizability of the neutron was determined to be γ⁽ⁿ⁾ _π = (58.6±4.0)×10^-4 fm ⁴. Received: 21 August 2002 / Accepted: 16 October 2002 / Published online: 11 February 2003 RID="a" ID="a"Part of the Doctoral Thesis. RID="b" ID="b"Present address: Physikalisch-Technische Bundesanstalt, Bundesallee 100, D-38116 Braunschweig. RID="a" ID="a"Part of the Doctoral Thesis. RID="b" ID="b"Present address: Physikalisch-Technische Bundesanstalt, Bundesallee 100, D-38116 Braunschweig. RID="c" ID="c"Part of the Habilitation Thesis. RID="d" ID="d"e-mail: schumacher@physik2.uni-goettingen.de Communicated by Th. Walcher     

Manifestations of hidden polarized strangeness of the nucleon in polarization phenomena for the process p +¯p → P0+ V0

The sensitivities of the one-spin and two-spin polarization observables for p+ˉp→P⁰+V⁰, with P⁰=π⁰, η,η′ and V⁰=ρ,ω,φ, based on the triplet enhancement hypothesis for strangeness production in pˉp collisions are analyzed. The analysis is carried out under special kinematical conditions where the P⁰ meson production angle in the CMS is equal to π/2, for which C-invariance of strong interactions reduces the number of independent amplitudes to three. Received: 17 September 1997     

Measurement of the generalized polarizabilities of the proton in virtual Compton scattering at MAMI

Virtual Compton scattering off the proton has been studied at Q ² = 0.33 (GeV/c)² at the MAMI accelerator in Mainz (Germany). The goal of the experiment is to measure the generalized polarizabilities (GPs) of the proton using the double polarized e p→e' p'γ reaction. This paper reports the measurement of the unpolarized photon electroproduction cross-section for the new data and the extraction of two linear combinations of GPs, which are essential for the analysis of the double spin asymmetry.     

General formalism in collinear regime for pseudoscalar meson production in NN collisions

The spin structure of the matrix element for the pseudoscalar meson production processes in nucleon-nucleon collisions is established in the collinear kinematical regime in terms of 3 independent scalar amplitudes. This result is valid for any reaction mechanism and for any energy of the colliding and the produced particles. The complete experiment for the full reconstruction of all 3 complex amplitudes must contain two different classes of polarization experiments. The polarization transfer coefficients can be used to determine the moduli of all 3 amplitudes, whereas the spin correlation coefficients for + collisions are sensitive to the relative phases of different amplitudes. Received: 13 August 2001 / Accepted: 20 September 2001     

Study of nucleon spin-dependent properties in the resonance region

In this paper, the spin-dependent structure functions of nucleon g ₁, and photoabsorption cross sections σ_1/2, σ_3/2 and σ_T in the resonance region are estimated based on the constituent quark model and the properties of the five phenomenological Breit-Wigner resonances P ₃₃(1232), S ₁₁(1535), D ₁₃(1520), P ₁₁(1440), and F ₁₅(1680). Our results are compared to the recent E143 data of the polarized structure functions g ₁(W ², Q ²) at points Q ²=0.5 GeV² and Q ²=1.2 GeV² and the data of the total inclusive photoabsorption cross sections. Received: 7 October 1997     

Threshold enhancement in η photoproduction from 2H and 4He

The photoproduction of η-mesons from ²H and ⁴He has been studied for energies close to the production thresholds. The experiments were carried out with the tagged photon beam of the Mainz MAMI accelerator. The η-mesons were detected via their two photon decays with the electromagnetic calorimeter TAPS. Total cross-sections, angular and momentum distributions of the η-mesons have been determined for both reactions. The total cross-sections in the threshold region show a large enhancement over the predictions of a participant-spectator model, indicating significant final-state interaction effects. The results are compared to recent model calculations taking into account nucleon-nucleon and nucleon-η final-state interaction effects on different levels of sophistication. Received: 23 November 2001 / Accepted: 11 February 2002     

KNΛ and KNΣ coupling constants from the Chiral Bag Model

The KNΛ and KNΣ coupling constants have been calculated in the framework of the Chiral Bag Model(CBM). We find −3.88 ≤g _KNΛ≤−3.67 and 1.15 ≤g _KNΣ≤ 1.24 by taking into account pseudoscalar mesons (π, K) and vector mesons (ρ, ω, K ^*) field effects. Particularly, it is shown that vector mesons make significant contributions to the coupling constants g _KNΛ and g _KNΣ. Our values are existing within the experimental limits compared to the phenomenological values extracted from the kaon photoproduction and kaon-nucleon scattering experiments. Also, form factors are suggested for the πNN, πNΔ, KNΛ and KNΣ couplings. Received: 17 August 1998 / Revised version: 22 December 1998     

The kaon form factor in the light-cone quark model

The electromagnetic form factor of the kaon meson is calculated in the light-cone formalism of the relativistic constituent quark model. The calculated K⁺ form factor is consistent with almost all of the available experimental data at low-energy scale, while other properties of the kaon could also be interrelated in this representation with reasonable parameters. Predictions of the form factors for the charged and neutral kaons at a higher-energy scale are also given, and we find the non-zero K⁰ form factor at Q ²≠ 0 due to the mass difference between the strange and down quarks inside K⁰. Received: 21 June 2002 / Accepted: 29 July 2002 / Published online: 3 December 2002 RID="a" ID="a"e-mail: mabq@phy.pku.edu.cn Communicated by A. Sch?fer     

Pion scalar density and chiral symmetry restoration at finite temperature and density

This paper is devoted to the evaluation of the pionic scalar density at finite temperature and baryonic density. We express the latter effect in terms of the nuclear response evaluated in the random phase approximation. We discuss the density and temperature evolution of the pionic density which governs the quark condensate evolution. Numerical evaluations are performed. Received: 7 December 1999 / Accepted: 8 May 2000     

The renormalon contribution to the current product j μ5 j S

The product of an axialvector and a scalar current and its relation to the chiral-odd distribution function h ₁ is discussed in the framework of the renormalon approach. Using a bag-model calculation for h ₁, we calculate its intrinsic uncertainty due to renormalon poles. The result is given as a function of Bjorken-x as well as for the first moments separately. Received: 9 November 1998 / Revised version: 22 January 1999     

K<Superscript>+</Superscript> production in proton-nucleus reactions and the role of momentum-dependent potentials

The production of K⁺-mesons in proton-nucleus collisions from 1.0 to 2.5GeV is analyzed with respect to one-step nucleon-nucleon ( NN → NYK ⁺) and two-step Δ-nucleon ( ΔN → K ⁺ YN) or pion-nucleon ( πN → K ⁺ Y) production channels on the basis of a coupled-channel transport approach (CBUU) including the kaon final-state interactions. The influence of momentum-dependent potentials for the nucleon, hyperon and kaon in the final state are studied as well as the importance of K⁺ elastic rescattering in the target nucleus. The transport calculations are compared to the experimental K⁺ spectra taken at LBL Berkeley, SATURNE, CELSIUS, GSI and COSY-Jülich. It is found that the momentum-dependent baryon potentials affect the excitation function of the K⁺ cross-section; at low bombarding energies of ∼ 1.0GeV the attractive baryon potentials in the final state lead to a relative enhancement of the kaon yield, whereas the net repulsive potential at bombarding energies ∼ 2GeV causes a decrease of the K⁺ cross-section. Furthermore, it is pointed out that especially the K⁺ spectra at low momenta (or kinetic energy T _K) allow to determine the in-medium K⁺ potential almost model independently due to a relative shift of the K⁺ spectra in kinetic energy that arises from the acceleration of the kaons when propagating out of the nuclear medium to free space, i.e. converting the potential energy to the kinetic energy of the free kaon. Received: 28 January 2002 / Accepted: 3 June 2002 / Published online: 19 November 2002 RID="a" ID="a"e-mail: Wolfgang.Cassing@theo.physik.uni-giessen.de Communicated by P. Schuck     

Dispersion relation formalism for virtual Compton scattering off the proton

We present in detail a dispersion relation formalism for virtual Compton scattering (VCS) off the proton from threshold into the Δ(1232)-resonance region. Such a formalism can be used as a tool to extract the generalized polarizabilities of the proton from both unpolarized and polarized VCS observables over a larger energy range. We present calculations for existing and forthcoming VCS experiments and demonstrate that the VCS observables in the energy region between pion production threshold and the Δ(1232)-resonance show an enhanced sensitivity to the generalized polarizabilities. Received: 14 March 2001 / Accepted: 18 June 2001     

Rare pionium decays and pion polarizability

We calculate the decay of pionium atoms into two photons. The pion polarizabilities give rise to a 10% correction to the corresponding decay width for pointlike pions. This opens the possibility to obtain the difference between the electric and magnetic polarizability of the charged pion from a future measurement of the branching fraction of pionium into two photons. For such an experiment the ππ-scattering lengths would have to be known to better than 5% precision. We also comment on the contribution of the axial anomaly to the decay of pionium into γπ⁰. Received: 1 March 1999 / Revised version: 31 May 1999     

Understanding the proton spin “puzzle” with a new “minimal” quark model

We investigate the spin structure of the nucleon in an extended Jaffe-Lipkin quark model. In addition to the conventional 3q structure, different (3q)(Q ) admixtures in the nucleon wave function are also taken into account. The contributions to the nucleon spin from various components of the nucleon wave function are discussed. The effect due to the Melosh-Wigner rotation is also studied. It is shown that the Jaffe-Lipkin term is only important when antiquarks are negatively polarized. We arrive at a new “minimal” quark model, which is close to the naive quark model, in order to understand the proton spin “puzzle”. Received: 4 November 2000 / Accepted: 23 October 2001     

Low-energy Compton scattering and the polarizabilities of the proton

Differential cross-sections for Compton scattering from the proton have been measured at the MAMI tagged photon facility using the TAPS setup. The data cover an angular range of θ^lab _γ = 59^°-155^° and photon energies ranging from 55 MeV to 165 MeV. Our results are in good agreement with those from previous experiments, but yield higher precision. Using dispersion relations the proton polarizabilities have been determined to be = [11.9±0.5_stat.±1.3_syst.±0.3_mod.] ^. 10^-4 fm ³ and = [1.2±0.7_stat.±0.3_syst.±0.4_mod.)] ^. 10^-4 fm ³. These results confirm the Baldin sum rule which was re-evaluated to be + = [13.8±0.4] ^. 10^-4 fm ³. We can also conclude that there is no significant additional asymptotic contribution to the backward spin polarizability γ_π beyond the t-channel π⁰-exchange. Received: 9 January 2001 / Accepted: 13 February 2001     

Measurement of GEp/GMp via polarization transfer at Q2 = 0.4 GeV /c2

The polarization transfer from longitudinally polarized electrons to protons in the elastic scattering p( e, e' p) has been measured around Q² = 0.4 (GeV/c)² with the three-spectrometer facility at the Mainz microtron MAMI. From this polarization transfer the ratio G _Ep/(G _Mp/μ_p) has been determined. The ratio is found to be slightly less than unity in agreement with recent results from other laboratories and from the Rosenbluth separation of cross-sections measured with unpolarized electrons. Received: 26 July 2001 / Accepted: 25 September 2001     

Observation of K+d correlations from pA collisions

Results of a first experiment on (K^+p) and (K^+d) correlations from proton-carbon (pC) and proton-deuteron (pd) interactions at beam energies above and much below the threshold for elementary kaon production in nucleon-nucleon reactions ( T _NN = 1580 MeV) are discussed. These data, obtained with the ANKE spectrometer at COSY-Jülich, provide first direct evidence for K⁺ production via the two-step mechanism and an indication for a cluster mechanism. It is shown that both processes contribute significantly in pC collisions at 1200 MeV, while they are strongly suppressed at 2300 MeV and also in pd-interactions at 1344 MeV. It is emphasized that the underlying kinematics can be exploited to distinguish between these reaction mechanisms. Received: 26 November 2002 / Accepted: 26 March 2003 / Published online: 20 May 2003     

Measurements of the spin rotation parameter A in the elastic pion-proton scattering in the D13(1700) resonance region

The spin rotation parameters A and R were measured for the elastic pion-proton scattering by the PNPI-ITEP collaboration in the D₁₃(1700) resonance region. The main goal of the experimental program is to resolve the current partial-wave analyses (PWA) uncertainties. Simultaneously with A and R the polarization parameter P was measured with the purpose to improve the experimental database and estimate systematic errors. The constraint which demands a smooth energy dependence of all π^- p transverse amplitude zeros in the complex plane together with the new experimental data on A parameter can lead to the conclusion that the Barrelet branch of “zero trajectories" is chosen improperly in PWA of the Carnegie-Mellon-Lawrence-Berkely-Laboratory groups at the range of the pion beam momentum near 1.0 GeV/c. The setup included a longitudinally polarized proton target with superconductive magnet, multiwire spark chambers and carbon polarimeter with thick filter. The experiment was performed at the ITEP proton synchrotron, Moscow. Received: 17 August 2001 / Accepted: 13 September 2001     

Covariant electroweak nucleon form factors in a chiral constituent-quark model

Results for the proton and neutron electric and magnetic form factors as well as the nucleon axial and induced pseudoscalar form factors are presented for the chiral constituent-quark model based on Goldstone-boson exchange dynamics. The calculations are performed in a covariant framework using the point-form approach to relativistic quantum mechanics. The direct predictions of the model yield a remarkably consistent picture of the electroweak nucleon structure. Received: 28 February 2002 / Accepted: 14 March 2002     

The effect of quark exchange in A = 3 mirror nuclei and neutron/proton structure functions ratio

By using the quark-exchange formalism, the realistic Faddeev wave function and the Fermi motion effect, we investigate the deep inelastic electron scattering from A = 3 mirror nuclei in the deep-valence region. The initial valence quark input is taken from the GRV's (Glück, Reya and Vogt) fitting procedure and the next-to-leading-order QCD evolution on F₂^p(x, Q²) which gives a very good fit to the available data in the (x, Q²)-plane. It is shown that the free neutron to proton structure functions ratio can be extracted from the corresponding EMC ratios for ³He and ³H mirror nuclei by using the self-consistent iteration procedure and the results are in good agreement with the other theoretical models as well as the present available experimental data and especially the projected data expected from the proposed 11GeV Jefferson Laboratory in the near future.