Study of the reaction γp → pπ0η

The reaction γp → pπ⁰η has been studied with the CBELSA detector at the tagged photon beam of the Bonn electron stretcher facility. The reaction shows contributions from Δ⁺(1232)η , N(1535)⁺π⁰ and pa ₀(980) as intermediate states. A partial-wave analysis suggests that the reaction proceeds via formation of six Δ -resonances, Δ(1600)P ₃₃ , Δ(1920)P ₃₃ , Δ(1700)D ₃₃ , Δ(1940)D ₃₃ , Δ(1905)F ₃₅ , Δ(2360)D ₃₃ , and two nucleon resonances N(1880)P ₁₁ and N(2200)P ₁₃ , for which pole positions and decay branching ratios are given.     

Tensor analyzing capacity of the γd → Δp reaction

The relativistic momentum approximation has been used to calculate the tensor analyzing capacity F₂₀ of the d p reaction. At high proton moments (p50 MeV/sec), the dependence of F₂₀ of the parameters of the n vertex becomes substantial, which shows that experiments on F₂₀ can provide information on the electromagnetic form factor of the n transition.Research performed with financial support from the Russian Fundamental Research Fund (project code 93-02-3514).Nuclear Physics Research Institute, Tomsk Polytechnical University. Translated from Izvestiya Vysshikh Uchebnykh Zavedenii, Fizika, No. 10, pp. 35–38, October, 1994.     

Measurement of the πmeson polarizabilities via the γp → γπ+n reaction

An experiment on the radiative ^{+}-meson photoproduction from the proton ( p ^{+}n) was carried out at the Mainz Microtron MAMI in the kinematic region 537MeV < E < 817MeV, 140^° 180^°. The ^{+}-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 ( s₁ < 5m²) 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 < s₁/m² < 15, -12 < t/m² < - 2), the difference of the electric () and the magnetic () polarizabilities has been determined. As a result we find . This result is at variance with recent calculations in the framework of chiral perturbation theory.     

BFKL Pomeron in the reaction γ ⊥ * (Q 2)p→ρ ⊥ 0 p at large Q 2 and W

The reaction γ _⊥ ^* (Q ²)p→ρ _⊥ ⁰ p has been studied at large Q ² and W ²/Q ² and low momentum transfers to the nucleon, k _⊥ ² —that is, in the region where the Pomeron exchange mechanism is operative. At sufficiently large Q ², the Pomeron interacts with quarks occurring at small distances, whereby the hard component of the Pomeron is separated, so that the process is governed by the Pomeron of perturbative QCD (BFKL Pomeron). Our calculations indicate that, in vector-meson electroproduction at low k _⊥ ² , the perturbative regime cannot set in fast because, for Q ²≤100 GeV² and, accordingly, for W ²/Q ²≤10⁷, comparatively large distances of $\rho _{q\bar q} > 0.2$ fm are important.     

Description of initial-and final-state-interaction effects for the reaction γp → π−Δ++ in the region of nucleon-resonance excitation

A model has been developed for describing the cross section for the reaction γ_r,v p → π^?Δ⁺⁺ in the energy region of nucleon-resonance excitation, and relevant calculations have been performed. The model takes into account the contribution of all reliably established resonances with masses less than 2 GeV and the contributions of nonresonance mechanisms in the approximation of Born diagrams. On the basis of data on the amplitudes of pion-nucleon interactions, a method has been evolved for phenomenologically describing the coupling of initial and final states with open inelastic channels. Calculations performed in this study represent a first step in developing a method for determining the electromagnetic form factors for nucleon resonances in exclusive reactions of pion-pair production on a nucleon by real and virtual photons.     

Study of the polarization degree for the p ⃗ p → ppη measurement with WASA

Understanding of the production processes of the η meson will strongly rely on the precise determination of spin observables. So far these observables have been determined only for few excess energies and with low statistics (Winter et al. Eur. Phys. J. A18, 355 2003; Czyzykiewicz et al. Phys. Rev. Lett. 98, 122003 2007; Balestra et al. Phys. Rev. C69, 064003 2004). In the year 2010 WASA detector was used for the measurement of the \(\overrightarrow {p}p\rightarrow pp\eta \) reaction with the polarized proton beam of COSY (Moskal and Hodana J. Phys. Conf. Ser 295, 012080 2011). The measurement was done for the excess energy of Q = 15 MeV and Q = 72 MeV. In total about 10⁶ events corresponding to the \(\overrightarrow {p}p\rightarrow pp\eta \) reaction have been collected.     

Measurement of the reaction γp → K0Σat photon energies up to 2.6 GeV⋆

The reaction p K⁰⁺ was measured in the photon energy range from threshold up to 2.6 GeV with the SAPHIR detector at the electron stretcher facility, ELSA, in Bonn. Results are presented on the reaction cross-section and the polarization of the ⁺ as a function of the kaon production angle in the centre-of-mass system, cos^c.m._K, and the photon energy. The cross-section is lower and varies less with photon energy and kaon production angle than that of p K⁺⁰. The ⁺ is polarized predominantly at cos^c.m._K 0. The data presented here are more precise than previous ones obtained with SAPHIR and extend the photon energy range to higher values. They are compared to isobar model calculations.     

A measurement of the branching ratio for the∑ + →pγ decay

We have measured the weak radiative decay branching ratio (∑ ⁺ →pγ)/(∑ ⁺ →pπ⁰). The experiment was carried out at Brookhaven National Laboratory using a low-energy separatedK ^? beam stopping in liquid hydrogen. The∑ ⁺ was tagged by observing the pion fromK ^? K ⁺ p→∑ ⁺π^? in a range telescope, and the gamma was detected in a segmented NaI detector. The result is (2.81±0.39 _?0.43 ^+0.21 )×10^?3, which is consistent with previous measurements. This corresponds to a branching ratio for (∑ ⁺ →pγ)/(∑ ⁺ → all) of (1.45±0.20 _?0.22 ^+0.11 )×10^?3, where the quoted errors are statistical and systematic, respectively. A total of 408 signal events were observed, doubling the previous world total.     

Observation of the decay Ks → 2γ and measurement of the decay rates KL → 2γ and KS → 2γ

The decay rates of K_L → 2γ and K_S → 2γ have been measured at the CERN SPS. The results are Γ(K_L→2γ)/χ(K_L→2π₀)=0.632±0.004±0.008 and Γ(K_S→2γ)/Γ(K_L→2γ)=2.3 ±1.0±0.4. This is the first observation of K_S→2γ decays.     

First measurement of the exclusive (p,e+e−) reaction

We present the first data on the exclusive (p,e⁺e⁻) reaction in a nucleus. Cross sections for populating the ground state and first excited state in the residual nucleus are presented for the reaction ¹¹B(p, e⁺e⁻)¹²C at 98 MeV. The e⁺e⁻ momentum vectors were measured with a high-resolution magnetic pair spectrometer. The distributions of the virtual photon invariant mass and of the direction of the electron in the virtual photon rest system are compared with a simple single-nucleon model. The (p,e⁺e⁻) cross sections are compared with corresponding data for the (p, γ) reaction obtained with the same apparatus. The (p,e⁺e⁻) cross sections show substantial enhancement over what is expected from the simple model.     

New measurement of the π → μνγ decay

We have measured the branching ratio of the radiative decay π → μνγ. The energies of the gamma-ray and of the muon were both measured and the Dalitz-plot distribution of the decay was obtained. The data agree well with the theoretical prediction (QED internal bremsstrahlung) down to a γ-ray energy of 1 MeV. The discrepancy reported in a previous experiment is not confirmed.     

Exclusive measurement of the pp → nnπ+π+ reaction at 1.1 GeV

First exclusive data for the $\ensuremath{pp \to nn\pi^+\pi^+}$ reaction have been obtained at CELSIUS with the WASA detector setup at a beam energy of T_p = 1.1 GeV. Total and differential cross-sections disagree with theoretical calculations, which predict the $ \Delta$ $ \Delta$ excitation to be the dominant process at this beam energy. Instead, the data require the excitation of one of the nucleons to a higher-lying $ \Delta$ state, preferably the $\ensuremath{\Delta(1600)P_{33}}$ , to be the leading process.     

Study of Fermilab data for the reaction π− p→ηn

Fermilab data for the angular distribution measurements of the reaction ^– p n are investigated and it is shown that a good fit with experiment is obtained by using a simple Regge pole model with phenomenological residue functions. The energy dependence in this case is found to be different from that for the d/dt/ data obtained at Serpukhov.Work supported by the Pakistan Science Foundation under contract No. P-PU-PHY(11/1).     

Effect of rescattering on polarization observables of the reaction γd → ppπ in the delta-resonance region

The effect of pion-nucleon and nucleon-nucleon rescatterings on polarization observables of the reaction γd → ppπ^? are calculated in the delta-isobar region. Pion-nucleon and nucleon-nucleon rescatterings are considered on the basis of a diagrammatic approach. Lorentz invariant expressions are used for the operators of photoproduction and of pion scattering on a free nucleon. A unitarization procedure in the K-matrix approximation is used for resonance partial-wave amplitudes. It is shown that, in the delta-resonance region, rescattering has a sizable effect on polarization observables of the reaction γd → ppπ^? at high momenta of final protons.     

Study of the reaction γp → K+Λ(1520) at photon energies up to 2.65 GeV

The reaction γp → K ⁺ Λ(1520) was measured in the energy range from threshold to 2.65 GeV with the SAPHIR detector at the electron stretcher facility ELSA in Bonn. The Λ(1520) production cross-section was analyzed in the decay modes pK ^?, $ n\bar K^0 $ , Σ ^± π ^?, and Λπ ⁺ π ^? as a function of the photon energy and the squared four-momentum transfer t . While the cross-sections for the inclusive reactions rise steadily with energy, the cross-section of the process γp → K ⁺ Λ(1520) peaks at a photon energy of about 2.0 GeV, falls off exponentially with t , and shows a slope flattening with increasing photon energy. The angular distributions in the t-channel helicity system indicate neither a K nor a K ^* exchange dominance. Rather, the rapid change of the angular distribution with energy suggests that, in addition to t -channel K and K^* exchange, amplitudes for formation of nucleon resonances in the s-channel play an important role.     

Effect of the Λ n interaction on the cross section of the reaction γd → K+Λ n

Taking into account the n interaction, a nonrelativistic graphical technique is used to calculate the differential cross section of the reaction d K⁺ n as a function of photon energy, K⁺-momentum, and angle of emission of the K⁺. The kinematic region in which variation of the n-scattering parameters gives the most noticeable effect is separated out.Translated from Izvestiya Vysshikh Uchebnykh Zavedenii, Fizika, No. 8, pp. 104–106, August, 1978.The author thanks G. M. Radutskii and V. A. Filimonov for many useful discussions.