arXiv subjects

V. Hejny

Publications and source records attributed to V. Hejny.

89 records · Page 5Linked to original sources

arXiv2006

Vector and tensor analysing powers in deuteron-proton breakup reactions at intermediate energies

Vector and tensor analysing powers of the d(pol)p->(pp)n (charge-exchange) and d(pol)p->(pn)p (non-charge-exchange) breakup reactions have been measured with the ANKE spectrometer at the COSY ring at a deuteron beam energy of 1170 MeV for small momentum transfers to the low excitation energy (pp) or (pn) systems. A quantitative understanding of the values of A_xx and A_yy for the charge-exchange reaction is provided by impulse approximation calculations. The data suggest that spin-flip isospin-flip transitions, which dominate the charge-exchange breakup of the deuteron, are also important in the non-charge-exchange reaction.

nucl-ex↗

arXiv2006

Near threshold eta meson production in the d+d->alpha+eta reaction

The d+d->alpha+eta reaction has been investigated near threshold using the ANKE facility at COSY-Juelich. Both total and differential cross sections have been measured at two excess energies, Q=2.6 MeV and 7.7 MeV, with a subthreshold measurement being undertaken at Q=-2.6 MeV to study the physical background. While consistent with isotropy at the lower energy, the angular distribution reveals a pronounced anisotropy at the higher one, indicating the presence of higher partial waves. Options for the decomposition into partial amplitudes and their consequences for determination of the s-wave eta-alpha scattering length are discussed.

nucl-ex↗

arXiv2005

Production of the 1S0 diproton in the pp -> pp pi0 reaction at 0.8 GeV

The pp -> pp pi0 differential cross section has been measured with the ANKE spectrometer at COSY-Juelich for pion cms angles between 0 and 15.4 degrees at a proton beam energy of 0.8 GeV. The selection of diproton pairs with an excitation energy E_{pp} < 3 MeV ensures that the final pp system is dominantly in the spin-singlet 1S0 state. The kinematics are therefore very similar to those of pp -> d pi+ but with different spin and isospin transitions. The results will thus provide a crucial extra test of pion production models in nucleon-nucleon collisions. The cross sections, which are over two orders of magnitude smaller than those of pp -> d pi+, show a forward dip, even stronger than that seen at lower energies. This behaviour is well reproduced in a theoretical model that includes P-wave Delta-N states.

nucl-ex↗

arXiv2005

a0+(980)-resonance production in the reaction pp -> dpi+eta close to the K(bar(K)) threshold

The reaction pp -> dpi+eta has been measured at a beam energy of T=2.65 GeV (p=3.46 GeV/c) using the ANKE spectrometer at COSY-Juelich. The missing mass distribution of the detected dpi+ pairs exhibits a peak around the eta mass on top of a strong background of multi-pion pp -> dpi+(n(pi)) events. The differential cross section d^4(sigma)/d(Omega_d)d(Omega_pi+)d(p_d)d(p_pi+) for the reaction pp -> dpi+eta has been determined model independently for two regions of phase space. Employing a dynamical model for the a0+ production allows one then to deduce a total cross section of sigma(pp -> da0+ -> dpi+eta)=(1.1 +/- 0.3_(stat) +/- 0.7_(syst)) microbarn for the production of pi+eta via the scalar a0+(980) resonance and sigma(pp -> dpi+eta) = (3.5 +/- 0.3_(stat) +/- 1.0_(syst)) microbarn for the non-resonant production. Using the same model as for the interpretation of recent results from ANKE for the reaction pp -> dK+(bar(K0)), the ratio of the total cross sections is sigma(pp -> d(K+(bar(K0)))_(L=0))/sigma(pp -> da0+ -> dpi+eta) = 0.029 +/- 0.008_(stat) +/- 0.009_(syst), which is in agreement with branching ratios in the literature.

nucl-ex↗

arXiv2004

Near-threshold production of omega mesons in the pn -> d omega reaction

The first measurement of the p n -> d omega total cross section has been achieved at mean excess energies of Q = 28 and 57 MeV by using a deuterium cluster-jet target. The momentum of the fast deuteron was measured in the ANKE spectrometer at COSY-Juelich and that of the slow "spectator" proton p(sp) from the p d -> p(sp) d omega reaction in a silicon telescope placed close to the target. The cross sections lie above those measured for p p -> p p omega but seem to be below theoretical predictions.

nucl-ex↗

arXiv2004

Inclusive K^+ meson production in proton-nucleus interactions

The production of K^+ mesons in pA (A = D, C, Cu, Ag, Au) collisions has been investigated at the COoler SYnchrotron COSY-Jülich for beam energies T_p = 1.0 - 2.3 GeV. Double differential inclusive pC cross sections at forward angles theta < 12 degrees as well as the target-mass dependence of the K^+ momentum spectra have been measured with the ANKE spectrometer. Far below the free NN threshold at T_{NN}=1.58 GeV the spectra reveal a high degree of collectivity in the target nucleus. From the target-mass dependence of the cross sections at higher energies, the repulsive in-medium potential of K^+ mesons can be deduced. Using pN cross-section parameterisations from literature and our measured pD data we derive a cross-section ratio of sigma(pn -> K^+ X) / sigma(pp -> K^+ X) ~ (3-4).

nucl-ex↗

arXiv2003

a0+(980)-resonance production in pp->dK+Kbar0 reactions close to threshold

The reaction pp->d K+ Kbar0 has been investigated at an excess energy of Q=46 MeV above the (K+ Kbar0) threshold with ANKE at COSY-Juelich. From the detected coincident dK+ pairs about 1000 events with a missing Kbar0 were identified, corresponding to a total cross section of sigma(pp->d K+ Kbar0)=(38 +/- 2(stat) +/- 14(syst)) nb. Invariant-mass and angular distributions have been jointly analyzed and reveal s-wave dominance between the two kaons, accompanied by a p-wave between the deuteron and the kaon system. This is interpreted in terms of a0+(980)-resonance production.

nucl-ex↗

arXiv2003

Threshold enhancement in eta photoproduction from 2H and 4He

The photoproduction of eta-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 eta-mesons were detected via their two photon decays with the electromagnetic calorimeter TAPS. Total cross sections, angular and momentum distributions of the eta-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-eta final state interaction effects on different levels of sophistication.

nucl-ex↗

arXiv2003

Exclusive measurement of coherent eta photoproduction from the deuteron

Coherent photoproduction of eta mesons from the deuteron has been measured from threshold up to incident photon energies of 750 MeV using the photon spectrometer TAPS at the tagged photon facility at the Mainz microtron MAMI. For the first time, differential coherent cross sections have been deduced from the coincident detection of the eta meson and the recoil deuteron. A missing energy analysis was used for the suppression of background events so that a very clean identification of coherent eta-photoproduction was achieved. The resulting cross sections agree with previous experimental results except for angles around 90 deg in the photon-deuteron cm-system where they are smaller. They are compared to various model calculations.

nucl-ex↗

arXiv2002

Exclusive measurements of quasi-free eta-photoproduction from deuterium

Quasi-free photoproduction of eta-mesons from the deuteron has been measured at the tagged photon facility of the Mainz microtron MAMI with the photon spectrometer TAPS for incident photon energies from the production threshold at 630 MeV up to 820 MeV. In a fully exclusive measurement eta-mesons and recoil nucleons were detected in coincidence. At incident photon energies above the production threshold on the free nucleon, where final state interaction effects are negligible, an almost constant ratio of sigma(n)/sigma(p)=0.66+/-0.10 was found. At lower incident photon energies the ratio rises due to re-scattering effects. The average ratio agrees with the value extracted from a comparison of the inclusive d(gamma,eta)X cross section to the free proton cross section via a participant - spectator model for the deuteron cross section (sigma(n)/sigma(p)=0.67+/-0.07). The angular dependence of the ratio agrees with the expected small deviation of neutron and proton cross section from isotropy due to the influence of the D13(1520) resonance. The energy dependence of the ratio in the excitation energy range of the S11(1535) resonance disfavors model predictions which try to explain this resonance as K-Sigma bound state.

nucl-ex↗

arXiv2002

Evidence of kaon nuclear and Coulomb potential effects on soft K+ production from nuclei

The ratio of forward K+ production on copper, silver and gold targets to that on carbon has been measured at proton beam energies between 1.5 and 2.3 GeV as a function of the kaon momentum p_K using the ANKE spectrometer at COSY-Juelich. The strong suppression in the ratios observed for p_K<200-250 MeV/c can be ascribed to a combination of Coulomb and nuclear repulsion in the K+A system. This opens a new way to investigate the interaction of K+-mesons in the nuclear medium. Our data are consistent with a K+A nuclear potential of V_K~20 MeV at low kaon momenta and normal nuclear density. Given the sensitivity of the data to the kaon potential, the current experimental precision might allow one to determine V_K to better than 3 MeV.

nucl-ex↗

arXiv2001

Test of low-energy theorems for p(\vec{gamma},π^{0})p in the threshold region

The photon asymmetry in the reaction p(\vecγ,π^{0})p close to threshold has been measured for the first time with the photon spectrometer TAPS using linearly polarized photons from the tagged-photon facility at the Mainz Microtron MAMI. The total and differential cross sections were also measured simultaneously with the photon asymmetry. This allowed determination of the S-wave and all three P-wave amplitudes. The low-energy theorems based on the parameter-free third-order calculations of heavy-baryon chiral perturbation theory for P1 and P2 agree with the experimental values.

nucl-ex↗

arXiv2001

Medium effects in the production and pi0 gamma decay of omega-mesons in pA collisions in the GeV region

The omega-resonance production and its pi0 gamma decay in p-A reactions close to threshold is considered within the Intranuclear Cascade (INC) Model. The pi0 gamma invariant mass distribution shows two components which correspond to the omega decay 'inside' and 'outside' the nucleus, respectively. The 'inside' component is distorted by medium effects, which introduce a mass shift as well as collisional broadening for the omega-meson and its decaying pion. The relative contribution of the 'inside' component is analyzed in detail for different kinematical conditions and nuclear targets. It is demonstrated that a measurement of the correlation in azimuthal angle between the pi0 and gamma momenta allows to separate events related to the 'inside' omega decay from different sources of background when uncorrelated pi0's and gamma's are produced.

nucl-th↗

arXiv2000

Studying the $ω$ properties in pA collisions via the $ω{\to}π^0γ$ decay

Within transport calculations we study the production and decay of $ω$-mesons in $pA$ reactions at COSY energies including elastic and inelastic $ωN$ rescattering, the $ω{\to}π^0γ$ Dalitz decay as well as $π^0 N$ rescattering. The resulting invariant $π^0 γ$ mass distributions indicate that in-medium modifications of the $ω$-meson may be observed experimentally.

nucl-th↗

arXiv1999

Elliptic flow of $η$ and $π^{0}$ Mesons in Heavy-Ion Collisions at 2 AGeV

Azimuthal distributions of $η$ and $π^{0}$ mesons emitted at midrapidity in collisions of 1.9 AGeV $^{58}$Ni+$^{58}$Ni and 2 AGeV $^{40}$Ca+$^{nat}$Ca are studied as a function of the number of projectile-like spectator nucleons. The observed anisotropy corresponds to a negative elliptic flow signal for $η$ mesons, indicating a preferred emission perpendicular to the reaction plane. In contrast, only small azimuthal anisotropies are observed for $π^{0}$ mesons. This may indicate that $η$ mesons freeze out earlier from the fire ball than pions.

nucl-ex↗

arXiv1999

Coherent π^\circ photoproduction from ^4He

Differential cross sections and beam asymmetries for coherent π^\circ photoproduction from ^4He in the Δenergy-range have been measured with high statistical and systematic precisions using both decay photons for identifying the process.The experiment was performed at the MAinz MIcrotron using the TAPS photon spectrometer and the Glasgow/Mainz tagged photon facility. The differential cross sections are in excellent agreement with predictions based on the DWIA if an appropriate parametrization of the Δ-nuclear interaction is applied. The beam asymmetries are interpreted in terms of degrees of linear polarization of collimated coherent bremsstrahlung. The expected increase of the degree of linear polarization with decreasing collimation angle is confirmed. Agreement with calculations is obtained on a few-percent level of precision in the maxima of the coherent peaks.

nucl-ex↗

arXiv1999

Azimuthal Anisotropy of $η$ and $π^{0}$ Mesons in Heavy-Ion Collisions at 2 AGeV

Azimuthal distributions of $η$ and $π^{0}$ mesons emitted at midrapidity in collisions of 1.9 AGeV $^{58}$Ni+$^{58}$Ni and 2 AGeV $^{40}$Ca+$^{nat}$Ca are studied as a function of the number of projectile-like spectator nucleons. The observed anisotropy corresponds to a negative elliptic flow signal for $η$ mesons, indicating a preferred emission perpendicular to the reaction plane. The effect is smallest in peripheral Ni+Ni collisions. In contrast, for $π^{0}$ mesons, elliptic flow is observed only in peripheral Ni+Ni collisions, changing from positive to negative sign with increasing pion transverse momentum.

nucl-ex↗