Cooler Synchrotron Research Articles

Invariant-mass distributions for the pp $ \rightarrow$ pp$ \eta$ reaction at Q = 10 MeV

Proton-proton and proton-eta invariant mass distributions and the total cross section for the pp to pp eta reaction have been determined near the threshold at an excess energy of Q=10 MeV. The experiment has been conducted using the COSY-11 detector setup and the cooler synchrotron COSY. The determined invariant mass spectra reveal significant enhancements in the region of low proton-proton relative momenta, similarly as observed previously at higher excess energies of Q=15.5 MeV and Q= 40MeV.

Measurement of the isospin-filtering dd $ \rightarrow$ 4HeK+K- reaction at Q = 39 MeV

The total cross-section for the dd $ \rightarrow$ 4HeK + K - reaction has been measured at a beam momentum of 3.7GeV/c, corresponding to an excess energy of 39MeV, which is the maximum possible at the Cooler Synchrotron COSY-Julich. A deuterium cluster-jet target and the ANKE forward magnetic spectrometer, placed inside the storage ring, have been employed in this investigation. We find a total cross-section of $ \sigma_{{{\rm tot}}}^{}$ < 14 pb, which brings into question the viability of investigating the dd $ \rightarrow$ 4Hea 0(980) reaction as a means of studying isospin violation.

Precision measurements of thepp→π+pnandpp→π+dreactions: Importance of long-range and tensor force effects

Inclusive measurements of pion production in proton-proton collisions in the forward direction were undertaken at 400 and 600 MeV at the cooler synchrotron (COSY) at Forschungszentrum Julich using the Big Karl spectrograph. The high resolution in the pi(+) momentum ensured that there was an unambiguous separation of the pp ->pi(+)d/pi(+)pn channels. Using these and earlier data, the ratio of the production cross sections could be followed through the Delta region and compared with the predictions of final state interaction theory. Deviations are strongly influenced by long-range terms in the production operator and the tensor force in the final pn system. These have been investigated in a realistic pp ->pi(+)d/pi(+)pn calculation that includes S reversible arrow D channel coupling between the final nucleons. A semiquantitative understanding of the observed effects is achieved. .

ETA-NUCLEUS INTERACTION STUDIES: EXPERIMENTAL EFFORTS

The idea that the η meson could form a bound system with nucleus was put forward long ago by Q.Haider and L.C.Liu and was based on the observation that the low energy η-nucleon interaction was attractive. Since then a number of theoretical calculations have been performed to the predict eta-nucleus bound states. Such bound states-the so called "eta-mesic nuclei" - are predicted to have finite widths and will eventually decay. The experimental confirmation of such a quasi-bound system would be a very useful tool for investigation of interaction between a short lived eta-meson and the nucleus. Since the eta-mesons can not be produced as a secondary beam, the eta-meson particles are available for investigations only as products of certain nuclear reactions where they appear as final state particles. The final state interaction(FSI) effects are the only source of information about the eta-meson interaction with the nucleus. Experiments were performed at the cooler synchrotron accelerator, COSY, Juelich using dedicated detection systems to study the eta-nucleus interaction. The present talk summarizes the status of previous experimental searches. Some preliminary results of the experiments performed at COSY on the eta-nucleus FSI studies are also presented here. The details of the experiments done at COSY, Juelich on eta-mesic nuclei searches along with recent results are reported in an another communication in this proceedings.

SOME EXPERIMENTAL TECHNIQUES AT STORAGE RINGS

Two experimental techniques that have been developed at the COoler SYnchrotron COSY-Jülich are presented: (i) The energy of a stored polarized proton or deuteron beam can be precisely determined by sweeping an rf magnetic dipole or solenoid field over a spin resonance. This perturbation induces a beam depolarization, which is maximal at the spin resonance's frequency. That frequency, together with the beam revolution frequency, determines the beam's kinematic γ factor, which can thus be measured with high accuracy. Therefore, the beam energy can be determined about one order of magnitude more precisely than with conventional methods based on orbit length measurements. The technique has been used at COSY for an experiment aiming at a high-precision measurement the mass of the η meson. (ii) The repeated passage of a coasting ion beam through a thin internal target leads to a beam-energy loss and a shift of its revolution frequency. This shift is proportional to the beam-target overlap and thus allows one to measure the target thickness and hence the luminosity during the corresponding experiment. This effect has been studied quantitatively with a 2.65 GeV proton beam impinging on a hydrogen cluster-jet target at the ANKE spectrometer. After a careful error evaluation the luminosity, could be determined with an accuracy of better than 5%.

HADRON PHYSICS AT COSY

One major focus of the physics studied at the COoler SYnchrotron COSY of the Forschungszentrum Jülich is the production of mesons and hyperons in the hadron-hadron scattering with the aim to investigate relevant production processes, interactions of the participating particles as well as symmetries and symmetry breaking. Due to the excellent experimental conditions at COSY also single- and double-polarization measurements can be performed with high reaction rates. This contribution gives an overview about the physics program which is currently under investigation at the three big COSY installations ANKE, COSY-TOF and WASA-at-COSY.

HADRON PHYSICS AT COSY

One major focus of the physics studied at the COoler SYnchrotron COSY of the Forschungszentrum Jülich is the production of mesons and hyperons in the hadron-hadron scattering with the aim to investigate relevant production processes, interactions of the participating particles as well as symmetries and symmetry breaking. Due to the excellent experimental conditions at COSY also single- and double-polarization measurements can be performed with high reaction rates. This contribution gives an overview about the physics program which is currently under investigation at the three big COSY installations ANKE, COSY-TOF and WASA-at-COSY.

Measurement of thepn→dK+K−total cross section close to threshold

Measurements of the pd{yields}p{sub sp}dK{sup +}K{sup -} reaction, where p{sub sp} is a spectator proton, have been undertaken at the Cooler Synchrotron COSY-Juelich by detecting a fast deuteron in coincidence with a K{sup +}K{sup -} pair in the ANKE facility. Although the proton beam energy was fixed, the moving target neutron allowed values of the nonresonant quasifree pn{yields}dK{sup +}K{sup -} total cross section to be deduced up to an excess energy {epsilon}{approx_equal}100 MeV. Evidence is found for the effects of K{sup -}d and KK final state interactions. The comparison of these data with those of pp{yields}ppK{sup +}K{sup -} and pp{yields}dK{sup +}K{sup 0} shows that all the total cross sections are very similar in magnitude.

Near-threshold production of theηmeson via the quasifreepn→pnηreaction

Total cross sections for the quasifree pn{yields}pn{eta} reaction in the range from the kinematical threshold up to 20 MeV excess energy have been determined. At threshold they exceed corresponding cross sections for the pp{yields}pp{eta} reaction by a factor of about 3 in contrast to the factor of 6 established for higher excess energies. To large extent, the observed decrease of the ratio {sigma}(pn{yields}pn{eta})/{sigma}(pp{yields}pp{eta}) toward threshold may be assigned to the different energy dependence of the proton-proton and proton-neutron final-state interactions. The experiment was conducted using a proton beam of the cooler synchrotron COSY and a cluster jet deuteron target. The proton-neutron reactions were tagged by the spectator proton whose momentum was measured for each event. Protons and neutron outgoing from the pn{yields}pn{eta} reaction have been registered by means of the COSY-11 facility, an apparatus dedicated for threshold meson production.

Neutron experiments with cryogenic methane hydrate and mesitylene moderators

In this article we describe the experimental results of a methane hydrate moderator as well as as mesitylene moderator operated at a temperature around 20K at the JESSICA (Julich Experimental Spallation Target Set-up In COSY Area) experiment at the Julich cooler synchrotron COSY. For the first time the cold neutron spectrum of a methane hydrate moderator was experimentally investigated. A comparison with a solid methane and an ice moderator atT = 20 K will be shown. MCNPX simulations with new developedS(α,β) scattering kernels will be compared with experimental data. The applicability of mesitylene and methane hydrate as cold moderators at spallation neutron sources will be discussed.

Study of the hadronic production of kaon pairs below the threshold for the ϕ meson

The near threshold production of K + K − pairs in proton-proton collisions has been investigated at the cooler synchrotron COSY below and above the threshold for the ϕ meson using the COSY-11 and ANKE facilities, respectively. The excitation function determined for the p p → p p K + K − reaction revealed a statistically significant enhancement close to the threshold which may plausibly be assigned to the influence of the K − p interaction. In addition, observed consistently by both groups, a strong enhancement at low values of the ratio of the K − p to K + p invariant mass distributions shows that the proton interacts much stronger with K − than with K + . In this report we focus on the measurements performed by the COSY-11 collaboration. We explain the experimental method used and present main results of completed analyses as well as a new qualitative elaboration of the p p K + K − events population on the Goldhaber plot. We conclude with the observation that event densities increase at the region where the influence from the K + K − interaction is expected.

Experimental Verification of Predicted Beam-Polarization Oscillations near a Spin Resonance

The Chao matrix formalism allows analytic calculations of a beam's polarization behavior inside a spin resonance. We recently tested its prediction of polarization oscillations occurring in a stored beam of polarized particles near a spin resonance. Using a 1.85 GeV/c polarized deuteron beam stored in the COoler SYnchrotron, we swept a new rf solenoid's frequency rather rapidly through 400 Hz during 100 ms, while varying the distance between the sweep's end frequency and the central frequency of an rf-induced spin resonance. Our measurements of the deuteron's polarization near and inside the resonance agree with the Chao formalism's predicted oscillations.

Performance Issues of the New DAQ System for WASA at COSY

For the operation of the WASA (Wide Angle Shower Apparatus) detector system at the Cooler Synchrotron COSY in Forschungszentrum Julich (FZJ) a new DAQ system has been developed, which conforms to the third generation of the DAQ systems at COSY. This comprises new readout electronics based on an optimized parallel bus with LVDS technology and FPGA-controlled event and buffer management. The new DAQ system has been successfully commissioned in autumn 2006. Further developments aiming at performance optimizations are in progress. The implementation status is presented and performance issues are discussed.

Updated analysis ofNNelastic scattering to 3 GeV

A partial-wave analysis of $NN$ elastic scattering data has been updated to include a number of recent measurements. Experiments carried out at the Cooler Synchrotron (COSY) by the EDDA Collaboration have had a significant impact above 1 GeV. Results are discussed in terms of the partial-wave and direct-reconstruction amplitudes.

A large acceptance scintillator detector with wavelength shifting fibre readout for search of [formula omitted]-nucleus bound states

A large acceptance scintillator detector with wavelength shifting optical fibre readout has been designed and built to detect the decay particles of η -nucleus bound system (the so-called η -mesic nuclei), namely, protons and pions. The detector, named as ENSTAR detector, consists of 122 pieces of plastic scintillator of various shapes and sizes, which are arranged in a cylindrical geometry to provide particle identification, energy loss and coarse position information for these particles. A solid angle coverage of ∼ 95 % of total 4 π is obtained in the present design of the detector. Monte Carlo phase space calculations performed to simulate the formation and decay of η -mesic nuclei suggest that its decay particles, the protons and pions are emitted with an opening angle of 150 ± 20 ∘ , and with energies in the range of 25–300 and 225–450 MeV, respectively. The detailed GEANT simulations show that ∼ 80 % of the decay particles (protons and pions) can be detected within ENSTAR. Several test measurements using alpha source, cosmic-ray muons, etc. have been carried out to study the response of ENSTAR scintillator pieces. The in-beam tests of fully assembled detector with proton beam of momentum 870 MeV / c from the Cooler synchrotron COSY have been performed. The test results show that the scintillator fibre design chosen for the detector has performed satisfactorily well. The present article describes the detector design, simulation studies, construction details and test results.

THE NEAR-THRESHOLD PRODUCTION OF ϕ-MESONS IN PROTON–NUCLEON COLLISIONS

The pp → ppϕ and quasi–free pn → dϕ reactions have been studied at the Cooler Synchrotron COSY–Jülich, using the internal beam and the ANKE facility. Total cross sections in the pp entrance channel have been determined at three excess energies ∊ in the range of 18.5–75.9 MeV. In case of the pn entrance channel the energy dependence of the cross section up to 80 MeV has been extracted by exploiting the intrinsic momentum of the neutron inside a deuterium target. Taken together with data for ω–production, a significant enhancement of the ϕ/ω ratio for both entrance channels of a factor 8 is found compared to predictions based on the Okubo–Zweig–Iizuka rule.

η AND η′ MESONS PRODUCTION AT COSY-11

The low emittance and small momentum spread of the proton and deuteron beams of the Cooler Synchrotron COSY combined with the high mass resolution of the COSY-11 detection system permit to study the creation of mesons in the nucleon-nucleon interaction down to the fraction of MeV with respect to the kinematical threshold. At such small excess energies, the ejectiles possess low relative momenta and are predominantly produced with the relative angular momentum equal to zero. Taking advantage of these conditions we have performed investigations aiming to determine the mechanism of the production of η and η′ mesons in the collision of hadrons as well as the hadronic interaction of these mesons with nucleons and nuclei. In this proceedings we address the ongoing studies of the spin and isospin dependence for the production of the η and η′ mesons in free and quasi-free nucleon-nucleon collisions. New results on the spin observables for the [Formula: see text] reaction, combined with the previously determined total cross section isospin dependence, reveal a statistically significant indication that the excitation of the nucleon to the S11(1535) resonance, the process which intermediates the production of the η meson in the nucleon-nuleon interactions, is predominantly due to the exchange of the π meson between the colliding nucleons.

STATUS OF WASA AT COSY

The WASA detector built and operated at the storage ring CELSIUS in Uppsala, was during 2005 moved to the COoler SYnchrotron COSY in Jülich where it is now being reassembled. The main physics concerns decays of η and η′ mesons and isospin violation in deuteron-deuteron interactions. Several modifications and upgrades are planned to adapt the WASA detector to the COSY environment and beam energy. The first commissioning run is scheduled to start August 2006. Here we report on the status of the installation work and upgrades planned for the WASA detector at COSY.

Precision Measurement of the Quasifreepn→dϕReaction Close to Threshold

The quasifree $pn\ensuremath{\rightarrow}d\ensuremath{\phi}$ reaction has been studied at the Cooler Synchrotron COSY---J\"ulich, using the internal proton beam incident on a deuterium cluster-jet target and detecting a fast deuteron in coincidence with the ${K}^{+}{K}^{\ensuremath{-}}$ decay of the $\ensuremath{\phi}$ meson. The energy dependence of the total and differential cross sections are extracted for excess energies up to 80 MeV by determining the Fermi momentum of the target neutron on an event-by-event basis. Though these cross sections are consistent with $s$-wave production, the kaon angular distributions show the presence of $p$ waves at quite a low energy. Production on the neutron is found to be stronger than on the proton but not by as much as for the $\ensuremath{\eta}$ meson.

Near-Threshold Production ofϕMesons inppCollisions

The pp->pp phi reaction has been studied at the Cooler Synchrotron COSY-Juelich, using the internal beam and ANKE facility. Total cross sections have been determined at three excess energies epsilon near the production threshold. The differential cross section closest to threshold at epsilon=18.5 MeV exhibits a clear S-wave dominance as well as a noticeable effect due to the proton-proton final state interaction. Taken together with data for pp omega-production, a significant enhancement of the phi/omega ratio of a factor 8 is found compared to predictions based on the Okubo-Zweig-Iizuka rule.