Search

New high precision total and differential cross sections are reported for the $dp\to {}^3\textrm{He}\,\eta$ reaction close to threshold. The measurements were performed using the magnetic spectrometer ANKE, which is an internal fixed target facility at the COSY cooler synchrotron. The data were taken for deuteron beam momenta between $3.14641~\textrm{GeV}/c$ and $3.20416~\textrm{GeV}/c$, which corresponds to the range in excess energy $Q$ for this reaction between $1.14~\textrm{MeV}$ and $15.01~\textrm{MeV}$. The normalization was established through the measurement in parallel of deuteron-proton elastic scattering and this was checked through the study of the $dp\to {}^3\textrm{He}\,\pi^0$ reaction. The previously indicated possible change of sign of the slope of the differential cross sections near the production threshold, which could be explained by a rapid variation of the $s$- and $p$-wave interference term, is not confirmed by the new data. The energy dependence of the total cross section and the $90^{\circ}$ slope parameter are well explained by describing the final state interaction in terms of a complex Jost function and the results are significant in the discussion of $\eta$-mesic nuclei. In combination with recently published WASA-at-COSY data [P. Adlarson $et\, al.$, Phys. Lett. B 782, 297 (2018)], a smooth variation of the slope parameter is achieved up to an excess energy of $80.9~\textrm{MeV}$.

The vector and tensor analysing powers in deuteron-proton elastic scattering have been measured in the forward hemisphere at deuteron kinetic energies of 1.2 GeV and 2.27 GeV using the ANKE spectrometer at the COSY storage ring. The results are compared with other experimental data and with predictions made within the framework of Glauber multiple scattering theory., Comment: 11 pages with 8 figures. To be submitted for publication

A previous publication has shown that the in-plane polarization (IPP) component of a polarized 0.97-GeV/c deuteron beam in the COSY storage ring may acquire a polarization half-life in excess of 1000 s through a combination of beam bunching, electron cooling (prior to any spin manipulation), sextupole field adjustment, and a limitation of the beam intensity. This paper documents further tests pointing to additional gains in the IPP lifetime if cooling is active throughout the beam store., 11 pages, 6 figures Submitted to Physical Review Accelerators and Beams

The motivation of the current R&D project is based upon the requirements of the JEDI international collaboration 1 aiming to measure Electric Dipole Moments (EDMs) of charged particles in storage rings. One of the most important elements of such an experiment will be a specially designed polarimeter with the detection system based on a modular inorganic scintillator (LYSO crystal) calorimeter. The calorimeter modules are read out by Silicon Photo Multipliers (SiPMs). This paper describes the development of a multi-channel power supply for the polarimeter modules, providing very stable and clean bias voltages for SiPMs. In order to ensure the best possible performance of SiPMs in conjunction with the crystal-based calorimeter modules and to guarantee the required level of calorimeter stability, several quality requirements have to be met by the power supply. Additionally, it is required to provide features including remote control via the network, ramping of the output voltage, measuring and sending the information about its output voltages and currents, etc. The obtained results demonstrate that the goals for the JEDI polarimeter are met. The developed hardware will be useful in other fields of fundamental and applied research, medical diagnostic techniques and industry, where SiPMs are used.

A low-energy spin-physics program is being developed at the COSY storage ring. To support the planned experimental activities, several experimental tools for polarized beams and targets have been developed. The paper presents the physics case and the readiness of the setup for its realization. QC 20190905. QC 20191203Although the hadron physics program has been officially terminated, various experimental activities are underway at the COSY storage ring of Forschungszentrum Jülich. Besides the feasibility studies related to the search for electric dipole moments [2], low-energy spin-physics experiments employing polarized beam and targets appear as an additional focus. This paper is devoted to these studies.

New precise unpolarised differential cross sections of deuteron–proton elastic scattering have been measured at 16 different deuteron beam momenta between pd=3120.17MeV/c and pd=3204.16MeV/c at the COoler SYnchrotron COSY of the Forschungszentrum Jülich. The data, which were taken using the magnetic spectrometer ANKE, cover the equivalent range in proton kinetic energies from Tp=882.2MeV to Tp=918.3MeV. The experimental results are analysed theoretically using the Glauber diffraction model with accurate nucleon–nucleon input. The theoretical cross section at Tp=900MeV agrees very well with the experimental one at low momentum transfers t. There are, however, significant deviations for |t|>0.2(GeV/c)2 that must be investigated further.

The reaction $p + d \rightarrow p + d + X$ was studied at 0.8-2.0 GeV proton beam energies with the ANKE magnetic spectrometer at the COSY synchrotron storage ring. The proton-deuteron pairs emerging with high momenta, 0.6-1.8 GeV/$c$, were detected at small angles with respect to the proton beam. Distribution over the reaction missing mass $M_x$ reveals a local enhancement near the threshold of the pion pair production specific for the so-called ABC effect. The enhancement has a structure of a narrow bump placed above a smooth continuum. The invariant mass of the $d\pi\pi$ system in this enhancement region exhibits a resonance-like peak at $M_{d\pi\pi} \approx 2.36$ GeV/$c^2$ with the width $\Gamma \approx 0.10$ GeV/$c^2$. A possible interpretation of these features is discussed., Comment: 14 pages, 16 figures, submitted to Eur. Phys. J. A. v2: Added references [42,43] in section IV.A. v3: revised version according to referee remarks v4: revised version according to referee remarks

Physical review accelerators and beams 21(4), 042002 (2018). doi:10.1103/PhysRevAccelBeams.21.042002, Published by American Physical Society, College Park, MD

This letter reports the successful use of feedback from a spin polarization measurement to the revolution frequency of a 0.97 GeV/$c$ bunched and polarized deuteron beam in the Cooler Synchrotron (COSY) storage ring in order to control both the precession rate ($\approx 121$ kHz) and the phase of the horizontal polarization component. Real time synchronization with a radio frequency (rf) solenoid made possible the rotation of the polarization out of the horizontal plane, yielding a demonstration of the feedback method to manipulate the polarization. In particular, the rotation rate shows a sinusoidal function of the horizontal polarization phase (relative to the rf solenoid), which was controlled to within a one standard deviation range of $\sigma = 0.21$ rad. The minimum possible adjustment was 3.7 mHz out of a revolution frequency of 753 kHz, which changes the precession rate by 26 mrad/s. Such a capability meets a requirement for the use of storage rings to look for an intrinsic electric dipole moment of charged particles.

In a previous publication, measurements of the depolarization of a stored proton beam by interaction with a co-propagating unpolarized electron beam at low relative energy have been presented and an upper limit of about 3 x 10(7) b for the electron-proton spin flip cross-section was determined. A refined analysis presented in this paper reduces the previous upper limit by a factor of three by the introduction of a new procedure that, also makes use of non-identified particles. QC 20140806

The deuteron tensor analysing power t_{20} of the d(pol) p --> 3He eta reaction has been measured at the COSY-ANKE facility in small steps in excess energy Q up to Q = 11 MeV. Despite the square of the production amplitude varying by over a factor of five through this range, t_{20} shows little or no energy dependence. This is evidence that the final state interaction causing the energy variation is not influenced by the spin configuration in the entrance channel. The weak angular dependence observed for t_{20} provides useful insight into the amplitude structure near threshold.

The transverse spin correlations A x , x and A y , y in the n → p → → d π 0 reaction have been measured for the first time in quasi-free kinematics at the COSY-ANKE facility using a polarised deuteron beam incident on a polarised hydrogen cell target. The results obtained for neutron energies close to 353 MeV and 600 MeV are in good agreement with the partial wave analysis of data on the isospin-related p p → d π + reaction, though the present results cover kinematic regions that are largely absent from existing data sets.

The charge-exchange break-up of polarised deuterons d → p → { p p } s n , where the final { p p } s diproton system has a very low excitation energy and hence is mainly in the S 0 1 state, is a powerful tool to probe the spin-flip terms in the proton–neutron charge-exchange scattering. Recent measurements with the ANKE spectrometer at the COSY storage ring at 1.6, 1.8, and 2.27 GeV have extended these studies into the pion-production regime in order to investigate the mechanism for the excitation of the Δ ( 1232 ) isobar in the d → p → { p p } s X reaction. Values of the differential cross section and two deuteron tensor analysing powers, A x x and A y y , have been extracted as functions of the momentum transfer to the diproton or the invariant mass M X of the unobserved system X. The unpolarised cross section in the high M X region is well described in a model that includes only direct excitation of the Δ isobar through undistorted one pion exchange. However, the cross section is grossly underestimated for low M X , even when Δ excitation in the projectile deuteron is included in the calculation. Furthermore, direct Δ production through one pion exchange only reproduces the angular dependence of the difference between the two tensor analysing powers.

In this paper, we demonstrate the connection between a magnetic storage ring with additional sextupole fields set so that the x and y chromaticities vanish and the maximizing of the lifetime of in-plane polarization (IPP) for a 0.97-GeV/c deuteron beam. The IPP magnitude was measured by continuously monitoring the down-up scattering asymmetry (sensitive to sideways polarization) in an in-beam, carbon-target polarimeter and unfolding the precession of the IPP due to the magnetic anomaly of the deuteron. The optimum operating conditions for a long IPP lifetime were made by scanning the field of the storage ring sextupole magnet families while observing the rate of IPP loss during storage of the beam. The beam was bunched and electron cooled. The IPP losses appear to arise from the change of the orbit circumference, and consequently the particle speed and spin tune, due to the transverse betatron oscillations of individual particles in the beam. The effects of these changes are canceled by an appropriate sextupole field setting., Comment: 18 pages, 18 figures, prepared for Phys. Rev. Accel. Beams

We report on measurements of the differential cross section $d\ensuremath{\sigma}/d\mathrm{\ensuremath{\Omega}}$ and the first measurement of the analyzing power ${A}_{y}$ in the $\mathrm{\ensuremath{\Delta}}(1232)$ excitation energy region of the reaction $pp\ensuremath{\rightarrow}{{pp}}_{s}{\ensuremath{\pi}}^{0}$ where ${{pp}}_{s}$ is a $^{1}S_{0}$ proton pair. The experiment has been performed with the ANKE spectrometer at COSY-J\"ulich. The data reveal a peak in the energy dependence of the forward ${{pp}}_{s}$ differential cross section, a minimum at zero degrees of its angular distribution, and a large analyzing power. The results present a direct manifestation of two two-baryon resonance-like states with ${J}^{P}={2}^{\ensuremath{-}}$ and ${0}^{\ensuremath{-}}$ and an invariant mass of $2.2\phantom{\rule{0.16em}{0ex}}\mathrm{GeV}/{c}^{2}$.

The PAX Collaboration has successfully performed a spin-filtering experiment with protons at the COSY-ring. The measurement allowed the determination of the spin-dependent polarizing cross section, that compares well with the theoretical prediction from the nucleon-nucleon potential. The test confirms that spin-filtering can be adopted as a method to polarize a stored beam and that the present interpretation of the mechanism in terms of the proton-proton interaction is correct. The outcome of the experiment is of utmost importance in view of the possible application of the method to polarize a beam of stored antiprotons. (C) 2012 Elsevier B.V. All rights reserved.

The differential cross section d σ / d Ω and analysing power A y of the p → p → { p p } s π 0 reaction have been measured at COSY-ANKE at 353 MeV. The final proton pair was detected at very low excitation energy, leading to an S -wave diproton, denoted here as { p p } s . The angular dependence of both d σ / d Ω and A y can be described in terms of s - and d -wave pion production. By using phase information from elastic pp scattering, unique solutions are derived for the corresponding amplitudes only as a result of the combined analysis of both experimental observables. The large d -wave term thus obtained is important for the tests of the current state-of-the-art chiral perturbation theory approach. The amplitudes will also help towards determining the isospin dependence of pion production in nucleon–nucleon collisions with diproton formation.

In order to test chiral perturbation theory and establish links between p-wave pion production in nucleon–nucleon collisions and low energy three-nucleon scattering, an extensive programme of experiments on pion production is currently underway at COSY-ANKE. The final proton pair is measured at very low excitation energy, leading to an S-wave diproton, denoted here as {pp}s. By using a deuterium target we have obtained data on the differential cross section and analysing power of the quasi-free p→n→{pp}sπ− reaction at 353 MeV. The spectator proton psp was either measured directly in silicon tracking telescopes or reconstructed using the momentum of a detected π−. Both observables can be described in terms of s-, p-, and d-wave pion production amplitudes. Taken together with the analogous data on the p→p→{pp}sπ0 reaction, full partial wave decompositions of both processes were carried out.

COSY has a history of highly successful operation of cooled beams and internal targets both with and without polarization and as such it constitutes a unique facility for hadronic probes on a world-wide scale. Starting from meson production and nucleon-nucleon scattering more and more experience in polarized beam/target manipulation and polarimetry is gained. Extending polarization physics to antiprotons requires pre-studies at COSY to be continued at the antiproton decelerator AD at CERN. As a long-term perspective, the search for electric dipole moments of the proton, deuteron, and 3He in a dedicated storage ring could provide a new method to push the limits in the quest for CP violation searches beyond the Standard Model.

The differential cross section and deuteron analysing powers of the $$ \vec d $$ p → {pp}n charge-exchange reaction have been measured with the ANKE spectrometer at the COSY storage ring. Using a deuteron beam of energy 1170MeV, data were obtained for small momentum transfers to a {pp} system with low excitation energy. A good quantitative understanding of all the measured observables is provided by the impulse approximation using known neutron-proton amplitudes. The proof of principle achieved here for the method suggests that measurements at higher energies will provide useful information in regions where the existing np database is far less reliable.

A first storage cell for the Polarized Internal gas Target (PIT) of the ANKE facility at COSY was produced from a Teflon-coated pure Aluminum foil and successfully used in the double polarized experiment with a 1.2 GeV polarized deuteron beam carried out at the COSY accelerator in January 2007. Prior to this experiment, the minimum possible cross section of the storage-cell beam tube at the interaction point at ANKE was determined, based on the COSY beam profile at different beam formation stages. Electron cooling combined with stacking and stochastic cooling have been studied as well. Experiments with N2 gas in the storage cell to simulate the background produced by beam interaction with the aluminum cell walls were performed. The analysis of the [Formula: see text] and [Formula: see text] reactions showed that events from the extended target can be clearly identified in the ANKE detector system. Taking into account the known intensity of the Atomic Beam Source (ABS), the expected target density in the experiment with a storage-cell beam tube of 15hor × 20vert × 390long mm3 and a 130 mm long feeding tube with 15 mm diameter reach values of 3.2 × 1013 at/cm2. The polarization, measured by the np → dπ0 reaction with the storage-cell, was 0.75 ± 0.06.

The p p → { p p } s π 0 differential cross section has been measured with the ANKE spectrometer at COSY-Julich for seven proton beam energies T p between 0.51 and 1.97 GeV. By selecting proton pairs with an excitation energy of less than 3 MeV it is ensured that the final { p p } s system is in the S 0 1 state. In the measured region of θ p p cm ≲ 18 ° , the data reveal a forward dip for T p ⩽ 1.4 GeV whereas a forward peaking is seen at 1.97 GeV. The energy dependence of the forward cross section shows a broad peak in the 0.6–0.8 GeV region, probably associated with Δ ( 1232 ) excitation, and a minimum at 1.4 GeV. Some of these features are similar to those observed for the spin–isospin partner reaction, p p → d π + . However, the ratio of the forward differential cross sections of the two reactions shows a significant suppression of single pion production associated with a spin-singlet final nucleon pair.

The pp ! pK + Y 0 reaction has been studied for hyperon masses m(Y 0 ) � 1540MeV/c 2 at COSY-Julich by using a 3.65GeV/c circulating proton beam incident on an in- ternal hydrogen target. Final states comprising two protons, one positively charged kaon and one negatively charged pion have been identified with the ANKE spectrom- eter. Such configurations are sensitive to the production of the ground stateand � 0 hyperons as well as the � 0 (1385) and �(1405) resonances. Applying invariant- and missing-mass techniques, the two overlapping excited states could be well sep- arated, though with limited statistics. The shape and position of the �(1405) distri- bution, reconstructed cleanly in the � 0 � 0 channel, are similar to those found from other decay modes and there is no obvious mass shift. This finding constitutes a challenging test for models that predict �(1405) to be a two-state resonance.

The observed variation of the total cross section for the dp -> He-3 eta reaction near threshold means that the magnitude of the s-wave amplitude falls very rapidly with the eta centre-of-mass momentum. It is shown here that recent measurements of the momentum dependence of the angular distribution imply a strong variation also in the phase of this amplitude. Such a behaviour is that expected from a quasi-bound or virtual eta He-3 state. The interpretation can be investigated further through measurements of the deuteron or proton analysing powers and/or spin-correlations. (C) 2007 Elsevier B.V. All rights reserved.

Inclusive K + production in proton–proton collisions has been measured at a beam energy of 2.16 GeV using the COSY-ANKE magnetic spectrometer. The resulting spectrum, as well as those corresponding to K + p and K + π + correlated pairs, can all be well described using consistent values of the total cross sections for the p p → K + p Λ , p p → K + p Σ 0 , and p p → K + n Σ + reactions. While the resulting values for Λ and Σ 0 production are in good agreement with world data, our value for the total Σ + production cross section, σ ( p p → K + n Σ + ) = ( 2.5 ± 0.6 stat ± 0.4 syst ) μb at an excess energy of e = 129 MeV , could only be reconciled with other recently published data if there were a highly unusual near threshold behaviour.

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.

The charge exchange of vector polarised deuterons on a polarised hydrogen target has been studied in a high statistics experiment at the COSY-ANKE facility at a deuteron beam energy of T d = 726 MeV . By selecting two fast protons at low relative energy E p p , the measured analysing powers and spin correlations are sensitive to interference terms between specific neutron–proton charge-exchange amplitudes at a neutron kinetic energy of T n ≈ 1 2 T d = 363 MeV . An impulse approximation calculation, which takes into account corrections due to the angular distribution in the diproton, describes reasonably the dependence of the data on both E p p and the momentum transfer. This lends broad support to the current neutron–proton partial wave solution that was used in the estimation.

A new method to determine the spin tune is described and tested. In an ideal planar magnetic ring, the spin tune - defined as the number of spin precessions per turn - is given by $\nu_s = \gamma G$ (gamma is the Lorentz factor, $G$ the magnetic anomaly). For 970 MeV/c deuterons coherently precessing with a frequency of ~120 kHz in the Cooler Synchrotron COSY, the spin tune is deduced from the up-down asymmetry of deuteron carbon scattering. In a time interval of 2.6 s, the spin tune was determined with a precision of the order $10^{-8}$, and to $1 \cdot 10^{-10}$ for a continuous 100 s accelerator cycle. This renders the presented method a new precision tool for accelerator physics: controlling the spin motion of particles to high precision is mandatory, in particular, for the measurement of electric dipole moments of charged particles in a storage ring., Comment: 6 pages, 6 figures

The pp → ppπ 0 differential cross section has been measured with the ANKE spectrometer at COSY–Julich for pion cms angles between 0 ◦ and 15.4 ◦ at a proton beam energy of 0.8 GeV. The selection of diproton pairs with an excitation energy Epp < 3 MeV ensures that the final pp system is dominantly in the spin-singlet 1 S0 state. The kinematics are therefore very similar to those of pp → dπ + but with different spin and isospin transitions. The cross sections are over two orders of magnitude smaller than those of pp → dπ + and show a forward dip that is even stronger than that seen at lower energies. The results should provide a crucial extra test of pion production models in nucleon–nucleon collisions.

The first measurement of the $pn\rightarrow d\omega$ total cross-section has been achieved at mean excess energies $Q\approx 28$ and 57 MeV by using a deuterium cluster-jet target. The momentum of the fast deuteron was measured in the ANKE spectrometer at COSY-Julich and that of the slow “spectator” proton $(p_{\rm {sp}})$ from the $pd\rightarrow p_{\rm {sp}}d\omega$ reaction in a silicon telescope placed close to the target. The cross-sections lie above those measured for $pp\rightarrow pp\omega$ but seem to be below theoretical predictions.

The analyzing power of the reaction p p→ppπ 0 has been measured at a beam energy of 390 MeV. The dependences of the analyzing power on the pion emission-angle and the relative momentum of two outgoing protons have been obtained. The angular dependence could be decomposed by Legendre polynomial and the relative contribution of the P 21 to the P 11 function is less than 4%. The momentum dependence of the analyzing power has been studied to obtain information about the pion-production mechanism. It has been deduced that pion production due to the long-range interaction plays an important role in the momentum dependence of the P -state amplitude.

The production of non-phi K+K- pairs by protons of 2.83 GeV kinetic energy on C, Cu, Ag, and Au targets has been investigated using the COSY-ANKE magnetic spectrometer. The K- momentum dependence of the differential cross section has been measured at small angles over the 0.2--0.9 GeV/c range. The comparison of the data with detailed model calculations indicates an attractive K- -nucleus potential of about -60 MeV at normal nuclear matter density at a mean momentum of 0.5 GeV/c. However, this approach has difficulty in reproducing the smallness of the observed cross sections at low K- momenta., Comment: 7 pages, 5 figures, 1 table

The spectrometer ANKE has been put into operation at the accelerator COSY of the Forschungszentrum Julich in spring 1998. An initial scientific goal is to study K + -production in pA collisions at subthreshold energies far below the free NN-threshold at T p =1.58 GeV. This requires the identification of K + -mesons in a background of pions and protons, about 10 6 times more intense. In this paper the sophisticated detection system and the software procedures for kaon identification are described. With the help of TOF, energy-loss and range measurements as well as the track information from wire chambers, it is possible to measure d 2 σ/ d Ω d p for deep subthreshold K + production at beam energies down to T p =1.0 GeV.

In order to provide the experimental tools necessary to carry out few-nucleon reaction studies with polarized beams and targets at the medium energy storage ring COSY at Julich, our group is currently developing a polarized internal storage-cell gas target for first use at the magnetic spectrometer ANKE. In this paper we give an overview about currently relevant aspects of the polarized deuteron breakup experiment.

The proton analysing power in $\vec{p}p$ elastic scattering has been measured at small angles at COSY-ANKE at 796 MeV and five other beam energies between 1.6 and 2.4 GeV using a polarised proton beam. The asymmetries obtained by detecting the fast proton in the ANKE forward detector or the slow recoil proton in a silicon tracking telescope are completely consistent. Although the analysing power results agree well with the many published data at 796 MeV, and also with the most recent partial wave solution at this energy, the ANKE data at the higher energies lie well above the predictions of this solution at small angles. An updated phase shift analysis that uses the ANKE results together with the World data leads to a much better description of these new measurements., Comment: 5 pages, 3 figures

This paper describes a time-marking system that enables a measurement of the in-plane (horizontal) polarization of a 0.97-GeV/c deuteron beam circulating in the Cooler Synchrotron (COSY) at the Forschungszentrum J\"ulich. The clock time of each polarimeter event is used to unfold the 120-kHz spin precession and assign events to bins according to the direction of the horizontal polarization. After accumulation for one or more seconds, the down-up scattering asymmetry can be calculated for each direction and matched to a sinusoidal function whose magnitude is proportional to the horizontal polarization. This requires prior knowledge of the spin tune or polarization precession rate. An initial estimate is refined by re-sorting the events as the spin tune is adjusted across a narrow range and searching for the maximum polarization magnitude. The result is biased toward polarization values that are too large, in part because of statistical fluctuations but also because sinusoidal fits to even random data will produce sizeable magnitudes when the phase is left free to vary. An analysis procedure is described that matches the time dependence of the horizontal polarization to templates based on emittance-driven polarization loss while correcting for the positive bias. This information will be used to study ways to extend the horizontal polarization lifetime by correcting spin tune spread using ring sextupole fields and thereby to support the feasibility of searching for an intrinsic electric dipole moment using polarized beams in a storage ring. This paper is a combined effort of the Storage Ring EDM Collaboration and the JEDI Collaboration., Comment: 28 pages, 15 figures, prepared for Physical Review ST - Accelerators and Beams

ANKE is a new experimental facility for the spectroscopy of products from proton-induced reactions on internal targets. It has recently been implemented in the accelerator ring of the cooler synchrotron COSY of the Forschungszentrum Julich (FZ-Julich), Germany. The device consists of three dipole magnets, various target installations and dedicated detection systems. It will enable a variety of hadron-physics experiments like meson production in elementary proton–nucleon processes and studies of medium modifications in proton–nucleus interactions.

The new spectrometer ANKE has been put into operation at the accelerator COSY of the Forschungszentrum Julich. An initial scientific goal is to study K+-production in pA collisions at subthreshold energies below the free NN-threshold of T = 1.58 GeV. First measurements of double differential cross sections in p12C collisions at emission angles around 0° have been performed at T = 1.0, 1.2 and 2.0 GeV. The challenge is to identify the kaons in a huge background of pions and protons, since the signal to background ratio decreases to about 10−6 at T = 1.0 GeV. For background suppression detectors and a trigger system based on energy-loss and time-of-flight measurements have been developed. In the analysis the decay of kaons (τ = 12.4 ns) stopped in the detection system into μ+ and π+ is exploited as well as the track information from the wire chambers.

The differential cross section for the exclusive dp -> 3He{\pi}+{\pi}- reaction has been measured with high resolution and large statistics over a large fraction of the backward 3He hemisphere at the excess energy 265 MeV using the COSY-ANKE magnetic spectrometer. Though the well-known ABC enhancement is observed in the {\pi}+{\pi}- spectrum, the differences detected between the {\pi}+3He and {\pi}-3He invariant-mass distributions show that there must be some isospin-one {\pi}{\pi} production even at relatively low excess energies. The invariant-mass differences are modeled in terms of the sequential decay N*(1440) -> Delta(1232){\pi} -> N{\pi}{\pi}.

Physical review / Special topics / Accelerators and beams 18(2), 020101 (2015). doi:10.1103/PhysRevSTAB.18.020101, Published by American Physical Society, {[S.l.]

The transverse spin correlations ${A}_{x,x}$ and ${A}_{y,y}$ have been measured in the $\stackrel{P\vec}{d}\stackrel{P\vec}{p}\ensuremath{\rightarrow}{p}_{\text{spec}}{{pp}}_{s}{\ensuremath{\pi}}^{\ensuremath{-}}$ reaction at COSY-ANKE at 353 MeV per nucleon. Here ${{pp}}_{s}$ denotes a proton-proton pair with low excitation energy, which is dominantly in the ${}^{1}{S}_{0}$ state. By measuring three protons in the final state it was possible to extract events where there was a spectator proton ${p}_{\text{spec}}$ so that the reaction could be interpreted in terms of quasifree $\stackrel{P\vec}{n}\phantom{\rule{0.16em}{0ex}}\stackrel{P\vec}{p}\ensuremath{\rightarrow}{{pp}}_{s}{\ensuremath{\pi}}^{\ensuremath{-}}$. The proton and neutron analyzing powers in this reaction were also deduced from this data set by averaging over the polarizations of the deuteron beam and hydrogen target, respectively. The values of ${A}_{y}$ were shown to be consistent with a refined analysis of our earlier results obtained with a polarized proton incident on a deuterium target. Taking these data in combination with our previous measurements of the differential cross sections and analyzing powers in the $\stackrel{P\vec}{p}p\ensuremath{\rightarrow}{{pp}}_{s\phantom{\rule{0.16em}{0ex}}}{\ensuremath{\pi}}^{0}$ reaction, a more robust partial wave decomposition was achieved. Three different acceptable solutions were found, and the only way of resolving this ambiguity without further theoretical input would be through a measurement of the mixed spin-correlation parameter ${A}_{x,z}$.

The $pp\ensuremath{\rightarrow}pp{K}^{+}{K}^{\ensuremath{-}}$ reaction was measured below the $\ensuremath{\phi}$ threshold at a beam energy of 2.568 GeV using the COSY-ANKE magnetic spectrometer. By assuming that the four-body phase space is distorted only by the product of two-body final-state interactions, fits to a variety of one-dimensional distributions permit the evaluation of differential and total cross sections. The shapes of the distributions in the $Kp$ and $Kpp$ invariant masses are reproduced only if the ${K}^{\ensuremath{-}}p$ interaction is even stronger than that found at higher energy. The cusp effect in the ${K}^{+}{K}^{\ensuremath{-}}$ distribution at the ${K}^{0}{\overline{K}}^{0}$ threshold is much more clear and some evidence is also found for coupling between the ${K}^{\ensuremath{-}}p$ and ${\overline{K}}^{0}n$ channels. However, the energy dependence of the total cross section cannot be reproduced by considering only a simple product of such pairwise final-state interactions.

In this paper we describe a Cherenkov counter of total internal reflection for particle separation in the momentum range where all types of particles radiate Cherenkov light. The Cherenkov counter prototype with the lucite radiator was tested on the secondary beam of the ITEP (Moscow) accelerator. Dependence of the photomultiplier pulse height on the particle entrance angle was clearly observed.

A method of direct determination of the deuteron D-state probability from the experiment, based on the d ↑ p → ppn reaction analyzis is suggested. Using the known results on the vector analyzing power in elastic NN-scattering, the values of the overall nucleon polarization and the deuteron vector polarization have been obtained. The probability of the deuteron D-state is estimated to be w D = 0.078 ± 0.046.

We plan to investigate sub-threshold K+ production in proton bombardment of atomic nuclei in order to study the reaction mechanism through a measurement of the double differential cross section and of K+ p and K+ d coincidences. The status of the theoretical studies is summarized, and the spectrometer to be installed in the COSY ring is briefly described.

New measurements are reported for the time dependence of the vertical polarization of a 0.97 GeV/c deuteron beam circulating in a storage ring and perturbed by an rf solenoid. The storage ring is the cooler synchrotron (COSY) located at the Forschungszentrum Jülich. The beam polarization was measured continuously using a 1.5 cm thick carbon target located at the edge of the circulating deuteron beam and the scintillators of the EDDA detector. An rf solenoid mounted on the ring was used to generate fields at and near the frequency of the 1-Gγ spin resonance. Measurements were made of the vertical beam polarization as a function of time with the operation of the rf solenoid in either fixed or continuously variable frequency mode. Using rf-solenoid strengths as large as 2.66×10^{-5} revolutions/turn, slow oscillations (∼1 Hz) were observed in the vertical beam polarization. When the circulating beam was continuously electron cooled, these oscillations completely reversed the polarization and showed no sign of diminishing in amplitude. But for the uncooled beam, the oscillation amplitude was damped to nearly zero within a few seconds. A simple spin-tracking model without the details of the COSY ring lattice was successful in reproducing these oscillations and demonstrating the sensitivity of the damping to the magnitude of the synchrotron motion of the beam particles. The model demonstrates that the characteristic features of measurements made in the presence of large synchrotron oscillations are distinct from the features of such measurements when made off resonance. These data were collected in preparation for a study of the spin coherence time, a beam property that needs to become long to enable a search for an electric dipole moment using a storage ring.

A value for the mass of the eta meson has been determined at the COSY-ANKE facility through the measurement of a set of deuteron laboratory beam momenta and associated 3He center-of-mass momenta in the d+p -> 3He+X reaction. The eta was then identified by the missing-mass peak and the production threshold determined. The individual beam momenta were fixed with a relative precision of 3 x 10^-5 for values around 3 GeV/c by using a polarized deuteron beam and inducing an artificial depolarizing spin resonance, which occurs at a well-defined frequency. The final-state momenta in the two-body d+p -> 3He+eta reaction were investigated in detail by studying the size of the 3He momentum ellipse with the forward detection system of the ANKE spectrometer. Final alignment of the spectrometer for this high precision experiment was achieved through a comprehensive study of the 3He final-state momenta as a function of the center-of-mass angles, taking advantage of the full geometrical acceptance. The value obtained for the mass, m(eta)=(547.873 +- 0.005(stat) +- 0.027(syst)) MeV/c^2, is consistent and competitive with other recent measurements, in which the meson was detected through its decay products., 11 pages, 11 figures, 3 tables, published version

The momentum spectra of K+ produced at small angles in proton-proton and proton-deuteron collisions have been measured at four beam energies, 1.826, 1.920, 2.020, and 2.650 GeV, using the ANKE spectrometer at COSY-Juelich. After making corrections for Fermi motion and shadowing, the data indicate that K+ production near threshold is stronger in pp- than in pn-induced reactions. However, most of this difference could be made up by the unobserved K0 production in the pn case., 6 pages, 4 figures, Submitted to PRC