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C. Wilkin

Publications and source records attributed to C. Wilkin.

At least 91 records · Page 5Linked to original sources
arXiv2006

Precision measurement of the quasi-free pn->dphi reaction close to threshold

The quasi-free pn->dphi reaction has been studied at the Cooler Synchrotron COSY-Juelich, using the internal proton beam incident on a deuterium cluster-jet target and detecting a fast deuteron in coincidence with the K+K- decay of the 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 low energy. Production on the neutron is found to be stronger than on the proton but not by as much as for the eta-meson.

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arXiv2006

Determination of Deuteron Beam Polarizations at COSY

The vector and tensor polarizations of a deuteron beam have been measured using elastic deuteron-carbon scattering at 75.6 MeV and deuteron-proton scattering at 270 MeV. After acceleration to 1170 MeV inside the COSY ring, the polarizations of the deuterons were checked by studying a variety of nuclear reactions using a cluster target at the ANKE magnet spectrometer placed at an internal target position of the storage ring. All these measurements were consistent with the absence of depolarization during acceleration and provide a number of secondary standards that can be used in subsequent experiments at the facility.

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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.

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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.

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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.

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arXiv2005

Measurement of the Vector and Tensor Polarisation of Proton and Deuteron Beams

Measurement of the d + d -> 4He + eta reaction using vector and tensor polarised beam has been performed at COSY using Big Karl magnetic spectrograph. The beam polarisation necessary for obtaining the vector and tensor analysing power for this reaction was measured. The method and the results of the tensor polarisation measurement of the deuteron beam are presented.

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arXiv2005

Spin Physics from COSY to FAIR

It is the aim of the ANKE-COSY spin collaboration to carry out a well directed programme of internationally competitive experiments involving polarised beams and targets, using the outstanding facilities available at the storage ring. These activities, at the same time, are good preparation for our participation in the PAX@FAIR project.

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arXiv2005

Evidence for an Excited Hyperon State in pp -> p K^+ Y^{0*}

Indications for the production of a neutral excited hyperon in the reaction pp -> p K^+ Y^{0*} are observed in an experiment performed with the ANKE spectrometer at COSY-Jülich at a beam momentum of 3.65 GeV/c. Two final states were investigated simultaneously, viz. Y^{0*} -> pi^+X^- and pi^-X^+, and consistent results were obtained in spite of the quite different experimental conditions. The parameters of the hyperon state are M(Y^{0*})= (1480 +/- 15) MeV/c^2 and Gamma(Y^{0*})= (60 +/- 15) MeV/c^2. The production cross section is of the order of few hundred nanobarns. Since the isospin of the Y^{0*} has not been determined here, it could either be an observation of the Sigma(1480), a one-star resonance of the PDG tables, or alternatively a Lambda hyperon. Relativistic quark models for the baryon spectrum do not predict any excited hyperon in this mass range and so the Y^{0*} may be of exotic nature.

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arXiv2005

Theoretical Description of the pd -> pd eta Reaction Near Threshold

The contributions of three different types of driving terms are included in the estimation of the pd -> pd eta reaction at low energies. Near threshold, it is predicted that a two-step model involving an intermediate pion should be the most important but, as the energy approaches the threshold for eta production in the free nucleon--nucleon reaction, a pick-up mechanism with a spectator proton should become dominant. The total cross sections are underestimated by about a factor of two compared to experimental data but the discrepancies in the angular distributions are more serious, with no sign in the data for the peaks corresponding to the pick--up diagram.

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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.

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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.

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arXiv2001

Spin-triplet final-state dominance in the pp -> pn pi+ reaction at 492 MeV

The near-forward cross section for the pp -> pn pi^+ reaction has been measured at 492 MeV by using the large acceptance ANKE magnetic spectrometer placed at an internal target position of the storage ring COSY-Juelich. Protons and pions emitted near zero degrees were detected in coincidence, and those with theta_pi <= 2 degrees and theta_p <= 2.5degrees were subjected to detailed analysis. Under these conditions, the excitation energy in the np system was below 3 MeV over the measured momentum range. This is the region of the np final-state-interaction peak, which was measured with a resolution of a fraction of an MeV. The shape of the peak allows one to conclude that the fraction of final spin-singlet np pairs is below about 10%. By using the results of scattering theory, this limit is confirmed through a comparison with the cross section for pp -> d pi^+. The smallness of the singlet contribution is consistent with trends seen in lower energy data.

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arXiv2000

Production of isoscalar pion pairs in the pd -> 3He pi pi reaction near threshold

The production near threshold of isoscalar pion pairs in the pd -> 3He(pi pi)^0 reaction is estimated in a two-step model which successfully describes the production of eta, omega and eta' mesons. A virtual pion beam, generated through an NN -> d pi reaction on one of the nucleons in the deuteron, produces a second pion via a pi N -> pi pi N reaction on the other nucleon. Using the same scale factor as for heavy meson production, the model reproduces the total pi^0 pi^0 production rate determined at an excess energy of 37 MeV. There are some indications in the data for a suppression of events with low pi pi masses, as in the pi^- p -> pi^0 pi^0 n reaction, and this is confirmed within the model. The model suggests that a significant fraction of the charged pion production in the p d -> 3He pi^+ pi^- reaction at Q=70 MeV might be associated with isoscalar pion pairs, though this does not explain the strong dependence observed on the pi^+ pi^- relative momentum angle.

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arXiv2000

Near-threshold eta production in the p d -> p d eta reaction

The total cross section of the p d -> p d eta reaction has been measured at two energies near threshold by detecting the final proton and deuteron in a magneti spectrometer. The values are somewhat larger than expected on the basis of two simple theoretical estimates.

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arXiv1999

Has charge symmetry breaking been observed in the d d -> alpha pi0 reaction?

Estimates are made of the d d -> alpha gamma gamma production cross sections in a model where each neutron-proton pair in the beam and target initiates an n p -> d gamma reaction. This approach, which successfully reproduces observables in two-pion production at intermediate energies, suggests that direct two-photon production could provide a very significant background to the measurement of the charge-symmetry-breaking (CSB) reaction d d -> alpha pi0. A non-vanishing CSB cross section has been reported which might be confused with such two-photon production under the given experimental conditions.

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arXiv1999

Higher Partial Waves in p+p->p+p+eta near Threshold

Exclusive measurements of the production of eta mesons in the p+p->p+p+eta reaction have been carried out at excess energies of 16 and 37 MeV above threshold. The deviations from phase space are dominated by the proton-proton final state interaction and this influences particularly the energy distribution of the eta meson. However, evidence is also presented at the higher energy for the existence of an anisotropy in the angular distributions of the eta meson and also of the final proton-proton pair, probably to be associated with D-waves in this system interfering with the dominant S-wave term. The sign of the eta angular anisotropy suggests that rho-exchange is important for this reaction.

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arXiv1999

Near-Threshold Production of omega Mesons in the pp -> pp omega Reaction

The total cross section for omega production in the pp -> pp omega reaction has been measured at five c.m. excess energies from 3.8 to 30 MeV. The energy dependence is easily understood in terms of a strong proton-proton final state interaction combined with a smearing over the width of the state. The ratio of near-threshold phi and omega production is consistent with the predictions of a one-pion-exchange model and the degree of violation of the OZI rule is similar to that found in the pi-p -> n omega/phi reactions.

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