arXiv subjects

C. Wilkin

Publications and source records attributed to C. Wilkin.

At least 55 records · Page 3Linked to original sources

arXiv2011

Comparison of inclusive K+ production in proton-proton and proton-neutron collisions

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.

nucl-ex↗

arXiv2011

Estimation of the ratio of the pn -> pn pi0 pi0/pn -> d pi0 pi0 cross sections

Evidence has recently been presented for the existence of a dibaryon of mass 2380 MeV/c^2 and width 70 MeV/c^2, which decays strongly into the d pi0 pi0 channel [M. Bashkanov et al., Phys.Rev.Lett. 102 (2009) 052301; P. Adlarson et al., arXiv:1104.0123]. The decay rate of such a hypothesised dibaryon into the {pn}_{I=0} pi0 pi0 channel is estimated in a weakly model-dependent way by using final state interaction theory. It is shown that, if the resonance exists, it should then show up as strongly in this channel as in d pi0 pi0. The sum of the two decay modes would saturate most of the inelasticity predicted in the relevant partial waves in the 2380 MeV/c^2 region.

nucl-th↗

arXiv2011

Measurement and analysis of the pp -> pp gamma reaction at 310 MeV

The pp -> pp gamma reaction has been studied at a beam energy of 310 MeV by detecting both final protons at the PROMICE-WASA facility and identifying the photon through the resulting missing-mass peak. The photon angular distribution in the center-of-mass system and those of the proton-proton relative momentum with respect to the beam direction and to that of the recoil photon were determined reliably up to a final pp excitation energy of Epp ~ 30 MeV. Except for very small Epp values, the behavior of these distributions with excitation energy is well reproduced by a new refined model of the hard bremsstrahlung process. The model reproduces absolutely the total cross section and its energy dependence to within the experimental and theoretical uncertainties.

nucl-ex↗

arXiv2010

The measurement of the pp->K+ n Sigma+ reaction near threshold

It is shown that a recent extraction of the total cross section for pp->K+ n Sigma+ from inclusive K+ production data is in conflict with experimental data on the exclusive pp->K+ p Lambda reaction. The result may be interpreted as an upper bound which is not inconsistent with the much lower values that already exist in the literature.

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arXiv2010

Measurement of the in-medium phi-meson width in proton-nucleus collisions

The production of phi mesons in the collisions of 2.83 GeV protons with C, Cu, Ag, and Au at forward angles has been measured via the phi -> K+K- decay using the COSY-ANKE magnetic spectrometer. The phi meson production cross section follows a target mass dependence of A^0.56+/-0.02 in the momentum region of 0.6-1.6 GeV/c. The comparison of the data with model calculations suggests that the in-medium phi width is about an order of magnitude larger than its free value.

nucl-ex↗

arXiv2010

Energy dependence of hard bremsstrahlung production in proton-proton collisions in the Delta(1232) region

Hard bremsstrahlung production in proton-proton collisions has been studied with the ANKE spectrometer at COSY-Juelich in the energy range of 353-800 MeV by detecting the final proton pair {pp}_s from the pp -> {pp}_s reaction with very low excitation energy. Differential cross sections were measured at small diproton c.m. angles from 0 to 20 degrees and the average over this angular interval reveals a broad peak at a beam energy around 650 MeV with a FWHM of about 220 MeV, suggesting the influence of Delta(1232)N intermediate states. Comparison with deuteron photodisintegration shows that the cross section for diproton production is up to two orders of magnitude smaller, due largely to differences in the selection rules.

nucl-ex↗

arXiv2010

High precision beam momentum determination in a synchrotron using a spin resonance method

In order to measure the mass of the eta meson with high accuracy using the d+p -> 3He+eta reaction, the momentum of the circulating deuteron beam in the Cooler Synchrotron COSY of the Forschungszentrum Juelich has to be determined with unprecedented precision. This has been achieved by studying the spin dynamics of the polarized deuteron beam. By depolarizing the beam through the use of an artificially induced spin resonance, it was possible to evaluate its momentum p with a precision of dp/p < 10-4 for a momentum of roughly 3 GeV/c. Different possible sources of error in the application of the spin resonance method are discussed in detail and its possible use during a standard experiment is considered.

physics.acc-ph↗

arXiv2010

The energy dependence of the pp->K+ n Sigma+ reaction close to threshold

The production of the Sigma+ hyperon through the pp->K+nSigma+ reaction has been investigated at four energies close to threshold, 1.826, 1.920, 1.958, and 2.020 GeV. At low energies, correlated K+pi+ pairs can only originate from Sigma+ production so that their measurement allows the total cross section for the reaction to be determined. The results obtained are completely consistent with the values extracted from the study of the K+-proton correlation spectra obtained in the same experiment. These spectra, as well as the inclusive K+ momentum distributions, also provide conservative upper limits on the Sigma+ production rates. The measurements show a Sigma+ production cross section that varies roughly like phase space and, in particular, none of the three experimental approaches used supports the anomalously high near-threshold pp->K+ nSigma+ total cross section previously reported [T. Rozek et al., Phys. Lett. B 643, 251 (2006)].

hep-ex↗

arXiv2009

Review of quasi-elastic charge-exchange data in the nucleon-deuteron breakup reaction

The available data on the forward charge exchange of nucleons on the deuteron up to 2GeV per nucleon are reviewed. The value of the inclusive nd->pnn/np->pn cross section ratio is sensitive to the fraction of spin-independent neutron-proton backward scattering. The measurements of the polarisation transfer in d(\vec{n},\vec{p})nn or the deuteron analysing power in p(\vec{d},pp)n in high resolution experiments, where the final nn or pp pair emerge at low excitation energy, depend upon the longitudinal and transverse spin-spin np amplitudes. The relation between these types of experiments is discussed and the results compared with predictions of the impulse approximation model in order to see what new constraints they can bring to the neutron-proton database.

nucl-ex↗

arXiv2009

The pd --> ^3He eta pi0 reaction at T_p = 1450 MeV

The cross section for the pd --> ^3He eta pi0 reaction has been measured at a beam energy of 1450 MeV using the WASA detector at the CELSIUS storage ring and detecting one ^3He and four photons from the decays of the two photons. The data indicate that the production mechanism involves the formation of the Delta(1232) isobar. Although the beam energy does not allow the full peak of this resonance to be seen, the invariant masses of all three pairs of final state particles are well reproduced by a phase space Monte Carlo simulation weighted with the p-wave factor of the square of the pi^0 momentum in the ^3Hepi^0 system.

nucl-ex↗

arXiv2009

Hard bremsstrahlung in the pp -> pp gamma reaction

The pp -> pp gamma reaction has been measured at a beam energy of 310 MeV by detecting both final protons in the PROMICE-WASA facility and identifying a missing-mass peak. For those events where the pp excitation is less than 3 MeV, the final diproton is almost purely in the 1S0 state and, under these conditions, there is complete coverage in the photon c.m. angle theta_gamma. The linear behaviour observed in cos^2(theta_gamma) shows that there is almost no influence of an E2 multipole at this energy, though the E1 and M2 must be rather similar in size.

nucl-ex↗

arXiv2009

Precision study of the d+p->3He+eta reaction for excess energies between 20 and 60 MeV

The differential and total cross sections for the d+p->3He+eta reaction have been measured at COSY-ANKE at excess energies of 19.5, 39.4, and 59.4 MeV over the full angular range. The results are in line with trends apparent from the detailed near-threshold studies and also agree with those from CELSIUS, though the present data have higher precision. While at 19.5 MeV the results can be described in terms of s- and p-wave production, by 59.4 MeV higher partial waves are required. Including the 19.5 MeV point together with the near-threshold data in a global s- and p-wave fit gives a poorer overall description of the data though the position of the pole in the 3He scattering amplitude, corresponding to the quasi-bound or virtual state, is hardly changed.

nucl-ex↗

arXiv2009

Measurement of the isospin-filtering dd -> 4He K+ K- reaction at Q=39 MeV

The total cross section for the dd -> 4He K+ K- reaction has been measured at a beam momentum of 3.7 GeV/c, corresponding to an excess energy of 39 MeV, which is the maximum possible at the Cooler Synchrotron COSY-Jülich. 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(tot) < 14 pb, which brings into question the viability of investigating the dd -> 4He a0(980) reaction as a means of studying isospin violation.

nucl-ex↗

arXiv2009

Precision measurements of the pp\to π^+pn and pp\to π^+d reactions: 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 COSY using the Big Karl spectrograph. The high resolution in the $π^+$ momentum ensured that there was an unambiguous separation of the $pp\to π^+d/π^+pn$ channels. Using these and earlier data, the ratio of the production cross sections could be followed through the $Δ$ 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 $pp\toπ^+d/π^+pn$ calculation that includes $S \rightleftharpoons D$ channel coupling between the final nucleons. A semi-quantitative understanding of the observed effects is achieved.

nucl-ex↗

arXiv2009

Exclusive measurement of two-pion production in the dd --> 4Hepipi reaction

The results from the first kinematically complete measurement of the dd --> 4Hepipi reaction are reported. The aim was to investigate a long standing puzzle regarding the origin of the peculiar pipi-invariant mass distributions appearing in double pion production in light ion collisions, the so-called ABC effect. The measurements were performed at the incident deuteron energies of 712 MeV and 1029 MeV, with the WASA detector assembly at CELSIUS in Uppsala, Sweden. We report the observation of a characteristic enhancement at low pipi-invariant mass at 712 MeV, the lowest energy yet. At the higher energy, in addition to confirming previous experimental observations, our results reveal a strong angular dependence of the pions in the overall centre of mass system. The results are qualitatively reproduced by a theoretical model, according to which the ABC effect is described as resulting from a kinematical enhancement in the production of the pion pairs from two parallel and independent NN--> dpi sub-processes.

nucl-ex↗

arXiv2009

Production of the omega meson in the pd --> 3He omega reaction at 1450 MeV and 1360 MeV

The production of $ω$ mesons in the $pd \to{}^3$He$ ω$ reaction has been studied at two energies near the kinematic threshold, $T_p=1450$ MeV and $T_p=1360$ MeV. The differential cross section was measured as a function of the $ω$ cm angle at both energies over the whole angular range. Whereas the results at 1360 MeV are consistent with isotropy, strong rises are observed near both the forward and backward directions at 1450 MeV. Calculations made using a two-step model with an intermediate pion fail to reproduce the shapes of the measured angular distributions and also underestimate the total cross sections.

nucl-ex↗

arXiv2009

Observation of an ABC effect in proton-proton collisions

The cross section for inclusive multipion production in the pp->ppX reaction was measured at COSY-ANKE at four beam energies, 0.8, 1.1, 1.4, and 2.0 GeV, for low excitation energy in the final pp system, such that the diproton quasi-particle is in the 1S0 state. At the three higher energies the missing mass Mx spectra show a strong enhancement at low Mx, corresponding to an ABC effect that moves steadily to larger values as the energy is increased. Despite the missing-mass structure looking very different at 0.8 GeV, the variation with Mx and beam energy are consistent with two-pion production being mediated through the excitation of two Delta(1232) isobars, coupled to S-- and D-- states of the initial pp system.

nucl-ex↗

arXiv2008

Measurement of the pn -> dK^+K^- total cross section close to threshold

Measurements of the pn -> p_spec dK^+K^- reaction, where p_spec is a spectator proton, have been undertaken at the Cooler Synchrotron COSY-Juelich by detecting a fast deuteron in coincidence with a K^+K^- pair in the ANKE facility. Although the proton beam energy was fixed, the moving target neutron allowed values of the non-resonant quasi-free pn -> dK^+K^- total cross section to be deduced up to an excess energy epsilon ~ 100 MeV. Evidence is found for the effects of K^-d and KKbar final state interactions. The comparison of these data with those of pp -> ppK^+K^- and pp -> dK^+Kbar shows that all the total cross sections are very similar in magnitude.

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