arXiv subjects
Publications and source records attributed to Colin Wilkin.
The experimental hadronic physics programme at the COoler SYnchrotron of the Forschungszentrum Juelich terminated at the end of 2014. After describing the accelerator and the associated facilities, a review is presented of the major achievements in the field realized over the twenty years of intense research activity.
We analyze the data on cross sections and asymmetries for the $pd (dp) \to η^3{\rm He}$ reaction close to threshold and look for bound states of the $η^3 {\rm He}$ system. Rather than parameterizing the scattering matrix, as is usually done, we develop a framework in which the $η^3 {\rm He}$ optical potential is the key ingredient, and its strength, together with some production parameters, are fitted to the available experimental data. The relationship of the scattering matrix to the optical potential is established using the Bethe-Salpeter equation and the $η^3 {\rm He}$ loop function incorporates the range of the interaction given by the empirical $^3 {\rm He}$ density. We find a local Breit Wigner form of the $η^3$He amplitude $T$ below threshold with a clear peak in $|T|^2$, which corresponds to an $η^3 {\rm He}$ binding of about 0.3 MeV and a width of about 3 MeV. By fitting the potential we can also evaluate the $η^3 {\rm He}$ scattering length, including its sign, thus resolving the ambiguity in the former analyses.
There is much speculation and a modest amount of evidence that certain mesons might form quasi-bound states with nuclei to produce really exotic states of matter. For this to be a practical possibility, the interaction between the meson and nucleons at low energies must be strong and attractive and the production rates "healthy". The conditions for this are surveyed for the light mesons. How this might lead to quasi-bound states is then discussed in a few typical cases.
The total and differential cross sections for associated strangeness production in the $pp \to pK^+K^-p$ and $pp \to pK^+π^0Σ^0$ reactions have been studied in a unified approach using an effective Lagrangian model. It is assumed that both the $K^-p$ and $π^0Σ^0$ final states originate from the decay of the $Λ(1405)$ resonance which was formed in the production chain $pp\to p(N^*(1535)\to K^+Λ(1405))$. The available experimental data are well reproduced, especially the ratio of the two total cross sections, which is much less sensitive to the particular model of the entrance channel. The significant coupling of the $N^*(1535)$ resonance to $Λ(1405) K$ is further evidence for large $s \bar{s}$ components in the quark wave function of the $N^*(1535)$ resonance.
New experimental data are presented on the energy dependence of the total cross sections for the np -> d eta and quasi-two-body pp -> pp eta reactions, where the final diproton is detected at very low excitation energy. Differential cross sections of pp -> pp eta away from threshold show the influence of higher partial waves and a partial wave decomposition is attempted.
The roles of the COSY-TOF and COSY-ANKE spectrometers in the measurement of strange meson production are briefly reviewed, mainly in connection with new results on the pp -> K^+ p Lambda, pp -> K^+ p Sigma^0 and pp -> K^+ n Sigma^+ reactions.
Two coupled channel effects connected with kaon pair production in proton-proton collisions are discussed. (1) Although there is ample evidence that the antikaon is strongly attracted to the recoil protons in pp -> K+pK-p, residual effects of the K+K- interaction are seen, including a possible cusp at the K0\bar{K}0 threshold. This is investigated within a simple K-matrix approach. (2) The production rates and invariant mass distributions for pp -> K+p{K-p} and pp -> K+p{pi0 Sigma0} are related using a separable potential description of the coupled K-p/pi0 Sigma0 channels. It can be plausibly argued that this pair of reactions is driven through the production of the Lambda(1405).
The existing experimental data on the d(n,p)nn and d(p,n)pp cross sections in the forward direction are reviewed in terms of the Dean sum rule. It is shown that the measurement of the ratio of the charge exchange on the deuteron to that on the proton might, if taken together with other experimental data, allow a direct construction of the np -> np scattering amplitude in the backward direction with few ambiguities.
The value of the $N^*(1535)Nρ$ coupling constant $g_{N^*Nρ}$ derived from the $N^*(1535) \to Nρ\to N ππ$ decay is compared with that deduced from the radiative decay $N^*(1535) \to N γ$ using the vector-meson-dominance model. On the basis of an effective Lagrangian approach, we show that the value of $g_{N^*Nρ}$ extracted from the available experimental data on the two decays are consistent, though the error bars are rather large.
A personal selection is made of the highlights of the COSY-11 physics program undertaken at the COoler SYnchrotron of the Forschungszentrum Juelich. This has been particularly rich in the field of strange and non-strange meson production in proton-proton and proton-deuteron collisions. The results are considered in relation to experiments carried out at other facilities and with respect to their impact on theory.
This is the summary of the 'International Workshop on eta--Nucleus Physics' held in Juelich, May 8--12, 2006. Each talk is represented by an extended abstract. At the same time the document provides a progress report of the corresponding working group.
The cross section for the dd -> 4He pi pi reaction is estimated near threshold in a two-step model where a pion created in a first interaction produces a second pion in a subsequent interaction. This approach, which describes well the rates of 2pi and eta production in the pd -> 3He pi pi and dd -> 4He eta reactions, leads to predictions that are much too low compared to experiment. Alternatives to this and the double-Delta model will have to be sought to explain these data.
The spin and isospin structure of the amplitudes and observables for K+Lambda production in nucleon-nucleon collisions in the near-threshold region is analysed. It is shown that, with reasonable values for the relative strengths of the pi and rho terms in a meson-exchange model, one expects production on the neutron to be significantly stronger than that on the proton. Negative values of the spin-transfer coefficient D_NN are also predicted due to pi-rho interference.
Data on the total cross sections for the pp -> pp eta, pn -> pn eta, and pn -> d eta reactions and the pp -> pp eta differential cross section near threshold are analysed in a one-meson-exchange model. After including initial and final-state nucleon-nucleon distortion, the magnitude and most of the energy dependence are well reproduced. It is found that the contribution of rho-exchange is larger than that of pi-exchange. With destructive rho/pi interference in the pp case, the model explains quantitatively the pp -> pp eta/pn -> pn eta cross section ratio and the slope of the pp -> pp eta differential cross section. Such an agreement would be destroyed by any significant eta-exchange term. The residual energy dependence may be associated with eta-nucleon rescattering that has not been taken into account. The pn -> pn eta/pn -> d eta ratio depends weakly upon the nature of the particle exchanges, being determined primarily by the nucleon-nucleon final state interactions. The proton analysing power is predicted to remain small in the low energy region.
Experimental data on the pp -> pn pi^+ reaction measured in an exclusive two-arm experiment at 800 MeV show a narrow peak arising from the strong proton-neutron final-state interaction. It was claimed, within the framework of a certain model, that this peak contained up to a 25% spin-singlet final state contribution. By comparing the data with those of pp -> d pi^+ in a largely model-independent way, it is here demonstrated that at all the angles measured the whole of the peak could be explained as being due to spin-triplet final states, with the spin-singlet being at most a few percent. Good qualitative agreement with the measured proton analysing power is also found within this approach.
A model for the d d -> alpha pi pi reaction, based on two parallel N N -> d pi reactions, is extended to incorporate a complete set of input amplitudes. While low-energy cross sections are underestimated, the rich structure observed in the alpha-particle momentum distributions for 0.8 < T_d < 1.9 GeV (the ABC effect) is extraordinarily well reproduced. In addition, a recent measurement of deuteron analyzing powers agrees quite well with our predictions, both in frequency and magnitude of the oscillations.
Near-threshold data on meson production in nucleon-nucleon scattering are analysed in the short-range limit which permits a quantitative comparison of the production in two and three-body final states. The relative strengths of different meson productions are broadly in line with the predictions of one-meson-exchange models and it is deviations from these which are indications of extra physics.
A personal choice is made of the highlights of the physics programme carried out at the Saturne-2 facility over the last twenty years.