arXiv subjects
Publications and source records attributed to J. A. Niskanen.
We present a new parameterization of the trinucleon wavefunction. As a novel feature a separable parameterization for the complete wavefunction is given. In this way any calculation that considers two body forces only is largely simplified. To demonstrate this we calculate the $π^3$He scattering length.
We study pion absorption on 3He employing trinucleon wave functions calculated from modern realistic NN interactions (Paris, CD Bonn). Even though the use of the new wave functions leads to a significant improvement over older calculations with regard to both cross section and polarization data, there are hints that polarization data with quasifree kinematics cannot be described by just two-nucleon absorption mechanisms.
Chiral effective field theory predicts a charge symmetry violating (CSB) amplitude for pion-nucleon scattering. This mechanism provides a very large contribution also to the CSB forward-backward asymmetry in the angular distribution of the reaction pn --> d pi0. This contribution was so large that it had a potential to cause a large effect also in CSB elastic NN scattering and to disturb its present understanding. However, it can be seen that, contrary to pion production, in this case the ud-quark mass difference and the electromagnetic contribution to the np-mass difference tend to cancel causing the total effect in the effective range parameters Delta a = a_pp - a_nn and Delta r_0 = r_0,pp - r_0,nn to be relatively small. In the lowest order and within the static approximation for the nucleons CSB pion-nucleon rescattering does not influence np scattering.
The effect of isospin breaking pion s-wave rescattering is included in elastic NN scattering at low energies using effective field theory. Although this mechanism gave a large contribution to charge symmetry breaking in np --> d pi0, the effect is rather small in pp vs. nn scattering parameters and in the 3H-3He binding energy difference. This smallness is caused by large cancellation of the up-down quark mass difference contribution and electromagnetic effects to the np mass difference.
Chiral effective field theory predicts a specific charge symmetry violating amplitude for pion production. This term is shown to provide the dominant contribution to the forward-backward asymmetry in the angular distribution for the reaction $pn\to dπ^0$, for reasonable values of the mass difference between down and up quarks, $δm_N$. Using a value $δm_N\approx$ 3 MeV leads to a prediction of a 10 standard deviation effect for a TRIUMF experiment.
The Watson-Migdal approximation scheme to take into account final state interactions is shown to give the actual threshold momentum dependence of the reaction $pp \to ppπ^0$. However, by an explicit plane wave replacement of the final state wave function it is stressed that not too much physical significance should be given to the proportionality coefficient extracted using this procedure. The plane wave approximation is not physically reliable even after introducing the Watson-Migdal or a more sophisticated final state interaction factor, since direct production (impulse term) is missed. Also with short range interactions there can be discrepancies of a factor two.
The model for charge symmetry breaking in the reaction np --> dpi^0 applied earlier around the Delta region is used to calculate the integrated forward-backward asymmetry of the cross section close to threshold. The mixing of the pi and eta mesons appears as strongly dominant at these energies. This contrasts elastic np scattering experiments, where the np mass difference in OPE dominates, or np --> dπ^0 closer to the Delta region.
We calculate the isospin symmetry violating effects to the reactions $pp\rightarrow dπ^+$ and $np\rightarrow dπ^0$ arising from the different hadron masses and from the Coulomb interaction between the positive pion and the deuteron. These effects are large enough in the cross section and analyzing power $A_y$ that they should be taken into account in comparisons of accurate experiments in different charge channels.
Eta-deuteron scattering lengths are calculated. A summation of the multiple scattering series is carried out and the result is checked against more involved calculations. The necessity to go beyond the fixed nucleon approximation is emphasized. It is shown that a quasibound or virtual state in the $η$-deuteron system may occur within the range of $η$-nucleon scattering lengths suggested by other experiments.
Using multiple scattering theory the scattering lengths of $η$ mesons on helium nuclei are calculated and checked against final state $η$ interactions from the $pd \rightarrow η^3$He and $dd \rightarrow η^4$He reactions. The existence of an $η^4$He quasibound state is indicated.
In a recent paper by C. J. Horowitz (Phys. Rev. C {\bf 48}, 2920 (1993)) a heavy meson exchange is incorporated into threshold NN --> d pi to enhance the grossly underestimated cross section. However, that calculation uses an unjustified assumption on the initial and final momenta, which causes an overestimate of this effect by a factor of 3--4. I point out that the inclusion of the Delta(1232) isobar increases the cross section significantly even at threshold.
Three-body photodisintegration of 3He is calculated in the photon energy range 200 - 400 MeV assuming quasifree absorption on np pairs both in initial quasideuteron and singlet configurations. The model includes the normal nucleonic current, explicit meson exchange currents and the Delta(1232)-isobar excitation. The total cross section is increased by a factor of about 1.5 compared with free deuteron photodisintegration. Well below and above the Delta region also some spin observables differ significantly from the ones of free deuteron disintegration due to the more compressed wave function of the correlated np pairs in 3He compared to the deuteron. The initial singlet state causes a significant change in the analyzing power Ay. These differences could presumably be seen at the conjugate angles where two-body effects are maximized and where photoreactions could complement similar pion absorption experiments. Figures by fax or post from janiskanen@phcu.helsinki.fi
The cross section of the reaction $^3$He$(γ,pn)p_{spec}$ in the energy range 150 - 450 MeV has been calculated in a quasifree two-body model and compared with a recent kinematically complete experiment. In comparison with free deuteron disintegration the cross section is increased by a factor of about 1.5 in reasonable agreement with the data for photon energies above 200 MeV. At lower energies the data deviate from the two-body behaviour and thus suggest a more important role of the spectator requiring a more exact treatment.