Three-nucleon Absorption Research Articles

Λdcorrelations from theHe4(stopped-K−,d) reaction

We have observed an intense high-energy component in an inclusive He4(stopped K−,d) spectrum. For back-to-back Λd pairs a prominent event cluster has been found just below the m4He+mK−−mn mass threshold in the Λd invariant mass spectrum of Λdn events, which is evidence for a three-nucleon absorption process of K− in He4. In addition, an appreciable strength is revealed below ~3220 MeV/c2. Well separated Σ0dn events show a peak similar to the case of Λdn.Received 5 September 2007DOI:https://doi.org/10.1103/PhysRevC.76.068202©2007 American Physical Society

Multinucleon pion absorption on4He into thepppnfinal state

Results from a 4π solid angle measurement of the reaction π+4He→pppn at incident pion energies of Tπ+= 70, 118, 162, 239, and 330 MeV are presented. Integrated cross sections are given for the reactions where three nucleons participate, leading to energetic (ppp) or (ppn) final states, and for states where four nucleons are involved (pppn). The two three-nucleon absorption modes were investigated in particular, and an energy dependent isospin ratio of the cross sections of σppn/σppp=3.6±1.3, 2.6±0.5, 1.8±0.3, 1.4±0.2, and 1.8±0.6 was determined from 70 to 330 MeV. The differential cross sections were described by a complete set of eight independent variables and compared to simple cascade and phase space models. From this analysis the contributions from initial state interactions to the multinucleon absorption cross sections were found to be more important at higher pion energies, while those from final state interactions are stronger at lower energies. However, both mechanisms combined were found to account for not more than one-third of the total pppn multinucleon yield. The remaining strength is reasonably well reproduced by phase space models, but shows a dependence on the incident pion's orbital angular momentum. The isospin structure of the (ppp) and (ppn) final states is not understood, nor are some structures in their distributions. The four-nucleon yield (pppn) was found to be weak (1–8 % of the total absorption cross section) and shows no evidence for a “double-Δ” excitation.Received 18 June 1997DOI:https://doi.org/10.1103/PhysRevC.56.1872©1997 American Physical Society

Study of three-nucleon mechanisms in the photodisintegration of 3He

The cross section of the 3He(γ,pp)n reaction has been measured for the first time over a wide photon energy and proton angular range (200 MeV ≤ E γ ≤ 800 MeV; 20° ≤ ϑ p lab ≤ 160°) using the large acceptance detector DAPHNE at the tagged photon facility of the MAMI microtron in Mainz. The wide kinematical coverage of the measurement has allowed a detailed analysis of three-nucleon absorption mechanisms. A model developed by Laget explains the main characteristics of the data in the Δ resonance region.

3He( gamma,pp)n cross sections with tagged photons below the Delta resonance energy.

Cross sections have been measured for the $^{3}\mathrm{He}$(\ensuremath{\gamma},pp)n reaction with tagged photons in the range ${\mathit{E}}_{\ensuremath{\gamma}}$ =161--208 MeV using the Saskatchewan-Alberta Large Acceptance Detector (SALAD). The protons were detected over a range of polar angles of 40\ifmmode^\circ\else\textdegree\fi{}--140\ifmmode^\circ\else\textdegree\fi{} and azimuthal angles of 0\ifmmode^\circ\else\textdegree\fi{}--360\ifmmode^\circ\else\textdegree\fi{} with an energy threshold of 40 MeV. Comparisons are made with a microscopic calculation which includes one-, two-, and three-nucleon absorption mechanisms. One- and two-nucleon processes, including final-state interactions, are unable to account for the measured cross sections. The addition of three-nucleon absorption diagrams gives roughly the right strength, but the distribution in phase space is in disagreement with the data. \textcopyright{} 1996 The American Physical Society.

Contribution of initial state interactions to the three-nucleon absorption of pions by 3He

The distributions of protons after the absorption of positive pions by 3He at 118, 162 and 239 MeV are presented. A decomposition of the three-nucleon cross section in terms of initial state interactions and other processes is given, based upon simple models. A clear signal of initial state interactions is visible in the data, diminishing at the lower energies.

Angular distributions following three-nucleon pion absorption.

We present an analysis of the energy and angular distributions of nucleons following three-nucleon pion absorption. Three-nucleon absorption is treated as a one-step process whose transition matrix element is constant with respect to the energy of the final state nucleons. The inclusion of partial wave amplitudes for incident pions of angular momentum less than two shows good agreement with the experimental data in the Delta-resonance region.

Negative pion absorption at rest in 3He.

The pnn and dn final states after the absorption of stopped pions in $^{3}\mathrm{He}$ have been measured in a kinematically complete experiment covering the whole phase space. Proton-neutron, deuteron-neutron, and neutron-neutron coincidences have been detected with a charged-particle hodoscope and large-area time-of-flight counters. For the first time, triple coincidences between the pionic K x rays and two particles have been measured thus selecting absorption from the atomic 1s state. It has been found that the absorption is dominated by the two-nucleon mechanism (2NA) on isoscalar nucleon pairs as indicted by the measured ratio of 6.3\ifmmode\pm\else\textpm\fi{}1.1 for back-to-back correlated nn and pn pairs. A substantial amount of the three-body final state pnn is affected by the final-state interaction (FSI) or leads to the two-particle final state dn. Ratios of 3.1\ifmmode\pm\else\textpm\fi{}0.4 for 2NA to FSI and of 4.2\ifmmode\pm\else\textpm\fi{}0.6 for the pnn to the dn final state are measured. No evidence has been found for three-nucleon absorption with an upper limit of %/${\mathrm{\ensuremath{\pi}}}_{\mathrm{stop}}^{\mathrm{\ensuremath{-}}}$.

Exclusive photodisintegration of 3He with polarized photons.

The first measurement of exclusive photodisintegration of He3 with polarized photons is reported. Cross sections and asymmetries for He3(γ→,pn)p and He3(γ→,pp)n at 235≤Eγ≤305 MeV are compared to a theoretical calculation which includes one-, two-, and three-nucleon photon, absorption. Two-nucleon absorption dominates the pn data, but is suppressed in the pp data. The pp data require the inclusion of three-nucleon absorption to describe the cross section and beam asymmetry over all momenta.Received 10 February 1994DOI:https://doi.org/10.1103/PhysRevLett.73.408©1994 American Physical Society

Pion absorption in medium and heavy nuclei

Pion absorption in the Δ resonance region is studied in terms of a microscopical model which includes two- and three-nucleon absorption mechanisms. Results of the model are compared with data on A(π, N)X, A(π, 2N)X and A( π, 3 N) X for several energies, nuclei and isospin channels finding an overall good agreement.

First measurement of the reaction 3He( gamma

The first measurement of the reaction He3(γ→,p)X using linearly polarized photons is reported. Cross sections and beam-polarization asymmetries for θplab=60°–100° and Eγ=195–304 MeV are compared with a microscopic calculation which includes one-, two-, and three-nucleon absorption mechanisms. One- and two-nucleon absorption alone fails to describe the data at low proton momenta. The inclusion of three-nucleon absorption significantly improves the comparison with the measured cross sections. However, some features of the asymmetry distributions are not explained.Received 9 August 1993DOI:https://doi.org/10.1103/PhysRevLett.72.617©1994 American Physical Society

Pion absorption in tritium at intermediate energies.

The angular distribution of nucleons from a kinematically complete measurement of pions absorbed in $^{3}\mathrm{H}$ on nucleon pairs with isospin T=0 and T=1 at incident pion energies of 119, 206, and 300 MeV are presented. The T=0 data are quite similar to pion absorption data in deuterium. The integrated absorption cross sections scale within errors with the number of isoscaler neutron-proton pairs. The T=1 cross sections, which correspond to absorption on dineutrons, are an order of magnitude smaller. The angular distributions for isovector absorption show a significant asymmetry, indicating interference effects between amplitudes with or without \ensuremath{\Delta} isobars in the intermediate states. The existence of a significant three-nucleon absorption component is established. The integrated absorption cross sections for processes involving two and three nucleons were determined. The neutron and proton momentum probability densities in $^{3}\mathrm{H}$ were determined from the spectator recoil resulting from quasifree two-nucleon pion absorption.

Multinucleon pion absorption in the 4He( pi +,ppp)n reaction.

Three-proton emission cross sections for the $^{4}\mathrm{He}$(${\mathrm{\ensuremath{\pi}}}^{+}$,ppp)n reaction were measured at an incident pion kinetic energy of ${\mathit{T}}_{\mathrm{\ensuremath{\pi}}}^{+}$=165 MeV over a wide angular range in a kinematically complete experiment. Angular correlations, missing momentum distributions, and energy spectra are compared with three- and four-body phase-space Monte Carlo calculations. The results provide strong evidence that most of the three-proton coincidences result from three-nucleon absorption. From phase-space integration the total three-nucleon absorption cross section is estimated to be ${\mathrm{\ensuremath{\sigma}}}^{3\mathit{N}}$=4.8\ifmmode\pm\else\textpm\fi{}1.0 mb. The cross section involving four nucleons is small and is estimated to be ${\mathrm{\ensuremath{\sigma}}}^{4\mathit{N}}$2 mb. On the scale of the total absorption cross section in $^{4}\mathrm{He}$, multinucleon pion absorption seems to represent only a small fraction.

Pion absorption in flight on 4He

We have measured double and triple coincidences following π + and π − absorption on 4He at various pion energies. Differential and integrated cross sections are determined for all possible absorption channels. We study and compare the two-nucleon absorption on nucleon pairs with isospin I = 0 and I = 1. In the dNN final-state two absorption mechanisms are identified, one with only one spectator nucleon and one with phase-space behavior. The existence of a three-nucleon absorption component with a spectator nucleon is established and a four nucleon phase-space-like contribution is also identified and found to be small. The total absorption cross section is directly determined by adding the integrated cross sections of all measured reaction channels.

Composite particle emission following negative pion absorption on 12C at T pi -=165 MeV.

Double differential and total cross sections of the ({pi}{sup {minus}},{ital pd}), ({pi}{sup {minus}},{ital dp}), and ({pi}{sup {minus}},{ital dd}) reactions on {sup 12}C were measured over a wide angular range at an incident pion kinetic energy of 165 MeV. The shape of the angular distributions of the differential cross sections of these reactions is in most cases similar to that of the {sup 12}C({pi}{sup {minus}},{ital pp}){ital X} reaction. These similarities seem to indicate a pickup mechanism in certain regions of phase space following three-nucleon absorption. Contributions from both three-nucleon absorption and two-step processes are found to be important for the above type of reactions.

Pion absorption in3He at 100, 120, and 145 MeV

π+ true absorption cross-section on3He has been measured at 100, 120, and 145 MeV. The maximal total cross-section values of the reaction π+3He→ppp turn out to be 10, 16, and 19 mb, respectively. About 16% of the cross-section corresponds to three-nucleon absorption. The measured angular distribution as a function of the opening angle between the two fast protons has been fitted with the Legendre polynomial expansionA0+A2P2(cos θ). The values of the coefficients turn out to beA0=(1.7±0.5) mb/sr, andA2=(1.4±0.5) mb/sr, in agreement with the values obtained in the study of the pion absorption on deuteron.

Multi-nucleon pion absorption in the 4He(π +, ppp)n reaction

Three-proton emission cross sections for the 4HE(π +, ppp)n reaction were measured at T π + = 165 MeV over a wide angular range in a kinematically complete experiment. The momentum distribution of the undetected neutron and angular correlations have been compared with three- and four-body phase space Monte Carlo calculations. The results indicate that most of the three-proton coincidences result from three nucleon absorption. The total three-nucleon absorption cross section is estimated to be σ 3N=4.5±1.0 mb from phase-space integration. This cross section does not account for the discrepancy between total absorption cross sections and two-nucleon absorption cross sections.

Multi-nucleon pion absorption in the 4He(π+,ppp)n reaction

Three-proton emission cross sections for the4He(π+, ppp)n reaction were measured at Tπ+=165 MeV over a wide angular range in a kinematically complete experiment. The momentum distribution of the undetected neutron and angular correlations have been compared with three- and four-body phase space Monte Carlo calculations. The results indicate that most of the three-proton coincidences result from three nucleon absorption. The total three-nucleon absorption cross section is estimated to be σ3N=4.5±1.0 mb from phase-space integration. This cross section does not account for the discrepancy between total absorption cross sections and two-nucleon absorption cross sections.

Three-nucleon pion absorption in carbon.

We have performed an experiment in which we have detected three protons in coincidence, following the absorption of positive pions in carbon. Data were collected with twelve plastic scintillator detectors, covering a fairly large region of the available phase space, at three incident pion kinetic energies: ${T}_{\ensuremath{\pi}}$=130, 180, and 228 MeV. Comparisons are made with phase space calculations which simulate quasifree three-nucleon and four-nucleon absorption mechanisms. The first of these provides an excellent description of the bulk of the data. Some of the observed events appear to come from quasideuteron absorption, followed by a final-state interaction of one of the outgoing protons. There is no evidence for absorption mechanisms involving more than three nucleons. Estimates are made for the contributions of the three-nucleon absorption mechanisms to the total pion absorption cross section. These are relatively small.

What is the mechanism for three-nucleon absorption of pions in 3He?

Experimental data for 3He(π +, pp)p at T π =120 MeV in a kinematical region dominated by three-nucleon absorption, where all three protons have obtained an appreciable amount of energy, are analyzed by means of variables specifically sensitive to different reaction mechanisms. The data are compared with predictions for two-step mechanisms and a genuine three-nucleon absorption mechanism. No evidence is found for such two-step processes as scattering or charge exchange preceding a two-nucleon absorption or as two-nucleon absorption followed by “Hard FSI”. The results, however, agree with a genuine three-nucleon absorption process, consistent with the recently measured phase-space-like behavior of this process.

New pion-absorption modes observed from triple coincidences in 4He.

The first triple-coincidence data resulting from pion absorption in $^{4}\mathrm{He}$ are reported. The existence of a three-nucleon absorption component in the four-nucleon channel is established. A four-nucleon phase-space-like contribution and nucleon-nucleon final-state interactions are also identified.