D. Vetterli

Total reaction cross sections for 20–30 MeV pions and the anomaly of pionic atoms

Abstract Total reaction cross sections of 20 MeV π− and 30 MeV π+ and π− have been measured for carbon and nickel targets. The experimental results are in very good agreement with calculations based on commonly accepted pion-nucleus potentials but disagree with calculations based on the potentials associated with the so-called pionic atom anomaly.

Threshold behaviour of the π+π− invariant mass in nuclei

We present π+π− invariant-mass distributions between 300 and 380 MeV for the 2H, 4He, 16O, 208Pb(π+, π+π−) reaction at an incident π+ energy of 280 MeV. A prominent feature is the shift of their maximum downward to the 2mπ threshold as A increases. This behaviour finds a qualitative explanation in the framework of a model that investigates the modification of the strength function for an interacting (ππ)I = L = 0 system embedded in the nuclear medium. Also, the π+π− invariant-mass cross sections are compared with the predictions of an A(π+, π+π−) model that uses a microscopical approach for the elementary process of pion production, πN → ππ N. Four-fold differential cross sections as a function of the energy of outgoing π+ and p, and missing-mass distributions for the A(π+, pπ−) reaction are also presented for some of the examined nuclei. The experimental results are discussed within the framework of the two available models.

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

Abstract 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.

Pion-induced pion production on deuterium: a quasifree process

Abstract A detailed experimental analysis of the π + d → π + π − pp in-plane coincidence data first presented by Rui et al. is compared to an expanded version of the Oset and Vicente-Vacas model for pion-induced pion production on a free nucleon. This extended model averages over Fermi motion to describe the assumed quasifree nature of the process occurring on the deuteron and includes nine additional diagrams to account for the N ∗ → N (ππ) p-wave reaction channels. Experimental effects such as pion energy loss in the target and in the detectors, pion decay and muon detection are investigated and incorporated into the comparison of experimental data and theory. Inclusion of Fermi motion was found to be essential to provide good agreement between data and model confirming the quasifree nature of the reaction. When compared to the total-cross-section measurements of Manley et al. , the free-reaction model yields a model-dependent estimate of the overall strength of the diagram containing the N ∗ → N (ππ) s-wave vertex.

π+π− coincidence measurement in the 4He(π+, π+π−) reaction at Tπ+ = 280 MeV

Abstract The pion induced pion production (π, 2π) reaction on deuterium has been studied at an incident pion energy of 280 MeV. The outgoing pions were detected in coincidence and the measured four-fold cross sections were compared with a πN → ππ N microscopic model, and, upon integration, with the available experimental total cross sections from the same reaction on H and 2 H. Finally, the results were directly compared with recent data from the 16 O(π + , π + π − ) reaction at the same incident energy.

Vector analyzing power iT11 in pi d elastic scattering at 49 MeV

Measurements of the vector analyzing power {ital iT}{sub 11} in {pi}{ital d} elastic scattering at 49 MeV have been performed using a dynamically polarized target and a magnetic spectrometer. Data at seven {pi}{sup +} laboratory scattering angles between 50{degree} and 130{degree} were taken together with a complementary measurement at 60{degree} for {pi}{sup {minus}}{ital d} elastic scattering. In general, we find agreement with models that include the {pi}{ital N} P{sub 11} amplitude and disagreement with models that exclude or suppress it.

Vector analyzing power {ital iT}{sub 11} in {pi}{ital d} elastic scattering at 49 MeV

Measurements of the vector analyzing power {ital iT}{sub 11} in {pi}{ital d} elastic scattering at 49 MeV have been performed using a dynamically polarized target and a magnetic spectrometer. Data at seven {pi}{sup +} laboratory scattering angles between 50{degree} and 130{degree} were taken together with a complementary measurement at 60{degree} for {pi}{sup {minus}}{ital d} elastic scattering. In general, we find agreement with models that include the {pi}{ital N} P{sub 11} amplitude and disagreement with models that exclude or suppress it.

Multinucleon pion absorption in the He4(,ppp)n reaction

Three-proton emission cross sections for the {sup 4}He({pi}{sup +},{ital ppp}){ital n} reaction were measured at an incident pion kinetic energy of {ital T}{sub {pi}}{sup +}=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 {sigma}{sup 3{ital N}}=4.8{plus minus}1.0 mb. The cross section involving four nucleons is small and is estimated to be {sigma}{sup 4{ital N}}{lt}2 mb. On the scale of the total absorption cross section in {sup 4}He, multinucleon pion absorption seems to represent only a small fraction.

Multinucleon pion absorption in theHe4(π+,ppp)nreaction

Three-proton emission cross sections for the {sup 4}He({pi}{sup +},{ital ppp}){ital n} reaction were measured at an incident pion kinetic energy of {ital T}{sub {pi}}{sup +}=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 {sigma}{sup 3{ital N}}=4.8{plus minus}1.0 mb. The cross section involving four nucleons is small and is estimated to be {sigma}{sup 4{ital N}}{lt}2 mb. On the scale of the total absorption cross section in {sup 4}He, multinucleon pion absorption seems to represent only a small fraction.

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

Three-proton emission cross sections for the {sup 4}He({pi}{sup +},{ital ppp}){ital n} reaction were measured at an incident pion kinetic energy of {ital T}{sub {pi}}{sup +}=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 {sigma}{sup 3{ital N}}=4.8{plus minus}1.0 mb. The cross section involving four nucleons is small and is estimated to be {sigma}{sup 4{ital N}}{lt}2 mb. On the scale of the total absorption cross section in {sup 4}He, multinucleon pion absorption seems to represent only a small fraction.