A systematic experimental study of inclusive pion double charge exchange in 4He has been undertaken. The reaction 4He(pi+,pi-)4p was observed at incident energies 120, 150, 180, 240 and 270 MeV; the 4He(pi-,pi+)4n reaction was observed at incident energies 180 and 240 MeV. At each incident energy, the doubly differential cross section was measured at three to five outgoing pion laboratory angles between 25 deg and 130 deg. At each angle, cross sections were measured over the range of outgoing pion energies from 10 MeV up to the kinematic limit for the reaction in which the final state consists of the oppositely charged pion plus four free nucleons. The spectra of outgoing pions are strikingly different from those observed for the inclusive double charge exchange reaction in heavier nuclei, but resemble those observed in the (pi-,pi+) reaction in 3He. The forward-angle spectra in the 3He and 4He reactions exhibit a prominent peak at high outgoing pion energies. Interpretation of the peaks in 3He (4He) as a three- (four-)nucleon resonance is ruled out by kinematic analysis. The results of a calculation, wherein the double charge exchange reaction is assumed to proceed as two sequential single charge exchange interactions, suggest that the high-energy peak is naturally explained by this double scattering mechanism. Non-static treatment of the pi-N interactions and the inclusion of nuclear binding effects appear to be important in reproducing the shape of the energy spectra at forward angles.

We have measured cross sections for the (pi (+), pi (+/-)p) reactions on H-3, He-4, Li-6 and Li-7 in quasi-free kinematics at incident pion beam energy 500 MeV. An enhancement of the (pi (+), pi (-)p) cross section in this kinematics is observed. If this is interpreted as due to quasi-free scattering from preexisting Delta components of the nuclear wave function, the extracted probabilities are in agreement with theoretical expectations. (C) 2001 Elsevier Science B.V. All rights reserved.

This paper presents a large solid angle measurement of the positive pion absorption cross section on ^4He and its decomposition into partial channels.

This paper presents a large solid angle measurement of the positive pion absorption cross section on ^4He and its decomposition into partial channels.

The absorption of positive pions on N and Ar has been studied for Tpi+= 118, 162, and 239 MeV. A precise determination has been made of the total absorption cross section for each energy and target nucleus. Comparisons are made with prior LADS results on H-2, He-3, and He-4. In addition, results are presented for the detected multiplicities of emitted protons, neutrons, and deuterons following pion absorption. [S0556-2813(99)06610-8].

Results from a 4 pi solid angle measurement of the inclusive reaction (pi(+), pi(0)) on H-2, He-3, and He-4 at incident pion energies of Tpi+ = 70, 118, 162, 239, and 330 MeV are presented. The single charge exchange total cross sections were determined, and are compared to previous results and simple models of pi-few-nucleon interactions. On the helium isotopes a strong damping of the cross sections in the Delta(1232) energy region is observed. Total cross sections of the breakup reaction pi(+) + H-2 --> pi(+) pn are also given. [S0556-2813(99)00708-6].

Results from a measurement of the reaction H-2(pi(+),pp)pi(o) at Tpi+ = 239 MeV are presented. Because two protons were detected, the measurement was sensitive to events which involved both nucleons in a substantial way. Differential distributions are compared to the predictions of simple models. An integrated cross section for SCX (single charge exchange), with the emission of two protons each with kinetic energy >25 MeV, is presented. [S0556-2813(98)06112-3].

Multinucleon absorption cross sections have been measured in 3He and 4He with three unbound, energetic nucleons in the final state. The ratios of the different 3NA cross sections are difficult to understand in terms of only two-step processes. Similar difficulties appear in the decomposition of the 3NA yield into mechanisms. Although a part of this yield is identified to be due to two-step processes, a large fraction is well described by a simple phase space model. (AIP)

Results from a 4 pi solid angle measurement of the reactions He-3(pi(+),ppp) and He-4(pi(+),ppp)n at incident pion energies of Tpi+ = 70, 118, 162, 239, and 330 MeV are presented. For He-3 the total absorption cross sections and their decomposition into two-proton and three-proton components are evaluated; for He-4 the three-proton absorption cross sections are given. The differential distributions of the three-proton multinucleon absorption mode of both nuclei are analyzed and compared to each other by making use of a complete set of variables. The data are investigated for signatures of initial and final state interactions: it is found that more than half of the three-proton yield cannot be accounted for by cascade mechanisms. The remaining strength shows dependence on the incident pion angular momentum, but also structures that are not explained by simple semiclassical models.

A Large Acceptance Detector System (LADS) has been designed and built at the Paul Scherrer Institute to study multiparticle final states following pion-nucleus absorption. It consists of a 28-sector cylinder of plastic scintillators of 1.6 m active length and 1.4 m diameter, two cylindrical wire chambers, and two 14-sector plastic scintillator end-caps which close each end. The nearly 4π solid angle coverage of this detector minimizes uncertainties associated with extrapolations over unmeasured regions of phase space. The design and the performance of the LADS detector are presented.

The absorption of 239 MeV positive pions on $^{3}\mathrm{He}$, $^{4}\mathrm{He}$, N, and Ar is studied. A strong enhancement is seen in the differential cross sections in the kinematic region associated with free $\ensuremath{\pi}\ensuremath{-}p$ scattering. This enhancement is interpreted as a clear signature of initial state interactions in the absorption of pions on nuclei.

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.

Inclusive measurements of pion double-charge--exchange in 3He[l] and 4He[2] in the A(1232) resonance region suggest the dominance of a two-step sequentim single-charge-exchange mechanism involving quasi-free nucleons. To investigate this reaction mechanism, we have observed protons in coincidence with the outgoing pion in r + + 4He --* ~r+ 4p at T~+ = 240 MeV. Pions were detected in a magnetic spectrometer at 0~= 32 ~ while protons were detected in a close--packed array of plastic scintillator telescopes covering 0 v = 67.5* 157.5 ~ on the same side of the beam as the spectrometer, and 8p = 22.5* 157.5 ~ on the opposite side. We will present preliminary results for the distributions in energy and angle of the coincident protons. Two-nucleon correlations have long been a subject of study in nuclear physics, from the point of view of medium modifications to the NN interaction as well as nuclear structure issues. Two-nucleon processes play a large role in the absorption of photons[3] and pions[4, 5, 6] by nuclei. The contributions of these multi-nucleon mechanisms are in many cases difficult to study directly, since these mechanisms are by nature second order and may be obscured by first order processes. Pion-induced double charge exchange (DCX), however, must proceed via interactions with at least two nucleons (since two nucleons must change charge during the reaction) and thus affords the study of two nucleon processes directly. Over the last decade, the cross section for inclusive DCX has been systematically studied over a range of target mass and beam energy, with emphasis on the energy region near the A(1232) resonance[l, 2, 7, 8, 9]. Several distinct features of these data are worthy of mention. The total cross section for DCX as a function of A obeys a scaling law (a function only of A and Z) based on competition between DCX and other channels[9]. Only the helium isotopes differ significantly from this scaling behavior. Despite this apparent universality in the total cross section, there is a distinct change in the appearance of 542 9 1995 American Institute of Physics S. F. Pate et al. 543

Measurements have been made of pi(+) absorption on He-3 at T-pi+ = 118, 162, and 239 MeV using the Large Acceptance Detector System. The nearly 4 pi solid angle coverage of this detector minimizes uncertainties associated with extrapolations over unmeasured regions of phase space. The total absorption cross section is reported. In addition, the total cross section is divided into components in which only two or all three nucleons play a significant role in the process. These are the first direct measurements of the total and three nucleon absorption cross sections.

The relative photodetachment cross section for decay into the H(N = 2) channel by the 1P-degrees shape resonance in H- was measured, as well as that for decay into all channels. The branching ratio sigma(N = 2)/sigma(total) was computed for a series of energies between 10.95 and 11.3 eV after normalizing the cross sections to theoretical peak amplitudes. The maximum branching ratio (almost-equal-to 0.8) appears at an energy about 20 meV higher than the central energy of the resonance. Results are compared with recent theoretical calculations.

A systematic study of inclusive pion double charge exchange is reported for nuclei between A=16 and A=208. The doubly differential cross sections for the A(${\mathrm{\ensuremath{\pi}}}^{+}$,${\mathrm{\ensuremath{\pi}}}^{\mathrm{\ensuremath{-}}}$) and A(${\mathrm{\ensuremath{\pi}}}^{\mathrm{\ensuremath{-}}}$,${\mathrm{\ensuremath{\pi}}}^{+}$) reactions in $^{16}\mathrm{O}$, $^{40}\mathrm{Ca}$, $^{103}\mathrm{Rh}$, and $^{208}\mathrm{Pb}$ were measured at incident energies between 120 and 270 MeV at three to five outgoing pion angles. Pion spectra were measured over the energy range from 10 MeV to the kinematic limit for double charge exchange. Integrating these spectra over outgoing pion energy generated angular distributions, and subsequently integrating these angular distributions generated total reaction cross sections. The shapes of the measured spectra are compared with those determined by the distribution of events in four-body phase space and with those predicted by a model in which the pion undergoes two sequential single charge exchanges on nucleons in a Fermi gas. The cross sections are compared with predictions of cascade calculations in which more than two interactions, including pion absorption and scattering without charge exchange, may occur.

At the peak of the delta resonance, the pion is the most strongly interacting particle we know. It follows that interactions between pions and nuclei at these energies all tend to involve multiple scattering to some degree. Multiple scattering includes different kinds of processes. In its simplest forms, it involves a sequence of interactions with single nucleons that take place as the pion bumbles its way through a nucleus. Much more interesting is the possibility that there are interactions that in some sense directly involve several nucleons at once. If there are such interactions, can we distinguish between them and the simpler, more prosaic forms of multiple scattering Experimental study of pion-nucleus reaction mechanisms has produced intriguing suggestions that truly multi-nucleon interactions do occur. They are the subject of this paper. To develop this subject I will focus on reactions that cannot happen at all without the essential participation of two or more nucleons.

Total reaction cross sections for inclusive pion double charge exchange have been obtained at incident pion energies of 180 and 240 MeV for $^{4}\mathrm{He}$, $^{6.7}\mathrm{Li}$, $^{9}\mathrm{Be}$, $^{12}\mathrm{C}$, $^{16}\mathrm{O}$, $^{40}\mathrm{Ca}$, $^{103}\mathrm{Rh}$, and $^{208}\mathrm{Pb}$. An explanation of the dependence of these cross sections upon the mass and charge of the target nucleus and the charge of the incident pion is suggested in terms of a phenomenological model in which successive single-charge-exchange processes compete with more probable processes such as quasifree scattering.

New experimental results on muon-catalyzed d-t fusion are reported. Unexpected target-density effects have been discovered both in dt\ensuremath{\mu} molecular formation in t\ensuremath{\mu}+${\mathrm{D}}_{2}$ collisions and in the effective sticking probability. The dt\ensuremath{\mu} formation rate is significantly enhanced at high density, presumably because of a strong three-body contribution. The origin of the observed reduction of the sticking probability at high density remains unclear. Both effects increase the number of fusions per muon that can be achieved; 150\ifmmode\pm\else\textpm\fi{}4(stat.)\ifmmode\pm\else\textpm\fi{}20(syst.) fusions per muon have been observed.