Results are presented on the measurement of 200-GeV/c ${\pi }^{-}p$ elastic scattering at $-t$ from 0.8 to 11 ${(\mathrm{G}\mathrm{e}\mathrm{V}/\mathit{c})}^2$. As $-t$ is increased, $\frac{d\sigma }{\mathrm{dt}}$ falls by ∼6 decades to a prominent dip at 4 ${(\mathrm{G}\mathrm{e}\mathrm{V}/\mathit{c})}^2$, followed by a second maximum and than a slow decrease with increasing $-t$.

Inclusive $e^{+}{e}^{-}$ production in 17-GeV/c ${\pi }^{-}p$ collisions has been measured. An excess of $e^{+}{e}^{-}$ pairs over those from known sources for $0.1<~m_{\mathrm{ee}}<~0.6$ GeV and $x<\mathrm{}0.5\mathrm{}$ was found. No evidence is found for enhancements in specific final states involving electrons and photons or charged particles. The photon multiplicity associated with these pairs is measured.

Measurements of the production cross sections at large values of transverse momentum ($P_T$) for ${\pi }^{\pm }$, $K^{\pm }$, $p$, and $\overline{p}$ in 200- and 300-GeV/c ${\pi }^{-}p$ collisions are presented. The dependences of the cross section on $P_T$, the transverse scaling variable $x_T=\frac{2P_T}{\sqrt{s}}$, and the production angle are discussed. The cross sections are compared with those from $\mathrm{pp}$ collisions and with theoretical predictions. While the ${\pi }^{-}p$ and $\mathrm{pp}$ data are quite different, both agree qualitatively with hard-scattering calculations.

A search for $\tau$ lepton production in a sample of 1241 neutrino interactions with use of a hybrid emulsion spectrometer in the Fermilab wide-band neutrino beam is reported. No such events are seen and an upper limit to the ${\nu }_{\mu }-\tau$ coupling of 0.63% (90% C.L.) is set. For ${\nu }_{\mu }-{\nu }_{\tau }$ oscillations this sets a limit of $|{m_{{\nu }_{\mu }}}^2-{m_{{\nu }_{\tau }}}^2|<\mathrm{}3.0\mathrm{}$ ${\mathrm{eV}}^2$ (90% C.L.) for maximum mixing.

An SU(5) ⊗ SU(5) model is presented which solves the strong $\mathrm{CP}$ and axion problems in a natural manner, and which reduces to three families with SU(3) ⊗ SU(2) ⊗ U(1) symmetry at low energies. The model also provides Majorana neutrino masses in an observable range.

The $n$ decay of the giant resonance region between 8.5 and 12.5 MeV in ${}_{}{}^{208}{}_{}{}^{}\mathrm{Pb}$ has been studied in an ($\alpha$,${\alpha }^{\prime }n$) coincidence experiment at $E_x=104\mathrm{}$ MeV. The energy resolution was sufficient to observe the decay into the different states of ${}_{}{}^{207}{}_{}{}^{}\mathrm{Pb}$ separately. The strong population of the ${\frac{13}{2}}^{+}$ state shows that large contributions of the total strength excited in the giant quadrupole resonance region have multipolarities ≥ 4. The $\alpha$ spectra coincident with the decay neutrons show fine structures in the giant quadrupole resonance region of ${}_{}{}^{208}{}_{}{}^{}\mathrm{Pb}$.

Measurements of the elastic scattering of ${}_{}{}^{12}{}_{}{}^{}\mathrm{C}$ on ${}_{}{}^{12}{}_{}{}^{}\mathrm{C}$ at ten laboratory energies between 120 and 290 MeV were made to determine the reaction cross section ${\sigma }_R$ and the total nuclear cross section ${\sigma }_T$. Analysis of these data and existing data at lower energies shows that ${\sigma }_R$ reaches a maximum at around 200 MeV in the laboratory whereas ${\sigma }_T$ shows little variation with bombarding energy.

Isomeric transitions, of $E3\mathrm{}$, $M2\mathrm{}$, and $\frac{E1\mathrm{}}{M2\mathrm{}}$ character, depopulating $\tilde{\pi }{h}_{\frac{9}{2}}\otimes \tilde{\nu }{i}_{\frac{13}{2}}$ bands in ${}_{}{}^{186,188,190}{}_{}{}^{}\mathrm{T}1\mathrm{}$, respectively, have been found with use of $\gamma$-ray spectroscopic techniques. The changes in multipolarity reflect successive changes in the bandhead spin values corresponding to level crossings within the $\tilde{\pi }{h}_{\frac{9}{2}}\otimes \tilde{\nu }{i}_{\frac{13}{2}}$ system. These crossings were predicted in the framework of the two-quasiparticle plus rotor model and interpreted as a direct consequence of the depletion of the $i_{\frac{13}{2}}$ shell.

It is pointed out that in a general class of gauge models, there exist new contributions to neutrinoless double $\beta$ transitions, that do not involve a Majorana neutrino but the decay of a doubly charged Higgs boson to electrons. Explicit calculations for the case ${}_{}{}^{48}{}_{}{}^{}\mathrm{Ca}$→${}_{}{}^{48}{}_{}{}^{}\mathrm{Ti}$ indicate that for reasonable choice of parameters, this new contribution may dominate over that involving light or heavy Majorana neutrinos.

Critical conditions for instability in the isovector unnatural-parity channel in nuclei are investigated in terms of the random-phase approximation. The particle-hole residual interaction consists of the $\pi$ and $\rho$ ($2\pi$) meson-exchange attractive components in addition to the short-range repulsive Landau term. It is found that the nonresonant part of the iterated two-pion exchange moves the characteristics of the nuclear phase transition closer to those for rho-meson condensation. This mode may dominate at high density.

The branching ratio for pair emission for the ground-state transition from the $J^{\pi }={\frac{1}{2}}^{-}$, 2789-keV level of ${}_{}{}^{21}{}_{}{}^{}\mathrm{Ne}$ has been measured relative to the branching ratio for the transition from the $J^{\pi }={\frac{1}{2}}^{+}$, 2796-keV level. The result indicates that the former transition is predominantly $E1\mathrm{}$ in multipolarity [$\delta \left(\frac{M2\mathrm{}}{E1\mathrm{}}\right)<\mathrm{}0.6\mathrm{}$] and therefore that measurements of $\gamma$-ray circular polarization for this transition are very sensitive to parity mixing in the spin-½ doublet.

A new method of calculating the one-loop self-energy contribution to the Lamb shift is presented. The technique relies on treating as perturbations certain terms in the equation for the Dirac Coulomb Green's function, in the absence of which the equation can be solved in terms of a simple integral representation. A new result for the $1S$ Lamb shift is obtained and compared with previous calculations.

A method is proposed to decelerate neutral atoms in a thermal beam for purposes of precision spectroscopy, atomic clocks, or loading into shallow traps. Rydberg atoms exhibit large electric dipole moments suggesting that inhomogenous fields can exert forces on them. The resultant change in kinetic energy is equal to the Stark shift of the Rydberg state. It has been found that a series of tandem electrode pairs can reduce atomic velocities by a factor of 10. Various limitations and constraints are discussed.

The charge-density distribution in multielectron bubbles is calculated with use of a generalized Thomas-Fermi method. The exchange and correlation effects are found crucial; these result in a density profile much narrower than that obtained by simple estimates. The observed stability of the bubbles is explained in terms of the nonlinear coupling between the radial and angular modes of oscillation. Above a threshold oscillation amplitude a period-doubling instability sets in.

The isochoric temperature derivative of the neutron liquid structure factor is shown to be more sensitive to the variation of O-D and D-D distances than the measurable structure factor itself. Careful determinations of this quantity have been made, with measurements on either side of the density maximum at 11.2°C. These results are compared to the computer data of Impey, Klein, and McDonald; significant discrepancies are found which highlight the differences between the computer model and real water.

Measurements of the temperature dependence of the longitudinal sound velocity and attenuation in solid ${}_{}{}^3{}_{}{}^{}\mathrm{He}$ show a frequency-dependent anomaly due to interactions between the ultrasonic waves and dislocations. By application of high-amplitude sound pulses at temperatures below about 0.4 K, the attenuation can be greatly reduced and the velocity changed. Upon warming, the velocity and attenuation return to their initial values by a thermally activated process whose activation energy appears to be that of mobile vacancies.

The cubic-to-tetragonal structural phase transition of KMn${\mathrm{F}}_3$ has been investigated by measurement of the phase diagrams with uniaxial pressure along [110] and [100]. Results indicate the existence of a tricritical point near $p=0.45\mathrm{}$ kbar with $p$ along [110], and two new consecutive tricritical points near $p=0.25\mathrm{}$ kbar with $p$ along [100].

A droplet theory of Ising systems is constructed, valid near space dimension $d=1\mathrm{}$, and based on a model incorporating only the configurational energy of surface tension. The droplet distribution function is calculated explicitly, with use of renormalization-group methods to control the droplet-shape fluctuation effects essential to its scaling form. Universal quantities are calculated; those (e.g., $\beta$) which reflect the droplet concentration have an essential singularity in $\epsilon =d-1\mathrm{}$.

A phase transition starting at 84 K and indicated by a marked color change of the sample has been found in the organic insulator tetrathiafulvalene-chloranil. Optical, infrared, and Raman measurements indicate that this is a reversible transition from a nominally neutral (N) solid to a nominally ionic (I) salt. Surprisingly, the N-I transition is not first order, but occurs over a broad temperature region (∼30 K), in which there is a coexistence of N and I molecules.

A model is presented describing how an organic solid transforms between a neutral and an ionic ground state. Whereas for a one-dimensional stack this transition is expected to be first order, the Coulomb interactions between stacks can cause the phase transition to occur over a broad temperature range by stabilizing an intermediate region in between the neutral and ionic phases, in which both neutral and ionized species coexist, as observed in tetrathiafulvalene-chloranil.