
^{3}He (1987TI07)Ground State:
μ = 2.127624 ± 0.0000011 nm, M  A = 14.93132 ± 0.00003 MeV. General properties of the ground state of the A = 3 system are under ^{3}H above. The wave function is predominantly Sstate (≈ 90%) with S'state (1  2%) and Dstate (≈ 9%) admixtures (1975FI08, 1980PA12, 1984CI05, 1984CI09). For ^{3}He the measured magnetic moment is μ = 2.127624 ± 0.0000011 nm (1978LEZA, 1978NE12). Calculations which include both impulse and pion exchange contributions (1985TO21) are adequate to explain magnetic moments of ^{3}He and ^{3}H. Results obtained with a sixquark bag model are also compatible with the data according to (1986BH05) which includes a discussion of other published calculations. Exchange current contributions to the ^{3}He magnetic properties are calculated by (1983ST11). See also reaction 9 (a). The rms charge and magnetic radii of ^{3}He determined from electron scattering (see reaction 9 (a)) are γ^{c}_{rms} = 1.976 ± 0.015 fm and γ^{m}_{rms} = 1.99 ± 0.06 fm. See the discussion under ^{3}H for comparisons with theory. The binding energy of ^{3}He is 7.718109 ± 0.000010 MeV (1985WA02). Calculations of the binding energy tend to underestimate the experimental results as is also true for ^{3}H. See references under ^{3}H above. Many calculations have been performed in an attempt to account for the ^{3}H  ^{3}He binding energy differences (≈ 0.76 MeV) and various methods are reviewed in (1986FRZU). Most twobody force calculations with realistic forces underestimate this difference, giving ≈ 0.64 MeV for the Coulomb energy of ^{3}He and the problem is not resolved by threebody force calculations (1986GIZS). Charge and magnetic form factors for ^{3}He have been determined in electron scattering experiments (see reaction 9 (a)). The ^{3}He magnetic form factor has a diffraction minimum at a higher value of q than predicted by impulse approximation calculations (1984FR16) but the isobar model of (1983ST11) satisfactorily accounts for the difference. The ^{3}He charge form factor has a diffraction minimum at q^{2} ≈ 11 fm^{2} and a very large secondary maximum. The charge densities derived from the data indicate a deep "hole" near the origin which remains a problem for theorists (1986FRZU). Impulse approximation calculations significantly underestimate the secondary maximum, and threebody force calculations have improved the predictions but have not solved the problem (1986GIZS). References not mentioned above or under the ^{3}H discussion, relating to the ^{3}H  ^{3}He binding energy difference and other charge asymmetry effects include ( 1975GI06, 1975ZA03, 1976ZA04, 1978BR03, 1978FR07, 1978FR09, 1978FR17, 1978SI15, 1980PA13, 1981SA04, 1981YA03, 1982BA41, 1982YA01, 1983SA17, 1983SH10, 1985BA24, 1985BA70, 1985BE57, 1985DE56, 1986BE39, 1986CH16, 1986FR23, 1986OS04, 1986SA02, 1986SC03, 1987DR04).
Experimental data on ^{2}H(p, γ)^{3}He and the inverse reaction prior to 1979 were reviewed in (1979BE2C, 1979TO2A). For the ^{3}He excitation energy region E_{x} < 30 MeV the existing data and theory are reviewd in the experimental papers of (1974MA18, 1979SK01). These papers also described attempts to resolved the discrepancies in the published cross section which are subtantial, e.g. at E_{x} near 12 MeV the measured differential cross sections for ^{3}He(γ, p)^{2}H range from 90 to 120 μb/sr. Measurements of (1979SK01) at E_{x} = 10.83 MeV gave 117 ± 11 μb/sr and 1.07 ± 0.11 mb respectively for the detailed balanced differential (θ_{lab} = 90°) and total cross section. The Tmatrix analysis of the differential cross sections and analyzing power data of (1979SK01) gave an E2 cross section of (12 ± 5)% of the total at E_{x} = 10.83 MeV which is ≈ 10 times the theoretical estimates of (1975FI08, 1981AU02). Measurements and calculations by (1983KI11) demonstrated the sensitivity of the extracted a_{2} angular distribution coefficients in the region E_{p} = 6.5  16 MeV to the inclusion of Dstate components in the ^{3}He wave function, and are consistent with 5  9% Dstate probabilities. This effect was incorporated in the analysis (1984KI06) of an improved data set (relative to (1979SK01)), and the results are consistent with an E2 strength of (2 ± 3)% of the total cross section at E_{x} = 10.8 MeV and an s = 3/2 (E1) strength of (3 ± 5)%, which could arise from the Dstate admixture in the ^{3}He ground state. This lower E2 strength is consistent with the electrodisintegration results of (1983SK01). Measurements of the vector analyzing power in ^{2}H(p, γ)^{3}He and ^{1}H(d, γ)^{3}He at very low excitation energies (E_{x} = 6 MeV) (1984KI14) indicated the presence of s = 3/2 capture strength and the results are consistent with an M1 strength amounting to 1  8% of the total cross section. The tensor analyzing power T_{20} in ^{1}H(d, γ) was measured (1985VE02) and compared with an effective twobody direct capture calculation which used the Faddeevgenerated ground state wave functions of (1984GI01). The results were in good qualitative agreement although the calculated T_{20} was about 20% too small when ^{3}He wave functions having 5  9% Dstate probabilities were assumed. The data were also analyzed to extract a D/S asymptotic ratio of 0.035 ± 0.01 in the ^{3}He wave function. This results is consistent with the range (0.038 < η < 0.050) calculated (1984GI01) for ^{3}He Dstate probabilities between 5 and 9%. The tensor analyzing power A_{yy} was measured (1985JO05, 1986JO06) and compared with a full Faddeev calculation using the Reid softcore interaction as a check of the ^{3}He Dstate component. The comparison shows that the calculated Dstate wave function is about 20% too large (in the 2  5 fm region). In the intermediate energy region, measurements at E_{p} = 337 and 576 MeV were made (1976HE2A) to compare with measurements of the inverse reaction for a test of time reversal invariance. The results were consistent with no violation but the conclution was a matter of controversy (1980NE03) because of discrepancies in existing data for ^{3}He(γ, p)^{2}H. The situation is reviewed in (1981FA2B) and most recently in (1985BR23). Additional measurements were made by (1980NE03, 1982AB09, 1984CA23, 1985CA42, 1987PI01) with none reporting any evidence for timereversal invariance violation. See also (1982BR12, 1983SO10). The most recent measurements were those of (1987PI01) and (1985BR23) which report final values which supercede the preliminary data of (1980NE03) and which are in agreement with those of (1984CA23). The data of (1984CA23, 1985CA42) were compared with several theoretical calculations showing that inclusion of mesonexchange current contributions are important in reproducing the cross sections, but the analyzing powers measured at E_{p} = 500 MeV were not explained by microscopic models. Comparison of the data of (1985BR23) wtih calculations showed that the contribution of delta effects is undramatic but must be included. The recent measurements of σ(θ) and A(θ) for E_{p} = 99.1, 150.3, 200.7 MeV (1987PI01) were well accounted for by a simple "quasideutero" model. See also (1984ME13). Theoretical work on the ^{2}H(p, γ)^{3}He reaction and its inverse has focused in large part on the effects on the cross sections and other observables of Dstate components in the ^{3}He bound state wave function. In (1973HE20) ^{3}He wave functions generated from Faddeev equations with separable Yamaguchi interactions were used in calculating cross sections, and it was concluded that the isotropic part of the cross section was unlikely to yield information on Dstate components in ^{3}He and ^{2}H. Realistic bound state wave functions obtained with NN interactions given by the Reid softcore potential were used in (1977CR01) in calculations over a wide energy region from threshold to 600 MeV. The results indicated substantial contributions of Dstate to both total and differential cross sections, but gave pronounced structure in the cross sections in disagreement with experiment. The calculations of (1981AU02) used the same physical input as (1977CR01) but used different methods and found no distinct signature of Dstate components in ^{3}He and ^{2}H for E_{γ} < 35 MeV. On the other hand the work of (1983KI11) mentioned above demonstrated the sensitivity of the differential cross section to Dstate effefcts. In addition, calculations reported in (1984AR07) used the Sasakawa wave function for ^{3}He and found that the tensor analyzing powers for the ^{1}H(d, γ)^{3}He are very sensitive to Dstate components in ^{3}He. The data were shown to be consistent with an asymptotic D to Sstate ratio of η = 0.029. At intermediate energies dispersion methods were used to calculate angular distributions for the ^{2}H(p, γ) reaction (1979PR12). Comparisons with data 52.5, 100, and 140 MeV gave reasonable agreement with angular and energy dependence.
Reviews of threebody breakup reactions are given in (1978SU2A) which includes 3 and 4body elastic scattering and breakup and emphasizes the precision of the measurements, and in (1978KU13) which contains an extensive discussion of threeparticle kinematics, experimental techniques, and the basic theoretical equations of threeparticle scattering as well as a review of existing data and theoretical work. See also (1978SL2A) which includes the ^{2}H(p, n)2p reaction in discussion of fewnucleon experiments and theory in general. The ^{2}H(p, n) excitation curve at 0° rises from threshold to ≈ 50 mb/sr at E_{p} = 7 MeV (1975FI08). The total cross section for breakup rises from threshold to ≈ 180 mb at E_{p} = 13 MeV and drops off gradually to ≈ 130 mb at E_{p} = 25 MeV. The particle spectra contain structure corresponding to pn and pp quasifree scattering (QFS) and to pn and pp final state interactions (FSI). The structure is similar to that of ^{2}H(n, p)2n. As noted in (1975FI08), the ratio of the np to pp QFS peaks varied between 2 and 3 depending on E_{p} in the region between 4.5  60 MeV and is larger than that of free nucleonnucleon cross sections especially at low energies. This topic is discussed in (1978KU13 and 1981BL09). Empirical rules for the energy behavior of this ratio are proposed in 1977FU05). Good agreement between the measured ratio with estimates based on the impulse approximation is obtained by (1979JA20). Agreement of pp and np QFS with the spectator model is obtained (1975WI29, 1976FE05) for spectator momenta up to ≈ 200 MeV/c, significant discrepancies are found at higher momenta. The measurements of (1982SH07) indicate that polarization effects on the spectator nucleon in pp QFS is quite different from that of pn QFS. It has been established (1975FI08, 1978KU13) that deuteron breakup measurements with complete kinematics allow the determination of the twonucleon scattering parameters with good accuracy and that the np and pp scattering lengths agree with the corresponding free scattering lengths. See also the section of this review on ^{2}H(n, p)2n and the references cited there. Comparisons with Faddeev calculations show that both the structure and absolute value of the cross sections are described very well (1975FI08) even if the twoparticle interaction is taken as a separable potential that reproduces the NN data only at low energies (1978KU13). A major aim of pd and nd breakup studies is to obtain information about the offshell behavior of nuclear forces and to explore the role of threebody forces. Much theoretical efforts has gone into attempting to separate the two. Numerical solutions of the integral equations of scattering theory were used by (1983ZA04) to study the effect of the form of the twoparticle interaction on the breakup amplitude. The authors of (1981SL02) studied pp correlation spectra at very low kinetic energy with Faddeev calculations which included Coulomb corrections and discuss the possible role of threebody forces. Calculations of cross sections and polarization observables in the approximation of pole and triangular diagrams reported in (1980GO03) gave satisfactory agreement with experiment for E_{p} = 200  340 MeV. Backward inelastic pd scattering for E_{p} = ≈ 1 GeV was calculated in (1978AM06) and attributed primarily to "triangle" diagrams with singlepion exchange. Observed asymmetry in the angle between the proton momentum transfer and the direction of the spectator nucleon was explored with the separable potential model (1977AL04). Calculations with several separable potentials by (1976HA38) were done to explore the shortrange behavior of the nuclear force and suggest that FSI angular distributions between 20 and 50 MeV would be useful. See also (1974ST19) for potential effects. Offshell and multiple scattering effects were explored (1975LH02) in analysis of E_{p} = 156 MeV data. Realistic potentials were used (1975IS06) in analysis of QFS data for E_{p} between 60 and 160 MeV. See also (1975DU13). Differences in pp and pn QFS were stuied (1975HA03) with an energydependentcore model. A general discussion and review of quasifree processes in fewbody systems is given in (1974SL04). See also (1974HA07, 1974HA36, 1974ME06).
A phaseshift analysis of pd elastic scattering based on measurements of differential cross section and proton and deuteron analyzing powers for energies below the breakup threshold was performed (1983HU08), and S and jsplit Pphases including the channelspin mixing, unsplit D and Fphases and SD tensor coupling were determined. A phaseshift analysis at the threebody threshold using Faddeev equations in configuration space was reported in (1980LA19). Doublet phases were extracted also (1975CH20), and a phaseshift analysis of combined crosssection and spincorrelation data was performed. An analysis of σ(total), σ(θ), and spin correlationparameter data for E_{p} = 1.1  1.7 GeV was reported in (1980HA50). Theoretical calculations of phase shifts have been done in a Faddeev formalism using different rankone separable interactions (1975CH20), or using Swave NN potentials in which the Coulomb interaction was incorporated (1983HU08). Doublet and quartet phase shifts for pd and nd scattering have been calculated for nucleon energies between 2 and 10 MeV using a method based on modified Faddeev differential equations in which the Coulomb interaction is included, and they are found to agree with experiment (1983KU08). Theoretical phase shifts for nd and pd scatterings in the ^{4}P, ^{2}P, ^{4}D and ^{2}D states near threshold were calculated (1977EY01) with an offshell model with Coulomb corrections taken into account approximately. The effective range functions calculated from this model (1977EY01) and from a simple onshell partial wave dispersion model (1977EY02) agree well with each other. The quartet scattering length and effective range reported in (1977EY01) are ^{4}a_{pd} = 10.9 fm and ^{4}γ_{pd} = 1.3 fm respectively; while the corresponding quantities obtained in (1977EY02) are ^{4}a_{pd} = 11 fm, ^{4}γ_{pd} = 1.44 fm. The doublet scattering length is ^{2}a_{pd} = 1.8 fm, while the parameters of the effective range expansion for the P and Dstates are listed in a table in (1977EY01). In (1976TI02) an analytic expression is derived for the difference in nd and pd quartet scattering lengths and a value ^{4}a_{pd} = 10.4 fm is calculated. In (1983FR21) a value of ^{4}a_{pd} = 14 fm and ^{2}a_{pd} ≈ 0 fm is calculated using a configuration space formalism of the Faddeev equations including the Coulomb interaction. This paper emphasizes the need for new lowenergy pd data. Differential cross sections and polarization data around 800 MeV incident proton energies have been analyzed by Glauber theory (1980WI07) and by noneikonal approximations to Glauber theory and multiple scattering theory (1979BL08, 1981BL13, 1983IR03). Noneikonal corrections are seen to be important in explaining tensor asymmetries (1980AL2C) and proton elastic scattering from polarized deuterons (1975GU17, 1979GU14). Data in the Coulomb interference region around 600 MeV are reproduced by taking into account spin effects (1981GA15) and the virtualdeuteron effect (1976GA36) in the Glauber model. Low energy (E_{p} ≈ 10 MeV) data have been analyzed in the Faddeev formalism (1975CH19, 1978GR04, 1981SP05, 1982SP03, 1983SA05). The analysis of (1981SP05) and the threebody calculation of (1981KO39) show that the data are sensitive to the Swave part of the deuteron wave function, and there is evidence for NN offshell effects in nucleondeuteron scattering. The difference in the ^{3}S  ^{3}D interactions, which do not appear in the lowenergy pd scattering clearly, appear distinctly at 65 MeV (1982KO34). One set of lowenergy data has been analyzed in terms of Legendre polynomials (1983GR05). Coulomb effects in pd scattering have been taken into account in the framework of multiple scattering theory by (1975FR13) and in a threebody formalism by (1976AL13, 1976TI02). See also (1983BL15). In a first attempt to include an approximation for the Coulomb effects in Faddeev calculations, the work of (1982DO07) took into account the influence of the asymptotic Coulomb phase shifts in pd and dp scattering. See Also (1981HA30). Calculations performed in (1981ZA06) at a few energies between 5 and 15 MeV predict differences between the nd and pd analyzing powers. Except for a small angle shift this was borne out by experiment (1982TO06). The effect of Coulomb distortion on the proton analyzing powers in elastic pd scattering is calculated by an effective twobody approximation that includes nd onshell information only, in (1983ZA01). The agreement with the measured analyzing power at 10 MeV is good but only fair at 14 MeV. A rigorous approach for solving the threebody Coulomb problem in configuration space based on Faddeev differential equations is presented in (1982PO08, 1983KU08). Differential cross sections at 2 MeV and 10 MeV agree well with experiment. Backward pd elastic scattering in the energy range 0.3  2 GeV has been extensively studied. See (1982IG2A) for a review. See also the study of (1974NO2A) who concluded no convincing evidence for N* isoscalar exchange or pionnucleon exchange beyond the usual nucleon transfer. A phenomenological analysis made in (1974DU05) suggests a mechanism in addition to onenucleon exchange. The cross sections are roughly reproduced by a calculation in the framework of the pole mechanism (1975KA27). Rescattering corrections to the singlenucleon exchange model were found to be important (1975LE21). Single scattering and nexchange are shown to account for largeq^{2} pd data (1976GU2A, 1979GU14, 1979GU2B). The deuteron charge form factor is predicted from the extracted twobody form factor. In (1976TE2A) a doubletriangle diagram is considered, while in (1977KO48, 1979KO2A) it is found sufficient to take into account rescattering with the deltaisobar in the intermediate and onenucleon exchange. Calculations based on the model of resonance onepion exchange produce cross sections which agree with experiment but depend strongly on the deuteron wace function (1977SM04). Absence of a peak in the cross section for energies greater than 1 GeV is predicted in (1979KO2A) where lightfrontdynamics is applied to describe the scattering. In (1974SH2B, 1977SH17, 1980JE03) calculations are performed in the light of the KermanKisslinger model using a generalized baryontransfer mechanism on the assumption that N*s exist in the deuteron and that the backward peak is caused by their exchange. The tensor polarization of the deuteron was calculated on the basis of a triangle diagram without free parameters and found to be small in agreement with experiment (1980VE07, 1981VE15). The contribution of the intermediate Δ(1236) resonance is found to dominate the cross section in the 0.3 to 1 GeV region in (1980TO10) where a twoloop diagram is evaluated. A double diagram with intermediate Δ(1232) excitation is used in (1981AN03) to reproduce cross sections at about 600 MeV. In (1981KO09, 1981KO17) it is found necessary to include tribaryon resonances in addition to quasiresonant contributions from the delta isobar, onenucleon exchange and nucleondeuteron single scattering to explain the data. Elastic pd backward scattering in the energy range 0.6  2.7 GeV has been measured and discussed in terms of onenucleon exchange and onepion exchange mechanisms (1982BE30). The experimental plateau in the 180° excitation function for energies > 1 GeV could be explained as excitation of the Δ(1950) in the intermediate state. Experimental data on the sensitivity to the proton polarization and to the deuteron alignment are described in (1981KA45, 1981KA29). In 1980GU16) it is shown that it is possible to explain backward pd scattering at intermediate energies in the framework of multiple scattering theory by searching for the "optimal" approximation for the formal exact solution of the problem. The analysis also permits an extraction of the deuteron twobody form factor for values of q^{2} which far exceed those measured in e, d elastic data. The accuracy of the "optimal" approximation is studied for a wide range of momentum transfer in (1983LE16). Cross sections and tensor analyzing powers in dp elastic scattering data at E_{d} = 10 and 20 MeV were used in a pole extrapolation technique to determine a value of ρ_{D} = 0.027 ± 0.005 for the asymptotic D to Sratio of the deuteron wave function (1978AM2B). Data at 35 MeV and 45 MeV (1979CO12) gave a value ρ_{D} = 0.023 ± 0.0013, while an analysis by (1980GR06) of data at ten different deuteron energies between 5 and 45 MeV gave ρ_{D} = 0.0259 ± 0.0007. Note however that the method has been criticized in more recent work as subject to large systematic errors (1981CO2C, 1983LO03, 1984BE2B, 1985PU01). A different method of pole extrapolation was used in (1986HO07) to obtain a D/S ratio of 0.0270 with a statistical error of 2%. The dependence of ρ_{D} on the asymptotic Sstate amplitude A_{s} was studied and reported in (1982AL04). Elastic dp scattering data in conjunction with data on pp and np elastic scattering between 10  26 GeV have been used in (1973ZO05) to determine the deuteron form factor. The ratio of pd inelastic/elastic differential cross sections has been calculated using Glauber theory in (1977DU02). Polarization data at 1 GeV have been analyzed in terms of Glauber theory and the spinorbit protonnucleon amplitude has been determined in (1980AL09). Parity nonconserving effects in low energy pd scattering which manifest themselves as an asymmetry in the total pd cross section for longitudinally polarized protons have been examined in (1978HE2A, 1978HE2B, 1979DE27, 1983KL05). The calculations of (1979DE27) indicate an asymmetry of about 0.8 x 10^{7}.
Measurements on both reactions 6 (a) and 6 (b) were carried out in a recent experiment (1984BE08, 1984BE36) in which π^{0} photproduction on a ^{3}H target was observed for the first time. Bremsstrahlung radiation from a 500 MeV electron beam was used, and the ^{3}H and ^{3}He recoils were detected. The energyaveraged differential cross section in the Δ(1232) resonance region was obtained as a function of momentum transfer both for ^{3}H(γ, π^{0}) and ^{3}He(γ, π^{0}). The averaged cross section is characterized by: (i) A steep falloff for  t  < 5 fm^{2} characteristic of a coherent process. The cross section falls from ≈ 10 μb/sr at  t  = 3 fm^{2} to ≈ 1.5 μb/sr at  t  ≈ 5 fm^{2}; (ii) A change in slope at  t  ≈ 5 fm^{2} attributed to the rescattering of the photoproduced pions; (iii) Equal cross sections for ^{3}He(γ, π^{0})^{3}He and ^{3}H(γ, π^{0})^{3}H suggesting direct production on one nucleon without final state interactions. The results are compared with an uncorrected impulse approximation calculation, and the agreement is good for  t  < / = 5 fm^{2}. The experiment also shows that the charge exchange reaction ^{3}He(γ, π^{+})^{3}H is down by about an order of magnitude from the π^{0} production. A number of theoretical investigations of the (γ, π^{0}) reaction on ^{3}H and other fewbody targets near threshold have been carried out. For early work see (1975FI08). It is noted in (1976KO04) that whereas charged pions are produced mainly on the nuclear surface, neutral pions can be produced coherently, and π^{0} photoproduction is in principle sensitive to the entire nuclear matter distribution. In addition, the small π^{0} photoproduction cross section for a single nucleon (≈ an order of magnitude smaller than that of charged pions) suggests a mechanism for π^{0} photoproduction in nuclei whereby a charged pion photoproduced on one nucleon can undergo chargeexchange scattering on another nucleon. This twonucleon mechanism is found (1976KO04) to be important. For a discussion of these and other aspects of the (γ, π^{0}) process in ^{1}H, ^{2}H, ^{3}H, and ^{4}He see the review of (1979DE2A). Other calculations of threshold π^{0} photoproduction on ^{3}He and ^{3}H are reported in (1978BO13, 1982BE25). Threshold effects in π^{0} photoproduction on ^{2}H and ^{3}He are treated in (1979LA21). The case of π^{0} photoproduction by linearly polarized photons on ^{3}H and ^{3}He has been treated in the impulse approximation (1979GA18) for E_{γ} = 180  700 MeV, and the asymmetry in the angular distribution has been evaluated with the use of multipole amplitudes. Finally, an investigation reported in (1984DR07) included both a calculation of π^{0} photoproduction for the ^{3}H/^{3}He isodoublet in terms of nucleons, and a calculation of π^{0} photoproduction for nucleons in terms of constituent quarks.
Early theoretical work on π^{+} photoproduction on ^{3}He is reviewed in (1975FI08). More recently several impulseapproximation calculations (1973LA39, 1975LA12, 1980TI01, 1983BA12) for incident photon energies that included the first pionnucleus resonance have been carried out. An estimate of rescattering terms is made in (1973LA39), and in (1975LA12) the contribution of meson exchange effects is explored. Calculations in impulse approximation of pion photoproduction near threshold have been reported in (1976OCZZ, 1981TI02, 1981TO13, 1983BA12). See also the review of (1979DE2A). The effects of pion momentum, Fermi momentum, and delta resonance terms on the cross section near threshold were studied in (1980DR05). Realistic wave functions generated from Reid softcore potentials with the Faddeev equations were used in the claculations reported in (1981TI02, 1981TO13, 1983BA12), while in (1980TI01) account was taken of Fermi motion on the resonance structure. In (1984DR07) threshold photoproduction of pions is calculated for nucleons in terms of constituent quarks and for ^{3}H and ^{3}He, in terms of nucleons. A determination of the π  ^{3}He  ^{3}H coupling constant from threshold pion photoproduction data was made in (1979LE09), and discussions of various methods for determining the coupling constant are presented in (1980DU01, 1984KL02). The related process of electroproduction of pions from ^{3}He (reaction (b)) has been studied experimentally and theoretically. In the work reported in (1978SK01) triton recoil cross sections were measured at incident electron energies and triton recoil energies corresponding to excitation energies near 20 MeV above the pion threshold. Model calculations were reported in (1977AS07). Calculations to describe coincident cross sections for coherent pion electroproduction in the impulse approximation using realistic wave functions were reported in (1981TI01).
Many excellent reviews exist in the literature on the photodisintegration of ^{3}He and ^{3}H nuclei (1979TO2A, 1979BE2C, 1977CI2A, 1975WE2A, 1976GO2A, 1975FI08). Since theoretical calculations are often made simultaneously for both nuclei, the section on ^{3}H should be consulted as well. References to the earliest theoretical calculations on the photodisintegration of ^{3}He may be found in the compilation of (1975FI08). The twobody breakup reaction ^{3}He(γ, p)^{2}H has been investigated and reported in (1977CR01). Cross sections at intermediate energies (E_{γ} < 600 MeV) are calculated using realistic bound state wave functions obtained with the Reid softcore potential, but neglecting final state interactions. In (1981AU02) an independent calculation using the same physical input as (1977CR01) is reported. Considerable differences in the results are found and discussed in detail. The importance of final state interactions and possible meson exchange effects is also discussed. Electromagnetic and pionecxhange contributions were studied in calculations for E_{lab} = 165 and 330 MeV (1976FI11). For E_{γ} < 40 MeV, cross sections are calculated (1975GI01) for the twobody photodisintegration of ^{3}He and ^{3}H in the electricdipole approximation. The calculations were performed within the context of exact threebody theory with the twonucleon interactions represented by swave spindependent separable potentials. The numerical results indicate: (i) the ^{3}He and ^{3}H 90°photodisintegration cross sections are essentially identical in shape; (ii) the ^{3}He(γ, d)p 90° differential cross section has a peak value of approximately 95 μb/sr. (See (1981FA03) for comparison with experiment where it is concluded that agreement is not notably good). In a later work (1976GI02) the same theory applied earlier (1975GI01) for the twobody breakup for ^{3}H and ^{3}He targets was applied to the threebody breakup reaction ^{3}He(γ, n)^{1}H^{1}H. The numerical results indicate: (i) the ^{3}He(γ, n)^{1}H^{1}H cross section has a peak value of one mb; (ii) the neutron spectra for ^{3}He(γ, n)^{1}H^{1}H and a proton spectrum for ^{3}He(γ, p)np peak sharply in the region of the strong pp final state interaction. In (1975FA05, 1976FA12) hyperspherical harmonics were used for continuum threebody states in the calculation of cross sections for E1 transitions in ^{3}He to final isospin3/2 states (trinucleon photoeffect). The use of a softcore potential gives fair agreement with calculations of (1976GI02). In (1978FA01) this work was extented by including an additional grand orbital in the final state and finding reasonable agreement with measured total cross sections. The same formalism has been used in (1978LE11) to calculate several new examples of the trinucleon photoeffect. Agreement with experiment was in general unsatisfactory. The method of hyperspherical harmonics has also been used (1977VO11) to calculate threebody photodisintegration cross sections of ^{3}He and ^{3}H, and good agreement with experiment was obtained. The suppression of the isospin1/2 threebody photodisintegration of ^{3}He is investigated and explained in (1979LE03), in which exact threebody calculations and evaluation of isospin sum rules are also reported. The role of mesonic and isobaric degrees of freedom for electromagnetic processes in light nuclei including ^{3}He is discussed in (1983AR21).
The elastic charge form factor has been measured for a range of q^{2} (fourmomentum squared) from 0.032 fm^{2} to 100 fm^{2}. Extension of the data beyond q^{2} = 20 fm^{2} shows the charge form factor continues to fall until q^{2} = 65 fm^{2} and then goes up slightly (1978AR05). Recent determinations of the rms radius of the charge density distribution in ^{3}He gave γ^{c}_{rms} = 1.976 ± 0.015 fm (1985OT02) and γ^{c}_{rms} = 1.93 ± 0.03 fm (1985MA12). Earlier evaluations gave γ^{c}_{rms} = 1.877 ± 0.019 fm (1984RE03), 1.935 ± 0.03 fm (1983DU01), 1.89 ± 0.05 fm (1977SZ02) and 1.88 ± 0.05 (1977MC03). The elastic magnetic form factor has been measured for q^{2} = 0.2  31.6 fm^{2}. The data of (1982CA15) define a diffraction minimum in the magnetic form factor at q^{2} = 18 fm^{2} and a second maximum at 25 fm^{2}. See also the very recent measurement of the isoscalar and isovector form factors for ^{3}H and ^{3}He for momentum transfers between ≈ 0.3 and 2.9 fm^{1} reported in (1987BE30). Values of the rms radius for ^{3}He obtained from the magnetic form factor measurements are in agreement with one another. The values for γ^{m}_{rms} are 1.93 ± 0.07 fm (1985MA12), 1.99 ± 0.06 fm (1985OT02), 1.935 ± 0.04 (1983DU01) and 1.95 ± 0.11 (1977MC03). No theory has yet been successful in reproducing the charge form factor data over the entire range of q^{2}. Inclusion of a sixquark admixture in the ^{3}He wave function results in better agreement with experimental data in the whole region of momentum transfer up to q^{2} = 36 fm^{2} as compared with pure nuclear models (1984BU24, 1984BU42). A second diffraction minimum is predicted at q^{2} = 50 fm^{2} (1978HA09, 1982HA09, 1984BU24). The effects on the calculated charge form factor of the nucleon polarization (1982DR01) and the pair current (1983BE08, 1983DR12) calculated in the constituent quark model have been discussed as have multiquark clusters (1982NA09, 1984HO22, 1985KI12, 1985MA24). The dimensionalscaling quark model is discussed in (1978CH2A). Isobar currents, mesonic exchange currents and other corrections of relativistic order (1976DU05, 1976HA33, 1976KL02, 1977HA03, 1977RI15, 1978HA09, 1978SI15, 1979GI08, 1979SA39, 1981FI05, 1981HA07, 1981TO08, 1982HA09, 1983AZ01, 1983GI11, 1983HA04, 1983HA18, 1983MA58, 1983SA28) have been examined. The ^{3}He charge density determined from the charge form factor is seen to exhibit a central depression when the protons are treated as point charges (1978SI2B). This feature of the charge density is not reproduced by nonrelativistic Faddeev calculations (1978SI2B), even when different potentials are used (1981FR15). A suggestion (1981NO04) based on a variational calculation using a simple trinucleon wave function, that the central depression could be attributed to a twopion exchange threebody force, is not borne out by a more rigorous Faddeev calculation in which a twopion exchange threebody force is added to a realistic nucleonnucleon interaction (1981TO08). A variational method was employed (1984HU09) to calculate the trinucleon ground state properties, and it was concluded that the charge form factor is rather insensitive to the addition of different threebody forces. See also (1986SA08). Inclusion of meson exchnage currents, isobaric processes and other corrections of relativistic order is able to reproduce the central depression (1978HA09, 1982HA09). The bearing of these ingredients on current conservation is discussed in (1986LI09). Arguments based on QCD presented in (1986AB02) indicate that the central depression is due to the presence of large hiddencolor components. Ambiguities in the point charge density are discussed in (1982HA30). Errors in the determination of the charge distribution are discussed in (1978BO21, 1981BO13, 1982BO08, 1984CO06). The magnetic form factor data are not theoretically reproduced over the entire range of q^{2}. Inclusion of meson exchange currents in the ^{3}He wave function is essential to explain the diffraction minimum (1980RI04, 1982CA15, 1984MA26). The importance of the Dstate of the trinucleon and the shortrange behavior of the S and Dstate wave functions in determining the magnetic form factors of the threebody system is shown in (1975BA08). The role of the ninequark state is discussed in (1985AN13). Parity violating asymmetries in elastic electronnucleus scattering are considered in (1981FI05). The effect of clustering on the electromagnetic form factors of ^{3}He and ^{3}H is studied in (1976TA06). A brief review of theories on elastic electron scattering by ^{3}He and ^{3}H is contained in the talks reported in (1977CI2A, 1978SI2B). See also (1977NE2A, 1984FR16, 1985BO44) for a discussion of ^{3}He electromagnetic form factors. Pionic contributions to veryforward elastic scattering are discussed in (1986KA01). An analysis of inclusive quasielastic electron scattering data which can be interpreted to imply an increase in the nucleon radius in ^{3}He compared to the free nucleon radius is presented in (1986MC03). For other elastic scattering work see also (1974AR09, 1977DI10, 1977DU01, 1982TO08, 1986KI10). In inelastic electron scattering experiments, a 2S  2S monopole transition has been observed (1975KA04, 1975KA28) and a possible excited state at 10 MeV has been discussed (1979JO02). The first experimental separation of the transverse and longitudinal response functions has been carried out and reported in (1980MC01). The structure function has been derived from experimental data (1982RO16), while longitudinal and transverse form factors were derived from data in (1984KO05). The inelastic electron scattering data cover a momentum transfer range 0.09 < q^{2} < 1 fm^{2} (1975KA28), q^{2} = 5 fm^{2} (1976MC01), 2.5 < q^{2} < 7.1 fm^{2} (1978KU11), 4 < q^{2} < 4.9 fm^{2} (1979DA14), 20 < q^{2} < 128 fm^{2} (1982RO16) and 1 < q^{2} < 2.5 fm^{2} (1984KO05). Theoretical fits to the data are made mainly using the impulse approximation (1981BI01) with wave functions calculated by the Faddeev technique (see (1976DI09) and the experimental papers). The spectral function is derived (1983ME03) in the plane wave impulse approximation. Contributions of mesonexchange currents and final state interactions to the longitudinal and transverse response functions of ^{3}He are estimated in (1985LA04). The effect on inelastic electron scattering from ^{3}He of meson exchange currents (1983BI05) and quark clusters (1981PI04, 1981PI2B) has been studied. Theoretical descriptions of yscaling effects in inclusive electron scattering are discussed in (1982BO30, 1983CI11, 1983CI14, 1986GU10). The question of loose quarks in nuclei was raised (1986OE02) in connection with structure function calculations. The use of quasielastic scattering of polarized electrons on polarized ^{3}He as a probe of the subdominant components of the ^{3}He wave function has been explored in (1982WO05, 1984BL02). The channels associated with two and threeparticle electrodisintegrations of ^{3}He have been separated using ^{3}He(e, e'p)^{2}H reactions (1981KO25, 1982GO06, 1982JA06, 1983GO01). The proton momentum distribution of ^{3}He (1982GO06, 1982JA06, 1983GO01) and spectral function (1980GO09, 1982JA06) have been determined and fitted with various theoretical models for the ^{3}He wave function and nucleonnucleon potential. A theoretical calculation in the framework of the Faddeev fromalism in which a oneterm swave spindependent separable interaction fitted to the twonucleon scattering data is used, is found capable of explaining twobody electrodisintegration of ^{3}He near threshold and possibly at higher excitation energies (1977HE03, 1977HE22). High resolution (e, e'p) experiments are reviewed in (1985DE56) where it is concluded that (e, e'd) data of (1985KE05) constitute evidence for direct coupling of the virtual photon with correlated nucleon pairs. Sum rules for electron scattering are discussed in (1979JO02, 1980MC01, 1981TO11) and in other references cited in (1975FI08). A recent calculation (1986EF01) involving sum rules in analysis of longitudinal (e, e') spectra gave agreememt with traditional descriptions involving only nucleon degrees of freedom. For other work see also (1978CI2A, 1979CI2A, 1982BO30, 1983CI14).
The photon spectrum for the radiative capture reaction (d) has been calculated (1981KL03, 1984KL02) using the impulse approximation with a realistic function and the elementary particle method. A relativistic calculation made in (1980FE02) disagrees with the calculations of (1978HW2B) which are based on current conservation and a special linearity hypothesis. In (1976BE04, 1980GO2D, 1981GM01) the elementary particle method is employed to analyze reaction (d). A review of muon capture by ^{3}He is contained in (1977PH2A).
The angular distributions for π^{±} scattering by ^{3}He at energies between 260  310 MeV comfirm the fixed angle (θ ≈ 75°) minimum seen at lower energies and show a deep minimum at θ ≈ 110° (1980KA17). The data of (1976SH2B, 1978FA06, 1980FA12, 1981KA17, 1984FO18) for reaction (a) are not reproduced at all energies by optical model calculations. A value of 1.95 fm for the magnetic radius of ^{3}He is extracted by the authors of (1980FA12) from their data. Estimates of the π^{}^{3}He  ^{3}H coupling constant are made in (1976SH2B, 1978FA06), and a theoretical value obtained in (1979LE09, 1984KL01). The observation of the violation of charge symmetry in π^{±} scattering on ^{3}H and ^{3}He in (1984NE01, 1984NEZY) is questioned by the authors of 1984KI13) who comment that the experimental results can be explained as the manifestation of multiquark resonances in interacting hadronic systems. See also (1986KI08). A theoretical model for the distribution of matter in the ^{3}He  ^{3}H system (1985BA24) is, however, able to explain the observed chargesymmetry violating effects. Offshell effects in pion ^{3}He scattering are examined in (1980MU16, 1984GM01). A theoretical optical potential including spin is used in (1975LA15, 1975LA19) to analyze π^{}  ^{3}He data and it is proposed that π^{}  ^{3}He scattering can provide information on the magnetic form factor of ^{3}He. A Glauber theory calculation reported in (1975GO29) examines the effects on π^{}  ^{3}He scattering of the details of the nuclear wave function and of a repulsive core. A model based on the use of a simple pionnucleus potential proposed in (1981NI05) has been used to determine the value of the nuclear mass radius of ^{3}He in the region of the Δ_{33} resonance. Scattering length for π  ^{3}He are calculated in work reported in (1978LO16, 1978TH2A, 1979BE13, 1979BE2C, 1981BE63, 1982MU13, 1983GE12, 1985BE56). In references reported here all experimental data for reaction (b) are analyzed using Glauber theory which gives good to fair fits. The authors of (1979KA02, 1982OR06) have performed optical model calculations as well and get an unsatisfactory fit to their data by both methods. Dominance of spinflip contributions is suggested by the data (1980BO03, 1982CO01, 1982KA02). The effective number of nucleons and Pauli blocking effects are deduced in (1982CO01) from measurements reported there of the continuum angular distributions. Pauli principle effects were also studied in (1986NA05). Optical model calculations (1982MA26) and Glauber model calculations (1980GE06) are used to show that the differential cross sections are insensitive to the magnetic form factor of ^{3}He. The importance of spinflip contributions first pointed out in (1975SP06) in a Glauber calculation was later confirmed in (1977LO13). Both reactions (a) and (b) are investigated theorectically using coupled Schrodinger equations (1982AV2A), fourparticle equations (1979BE13, 1980BE55) modified in the spirit of the impulse approximation (1981BE46), and optical potentials (1976MA11, 1977LA06, 1980WA09). The effect of centerofmass correlations and intermediate states of π  ^{3}He scattering can be ignored (1981ME14). A review of these relations is contained in (1978NE2B). Proton spectra for the reactions (c) and (d) with emphasis on the kinematic region of twonucleon pion absorption have been measured and reported in (1980KA37, 1981KA41, 1981KA43) and (1983KA14) which reports measurements of quasifree scattering as well. The isospin dependence of pion absorption by nucleon pairs has been studied experimentally in work reported in (1981AS10, 1984MO03). The strong suppression of pion absorption by nucleon pairs having isospin equal to one, in the resonance region, is explained by theories based on the deltaisobar intermediate excitations (1982LE18, 1982TO18, 1984SI03). See also (1985OH09, 1986MA21). The same suppression of absorption on isospinone nucleon pairs for low energy swave pions has been used to relate the 1s absorption width in the pionic atom ^{3}H to that of ^{3}He (1985WE04). See also (1980SC24). Reactions (d) and (e) with stopped pions have been measured (1982GO04) and branching ratios deduced for all observed final states. Theories for pion absorption on ^{3}He at threshold have been proposed in work (1978JA02) based on the twonucleon absorption model, and in (1980AV2A) in which coupled Schrodinger equations are solved. The energy dependence of the cross sections for reaction (e) measured for pion energies between 50  575 MeV suggests the formation of an isospin equal to 1/2πN resonance in the intermediate state (1981KA26, 1981OR01, 1982OR06). The photon spectrum from radiative and charge exchange capture of pions in ^{3}He was measured (1974TR2A) and a value of the Panofsky ratio P_{3} = 2.68 ± 0.13 was obtained. An impulse approximation calculation gives P_{3} = 2.82 (1978GI13). The Panofsky ratio and photopion production crosss section at threshold on ^{3}He are investigated in a softpion approach to the ^{3}He  ^{3}H weak axialvector form factor reported in (1978GO11, 1980GO2D). See (1977BA2A) for a review on radiative pion capture reactions. Information about the interaction between pions and nuclei at low relative momenta has been extracted from measurements of Xrays from pionic ^{3}He and reported in (1977AB2A, 1978MA12, 1980MA20, 1982BA43, 1983BA39, 1984SC09). For a discussion of reaction (f) see reaction 2 in ^{3}n.
