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Double differential and total cross sections of the (${\mathrm{\pi }}^{\mathrm{-}}$,pd), (${\mathrm{\pi }}^{\mathrm{-}}$,dp), and (${\mathrm{\pi }}^{\mathrm{-}}$,dd) reactions on ${}_{}{}^{12}{}_{}{}^{}\mathrm{C}$ 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 ${}_{}{}^{12}{}_{}{}^{}\mathrm{C}$(${\mathrm{\pi }}^{\mathrm{-}}$,pp)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.

Differential cross sections for the process ${\pi }^{-}p\to \gamma n$ have been measured at Brookhaven National Laboratory’s Alternating Gradient Synchrotron with the Crystal Ball multiphoton spectrometer. Measurements were made at 18 pion momenta from 238 to $748\mathrm{MeV}∕c$, corresponding to $E_{\gamma }$ for the inverse reaction from 285 to $769\mathrm{MeV}$. The data have been used to evaluate the $\gamma n$ multipoles in the vicinity of the $N\left(1440\right)$ resonance. We compare our data and multipoles to previous determinations.

The reaction ${}_{}{}^{12}{}_{}{}^{}C\left({e,e}^{\prime }\mathrm{pp}\right)$ has been studied at an energy transfer $\omega =212\mathrm{MeV}$ and a three-momentum transfer $|\mathit{q}|=70\mathrm{MeV}/c$. The measured missing-energy spectrum shows a signature for knockout of proton pairs from $(1p{)}^2$, $(1p,1s)$, and $(1s{)}^2$ states. A comparison of the data with a calculation, in which different processes leading to two-nucleon knockout are accounted for, shows that the measured cross section for the knockout of a $(1p{)}^2$ pair can largely be attributed to short-range nucleon-nucleon correlations.

We have made the first measurements of the virtual Compton scattering (VCS) process via the $\mathrm{H}$($e$, $e^{\text{'}}p$)$\gamma$ exclusive reaction in the nucleon resonance region, at backward angles. Results are presented for the $W$-dependence at fixed $Q^2=1\hspace{0.5em}{\mathrm{GeV}}^2$ and for the $Q^2$ dependence at fixed $W$ near 1.5 GeV. The VCS data show resonant structures in the first and second resonance regions. The observed $Q^2$ dependence is smooth. The measured ratio of $\mathrm{H}$($e$, $e^{\text{'}}p$)$\gamma$ to $\mathrm{H}$($e$, $e^{\text{'}}p$)${\pi }^0$ cross sections emphasizes the different sensitivity of these two reactions to the various nucleon resonances. Finally, when compared to real Compton scattering (RCS) at high energy and large angles, our VCS data at the highest $W$ ($1.8\text{−}1.9$ GeV) show a striking $Q^2$ independence, which may suggest a transition to a perturbative scattering mechanism at the quark level.

We present a measurement of the spin-dependent cross sections for the ${}^3\overrightarrow{\mathrm{He}}(\overrightarrow{e},e^{\prime })X$ reaction in the quasielastic and resonance regions at a four-momentum transfer $0.1\le Q^2\le 0.9{\mathrm{GeV}}^2$. The spin-structure functions have been extracted and used to evaluate the nuclear Burkhardt-Cottingham and extended Gerasimov-Drell-Hearn sum rules for the first time. The data are also compared to an impulse approximation calculation and an exact three-body Faddeev calculation in the quasielastic region.

Reaction $K^{-}p\to {\pi }^0{\pi }^0\Lambda$ was measured at eight incident $K^{-}$ momenta between 514 and $750\text{MeV}∕c$ using the Crystal Ball multiphoton spectrometer. The reaction dynamics are displayed in total cross sections, Dalitz plots, invariant-mass spectra, production angular distributions, and the $\Lambda$ polarization. The ${\pi }^0{\pi }^0\Lambda$ production is dominated by the ${\pi }^0{\Sigma }^0\left(1385\right)$ intermediate state; no trace of other light ${\Sigma }^{\ast }$ states is observed, and the role of the $f_0\left(600\right)$ meson appears to be insignificant. A striking similarity is seen between $K^{-}p\to {\pi }^0{\pi }^0\Lambda$ and ${\pi }^{-}p\to {\pi }^0{\pi }^{0}n$; this can be understood as a consequence of dynamical flavor symmetry.

Structure functions, as measured in lepton-nucleon scattering, have proven to be very useful in studying the partonic dynamics within the nucleon. However, it is experimentally difficult to separately determine the longitudinal and transverse structure functions, and consequently there are substantially less data available in particular for the longitudinal structure function. Here, we present separated structure functions for hydrogen and deuterium at low four-momentum transfer squared, $Q^2<1{\mathrm{GeV}}^2$, and compare them with parton distribution parametrization and $k_T$ factorization approaches. While differences are found, the parametrizations generally agree with the data, even at the very low-$Q^2$ scale of the data. The deuterium data show a smaller longitudinal structure function and a smaller ratio of longitudinal to transverse cross section, $R$, than the proton. This suggests either an unexpected difference in $R$ for the proton and the neutron or a suppression of the gluonic distribution in nuclei.

We present data on the inclusive scattering of polarized electrons from a polarized ${}^{\mathrm{3}}\mathrm{H}\mathrm{e}$ target at energies from 0.862 to 5.06 GeV, obtained at a scattering angle of $15.5°$. Our data include measurements from the quasielastic peak, through the nucleon resonance region, and beyond, and were used to determine the virtual photon cross-section difference ${\sigma }_{1/2}-{\sigma }_{3/2}$. We extract the extended Gerasimov-Drell-Hearn integral for the neutron in the range of four-momentum transfer squared $Q^2$ of $0.1–0.9{\mathrm{G}\mathrm{e}\mathrm{V}}^2$.

We report on the measurement of spin density matrix elements of the $\mathrm{\Lambda }\left(1520\right)$ in the photoproduction reaction $\gamma p\to \mathrm{\Lambda }\left(1520\right)K^{+}$, via its subsequent decay to $K^{-}p$. The measurement was performed as part of the GlueX experimental program in Hall D at Jefferson Laboratory using a linearly polarized photon beam with $E_{\gamma }=8.2\mathrm{GeV}\text{–}8.8\mathrm{GeV}$. These are the first such measurements in this photon energy range. Results are presented in bins of momentum transfer squared, $-(t-t_{\text{0}})$. We compare the results with a Reggeon exchange model and determine that natural exchange amplitudes are dominant in $\mathrm{\Lambda }\left(1520\right)$ photoproduction.

Data are presented for the reaction ${\pi }^{-}p\to {\pi }^0{\pi }^{0}n$ in the range from threshold to $p_{{\pi }^{-}}=750\mathrm{M}\mathrm{e}\mathrm{V}/c$. The systematics of the data and multipole analyses are examined for sensitivity to a $f_0(600)$ (“$\sigma$”) meson. A one-pion-exchange mechanism is found to be very weak, or absent. The reaction appears to become dominated by sequential ${\pi }^0$ decays through the $\Delta (1232)$ resonance as the beam momentum increases, along with substantial interference effects from several competing mechanisms.

We have measured the spin structure functions $g_1$ and $g_2$ of ${}^3\mathrm{H}\mathrm{e}$ in a double-spin experiment by inclusively scattering polarized electrons at energies ranging from 0.862 to 5.058 GeV off a polarized ${}^3\mathrm{H}\mathrm{e}$ target at a $15.5°$ scattering angle. Excitation energies covered the resonance and the onset of the deep inelastic regions. We have determined for the first time the $Q^2$ evolution of ${\Gamma }_1(Q^2)={\int }_0^1{g}_1(x,Q^2)dx$, ${\Gamma }_2(Q^2)={\int }_0^1{g}_2(x,Q^2)dx$, and $d_2(Q^2)={\int }_0^1{x}^2[2g_1(x,Q^2)+3g_2(x,Q^2)]dx$ for the neutron in the range $0.1\le Q^2\le 0.9{\mathrm{G}\mathrm{e}\mathrm{V}}^2$ with good precision. ${\Gamma }_1(Q^2)$ displays a smooth variation from high to low $Q^2$. The Burkhardt-Cottingham sum rule holds within uncertainties and $d_2$ is nonzero over the measured range.

Based on $334{\mathrm{pb}}^{-1}$ of photoproduction data collected with the GlueX detector at Jefferson Lab, we have measured for the first time the cross section of the exclusive reaction $\gamma +p\to \varphi (1020){\pi }^{+}{\pi }^{-}p$ by reconstructing the final state $K^{+}{K}^{-}{\pi }^{+}{\pi }^{-}p$ produced with a photon beam of energies between 8.0 and 11.6 GeV. Based on the measured differential cross section, we have performed a search for the strangeoniumlike exotic candidate $Y(2175)$, recently renamed to $\varphi (2170)$. This state has been reported by different $e^{+}{e}^{-}$ annihilation experiments and it is addressed here for the first time in a photoproduction experiment. We do not find evidence for this state when using the resonance parameters quoted by the Particle Data Group and provide upper limits on the photoproduction cross section. Instead, we find a structure at a mass of $m(\varphi {\pi }^{+}{\pi }^{-})=2.24\mathrm{GeV}/c^2$ with a statistical significance of about $5\sigma$. The parameters of this structure differ from those quoted by the Particle Data Group for the $\varphi (2170)$ and are consistent with a previous observation in $e^{+}{e}^{-}$ annihilation. In addition, there is evidence for a second structure at $1.82\mathrm{GeV}/c^2$.

The differential cross section for η production in reaction ${\pi }^{-}p\to \eta n$ has been measured over the full angular range at seven incident ${\pi }^{-}$ beam momenta from threshold to $p_{{\pi }^{-}}=747$ MeV/c using the Crystal Ball multiphoton spectrometer. The angular distributions are S wave dominated. At 10 MeV/c above threshold, a small D-wave contribution appears that interferes with the main S wave. The total η production cross section ${\sigma }^{\mathrm{tot}}$ is obtained by integration of $d\sigma /d\Omega$. Starting at threshold, ${\sigma }^{\mathrm{tot}}$ rises rapidly, as expected for S-wave-dominated production. The features of the ${\pi }^{-}p\to \eta n$ cross section are strikingly similar to those of the SU(3) flavor-related process $K^{-}p\to \eta \Lambda$. Comparison of the ${\pi }^{-}p\to \eta n$ reaction is made with η photoproduction.

We have determined the quadratic slope parameter $\alpha$ for $\eta \to 3{\pi }^0$ to be $\alpha =-0.031\mathrm{}\left(4\right)$ from a $99\%$ pure sample of ${10}^6\eta \to 3{\pi }^0$ decays produced in the reaction ${\pi }^{-}p\to n\eta$ close to the $\eta$ threshold using the Crystal Ball detector at the AGS. The result is four times more precise than the present world data and disagrees with current chiral perturbation theory calculations by about four standard deviations.

The first spin-transfer observables for the πd→pp reaction have been measured at a number of energies spanning the Δ resonance in this system. These parameters correspond to ${\mathit{K}}_{\mathit{S}\mathit{L}}$ and ${\mathit{K}}_{\mathit{S}\mathit{S}}$ of the pp→dπ reaction for incident proton energies ranging from 600 to 800 MeV. Such data can provide an important constraint on the determination of the partial-wave amplitudes describing this fundamental reaction. The discrepancies between our data, theoretical predictions, and values calculated from published partial-wave amplitudes demonstrate the need for further work in this area.

Virtual Compton scattering (VCS) on the proton has been studied at the Jefferson Laboratory using the exclusive photon electroproduction reaction $ep\to ep\gamma$. This paper gives a detailed account of the analysis which has led to the determination of the structure functions $P_{LL}-P_{TT}/\epsilon$ and $P_{LT}$ and the electric and magnetic generalized polarizabilities (GPs) ${\alpha }_E\left(Q^2\right)$ and ${\beta }_M\left(Q^2\right)$ at values of the four-momentum transfer squared $Q^2=0.92$ and 1.76 GeV${}^2$. These data, together with the results of VCS experiments at lower momenta, help building a coherent picture of the electric and magnetic GPs of the proton over the full measured $Q^2$ range and point to their nontrivial behavior.

The generalized forward spin polarizabilities ${\gamma }_0$ and ${\delta }_{LT}$ of the neutron have been extracted for the first time in a $Q^2$ range from 0.1 to $0.9{\mathrm{G}\mathrm{e}\mathrm{V}}^2$. Since ${\gamma }_0$ is sensitive to nucleon resonances and ${\delta }_{LT}$ is insensitive to the $\Delta$ resonance, it is expected that the pair of forward spin polarizabilities should provide benchmark tests of the current understanding of the chiral dynamics of QCD. The new results on ${\delta }_{LT}$ show significant disagreement with chiral perturbation theory calculations, while the data for ${\gamma }_0$ at low $Q^2$ are in good agreement with a next-to-leading-order relativistic baryon chiral perturbation theory calculation. The data show good agreement with the phenomenological MAID model.

Reaction ${\pi }^{-}p\to {\pi }^0{\pi }^{0}n$ has been measured with high statistics in the beam momentum range $270–750\text{MeV}∕c$. The data were obtained using the Crystal Ball multiphoton spectrometer, which has $93\%$ of $4\pi$ solid angle coverage. The dynamics of the ${\pi }^{-}p\to {\pi }^0{\pi }^{0}n$ reaction and the dependence on the beam energy are displayed in total cross sections, Dalitz plots, invariant-mass spectra, and production angular distributions. Special attention is paid to the evaluation of the acceptance that is needed for the precision determination of the total cross section ${\sigma }_t({\pi }^{-}p\to {\pi }^0{\pi }^{0}n)$. The energy dependence of ${\sigma }_t({\pi }^{-}p\to {\pi }^0{\pi }^{0}n)$ shows a shoulder at the Roper resonance [i.e., the $N\left(1440\right){\frac{1}{2}}^{+}$], and there is also a maximum near the $N\left(1520\right){\frac{3}{2}}^{-}$. It illustrates the importance of these two resonances to the ${\pi }^0{\pi }^0$ production process. The Dalitz plots are highly nonuniform; they indicate that the ${\pi }^0{\pi }^{0}n$ final state is dominantly produced via the ${\pi }^0{\Delta }^0\left(1232\right)$ intermediate state. The invariant-mass spectra differ much from the phase-space distributions. The production angular distributions are also different from the isotropic distribution, and their structure depends on the beam energy. For beam momenta above $550\text{MeV}∕c$, the density distribution in the Dalitz plots strongly depends on the angle of the outgoing dipion system (or equivalently on the neutron angle). The role of the $f_0\left(600\right)$ meson (also known as the $\sigma$) in ${\pi }^0{\pi }^{0}n$ production remains controversial.

A new upper limit is presented for the branching ratio (BR) of $\eta \to 3\gamma$. This decay is not allowed by charge-conjugation invariance. We have determined that $\mathit{BR}(\eta \to 3\gamma )<4\times {10}^{-5}$. We also obtained $\mathit{BR}(\eta \to {\pi }^0\gamma \to 3\gamma )<9\times {10}^{-5}$; this decay mode is absolutely forbidden, as it violates conservation of angular momentum. Finally, we have established that $\mathit{BR}(\eta \to {\pi }^0{\pi }^0\gamma \gamma )<1.2\times {10}^{-3}$ in a restricted diphoton-energy region. All three upper limits are at the 90% confidence level. They were obtained by using the Crystal Ball multiphoton spectrometer at the AGS and are based on the analysis of 28 million η mesons produced in the ${\pi }^{-}p\to \eta n$ reaction close to threshold.

New results on the rare, doubly radiative decay $\eta \to {\pi }^0\gamma \gamma$ have been obtained from a revised analysis of the Crystal Ball experiment performed at the AGS. The analysis yields the first information on the dependence of the decay width, $\Gamma (\eta \to {\pi }^0\gamma \gamma )$, on the two-photon invariant mass squared, $m^2(\gamma \gamma )$. A re-evaluation of the branching ratio is also made, $\mathit{BR}(\eta \to {\pi }^0\gamma \gamma )=(2.21\pm 0.{24}_{\mathrm{stat}}\pm 0.{47}_{\mathrm{syst}})\times {10}^{-4}$; it implies that the decay width is $\Gamma (\eta \to {\pi }^0\gamma \gamma )=0.285\pm 0.{031}_{\mathrm{stat}}\pm 0.{061}_{\mathrm{syst}}$ eV. These results are close to predictions based on chiral perturbation theory with vector-meson dominance.