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Precise new data for the reaction K−p→π0Λ are presented in the c.m. energy range 1565 to 1600 MeV. Our analysis of these data sheds new light on claims for the Σ(1580) 3 2 − resonance, which (if it exists with the specified quantum numbers) must be an exotic baryon because of its very low mass. Our results show no evidence for this state.

We report on a measurement ofthe neutron detection efficiency in NaI crystals in the Crystal Ball (CB)

We report on an investigation of the ({pi}{sup -}, {pi}{sup 0}{pi}{sup 0}) reaction by means of the {pi}{sup 0}{pi}{sup 0} invariant mass distributions from {pi}{sup -} interactions on H, D, C, Al, and Cu targets at {rho}{sub {pi}{sup -}} = 0.408 GeV/c. The sharp, strong peak in the {pi}{sup +}{pi}{sup -} invariant mass near 2m{sub {pi}} reported by the CHAOS Collaboration is not seen in our {pi}{sup 0}{pi}{sup 0} data. However, we do observe a change in the shape of the {pi}{sup 0}{pi}{sup 0} invariant mass spectrum for the different targets, indicating that the {pi}{sup 0}{pi}{sup 0} interaction diminishes in the nuclear medium as represented by nuclei D, C, Al, and Cu, compared to hydrogen.

Angular distributions for the differential cross section and three deuteron analyzing powers {ital iT}{sub 11}, {ital T}{sub 20}, and {ital T}{sub 22} of the reaction {ital d}{searrow}{ital p}{r_arrow}{sup 3}He{pi}{sup 0} have been measured over the whole angular domain at 20 energies close to threshold (0.03{lt}{ital T}{sub {pi}}{sup c.m.}{lt}10.2 MeV). The differential cross section and tensor analyzing power {ital T}{sub 20} both show strong variation in energy and angle due to interference between {ital S}- and {ital P}-wave pion production, whereas {ital iT}{sub 11} and {ital T}{sub 22} remain consistent with zero over the whole experimental range. All the data at different energies and angles fall on universal curves when plotted as functions of the single variable {ital p}{sub {pi}}cos{Theta}, evaluated in the c.m. The broad features of the results are in line with theoretical expectations. {copyright} {ital 1996 The American Physical Society.}

The first direct, precision measurement of the branching ratio for the decay of the \ensuremath{\eta} meson into two photons, B(\ensuremath{\eta}\ensuremath{\rightarrow}\ensuremath{\gamma}\ensuremath{\gamma})\ensuremath{\equiv}\ensuremath{\Gamma}(\ensuremath{\eta}\ensuremath{\rightarrow}\ensuremath{\gamma}\ensuremath{\gamma})/${\mathrm{\ensuremath{\Gamma}}}_{\mathrm{tot}}$(\ensuremath{\eta}), is presented. The systematic undertainties are quite different than in previous indirect measurements. Using the SATURNE II proton beam and a liquid-deuterium target, the \ensuremath{\eta} mesons were produced in the reaction pd${\ensuremath{\rightarrow}}^{3}$He \ensuremath{\eta} at 1.5 MeV above threshold, and tagged by the detection of the recoil $^{3}\mathrm{He}$ in the magnetic spectrometer SPES2. The two decay photons were detected in two BGO electromagnetic calorimeters located at 65\ifmmode^\circ\else\textdegree\fi{} left and right of the beam axis. A lead collimator placed in front of the right calorimeter defined the detector acceptance. The analysis yielded B(\ensuremath{\eta}\ensuremath{\rightarrow}\ensuremath{\gamma}\ensuremath{\gamma})=0.3949\ifmmode\pm\else\textpm\fi{}0.0017(stat)\ifmmode\pm\else\textpm\fi{}0.0030(syst). \textcopyright{} 1995 The American Physical Society.

The total cross section for {gamma}{ital p}{r_arrow}{eta}{ital p} near threshold has been measured using the PHOENICS tagging system at the ELSA electron facility of the Physikalisches Institut der Universitaet Bonn. The photons are created by bremsstrahlung, and are tagged by measuring the momentum of each electron after the photon has been emitted. The recoil proton from {gamma}{ital p}{r_arrow}{eta}{ital p} is detected by the AMADEUS counter setup in coincidence with the tagging system. Data were taken with AMADEUS at 3.3{degree} in the laboratory, where the large Jacobian increases our event rate so that we obtain the cross section from threshold ({ital E}{sub {gamma}}=707.2 MeV) to {ital E}{sub {gamma}}{congruent}720 MeV with adequate statistics. The {gamma}{ital p}{r_arrow}{eta}{ital p} events are identified by kinematics, {ital dE}/{ital dx}, and timing information. We find that in our energy region the production cross section is consistent with {ital S}-wave production.

Measured values of the differential cross sections for pion-nucleon charge exchange, ${\ensuremath{\pi}}^{\ensuremath{-}}p\ensuremath{\rightarrow}{\ensuremath{\pi}}^{0}n$, are presented for ${\ensuremath{\pi}}^{\ensuremath{-}}$ momenta of $103,112,120,130,139,152$, and $178$ MeV/$c$. Complete angular distributions were obtained by using the Crystal Ball detector at the Alternating Gradient Synchrotron at Brookhaven National Laboratory. Statistical uncertainties of the differential cross sections vary from $3$ to $6%$ in the backward angle region and from $6$ to about $20%$ in the forward region with the exception of the two most forward angles. The systematic uncertainties are estimated to be about $3%$ for all momenta.

Measured values of the differential cross sections for pion-nucleon charge exchange, π-p→π0n, are presented for π- momenta of 103, 112, 120, 130, 139, 152, and 178 MeV/c. Complete angular distributions were obtained by using the Crystal Ball detector at the Alternating Gradient Synchrotron at Brookhaven National Laboratory. Statistical uncertainties of the differential cross sections vary from 3 to 6% in the backward angle region and from 6 to about 20% in the forward region with the exception of the two most forward angles. The systematic uncertainties are estimated to be about 3% for all momenta.

The reaction K{sup -}p{yields}{sigma}{sup 0}{pi}{sup 0}{yields} neutrals was studied with the Crystal Ball detector at the BNL Alternating Gradient Synchrotron for beam momenta between 514 and 750 MeV/c. The photons from {sigma}{sup 0} and {pi}{sup 0} decays were detected, as well as the neutron in a small fraction of the events. The differential cross section and the {sigma}{sup 0} polarization are reported at eight momenta and nearly the full angular range. Total cross sections are derived from Legendre polynomial fits to the data. The measurements presented are of considerably higher precision than previous results.

A new upper limit is presented for the branching ratio (BR) of {eta}{yields}3{gamma}. This decay is not allowed by charge-conjugation invariance. We have determined that BR({eta}{yields}3{gamma})

The branching ratio (BR) for the rare decay $\ensuremath{\eta}\ensuremath{\rightarrow}{\ensuremath{\pi}}^{0}\ensuremath{\gamma}\ensuremath{\gamma}$ was measured with the Crystal Ball multiphoton spectrometer. The result, BR$(\ensuremath{\eta}\ensuremath{\rightarrow}{\ensuremath{\pi}}^{0}\ensuremath{\gamma}\ensuremath{\gamma})=(3.5\ifmmode\pm\else\textpm\fi{}0.{7}_{\mathrm{stat}}\ifmmode\pm\else\textpm\fi{}0.{6}_{\mathrm{syst}})\ifmmode\times\else\texttimes\fi{}{10}^{\ensuremath{-}4}$, is in agreement with calculations of chiral perturbation theory to third order. We have used a sample of 28 million \ensuremath{\eta} mesons produced at the BNL Alternating Gradient Synchrotron using the ${\ensuremath{\pi}}^{\ensuremath{-}}p\ensuremath{\rightarrow}\ensuremath{\eta}n$ reaction close to threshold. We detail the intricate extraction of the signal, which has about 1.6 thousand $\ensuremath{\eta}\ensuremath{\rightarrow}{\ensuremath{\pi}}^{0}\ensuremath{\gamma}\ensuremath{\gamma}$ events, from the overwhelming background of $\ensuremath{\eta}\ensuremath{\rightarrow}3{\ensuremath{\pi}}^{0}$ decays and from the ${\ensuremath{\pi}}^{\ensuremath{-}}p\ensuremath{\rightarrow}{\ensuremath{\pi}}^{0}{\ensuremath{\pi}}^{0}n$ reaction.

We report a new measurement of the differential cross section for {pi}{sup -}p{yields}{pi}{sup 0}n from p{sub {pi}}=649 to 752 MeV/c, which is around the opening of the {eta} channel (685 MeV/c). Our data support the main features of the {pi}{sup -}p charge-exchange differential cross sections generated by the SAID {pi}N partial-wave analysis. The opening of the {eta} channel has a clear effect on the shape of the excitation function for d{sigma}({pi}{sup -}p{yields}{pi}{sup 0}n), which is most noticeable in the backward direction.

The differential cross section for {eta} production in reaction {pi}{sup -}p{yields}{eta}n has been measured over the full angular range at seven incident {pi}{sup -} beam momenta from threshold to p{sub {pi}}{sub {sup -}}=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 {eta} production cross section {sigma}{sup tot} is obtained by integration of d{sigma}/d{omega}. Starting at threshold, {sigma}{sup tot} rises rapidly, as expected for S-wave-dominated production. The features of the {pi}{sup -}p{yields}{eta}n cross section are strikingly similar to those of the SU(3) flavor-related process K{sup -}p{yields}{eta}{lambda}. Comparison of the {pi}{sup -}p{yields}{eta}n reaction is made with {eta} photoproduction.

The study of the light baryon resonances, in particu-lar the determination of their electromagnetic couplings,is undergoing a resurgence driven by a stream of new,high-precision data emanating from experimental pro-grams at modern photon facilities. The main propertiesof the ∆(1232) resonance, the mass, pole value, masssplittings, width, and branching ratio for different decay

Reaction K{sup -}p{yields}{pi}{sup 0}{pi}{sup 0}{sigma}{sup 0} was measured with the Crystal Ball multiphoton spectrometer at eight K{sup -} momenta from 514 to 750 MeV/c. Dynamics and the energy dependence of the reaction are shown by Dalitz plots, invariant mass spectra, production angular distributions, and total cross sections. The dipion invariant-mass spectra for K{sup -}p{yields}{pi}{sup 0}{pi}{sup 0}{sigma}{sup 0} show an enhancement in the low mass region; the opposite is the case for the K{sup -}p{yields}{pi}{sup 0}{pi}{sup 0}{lambda} and {pi}{sup -}p{yields}{pi}{sup 0}{pi}{sup 0}n reactions. There is no direct evidence for a f{sub 0}(600)-meson contribution to {pi}{sup 0}{pi}{sup 0} production. Everywhere {sigma}{sub t}(K{sup -}p{yields}{pi}{sup 0}{pi}{sup 0}{sigma}{sup 0})<

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

We report a new measurement of the ${\ensuremath{\pi}}^{\ensuremath{-}}\stackrel{\ensuremath{\rightarrow}}{p}3{\ensuremath{\pi}}^{0}n$ total cross section from threshold to ${p}_{\ensuremath{\pi}}=0.75\mathrm{GeV}/c.$ The cross section near the $N(1535){\frac{1}{2}}^{\ensuremath{-}}$ resonance is only a few $\ensuremath{\mu}\mathrm{b}$ after subtracting the large $\stackrel{\ensuremath{\rightarrow}}{\ensuremath{\eta}}3{\ensuremath{\pi}}^{0}$ background associated with ${\ensuremath{\pi}}^{\ensuremath{-}}\stackrel{\ensuremath{\rightarrow}}{p}\ensuremath{\eta}n.$ A simple analysis of our data results in the estimated branching fraction $\mathcal{B}[{S}_{11}\ensuremath{\rightarrow}\ensuremath{\pi}N(1440){\frac{1}{2}}^{+}]=(8\ifmmode\pm\else\textpm\fi{}2)%.$ This is the first such estimate obtained with a three-pion production reaction.

Data are presented for the reaction pi(-)p--pi(0)pi(0)n in the range from threshold to p(pi(-))=750 MeV/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.

The first results obtained in 1998 by the Crystal Ball collaboration from a measurement of the total and differential cross sections for the reaction pi(-)p --> etan are presented.These new experimental results for the total cross sections are compared with the predictions of the K-matrix model for pion- nucleon scattering.The angular distribution at momenta near the reaction threshold (685 MeV/c) is determined by the S-wave contribution.The P-wave contribution begins to manifest itself from a momentum of 720 MeV/c.

Recently the Crystal Ball Collaboration measured precise new data for the near-threshold reaction K(-)p-->etaLambda, which is dominated by formation of the Lambda(1670)1 / 2(-). In this Letter, we present results of a unitary, multichannel analysis that incorporates the new Crystal Ball data. For our preferred fit, we obtain mass M = 1673+/-2 MeV, width Gamma = 23+/-6 MeV, and elasticity x = 0.37+/-0.07. This elasticity is significantly larger than previously recognized. Resonance parameters of our preferred fit are in striking agreement with the quark-model predictions of Koniuk and Isgur.

We have determined the quadratic slope parameter $\ensuremath{\alpha}$ for $\ensuremath{\eta}\ensuremath{\rightarrow}3{\ensuremath{\pi}}^{0}$ to be $\ensuremath{\alpha}\phantom{\rule{0ex}{0ex}}=\phantom{\rule{0ex}{0ex}}\ensuremath{-}0.031(4)$ from a $99%$ pure sample of ${10}^{6}\ensuremath{\eta}\ensuremath{\rightarrow}3{\ensuremath{\pi}}^{0}$ decays produced in the reaction ${\ensuremath{\pi}}^{\ensuremath{-}}p\ensuremath{\rightarrow}n\ensuremath{\eta}$ close to the $\ensuremath{\eta}$ 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.

We present measurements of the differential and total cross sections and the {Lambda} polarization for the reaction K{sup -}p{yields}{eta}{Lambda} from threshold to p{sub K}-=770 MeV/c, with much better precision than previous measurements. Our cross-section data show a remarkable similarity to the SU(3) flavor-related {pi}{sup -}p{yields}{eta}n cross-section results. The reaction K{sup -}p{yields}{eta}{Lambda} at threshold is dominated by formation of the intermediate {Lambda}(1670)1/2- state.

We have determined the quadratic slope parameter alpha for eta--3pi(0) to be alpha = -0.031(4) from a 99% pure sample of 10(6)eta--3pi(0) decays produced in the reaction pi(-)p--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.

We report on an investigation of the (pi(-),pi(0)pi(0)) reaction by means of measurements of the pi(0)pi(0) invariant mass distributions from pi(-) interactions on H, D, C, Al, and Cu targets at p(pi(-)) = 0.408 GeV/c. The sharp, strong peak in the pi(+)pi(-) invariant mass near 2m(pi) reported by the CHAOS Collaboration is not seen in our pi(0)pi(0) data. However, we do observe a change in the shape of the pi(0)pi(0) invariant mass spectrum for the different targets, indicating that the pi(0)pi(0) interaction diminishes in the nuclear medium as represented by nuclei D, C, Al, and Cu, compared to hydrogen.

Yields of the reaction it π − p → ηn have been measured in the near—threshold region. The experiment was carried out by means of detecting neutron produced in this reaction and measuring their energies by TOF—technique. Measured momentum dependences of the reaction yield are compared with calculated ones.

As a part of our project to measure an almost complete set of proton‐deuteron elastic scattering observables at E=22.7 MeV we present data for several proton to deuteron vector and tensor polarization transfer coefficients. We compare them with the rigorous three‐body calculation using Paris, Nijmegen, Bonn‐A and Bonn‐B potentials.

We have measured polarization transfer observables Ky′y (θ), Kx′x (θ) and Kx′z(θ) for proton-proton scattering at 25.68 MeV with an accuracy of 0.007–0.012. Phase shift analyses of these data together with differential cross section and analyzing power data at the same energy yield an essentially model independent determination of the 1S0, 3PJ and e2 parameters at this energy. These parameters provide a test of nucleon-nucleon potential models and favour Paris and Nijmegen potentials.

The research programs of the UCLA Particle and Nuclear Physics Research Group, the research objectives, results of experiments, the continuing activities and new initiatives are presented. The primary goal of the research is to test the symmetries and invariances of particle/nuclear physics with special emphasis on investigating charge symmetry, isospin invariance, charge conjugation, and CP. Another important part of our work is baryon spectroscopy, which is the determination of the properties (mass, width, decay modes, etc.) of particles and resonances. We also measure some basic properties of light nuclei, for example the hadronic radii of {sup 3}H and {sup 3}He. Special attention is given to the eta meson, its production using photons, electrons, {pi}{sup {plus_minus}}, and protons, and its rare and not-so-rare decays. In Section 1, the physics motivation of our research is outlined. Section 2 provides a summary of the research projects. The status of each program is given in Section 3. We discuss the various experimental techniques used, the results obtained, and we outline the plans for the continuing and the new research. Details are presented of new research that is made possible by the use of the Crystal Ball Detector, a highly segmented NaI calorimeter and spectrometermore » with nearly 4{pi} acceptance (it was built and used at SLAC and is to be moved to BNL). The appendix contains an update of the bibliography, conference participation, and group memos; it also indicates our share in the organization of conferences, and gives a listing of the colloquia and seminars presented by us.« less

The polarization-Transfer observables Kyy1(θ), Kxx1(θ) and Kzx1(θ) for proton-proton scattering have been measured at Ep = 25.68MeV. A simultaneous phase-shift analysis of these new data with differential cross section and analyzing power data at the same energy resulted in a very accurate determination of the p-wave phase-shift combinations ΔC, ΔLS and ΔT and of the 3F2−3P2 mixing parameter ϵ2. With this complete set of high precision data a critical test of microscopic NN-potential models has been performed.

The polarization transfer in p-d elastic scattering was shown to be sensitive to the details of the tensor force in the N-N interaction. Therefore measurements of the polarization transfer coefficients in this scattering have been carried out at an incident proton energy of 22.7 MeV. The data are compared to rigorous Faddeev calculations using different N-N potentials. The observable most sensitive to details of the potentials is the nucleon-nucleon polarization transfer coefficient K y y′ (Wolfenstein’s depolarization parameter D). The agreement of the K y y′ data with the calculations using the Bonn A potential is much better than with the Paris or Bonn B potentials. Results of recent measurements of K z x′ are presented and compared to the calculations. Measurements of the polarization transfer to the recoil deuterons are in progress.

Proton-deuteron elastic scattering has been investigated at Ep =22.7 MeV by comparison of rigorous Faddeev calculations with experimental results. The observable most sensitive to the tensor force is the nucleon-nucleon polarization transfer coefficient Kyy′. The new angular distribution of Kyy′ clearly favours the tensor force of the Bonn A potential, which is weaker than the one of the Paris potential.

A new general method for the precise calibration of beam polarization or analyzing power in spin-12-spin-0 elastic scattering has been developed. This absolute calibration method uses the double scattering technique in connection with modern polarized ion source technology. It is based on an incident beam with at least two different polarization states and is independent of beam energy and scattering angle. The application to p-α elastic scattering at 25.68 MeV and a lab, angle of 117.5° is described. The result is a new determination of the analyzing power to an accuracy of better than 1%, i.e. Ay = 0.8119 ± 0.0076. Systematic errors are extensively discussed.