Search

Photoproduction of two neutral pions off the proton is studied using linearly polarized photons, and the polarization observables I-s and I-c are measured for the first time. These two observables are unique to multi-meson final states; they characterize correlations between the linear photon polarization and the direction of outgoing single particles in photoproduction of three-body final states. The I-s and I-c distributions suggest that, in the 1.8 to 2.0 GeV mass region, the N(1520) 3/2(-)pi intermediate state is reached with reaction dynamics consistent with a dominant J(P) = 3/2(+) wave. These data are included in the Bonn-Gatchina (BnGa) partial wave analysis which is based on a large variety of data; the analysis confirms a significant contribution from the reaction chain gamma p -> N(1900)3/2(+) -> N(1520)3/2(-)pi(0) -> p pi(0)pi(0). (C) 2015 The Authors. Published by Elsevier B.V.

The reaction γ p → K S 0 Σ + is studied in the photon energy range from threshold. Linearly polarised photon beams from coherent bremsstrahlung enabled the first measurement of photon beam asymmetries in this reaction up to E γ = 1650 MeV . In addition, the recoil hyperon polarisation was determined through the asymmetry in the weak decay Σ + → p π 0 up to E γ = 2250 MeV . The data are compared to partial wave analyses, and the possible impact on the interpretation of a recently observed prominent structure in the cross section near the K ⁎ thresholds is discussed.

The future deep-sea neutrino telescope of multi cubic-km size, KM3NeT, has been designed for an efficient search for high energy neutrinos originating from galactic and extragalactic sources. The detection principle relies on the measurement of Cherenkov light emitted from relativistic charged secondary particles caused by the interaction of neutrinos with matter inside or surrounding the active detection volume. In order to provide a homogeneous photon acceptance and to reduce the environmental background by local coincidences between neighbouring photo sensors, a digital optical module (DOM) containing an array of 31 3-in, diameter photomultiplier tubes (PMTs) has been designed. Optimum performance requires sensitivity to single-photo electrons, high collection efficiency at low dark noise, homogeneous photo-cathode response and excellent timing properties. We have studied the response to single photo electrons of a newly developed 3-in. diameter PMT from ET Enterprises Ltd. A 2D-scanning system with a picosecond laser illuminating various positions on the photo-cathode surface was employed to study the timing and homogeneity of the PMT. Results of these investigations indicate good photo-cathode homogeneity, low dark noise on the sub-kHz level, and an average transit-time spread below 2 ns. Simulations indicate a significantly improved signal-to-background ratio in the multi-PMT DOM as compared to a triplet of optical modules each housing a single 10-in. PMT. (C) 2012 Elsevier B.V. All rights reserved.

The photoproduction of η ′ -mesons off different nuclei has been measured with the CBELSA/TAPS detector system for incident photon energies between 1500–2200 MeV. The transparency ratio has been deduced and compared to theoretical calculations describing the propagation of η ′ -mesons in nuclei. The comparison indicates a width of the η ′ -meson of the order of Γ = 15 – 25 MeV at ρ = ρ 0 for an average momentum p η ′ = 1050 MeV / c , at which the η ′ -meson is produced in the nuclear rest frame. The inelastic η ′ N cross section is estimated to be 3–10 mb. Parameterizing the photoproduction cross section of η ′ -mesons by σ ( A ) = σ 0 A α , a value of α = 0.84 ± 0.03 has been deduced.

Several new methods for determining the depth of interaction (DOI) of annihilation photons in monolithic scintillator detectors with single-sided, multi-pixel readout are investigated. The aim is to develop a DOI decoding method that allows for practical implementation in a positron emission tomography system. Specifically, calibration data, obtained with perpendicularly incident gamma photons only, are being used. Furthermore, neither detector modifications nor a priori knowledge of the light transport and/or signal variances is required. For this purpose, a clustering approach is utilized in combination with different parameters correlated with the DOI, such as the degree of similarity to a set of reference light distributions, the measured intensity on the sensor pixel(s) closest to the interaction position and the peak intensity of the measured light distribution. The proposed methods were tested experimentally on a detector comprised of a 20 mm x 20 mm x 12 mm polished LYSO: Ce crystal coupled to a 4 x 4 multi-anode photomultiplier. The method based on the linearly interpolated measured intensities on the sensor pixels closest to the estimated (x, y)-coordinate outperformed the other methods, yielding DOI resolutions between similar to 1 and similar to 4.5 mm FWHM depending on the DOI, the (x, y) resolution and the amount of reference data used.

A 2 pi hemispherical detector consisting of 64 CsI(Tl) scintillator modules covering the angular range of 80 degrees -160 degrees has been constructed. This detector is employed as the Inner Shell of the Plastic Ball detector and was used in two experimental programs concerning the study of pionic fusion in light-ion systems. The geometry and construction are presented. The improved response of the combined detectors for photons are discussed. An energy calibration method using high energy cosmic muons for the Inner Shell and the Plastic Ball has been developed and the calibration results are compared to the results of the same method in the Monte-Carlo simulation. The improved acceptance of the combined detectors for the pi(0) detection in the (6)Li((4)He, pi(0))(10)B* experiment are presented and discussed. (C) 2010 Elsevier B.V. All rights reserved.

The polarization observable I^s, a feature exclusive to the acoplanar kinematics of multi-meson final states produced via linearly polarized photons, has been measured for the first time. Results for the reaction g p -> p pi0 eta are presented for incoming photon energies between 970 MeV and 1650 MeV along with the beam asymmetry I^c. The comparably large asymmetries demonstrate a high sensitivity of I^s to the dynamics of the reaction. Fits using Bonn-Gatchina partial wave analysis demonstrate that the new polarization observables carry significant information on the contributing partial waves., 11 pages, 6 figures, v2 to appear in Phys. Lett. B

We have investigated the timing performance of Hamamatsu Multi-Pixel Photon Counter (MPPC) photosensors in light of their use in time-of-flight (TOF) positron emission tomography detectors. Measurements using picosecond laser pulses show a single photo-electron root-mean-square (RMS) timing resolution down to about 100ps. In coincidences of 511 keV photons detected with an LYSO crystal coupled to a MPPC and a BaF(2) detector, an optimum FWHM timing resolution of 600ps was obtained with leading edge time pickoff at the 1-1.5 photo-electron level. By optimizing the LYSO/MPPC coupling, this can be improved by a factor of 2. We further conclude that the use of stored digitized pulses allows great flexibility and efficiency in developing data analysis algorithms. (C) 2009 Elsevier B.V. All rights reserved.

The photoproduction of 2$\pi^0$ mesons off protons was studied with the Crystal Barrel/TAPS experiment at the electron accelerator ELSA in Bonn. The energy of photons produced in a radiator was tagged in the energy range from 600\,MeV to 2.5\,GeV. Differential and total cross sections and $p\pi^0\pi^0$ Dalitz plots are presented. Part of the data was taken with a diamond radiator producing linearly polarized photons, and beam asymmetries were derived. Properties of nucleon and $\Delta$ resonances contributing to the $p\pi^0\pi^0$ final state were determined within the BnGa partial wave analysis. The data presented here allow us to determine branching ratios of nucleon and $\Delta$ resonances for their decays into $p\pi^0\pi^0$ via several intermediate states. Most prominent are decays proceeding via $\Delta(1232)\pi$, $N(1440)1/2^+\pi$, $N(1520)3/2^-\pi$, $N(1680)5/2^+\pi$, but also $pf_0(500)$, $pf_0(980)$, and $pf_2(1270)$ contribute to the reaction.

A series of nucleon-nucleon bremsstrahlung (NN gamma) experiments at 190 MeV incident beam energy have been performed at KVI in order to gain more insight into the dynamics governing the bremsstrahlung reaction. After initial measurements wherein the bremsstrahlung process was studied far away from the elastic limit, a new study was used to probe the process nearer to the elastic limit by measuring at lower photon energies. Measured cross sections and analyzing powers are compared with the predictions of a microscopic model and those of two soft-photon models. The theoretical calculations overestimate the data by up to approximate to 30%, for some kinematics.

The azimuthal distributions of photons and charged particles with respect to the event plane are investigated as a function of centrality in Pb-208 + Pb-208 collisions at 158 (.) A GeV/c in the WA98 experiment at the CERN SPS. The anisotropy of the azimuthal distributions is characterized using a Fourier analysis. For both the photon and charged particle distributions the first two Fourier coefficients are observed to decrease with increasing centrality. The observed anisotropies of the photon distributions compare well with the expectations from the charged particle measurements for all centralities.

Hard-photon (E$_{\gamma}>$ 30 MeV) emission from second-chance nucleon-nucleon Bremsstrahlung collisions in intermediate energy heavy-ion reactions is studied employing a realistic thermal model. Photon spectra and yields measured in several nucleus-nucleus reactions are consistent with an emission from hot nuclear systems with temperatures $T\approx$ 4 - 7 MeV. The corresponding caloric curve in the region of excitation energies $\epsilon^\star\approx$ 3{\it A} - 8{\it A} MeV shows lower values of $T$ than those expected for a Fermi fluid., Comment: 13 pages, 3 figures. To appear in Physics Letters B

The response of PbWO4 to high-energy protons has been investigated in two test experiments at the proton facilities Cosy and AGOR. The determined energy resolution below 360 MeV of sigma/E = 1.44%/E-gamma(1/2) + 1.97% is comparable to the known photon response. Measured spectra of inelastically scattered protons below 85 MeV are even compared to similar distributions obtained for BaF2- and CeF3-detectors. (C) 2002 Elsevier Science B.V. All rights reserved.

Photoproduction off protons of the pπ 0 η three-body final state was studied with the Crystal Barrel/TAPS detector, at the electron stretcher accelerator ELSA in Bonn, for incident energies from the π 0 η production threshold up to 2.5 GeV. Differential cross sections and the total cross sections are presented. The use of linearly polarized photons gives access to the polarization observables Σ, I s , and I c , the latter two characterize beam asymmetries in case of three-body final states. Δ(1232)η, N(1535)1/2− π, and pa 0(980) are the dominant isobars contributing to the reaction. The partial wave analysis confirms the existence of some nucleon and Δ resonances, for which so far only fair evidence was reported. A large number of decay modes of known nucleon and Δ resonances is presented. It is shown that detailed investigations of decay branching ratios may provide a key to unravelling the structure of nucleon and Δ resonances.

The production of hard photons and neutral pions in 190 MeV proton induced reactions on C, Ca, Ni, and W targets has been for the first time concurrently studied. Angular distributions and energy spectra up to the kinematical limit are discussed and the production cross-sections are presented. From the target mass dependence of the cross-sections the propagation of pions through nuclear matter is analyzed and the production mechanisms of hard photons and primordial pions are derived. It is found that the production of subthreshold particles proceeds mainly through first chance nucleon-nucleon collisions. For the most energetic particles the mass scaling evidences the effect of multiple collisions., Comment: submitted to Phys. Lett. B

A measurement of direct photon production in {sup 208}Pb+ {sup 208}Pb collisions at 158A GeV has been carried out in the CERN WA98 experiment. The invariant yield of direct photons in central collisions is extracted as a function of transverse momentum in the interval 0.5 1.5 GeV/c . The result constitutes the first observation of direct photons in ultrarelativistic heavy-ion collisions. It could be significant for diagnosis of quark-gluon-plasma formation.

Photons originating from coherent bremsstrahlung have been measured over a large dynamic range for the reaction of 200 MeV alpha particles with protons. At low photon energies the bremsstrahlung spectrum exhibits the classical behavior with an approximate 1/E-gamma shape. At higher photon energies there is a pronounced contribution from capture into the unbound ground state and first excited state of Li-5. These results allow one, for the first time, to test theoretical models for a consistent description of bremsstrahlung and radiative capture in a complex system. Calculations predict both features qualitatively but fail to account for their relative importance.

We report on a measurement of subthreshold eta and pi(0) mesons in the reaction Ar + Ca at 180A MeV. We find that the ratio of the eta to pi(0) meson-production cross section is more than a factor of 20 smaller than the one expected from threshold-energy scalings of meson production. In addition, the multiplicity of high m(t) pi(0) increases faster with the centrality of the reaction than the multiplicity of the bulk of pi(0) mesons. This behavior is explained by the rescattering of pi mesons in nuclear matter at the origin of most energetic particles.

In photoproduction experiments, a large number of final states yielding various resonance contributions are accessible. To extract resonance parameters via partial-wave analysis not only the measurement of differential cross-sections is necessary, but also the determination of polarization observables. At the electron accelerator ELSA (Bonn) the coherent bremsstrahlung method was used to generate a linearly polarized photon beam. Using the CBELSA/TAPS detector setup, the beam asymmetry Σ in the reaction γp → pπ 0 η was determined as a function of various masses and angles for photon energies between 970MeV and 1650MeV.

The impact-parameter dependence of π o - and η-meson production is reported for the system 40 Ar+ nat Ca at a beam energy of 800 A MeV. Scaling of the meson abundances with the transverse mass is observed. The experimental results are compared to calculations within the BUU model.

The PANDA detection system at FAIR, Germany, is designed to study antiproton-proton annihilations, in order to investigate among others the realm of charm-meson states and glueballs, which has still much to reveal. The yet unknown properties of this field are to be unraveled through studying QCD phenomena in the non-perturbative regime. The multipurpose PANDA detector will be capable of tracking, calorimetry, and particle identification, and is foreseen to run at high luminosities providing average reaction rates up to 20 Million interactions/s. The envisaged physics program requires measurements of photons and charged particles with excellent energy, position, and time resolutions. The electromagnetic calorimeter (EMC) will serve as one of the basic components of the detector setup and comprises cooled Lead-Tungstate (PbWO4) crystals. This paper presents the mechanical design of the Forward End-cap calorimeter and analyses the response of this detector component in conjunction with the full EMC and the complete PANDA detector. The simulation studies are focused on the performance of the planned EMC with respect to the energy and spatial resolution of the reconstructed photons. Results of the Monte Carlo simulations, excluding very low-energy photons, have been validated by data obtained from a prototype calorimeter and shown to fulfil the requirements imposed by the PANDA physics program., Comment: 22 Pages, 24 Figures, 4 Tables

Measurements of direct photon production in p + Pb and p + C collisions at root(NN)-N-s = 17.4 GeV are presented. Upper limits on the direct photon yield as a function of p(T) are derived and compared to the results for Pb + Pb collisions at root(NN)-N-s = 17.3 GeV. The production of the eta meson, which is an important input to the direct photon signal extraction, has been determined in the eta -> 2 gamma channel for p + C collisions at root(NN)-N-s = 17.4 GeV. (c) 2012 Elsevier B.V. All rights reserved.

Hard photons were measured in coincidence with projectilelike fragments in peripheral reactions of Ar-36 + Tb-159 at E/A = 44 MeV. The probability for hard photon production was found to scale linearly with the transferred mass and to depend on the direction of the transfer, thus indicating the relative importance of one- and two-body dissipation in peripheral reactions.

The future cubic kilometre scale neutrino telescope KM3NeT will employ a novel type of a Digital Optical Module (DOM), developed during the recent FP6 Design Study. A pressure-resistant glass sphere hosts 31 photomultiplier tubes (PMTs) of 3-in, diameter, together with all the electronics for high-voltage generation and signal readout. The optical module forms a complete stand-alone detector that. is connected to the outside world via a single optical fibre and two copper conductors providing electrical power. The advantages of using multiple small PMTs in the same DOM are the higher quantum efficiency (>30% expected), smaller transit time spread, better two-photon separation capability and directional sensitivity. Moreover, a longer operating lifetime is expected than for large PMTs due to the accumulation of less charge on the anode. In addition, small PMTs are insensitive to the Earth's magnetic field and do not require mu-metal shielding. In order to maximise the detector sensitivity, each PMT will be surrounded by an expansion cone collecting photons that would normally miss the photocathode. Such an expansion cone consists of an aluminium ring filled with silicone gel. An increase in the overall sensitivity, integrated over all angles of incidence, was estimated to be about 27%. Monte-Carlo simulations have shown that a detector configuration with multi-PMT DOMs requires three times less OMs to achieve the same performance as conventional OMs hosting 10-in. PMTs. Prototype DOMs are currently being built by the KM3NeT consortium. (C) 2011 Elsevier B.V. All rights reserved.

The probability for Bremsstrahlung has been measured as function of the mass of the projectile-like fragment in peripheral reactions of 36 Ar + 159 Tb at 44 MeV/nucleon. The Bremsstrahlung probability is found to depend on the amount of mass transferred and the direction of the transfer.

The PANDA collaboration at FAIR, Germany, will employ antiproton annihilations to investigate yet undiscovered charm-meson states and glueballs. The aim is to study QCD phenomena in the non-perturbative regime and to unravel the origin of hadronic masses. A multi-purpose detector for tracking, calorimetry and particle identification is presently being developed to run at high luminosities providing up to 2 x 10(7) interactions/s. One of the crucial components of the PANDA spectrometer is the Electromagnetic Calorimeter, composed of cooled PbWO(4) crystals. This paper describes construction and performance of a fully functioning prototype of this calorimeter. The performance was determined from measurements exploiting cosmic muons and high-energy tagged photons from the MAMI-C electron accelerator. The response measurements were carried out using sampling ADCs and, for comparison, charge-integrating ADCs. The achieved results validate the usage of sampling ADCs with a moderate sampling frequency, provide the energy resolution as foreseen in the Technical Design Report of the full calorimeter, and secure event correlation by achieving a good timing resolution through digital analysis of the sampled signals. (C) 2011 Elsevier B.V. All rights reserved.

Photoproduction of neutral pions has been studied with the CBELSA/TAPS detector in the reaction $\gamma p\to p\pi^0$ for photon energies between 0.85 and 2.50 GeV. The $\pi^0$ mesons are observed in their dominant neutral decay mode: $\pi^0\to\gamma\gamma$. For the first time, the differential cross sections cover the very forward region, $\theta_{\rm c.m.}, Comment: 12 pages, 9 figures, minor modifications to PWA section

Differential cross sections of the reaction $p(d,^3{\rm He})\gamma$ have been measured at deuteron laboratory energies of 110, 133 and 180 MeV. The data were obtained with a coincidence setup measuring both the outgoing $^3$He and the photon. The data are compared with modern calculations including all possible meson-exchange currents and two- and three- nucleon forces in the potential. The data clearly show a preference for one of the models, although the shape of the angular distribution cannot be reproduced by any of the presented models., Comment: 6 pages, 6 figures, accepted for publication in EPJA

Using a 3790 module lead glass photon spectrometer π° and η mesons have been measured in 200 A · GeV S + Au collisions by invariant mass reconstruction. Centrality selected transverse momentum π° spectra have been obtained for 0.5 ≤ pT ≤ 4.0 GeV/c displaying a much flatter slope for central compared to peripheral data. The problems of extracting single photon to π° yields from the data are discussed and the total error of their determination is given for present and future experiments. It is concluded that an accuracy of about 7 % can be obtained in the γπ° ratio.

In this work we present a measurement setup for the determination of scintillation pulse shapes of fast scintillators. It is based on a time-correlated single photon counting approach that utilizes correlation between 511 keV annihilation photons to produce start and stop signals in two, separate crystals. The measurement is potentially cost effective and simple to set up while maintaining an excellent timing resolution of 120 ps. Apart from these advantages this method probes the entire volume of the crystal under investigation rather than a thin layer at the surface. Here, we present a proof-of-concept and first results for the time constants of LYSO.

Data on the photoproduction of $\ensuremath{\omega}$ mesons on nuclei have been reanalyzed in a search for in-medium modifications. The data were taken with the crystal barrel (CB)/two-arm photon spectrometer (TAPS) detector system at the ELectron Stretcher Anlage (ELSA) accelerator facility in Bonn. First results from the analysis of the data set were published by Trnka et al. [Phys. Rev. Lett. 94, 192303 (2005)], claiming a lowering of the $\ensuremath{\omega}$ mass in the nuclear medium by 14$%$ at normal nuclear matter density. The extracted $\ensuremath{\omega}$ line shape was found to be sensitive to the background subtraction. For this reason a reanalysis of the same data set has been initiated, and a new method has been developed to reduce the background and to determine the shape and absolute magnitude of the background directly from the data. Details of the reanalysis and of the background determination are described. The $\ensuremath{\omega}$ signal on the Nb target, extracted in the reanalysis, does not show a deviation from the corresponding line shape on a ${\mathrm{LH}}_{2}$ target, measured as reference. The earlier claim of an in-medium mass shift is thus not confirmed. The sensitivity of the $\ensuremath{\omega}$ line shape to different in-medium modification scenarios is discussed.

The response of a silicon photomultiplier (SiPM) to optical signals is inherently nonproportional due to saturation, afterpulsing, and crosstalk. Existing models of the SiPM response do not account for all of these effects, and therefore, these models are not sufficiently accurate for many applications. In this work, a comprehensive model of the SiPM response is developed that is generally applicable to exponentially decaying light pulses and that can be simplified in the case of very short (e.g., laser) light pulses. The model accounts for the total number and the temporal distribution of the incident photons as well as for the relevant SiPM parameters, viz. the recovery time, afterpulsing, crosstalk, and their cross correlations. The model is shown to correspond well with measurements on a SiPM-based scintillation detector. Furthermore, it is shown to be in agreement with several cases for which the SiPM response is known a priori. Having thus validated the model, its use is demonstrated by predicting the response of the Hamamatsu multipixel photon counter (MPPC) S10362-33-050C SiPM to several different scintillators.

Photoproduction of neutral pions has been studied with the CBELSA/TAPS detector for photon energies between 0.92 and 1.68 GeV at the electron stretcher accelerator ELSA. The beam asymmetry Sigma has been extracted for 115 degrees > theta(c.m.) > 155 degrees of the pi(0) meson and for theta(c.m.) > 60 degrees. The new beam-asymmetry data cover the very forward region, which extend previously published data for the same reaction by our collaboration and improve the world database for photon energies above 1.5 GeV. The angular dependence of Sigma shows overall good agreement with the SAID parametrization.

Differential cross sections of neutral pions emitted in {sup 181}Ta+{sup 197}Au collisions at a beam energy of 39.5A MeV have been measured with the two-arm photon spectrometer (TAPS). The kinetic energy and transverse momentum spectra of neutral pions cannot be properly described in the framework of the thermal model, nor when the reabsorption of pions is accounted for in a phenomenological model. However, high energy and high momentum tails of the pion spectra can be well fitted through thermal distributions with unexpectedly soft temperature parameters below 10 MeV.

The response of a Silicon Photomultiplier (SiPM) is affected by afterpulsing, crosstalk, and saturation. Existing models for the SiPM response do not account for all of these effects and, therefore, these models are not sufficiently accurate for many applications. Here we present a model for the SiPM response that is generally applicable to light pulses with a finite, exponential decay. It accounts for the total number and the temporal distribution of the incident photons as well as the relevant SiPM parameters, viz. the recovery time, afterpulsing, and crosstalk, and their cross-correlations. The model is validated by comparison to measurements on a scintillation detector, showing good correspondence.

A search for direct photon production has been performed using p and 16O projectiles at 60 and 200 A-GeV interacting with a C and a Au target. Photon and π0 spectra have been measured in the pseudorapidity range 1.5 ⩽ η ⩽ 2.1 for the transverse momentum region 0.4 GeV/c ⩽ PT ⩽ 3 GeV/c with the lead-glass spectrometer SAPHIR. An upper limit of 15 % at the 90 % CL for the direct photon signal relative to the neutral pion production is obtained from the measured photon spectra and the Monte Carlo simulations of the hadronic background based on reconstructed π0 and η spectra. Consequences for a possible phase transition to a quark-gluon plasma (QGP) are discussed.

The photoproduction of {omega} mesons off protons has been studied at the Bonn ELSA accelerator from threshold to E{sub {gamma}}=1700 MeV. Linearly polarized beams were produced via coherent bremsstrahlung. Large photon asymmetries in excess of 50% were obtained. For the first time the pion asymmetries associated with the {omega}{yields}{pi}{sup 0}{gamma} decay were measured and found close to zero. The asymmetries indicate s-channel resonance formation on top of t-channel exchange processes.

Precise data for quasi-free photoproduction of η-mesons off the deuteron have been measured at the Bonn ELSA accelerator with the combined Crystal Barrel/TAPS detector for incident photon energies up to 2.5GeV. The η-mesons have been detected in coincidence with recoil protons and neutrons. Possible nuclear effects like Fermi motion and re-scattering can be studied via a comparison of the quasi-free reaction off the bound proton to η-production off the free proton. No significant effects beyond the folding of the free cross-section with the momentum distribution of the bound protons have been found. These Fermi motion effects can be removed by an analysis using the invariant mass of the η-nucleon pairs reconstructed from the final-state four-momenta of the particles. The total cross-section for quasi-free η-photoproduction off the neutron reveals even without correction for Fermi motion a pronounced bump-like structure around 1GeV of incident photon energy, which is not observed for the proton. This structure is even narrower in the invariant-mass spectrum of the η-neutron pairs. Position and width of the peak in the invariant-mass spectrum are W ≈ 1665 MeV and FWHM Γ ≈ 25 MeV. The data are compared to the results of different models.

Results from 16 O- and 32 S-induced reactions obtained by the WA80 collaboration at the CERN SPS are presented with reference to global event characteristics such as collision geometry, the degree of nuclear stopping, and the energy density attained. Transverse momentum spectra of neutral pions and of direct photons are also presented. At an accuracy within 15% limits, all observed photons are accounted for by known hadronic decays.

Results are presented from 60 and 200 A GeV p, 16O and 32S projectiles with C, Cu, Ag and Au nuclei. Energy spectra at zero degrees and transverse energy distributions in the pseudorapidity interval 2.4 to 5.5 are shown. Transverse momentum distributions of inclusive photons and neutral pions at midrapidity measured with a lead glass array are also presented. For all target projectile combinations an increase in average pT is observed for small entropy values, which is deduced from the central multiplicity density. Different from proton induced reactions, 200 A GeV 16O + Au data show a plateau like region at large values of entropy density characteristic for a mixed phase.

Properties of the Roper resonance, the first scalar excitation of the nucleon, are determined. Pole positions and residues of the $P_{11}$ partial wave are studied in a combined analysis of pion- and photo-induced reactions. We find the Roper pole at $\{(1371\pm7)-i(92\pm10)\}$ MeV and an elasticity of $0.61\pm 0.03$. The largest decay coupling is found for the $N\sigma$ ($\sigma=(\pi\pi)$-$S$-wave). The analysis is based on new data on $\gamma p\to p\pi^0\pi^0$ for photons in the energy range from the two-pion threshold to 820 MeV from TAPS at Mainz and from 0.4 to 1.3 GeV from Crystal Barrel at Bonn and includes further data from other experiments. The partial wave analysis excludes the possibility that the Roper resonance is split into two states with different partial decay widths., Comment: 14 pages, 6 figures

The response of PbWO/sub 4/ to high-energy protons has been investigated in two test experiments at the proton beam facilities COSY and AGOR. The determined energy resolution below 360 MeV of /spl sigma//E = 0.97%/E/sub /spl gamma///sup 1/2/ + 3.33% is comparable to the previously deduced photon response. Energy spectra of inelastically scattered protons below 85 MeV are compared to similar distributions obtained for BaF/sub 2-/ and CeF/sub 3-/ detectors, respectively. The comparison to the response of charged pions indicates a strong quenching effect of the scintillation light for hydrogen isotopes.

Single pi(0) photoproduction has been studied with the CB-ELSA experiment at Bonn using tagged photon energies between 0.3 and 3.0 GeV. The experimental setup covers a very large solid angle of similar to98% of 4pi. Differential cross sections dsigma/dOmega have been measured. Complicated structures in the angular distributions indicate a variety of different resonances being produced in the s channel intermediate state gammap-->N(*)(Delta(*))-->ppi(0). A combined analysis including the data presented in this letter along with other data sets reveals contributions from known resonances and evidence for a new resonance N(2070)D(15).

Total and differential cross sections for the reaction p(gamma,eta)p have been measured for photon energies in the range from 750 MeV to 3 GeV. The low-energy data are dominated by the S-11 wave which has two poles in the energy region below 2 GeV. Eleven nucleon resonances are observed in their decay into peta. At medium energies we find evidence for a new resonance N(2070)D-15 with (M,Gamma)=(2068+/-22, 295+/-40) MeV. At gamma energies above 1.5 GeV, a strong peak in the forward direction develops, signaling the exchange of vector mesons in the t channel.

Directed and elliptic flow of inclusive photons near mid-rapidity in $158 $A GeV Pb+Pb collisions has been studied. The data have been obtained with the photon spectrometer LEDA of the WA98 experiment at the CERN SPS. The flow strength has been measured for various centralities as a function of $p_T$ and rapidity over $0.18 < p_T < 1.5 \mathrm{GeV}/c$ and $2.3 < y < 2.9$. The angular anisotropy has been studied relative to an event plane obtained in the target fragmentation region that shows the elliptic flow to be in-plane. The elliptic flow has also been studied using two-particle correlations and shown to give similar results. A small directed flow component is observed. Both the directed and elliptic flow strengths increase with $p_T$. The photon flow results are used to estimate the corresponding neutral pion flow., 28 pages, 10 figures

Two-particle correlations of direct photons were measured in central 208Pb+208Pb collisions at 158A GeV. The invariant interferometric radii were extracted for 100

With the combined setup of the Crystal Barrel and TAPS photonspectrometers at ELSA in Bonn we have studied photonuclear reactions on protons and deuterons. From the series of experiments on single and multiple neutral meson emission we concentrate here on the hyperon production off protons and deuterons, and in particular on the K(0)Sigma(+) decay channel. The reaction is characterized by the final state of 6 photons and a forward emitted proton. We report on results of simulations to demonstrate the feasibility of the experiment. First results on the identification of kaons and hyperons are presented.

Results on the study of localized fluctuations in the multiplicity of charged particles and photons produced in 158A GeV/c Pb+Pb collisions are presented for varying centralities. The charged versus neutral particle multiplicity correlations in common phase space regions of varying azimuthal sizes are analyzed by two different methods. Various types of mixed events are constructed to probe fluctuations arising from different sources. The measured results are compared to those from simulations and from mixed events. The comparison indicates the presence of nonstatistical fluctuations in both the charged particle and photon multiplicities in limited azimuthal regions. However, no correlated charged-neutral fluctuations, a possible signature of formation of disoriented chiral condensates, are observed. An upper limit on the production of disoriented chiral condensates is set.

Event-by-event fluctuations in the multiplicities of charged particles and photons, and the total transverse energy in 158A GeV Pb+Pb collisions are studied for a wide range of centralities. For narrow centrality bins the multiplicity and transverse energy distributions are found to be near perfect Gaussians. The effect of detector acceptance on the multiplicity fluctuations has been studied and demonstrated to follow statistical considerations. The centrality dependence of the charged particle multiplicity fluctuations in the measured data has been found to agree reasonably well with those obtained from a participant model. However, for photons the multiplicity fluctuations have been found to be lower compared to those obtained from a participant model. The multiplicity and transverse energy fluctuations have also been compared to those obtained from the VENUS event generator.

Recent improvements in the performance of PbWO4, a high-density inorganic scintillator, offer the possibility to design very compact, large-acceptance electromagnetic calorimeters with excellent timing and good energy resolution, applicable also for photon energies below 1 Gev. Such a detection system is planned to be built for the internal magnetic spectrometer ANKE (Apparatus for Studies of Nucleon and Kaon Ejectiles) at the Cooler Synchrotron (Cosy) Julich. The limited space and stray magnetic fields of ANKE place severe boundary conditions, which have to be taken into account for detector layout and the choice of photo sensors. (C) 2002 Elsevier Science B.V. All rights reserved.