Search

The in-medium properties of the ${\mathrm{D}}_{13}(1520)$ nucleon resonance were studied via photoproduction of ${\ensuremath{\pi}}^{0}$ mesons from nuclei (C, Ca, Nb, Pb) with the TAPS detector at the Mainz Microton accelerator. The inclusive (single and multiple pion production) data disagree with model predictions which explain the disappearance of the second resonance bump in total photoabsorption via a medium modification of the ${\mathrm{D}}_{13}\ensuremath{\rightarrow}\mathrm{N}\ensuremath{\rho}$ decay. The exclusive single ${\ensuremath{\pi}}^{0}$ production data show no broadening of the resonance structure beyond Fermi smearing. Both results together cast doubt on attempts to explain the vanishing of the second resonance bump for nuclei by a broadening of the ${\mathrm{D}}_{13}$ resonance.

Differential cross-sections for the reactions d(gamma,pi (o))d and d(gamma,pi (o))pn have been measured at MAMI with the TAPS detector setup in the energy range 140 MeV > E- > 306 MeV. By use of the Glasgow tagging spectrometer an 0.8 MeV energy resolution for photons incident on the target was achieved. The (o) missing energy resolution was sufficient for a reliable separation of coherent and incoherent channels. The data for the break-up channel exhibit very strong final state interaction effects, whereas the observed angular dependence of the inclusive process d(gamma, pi (o))X is in quantitative agreement with predictions for a quasi-free process. The observed absolute d(gamma, pi (o))X cross-sections, on, the other hand, are significantly smaller than predicted by the quasi-free process for E-gamma greater than or similar to 250 MeV. Associating this failure with the pi (o) photoproduction on the neutron would suggest that its cross-section is up to 25% below the presently believed value.

Differential cross-sections for Compton scattering from the proton have been measured at the MAMI tagged photon facility using the TAPS setup. The data cover an angular range of θlab γ = 59°-155° and photon energies ranging from 55 MeV to 165 MeV. Our results are in good agreement with those from previous experiments, but yield higher precision. Using dispersion relations the proton polarizabilities have been determined to be = [11.9±0.5stat.±1.3syst.±0.3mod.] . 10-4 fm 3 and = [1.2±0.7stat.±0.3syst.±0.4mod.)] . 10-4 fm 3. These results confirm the Baldin sum rule which was re-evaluated to be + = [13.8±0.4] . 10-4 fm 3. We can also conclude that there is no significant additional asymptotic contribution to the backward spin polarizability γπ beyond the t-channel π0-exchange.

Precise angular distributions have been measured for the first time for the photoproduction of π0 mesons off neutrons bound in the deuteron. The effects from nuclear Fermi motion have been eliminated by a complete kinematic reconstruction of the final state. The influence of final-state-interaction effects has been estimated by a comparison of the reaction cross section for quasifree protons bound in the deuteron to the results for free protons and then applied as a correction to the quasifree neutron data. The experiment was performed at the tagged photon facility of the Mainz Microtron MAMI with the Crystal Ball and TAPS detector setup for incident photon energies between 0.45 and 1.4 GeV. The results are compared to the predictions from reaction models and partial-wave analyses based on data from other isospin channels. The model predictions show large discrepancies among each other and the present data will provide much tighter constraints. This is demonstrated by the results of a new analysis in the framework of the Bonn-Gatchina coupled-channel analysis which included the present data.

The gamma p -> K-0 Sigma(+) reaction has been measured from threshold to E-gamma = 1.45 GeV (W-CM = 1.9 GeV) using the Crystal Ball and TAPS multiphoton spectrometers together with the photon tagging facility at the Mainz Microtron MAMI. In the present experiment, this reaction was searched for in the 3 pi(0)p final state, by assuming K-S(0) -> pi(0)pi(0) and Sigma(+) -> pi(0)p. The experimental results include total and differential cross sections as well as the polarization of the recoil hyperon. The new data significantly improve empirical knowledge about the gamma p -> K-0 Sigma(+) reaction in the measured energy range. The results are compared to previous measurements and model predictions. It is demonstrated that adding the present gamma p -> K-0 Sigma(+) results to existing data allowed a better description of this reaction with various models. (Less)

The photoproduction of two neutral pions from the deuteron has been studied for incident photon energies from 200 MeV to 820 MeV with the TAPS spectrometer at MAMI (Mainz Microtron). The total cross-section was determined and used to deduce the cross-section from the neutron. Due to the good statistical quality of the data Dalitz plots for the three particles in the exit channel (pi (0)pi N-0) could be constructed. The invariant mass distributions derived from them are presented in this paper. They indicate that the important reaction mechanism in the second resonance region is a sequential decay pattern involving the population of the Delta (1232)-resonance as an intermediate state.

Double neutral pion photoproduction from the proton has been measured at MAMI for photon energies between threshold and 820 MeV. The reaction was identified by an invariant mass and missing mass analysis. From threshold up to 370 MeV the total cross-section does not exceed 30 nb. For higher energies it shows a smooth rise until it reaches a maximum of about 10 μb at Eγ = 740 MeV. Dalitz plots of m2(π0π0) versusm2(p,π0) for seven bins of incident photon energy have been analysed. For Eγ > 610 MeV, a strong contribution of a sequential decay is observed with the Δ(1232)-resonance as intermediate state. A comparison to model calculations shows that these sequential decays presumably originate from the D13(1520) and also the P11(1440)-resonance.

Coherent and incoherent pi(0) photoproduction off the deuteron has been measured for incident quasi-monoenergetic photons in the energy range from pi(0) threshold up to 300 MeV using the TAPS setup together with the Glasgow tagger at MAMI. Integral and differential cross sections over the full angular range have been obtained. Coherent and incoherent production are found to exhibit very different angle and energy dependences. The sum of both processes, the total pi(0) production off the deuteron is quantitatively extremely well described by a pure quasifree model, whereas the data for coherent and incoherent channels necessitate strong FSI effects for their description. Finally upper limits for the production of dibaryon resonances in this reaction have bees obtained.

Photoproduction of neutral pions from the deuteron has been studied for incident photon energies from 200 MeV to 792 MeV with the TAPS detector at the Mainz MAMI accelerator. Total and differential cross sections covering the full angular range have been obtained for coherent and incoherent single πo-photoproduction. Good agreement between model predictions and the data was found for the coherent process. The incoherent cross section in the energy region of the Δ(1232)-resonance is overestimated by existing models. A comparison to model predictions indicates that final state interaction effects are much more important than for the coherent reaction. However, the angular dependence of the data in the Δ-peak region follows the pattern expected from the dominant excitation of the M1+-multipole on the free nucleon. The energy and angular dependence of single πo-photoproduction in the second resonance region is remarkably different from the reaction on the free proton, indicating a strong nuclear effect. Finally the total cross section for double πo-photoproduction from the deuteron has been measured for the first time and was used to estimate the cross section for double πo-photoproduction from the neutron.

Differential cross sections and beam asymmetries for coherent \pi^\circ photoproduction from ^4He in the \Delta energy-range have been measured with high statistical and systematic precisions using both decay photons for identifying the process.The experiment was performed at the MAinz MIcrotron using the TAPS photon spectrometer and the Glasgow/Mainz tagged photon facility. The differential cross sections are in excellent agreement with predictions based on the DWIA if an appropriate parametrization of the \Delta-nuclear interaction is applied. The beam asymmetries are interpreted in terms of degrees of linear polarization of collimated coherent bremsstrahlung. The expected increase of the degree of linear polarization with decreasing collimation angle is confirmed. Agreement with calculations is obtained on a few-percent level of precision in the maxima of the coherent peaks.

Photoproduction of η mesons from 4He has been measured for the first time. The experiment was performed at the Mainz Microtron (MAMI) using the TAPS photon spectrometer and the Glasgow/Mainz tagged photon facility. η mesons were identified in coincidence with recoil nucleons. Total and differential cross sections are presented in the photon energy range between threshold and 818 MeV. The exclusive data are used to determine the ratio of the elementary production cross sections on bound neutrons and protons, σn/σp= 0.68 ± 0.02 ± 0.09. In addition, upper limits for the total coherent cross section have been derived.

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 nucleus-nucleus collisions at bombarding energies on the order of 1 AGeV nuclear matter can be compressed to similar densities as encountered in stellar processes, i.e. to 2–3 times normal density, Experimental data providing information on the space-time evolution of these collisions are presented: the properties of hadrons in the hot and compressed nuclear medium in the high-density phase, collective flow phenomena during the expansion phase, and the hadrochemical composition of the collision system in the final stage of the reaction at freeze-out are discussed. Future directions in the heavy-ion reaction program are indicated.

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 neutral pi(o) and eta mesons are studied in Au-197-Au-197 collisions at an incident energy of 800A MeV, substantially below the threshold for eta production in N-N collisions. While the gross pi(o) multiplicity increases almost linearly with the number of participant nucleons, the multiplicities of eta and hard pi(o) mesons show a stronger than linear dependence. The nonlinearity is governed by the average transverse-mass excess [m(t)] - (root s - 2m(N)) of the mesons and is insensitive to their final-state interaction in the nuclear medium.

The Dalitz-decay contributions of pi(0) and eta mesons to the di-electron invariant-mass spectrum at 1A GeV have been obtained from a systematics of inclusive meson production cross sections measured for the collision systems C-12 + C-nat and Ar-40, Ca-40 + Ca-nat in the bombarding-energy range of 0.8-2.0 A GeV. These results are compared with the recently published di-electron mass spectra of the DLS collaboration. Systematic errors and angular-distribution effects are discussed. We conclude that the low-mass part of the DLS data cannot be explained by the Dalitz decays of light neutral mesons only.

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 photoproduction of two neutral pions off the proton has been studied for incident photon energies between E γ = 309 MeV (threshold) and 792 MeV with the TAPS photon spectrometer at the Mainz Microtron MAMI. Both 3γ- and 4γ-events have been analysed in order to deduce the total p ( γ , 2 π 0 ) p cross section. The total cross section is less than 300 nb between threshold and ⋍ 460 MeV, beyond which it starts to rise to a maximum value of about 11 μb at around 750 MeV. For the highest energies it starts to decrease again. The 4γ-sample has been used to reconstruct 2 neutral pions from the above reaction channel by an invariant mass and a missing mass analysis. After reconstruction of the unobserved proton, a Dalitz-plot of m π 0 π 0 2 vs. m pπ 0 2 has been obtained, which indicates that the decay chain includes the Δ(1232)-resonance as an intermediate state.

The first measurement of the helicity dependence of the photoproduction cross section of single neutral pions off protons is reported for photon energies from 600 to 2300 MeV, covering nearly the full solid angle. The data are compared to predictions from the SAID, MAID, and BnGa partial wave analyses. Strikingly large differences between data and predictions are observed, which are traced to differences in the helicity amplitudes of well-known and established resonances. Precise values for the helicity amplitudes of several resonances are reported.

We report the first large-acceptance measurement of polarization transfer from a polarized photon beam to a recoiling nucleon. The measurement pioneers a novel polarimetry technique, which can be applied to many other nuclear and hadron physics experiments. The commissioning reaction of 1H(γ, p)π0 in the range 0.4Eγ1.4 GeV validates the technique and provides essential new data to constrain the excitation spectrum of the nucleon.

Data on inclusive pion and $\ensuremath{\eta}$ production measured with the dielectron spectrometer HADES in the reaction $p+{}^{93}\mathrm{Nb}$ at a kinetic beam energy of 3.5 GeV are presented. Our results, obtained with the photon-conversion method, supplement the rather sparse information on neutral-meson production in proton-nucleus reactions existing for this bombarding energy regime. The reconstructed ${e}^{+}{e}^{\ensuremath{-}}{e}^{+}{e}^{\ensuremath{-}}$ transverse-momentum and rapidity distributions are confronted with transport-model calculations, which account fairly well for both ${\ensuremath{\pi}}^{0}$ and $\ensuremath{\eta}$ production.

New results on the differential cross section in deuteron-proton elastic scattering are obtained at the deuteron kinetic energy of 2.5 GeV with the HADES spectrometer. The angular range of 69°-125° in the center of mass system is covered. The obtained results are compared with the relativistic multiple scattering model calculation using the CD-Bonn deuteron wave function. The data at fixed scattering angles in the c.m. are in qualitative agreement with the constituent counting rules prediction.

This document describes the technical layout and the expected performance of the Straw Tube Tracker (STT), the main tracking detector of the $\bar{P}$ANDA target spectrometer. The STT encloses a Micro-Vertex-Detector (MVD) for the inner tracking and is followed in beam direction by a set of GEM stations. The tasks of the STT are the measurement of the particle momentum from the reconstructed trajectory and the measurement of the specific energy loss for a particle identification. Dedicated simulations with full analysis studies of certain proton-antiproton reactions, identified as being benchmark tests for the whole $\bar{P}$ANDA scientific program, have been performed to test the STT layout and performance. The results are presented, and the time lines to construct the STT are described.

Total and differential cross sections for photoproduction of eta mesons from C-12, Ca-40, Nb-93 and Pb-nat have been obtained up to 790 MeV incident photon energy at the Mainz Microtron (MAMI) with the TAPS spectrometer. The absorption cross section sigma(eta N)(abs) = (30 +/- 2.5 +/- 6) mb of eta mesons in nuclear matter and the absorption length lambda(eta) = (2.0 +/- 0.2 +/- 0.4) fm are extracted. No significant depletion of the S(1)1(1535) strength in the eta photoproduction on nuclei is observed.

Differential and total cross sections for the photoproduction of neutral pions from the proton have been measured for incident photon energies from 140–270 MeV, using the photon spectrometer TAPS at the tagged photon beam of the 855 MeV Mainz Microtron. The energy dependence of the s- and p-wave multipoles close to threshold was deduced from a multipole fit and a multipole analysis. The extracted s-wave amplitude E 0+ at threshold is found to be significantly smaller than the prediction of the classical low energy theorems, but is in reasonable agreement with the chiral perturbation theory.

Near threshold photoproduction ofη-mesons from the proton has been measured at the MAMI accelerator with the TAPS spectrometer. The mass of theη-meson was deduced from the threshold energy forη-photoproduction. The result of m η =(547.12 ± 0.06 ± 0.25) MeV supports the low value of theη-mass reported from a dp →3Heη measurement at SATURNE in 1992. Theη-decay branching ratio Γ3π0/Γγγ was measured to be (0.832±0.005±0.012).

Absolute differential and integrated-over-angle cross sections for the reactions 12 C( γ , π 0 ) 12 C and 13 C( γ , π 0 ) 13 C leaving the residual nuclei in the ground state have been measured with tagged bremsstrahlung photons in the near threshold region (170.1 MeV ≤ E γ ≤ 176.8 MeV). This reaction mechanism is sensitive to the whole nuclear volume and the coherent processes which occur are revealed by comparing the extracted cross sections for both isotopes with each other and with theoretical calculations.

Precise total cross-sections and invariant-mass distributions have been measured for photoproduction of pion pairs off the proton producing $p\pi^0\pi^0$ and $n\pi^+\pi^0$ final states from the threshold region up to 800 MeV incident photon energy. Additionally, beam helicity asymmetries have been measured in the second resonance region (550 MeV - 820 MeV). The experiment was performed at the tagged photon beam of the Mainz MAMI accelerator with the Crystal Ball and TAPS detectors combined to give an almost 4$\pi$ solid-angle electromagnetic calorimeter. The results are much more precise than any previous measurements and confirm the chiral perturbation theory predictions for the threshold behavior of these reactions. In the second resonance region, the invariant-mass distributions of meson-meson and meson-nucleon pairs are in reasonable agreement with model predictions, but none of the models reproduce the asymmetries for the mixed-charge channel., Comment: accepted for publication in Eur. Phys. J. A

The gamma p -> pi(0)pi(0) p reaction has been measured from threshold to 1.4 GeV using the Crystal Ball and TAPS photon spectrometers together with the photon tagging facility at the Mainz Microtron. The experimental results include total and differential cross sections as well as specific angular distributions, which were used to extract partial-wave amplitudes. In particular, the energy region below the D-13(1520) resonance was studied.

We present results of an exclusive measurement of the first-excited state of the $\ensuremath{\Sigma}$ hyperon, $\ensuremath{\Sigma}{(1385)}^{+}$, produced in $p+p\ensuremath{\rightarrow}{\ensuremath{\Sigma}}^{+}+{K}^{+}+n$ at $3.5\phantom{\rule{0.28em}{0ex}}\mathrm{GeV}$ beam energy. The extracted data allow the detailed study of the invariant mass distribution of the $\ensuremath{\Sigma}{(1385)}^{+}$. The mass distribution is well described by a relativistic Breit-Wigner function with a maximum at ${m}_{0}=1383.2\ifmmode\pm\else\textpm\fi{}0.9\phantom{\rule{0.28em}{0ex}}\mathrm{MeV}/{c}^{2}$ and a width of $40.2\ifmmode\pm\else\textpm\fi{}2.1\phantom{\rule{0.28em}{0ex}}\mathrm{MeV}/{c}^{2}$. The exclusive production cross section comes out to be $22.27\ifmmode\pm\else\textpm\fi{}0.89\ifmmode\pm\else\textpm\fi{}1.{56}_{\ensuremath{-}2.10}^{+3.07}\phantom{\rule{0.28em}{0ex}}\ensuremath{\mu}b$. Angular distributions of the $\ensuremath{\Sigma}{(1385)}^{+}$ in different reference frames are found to be compatible with the hypothesis that $33%$ of $\ensuremath{\Sigma}{(1385)}^{+}$ result from the decay of an intermediate broad ${\ensuremath{\Delta}}^{++}$ excitation at about $2000\phantom{\rule{0.28em}{0ex}}\mathrm{MeV}/{c}^{2}$.

The first measurement is reported of the double-polarization observable G in photoproduction of neutral pions off protons, covering the photon energy range from 620 to 1120 MeV and the full solid angle. G describes the correlation between the photon polarization plane and the scattering plane for protons polarized along the direction of the incoming photon. The observable is highly sensitive to contributions from baryon resonances. The new results are compared to the predictions from SAID, MAID, and BnGa partial wave analyses. In spite of the long-lasting efforts to understand {\gamma}p -> p{\pi} 0 as the simplest photoproduction reaction, surprisingly large differences between the new data and the latest predictions are observed which are traced to different contributions of the N (1535) with spin-parity J^P = 1/2^- and N (1520) with J^P = 3/2^- . In the third resonance region, where N (1680) with J^P = 5/2^+ production dominates, the new data are reasonably close to the predictions., Comment: Submitted for publication in PRL

The Two Arm Photon Spectrometer TAPS - comprising 384 plastic-BaF2 scintillator telescopes - was tested with monochromatic photons in the energy range between 45 and 790 MeV. The energy resolution for a collimated photon beam hitting the central detector module was determined to sigma/E = 0.59% X E(gamma)-1/2 + 1.9% (E, given in GeV). For the the fast scintillation component alone sigma/E = 0.79% X E(gamma)-1/2 + 1.8% has been measured. The position resolution of the point of impact amounts to DELTAx = 2 cm (FWHM) at the highest energies which corresponds to 30% of the diameter of an individual module. Monte Carlo simulations using the code GEANT3 are in good agreement with the experimental results.

Transverse momentum spectra of \ensuremath{\eta} mesons have been measured near the free nucleon-nucleon production threshold in the heavy ion reactions $^{40}\mathrm{Ar}$+ $^{\mathrm{nat}}\mathrm{Ca}$, $^{86}\mathrm{Kr}$ + $^{\mathrm{nat}}\mathrm{Zr}$, and $^{197}\mathrm{Au}$+ $^{197}\mathrm{Au}$ at 1.0A GeV and also in $^{40}\mathrm{Ar}$+ $^{\mathrm{nat}}\mathrm{Ca}$ at 1.5A GeV. The measured transverse momentum distributions are compared to model calculations. The relative abundance of \ensuremath{\Delta}(1232) and N(1535) resonances excited in the collision is deduced. A comparison to pion data reveals scaling with the transverse mass of the emitted meson.

The production of neutral pions has been studied in the reactions 40 Ar + nat Ca , 86 Kr + nat Zr and 197 Au + 197 Au at 1 A GeV. For high energy pions emitted from the heavier systems a steeper than linear rise of the pion multiplicity with the centrality of the reaction is observed, indicating a pion production process other than binary nucleon-nucleon collisions. At low transverse momenta an enhancement of the π 0 -multiplicity increasing with the mass of the collision system is found. Systematic discrepancies between the experimental results and recent BUU, QMD and Cascade calculations are discussed.

G. Agakishiev6, A. Balanda3, D. Belver16, A. Belyaev6, A. Blanco2, M. Bohmer9, J. L. Boyard14, P. Cabanelas16, E. Castro16, S. Chernenko6, J. Diaz17, A. Dybczak3, E. Epple8, L. Fabbietti8, O. Fateev6, P. Finocchiaro1, P. Fonte2,b, J. Friese9, I. Frohlich7, T. Galatyuk7,c, J. A. Garzon16, A. Gil17, M. Golubeva12, D. Gonzalez-Diazd , F. Guber12, M. Gumberidze14, T. Hennino14, R. Holzmann4, P. Huck9, A. Ierusalimov6, I. Iori11, f , A. Ivashkin12, M. Jurkovic9, B. Kampfer5,e, T. Karavicheva12, I. Koenig4, W. Koenig4, B. W. Kolb4, G. Korcyl3, G. Kornakov16, R. Kotte5, A. Kozuch3,g, A. Krasa15, F. Krizek15, R. Krucken9, H. Kuc3,14, W. Kuhn10, A. Kugler15, A. Kurepin12, A. Kurilkin6, P. Kurilkin6, P. Kahlitz5, V. Ladygin6, J. Lamas-Valverde16, S. Lang4, K. Lapidus8, T. Liu14, L. Lopes2, M. Lorenz7, L. Maier9, A. Mangiarotti2, J. Markert7, V. Metag10, B. Michalska3, J. Michel7, C. Muntz7, L. Naumann5, Y. C. Pachmayer7, M. Palka7, Y. Parpottas13, V. Pechenov4, O. Pechenova7, J. Pietraszko7, W. Przygoda3, B. Ramstein14, A. Reshetin12, J. Roskoss10, A. Rustamov4, A. Sadovsky12, P. Salabura3, A. Schmaha, J. Siebenson8, Yu.G. Sobolev15, S. Spataroh, H. Strobele7, J. Stroth7,4, C. Sturm4, A. Tarantola7, K. Teilab7, P. Tlusty15, M. Traxler4, R. Trebacz3, H. Tsertos13, T. Vasiliev6, V. Wagner15, M. Weber9, J. Wustenfeld5, S. Yurevich4, Y. Zanevsky6 (HADES collaboration)

Here we correct a factor of 2 missed in one of figures and the corresponding sentence in the text. It does not change any conclusions.

Precise data for quasi-free photoproduction of $\eta$ 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 2.5 GeV. The $\eta$-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 $\eta$-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 $\eta$-nucleon pairs reconstructed from the final state four-momenta of the particles. The total cross section for quasi-free $\eta$-photoproduction off the neutron reveals even without correction for Fermi motion a pronounced bump-like structure around 1 GeV of incident photon energy, which is not observed for the proton. This structure is even narrower in the invariant mass spectrum of the $\eta$-neutron pairs. Position and width of the peak in the invariant mass spectrum are $W\approx 1665$ MeV and FWHM $\Gamma\approx 25$ MeV. The data are compared to the results of different models., Comment: accepted for publication in Eur. Phys. J. A

The multiplicities of p and \ensuremath{\alpha} particles detected in coincidence with fragments emitted in fully relaxed collisions in the reactions of 18.5A MeV $^{136}\mathrm{Xe}$${+}^{48}$Ti have been measured for different exit channel mass asymmetries. A kinematic source analysis of the spectra and angular distributions of the light particles has been used to separate the total multiplicities into prescission and postscission contributions. From these results, the excitation energies at scission are determined using an empirical technique based upon previous measurements of light charged particle multiplicities observed in coincidence with evaporation residues. These excitation energies are found to decrease from \ensuremath{\sim}400 MeV to 110 MeV as the fragment mass asymmetry, ${\mathit{A}}_{\mathit{H}}$/${\mathit{A}}_{\mathit{L}}$, varies from 4.8 to 1.0. A corresponding increase of the mean lifetime of the scissioning nucleus from \ensuremath{\sim}5\ifmmode\times\else\texttimes\fi{}${10}^{\mathrm{\ensuremath{-}}22}$ s to \ensuremath{\sim}1\ifmmode\times\else\texttimes\fi{}${10}^{\mathrm{\ensuremath{-}}20}$ s is derived using calculated statistical model decay widths. The extent to which this variation of lifetime with mass asymmetry may be attributed to completely damped deep inelastic collisions or to dynamic delays in the decay of a compound nucleus is discussed as is the need for inclusion of dynamics in the deexcitation calculations for hot nuclei. Observed three fragment events are also discussed.

Recent results on π 0 , π ± , ν, K ± and p - production obtained at SIS are summarized. Systematics for particle spectra and production probabilities have been established. The subthreshold production of ν, K+ and p has been observed and the importance of multi-step production processes is emphasized. At bombarding energies of 1–2 GeV/u there is evidence for a gradual transition to resonance matter with about 30% of the baryons excited to Δ,N∗ resonances corresponding to a density ϱN · Δ ≈ ϱ0. The future perspectives of the field, in particular the potential of dilepton emission in heavy ion collisions is outlined.

We present results from an experimental study of reabsorption effects in subthreshold pi0 production in the reaction Xe-129 + Au-197 at 44 MeV/nucleon. Within the picture of pion generation in nucleon-nucleon scattering we deduce, from our data and from a comparison with the systematics of production cross sections available for lighter reaction systems, information on the pi0 absorption length in nuclear matter. For the pi0 kinetic-energy range congruent-to 5-100 MeV the energy-averaged lambda(abs) and its momentum dependence are obtained, and compared with optical-model calculations.

An overview is given over recent π 0 , π ± , η, K ± and p -production experiments performed at SIS where subthreshold production of η, K + and p has been observed. Systematics for particle spectra and production probabilities have been established and the importance of multi-step meson production processes involving baryon resonances as energy storage is discussed. At bombarding energies of 1–2 GeV/u a gradual transition to resonance matter seems to occur with about 30% of the baryons excited to Δ,N ∗ resonances corresponding to a density ρ N ∗ , Δ ≈ ρ 0 . The future perspectives of the field, in particular the potential of dilepton emission in heavy ion collisions is outlined.

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 gamma p - = W >= 1.87 GeV). Bremsstrahlung photons produced by the 1.5-GeV electron beam of the Mainz Microtron MAMI-C and momentum analyzed by the Glasgow tagging spectrometer were used for the eta-meson production. Our accumulation of 3.8 x 10(6) gamma p -< eta p -< 3 pi(0) p -< 6 gamma p events allows a detailed study of the reaction dynamics. The gamma p -< eta p differential cross sections were determined for 120 energy bins and the full range of the production angles. Our data show a dip near W = 1680 MeV in the total cross section caused by a substantial dip in eta production at forward angles. The data are compared to predictions of previous SAID and MAID partial-wave analyses and to the latest SAID and MAID fits that included our data.

Results on $\ensuremath{\Lambda}p$ femtoscopy are reported at the lowest energy so far. At a beam energy of $1.76A$ GeV, the reaction $\mathrm{Ar}+\mathrm{KCl}$ was studied with the High Acceptance Di-Electron Spectrometer (HADES) at SIS18/GSI. A high-statistics and high-purity $\ensuremath{\Lambda}$ sample was collected, allowing for the investigation of $\ensuremath{\Lambda}p$ correlations at low relative momenta. The experimental correlation function is compared to corresponding model calculations allowing the determination of the space-time extent of the $\ensuremath{\Lambda}p$ emission source. The $\ensuremath{\Lambda}p$ source radius is found to be slightly smaller than the $\mathit{pp}$ correlation radius for a similar collision system. The present $\ensuremath{\Lambda}p$ radius is significantly smaller than that found for $\mathrm{Au}+\mathrm{Au}/\mathrm{Pb}+\mathrm{Pb}$ collisions in the AGS, SPS, and RHIC energy domains but larger than that observed for electroproduction from He. Taking into account all available data, we find the $\ensuremath{\Lambda}p$ source radius to increase almost linearly with the number of participants to the power of one-third.

The reaction gamma p -> p pi0 gamma' has been measured with the Crystal Ball / TAPS detectors using the energy-tagged photon beam at the electron accelerator facility MAMI-B. Energy and angular differential cross sections for the emitted photon gamma' and angular differential cross sections for the pi0 have been determined with high statistics in the energy range of the Delta+(1232) resonance. Cross sections and the ratio of the cross section to the non-radiative process gamma p -> p pi0 are compared to theoretical reaction models, having the anomalous magnetic moment kappa_Delta+ as free parameter. As the shape of the experimental distributions is not reproduced in detail by the model calculations, currently no extraction of kappa_Delta+ is feasible., 14 pages, 19 figures

Based on the systematics for hard photon production a scheme is suggested which relates the hard-photon multiplicity observed in intermediate energy heavy ion reactions to the impact parameter. The method is applied to data obtained in an exclusive experiment of the reaction $^{40}\mathrm{Ar}$${+}^{158}$Gd at 44 MeV/nucleon, demonstrating the usefulness of this new approach.

The reaction γd↦π0 X has been measured with TAPS at MAMI in the energy range E γ = 140-300 MeV. Using the Glasgow tagging spectrometer a photon energy resolution of 0.8 MeV was achieved. The energy excitation functions of integral and differential total cross-sections show no structures of statistical significance > 2σ. Upper limits for the production of narrow isoscalar or isovector dibaryons with masses m? 2100 MeV/c2 were deduced. They are in the range 2-5 μb averaged over the 0.8 MeV energy resolution.