Search Results (53 total)

Quasifree photoproduction of η mesons off nucleons bound in the deuteron has been measured with the CBELSA/TAPS detector for incident photon energies up to 2.5 GeV at the Bonn ELSA accelerator. The η mesons have been detected in coincidence with recoil protons and recoil neutrons, which allows a detailed comparison of the quasifree n(γ,η)n and p(γ,η)p reactions. The excitation function for η production off the neutron shows a pronounced bumplike structure at W=1.68  GeV (E_γ≈1  GeV), which is absent for the proton.

Information on hadron properties in the nuclear medium has been derived from the photoproduction of ω mesons on the nuclei C, Ca, Nb, and Pb using the Crystal Barrel/TAPS detector at the ELSA tagged photon facility in Bonn. The dependence of the ω-meson cross section on the nuclear mass number has been compared with three different types of models: a Glauber analysis, a Boltzmann-Uehling-Uhlenbeck analysis of the Giessen theory group, and a calculation by the Valencia theory group. In all three cases, the inelastic ω width is found to be 130–150  MeV/c² at normal nuclear matter density for an average 3-momentum of 1.1  GeV/c. In the rest frame of the ω meson, this inelastic ω width corresponds to a reduction of the ω lifetime by a factor ≈30. For the first time, the momentum dependent ωN cross section has been extracted from the experiment and is in the range of 70 mb.

We present the first detailed measurement of incoherent photoproduction of neutral pions to a discrete state of a residual nucleus. The ¹²C(γ,π⁰)¹²C_4.4  MeV^* reaction has been studied with the Glasgow photon tagger at MAMI employing a new technique which uses the large solid angle Crystal Ball detector both as a π⁰ spectrometer and to detect decay photons from the excited residual nucleus. The technique has potential applications to a broad range of future nuclear measurements with the Crystal Ball and similar detector systems elsewhere. Such data are sensitive to the propagation of the Δ in the nuclear medium and will give the first information on matter transition form factors from measurements with an electromagnetic probe. The incoherent cross sections are compared to two theoretical predictions including a Δ-hole model.

The invariant-mass spectrum of e⁺e^- pairs produced in ¹²C+¹²C collisions at an incident energy of 2 GeV per nucleon has been measured for the first time. The measured pair production probabilities span over 5 orders of magnitude from the π⁰-Dalitz to the ρ/ω invariant-mass region. Dalitz decays of π⁰ and η account for all the yield up to 0.15  GeV/c², but for only about 50% above this mass. A comparison with model calculations shows that the excess pair yield is likely due to baryon-resonance and vector-meson decays. Transport calculations based on vacuum spectral functions fail, however, to describe the entire mass region.

The photoproduction of ω mesons on nuclei has been investigated using the Crystal Barrel/TAPS experiment at the ELSA tagged photon facility in Bonn. The aim is to study possible in-medium modifications of the ω meson via the reaction γ+A→ω+X→π⁰γ+X^′. Results obtained for Nb are compared to a reference measurement on a LH₂ target. While for recoiling, long-lived mesons (π⁰, η, and η^′), which decay outside of the nucleus, a difference in the line shape for the two data samples is not observed, we find a significant enhancement towards lower masses for ω mesons produced on the Nb target. For momenta less than 500  MeV/c an in-medium ω meson mass of M_medium=[722_-4⁺⁴(stat)_-5⁺³⁵(syst)]  MeV/c² has been deduced at an estimated average nuclear density of 0.6ρ₀.

A Reply to the Comment by C. Hanhart.

The photoproduction of η-mesic ³He has been investigated using the TAPS calorimeter at the Mainz Microtron accelerator facility MAMI. The total inclusive cross section for the reaction γ³He→ηX has been measured for photon energies from threshold to 820 MeV. The total and angular differential coherent η cross sections have been extracted up to energies of 745 MeV. A resonancelike structure just above the η production threshold with an isotropic angular distribution suggests the existence of a resonant quasibound state. This is supported by studies of a competing decay channel of such a quasibound η-mesic nucleus into π⁰pX. A binding energy of (-4.4±4.2)  MeV and a width of (25.6±6.1)  MeV is deduced for the quasibound η-mesic state in ³He.

Chemical and thermal freeze-out of the hadronic fireball formed in symmetric collisions of light, intermediate-mass, and heavy nuclei at beam energies between 0.8A GeV and 2.0A GeV are discussed in terms of an equilibrated, isospin-symmetric ideal hadron gas with grand-canonical baryon-number conservation. For each collision system the baryochemical potential μ_B and the chemical freeze-out temperature T_c are deduced from the inclusive π⁰ and η yields which are augmented by interpolated data on deuteron production. With increasing beam energy μ_B drops from 800 MeV to 650 MeV, while T_c rises from 55 MeV to 90 MeV. For given beam energy μ_B grows with system size, whereas T_c remains constant. The centrality dependence of the freeze-out parameters is weak as exemplified by the system Au+Au at 0.8A GeV. For the highest beam energies the fraction of nucleons excited to resonance states reaches freeze-out values of nearly 15%, suggesting resonance densities close to normal nuclear density at maximum compression. In contrast to the particle yields, which convey the status at chemical freeze-out, the shapes of the related transverse-mass spectra do reflect thermal freeze-out. The observed thermal freeze-out temperatures T_th are equal to or slightly lower than T_c, indicative of nearly simultaneous chemical and thermal freeze-out.

The reaction γp→π⁰γ^′p has been measured with the TAPS calorimeter at the Mainz Microtron accelerator facility MAMI for energies between sqrt[s]=1221–1331  MeV. The cross section’s differential in angle and energy have been determined for the photon γ^′ in three bins of the excitation energy. This reaction channel provides access to the magnetic dipole moment of the Δ⁺(1232) resonance and, for the first time, a value of μ_Δ⁺=[2.7_-1.3^+1.0(stat)±1.5(syst)±3(theor)]μ_N has been extracted.

Differential cross sections of the reactions (γ,π⁰π⁰) and (γ,π⁰π^+/-) have been measured for several nuclei (¹H,¹²C, and ^natPb) at an incident-photon energy of E_γ=400–460  MeV at the tagged-photon facility at MAMI-B using the TAPS spectrometer. A significant nuclear-mass dependence of the ππ invariant-mass distribution is found in the π⁰π⁰ channel. This dependence is not observed in the π⁰π^+/- channel and is consistent with an in-medium modification of the ππ interaction in the I=J=0 channel. The data are compared to π-induced measurements and to calculations within a chiral-unitary approach.

The photon asymmetry in the reaction 1H(γ→,π0)1H close to threshold has been measured for the first time with the photon spectrometer TAPS using linearly polarized photons from the tagged-photon facility at the Mainz Microtron MAMI. The total and differential cross sections were also measured simultaneously with the photon asymmetry. This allowed determination of the S-wave and all three P-wave amplitudes. The values obtained at threshold are E0+  =  [-1.33±0.08(stat)±0.03(syst)]×10-3/mπ+, P1  =  [9.47±0.08(stat)±0.29(syst)]×10-3q/mπ+2, P2  =  [-9.46±0.1(stat)±0.29(syst)]×10-3q/mπ+2, and P3  =  [11.48±0.06(stat)±0.35(syst)]×10-3q/mπ+2.

The reaction γp→π⁺π⁰n has been measured at MAMI for photon energies up to 820 MeV. Invariant mass spectra of the particles in the final state (π⁺n), (π⁰n), (π⁺π⁰) have been determined for several bins of incident photon energy. Differences in π⁺π⁰ and simultaneously measured π⁰π⁰ invariant mass distributions are assigned to a ρ branch of the D₁₃(1520) nucleon resonance.

The in-medium properties of the D₁₃(1520) nucleon resonance were studied via photoproduction of π⁰ 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 D₁₃→Nρ decay. The exclusive single π⁰ 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 D₁₃ resonance.

We report on a measurement of subthreshold η and π⁰ mesons in the reaction Ar+Ca at 180A MeV. We find that the ratio of the η to π⁰ meson-production cross section is more than a factor of 20 smaller than the one expected from threshold-energy scaling of meson production. In addition, the multiplicity of high m_t π⁰ increases faster with the centrality of the reaction than the multiplicity of the bulk of π⁰ mesons. This behavior is explained by the rescattering of π mesons in nuclear matter at the origin of most energetic particles.

The neutral π⁰ and η mesons are studied in ¹⁹⁷Au-¹⁹⁷Au collisions at an incident energy of 800A MeV, substantially below the threshold for η production in N-N collisions. While the gross π⁰ multiplicity increases almost linearly with the number of participant nucleons, the multiplicities of η and hard π⁰ mesons show a stronger than linear dependence. The nonlinearity is governed by the average transverse-mass excess 〈m_t〉-(sqrt[s]-2m_N) of the mesons and is insensitive to their final-state interaction in the nuclear medium.

The Dalitz-decay contributions of π⁰ and η 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 ¹²C+^natC and ⁴⁰Ar, ⁴⁰Ca+^natCa 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.

A Reply to the Comment by N. C. Mukhopadhyay, J.-F. Zhang, and M. Benmerrouche.

We have measured precise total and differential cross sections for the reaction γp→pη from threshold to 790 MeV at the MAMI accelerator in Mainz with the neutral meson spectrometer TAPS. Resonance parameters of the S₁₁(1535) resonance and the electromagnetic coupling γp→S₁₁ have been extracted from the data. Contributions from the D₁₃(1520) resonance to η photoproduction in the threshold region have been identified for the first time via interference terms in the angular distributions.

We present data on hard-photon production in ³⁶Ar+¹⁹⁷Au and ³⁶Ar+¹²C collisions at 95 MeV/nucleon. The photon source velocity β_S is investigated as a function of system size, photon energy, and impact parameter. In the heavy system, β_S is found to be sizably smaller than β_beam/2, except for very peripheral reactions. This unambiguously demonstrates that at intermediate bombarding energies photon emission is not solely dominated by first-chance p-n collisions, but carries also information on the later, stopping phase of the reaction.

Transverse momentum spectra of η mesons have been measured near the free nucleon-nucleon production threshold in the heavy ion reactions ⁴⁰Ar+ ^natCa, ⁸⁶Kr + ^natZr, and ¹⁹⁷Au+ ¹⁹⁷Au at 1.0A GeV and also in ⁴⁰Ar+ ^natCa at 1.5A GeV. The measured transverse momentum distributions are compared to model calculations. The relative abundance of Δ(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.

Intermediate mass fragment (IMF) production was studied in the inverse kinematics reaction ⁵⁸Ni+²⁷Al at 15.8 MeV/nucleon bombarding energy using the Heavy Ion Light Ion (HILI) detector. Exit channels with up to 4 IMFs have been observed. The three-body events can be ascribed to different production mechanisms: (i) incomplete momentum transfer with projectile spectator breakup, (ii) sequential fission following a deep inelastic reaction, and (iii) fast sequential fission (i.e., with Coulomb final-state interaction) in competition with a prompt breakup out of a toroidal configuration. The identified ternary reaction mechanisms, covering an extremely wide dynamic range, could be observed simultaneously; a subgroup of events is compatible with an instantaneous breakup of a highly excited nuclear system.

The multiplicities of p and α particles detected in coincidence with fragments emitted in fully relaxed collisions in the reactions of 18.5A MeV ¹³⁶Xe+⁴⁸Ti 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 ∼400 MeV to 110 MeV as the fragment mass asymmetry, A_H/A_L, varies from 4.8 to 1.0. A corresponding increase of the mean lifetime of the scissioning nucleus from ∼5×10^-22 s to ∼1×10^-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.

The azimuthal angle distributions of neutral pions at midrapidity from Au+Au reactions at 1 GeV/nucleon incident energy have been measured. An enhanced emission of π⁰’s perpendicular to the reaction plane is observed. The azimuthal asymmetry is dependent on the π⁰ momentum: the π⁰ spectrum perpendicular to the reaction plane is harder than in the reaction plane. The strength of the observed asymmetry appears to be more pronounced for π⁰ than for charged particles and neutrons.

We present results from an experimental study of reabsorption effects in subthreshold π⁰ production in the reaction ¹²⁹Xe+¹⁹⁷Au 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 π⁰ absorption length in nuclear matter. For the π⁰ kinetic-energy range ≃5–100 MeV the energy-averaged λ_abs and its momentum dependence are obtained, and compared with optical-model calculations.