Prompt fission induced by antiproton annihilation at rest on heavy nuclei

Antiproton induced fission probabilities of U²³⁸, Bi²⁰⁹, Pb²⁰⁸ and Au¹⁷⁷ are reported together with the mass distribution of the fission fragments in the U²³⁸ and Bi²⁰⁹ cases. The charged particles multiplicities observed in co-incidence with fission have, also, been measured for U and Bi and are presented.Dedicated to Peter Armbruster on the occasion of his 60th birthday in recognition of his important contribution to modern Nuclear Physics     

Pion multiplicity distribution in antiproton-proton annihilation at rest

The pion multiplicity distribution is widely believed to reflect the statistical aspects of annihilation at rest. We try to reproduce it in a grand canonical picture with explicit conservation of electric charge, isospin, total angular momentum, and the parity quantum numbersP, C, andG via the projection operator formalism. Bose statistics is found to be non-negligible, particularly in fixing the interaction volume. The calculated pion multiplicity distribution for n =5 turns out to depend strongly on the conservation of the angular momentum and connected quantum numbers, as well as on the spin state occupation inS-wave annihilation. However, the empirical Gaussian pion multiplicity distribution cannot be reproduced. This calls in question either the statistical ansatz or the rather old data themselves.Work supported in part by DFG, BMFT and GSI     

Neutron emission from antiproton annihilation at rest in uranium

We have observed a large yield (5.77±0.16/ann.) of neutrons from antiproton annihilation at rest in a thin uranium target. Significant components of fission, evaporation and direct neutrons comprise this total. The large fission yield (2.50±0.10/ann.), as well as the high temperature (94±10 MeV) of the direct component, have not been anticipated in the intranuclear cascade model.     

Quark deconfinement in antiproton annihilation at rest on light nuclei

The production of low-energy antimatter provides unique opportunities to search for new physics in an unexplored regime. Testing gravitational interactions with antimatter is one such opportunity. Here a scenario based on Lorentz and CPT violation in the Standard-Model Extension is considered in which anomalous gravitational effects in antimatter could arise.     

Nuclear gamma-ray emission from antiproton annihilation at rest in uranium

We have observed low energy nuclear gamma-rays, which we interpret as coming from fission fragments, from antiproton annihilation at rest in a uranium target. The high gamma-ray and previously reported fission neutron temperatures, as well as large relative neutron-to-gamma ray yields, suggest a high level of excitation of the fission fragments.     

N\bar N annihilation at rest into five pions

annihilation at rest into five pions was studied in the ASTERIX spectrometer by stopping antiprotrons from LEAR in a H₂ or D₂ gas target. In annihilations in H₂ leading to the ^+–+–0 final state, the invariant mass spectra of two, three, and four pions show no evidence for any new narrow states. In the difference of the four pion mass spectra recoiling against a ^– and those recoiling against a ⁺ shows a resonance-like structure ( (1480)), confirming a previous analysis with a smaller event sample. The (1480) mass is shown to depend strongly on the momentum of the spectator proton. In H₂, we searched for the (1480) in the reaction (1480), (1480) ^+–+– to clarify whether the (1480) is identical to the 4-mode of thef ₂ (1520). No evidence for this decay mode was found. A prominent resonance in innihilation into five pions is the meson. We find two quasi-two-body intermediate states: and (1235). The mass and width of theb ₁ (1235) are determined to be:

Multinucleon phenomena and proton spectra characterizing antiproton annihilation at rest in nuclei

Spectra of protons with momenta greater than 500 MeV/c following antiproton annihilation at rest in carbon and uranium nuclei have been measured. They are compared to recent results from other experiments and to predictions of an intranuclear cascade model. Evidence is presented for a substantialB>0 component in the annihilation process, and an estimate of the magnitude of that component is made.     

Observation and analysis ofE mesons in\bar p p annihilation at rest in H2 gas

Antiproton-proton annihilation at rest in a gaseous H₂ target at NTP into the final state π⁺ π^? K ^± π^? (K ⁰) with an undetectedK ⁰ or \(\bar K^0 \) has been investigated. We observe theE(1420) resonance in the invariant mass spectrum (K ⁰)_miss K ^± π^? with massM _E =1413±8 MeV/c² and widthГ ^E =62 ± 16MeV/c² and find evidence for the production of thef ₁(1285). The absolute branching ratio of \(\bar p\) p → π⁺ π^? E ⁰,E ⁰ →K ₀ ^L K ^± π ^? at (61±6)%P wave annihilation is (3.0±0.9)·10^?4 of all annihilations. The observed suppression of theE production fromP wave with respect to theS wave together with some simple selection rules suggest that the quantum numbers of theE(1420) areJ ^pc=0^?+ and not I⁺⁺.     

Proton-antiproton annihilation at rest into two mesons and the quark line rule

Proton-antiproton annihilation at rest into two mesons is analysed in a model describing annihilation by quark lines. The model allows to fit existing data with two parameters. The parameters describe the amplitudes for diagrams with and without rearrangement of quark lines.     

OBELIX results on Kaonic Nuclear Clusters production by antiproton annihilation at rest on 4He

Experimental results recently obtained by OBELIX on the possible production of Kaonic Nuclear Clusters by $\overline{p}$ annihilation on ⁴He are critically revised and compared with analogous results from experiments with stopped K^??. The use of $\overline{p}$ instead than K^?? at rest on nuclei in the search of the existence and properties of such objects seems better, both from the experimental side and the discovery potential.     

Fast deuteron production from antiproton annihilation at rest on carbon and uranium targets

Deuteron yields from 20–350 MeV kinetic energy in antiproton annihilation at rest in¹²C and²³⁸U targets are compared with a coalescence model calculation. Agreement of the data with the model up to approximately 80 MeV is good. However, from 80 to 350 MeV the model increasingly underestimates the yield, by as much as on order of magnitude. These results, along with previously reported failures to explain proton spectra with first order rescattering of pions, suggest that other mechanisms are manifest in relatively rare, high energy light nucleus emission, e.g.B > 0 annihilations or possible six quark interactions.Work supported in part by the U.S. Air Force Office of Scientific Research and the National Science Foundation     

Energy transfer by intranuclear cascade of pi-zeros produced in antiproton annihilation at rest in nuclear targets

The energy transfer by pi-zeros in the intranuclear cascade initiated by antiproton annihilation at rest in carbon and uranium has been measured to be 28 ± 43 and 232 ± 33 MeV respectively. We estimate the total energy transfer due to all pions to be 75 ± 53 and 447 ± 42 MeV respectively. The uranium value is ～ 17% larger than a recent theoretical prediction. No predictions for carbon are available. Given the expected increase in efficiency of energy transfer from antiproton beams at ～ 3 GeV/c momentum, the prospects for initiating multifragmentation and disintegration of heavy nuclei at these energies appear good.     

$\overline pp$ annihilation into four charged pions at rest and in flight

The spin-parity analysis of the data on the annihilation reaction at rest in liquid and in gaseous hydrogen at 3 bar pressure and in flight at momentum of MeV/c, collected by the Obelix spectrometer at the LEAR complex of CERN, is presented. The relative branching ratios and are obtained. It is also shown that the inclusion of the exotic meson , J^PC = 1^{- +} , mass and width , GeV/c², in its decay to , improves the fit and some implications of these results are briefly discussed. The relative S and P-wave annihilation percentages in four charged pions at two target densities are obtained.Received: 2 December 2003, Published online: 23 April 2004     

Evidence for the production of heavy vector mesons in antiproton-proton annihilation at rest

Stopping antiprotons in a hydrogen target, 26 electron pairs have been found, which can be attributed to the reaction $\text{p}\text{p}\text{→}\text{V}{}^0\text{(}\text{e}{}^{\mathrm{+}}\text{e}{}^{\mathrm{?}}\text{)+π}{}^0$. The analysis of the data supports the existence of the ρ″ (1600) and the ρ′ (1250) mesons.     

Charged pion spectra and energy transfer following antiproton annihilation at rest in carbon and uranium

The momentum spectra of charged pions following antiproton annihilation at rest in carbon and uranium have been measured. This information complements our previous measurement of the neutral pion spectra. The total charged pion multiplicity is 2.84±0.10 and 2.47±0.09 for carbon and uranium, respectively, in good agreement with recent INC model predictions of 2.96 and 2.48 for the same quantities. However, structures predicted by the model near 200 MeV/c and 300 MeV/c related to delta-resonance production are not seen in the data. The total energy transfers to the nucleus are calculated to be 119±59 MeV(carbon) and 455±50 MeV(uranium). The possibility of exciting multifragmentation with a¯p beam impinging on heavy nuclei is discussed.     

A search for QGP formation in antiproton–He annihilation at rest

The dependence of the strangeness production on the number of nucleons involved in the annihilation process is investigated experimentally in and ⁴He annihilation at rest. In He, annihilations with the involvement of one and several nucleons (B=0 and B1 baryonic number, respectively) are identified. Strangeness enhancement factors, i.e., ratios of K^±, , K^0* and ϕ meson yields in ⁴He to the same yields in H, are derived for final states containing 4 charged mesons (2π⁺2π⁻, π⁺π⁻πK^±, π⁺π⁻K⁺K⁻). This work completes a previous investigation concerning charged kaon production in meson final states without neutral mesons. The results are compared with our previous ones and with results found in experiments on heavy-ion collisions. It is put in evidence that the strangeness content in the energy blob created by the annihilation has a lower or higher intensity depending on the reaction channel: for instance, π⁺π⁻πK^± production increases with B and π⁺π⁻K⁺K⁻ production decreases. The maximum enhancement factor (about 22) is found for π⁺π⁻π⁻K⁺ without neutral mesons. This value (as well as that concerning π⁺π⁺π⁻K⁻) is definitely higher than the values predicted by theoretical investigations based on the formation of a highly excited hadronic gas in the annihilation on few nucleons and values found in heavy-ion collisions. Is it the signature of the formation of quark–gluon plasma?     

Strangeness production inp¯p annihilation at rest and the quark — diquark structure of nucleons

The importance of approximate isospin selection rules observed inN¯ N annihilation at rest into two strange mesons is emphasized. The selection rules are traced back to a symmetry between amplitudes describing annihilation by the flux of quarks. Presumably the symmetry reflects the symmetry of ¯qq annihilation vertices. The relation of the symmetry to a quark — diquark substructure of the nucleon is discussed.     

Antiproton-proton annihilation at rest in H2 gas into π+ π− π0

Zeitschrift für Physik C Particles and Fields - Annihilation of antiprotonic hydrogen atoms into π+ π? π0 is studied by stopping antiprotons from LEAR in hydrogen gas. Two...