Antiproton Absorption Research Articles

Charmonium production in antiproton-nucleus reactions at low energies

The J/\Psi(1S) and \Psi'(2S) production near threshold in antiproton-nucleus reactions is calculated on the basis of the Glauber model. The model takes into account the antiproton (pre)absorption, proton Fermi motion, and charmonium formation length. We confirm earlier prediction that the charmonium production in $\bar pA$ collisions at $p_{lab}=3-10$ GeV/c is not influenced by formation length effects and is very well suited to determine the genuine charmonium-nucleon dissociation cross sections. The comparison is performed with the J/\Psi photoproduction at high energies, where formation length effects play a very important role. However, we demonstrate that the detailed structure of the proton and neutron density profiles has to be taken into account, if one wants to extract information on the J/\Psi N dissociation cross section from J/\Psi transparency ratios. These studies are relevant for the upcoming PANDA experiment at FAIR.

Probing of compact baryonic configurations in nuclei inA(p,p¯)Xreactions and antiproton formation length in nuclear matter

Inclusive cross sections $\sigma^A=Ed^3{\sigma(X,P_t^2)/d^3p}$ of antiproton and negative pion production on Be, Al, Cu and Ta targets hit by 10 GeV protons were measured at the laboratory angles of 10.5$^{\circ}$ and 59$^{\circ}$. Antiproton cross sections were obtained in both kinematically allowed and kinematically forbidden regions for antiproton production on a free nucleon. The antiproton cross section ratio as a function of the longitudinal variable $X$ exhibits three separate plateaus which gives evidence for the existence of compact baryon configurations in nuclei-small-distance scaled objects of nuclear structure. Comparability of the measured cross section ratios with those obtained in the inclusive electron scattering off nuclei suggests a weak antiproton absorption in nuclei. Observed behavior of the cross section ratios is interpreted in the framework of a model considering the hadron production as a fragmentation of quarks (antiquarks) into hadrons. It has been established that the antiproton formation length in nuclear matter can reach the magnitude of 4.5 fm.

Antiproton-nucleus collisions simulation within a kinetic approach with relativistic mean fields

The Giessen Boltzmann-Uehling-Uhlenbeck transport model with relativistic mean fields is used to simulate $\overline{p}$-nucleus collisions. Antiproton absorption cross sections and momentum distributions of annihilation products are calculated by varying the $\overline{p}$ coupling strength to the mean meson fields. Parameters of the antiproton-nucleus optical potential are extracted from the comparison of the model calculations with experimental data.

Medium energy antiproton absorption, a tool to study neutron halo nuclei

A novel method is proposed to measure the difference of neutron and proton density distributions of nuclei far off stability by medium energy antiproton absorption. The radioactive nuclei are produced by projectile fragmentation at medium energies (400–800) A MeV, separated and stored in a medium energy storage ring. The antiprotons are produced with 30 GeV protons, accumulated, cooled, decelerated and stored at energies of about 5 MeV in a small storage ring. The medium energy radioactive beams are brought to head on collisions with the low energy antiproton beam in an interaction section of both rings. The coasting radioactive beam and the reaction products from the collisions with the antiprotons are observed with the Schottky noise frequency analysis method, which allows one to separate all reaction products and measure their production rates as well as the loss rate of the primary beam. From these observations, the total and partial reaction cross sections may be determined, even with a small number of rare ion species in the ring. An analysis of the total and partial reaction cross sections will allow to determine both 〈 r p 2〉 as well as 〈 r n 2〉 for nuclei far off the stability.

Gold fragmentation induced by stopped antiprotons

A natural gold target was irradiated with the antiproton beam from the low-energy antiproton ring at CERN. Antiprotons of $200\mathrm{MeV}/c$ momentum were stopped in a thick target, products of their annihilations on Au nuclei were detected using the off-line $\ensuremath{\gamma}$-ray spectroscopy method. In total, yields for 114 residual nuclei were determined, providing a dataset to deduce the complete mass and charge distribution of all products with $A\ensuremath{\gtrsim}20$ from a fitting procedure. The contribution of evaporation and fission decay modes to the total reaction cross section as well as the mean mass loss were estimated. The fission probability for Au absorbing antiprotons at rest was determined to be equal to $(3.8\ifmmode\pm\else\textpm\fi{}0.5)%,$ in good agreement with an estimation derived using other techniques. The mass-charge yield distribution was compared with the results obtained for proton and pion induced gold fragmentation. On average, the energy released in $\overline{p}$ annihilation is similar to that introduced by $\ensuremath{\approx}1\mathrm{GeV}$ protons. However, compared to proton bombardment products, the yield distribution of antiproton absorption residues in the $N\ensuremath{-}Z$ plane is clearly distinct. The data for antiprotons exhibit also a substantial influence of odd-even and shell effects.

Composition of the nuclear periphery from antiproton absorption using short-lived residual nuclei

The neutron-to-proton density ratio at the nuclear periphery was determined for six targets from the yield of nuclei with mass ${(A}_{t}\ensuremath{-}1)$ generated by antiproton annihilation on targets with mass number ${A}_{t}.$ A new setup allowed us to measure this yield for nuclei with half-lives down to 6 s. The experiment confirmed the negative correlation between the peripheral neutron-to-proton density ratio and the neutron binding energy, which had been previously found. Results of Hartree-Fock-Bogoliubov and relativistic Hartree-Bogoliubov calculations show good agreement for a large number of nuclei. By comparing the measured values with those derived from Hartree-Fock-Bogoliubov calculations, the ratio R of the imaginary parts of the effective scattering lengths of antiprotons on bound neutrons and protons was deduced to be $\mathcal{R}=0.69(4).$ For the first time yields of several excited states of some ${(A}_{t}\ensuremath{-}1)$ nuclei have been investigated.

Composition of the nuclear periphery from antiproton absorption

Thirteen targets with mass numbers from 58 to 238 were irradiated with the antiproton beam from the Low Energy Antiproton Ring facility at CERN leading to the formation of antiprotonic atoms of these heavy elements. The antiproton capture at the end of an atomic cascade results in the production of more or less excited residual nuclei. The targets were selected with the criterion that both reaction products with one nucleon less than the proton and number of the target be radioactive. The yield of these radioactive products after stopped-antiproton annihilation was determined using gamma-ray spectroscopy techniques. This yield is related to the proton and density in the target nucleus at a radial distance corresponding to the antiproton annihilation site. The experimental data clearly indicate the existence of a neutron-rich nuclear periphery, a neutron halo, strongly correlated with the target separation energy Bn and observed for targets with Bn < 10 MeV. For two-target nuclei 106Cd and 144Sm, with larger binding energies, a proton-rich nuclear periphery was observed. Most of the experimental data are in reasonable agreement with calculations based on current antiproton-nucleus and pion-nucleus interaction potentials and on nuclear densities deduced with the help of the Hartree-Fock-Bogoliubov approach. This approach was, however, unable to account for the 106Cd and 144Sm results.

Multi-nucleon mechanism for subthreshold antiproton production in proton-nucleus reactions

We study a many-body (multipion) mechanism for ¯p subthreshold production in proton-nucleus collisions. The incoming proton and the interacting nucleons in the target act as sources of pions that merge to produce a nucleon-antiproton pair. The calculation takes fully into account the initial state interaction and antiproton absorption in the final state. The cooperation of several nucleons in the initial state reduces the threshold for the reaction allowing for production rates that may account for 15–50% of the total ¯p production. We point out that this contribution should be added to the primary ¯p production mechanism due toΔ formation and rescattering, as found by other groups, and should be taken into account when trying to extract information about ¯p properties in the nuclear medium.

Neutron halo in heavy nuclei from antiproton absorption.

A new method to study the nuclear periphery using antiproton annihilation was applied to nine isotopes with mass numbers between 58 and 238. The method makes use of the detection of the radioactive annihilation products one unit lower in mass number than the target. A clear neutron halo effect, strongly correlated with the neutron binding energy, was observed in some nuclei. The experimental results are in qualitative agreement with calculations of proton and neutron densities at the nuclear periphery based on either a simple asymptotic density model or a more complex Hartree-Fock approach.

Signature of a neutron halo in 232Th from antiproton absorption

We demonstrate a new method for the investigation of the neutron to proton density ratio at the nuclear surface. The method relies on the radiochemical determination of some residual nuclei after the annihilation of stopped antiprotons. A clear neutron halo effect has been observed with the 232Th target.

A silicon multiplicity detector system for an experiment on the interaction of antiprotons with nuclei at BNL

A large-angle multiplicity detector (LAMD) system has been developed and used in the BNL experiment E854, relating to antiproton-nucleus interactions. The purpose of the BNL experiment E854 was to search for the predicted formation of hot hadronic matter by energetic antiproton absorption on a range of nuclei. This experiment probed the high-temperature-low-density region of the nuclear matter phase diagram. The LAMD system performed well with an energetic antiproton beam. Charged particle multiplicity distributions from p annihilations were measured. The design and performance of the LAMD system are outlined.< <ETX xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">&gt;</ETX>

Fission fragment distribution following antiproton absorption at rest on238U

The distribution of nuclei resulting from the annihilation of stopped antiprotons on238U has been studied by an off-line measurement of the residual radioactivity. It was found that the probability of fission exceeds 84% (68% c.l.). The charge and mass distributions of the fission products can be separated into two parts, a more frequent symmetric component originating from high energy fission (excitation energy between 100 MeV and 1 GeV) and an asymmetric component from low energy fission (excitation energy less than 40 MeV).

Antiproton annihilation at rest in nitrogen and deuterium gas.

Results on antiproton absorption at rest in gaseous nitrogen and deuterium are presented from an analysis of approximately ${10}^{6}$ events each taken with a magnetic spectrometer. Inclusive features such as pion and proton multiplicities and spectra are presented. Data relating to absorption modes requiring more than one nucleon, such as the \ensuremath{\Lambda} yield, the \ensuremath{\Lambda} spectrum, and the exclusive deuterium channels p\ifmmode\bar\else\textasciimacron\fi{}d\ensuremath{\rightarrow}${\ensuremath{\pi}}^{\mathrm{\ensuremath{-}}}$p, \ensuremath{\Lambda}${K}^{+}$${\ensuremath{\pi}}^{\mathrm{\ensuremath{-}}}$ are discussed. The fully reconstructable channels p\ifmmode\bar\else\textasciimacron\fi{}d\ensuremath{\rightarrow}${\mathrm{\ensuremath{\pi}}}^{+}$${\mathrm{\ensuremath{\pi}}}^{\mathrm{\ensuremath{-}}}$${\mathrm{\ensuremath{\pi}}}^{\mathrm{\ensuremath{-}}}$p,${\mathrm{\ensuremath{\pi}}}^{+}$${\mathrm{\ensuremath{\pi}}}^{+}$ ${\mathrm{\ensuremath{\pi}}}^{\mathrm{\ensuremath{-}}}$${\mathrm{\ensuremath{\pi}}}^{\mathrm{\ensuremath{-}}}$${\mathrm{\ensuremath{\pi}}}^{\mathrm{\ensuremath{-}}}$p also show a high-energy proton tail unaccounted for by single nucleon rescattering mechanisms.

Nuclear antiproton absorption involving more than one nucleon

Nuclear antiproton absorption involving more than one nucleon

A summary of recent antiprotonic atom experiments

The outcome of the measurements on antiprotonic atoms performed in recent years is summarized. The experiments, done at the low-energy antiproton ring (LEAR), mainly studied the strong interaction of antiprotons with nucleons. The principle goals of the investigations were the measurement of level energies and widths in antiprotonic atoms with the aim of getting information on the overall antiproton-nucleus and antiproton-nucleon interactions in general, and the study of the spin and isospin dependence of that force. Moreover, the magnetic moment of the antiproton was determined with a factor of 2 higher precision than previously known. Furthermore, the antiproton absorption was studied in detail. The analysis of the strong-interaction effects in terms of an antiproton-nucleon potential is in progress.

Nuclear absorption of stopped antiprotons: Multipion-nucleus interactions

It is proposed to use the nuclear absorption of stopped antiprotons to study the interaction arising when several pions enter a nucleus simultaneously. A model is constructed for the incoherent multipion-nucleus interaction, when the annihilation pions interact with the nucleus independently of each other. Energy spectra and the multiplicities of secondary particles generated after the antiproton absorption are compared with the experimental data. Distributions of the number of pions interacting with the nucleus are calculated as well as the distributions of residual nuclei with excitation energy, momentum, angular momentum and isotope yield. It is shown that the effect of the excitation of high-spin isomers must be observed in the antiproton absorption. The example of the two-nucleon absorption of η- and ω-mesons is used to discuss the possibility of investigating interactions of pionic resonances with nuclei during the process of the annihilation of antiprotons.

Observation of antiprotonic x rays inLi6andHe4

We have observed the $3d\ensuremath{\rightarrow}2p$ x ray in antiprotonic atoms of $^{4}\mathrm{He}$. The same transition was absent in antiprotonic $^{6}\mathrm{Li}$ but the $4f\ensuremath{\rightarrow}3d$ line was present. The data are analyzed for strong interaction widths.NUCLEAR REACTIONS Experimental antiproton absorption probabilities from atomic states in $^{4}\mathrm{He}$ and $^{6}\mathrm{Li}$; exotic atoms.

Determination of the neutron halo from antiproton absorption

We investigate the size of neutron halo in Pb implied by the recent p̄-annihilation experiment of Bugg et al. We find that the answer depends sensitively on the strong interaction parameters determined by p̄ X-ray experiments.

Evidence for a Neutron Halo in Heavy Nuclei from Antiproton Absorption

From a study of stopping antiprotons in a variety of elements located in a hydrogen bubble chamber, we find evidence for the existence of a neutron fringe in heavy nuclei.