Low Energy Antiproton Research Articles

The cosmic-ray antiproton spectrum from dark matter annihilation and its astrophysical implications - A new look

The spectrum of antiprotons from dark matter annihilation are calculated using the Lund Monte Carlo program, and simple analytic expressions for the spectrum and low-energy antiproton/proton ratio are derived. Comparing the results with recent upper limits on low energy antiprotons, it is concluded that the reported 4-13 GeV antiproton flux cannot be accounted for by dark matter annihilation. The new upper limits do not provide useful constraints on dark matter particles. They restrict the annihilation rate and imply that annihilation gamma ray and e(+) fluxes would be far below the fluxes produced by cosmic-ray collisions. It may be possible to look for a dark matter halo annihilation signal at antiprotons energies below 0.5 GeV, where the flux from cosmic-ray collisions is expected to be negligible.

Experimental limit on low energy antiprotons in the cosmic radiation

We report results from the Low Energy Antiproton Experiment (LEAP), a balloon-borne instrument which was flown in August, 1987. We find no evidence of antiproton fluxes in the kinetic energy range 120 MeV to 360 MeV, top of the atmosphere. The 90 percent confidence upper limit on the antiproton/proton ratio in this energy range is 3.5 × 10 −5. In particular, this new experiment places an upper limit on the flux almost an order of magnitude below the reported flux of Buffington et al.

Adronnaya fizika na puchkakh antiprotonov nizkikh energii

Adronnaya fizika na puchkakh antiprotonov nizkikh energii

Interactions of antiprotons with atoms and molecules

Antiproton beams of relatively low energies (below hundreds of MeV) have recently become available. The present article discusses the significance of those beams in the contexts of radiation physics and of atomic and molecular physics. Studies on individual collisions of antiprotons with atoms and molecules are valuable for a better understanding of collisions of protons or electrons, a subject with many applications. An antiproton is unique as a stable, negative heavy particle without electronic structure, and it provides an excellent opportunity to study atomic collision theory. Comparison of the stopping powers of a material for an antiproton and a proton at the same speed will be the most clearcut approach to the Barkas effect. The moderation of an antiproton in matter is roughly similar to that of negative pions and muons, but some differences remain to be elucidated. Full discussion of the interactions of a low-energy antiproton with an atom or molecule must consider the adiabatic potential determined by the electronic motion in the field of the antiproton and nucleus at rest. When an antiproton approaches an atom or molecule sufficiently closely, then some of the atomic or molecular electrons no longer remain bound and ooze out with extremely low kinetic energies; thus, the atom or molecule becomes ionized, often multiply ionized. This mechanism of ionization is absent for a proton. Upon complete moderation, an antiproton is eventually captured by the Coulomb field of a nucleus and thus an antiprotonic atom is formed. The present article also touches upon some problems related to condensed matter. For instance, the possibility or chanelling of antiproton beams in a crystal is considered.

Experiments at the Low-Energy Antiproton Ring (Lear)

This article summarizes the first round of experiments at the Low- Energy Antiproton Ring (LEAR) at CERN. By providing for the first time an intense and clean source of low-energy antiprotons, LEAR allows a great improvement of many investigations. Antiproton-proton scatering and reaction, resonances in antiproton-proton interactions, antiprotonic atoms, and antiproton-nucleus interaction are discussed in this review. (AIP)

Characteristics of close collisions of an antiproton with atoms.

We discuss low-energy antiproton ($\overline{p}$) collisions with H and He on the basis of adiabatic potentials and with the use of a molecular-orbital expansion method. Every electronic state for the $\overline{p}\ensuremath{-}\mathrm{H}$ system merges into the continuum at a certain internuclear separation, and this separation is farther and farther out for higher and higher excited states. (This fact was first pointed out by E. Fermi and E. Teller [Phys. Rev. 72, 399 (1947)], who specifically considered the ground state of a system consisting of a meson and a hydrogen atom.) For the $\overline{p}\ensuremath{-}\mathrm{H}\mathrm{e}$ system, as for to the $\overline{p}\ensuremath{-}\mathrm{H}$ system, all electronic states cross over into the continuum of the ${\mathrm{He}}^{+}+{\mathrm{le}}^{\ensuremath{-}}$ state at a finite separation. These findings lead us to suggest that a close encounter of $\overline{p}$ with an atom should result, at a high probability, in ionization with near-zero electron energy and a wide range of ejected-electron angle. This mechanism of ionization is absent for collisions of protons ($p$), and may account in part for the greater double-ionization cross section of He for $\overline{p}$ than for $p$ at the same energy, which was found in a recent experiment.

Polarization at small angles in antiproton-carbon elastic scattering at lear energies

Abstract A double-scattering experiment of antiprotons on carbon has been carried out at the Low-Energy Antiproton Ring (LEAR) at CERN, to measure the polarization parameter A p C in antiproton-carbon elastic scattering at small angles. The polarization parameter has been inferred from the azimuthal distribution of the antiprotons after the second scattering. Data have also been collected with a liquid-hydrogen target as the second scatterer, thus allowing the sign of A p C to be determined. The experiment has been performed at two momenta of the extracted antiproton beam, 800 and 1100 MeV/c. A small positive value of the polarization has been observed, compatible with energy independence and a linear increase with the momentum transfer q. Parametrizing A p C as acq, we get ac = +0.72 0.10+0.09(GeV/c)−1. This result is compared with potential model predictions for NN amplitudes through a Glauber theory calculation.

New limit on the low-energy antiproton/proton ratio in the Galactic cosmic radiation.

Results are presented from a balloon-borne apparatus searching for low-energy antiprotons in the Galactic cosmic rays. For energies less than 640 MeV at the top of the atmosphere, no cosmic-ray antiprotons were observed. This yields an upper limit to the antiproton/proton ratio of 0.000046 at the 85-percent confidence level.

A calorimeter coupled with a magnetic spectrometer for the detection of primary cosmic antiprotons

A tracking calorimeter made of 3200 brass streamer tubes together with 3200 pick-up strips has been built to complement a magnetic spectrometer in order to detect cosmic antiprotons in space. The characteristics of such a calorimeter, the results of a preliminary test of a prototype as well as the properties of the whole apparatus are presented. The apparatus, designed to operate on a balloon at an altitude of about 40 km, can be considered as a second generation detector, capable in principle to solve the problem of the presence of low energy (≤1 Ge V/c) antiprotons in the cosmic rays which is still open because of the disagreement between the existent experimental data.

Is there an antimatter-universe or antigalaxy?

The very recent measurements on the ratio of cosmic antiproton to proton have contradicted seriously the previous measurements of the same for low-energy antiprotons. The controversial situation has been described and its implication for the validity of the postulate about the existence of antigalaxy has been examined. Although a very decisive conclusion cannot yet be made, a two-component behaviour of the ratio cannot, however, be excluded. Moreover, this two-component proposition seems to have better prospectsvis-a-vis the high-energy measurements of the ratio.

First direct observation of strong interaction spin-orbit effects in antiprotonic atoms

The strong ¯p-nucleus spin-orbit interaction was investigated in a measurement of the strong-interaction effects of the 9→8 transition in ¯p174Yb at the Low-Energy Antiproton Ring (LEAR) at CERN. This measurement was part of an experimental programme where, for the first time, the fine-structure components of the last observable X-ray transition in a ¯p atom, which carries information on the strong ¯p-nucleus interaction, were resolved and studied individually. The observed splitting ΔEexp=2408±26 eV consists of the electromagnetic fine-structure splitting ΔEFS=2350 eV and an additional splitting Δɛ=58±26 eV. In addition, one finds a significant difference in the level widths of Δ=195±59 eV with the larger value⇊=1216±41 eV for the lower fine-structure level. This experiment follows an earlier measurement on ¯p138Ba, where the transition 8→7 is influenced by the strong interaction. In this case, however, the fine-structure components could not be resolved. The results for174Yb may be attributed to a spin-orbit (LS) term in the complex strong-interaction potential.

Low-energy antiprotons as a new probe of nuclear reaction mechanism

The experimental data of LEAR, KEK and BNL on low-energy antiproton interaction with nuclei are analysed in the framework of the Glauber-Sitenko approach. It is shown that antiproton-nucleus interaction gives new information on the nuclear reaction mechanism, on nuclear structure and on parameters of the p N amplitude.

Experiments in the [formula omitted]-system at LEAR

The Low Energy Antiproton Ring (LEAR) at CERN is briefly discussed with special emphasis on cooling techniques. Some of the results of the first round of LEAR experiments concerning pp -annihilation at rest and meson-spectroscopy are reviewed as well as experiments planned for the future.

The LEAR Electron Cooler: Recent Improvements and Tests

Recent work on the electron cooling device at the CERN Low Energy Antiproton Ring (LEAR) is summarized. This included an improvement of the collector performance and of the vacuum. Relations between the different cooler parameters were measured with a view to simplifying the control system. Results of vacuum tests performed before the installation in LEAR are presented, and a short description of the status of the cooling device after its installation is given.

First Capture of Antiprotons in an Ion Trap: Progress Toward a Precision Mass Measurement and Antihydrogen

Antiprotons from the Low Energy Antiproton Ring of CERN are slowed from 21 MeV to below 3 keV by being passed through 3 mm of material, mostly Be. While still in flight, the kilo-electron volt antiprotons are captured in a Penning trap created by the sudden application of a 3-kV potential. Antiprotons are held for 100 s and more. Prospects are now excellent for much longer trapping times under better vacuum conditions. This demonstrates the feasibility of a greatly improved measurement of the inertial mass of the antiproton and opens the way to other intriguing experiments. The possibility of producing antihydrogen by merging cold, trapped plasmas of positrons and antiprotons is discussed.

Electron Cooling of Stored Ions

The experimental conditions of electron cooling in storage rings with magnetically confined electron beams are discussed. Particular attention is paid to "fast" electron cooling, which will be important for low-energy ion storage rings. Measurements of cooling rates and of equilibrium ion beam temperatures are reviewed. A short report is given on the recent development work for electron cooling at the CERN Low-Energy Antiproton Ring (LEAR), and on related electron beam diagnostic experiments. Conclusions are drawn in view of electron cooling at future ion storage rings.

CP invariance in the reaction [formula omitted] at 1.546 GeV/ c

In a study of hyperon production with antiprotons at the CERN Low-Energy Antiproton Ring (LEAR), we have measured the ratio A=(α + α)/(α− α) . Here α and α are the decay asymmetry parameters of the Λ and Λ non-leptonic charged decays. The hyperons were produced with identical polarization in the reaction pp→ ΛΛ at an incident p momentum of 1.546 GeV/c. The result, A=− 0.07 ± 0.09, is consistent with zero, as required from CP invariance.

Dealing with high gas loads in uhv systems: some recent developments at CERN (LEAR)

Particle storage rings and accelerators with uhv systems providing pressures less that 10 −11 mbar are or have been conceived with now well-established standards and technologies. Among other things, a key to success is the careful choice of materials and treatments to reduce the outgassing. The growing interest in low energy particle storage rings equipped with exotic and high gas load equipment such as electron coolers and internal gas jet targets has triggered the search for new solutions in uhv techniques. Some recent developments around the CERN Low Energy Antiproton Ring (LEAR) involve the design of new pumps using low temperature non-evaporable getters (NEG). These are comparable in pumping speed performance with titanium sublimation pumps, but have a much increased pressure and capacity range. The design principles, Monte Carlo calculation methods, and preliminary performance are presented.

First capture of antiprotons in a Penning trap: A kiloelectronvolt source.

Antiprotons from the Low Energy Antiproton Ring of CERN are slowed from 21 MeV to below 3 keV by being passed through 3 mm of material, mostly Be. While still in flight, the kiloelectronvolt antiprotons are captured in a Penning trap created by the sudden application of a 3-kV potential. Antiprotons are held for 100 s and more. Prospects are now excellent for much longer trapping times under better vacuum conditions. This demonstrates the feasibility of a greatly improved measurement of the inertial mass of the antiproton and opens the way to other intriguing experiments.

Low energy antiproton-nucleus annihilation radius selection using an active silicon detector/target

The use of an intrinsic silicon transmission detector to experimentally select the annihilation radius of low energy antiprotons in silicon nuclei is described. Through Monte Carlo modeling of the intranuclear and evaporation cascades following the annihilation, the energy deposited in the detector is correlated to the annihilation radius in the silicon nucleus. By selecting events with sufficiently large energy deposition, annihilations occurring on average within the half nuclear density radius are distinguished.