Search Results (24 total)

We demonstrate temporally controlled modulation of cold antihydrogen production by periodic RF heating of a positron plasma during antiproton-positron mixing in a Penning trap. Our observations have established a pulsed source of atomic antimatter, with a rise time of about 1 s, and a pulse length ranging from 3 to 100 s. Time-sensitive antihydrogen detection and positron plasma diagnostics, both capabilities of the ATHENA apparatus, allowed detailed studies of the pulsing behavior, which in turn gave information on the dependence of the antihydrogen production process on the positron temperature T. Our data are consistent with power law scaling T^-1.1±0.5 for the production rate in the high temperature regime from ∼100  meV up to 1.5 eV. This is not in accord with the behavior accepted for conventional three-body recombination.

Antihydrogen can be synthesized by mixing antiprotons and positrons in a Penning trap environment. Here an experiment to stimulate the formation of antihydrogen in the n=11 quantum state by the introduction of light from a CO₂ continuous wave laser is described. An overall upper limit of 0.8% with 90% C.L. on the laser-induced enhancement of the recombination has been found. This result strongly suggests that radiative recombination contributes negligibly to the antihydrogen formed in the experimental conditions used by the ATHENA Collaboration.

We present evidence showing how antiprotonic hydrogen, the quasistable antiproton (p̅ )-proton bound system, has been synthesized following the interaction of antiprotons with the molecular ion H₂⁺ in a nested Penning trap environment. From a careful analysis of the spatial distributions of antiproton annihilation events, evidence is presented for antiprotonic hydrogen production with sub-eV kinetic energies in states around n=70, and with low angular momenta. The slow antiprotonic hydrogen may be studied using laser spectroscopic techniques.

Antihydrogen is formed when antiprotons are mixed with cold positrons in a nested Penning trap. We present experimental evidence, obtained using our antihydrogen annihilation detector, that the spatial distribution of the emerging antihydrogen atoms is independent of the positron temperature and axially enhanced. This indicates that antihydrogen is formed before the antiprotons are in thermal equilibrium with the positron plasma. This result has important implications for the trapping and spectroscopy of antihydrogen.

Production of antihydrogen atoms by mixing antiprotons with a cold, confined, positron plasma depends critically on parameters such as the plasma density and temperature. We discuss nondestructive measurements, based on a novel, real-time analysis of excited, low-order plasma modes, that provide comprehensive characterization of the positron plasma in the ATHENA antihydrogen apparatus. The plasma length, radius, density, and total particle number are obtained. Measurement and control of plasma temperature variations, and the application to antihydrogen production experiments are discussed.

We comment on the recent paper published by the Obelix Collaboration on protonium annihilation into π⁰π⁰ at rest in a liquid hydrogen target [Phys. Rev. D 65, 012001 (2002)], with particular reference to the discrepancy with the results obtained by the Crystal Barrel Collaboration.

This biennial Review summarizes much of Particle Physics. Using data from previous editions, plus 2205 new measurements from 667 papers, we list, evaluate, and average measured properties of gauge bosons, leptons, quarks, mesons, and baryons. We also summarize searches for hypothetical particles such as Higgs bosons, heavy neutrinos, and supersymmetric particles. All the particle properties and search limits are listed in Summary Tables. We also give numerous tables, figures, formulae, and reviews of topics such as the Standard Model, particle detectors, probability, and statistics. This edition features expanded coverage of CP violation in B mesons and of neutrino oscillations. For the first time we cover searches for evidence of extra dimensions (both in the particle listings and in a new review). Another new review is on Grand Unified Theories. A booklet is available containing the Summary Tables and abbreviated versions of some of the other sections of this full Review. All tables, listings, and reviews (and errata) are also available on the Particle Data Group website: http://pdg.lbl.gov.

This report reviews the achievements of the Crystal Barrel experiment at the Low-Energy Antiproton Ring (LEAR) at CERN. During seven years of operation Crystal Barrel has collected very large statistical samples in p̅ p annihilation, especially at rest and with emphasis on final states with high neutral multiplicity. The measured rates for annihilation into various two-body channels and for electromagnetic processes have been used to test simple models for the annihilation mechanism based on the internal quark structure of hadrons. The production of φ mesons is larger than predicted in several annihilation channels. Important contributions to the spectroscopy of light mesons have been made. The exotic ρ̂(1405) meson with quantum numbers J^PC=1^-+ has been observed in its ηπ decay mode. Two 2^-+ isoscalar mesons η₂(1645) and η₂(1870), and the 0^-+ isoscalar meson η(1410) have been observed in the ηππ decay channel. From three-body annihilations three 0⁺⁺ mesons, a₀(1450), f₀(1370), and f₀(1500) have been established in various decay modes. One of them, f₀(1500), may be identified with the expected ground-state scalar glueball.

This report reviews the achievements of the Crystal Barrel experiment at the Low-Energy Antiproton Ring (LEAR) at CERN. During seven years of operation Crystal Barrel has collected very large statistical samples in p̅ p annihilation, especially at rest and with emphasis on final states with high neutral multiplicity. The measured rates for annihilation into various two-body channels and for electromagnetic processes have been used to test simple models for the annihilation mechanism based on the internal quark structure of hadrons. The production of φ mesons is larger than predicted in several annihilation channels. Important contributions to the spectroscopy of light mesons have been made. The exotic ρ̂(1405) meson with quantum numbers J^PC=1^-+ has been observed in its ηπ decay mode. Two 2^-+ isoscalar mesons η₂(1645) and η₂(1870), and the 0^-+ isoscalar meson η(1410) have been observed in the ηππ decay channel. From three-body annihilations three 0⁺⁺ mesons, a₀(1450), f₀(1370), and f₀(1500) have been established in various decay modes. One of them, f₀(1500), may be identified with the expected ground-state scalar glueball.

We present a measurement and partial wave analysis of the final state K_LK^±π^∓ of p̅ p annihilation at rest in liquid hydrogen. This reaction is important for the study of the K̅ K decay mode of scalar resonances, in particular, the isovectors a₀(980) and a₀(1450). The determination of the a₀(1450) production also fixes the K̅ K coupling of the isoscalar f₀(1500) which is discussed as a glueball. We find B(p̅ p→a₀(1450)π;a₀(1450)→K̅ K)=(8.88±1.68)×10^-4 and hence B(p̅ p→f₀(1500)π;f₀(1500)→K̅ K)=(4.52±0.36)×10^-4. The mass and width of a₀(1450) are m=1480±30 MeV/c² and Γ=265±15 MeV/c², respectively. For a₀(980) we determine the relative ratio B(p̅ p→a₀π;a₀→K̅ K)/B(p̅ p→a₀π;a₀→πη) =0.23±0.05 and its relative coupling, K̅ K to πη, is 1.03±0.14.

This biennial review summarizes much of Particle Physics. Using data from previous editions, plus 1900 new measurements from 700 papers, we list, evaluate, and average measured properties of gauge bosons, leptons, quarks, mesons, and baryons. We also summarize searches for hypothetical particles such as Higgs bosons, heavy neutrinos, and supersymmetric particles. All the particle properties and search limits are listed in Summary Tables. We also give numerous tables, figures, formulae, and reviews of topics such as the Standard Model, particle detectors, probability, and statistics. A booklet is available containing the Summary Tables and abbreviated versions of some of the other sections of this full Review.

Following the discovery of two new scalar mesons f₀(1370) and f₀(1500) at the Low Energy Antiproton Ring at CERN, we argue that the observed properties of this pair are incompatible with them both being QQ¯ mesons. We show instead that f₀(1500) is compatible with the ground state glueball expected around 1500 MeV mixed with the nearby states of the 0⁺⁺QQ¯ nonet. Tests of this hypothesis include the prediction of a further scalar state f₀^′(1500–1800) which couples strongly to KK¯, ηη, and ηη′. Signatures for a possible tensor glueball at ∼2 GeV are also considered. © 1995 The American Physical Society.

We report on a first measurement of the polarized-target asymmetry of the pion-proton bremsstrahlung cross section (π⁺p→π⁺pγ). As in previous cross section measurements the pion energy (298 MeV) and the detector geometry for this experiment was chosen to optimize the sensitivity to the radiation from the magnetic dipole moment of the Δ⁺⁺(1232) resonance μ_Δ. Comparison to a recent isobar model for pion-nucleon bremsstrahlung yields μ_Δ=(1.62±0.18) μ_p, where μ_p is the proton magnetic moment. Since the asymmetry depends less than the cross section on the choice of the other input parameters for the model, their uncertainties affect this analysis by less than the experimental error. However the theory fails to represent both the cross section and the asymmetry data at the highest photon energies. Hence further improvements in the calculations are needed before the model dependence of the magnetic moment analysis can be fully assessed. The present result agrees with bag-model corrections to the SU(6) prediction μ_Δ=2μ_p. As a by-product, the analyzing power for elastic π⁺p scattering at 415 MeV/c was also measured. This second result is in good agreement with phase shift calculations.

We describe the details of an experiment designed to search for the admixture of massive neutrinos into electron-flavor neutrinos by observation with high resolution of the positron momentum spectrum in π→e+ν decay. No indication was found for such an admixture and the corresponding mixing amplitude was constrained below the U_ei²≤10^-6 region for neutrinos of ∼30 MeV/c² mass.

First data are presented for the polarized-target asymmetry in the reaction π⁺p→π⁺pγ at an incident pion energy of 298 MeV. The geometry was chosen to maximize the sensitivity to the radiation of the magnetic dipole moment μ_Δ of the Δ⁺⁺(1232 MeV). A fit of the asymmetry in the cross section d⁵σ/dΩ_π dΩ_γ dk as a function of the photon energy k to predictions from a recent isobar-model calculation with μ_Δ as the only free parameter yields μ_Δ=1.64(±0.19expΔ,±0.14 theor)μ_p. Though this value agrees with bag-model corrections to the SU(6) prediction μ_Δ=2μ_p, further clarifications on the model dependence of the result are needed, in particular since the isobar model fails to describe both the cross section and the asymmetry at the highest photon energies.

Results on antiproton absorption at rest in gaseous nitrogen and deuterium are presented from an analysis of approximately 10⁶ 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 Λ yield, the Λ spectrum, and the exclusive deuterium channels p¯d→π^-p, ΛK⁺π^- are discussed. The fully reconstructable channels p¯d→π⁺π^-π^-p,π⁺π⁺ π^-π^-π^-p also show a high-energy proton tail unaccounted for by single nucleon rescattering mechanisms.

We have measured the fivefold differential cross section d⁵σ/dΩ_πdΩ_γdE_γ for the process π⁺p→π⁺pγ with incident pions of energy 299 MeV. The angular regions for the outgoing pions (55°≤θ_lab^π≤95°), and photons (θ_{lab^γ=241°±10}°) in coplanar geometry are selected to maximize the sensitivity to the radiation from the magnetic dipole moment of the Δ⁺⁺(1232) resonance. At low photon energies, the data agree with the soft-photon approximation to pion-proton bremsstrahlung. At forward pion angles the data agree with older data and with the latest theoretical calculations for 2.3μ_p≤μ_Δ≤3.3μ_p. However at more backward pion angles where no data existed, the predictions fail.

Relative cross sections have been measured for p̅ p annihilation by a new technique in the invariant-mass range 1920-1960 MeV. The 90%-confidence upper limit on simple Breit-Wigner enhancements in the S region (between 1930 and 1942 MeV) of width 4 MeV or narrower is 12 mb MeV, assuming a simple dependence with momentum of the nonresonant cross section.

A missing-mass experiment in pd→NX, where X is the system whose mass M_X is measured, was performed for interactions of antiprotons at rest and in flight below 500 MeV/c. The 4-standard-deviation upper limit for the production of narrow states, in the mass interval M_X=1650-1930 MeV/c², is 2 mb for annihilations in flight and 0.3% of the annihilations at rest.

The Wolfenstein parameters R_t and A_t have been measured in free np elastic scattering at 220, 325, 425, and 495 MeV at 10° intervals in the center-of-mass range 60 to 160°, with errors typically ±0.05.

NUCLEAR REACTIONS p(n→,p→)n, E=220, 325, 425, and 495 MeV, measured R_t(θ), A_t(θ).

Measurements over an extensive angular range of the D_t and P parameters in free neutron-proton elastic scattering at laboratory energies of 220, 325, 425, and 495 MeV are reported. Experimental and analytical details are given.

NUCLEAR REACTIONS p(n→, p→)n, E=220, 325, 425, and 495 MeV; measured D_t(θ), P(θ).

Data on D_t, R_t, A_t, and P lead to unique and accurate np phase shift solutions at 210, 325, 425, and 515 MeV. The I=O, D, and G waves show large systematic deviations from current theoretical models.

NUCLEAR REACTIONS p(n,n)p, E=210, 325, 425, and 515 MeV; phase shift analysis.