D. Bailey

pp annihilation into π+π− and K+K− from atomic p states

Abstract We have obtained the branching ratios for p p annihilation at rest into π+π− and K+K− in a pure p p initial angular momentum state L = 1. A gaseous hydrogen target at normal pressure and temperature was used and events associated with transitions of the antiprotonic atom to the 2p level were selected by detecting the Balmer X-ray series. The branching ratios for p p annihilation into π+π− and K+K− from the 2p state are (4.81 ± 0.49) × 10−3 and (2.87 ± 0.51) × 10−4, respect The pion yield is slightly larger than in liquid hydrogen, where L = 0 annihilation dominates, while the kaon yield is suppressed by a factor of four. Using these and previous data, we derive the branching ratios for pp annihilation into all ππ and K K modes from S and P states. A measurement in gaseous hydrogen, without X-ray requirement, yields the branching ratios (4.30 ± 0.14) × 10−3 and (6.92 ± 0.41) × 10−4. With the known branching ratios of (3.33 ± 0.17) × 10−3 and (1.01 ± 0.05) × 10−3 in liquid hydrogen, we find that (50.3 ± 6.4)% of all annihilations in gas at NTP occur in the initial angular momentum state L = 1.

Observation of an isoscalar meson ax(1565) in annihilation of the pp-atom from P states

Abstract A new isoscalar ππ resonance AX(1565) has been observed at a mass of 1565±10 MeV/c2 and a width of 170±20 MeV/c2. The resonance is produced in p p annihilation at rest into π+π−π0 from P states of antiprotonic hydrogen atoms formed b y stopping antiprotons in hydrogen gas at normal temperature and pressure. Annihilation from P states is identified by coincident observations of the LX-ray series emitted in the cascade of p p atoms. The resonance is seen as striking peak in the π+π− invariant mass spectrum. The phase of the π+π− D wave shows a resonance-like behaviour. Hence we assign the quantum numbers IG(JPC) = 0+(2++) to the resonance.

Measurement of the strong-interaction shift and broadening of the ground state of the pp̄ atom☆☆☆

Abstract The Kα X-rays from pp atoms formed in H2 gas at normal temperature and pressure are unambiguously identified by coincidences with L X-rays populating the 2P level. Background due to inner bremsstrahlung is suppressed by selecting events annihilating into neutral final states only. The Kα line is observed with a significance of more than 25 standard deviations at an energy of 8.67 (15) keV. From fits to the Kα line we obtain a strong-interaction shift and width of the 1S level, averaged over the unresolved spin singlet and triplet contributions, of ΔE + iΓ/2 = [−0.70(15) + i0.80(20)] keV.

pp annihilation into K0K0 in hydrogen gas

Abstract The annihilation channels p p→KsKs and KsKL at rest have been observed in hydrogen gas at normal temperature and pressure. The channel KsKs has also been observed from the atomic 2p orbital. The branching fraction for p p→K0 K 0 from initial L = 1 angular momentum p p states is (8.8±2.3)×10−5 and is suppressed by one order of magnitude compared to L = 0 states.

First observation of K X-rays from pp atoms☆☆☆

Abstract K and L series X-rays have been observed from p p atoms formed in hydrogen gas at NTP in association with annihilations into neutral particles. The total K X-ray yield is (6.5 ± 3.2) × 10 −3 per stopped antiproton. A model-dependent fit of the K X-ray spectrum gives a K α energy of 8.9±0.3 keV, corresponding to a strong interaction shift of (−0.5±0.3) keV.

X-rays from proton-antiproton annihilation into charged final states☆

Abstract X-rays originating in the deexcitation of pp atoms and their subsequent annihilation into two charged and any number of neutral mesons have been measured with a position and energy-sensitive drift proportional chamber. The atomic L-lines and the inner bremsstrahlung are clearly observed. The theoretically predicted differential probability for production of inner bremsstrahlung is confirmed. Additional features in the X-ray spectrum are attributed to K-lines and a nearly flat background which is probably due to Compton scattering in the detector of higher energy X- and γ-rays. The absolute yield of L-lines per antiproton stop is YL =0.11 (2), the corresponding value for K-lines is KK = 0.0014 (6).

Cascade time of antiprotons in gaseous hydrogen

Abstract Antiprotons moderated in H 2 are captured in the Coulomb field of protons forming pp atoms. The time which elapsed between capture into atomic orbits and annihilation has been measured in H 2 at NPT. A cascade time of (5.1±0.7) ns was determined.

The ASTERIX spectrometer at LEAR

The ASTERIX spectrometer has been used to study the formation and the ground state of the pp atom and exclusive final states of pp annihilation at rest in a H2 gas target at NTP, using antiprotons from the Low Energy Antiproton Ring (LEAR) at CERN. The article describes the mechanical construction of the apparatus, the trigger logic, the chamber readout, and the data acquisition system. The detector calibration and the offline processing of the 55 million reconstructed events are illustrated. An overview is given about the new physics results obtained by the experiment.

Search for monochromatic pion emission in pp annihilation from atomic p states

Abstract Narrow states observable through the emission of monoenergetic charged pions have been searched for in p p annihilation at rest in a gaseous hydrogen target where annihilation from atomic angular momentum L = 1 states dominates. No structure is observed. The 5σ upper limit for the production of narrow states in the mass range 1100–1670 MeV is 2 × 10 −3 of all annihilations.

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

AbstractAntiproton-proton annihilation at rest in a gaseous H2 target at NTP into the final state π+ π−K± π∓ (K0) with an undetectedK0 or