Ugo Gastaldi

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.

φ production in pp annihilation at rest

Abstract We report on measurements of the branching ratios for φ production in p p annihilation at rest in H 2 gas. Branching ratios for φπ 0 , φη, φϱ 0 , φω and φπ + π − are determined for two data sets with different contributions of annihilations from S and P states of the p p system. The branching ratios are compared to corresponding annihilation modes where the φ is replaced by an ω meson. We conclude that φ production is in most reactions enhanced with respect to the expectation based on the OZI rule. In annihilations from the spin triplet ground state of antiprotonic hydrogen into φπ the OZI rule is violated dramatically.

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.

The PVLAS experiment: measuring vacuum magnetic birefringence and dichroism with a birefringent Fabry–Perot cavity

Vacuum magnetic birefringence was predicted long time ago and is still lacking a direct experimental confirmation. Several experimental efforts are striving to reach this goal, and the sequence of results promises a success in the next few years. This measurement generally is accompanied by the search for hypothetical light particles that couple to two photons. The PVLAS experiment employs a sensitive polarimeter based on a high finesse Fabry–Perot cavity. In this paper we report on the latest experimental results of this experiment. The data are analysed taking into account the intrinsic birefringence of the dielectric mirrors of the cavity. Besides a new limit on the vacuum magnetic birefringence, the measurements also allow the model-independent exclusion of new regions in the parameter space of axion-like and milli-charged particles. In particular, these last limits hold also for all types of neutrinos, resulting in a laboratory limit on their charge.

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.

The πd scattering length from Kα X-rays

Abstract X-rays from the K α transition (2P → 1S) of the π − d mesic atom have been observed. Their energy, 2592.8 −2.0 +1.6 , has been measured by the critical absorber technique, using the M V absorption edge in bismuth. The strong interaction shift in the 1S state is −4.8 eV, corresponding to a scattering length a ( π d) = −(0.052 −0.017 +0.022 ) m π −1 , in agreement with recent calculations. The intensity ratio K α /K total = 0.548 ± 0.015.

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.