Protonium Research Articles

Protonium formation in the excited state in laser assisted antiproton-hydrogen atom collisions

A detailed analysis using the Born approximation is given of protonium formation in the first excited state ( n = 2) in laser assisted antiproton hydrogen atom collisions adopting the semi-classical model. The projectile-laser-field interaction and the dressing effects on the hydrogen atom and the protonium atom have been considered to first order in the A · p gauge. The differential cross sections for protonium ( n = 2) formation have been presented for no-photon and one-photon exchange ( l = 0, ± 1) at 50, 100 and 200 keV energies with laser frequency ω = 0.043 a.u. and laser field strength E 0 = 0.002 a.u. along with the field free results. Two interesting characteristics are noted. The differential cross sections increase slowly with the scattering angle unlike the case of electron capture by heavy particles. Secondly, contributions from the entire angular region (0°–180°) are important for the calculation of cross sections, which is a familiar characteristic in light particle scattering.

Progress in the physics of exotic atoms at CERN

In the last few years great progress has been made at the Low Energy Antiproton Ring of CERN in three fields of exotic atoms: (i) laser spectroscopy of the anomalously long-living antiprotonic helium atoms; (ii) high-precision X-ray spectroscopy of antiprotonic hydrogen and helium isotopes and (iii) production and spectroscopy of antihydrogen. Results of the first two experiments and prospects of making the third one are summarised in the present review.

Precision measurement of antiprotonic hydrogen and deuterium X-rays

X-rays from antiprotonic hydrogen and deuterium have been measured at low pressures. Using the cyclotron trap, a 105 MeV/c antiproton beam from LEAR was stopped with an efficiency of 86% in 30 mbar hydrogen gas in a volume of only 100 cm3. The X-rays were measured with Si(Li) detectors and a Xe-CH4 drift chamber. The strong interaction shift and broadening of the Lymanα transition and the spin-averaged 2p width in antiprotonic hydrogen was measured with unprecedented accuracy. The triplet component of the ground state in antiprotonic hydrogen was determined for the first time.

φ production in [formula omitted] annihilation at rest

We report on measurements of the branching ratios for φ production in pp 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 pp 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.

N -N Annihilation intoKK from atomicS-states

We investigate the initial state interaction for the antiprotonic hydrogen (protonium) annihilation intoKK. We calculate thepp-n¯n mixing in the annihilation region and underline that the initial state interaction drastically changes the results of the isospin analysis of theN¯N→K¯K amplitude.

Updating the effective range expansion of low-energyN $$\bar N$$ -scattering

Low energyN¯N scattering is reanalyzed using the new data on annihilation cross-sections from CERN [1], the¯np-absorption measurement from BNL [2] and the annihilation width of the 2p state in antiprotonic hydrogen [3]. Imposing the condition that the imaginary part of the effective range is negative we find low energy scattering parameters which interpolate between the existing ITEP [4] and CERN/Heidelberg [5] values.

Antiprotonic Hydrogen: From Atomic Capture to Annihilation

Antiprotons stopping in H2 form antiprotonic hydrogen, atoms of protons and antiprotons bound by their electromagnetic interaction. Antiprotonic hydrogen atoms are formed in high Rydberg states from which they deexcite by external Auger processes and by radiative transitions to the low-lying energy levels. The levels are shifted and broadened due to strong interactions between the two particles. The strong interaction shift and width of the 1S ground state and the width of the 2P state have been determined in recent experiments at the Low-Antiproton-Ring (LEAR) at CERN. The strong interaction effects can be understood quantitatively by including a meson exchange potential (known from nuclear forces) in the Hamiltonian. This meson exchange potential is known to require the existence of deuteron-like bound states of protons and antiprotons, often called baryonium. Baryonium resonances had beed searched for in numerous experiments but without any conclusive evidence for their existence. It is shown how a detailed understanding of the atomic cascade before annihilation has helped to find a new resonance which is likely such a baryonium resonance.

Annihilation of antiprotons at rest in H2 and D2: S versus P state capture

Abstract We discuss the fraction of P wave annihilations in antiprotonic hydrogen and deuterium atoms. In the case of antiprotonic hydrogen, experimental data and the results of a model for the atomic cascade are in good agreement. For antiprotonic deuterium there are conflicting experimental results: one high statistics experiment found that P wave annihilation in liquid D 2 is small, in agreement with cascade calculations. Several low statistics experiment found, however, P wave contributions of about 50%. Possible origins of this discrepancy are discussed.

Antiprotonic hydrogen. Initial state interactions

Strong interactions between proton and antiprotons in antiprotonic hydrogen atoms lead to measurable shifts and widths of the S and P levels. In addition, they introduce a n n component into the p p wave function. This breakdown of isospin symmetry - predicted by models of strong interactions in p p atoms - is determined from experimental data on p p annihilation into two mesons. A comparison with the theoretical results shows good agreement.

Evidence for a multiquark state in [formula omitted] annihilation at rest

We have observed an isoscalar tensor meson at a mass of 1565 ± 10 MeV/ c 2 and a width of 170 ± 20 MeV/ c 2. The resonance is seen in pp annihilation at rest into π + π − π 0 from P states of antiprotonic hydrogen atoms.

Antiprotonic hydrogen: From atomic capture to annihilation

We present a model of the cascade of antiprotonic hydrogen ( pp ) atoms. Stark mixing and external Auger rates are calculated by simulation of the collision processes between pp atom and hydrogen. The calculations show that the results are insensitive to variations of the pp velocity which is not calculated within the model. Therefore no free parameter is needed. The model describes the intensities of the characteristic lines, the fractional annihilation of S- and P-states, and the cascade time. The results agree with recent experiment data obtained at LEAR.

Observation of an isoscalar meson ax(1565) in annihilation of the [formula omitted] from P states

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 pp 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 pp 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 balmer and lyman X-rays in antiprotonic hydrogen isotopes at pressures below 300 hPa

X-Rays of Balmer and Lyman transitions in antiprotonic hydrogen and of Balmer transitions in antiprotonic deuterium were observed at pressures below 300 hPa using Si(Li) semiconductor detectors. The measurement was performed at the LEAR-facility at a beam momentum of 202 MeV/c. In order to stop antiprotons in a low pressure gaseous target with high efficiency, a novel technique, the cyclotron trap has been used. Absolute yields were determined and compared with cascade calculations. A distinct difference in the cascade of antiprotonic hydrogen and deuterium is found. The parameters of strong interaction in antiprotonic hydrogen are determined to beɛ 1s=−(620±100) eV,Γ 1s=(1130±170) eV andΓ 2p=(32±10) meV.

K-series X-rays from anti-protonic hydrogen and deuterium

Energies and yields of K-series X-rays from pp atoms have been measured with two gas scintillation proportional detectors at target gas densities of 0.25 and 0.92 ρ STP. Values for the strong interaction shift and width of the 1s state obtained from the X-ray spectra are compared with the predictions of a variety of theoretical models. Upper limits for the yield of K-series X-rays from p̄d atoms have also been obtained.

Coupled-channel effective-range expansion of low energy [formula omitted] scattering

The effective-range expansion is applied to the N¯N interactions at low energy. The coupled-channel formula for p¯p and n¯n interactions is composed of the scattering length and the effective range for s-wave, and the scattering volume and the effective range for p-wave. Experimental data on N¯N cross-sections below 300 MeV/c as well as the energy shift and the width of antiprotonic hydrogen are fitted. The s-wave scattering length and the p-wave effective range are determined with the presently available data although other parameters are strongly correlated in the fit.

Effective range analysis of low-energy nucleon-antinucleon interaction

An effective range expansion is developed for the N N system incorporating annihilation, charge-exchange and Coulomb interaction. By fitting the LEAR data on elastic scattering and charge-exchange differential cross sections the N N scattering lengths and effective ranges are obtained. These solutions reproduce also the data on antiprotonic hydrogen, antineutron-proton total and annihilation cross sections and the behavior of the forward amplitude real-to-imaginary ratio.

Measurement of X-rays from anti-protonic hydrogen and deuterium

Abstract The yields of K and L series X-rays from pp atoms and of the L series X-rays from pd atoms have been measured at gas densities of 0.25, 0.92, 2.0 and 10.0ρ STP . For pp atoms the strong interaction shift and width for the 1s state and the width of the 2p state have been measured. The results are in good agreement with the predictions of potential models.

Recent Results on Antiprotonic Atoms using a Cyclotron Trap at LEAR

A new technique to stop negatively charged particles inside a cyclotron field is motivated and presented. Its ability to produce a sufficient number of antiprotonic atoms is demonstrated with experimental results at target gas pressures previously not attainable. Thus the knowledge of the cascade process in antiprotonic hydrogen could be extended considerably and the strong interaction parameters in this system could be measured with good precision. It could also be shown that antiprotonic noble gas atoms up to Krypton are ionized completely. Moreover, the possibility to extract slow antiprotons out of the center of the cyclotron trap is discussed shortly.

X-rays from anti-protonic hydrogen and deuterium

Antiprotons from the LEAR facility at CERN were stopped in targets of gaseous H 2 or D 2. Yields of L X-rays were measured. K-series X-rays from p-p atoms were observed. The measured shift and width for the 1s level are ΔE 1s = −0.73±0.15 keV. and Γ 1s = 0.85±0.39 keV.

Results on the measurement ofK-series X-rays from antiprotonic hydrogen

The X-ray spectrum emerging from a liquid-hydrogen target when stopping antiprotons was measured with high resolution in order to determine the strong interaction shift and width of the atomic 1s level. Owing to strong Stark quenching only weak structures could be observed. The upper limit of the yield of any one of theK-series transitions is 3.5×10−4 (95% C.L.) per stopped antiproton, assuming no broadening of the 1s level. Possible identifications of the weak lines with theK-series transitions are discussed.