Cyclotron trap

The cyclotron trap was developed at SIN/PSI to increase the stopping density of negatively charged particle beams for the formation of exotic atoms in low pressure gases. A weak focusing magnetic field, produced by superconducting solenoids, is used. Particles are injected radially through the fringe field to a moderator, which decelerates them into orbits bound by the field. Further deceleration by moderators and/or low-pressure gases leads the particles to the centre of the device, where they can be stopped or eventually extracted. Experiments became feasible with this technique, such as those dealing with pionic hydrogen/deuterium at SIN/PSI. Muonic hydrogen laser experiments also became possible with the extraction of muons from the cyclotron trap. The formation of antiprotonic hydrogen in low pressure targets led to successful experiments at LEAR/CERN.

Pionic hydrogen and deuterium

The measurement of strong-interaction shift and broadening in pionic hydrogen and deuterium yields pion-nucleon scattering lengths as well as the threshold pion-production strength on isoscalar NN pairs. Results from recent high-resolution experiments at PSI using crystal spectrometers allow important comparisons with the outcome of the modern low-energy description of QCD within the framework of effective field theories.

The $\pi^0$ mass and the first experimental verification of Coulomb de-excitation in pionic hydrogen

The most precise value for the \pi^0π0 mass was obtained from the measurement of the mass difference m_{\pi^-}-m_{\pi^0} = 4.593\,64(48)mπ−−mπ0=4.59364(48),MeV/c^22 in the charge exchange reaction \pi^-π−p \rightarrow \pi^0→π0n at PSI. With the most precise charged pion mass value, m_{\pi^+} = 139.570\,21(14)mπ+=139.57021(14),MeV/c^22 and the validity of the CPT theorem (m_{\pi^-} = m_{\pi^+}mπ−=mπ+), a value m_{\pi^0} = 134.976\,57(50)mπ0=134.97657(50),MeV/c^22 is obtained. The measurements also revealed, for the first time, evidence of an unexpectedly large contribution from Coulomb de-excitation states during the pionic atom cascade.

Redetermination of the strong-interaction width in pionic hydrogen

The hadronic width of the ground state of pionic hydrogen has been redetermined by X-ray spectroscopy to be $$\varGamma ^{\pi \mathrm {H}}_{1s}=(856\,\pm \,16_\mathrm{stat}\,\pm \,22_\mathrm{sys})$$ Γ 1 s π H = ( 856 ± 16 stat ± 22 sys ) meV. The experiment was performed at the high-intensity low-energy pion beam of the Paul Scherrer Institute by using the cyclotron trap and a high-resolution Bragg spectrometer with spherically bent crystals. Coulomb de-excitation was studied in detail by comparing its influence on the line shape by measuring the three different transitions K$$\alpha $$ α , K$$\beta $$ β , and K$$\gamma $$ γ at various hydrogen densities. The pion-nucleon scattering lengths and other physical quantities extracted from pionic-atom data are in good agreement with the results obtained from pion-nucleon and nucleon-nucleon scattering experiments and confirm that a consistent picture is achieved for the low-energy pion-nucleon sector with respect to the expectations of chiral perturbation theory.