Investigation at a
φ
-factory can shed light on several debated issues in particle physics. We discuss: (i) recent theoretical development and experimental progress in kaon physics relevant for the ...Standard Model tests in the flavor sector, (ii) the sensitivity we can reach in probing CPT and Quantum Mechanics from time evolution of entangled-kaon states, (iii) the interest for improving on the present measurements of non-leptonic and radiative decays of kaons and
η
/
η
′ mesons, (iv) the contribution to understand the nature of light scalar mesons, and (v) the opportunity to search for narrow di-lepton resonances suggested by recent models proposing a hidden dark-matter sector. We also report on the
e
+
e
−
physics in the continuum with the measurements of (multi)hadronic cross sections and the study of
γ
γ
processes.
First exclusive data for the
reaction have been obtained at CELSIUS with the WASA detector setup at a beam energy of T
p
= 1.1 GeV. Total and differential cross-sections disagree with theoretical ...calculations, which predict the
excitation to be the dominant process at this beam energy. Instead, the data require the excitation of one of the nucleons to a higher-lying
state, preferably the
, to be the leading process.
The ABC effect-a puzzling low-mass enhancement in the pipi invariant mass spectrum, first observed by Abashian, Booth, and Crowe-is well known from inclusive measurements of two-pion production in ...nuclear fusion reactions. Here we report on the first exclusive and kinematically complete measurements of the most basic double-pionic fusion reaction pn-->dpi;{0}pi;{0} at beam energies of 1.03 and 1.35 GeV. The measurements, which have been carried out at CELSIUS-WASA, reveal the ABC effect to be a (pipi)_{I=L=0} channel phenomenon associated with both a resonancelike energy dependence in the integral cross section and the formation of a DeltaDelta system in the intermediate state. A corresponding simple s-channel resonance ansatz provides a surprisingly good description of the data.
We report on an exclusive and kinematically complete high-statistics measurement of the basic double-pionic fusion reaction pn→dπ(0)π(0) over the full energy region of the ABC effect, a pronounced ...low-mass enhancement in the ππ-invariant mass spectrum. The measurements, which cover also the transition region to the conventional t-channel ΔΔ process, were performed with the upgraded WASA detector setup at COSY. The data reveal the Abashian-Booth-Crowe effect to be uniquely correlated with a Lorentzian energy dependence in the integral cross section. The observables are consistent with a narrow resonance with m=2.37 GeV, Γ≈70 MeV and I(J(P))=0(3(+)) in both pn and ΔΔ systems. Necessary further tests of the resonance interpretation are discussed.
The reaction pd→He3η at threshold was used to provide a clean source of η mesons for decay studies with the WASA detector at CELSIUS. The branching ratio of the decay η→π+π−e+e− is measured to be ...(4.3±1.3±0.4)×10−4.
The ABC effect–an intriguing low-mass enhancement in the
π
π
invariant mass spectrum–is known from inclusive measurements of two-pion production in nuclear fusion reactions. First exclusive ...measurements carried out at CELSIUS-WASA for the fusion reactions leading to d or
3He reveal this effect to be a
σ
-channel phenomenon associated with the formation of a
Δ
Δ
system in the intermediate state and combined with a resonance-like behavior in the total cross-section. Together with the observation that the differential distributions do not change in shape over the resonance region the features fulfill the criteria of an isoscalar s-channel resonance in
p
n
and
N
N
π
π
systems, if the two emitted nucleons are bound. It obviously is robust enough to survive in nuclei as a dibaryonic resonance configuration. In this context also the phenomenon of
N
Δ
resonances is reexamined.
The two-pion production in pp-collisions has been investigated at CELSIUS in exclusive measurements from threshold up to Tp=1.36 GeV. Total and differential cross sections have been obtained for the ...channels pnπ+π0, ppπ+π−, ppπ0π0 and also nnπ+π+. For intermediate incident energies Tp>1 GeV, i.e. in the region which is beyond the Roper excitation but at the onset of ΔΔ excitation, the total ppπ0π0 cross section falls behind theoretical predictions by as much as an order of magnitude near 1.2 GeV, whereas the nnπ+π+ cross section is a factor of five larger than predicted. An isospin decomposition of the total cross sections exhibits a s-channel-like energy dependence in the region of the Roper excitation as well as a significant contribution of an isospin 3/2 resonance other than the Δ(1232). As possible candidates the Δ(1600) and the Δ(1700) are discussed.
Low-energy spectator protons (p
s) from quasi-free proton–neutron interactions on deuterium have been measured for the pd→dπ
0p
s and pd→pnπ
0p
s reactions at the CELSIUS storage ring of the The ...Svedberg Laboratory. The experiment used 320 MeV protons incident on a deuterium cluster-jet target. The spectator protons were detected inside the scattering chamber using a silicon detector system which is compatible with a high vacuum. The low target density allows the spectator proton emerge without undergoing significant secondary scattering. Its measurement is therefore a useful tag upon such reactions, allowing cross-sections for quasi-free reactions to be deduced at well-defined centre-of-mass energies. The comparison with the simulation for the pd→dπ
0p
s reaction shows that the energy dependence of the pn→dπ
0 cross-section can be extracted near the threshold. The method greatly improves the experimental resolution achievable in quasi-free p–n reaction studies.
Charge symmetry breaking (CSB) observables are a suitable experimental tool to examine effects induced by quark masses on the nuclear level. Previous high precision data from TRIUMF and IUCF are ...currently used to develop a consistent description of CSB within the framework of chiral perturbation theory. In this work the experimental studies on the reaction dd→He4π0 have been extended towards higher excess energies in order to provide information on the contribution of p-waves in the final state. For this, an exclusive measurement has been carried out at a beam momentum of pd=1.2 GeV/c using the WASA-at-COSY facility. The total cross section amounts to σtot=(118±18stat±13sys±8ext) pb and first data on the differential cross section are consistent with s-wave pion production.
.
In most investigations the Roper resonance is sensed only very indirectly via complex partial-wave analyses. We find indications for its excitation in the invariant
n
π
+
mass spectrum of the
pp
→
...np
π
+
reaction at
M
≈ 1360 MeV with a width of ≈ 150 MeV . The values fit very favorably to the most recent phase shift results as well as to the observations at BES. In the near-threshold two-pion production
pp
→
pp
π
0
π
0
, where the Roper excitation and its subsequent decays via the routes
N
*
→ Δπ →
N
ππ and
N
*
→
N
σ are the only dominant processes, we find its direct decay into the
N
σ channel to be the by far dominating decay process --in favor of a monopole excitation of the Roper resonance.
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