The NNBAR experiment for the European Spallation Source will search for free neutrons converting to antineutrons with an expected sensitivity improvement of three orders of magnitude compared to the ...last such search. This paper describes both the simulations of a key component for the experiment, the neutron optical reflector and the expected gains in sensitivity.
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.
.
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.
Frozen microspheres of hydrogen (pellets) are used as targets in the hadron physics experiment WASA (Forschungszentrum Julich, Germany) 1 and will also be used in the future PANDA experiment at FAIR ...(GSI, Darmstadt, Germany) 2. The interaction region is defined by the overlap of the pellet stream and the accelerator beam and has a size of a few millimeters. One would like to know the interaction point more precisely, to have better possibility to reconstruct particle tracks and events. One would also like to suppress background events that do not originate in a pellet, but e.g. may occur in residual gas in the beam pipe. A solution is provided by the presented pellet tracking system, for which a prototype 3 has been developed in Uppsala. The goal is to track individual pellets in order to know their position at the time of an interaction. The design of such a system, simulation techniques and results are presented.
A system for optical tracking of frozen hydrogen microsphere targets (pellets) has been designed. It is intended for the upcoming hadron physics experiment PANDA at FAIR, Darmstadt, Germany. With ...such a tracking system one can reconstruct the positions of the individual pellets at the time of a hadronic interaction in the offline event analysis. This gives information on the position of the primary interaction vertex with an accuracy of a few 100 mu m, which is very useful e.g. for reconstruction of charged particle tracks and secondary vertices and for background suppression. A study has been done at the WASA detector setup (Forschungszentrum Julich, Germany) to check the possibility of classification of hadronic events as originating in pellets or in background. The study has been done based on the instantaneous rate a Long Range TDC which was used to determine if a pellet was present in the accelerator beam region. It was clearly shown that it is possible to distinguish the two event classes. Also, an experience was gained with operation of two synchronized systems operating in different time scales, as it will also be the case with the optical pellet tracking.
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.
Frozen microspheres of hydrogen (pellets) will be one of the target types for the future hadron physics experiment PANDA at FAIR (GSI, Darmstadt, Germany)
1
. Pellets with a diameter of 25–
μ
m are ...generated about 3 meters above the interaction region, to which they travel with a velocity around 80 m/s inside a narrow pipe. The interaction region is defined by the overlap of the pellet stream and the accelerator beam and has a size of a few millimeters. One would like to know the interaction point more precisely, to have better possibilities to reconstruct particle tracks and events e.g. in charmonium decay studies. One would also like to suppress background events that do not originate in a pellet, but e.g. may occur in rest gas, that is present in the beam pipe. A solution is provided by the presented pellet tracking system together with a target operation mode that provides one and only one pellet in the interaction region most of the time. The goal is to track individual pellets in order to know their position with a resolution of a few tenths of a millimeter at the time of an interaction. The system must also be highly efficient and provide tracking information for essentially all pellets that pass the interaction region. Presented results from the design studies show that the goals can be fulfilled by this solution.
The presently world largest data sample for π0→γe+e− decays studies containing nearly 5×105 events was collected using the WASA detector at COSY. A search for a dark photon U produced in the ...π0→γU→γe+e− decay from the pp→ppπ0 reaction was carried out. An upper limit on the square of the U−γ mixing strength parameter ϵ2 of 5×10−6 at 90% CL was obtained for the mass range 20 MeV<MU<100 MeV. This result together with other recent experimental limits significantly reduces the MUvs. ϵ2 parameter space which could explain the presently seen deviation between the Standard Model prediction and the direct measurement of the anomalous magnetic moment of the muon.
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.
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