We report on a high-statistics measurement of the basic double pionic fusion reaction \(pn \to d\pi^0\pi^0\) over the energy region of the so-called ABC effect, a pronounced low-mass enhancement in ...the \(\pi\pi\)-invariant mass spectrum. The measurements were performed with the WASA detector setup at COSY. The data reveal the ABC effect to be associated with a Lorentzian shaped energy dependence in the integral cross section. The observables are consistent with a resonance with \(I(J^P) =0(3^+)\) in both \(pn\) and \(\Delta\Delta\) systems. Necessary further tests of the resonance interpretation are discussed.
An exclusive measurement of the dd -> 3He n \pi 0 reaction was carried at a beam momentum of p = 1.2 GeV/c using the WASA-at-COSY facility. For the first time data on the total cross section as well ...as differential distributions were obtained. The data are described with a phenomenological approach based on a combination of a quasi-free model and a partial wave expansion for three-body reaction. The total cross section is found to be \sigma(tot) = (2.89 +- 0.01(stat) +- 0.06(sys) +- 0.29(norm)) \mu b. The contribution of the quasi-free processes (with the neutron being target or beam spectator) accounts for 38% of the total cross section and dominates the differential distributions in specific regions of the phase space. The remaining part of the cross section can be described within a partial wave decomposition indicating the significance of p-wave contributions in the final state.
Exclusive measurements of the quasi-free \(pn \to pp\pi^0\pi^-\) reaction have been performed by means of \(pd\) collisions at \(T_p\) = 1.2 GeV using the WASA detector setup at COSY. Total and ...differential cross sections have been obtained covering the energy region \(\sqrt s\) = (2.35 - 2.46) GeV, which includes the region of the ABC effect and its associated resonance structure. No ABC effect, {\it i.e.} low-mass enhancement is found in the \(\pi^0\pi^-\)-invariant mass spectrum -- in agreement with the constraint from Bose statistics that the isovector pion pair can not be in relative s-wave. At the upper end of the covered energy region \(t\)-channel processes for Roper, \(\Delta(1600)\) and \(\Delta\Delta\) excitations provide a reasonable description of the data, but at low energies the measured cross sections are much larger than predicted by such processes. Adding a resonance amplitude for the resonance at \(m\)=~2.37 GeV with \(\Gamma\) =~70 MeV and \(I(J^P)=~0(3^+)\) observed recently in \(pn \to d\pi^0\pi^0\) and \(pn \to d\pi^+\pi^-\) reactions leads to an agreement with the data also at low energies.
The presently world largest data sample of pi0 --> gamma e+e- decays containing nearly 5E5 events was collected using the WASA detector at COSY. A search for a dark photon U produced in the pi0 --> ...gamma U --> gamma e+e- decay from the pp-->pp\pi^0 reaction was carried out. An upper limit on the square of the U-gamma mixing strength parameter epsilon^2 of 5e-6 at 90% CL was obtained for the mass range 20 MeV <M_U< 100 MeV. This result together with other recent experimental limits significantly reduces the M_U vs. \epsilon^2 parameter space preferred by the measured value of the muon anomalous magnetic moment.
We consider Dirichlet spaces with superharmonic weights. This class contains both the harmonic weights and the power weights. Our main result is a characterization of the Dirichlet spaces with ...superharmonic weights that can be identified as de Branges–Rovnyak spaces. As an application, we obtain the dilation inequality
D
ω
(
f
r
)
≤
2
r
1
+
r
D
ω
(
f
)
(
0
≤
r
<
1
)
,
where
D
ω
denotes the Dirichlet integral with superharmonic weight
ω
, and
f
r
(
z
)
:
=
f
(
r
z
)
is the
r
-dilation of the holomorphic function
f
.
.
The
Λ
polarization, the analyzing power, and the
Λ
spin transfer coefficient of the reaction
pp
→
pK
+
Λ
were measured at beam momenta of 2.70GeV/c and 2.95GeV/c corresponding to excess energies of ...122MeV and 204MeV. While the analyzing power and the spin transfer coefficient do not change significantly with the excess energy, the
Λ
polarization varies strongly and changes its sign. As this is the first measurement of polarization observables below an excess energy of 200MeV, the change of the sign of the
Λ
polarization was not observed before. The high statistics of the data (
≈
200 k events for each momentum) enables detailed studies of the dependence of the
Λ
polarization and the analyzing power on the center-of-mass momentum of the particles. The results of the spin transfer coefficient are in qualitative agreement with the DISTO experiment. The
Λ
polarization data of 2.95GeV/c are only conform with the DISTO experiment, while both the 2.70GeV/c and 2.95GeV/c data differ strongly from all previous measurements, whether exclusive or inclusive.
Measurements of the analyzing power for the pp-->pp eta reaction have been performed at excess energies of Q=10 and 36 MeV. The determined analyzing power is essentially consistent with zero, ...implying dominance of the s wave at both excess energies. The angular dependence of the analyzing power, combined with the isospin dependence of the total cross section for the eta meson production in nucleon-nucleon collisions, reveal that the excitation of the nucleon to the S11(1535) resonance is predominantly due to the exchange of the pi meson between the colliding nucleons.
.
A new high-precision measurement of the reaction
at a beam momentum of 2.95 GeV/
c
with more than 200 000 analyzed events allows a detailed analysis of differential observables and their ...inter-dependencies. Correlations of the angular distributions with momenta are examined. The invariant mass distributions are compared for different regions in the Dalitz plots. The cusp structure at the
threshold is described with the Flatté formalism and its variation in the Dalitz plot is analyzed.
The Σ+ hyperon production was measured at the COSY-11 spectrometer via the pp→nK+Σ+ reaction at excess energies of Q=13 MeV and Q=60 MeV. These measurements continue systematic hyperon production ...studies via the pp→pK+Λ/Σ0 reactions where a strong decrease of the cross section ratio close-to-threshold was observed. In order to verify models developed for the description of the Λ and Σ0 production we have performed the measurement on the Σ+ hyperon and found unexpectedly that the total cross section is by more than one order of magnitude larger than predicted by all anticipated models. After the reconstruction of the kaon and neutron four momenta, the Σ+ is identified via the missing mass technique. Details of the method and the measurement will be given and discussed in view of theoretical models.
PDF
