Spin density matrix elements have been determined for exclusive omega meson production on hydrogen and deuterium targets, in the kinematic region of 1.0 < Q super(2) < 10.0 GeV super(2), 3.0 < W < ...6.3 GeV and -t' < 0.2 GeV super(2). The data, from which SDMEs are determined, were accumulated with the HERMES forward spectrometer during the running period of 1996 to 2007 using the 27.6 GeV electron or positron beam of HERA. A sizable contribution of unnatural parity exchange amplitudes is found for exclusive omega meson production.
Exclusive measurements of the quasi-free pn→ppπ− and pp→ppπ0 reactions have been performed by means of pd collisions at Tp=1.2 GeV using the WASA detector setup at COSY. Total and differential cross ...sections have been obtained covering the energy region Tp=0.95–1.3 GeV (s=2.3–2.46 GeV), which includes the regions of Δ(1232), N⁎(1440) and d⁎(2380) resonance excitations. From these measurements the isoscalar single-pion production has been extracted, for which data existed so far only below Tp=1 GeV. We observe a substantial increase of this cross section around 1 GeV, which can be related to the Roper resonance N⁎(1440), the strength of which shows up isolated from the Δ resonance in the isoscalar (Nπ)I=0 invariant-mass spectrum. No evidence for a decay of the dibaryon resonance d⁎(2380) into the isoscalar (NNπ)I=0 channel is found. An upper limit of 180 μb (90% C.L.) corresponding to a branching ratio of 9% has been deduced.
New data on both total and differential cross sections of the production of η mesons in proton–deuteron fusion to He3η in the excess energy region 13.6MeV≤Qη≤80.9MeV are presented. These data have ...been obtained with the WASA-at-COSY detector setup located at the Forschungszentrum Jülich, using a proton beam at 15 different beam momenta between pp=1.60GeV/c and pp=1.74GeV/c. While significant structure of the total cross section is observed in the energy region 20MeV≲Qη≲60MeV, a previously reported sharp variation around Qη≈50MeV cannot be confirmed. Angular distributions show the typical forward-peaking that was noted earlier. For the first time, it is possible to study the development of these angular distributions with rising excess energy over a wide interval.
Analyses of inclusive nonelastic nucleon emission use inconsistently the gradual absorption into quasibound compound states together with the onestep direct reactions to describe experiments. We ...perform a sum rule analysis to show that the onestep reactions are misinterpreted.
Abstract We report on a measurement of Spin Density Matrix Elements (SDMEs) in hard exclusive $$\omega $$ ω meson muoproduction on the proton at COMPASS using 160 GeV/c polarised $$ \mu ^{+}$$ μ + ...and $$ \mu ^{-}$$ μ - beams impinging on a liquid hydrogen target. The measurement covers the range 5.0 GeV/ $$c^2$$ c 2 $$< W<$$ < W < 17.0 GeV/ $$c^2$$ c 2 , with the average kinematics $$\langle Q^{2} \rangle =$$ ⟨ Q 2 ⟩ = 2.1 (GeV/c) $$^2$$ 2 , $$\langle W \rangle = 7.6$$ ⟨ W ⟩ = 7.6 GeV/ $$c^2$$ c 2 , and $$\langle p^{2}_{\mathrm{T}} \rangle = 0.16$$ ⟨ p T 2 ⟩ = 0.16 (GeV/c) $$^2$$ 2 . Here, $$Q^2$$ Q 2 denotes the virtuality of the exchanged photon, W the mass of the final hadronic system and $$p_T$$ p T the transverse momentum of the $$\omega $$ ω meson with respect to the virtual-photon direction. The measured non-zero SDMEs for the transitions of transversely polarised virtual photons to longitudinally polarised vector mesons ( $$\gamma ^*_T \rightarrow V_L$$ γ T ∗ → V L ) indicate a violation of s-channel helicity conservation. Additionally, we observe a sizeable contribution of unnatural-parity-exchange (UPE) transitions that decreases with increasing W. The results provide important input for modelling Generalised Parton Distributions (GPDs). In particular, they may allow to evaluate in a model-dependent way the contribution of UPE transitions and assess the role of parton helicity-flip GPDs in exclusive $$\omega $$ ω production.
We report on the experimental search for the bound state of an eta meson and He-3 nucleus performed using the WASA-at-COSY detector setup. In order to search for the eta-mesic nucleus decay, the pd ...-> (3)He2(gamma) and pd -> (3)He6 gamma channels have been analysed. These reactions manifest the direct decay of the ti meson bound in a He-3 nucleus. This non-mesonic decay channel has been considered for the first time. When taking into account only statistical errors, the obtained excitation functions reveal a slight indication for a possible bound state signal corresponding to a He-3-eta nucleus width Gamma above 20 MeV and binding energy B-s between 0 and 15 MeV. However, the determined cross sections are consistent with zero in the range of the systematic uncertainty. Therefore, as final result we estimate only the upper limit for the cross section of the eta-mesic He-3 nucleus formation followed by the eta meson decay which varies between 2 nb and 15 nb depending on possible bound state parameters.
We report on the search for the rare decay η→π0e+e− which is of interest to study C violation in the electromagnetic interaction which would indicate contributions from physics beyond the Standard ...Model, since the allowed decay via a two-photon intermediate state is strongly suppressed. The experiment has been performed using the WASA-at-COSY installation, located at the COSY accelerator of the Forschungszentrum Jülich, Germany. In total 3×107 events of the reaction pd→3Heη have been recorded at an excess energy of Q=59.8MeV. Based on this data set the C parity violating decay η→π0γ⁎→π0e+e− via a single-photon intermediate state has been searched for, resulting in new upper limits of Γ(η→π0e+e−)/Γ(η→π+π−π0)<3.28×10−5 and Γ(η→π0e+e−)/Γ(η→all)<7.5×10−6 (CL=90%), respectively.
This letter reports a first quantitative analysis of the contribution of higher partial waves in the charge symmetry breaking reaction dd→4Heπ0 using the WASA-at-COSY detector setup at an excess ...energy of Q=60MeV. The determined differential cross section can be parametrized as dσ/dΩ=a+bcos2θ⁎, where θ⁎ is the production angle of the pion in the center-of-mass coordinate system, and the results for the parameters are a=(1.55±0.46(stat)+0.32−0.8(syst))pb/sr and b=(13.1±2.1(stat)−2.7+1.0(syst))pb/sr. The data are compatible with vanishing p-waves and a sizable d-wave contribution. This finding should strongly constrain the contribution of the Δ isobar to the dd→4Heπ0 reaction and is, therefore, crucial for a quantitative understanding of quark mass effects in nuclear production reactions.
Inclusive non-elastic nucleon emission is usually described as a one-step direct reaction followed by gradual absorption of the remaining flux into the quasibound particle-hole states of the ...multistep compound reaction chain. We present a sum rule analysis of the cross sections calculated in the framework of the quantal theory of Feshbach, Kerman and Koonin which shows that the experimental data cannot be explained by one-step direct reactions only. The closed-form one-step reaction cross section is expressed as a sum of contributions due to incoherent excitation of particle-hole pairs of orbital angular momentum transfer
l > 4 and coherent excitation of collective vibrations of multipolarity not exceeding
λ = 4. The calculation of multistep cross sections with the non-normal DWBA matrix elements is discussed.
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