Resonance-like coherent production of a pion pair in the reaction pd → pdππ in the GeV region (original) (raw)
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The European Physical Journal A
The reaction p + d → p + d + X was studied at 0.8-2.0 GeV proton beam energies with the ANKE magnetic spectrometer at the COSY synchrotron storage ring. The proton-deuteron pairs emerging with high momenta, 0.6-1.8 GeV/c, were detected at small angles with respect to the proton beam. Distribution over the reaction missing mass Mx reveals a local enhancement near the threshold of the pion pair production specific for the so-called ABC effect. The enhancement has a structure of a narrow bump placed above a smooth continuum. The invariant mass of the dππ system in this enhancement region exhibits a resonance-like peak at M dππ ≈ 2.36 GeV/c 2 with the width Γ ≈ 0.10 GeV/c 2. A possible interpretation of these features is discussed.
Progress in Particle and Nuclear Physics, 2008
The ABC effect -an intriguing low-mass enhancement in the ππ 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 3 He reveal this effect to be a σ -channel phenomenon associated with the formation of a ∆∆ system in the intermediate state and combined with a resonance-like behavior in the total cross-section. Together with the * Corresponding author. isoscalar s-channel resonance in pn and N N ππ 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 ∆ resonances is reexamined.
Exclusive measurement of two-pion production in the reaction
Nuclear Physics A, 2009
The results from the first kinematically complete measurement of the dd → 4 He ππ reaction are reported. The aim was to investigate a long standing puzzle regarding the origin of the peculiar ππ-invariant mass distributions appearing in double pion production in light ion collisions, the so-called ABC effect. The measurements were performed at the incident deuteron energies of 712 MeV and 1029 MeV, with the WASA detector assembly at CELSIUS in Uppsala, Sweden. We report the observation of a characteristic enhancement at low ππ-invariant mass at 712 MeV, the lowest energy yet. At the higher energy, in addition to confirming previous experimental observations, our results reveal a strong angular dependence of the pions in the overall centre of mass system. The results are qualitatively reproduced by a theoretical model, according to which the ABC effect is described as resulting from a kinematical enhancement in the production of the pion pairs from two parallel and independent NN → dπ sub-processes.
Coherent pion production in proton–deuteron collisions
Physics Letters B, 2016
Values of the proton analysing power in the pd → 3 He π 0 / 3 H π + reactions at 350-360 MeV per nucleon were obtained by using a polarised proton beam incident on a deuterium cluster-jet target and with a polarised deuteron beam incident on a target cell filled with polarised hydrogen. These results have a much larger angular coverage than existing data. First measurements are also presented of the deuteron vector analysing power and the deuteron-proton spin correlations. Data were also obtained on the deuteron-proton spin correlation and proton analysing power at small angles at 600 MeV per nucleon, though the angular coverage at this energy was much more restricted even when using a deuteron beam. By combining the extrapolated values of the spin correlations to the forward or backward directions with published measurements of the deuteron tensor analysing powers, the relative phases between the two non-vanishing amplitudes were evaluated.
ΔΔ excitation in proton–proton induced production
Physics Letters B, 2011
Exclusive measurements of the pp → ppπ 0 π 0 reaction have been performed at CELSIUS/WASA at energies from threshold up to Tp = 1.3 GeV. Total and differential cross sections have been obtained. Here we concentrate on energies Tp ≥ 1 GeV, where the ∆∆ excitation becomes the leading process. No evidence is found for a significant ABC effect beyond that given by the conventional t-channel ∆∆ excitation. This holds also for the double-pionic fusion to the quasibound 2 He. The data are compared to model predictions, which are based on both pion and ρ exchange. Total and differential cross sections are at variance with these predictions and call for a profound modification of the ρ-exchange. A phenomenological modification allowing only a small ρ exchange contribution leads to a quantitative description of the data.
Coincidence Measurements of Single-Pion Electroproduction near the Δ (1236) Resonance
Physical Review D, 1971
Differential cross sections for the reactions e-+ p + e-+p+rO and e-+p + e-+n+x+ have been measured near the A(1236) resonance a t four-momentum transfers of 0.05, 0.13, 0.25, and 0.4 (G e v /~)~. A few measurements of the r+ angular distribution have been obtained at a four-momentum transfer of 0.6 (GeV/c)Z. Cross sections for the x O reaction are compared with dispersion-theory predictions a t several pion-nucleon c.m. energies for each four-momentum transfer. A phenomenological analysis of the ao results leads to the determination of the magnetic dipole and electric quadrupole partial-wave amplitudes and the yNA transition form factor. Evidence is found for the existence of a significant scaler-transverse interference term in the cross section, which is tentatively associated with the resonant scaler quadrupole interaction. Cross sections for a+ electroproduction are compared with dispersion theories using the pion form factor as a free parameter. The results suggest a form factor similar to that of the proton.-1 fit to the form-factor results, using the p-dominance model, requires m,=560f 80 MeV. The rms pion charge radius is estimated to be t=0.86~k0.14 F.
Beam-Helicity Asymmetries in Double-Pion Photoproduction off the Proton
Physical Review Letters, 2009
Beam-helicity asymmetries have been measured at the MAMI accelerator in Mainz in the three isospin channels γp → π + π 0 n, γp → π 0 π 0 p and γp → π + π − p . The circularly polarized photons, produced from bremsstrahlung of longitudinally polarized electrons, were tagged with the Glasgow magnetic spectrometer. Charged pions and the decay photons of π 0 mesons were detected in a 4π electromagnetic calorimeter which combined the Crystal Ball detector with the TAPS detector. The precisely measured asymmetries are very sensitive to details of the production processes and are thus key observables in the modeling of the reaction dynamics.
Two-pion production in proton-proton collisions with a polarized beam
The European Physical Journal A, 2008
The two-pion production reaction pp → ppπ + π − was measured with a polarized proton beam at Tp ≈ 750 and 800 MeV using the short version of the COSY-TOF spectrometer. The implementation of a delayed-pulse technique for Quirl and central calorimeter provided positive π + identification in addition to the standard particle identification, energy determination as well as time-of-flight and angle measurements. Thus all four-momenta of the emerging particles could be determined with 1-4 overconstraints. Total and differential cross-sections as well as angular distributions of the vector analyzing power have been obtained. They are compared to previous data and theoretical calculations. In contrast to predictions we find significant analyzing-power values up to Ay = 0.3. The data taken in the energy region of the excitation of the Roper resonance confirm that its dominant ππ decay channel is N * → N σ. PACS. 13.75.Cs Nucleon-nucleon interactions (including antinucleons, deuterons, etc.) -14.20.Gk Baryon resonances with S = 0 -25.10.+s Nuclear reactions involving few-nucleon systems -25.40.Ep Inelastic proton scattering a
A Study of Proton-Induced Pion Production on C at Intermediate Energies via Recoil Detection
This work describes a study of proton-induced pion production from 12 C, for proton energies in the range E p = 166-350 MeV. The (p, π) reaction is particularly useful as a means by which the π-N interaction mechanism within a bound nuclear system can be sampled. Of further interest, double-pion production processes (p, ππ) near threshold in nuclei may be sensitive to any breaking of the underlying chiral symmetry. A full description of the pion-production process in nuclei is a key step toward understanding the long-range part of the nuclear strong force.