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Register a new userMeasurement of the $vec{n}p to dpi^0pi^0$ Reaction with Polarized Beam in the Region of the $d^*(2380)$ Resonance
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We report on a high-statistics measurement of the most basic double pionic fusion reaction $vec{n}p to dpi^0pi^0$ over the energy region of the $d^*(2380)$ resonance by use of a polarized deuteron beam and observing the double fusion reaction in the quasifree scattering mode. The measurements were performed with the WASA detector setup at COSY. The data reveal substantial analyzing powers and confirm conclusions about the $d^*$ resonance obtained from unpolarized measurements. We also confirm the previous unpolarized data obtained under complementary kinematic conditions.
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Exclusive measurements of the quasi-free $np to nppi^0pi^0$ reaction have been performed by means of $dp$ collisions at $T_d$ = 2.27 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 $d^*(2380)$ resonance. Adding the $d^*$ resonance amplitude to that for the conventional processes leads to a reasonable description of the data. The observed resonance effect in the total cross section is in agreement with the predictions of Faldt and Wilkin as well Albadajedo and Oset. The ABC effect, {it i.e.} the low-mass enhancement in the $pi^0pi^0$-invariant mass spectrum, is found to be very modest - if present at all, which might pose a problem to some of its interpretations.
Exclusive measurements of the quasi-free $pn to pppi^0pi^-$ 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 $pi^0pi^-$-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 $DeltaDelta$ 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 $pn to dpi^0pi^0$ and $pn to dpi^+pi^-$ reactions leads to an agreement with the data also at low energies.
The recoil proton polarization has been measured in the p (vec e,evec p) pi^0 reaction in parallel kinematics around W = 1232 MeV, Q^2 = 0.121 (GeV/c)^2 and epsilon = 0.718 using the polarized c.w. electron beam of the Mainz Microtron. Due to the spin precession in a magnetic spectrometer, all three proton polarization components P_x/P_e = (-11.4 pm 1.3 pm 1.4) %, P_y = (-43.1 pm 1.3 pm 2.2) %, and P_z/P_e = (56.2 pm 1.5 pm 2.6) % could be measured simultaneously. The Coulomb quadrupole to magnetic dipole ratio CMR = (-6.4pm 0.7_{stat}pm 0.8_{syst}) % was determined from P_x in the framework of the Mainz Unitary Isobar Model. The consistency among the reduced polarizations and the extraction of the ratio of longitudinal to transverse response is discussed.
We report the first beam-target double-polarization asymmetries in the $gamma + n(p) rightarrow pi^- + p(p)$ reaction spanning the nucleon resonance region from invariant mass $W$= $1500$ to $2300$ MeV. Circularly polarized photons and longitudinally polarized deuterons in $H!D$ have been used with the CLAS detector at Jefferson Lab. The exclusive final state has been extracted using three very different analyses that show excellent agreement, and these have been used to deduce the {it{E}} polarization observable for an effective neutron target. These results have been incorporated into new partial wave analyses, and have led to significant revisions for several $gamma nN^*$ resonance photo-couplings.
We report the first large-acceptance measurement of the beam-spin asymmetry for deuteron photodisintegration ($vec{gamma} dto pn$) in the photon energy range $400<E_{gamma}<630$~MeV. The measurement provides important new constraints on the mechanisms of photodisintegration above the delta resonance and on the photocoupling of the recently discovered $d^*(2380)$ hexaquark.
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