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Register a new userResonance structure in the {gamma}{gamma} and $pi^0pi^0$ systems in dC interactions
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Kh. U. Abraamyan
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Along with $pi^0$ and {eta} mesons, a resonance structure in the invariant mass spectrum of two photons at M{gamma}{gamma} = 360 pm 7 pm 9 MeV is observed in the reaction d + C rightarrow {gamma} + {gamma} + X at momentum 2.75 GeV/c per nucleon. Estimates of its width and production cross section are {Gamma} = 64 pm 18 MeV and $sigma_{gammagamma}$ = 98 pm 24 {mu}b, respectively. The collected statistics amount to 2339 pm 340 events of 1.5 cdot 10^6 triggered interactions of a total number ~ 10^12 of dC-interactions. The results on observation of the resonance in the invariant mass spectra of two $pi^0$ mesons are presented: the data obtained in the d + C rightarrow {gamma} + {gamma} reaction is confirmed by the d + C rightarrow $pi^0$ + $pi^0$ reaction: $M_{pi^0pi^0}$ = 359.2 pm 1.9 MeV, {Gamma} = 48.9 pm 4.9 MeV; the ratio of Br(Rrightarrow{gamma}{gamma}) / Br(Rrightarrow$pi^0pi^0$) = (1.8 {div} 3.7)cdot10^-3.
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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.
Coherent double neutral-pion photoproduction on the deuteron, $gamma{d}${$to$}$pi^0pi^0{d}$, has been experimentally studied at incident photon energies ranging from 0.75 to 1.15 GeV. The total cross section as a function of the $gamma{d}$ center-of-mass energy shows resonance-like behavior, which peaks at approximately 2.47 and 2.63 GeV. The measured angular distribution of deuteron emission is rather flat, which cannot be reproduced by the kinematics of quasi-free $pi^0pi^0$ production with deuteron coalescence. In $pi^0d $ invariant-mass distributions, a clear peak is observed at $2.14{pm}0.01$ GeV$/c^2$ with a width of $0.09{pm}0.01$ GeV$/c^2$. The spin-parity of this state is restricted to $1^+$, $2^+$ or $3^-$ from the angular distributions of the two $pi^0$s. The present work shows strong evidence for the existence of an isovector dibaryon resonance with a mass of 2.14 GeV$/c^2$. The $2^+$ assignment is consistent with the theoretically predicted ${cal{D}}_{12}$ state, and also with the energy dependence of the $pi{d}$ partial-wave amplitude $^3!P_2$ for the $pi^{pm}d${$to$}$pi^{pm}d$ and $pi^+d${$to$}${pp}$ reactions.
We revisit the coupled channel $Kbar{K}$ interactions and dynamically generate the resonances $f_0(980)$ and $a_0(980)$ within both the isospin and the physical bases. The $f_0(980)-a_0(980)$ mixing effects are generated in the scattering amplitudes of the coupled channels with the physical basis, which exploits the important role of the $Kbar{K}$ channel in the dynamical nature of these resonances. With the scattering amplitudes obtained, we investigate the $f_0(980)$ and $a_0(980)$ contributions to the $J/psito gammaetapi^0$, $J/psito gammapi^+pi^-$ and $J/psito gammapi^0pi^0$ radiative decays through the final-state interactions. We obtain the corresponding branching fractions $Br(J/psito gamma a_0(980) to gammaetapi^0) = (0.47pm0.05) times 10^{-7}$, $Br(J/psito gamma f_0(980) to gammapi^+pi^-) = 0.37 times 10^{-7} - 1.98 times 10^{-6}$, $Br(J/psito gamma f_0(980) to gammapi^0pi^0) = 0.18 times 10^{-7} - 9.92 times 10^{-7}$, and predict $Br(J/psito gamma a_0(980)) = 1.72 times 10^{-8} - 3.07times 10^{-7}$ and $Br(J/psito gamma f_0(980)) = 1.86 times 10^{-8} - 1.89times 10^{-5}$. These fractions are within the upper limits of the experimental measurements.
Data on the photoproduction of $omega$ mesons on nuclei have been re-analyzed in a search for in-medium modifications. The data were taken with the Crystal Barrel(CB)/TAPS detector system at the ELSA accelerator facility in Bonn. First results from the analysis of the data set were published by D. Trnka et al. in Phys. Rev. Lett 94 (2005) 192303 cite{david}, claiming a lowering of the $omega$ mass in the nuclear medium by 14$%$ at normal nuclear matter density. The extracted $omega$ line shape was found to be sensitive to the background subtraction. For this reason a re-analysis of the same data set has been initiated and a new method has been developed to reduce the background and to determine the shape and absolute magnitude of the background directly from the data. Details of the re-analysis and of the background determination are described. The $omega$ signal on the $Nb$ target, extracted in the re-analysis, does not show a deviation from the corresponding line shape on a $LH_2$ target, measured as reference. The earlier claim of an in-medium mass shift is thus not confirmed. The sensitivity of the $omega$ line shape to different in-medium modification scenarios is discussed.
A new resonance structure at M = 355 pm 6 pm 9 MeV is observed in the invariant mass spectrum of two gamma-quanta produced in the reaction d + C => gamma + gamma + x at momentum 2.75 GeV/c per nucleon. Preliminary estimates of its width and cross section are Gamma = 41 pm 12 MeV and sigma_{gamma-gamma} = 0.6 mkb. The collected statistics is 2680 pm 310 events of 1.5 10^6 triggered interactions of a total number 3 10^{12} dC-interactions.
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