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Register a new userPhotoproduction of the $omega$ meson off the proton near threshold
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An experimental study of $omega$ photoproduction on the proton was conducted by using the Crystal Ball and TAPS multiphoton spectrometers together with the photon tagging facility at the Mainz Microtron MAMI. The $gamma ptoomega p$ differential cross sections are measured from threshold to the incident-photon energy $E_gamma=1.40$ GeV ($W=1.87$ GeV for the center-of-mass energy) with 15-MeV binning in $E_gamma$ and full production-angle coverage. The quality of the present data near threshold gives access to a variety of interesting physics aspects. As an example, an estimation of the $omega N$ scattering length $alpha_{omega p}$ is provided.
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The differential cross sections and unpolarized spin-density matrix elements for the reaction $gamma pto pomega$ were measured using the CBELSA/TAPS experiment for initial photon energies ranging from the reaction threshold to 2.5 GeV. These observables were measured from the radiative decay of the $omega$ meson, $omegatopi^0gamma$. The cross sections cover the full angular range and show the full extent of the $t$-channel forward rise. The overall shape of the angular distributions in the differential cross sections and unpolarized spin-density matrix elements are in fair agreement with previous data. In addition, for the first time, a beam of linearly-polarized tagged photons in the energy range from 1150 MeV to 1650 MeV was used to extract polarized spin-density matrix elements. These data were included in the Bonn-Gatchina partial wave analysis (PWA). The dominant contribution to $omega$ photoproduction near threshold was found to be the $3/2^+$ partial wave, which is primarily due to the sub-threshold $N(1720),3/2^+$ resonance. At higher energies, pomeron-exchange was found to dominate whereas $pi$-exchange remained small. These $t$-channel contributions as well as further contributions from nucleon resonances were necessary to describe the entire dataset: the $1/2^-$, $3/2^-$, and $5/2^+$ partial waves were also found to contribute significantly.
Photoproduction of the $omega$ meson on the proton has been experimentally studied near the threshold. The total cross sections are determined at incident energies ranging from 1.09 to 1.15 GeV. The 1/2 and 3/2 spin-averaged scattering length $a_{omega p}$ and effective range $r_{omega p}$ between the $omega$ meson and proton are estimated from the shape of the total cross section as a function of the incident photon energy: $a_{omega p} = left(-0.97^{+0.16_{rm stat}}_{-0.16_{rm stat}}{}^{+0.03_{rm syst}}_{-0.00_{rm syst}}right)+i left(0.07^{+0.15_{rm stat}}_{-0.14_{rm stat}}{}^{+0.17_{rm syst}}_{-0.09_{rm syst}}right)$ fm and $r_{omega p}=left(+2.78^{+0.68_{rm stat}}_{-0.54_{rm stat}}{}^{+0.11_{rm syst}}_{-0.13_{rm syst}}right)+ileft(-0.01^{+0.46_{rm stat}}_{-0.50_{rm stat}}{}^{+0.07_{rm syst}}_{-0.00_{rm syst}}right)$ fm, resulting in a repulsive force. The real and imaginary parts for $a_{omega p}$ and $r_{omega p}$ are determined separately for the first time. A small $P$-wave contribution does not affect the obtained values.
Photoproduction of $phi$-meson on protons was studied by means of linearly polarized photons at forward angles in the low-energy region from threshold to $E_{gamma}$= 2.37 GeV. The differential cross sections at $t = -|t|_{min}$ do not increase smoothly as $E_{gamma}$ increases, but show a local maximum at around 2.0 GeV. The angular distributions demonstrate that $phi$-mesons are photo-produced predominantly by helicity-conserving processes, and the local maximum is not likely due to unnatural-parity processes.
We report on the measurement of the $gamma p rightarrow J/psi p$ cross section from $E_gamma = 11.8$ GeV down to the threshold at $8.2$ GeV using a tagged photon beam with the GlueX experiment. We find the total cross section falls toward the threshold less steeply than expected from two-gluon exchange models. The differential cross section $dsigma /dt$ has an exponential slope of $1.67 pm 0.39$ GeV$^{-2}$ at $10.7$ GeV average energy. The LHCb pentaquark candidates $P_c^+$ can be produced in the $s$-channel of this reaction. We see no evidence for them and set model-dependent upper limits on their branching fractions $mathcal{B}(P_c^+ rightarrow J/psi p)$ and cross sections $sigma(gamma p to P_c^+)timesmathcal{B}(P_c^+ to J/psi p) $.
The reaction $gamma p to pi^0 pi^0 p$ has been measured using the TAPS BaF$_2$ calorimeter at the tagged photon facility of the Mainz Microtron accelerator. Chiral perturbation theory (ChPT) predicts that close to threshold this channel is significantly enhanced compared to double pion final states with charged pions. In contrast to other reaction channels, the lower order tree terms are strongly suppressed in 2$pi^0$ photoproduction. The consequence is the dominance of pion loops in the 2$pi^0$ channel close to threshold - a result that opens new prospects for the test of ChPT and in particular its inherent loop terms. The present measurement is the first which is sensitive enough for a conclusive comparison with the ChPT calculation and is in agreement with its prediction. The data also show good agreement with a calculation in the unitary chiral approach.
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