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Register a new userNear-threshold Photoproduction of Phi Mesons from Deuterium
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We report the first measurement of the differential cross section on $phi$-meson photoproduction from deuterium near the production threshold for a proton using the CLAS detector and a tagged-photon beam in Hall B at Jefferson Lab. The measurement was carried out by a triple coincidence detection of a proton, $K^+$ and $K^-$ near the theoretical production threshold of 1.57 GeV. The extracted differential cross sections $frac{dsigma}{dt}$ for the initial photon energy from 1.65-1.75 GeV are consistent with predictions based on a quasifree mechanism. This experiment establishes a baseline for a future experimental search for an exotic $phi$-N bound state from heavier nuclear targets utilizing subthreshold/near-threshold production of $phi$ mesons.
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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.
The differential cross sections and decay angular distributions for coherent $phi$-meson photoproduction from helium-4 have been measured for the first time at forward angles with linearly polarized photons in the energy range $E_{gamma} = text{1.685-2.385 GeV}$. Thanks to the target with spin-parity $J^{P} = 0^{+}$, unnatural-parity exchanges are prohibited, and thus natural-parity exchanges can be investigated clearly. The decay asymmetry with respect to photon polarization is shown to be very close to the maximal value. This ensures the dominance ($> 94%$) of natural-parity exchanges in this reaction. To evaluate the contribution from natural-parity exchanges to the forward cross section ($theta = 0^circ$) for the $gamma p rightarrow phi p$ reaction near the threshold, the energy dependence of the forward cross section ($theta = 0^circ$) for the $gamma {^{4}text{He}} rightarrow phi {^{4}text{He}}$ reaction was analyzed. The comparison to $gamma p rightarrow phi p$ data suggests that enhancement of the forward cross section arising from natural-parity exchanges, and/or destructive interference between natural-parity and unnatural-parity exchanges is needed in the $gamma p rightarrow phi p$ reaction near the threshold.
The photoproduction of $omega$ mesons on LH_2, C and Nb has been studied for incident photon energies from 900 to 1300 MeV using the CB/TAPS detector at ELSA. The $omega$ line shape does not show any significant difference between the LH_2 and the Nb targets. The data are compared with results of transport calculations that predict a sensitivity of the $omega$ line shape to in-medium modifications near the production threshold on a free nucleon of $E_{gamma}^{lab}$ = 1109 MeV.}
We propose a simple meson-exchange model of the photoproduction of $rho$- and $omega$-mesons off protons near threshold ($E_gamma$ less than 2 GeV). This model provides a good description of the available data and implies a large $rho$-nucleon interaction in the scalar channel ($sigma$-exchange). We use this phenomenological interaction to estimate the leading contribution to the self-energy of $rho$-mesons in matter. The implications of our calculation for experimental studies of the $rho$-meson mass in nuclei are discussed.
The LEPS/SPring-8 experiment made a comprehensive measurement of the spin-density matrix elements for $gamma p to phi p$, $gamma d to phi p n$ and $gamma d to phi d$ at forward production angles. A linearly polarized photon beam at $E_{gamma}$=1.6-2.4 GeV was used for the production of $phi$ mesons. The natural-parity Pomeron exchange processes remains dominant nearthreshold. The unnatural-parity processes of pseudoscalar exchange is visible in the production from nucleons but is greatly reduced in the coherent production from deuterons. There is no strong $E_{gamma}$-dependence, but some dependence on momentum-transfer. A small but finite value of the spin-density matrix elements reflecting helicity-nonconserving amplitudes in the $t$-channel is observed.
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