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Register a new userFirst measurement of coherent $phi$-meson photoproduction from helium-4 near threshold
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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.
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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 cross section and decay angular distributions for the coherent phi meson photoproduction on the deuteron have been measured for the first time up to a squared four-momentum transfer t =(p_{gamma}-p_{phi})^2 =-2 GeV^2/c^2, using the CLAS detector at the Thomas Jefferson National Accelerator Facility. The cross sections are compared with predictions from a re-scattering model. In a framework of vector meson dominance, the data are consistent with the total phi-N cross section sigma_{phi N} at about 10 mb. If vector meson dominance is violated, a larger sigma_{phi N} is possible by introducing larger t-slope for the phi N to phi N process than that for the gamma N to phi N process. The decay angular distributions of the phi are consistent with helicity conservation.
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.
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.
We report measurements of differential cross sections and decay asymmetries of incoherent $phi$-meson photoproduction from the deuteron at forward angles using linearly polarized photons at Eg=1.5-2.4 GeV. The nuclear transparency ratio for the deuteron shows a large suppression, and is consistent with the A-dependence of the ratio observed in a previous measurement with nuclear targets. The reduction for the deuteron cannot be adequately explained in term of isospin asymmetry. The present results suggest the need of refining our understanding of the $phi$-N interaction within a nucleus.
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