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Register a new userPhotoproduction of the $f_2(1270)$ meson using the CLAS detector
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The quark structure of the $f_2(1270)$ meson has, for many years, been assumed to be a pure quark-antiquark ($qbar{q}$) resonance with quantum numbers $J^{PC} = 2^{++}$. Recently, it was proposed that the $f_2(1270)$ is a molecular state made from the attractive interaction of two $rho$-mesons. Such a state would be expected to decay strongly to final states with charged pions, due to the dominant decay $rho to pi^+ pi^-$, whereas decay to two neutral pions would likely be suppressed. Here, we measure for the first time the reaction $gamma p to pi^0 pi^0 p$, using the CLAS detector at Jefferson Lab for incident beam energies between 3.6-5.4~GeV. Differential cross sections, $dsigma / dt$, for $f_2(1270)$ photoproduction are extracted with good precision, due to low backgrounds, and are compared with theoretical calculations.
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The $f_1(1285)$ meson with mass $1281.0 pm 0.8$ MeV/$c^2$ and width $18.4 pm 1.4$ MeV (FWHM) was measured for the first time in photoproduction from a proton target using CLAS at Jefferson Lab. Differential cross sections were obtained via the $etapi^{+}pi^{-}$, $K^+bar{K}^0pi^-$, and $K^-K^0pi^+$ decay channels from threshold up to a center-of-mass energy of 2.8 GeV. The mass, width, and an amplitude analysis of the $etapi^{+}pi^{-}$ final-state Dalitz distribution are consistent with the axial-vector $J^P=1^+$ $f_1(1285)$ identity, rather than the pseudoscalar $0^-$ $eta(1295)$. The production mechanism is more consistent with $s$-channel decay of a high-mass $N^*$ state, and not with $t$-channel meson exchange. Decays to $etapipi$ go dominantly via the intermediate $a_0^pm(980)pi^mp$ states, with the branching ratio $Gamma(a_0pi text{ (no} bar{K} Ktext{)}) / Gamma(etapipi text{(all)}) = 0.74pm0.09$. The branching ratios $Gamma(K bar{K} pi)/Gamma(etapipi) = 0.216pm0.033$ and $Gamma(gammarho^0)/Gamma(etapipi) = 0.047pm0.018$ were also obtained. The first is in agreement with previous data for the $f_1(1285)$, while the latter is lower than the world average.
Photoproduction cross sections are reported for the reaction $gamma pto peta$ using energy-tagged photons and the CLAS spectrometer at Jefferson Laboratory. The $eta$ mesons are detected in their dominant charged decay mode, $etato pi^+pi^-pi^0$, and results on differential cross sections are presented for incident photon energies between 1.2 and 4.7 GeV. These new $eta$ photoproduction data are consistent with earlier CLAS results but extend the energy range beyond the nucleon resonance region into the Regge regime. The normalized angular distributions are also compared with the experimental results from several other experiments, and with predictions of $eta$ MAID,2018 and the latest solution of the Bonn-Gatchina coupled-channel analysis. Differential cross sections $dsigma/dt$ are presented for incident photon energies $E_gamma > 2.9$ GeV ($W > 2.5$ GeV), and compared with predictions which are based on Regge trajectories exchange in the $t$-channel (Regge models). The data confirm the expected dominance of $rho$, $omega$ vector-meson exchange in an analysis by the Joint Physics Analysis Center.
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.
The photoproduction of $omega$ mesons off the proton has been studied in the reaction $gamma pto p,omega$ using the CEBAF Large Acceptance Spectrometer (CLAS) and the frozen-spin target (FROST) in Hall B at the Thomas Jefferson National Accelerator Facility. For the first time, the target asymmetry, $T$, has been measured in photoproduction from the decay $omegatopi^+pi^-pi^0$, using a transversely-polarized target with energies ranging from just above the reaction threshold up to 2.8 GeV. Significant non-zero values are observed for these asymmetries, reaching about 30-40% in the third-resonance region. New measurements for the photon-beam asymmetry, $Sigma$, are also presented, which agree well with previous CLAS results and extend the world database up to 2.1 GeV. These data and additional $omega$-photoproduction observables from CLAS were included in a partial-wave analysis within the Bonn-Gatchina framework. Significant contributions from $s$-channel resonance production were found in addition to $t$-channel exchange processes.
The transition gamma_{v}pN^* amplitudes (electrocouplings) for prominent excited nucleon states obtained in a wide area of photon virtualities offer valuable information for the exploration of the N^* structure at different distances and allow us to access the complex dynamics of non-perturbative strong interaction. The current status in the studies of gamma_{v}pN^* electrocouplings from the data on exclusive meson electroproduction off protons measured with the CLAS detector at Jefferson Lab is presented. The impact of these results on exploration of the N^* structure is discussed.
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