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تسجيل مستخدم جديدMeasurement of the $eta$ and $omega$ Dalitz decays transition form factors in p-A collisions at 400 GeV/$c$ with the NA60 apparatus
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The NA60 experiment has studied low-mass muon pairs production in proton-nucleus collisions at 400 GeV/$c$ at the CERN SPS. The mass spectrum is well described by the superposition of the two-body and Dalitz decays of the light neutral mesons $eta$, $rho$, $omega$, $eta$ and $phi$. The large collected statistics allows to isolate the contributions due to the Dalitz decays of the $eta$ and $omega$ mesons, from which the electromagnetic transition form factors of the two mesons have been extracted. The found values agree with the previous available measurements, improving their uncertainty thanks to the higher statistics. The results thus confirm the discrepancy with the prediction of the Vector Meson Dominance model in the case of the electromagnetic form factor of the $omega$ meson.
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The NA60 experiment has studied low-mass muon pair production in proton-nucleus collisions with a system of Be, Cu, In, W, Pb and U targets, using a 400 GeV proton beam at the CERN SPS. The transverse momentum spectra of the $rho/omega$ and $phi$ mes
ons are measured in the full $p_mathrm{T}$ range accessible, from $p_mathrm{T} = 0$ up to 2 GeV/$c$. The nuclear dependence of the production cross sections of the $eta$, $omega$ and $phi$ mesons has been found to be consistent with the power law $sigma_mathrm{pA} propto mathrm{A}^alpha$, with the $alpha$ parameter increasing as a function of $p_mathrm{T}$ for all the particles, and an approximate hierarchy $alpha_eta approx alpha_phi > alpha_omega$. The cross section ratios $sigma_eta/sigma_omega$, $sigma_rho/sigma_omega$ and $sigma_phi/sigma_omega$ have been studied as a function of the size A of the production target, and an increase of the $eta$ and $phi$ yields relative to the $omega$ is observed from p-Be to p-U collisions.
The Dalitz decays eta -> e^+e^-g and omega -> pi^0 e^+e^- have been measured in the g p -> eta p and g p -> omega p reactions, respectively, with the A2 tagged-photon facility at the Mainz Microtron, MAMI. The value obtained for the slope parameter o
f the electromagnetic transition form factor of eta, Lambda^{-2}_eta=(1.97+/-0.11_tot) GeV^{-2}, is in good agreement with previous measurements of the eta -> e^+e^-g and eta -> mu^+mu^-g decays. The uncertainty obtained in the value of Lambda^{-2}_eta is lower than in previous results based on the eta -> e^+e^-g decay. The value obtained for the omega slope parameter, Lambda^{-2}_omega_pi^0 = (1.99+/-0.21_tot) GeV^{-2}, is somewhat lower than previous measurements based on omega -> pi^0 mu^+mu^-, but the results for the omega transition form factor are in better agreement with theoretical calculations, compared to earlier experiments.
The NA60 experiment has studied low-mass muon pair production in proton-nucleus collisions with a system of Be, Cu, In, W, Pb and U targets using a 400 GeV/$c$ proton beam at the CERN SPS. The mass spectrum is well described by the superposition of t
he two-body and Dalitz decays of the light neutral mesons $eta$, $rho$, $omega$, $eta$ and $phi$. A new high-precision measurement of the electromagnetic transition form factors of the $eta$ and $omega$ mesons is presented, complemented with a measurement of the temperature parameter of the $rho$ meson in cold nuclear matter. The $pt$ spectra for the $omega$ and $phi$ mesons are extracted in the full $pt$ range accessible, up to $pt = 2$ GeV/$c$. The nuclear dependence of the production cross sections for the $eta$, $omega$ and $phi$ mesons has been investigated in terms of the power law $sigma_mathrm{pA} propto mathrm{A}^alpha$, and the $alpha$ parameter was studied as a function of $pt$.
Dalitz decays of a hyperon resonance to a ground-state hyperon and an electron-positron pair can give access to some information about the composite structure of hyperons. We present expressions for the multi-differential decay rates in terms of gene
ral transition form factors for spin-parity combinations J^P = 1/2^+/-, 3/2^+/- of the hyperon resonance. Even if the spin of the initial hyperon resonance is not measured, the self-analyzing weak decay of the final ground-state hyperon contains information about the relative phase between combinations of transition form factors. This relative phase is non-vanishing because of the unstable nature of the hyperon resonance. If all form factor combinations in the differential decay formulae are replaced by their respective values at the photon point, one obtains a QED type approximation, which might be interpreted as characterizing hypothetical hyperons with point-like structure. We compare the QED type approximation to a more realistic form factor scenario for the lowest-lying singly-strange hyperon resonances. In this way we explore which accuracy in the measurements of the differential Dalitz decay rates is required in order to distinguish the composite-structure case from the pointlike case. Based on the QED type approximation we obtain as a by-product a rough prediction for the ratio between the Dalitz decay width and the corresponding photon decay width.
The Dalitz decay eta -> e^+ e^- gamma has been measured in the gamma p -> eta p reaction with the Crystal Ball and TAPS multiphoton spectrometers, together with the photon tagging facility at the Mainz Microtron MAMI. The experimental statistic used
in this work is one order of magnitude greater than in any previous measurement of eta -> e^+ e^- gamma. The value obtained for the slope parameter 1/Lambda^2 of the eta transition form factor, 1/Lambda^2 = (1.95 +/- 0.15_stat +/- 0.10_syst) [1/GeV^2], is in good agreement with recent measurements conducted in eta -> e^+ e^- gamma and eta -> mu^+ mu^- gamma decays, as well as with recent form-factor calculations. The uncertainty obtained in the value of 1/Lambda^2 is lower compared to results from previous measurements of the eta -> e^+ e^- gamma decay.
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