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Register a new userEnergy dependence of $e^+e^- to 6pi$ and $e^+e^- to Nbar{N}$ cross sections near the $Nbar{N}$ threshold
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Using recent BABAR, CMD-3 and SND data, the sum of $e^+e^- to 3(pi^+pi^-), 2(pi^+pi^-pi^0), pbar{p}, nbar{n}$ cross sections is obtained. Unlike $e^+e^- to 3(pi^+pi^-)$ and $e^+e^- to 2(pi^+pi^-pi^0)$ processes, no structures in total cross section are found near the $Nbar{N}$ threshold within the limits of measurement errors.
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The process $e^+e^- rightarrow Nbar N$ is studied nearby a threshold with account for polarizations of all initial and final particles. The nucleon polarization $bm zeta^N$ reveals a strong energy dependence due to that of the nucleon electromagnetic form factors $G_E(Q^2)$ and $G_M(Q^2)$ caused by the final-state interaction of nucleons. It is shown that the modulus of the ratio of these form factors and their relative phase can be determined by measuring $bm zeta^N$ along with the differential cross section. The polarization degree is analyzed using Paris $Nbar N$ optical potential for calculation of the form factors. It turns out that $|bm zeta^N|$ is high enough in a rather wide energy range above the threshold. Being especially high for longitudinally polarized beams, $|bm zeta^N|$ is noticeable even if both $e^+e^-$ beams are unpolarized.
We use the Paris nucleon-antinucleon optical potential for explanation of experimental data in the process $e^+e^- rightarrow pbar p$ near threshold. It turns out that final-state interaction due to Paris optical potential allows us to reproduce available experimental data. It follows from our consideration that the isoscalar form factor is much larger than the isovector one.
The energy dependence of the cross sections of $pbar p$, $nbar n$, and meson production in $e^+e^-$ annihilation in the vicinity of the $pbar p$ and $nbar n$ thresholds is studied. The proton-neutron mass difference and the $pbar p$ Coulomb interaction are taken into account. The values of the cross sections are very sensitive to the parameters of the optical potential. It is shown that the commonly accepted factorization approach for the account of the Coulomb interaction does not work well enough in the vicinity of the threshold due to the finite size of the optical potential well.
Unexpected features of the BaBar data on e+e- in baryon-antibaryon cross sections are discussed. These data have been collected, with unprecedented accuracy, by means of the initial state radiation technique, which is particularly suitable in giving good acceptance and energy resolution at threshold. A striking feature observed in the BaBar data is the non-vanishing cross section at threshold for all these processes. This is the expectation due to the Coulomb enhancement factor acting on a charged fermion pair. In the case of e+e- in proton-antiproton it is found that Coulomb final state interactions largely dominate the cross section and the form factor is |G^p(4M^2_p)|~1, which could be a general feature for baryons. In the case of neutral baryons an interpretation of the non-vanishing cross section at threshold is suggested, based on quark electromagnetic interaction and taking into account the asymmetry between attractive and repulsive Coulomb factors. Besides strange baryon cross sections are compared to U-spin invariance predictions.
A simultaneous fit is performed to the $e^+e^-to Lambda_c^+Lambda_c^-$ cross section data measured by Belle and BESIII from threshold up to 5.4 GeV. In order to accommodate both the BESIII measurement near threshold and the Belle observation of a resonance $Y(4630)$, we build a composite PDF with a Breit-Wigner resonance and a continuum contribution to model the full cross section line shape of $e^+e^-to Lambda_c^+Lambda_c^-$. The fit gives a mass of $M=[4636.1_{-7.2}^{+9.8} ($stat$)pm 8.0($syst$)]$~MeV/$c^2$, a width of $Gamma_{rm tot}=[34.5_{-16.2}^{+21.0} ($stat$)pm 5.6($syst$)]$~MeV, and $Gamma_{e^+ e^-}mathcal{B}[Y(4630)toLambda_c^+Lambda_c^-]=[18.3_{-6.1}^{+8.8} ($stat$)pm 1.1($syst$)]$~eV/$c^2$ for the resonance. The width of $Y(4630)$ from our study is narrower than the previous Belle fit. The mass and width of $Y(4630)$ also show good agreement with a vector resonance $Y(4626)$ recently observed in $D_s^+D_{s1}(2536)^-$ by Belle.
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