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Register a new userObservation of a near-threshold enhancement in the $Lambdabar{Lambda}$ mass spectrum from $e^+e^-tophiLambdabar{Lambda}$ at $sqrt{s}$ from 3.51 to 4.60 GeV
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The process $e^+ e^- to phi Lambda bar{Lambda}$ is studied using data samples collected with the BESIII detector at the BEPCII collider at center-of-mass energies $sqrt{s}$ ranging from $3.51$ to $4.60~{rm GeV}$ . An intermediate resonance structure is observed near the threshold of $Lambda bar{Lambda}$. It has a mass of $(2262 pm 4 pm 28)~{rm{MeV}}/c^{2}$ and a width of $(72 pm 5 pm 43)~rm{MeV}$, where the quoted uncertainties are statistical and systematic, respectively. The $J^{PC}$ quantum numbers of $0^{-+}$ and $0^{++}$ are rejected, while other $J^{PC}$ hypotheses are possible, according to the helicity angle study. The energy-dependent cross section of the $e^+ e^- to phi Lambda bar{Lambda}$ process is measured for the first time in this energy region, and contributions from excited $psi$ states and vector charmonium-like $Y$-states are investigated.
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The process $e^{+}e^{-}rightarrowLambdabar{Lambda}$ is studied using data samples at $sqrt{s}=2.2324$, 2.400, 2.800 and 3.080 GeV collected with the BESIII detector operating at the BEPCII collider. The Born cross section is measured at $sqrt{s}$=2.2324 GeV, which is 1.0 MeV above the $Lambdabar{Lambda}$ mass threshold, to be $305pm45^{+66}_{-36}$ pb, where the first uncertainty is statistical and the second systematic. The substantial cross section near threshold is significantly larger than that expected from theory, which predicts the cross section to vanish at threshold. The Born cross sections at $sqrt{s}$=2.400, 2.800 and 3.080 GeV are measured and found to be consistent with previous experimental results, but with improved precision. Finally, the corresponding effective electromagnetic form factors of $Lambda$ are deduced.
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.
The near-threshold $e^+e^- to Lambdabar{Lambda}$ reaction is studied with the assumption that the production mechanism is due to a near-$Lambda bar{Lambda}$-threshold resonance. The cross section of $e^+e^- to Lambdabar{Lambda}$ reaction is parametrized in terms of the electromagnetic form factors of $Lambda$ hyperon, which are obtained within the vector meson dominance model. It is shown that the contribution to the $e^+e^- to Lambdabar{Lambda}$ reaction from a new narrow state with quantum numbers $J^{PC}=1^{--}$ is dominant for energies very close to threshold. The mass of this new state is about 2232 MeV, which is very close to the mass threshold of $Lambda bar{Lambda}$, while its width is just a few MeV. This solves the problem that all previous calculations seriously underestimate the near-threshold total cross section of the $e^+e^- to Lambdabar{Lambda}$ reaction.
The process $e^{+}e^{-} to phi eta^{prime}$ has been studied for the first time in detail using data sample collected with the BESIII detector at the BEPCII collider at center of mass energies from 2.05 to 3.08 GeV. A resonance with quantum numbers $J^{PC}=1^{--}$ is observed with mass $M$ = (2177.5 $pm$ 4.8 (stat) $pm$ 19.5 (syst)) MeV/${ it{c}^{mathrm{2}}}$ and width $Gamma$ = (149.0 $pm$ 15.6 (stat) $pm$ 8.9 (syst)) MeV with a statistical significance larger than 10$sigma$. The observed structure could be identified with the $phi(2170)$, then the ratio of partial width between the $phi eta^{prime}$ by BESIII and $phi eta$ by BABAR is ($mathcal{B}^{R}_{phi eta}Gamma^{R}_{ee})/{(mathcal{B}^{R}_{phi eta^{prime}}Gamma^{R}_{ee})}$ = 0.23 $pm$ 0.10 (stat) $pm$ 0.18 (syst), which is smaller than the prediction of the $sbar{s}g$ hybrid models by several orders of magnitude.
A neutral structure in the $Dbar{D}^{*}$ system around the $Dbar{D}^{*}$ mass threshold is observed with a statistical significance greater than 10$sigma$ in the processes $e^{+}e^{-}rightarrow D^{+}D^{*-}pi^{0}+c.c.$ and $e^{+}e^{-}rightarrow D^{0}bar{D}^{*0}pi^{0}+c.c.$ at $sqrt{s}$ = 4.226 and 4.257 GeV in the BESIII experiment. The structure is denoted as $Z_{c}(3885)^{0}$. Assuming the presence of a resonance, its pole mass and width are determined to be ($3885.7^{+4.3}_{-5.7}$(stat)$pm 8.4$(syst)) MeV/$c^{2}$ and ($35^{+11}_{-12}$(stat)$ pm 15$(syst)) MeV, respectively. The Born cross sections are measured to be $sigma(e^{+}e^{-}to Z_{c}(3885)^{0}pi^{0}, Z_{c}(3885)^{0} to Dbar{D}^{*})=(77 pm 13$(stat)$pm 17$(syst)) pb at 4.226 GeV and ($47 pm 9$(stat)$ pm 10$(syst)) pb at 4.257 GeV. The ratio of decay rates $frac{mathcal{B}({Z_{c}(3885)^{0} to D^{+}D^{*-}+c.c.})}{mathcal{B}({Z_{c}(3885)^{0} to D^{0}bar{D}^{*0}+c.c.})}$ is determined to be $0.96 pm 0.18$(stat)$pm 0.12$(syst), consistent with no isospin violation in the process $Z_{c}(3885)^{0}to Dbar{D}^*$.
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