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Register a new userFirst Measurements of Timelike Form Factors of the Hyperons, Lambda0, Sigma0, Sigma+, Xi0, Xi-, and Omega-, and Evidence of Diquark Correlations
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Using 805 pb^-1 of e+e- annihilation data taken with the CLEO-c detector at psi(3770), sqrt{s}=3770 MeV, we report the first measurements of the electromagnetic form factors of the Lambda0, Sigma0, Sigma+, Xi0, Xi-, and Omega- hyperons for the large timelike momentum transfer of |Q^2|=14.2 GeV^2. The form factors for the different hyperons are found to vary by nearly a factor two. It is found that |G_M(Lambda0)|=1.66(24) x |G_M(Sigma0)|. The Lambda0 and Sigma0 hyperons have the same uds quark content, but differ in their isospin, and therefore the spin of the $ud$ quark pair. It is suggested that the spatial correlation implied by the singlet spin--isospin configuration in the Lambda0 is an example of strong diquark correlations in the Lambda0, as anticipated by Jaffe and Wilczek. Improved measurements of the branching fractions of psi(2S) -> p pbar and hyperon--antihyperon pairs are also reported.
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Using $e^+e^-$ annihilation data taken at the CESR collider with the CLEO-c detector, measurements of hyperon pair production cross sections and elastic and transition electromagnetic form factors have been made at the charmonium resonances: $psi(2S)$, $sqrt{s}=3.69$ GeV, $|Q^2|=13.6$ GeV$^2$, $mathcal{L}=48$~pb$^{-1}$; $psi(3770)$, $sqrt{s}=3.77$ GeV, $|Q^2|=14.2$ GeV$^2$, $mathcal{L}=805$~pb$^{-1}$; and $psi(4170)$, $sqrt{s}=4.17$ GeV, $|Q^2|=17.4$ GeV$^2$, $mathcal{L}=586$~pb$^{-1}$. %High efficiency particle identification has resulted in good statistical precision in the results. Results with good statistical precision are obtained with high efficiency particle identification. Systematics of pair production cross sections, and form factors with respect to the number of strange quarks in the hyperons are studied, and evidence is presented for effects of diquark correlations in comparative results for $Lambda^0$ and $Sigma^0$, both of which have the same $uds$ quark content.
The electromagnetic decays of the Sig0(1385) and Lambda(1520) hyperons were studied in photon-induced reactions gamma p -> K+ Lambda(1116)gamma in the CLAS detector at the Thomas Jefferson National Accelerator Facility. We report the first observation of the radiative decay of the Sig0(1385) and a measurement of the Lambda(1520) radiative decay width. For the Sig0(1385) -> Lambda(1116)gamma transition, we measured a partial width of 479+/-120(stat)+81-100(sys) keV, larger than all of the existing model predictions. For the Lambda(1520) -> Lambda(1116)gamma transition, we obtained a partial width of 167+/-43(stat)+26-12(sys) keV.
The Born cross sections of the $e^{+}e^{-}toSigma^{+}bar{Sigma}^{-}$ and $e^{+}e^{-}toSigma^{-}bar{Sigma}^{+}$ processes are determined with high precision for center-of-mass energy from 2.3864 to 3.0200 GeV with the BESIII detector. Nonzero cross sections near threshold are observed. The resulting ratio of effective form factors for the $Sigma^{+}$ and $Sigma^{-}$ is consistent with 3, agreeing with the ratio of the incoherent sum of the squared charges of the $Sigma^{+}$ and $Sigma^{-}$ valence quarks, but disagreeing with various theoretical predictions. In addition, ratios of the $Sigma^{+}$ electric and magnetic form factors, $|G_{E}/G_{M}|$, are obtained at three center-of-mass energies through an analysis of the angular distributions. These measurements, which are studied for the first time in the off-resonance region, provide precision experimental input for understanding baryonic structure. The observed novel features of the $Sigma^{pm}$ form factors require a new theoretical description for the hyperons.
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 general 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.
Using a data sample of 980 fb$^{-1}$ collected with the Belle detector at the KEKB asymmetric-energy $e^+e^-$ collider, we study the processes of $Xi^0_cto Lambdabar K^{*0}$, $Xi^0_cto Sigma^0bar K^{*0}$, and $Xi^0_cto Sigma^+K^{*-}$ for the first time. The relative branching ratios to the normalization mode of $Xi^0_ctoXi^-pi^+$ are measured to be $${cal B}(Xi^0_cto Lambdabar K^{*0})/{cal B}(xicto Xi^-pi^+)=0.18pm0.02({rm stat.})pm0.01({rm syst.}),$$ $${cal B}(Xi^0_cto Sigma^0bar K^{*0})/{cal B}(xicto Xi^-pi^+)=0.69pm0.03({rm stat.})pm0.03({rm syst.}),$$ $${cal B}(Xi^0_cto Sigma^+K^{*-})/{cal B}(xicto Xi^-pi^+)=0.34pm0.06({rm stat.})pm0.02({rm syst.}),$$ where the uncertainties are statistical and systematic, respectively. We obtain %measure the branching fractions of $Xi^0_cto Lambdabar K^{*0}$, $Xi^0_cto Sigma^0bar K^{*0}$, and $Xi^0_cto Sigma^+K^{*-}$ to be $${cal B}(Xi^0_cto Lambdabar K^{*0})=(3.3pm0.3({rm stat.})pm0.2({rm syst.})pm1.0({rm ref.}))times10^{-3},$$ $${cal B}(Xi^0_cto Sigma^0bar K^{*0})=(12.4pm0.5({rm stat.})pm0.5({rm syst.})pm3.6({rm ref.}))times10^{-3},$$ $${cal B}(Xi^0_cto Sigma^+K^{*-})=(6.1pm1.0({rm stat.})pm0.4({rm syst.})pm1.8({rm ref.}))times10^{-3},$$ where the uncertainties are statistical, systematic, and from ${cal B}(xic to Xi^-pi^+)$, respectively. The asymmetry parameters $alpha(Xi^0_cto Lambdabar K^{*0})$ and $alpha(Xi^0_cto Sigma^+K^{*-})$ are $0.15pm0.22({rm stat.})pm0.04({rm syst.})$ and $-0.52pm0.30({rm stat.})pm0.02({rm syst.})$, respectively, where the uncertainties are statistical followed by systematic.
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