http://cds.cern.ch/record/1292820/files/CERN-PH-EP-2010-032.pdf

New precise measurements of the Ξ0→Λγ and Ξ0→Σ0γ decay asymmetries

Weak radiative hyperon decays, important to test the strong interaction and relevant in searches for beyond the standard model physics, have remained puzzling both experimentally and theoretically for a long time. The recently updated branching fraction and first measurement of the asymmetry parameter of Λ→nγ by the BESIII Collaboration further exacerbate the issue, as none of the existing predictions can describe the data. We show in this work that the covariant baryon chiral perturbation theory, with constraints from the latest measurements of hyperon non-leptonic decays, can well describe the BESIII data. The predicted branching fraction and asymmetry parameter for Ξ-→Σ-γ are also in agreement with the experimental data. We note that a more precise measurement of the asymmetry parameter, which is strongly constrained by chiral symmetry and related with that of Σ+→pγ, is crucial to test Hara's theorem. We further predict the branching fraction and asymmetry parameter of Σ0→nγ, whose future measurement can serve as a highly nontrivial check on our understanding of weak radiative hyperon decays and on the covariant baryon chiral perturbation theory.

Weak radiative hyperon decays in covariant baryon chiral perturbation theory.


 Polarized hyperon-antihyperon pairs shed light on various unresolved puzzles in contemporary physics: How the strong interaction confines quarks into hadrons, how accurately the Standard Model describes microcosmos and even why our universe consists of so much more matter than antimatter. Thanks to their weak, parity violating decays, hyperons reveal their spin properties. This can be exploited e.g. the decomposition of the electromagnetic structure of hyperons, precision tests of flavour symmetry and searches for CP violation. At the BESIII experiment at BEPC-II, Beijing, China, hyperon-antihyperon pairs can be produced in abundance. Recently collected large data samples have triggered the development of new methods that provide unprecedented precision and a plethora of new results have emerged. When applied at future high-intensity facilities like PANDA and STCF, precision physics will be taken to a new level which can contribute to the solution to the aforementioned puzzles. Content from this work may be used under the terms of the Creative Commons Attribution 3.0 licence. Any further distribution of this work must maintain attribution to the author(s) and the title of the work, journal citation and DOI. Article funded by SCOAP3 and published under licence by Chinese Physical Society and the Institute of High Energy Physics of the Chinese Academy of Science and the Institute of Modern Physics of the Chinese Academy of Sciences and IOP Publishing Ltd.

pdf/production-and-decay-of-polarized-hyperon-antihyperon-pairs-omikthn1.pdf

Production and decay of polarized hyperon-antihyperon pairs

The HyperCP experiment (Fermilab E871) has the largest recorded sample of Ω− hyperon decays, allowing new searches for three rare decay modes, each with five charged tracks in the final state. Results are presented for Ω− → Ξ− π+ π−; Ω− → Ξ0(1530)π−; and the flavor-changing neutral-current decay Ω− → Ξ− π+π−. Normalization of these measurements were to the decay Ω− → ΛK− with a five-track topology similar to the signal mode with the subsequent decay of Λ → pπ− and K− → π−π+π− in the decay volume.

Ω− hyperon decay modes with the hyperCP experiment at Fermilab

Using (10 087±44)×10^{6} J/ψ events collected with the BESIII detector, the radiative hyperon decay Σ^{+}→pγ is studied at an electron-positron collider experiment for the first time. The absolute branching fraction is measured to be (0.996±0.021_{stat}±0.018_{syst})×10^{-3}, which is lower than its world average value by 4.2 standard deviations. Its decay asymmetry parameter is determined to be -0.652±0.056_{stat}±0.020_{syst}. The branching fraction and decay asymmetry parameter are the most precise to date, and the accuracies are improved by 78% and 34%, respectively. 

/pdf/precision-measurement-of-the-decay-mml-math-xmlns-mml-http-3ffk3eb1.pdf

Precision Measurement of the Decay 
<mml:math xmlns:mml="http://www.w3.org/1998/Math/MathML" display="inline"><mml:mrow><mml:msup><mml:mrow><mml:mi mathvariant="normal">Σ</mml:mi></mml:mrow><mml:mrow><mml:mo>+</mml:mo></mml:mrow></mml:msup><mml:mo stretchy="false">→</mml:mo><mml:mi>p</mml:mi><mml:mi

We have studied the rare weak radiative hyperon decay Xi degrees -->Sigma degrees gamma in the KTeV experiment at Fermilab. We have identified 4045 signal events over a background of 804 events. The dominant Xi degrees -->Lambdapi degrees decay, which was used for normalization, is the only important background source. An analysis of the acceptance of both modes yields a branching ratio of B(Xi degrees -->Sigma degrees gamma)/B(Xi degrees -->Lambdapi degrees ) = (3.34+/-0.05+/-0.09)x10(-3). By analyzing the final state decay distributions, we have also determined that the Sigma degrees emission asymmetry parameter for this decay is alpha(XiSigma) = -0.63+/-0.09.

Measurement of the branching ratio and asymmetry of the decay Ξ° → σ°γ

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Measurement of the branching ratio and asymmetry of the decay Ξ° → σ°γ