Hyperon Decays Research Articles

Pursue of CP violation in hyperon decay

In this paper, I review the status of CP violation in hyperon decay in light of recent progress by BESIII and the anticipated improvements at the super tau-charm facility. I emphasize the complementarity between kaons and hyperons for studying CP violation in [Formula: see text] processes.

Production and decay of hyperons in a transversely polarized electron-positron collider

The self-polarization of relativistic electrons or positrons moving in a magnetic field at a storage ring occurs through the emission of spin-flip synchrotron radiation, known as the Sokolov-Ternov effect. The resulting transverse polarizations of the colliding electrons and positrons, away from the depolarization resonances, allow for precise investigation of the spin entangled hyperon-antihyperon pairs via virtual photon or charmonium decay. The feasibility study reveals a promising increase in the statistical sensitivity of the CP violation signal after considering the transverse polarizations of the lepton beams. Published by the American Physical Society 2024

Hyperon physics at BESIII

Spin polarization and entanglement are utilized by the BESIII experiment to learn more about the production and decay properties of hyperon-antihyperon pairs in a series of recent analyses of its unprecedented datasets at the J/\psiJ/ψ and \psi'ψ′ resonances. This has lead to the observation of significant transverse polarisation in decays of J/\psiJ/ψ or \psi'ψ′ into \Lambda \bar{\Lambda}ΛΛ‾, \Sigma \bar{\Sigma}ΣΣ‾, \Xi \bar{\Xi}ΞΞ‾, and \Omega \bar{\Omega}ΩΩ‾. Because of the non-zero polarization, the decay parameters for the most common hadronic weak decays of both hyperons and antihyperons could be determined independently for the first time. Comparing these to each other yields precise tests of direct, \Delta S = 1ΔS=1 CP-violation that complement measurements of kaon decays.

Hyperon production and interaction studies in protonproton scattering with HADES

Hyperons and their interactions with baryonic matter may play a key role in our understanding of the core of neutron stars. Moreover, the internal structure of hyperons and their excitations are of great interest. Their structure can be probed by exploiting their couplings to radiative probes such as by measuring electromagnetic transition form factors. HADES (High-Acceptance Di-Electron Spectrometer) at GSI collected high-statistics proton-proton scattering data in 2022 at √s = 3.48 GeV. The Λ - Λ interaction is currently being studied in this data along with production of Σ states where the Dalitz decay of hyperons might be observed for the first time. In addition, the reaction pp → pK+K+Ξ−[π−Λ[pπ−]] has been studied to probe the production mechanism of double strangeness near its production threshold. This proceeding addresses the hyperon physics case and discusses the ongoing Ξ− analysis. A preliminary upper limit for the cross section at √s = 3.48 GeV is presented taking into account only statistical uncertainties.

The FAIR Phase-0 Hyperon Program at HADES

Abstract. Hyperons are a unique probe to study the non-perturbative aspects of the strong interaction. At HADES, they are produced in proton or pion induced reactions at kinetic energies up to 4.5 GeV. Already in the past, HADES has shown its potential for hyperon physics, including λ polarization, λ-N interaction and measurements of the λ(1405) and λ(1520) line-shapes. The HADES detector has recently been extended with a forward detector, partly developed for the PANDA experiment, extending the acceptance for hyperon channels at forward angles. The PANDA@HADES initiative gives the opportunity for an even richer hyperon program. The current main objectives are the production of hyperon resonances, electromagnetic decays of hyperons with special focus on hyperon Dalitz decays and double strangeness production, including a λ – λ interaction study and ξ− production. First results from the ongoing studies promise a successful execution of the program. In the future, there is the possibility for a pion beam experiment with HADES, enabling further hyperon studies.

Precision Measurement of the Decay Σ^{+}→pγ in the Process J/ψ→Σ^{+}Σ[over ¯]^{-}.

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.

CP Asymmetry in the Ξ Hyperon Sector

The Standard Model of particle physics has achieved great success in describing the fundamental particles and their interactions, but there are still some issues that have not been addressed yet. One of the key puzzles is to figure out why there is so much more matter than antimatter in the Universe, regarded as the CP asymmetry. The Ξ hyperon with strangeness S=−2, sometimes so-called the doubly-strange baryon, can provide key information to probe the asymmetry of the matter and antimatter. In this review, we discuss the studies of CP asymmetry in Ξ hyperon decay at E756, HyperCP and BESIII experiments.

Discrete symmetry tests using hyperon-antihyperon pairs

The BESIII experiment has accumulated millions of spin entangled hyperon–antihyperon pairs from the decays of the J/ψ and ψ(2S ) mesons produced in electron–positron collisions. Using multi-dimensional methods to analyze the angular distributions in the sequential decays the hyperon and antihyperon decay amplitudes are studied and tests of the combined charge-conjugation–parity (CP) symmetry are performed. Implications of the recent results using J/ψ decays into Λ$ \bar \Lambda $ and Ξ−$ \bar {\Xi}^{+} $ are presented. For the cascade decay chain, the exclusive measurement allows for three independent CP tests and the first direct determination of the weak phases.

Measurement of the Absolute Branching Fraction and Decay Asymmetry of Λ→nγ.

The radiative hyperon decay Λ→nγ is studied using (10087±44)×10^{6} J/ψ events collected with the BESIII detector operating at BEPCII. The absolute branching fraction of the decay Λ→nγ is determined to be (0.832±0.038_{stat}±0.054_{syst})×10^{-3}, which is a factor of 2.1 lower and 5.6 standard deviations different than the previous measurement. By analyzing the joint angular distribution of the decay products, the first determination of the decay asymmetry α_{γ} is reported with a value of -0.16±0.10_{stat}±0.05_{syst}.

Weak radiative hyperon decays in covariant baryon chiral perturbation theory

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.

Light hyperon polarization and CP test at BESIII

Hyperon is an ideal probe to study the QCD in the region of non-perturbative to perturbative. Using 1.31 × 109 J/ψ and 448.1 × 106 ψ(3686) events, the hyperon anti-hyperon pair production are well studied. The hyperon polarization and its decay parameter have been measured in high precision through their quantum entangle system. By comparing the decay parameters in hyperon and anti-hyperon decays, the CP conservation observables has been measured which is important to test the Standard Model and search for New Physics.

ΔL=2 hyperon decays induced by Majorana neutrinos and doubly charged scalars

Searches for $\mathrm{\ensuremath{\Delta}}L=2$ decays are crucial to disentangle the Dirac vs Majorana nature of neutrinos. While neutrinoless nuclear double beta decays are the most promising tool to look for dielectron signals of $\mathrm{\ensuremath{\Delta}}L=2$, searches of dilepton channels with other flavors are far less limited by statistics. We revisit the calculation of $\mathrm{\ensuremath{\Delta}}L=2$ decays of hyperons ${B}_{i}^{\ensuremath{-}}\ensuremath{\rightarrow}{B}_{f}^{+}{\ensuremath{\ell}}^{\ensuremath{-}}{{\ensuremath{\ell}}^{\ensuremath{'}}}^{\ensuremath{-}}$. We compute the rates of these decays by including the hyperon form factors, which yields finite results in the one-loop model mechanism involving Majorana neutrinos (long-range contributions). In addition, we study the short-range contributions to these kinds of processes in two appealing scenarios. First, we consider the effects of heavy Majorana neutrinos in low-scale seesaw models that involve a minimal parametrization with only two heavy Majorana states. Second, we study an alternative model where $\mathrm{\ensuremath{\Delta}}L=2$ decays are induced by a scalar boson coupled to dileptons to provide additional predictions.

Hyperon physics at HADES as a fair phase-0 experiment

The HADES experiment investigates the nuclear matter and the properties of baryonic resonances. As part of the FAIR Phase-0 program, HADES is being upgraded to enable a wide range of experiments, including investigating electromagnetic decays of hyperons produced in proton-induced reactions. Feasibility studies show that the newly installed forward detectors are crucial for hyperon reconstruction. The detector upgrade and feasibility studies in preparation for the upcoming beamtimes are presented.

One loop mechanism for neutrinoless double beta hyperon decay

Motivated by the large dataset to be accumulated of hyperon pairs produced in decays of $\mathcal{O}(10^{10})$ $J/\psi$ and $\psi^\prime$ charmonia states in the BES-III collaboration, we revisited the predictions of $\Delta L=2$ decays of hyperons $B^-_i\to B^+_f\ell^-\ell'^-$ in the one-loop model mechanism involving Majorana neutrinos previously presented in \cite{Barbero:2002wm}. Unlike the previous work, by modeling the momentum transfer dependence of the hyperon form factors in the computation we provide finite results for the loop integration. Furthermore, since we keep finite masses for the neutrinos throughout the calculations, we are able to consider the effects of heavy Majorana neutrinos. Thus, our results are applied to a simple model that involves two Majorana heavy neutrinos in the framework of a low-scale seesaw model. In order to provide and compare additional predictions, we study an alternative model where $\Delta L=2$ decays are induced by the short-range effects of a scalar boson coupled to di-leptons.

Contributions from Λ hyperons to nucleosynthesis in kilonovae

We study the expansion of neutron star (NS) matter containing hyperons $\mathrm{\ensuremath{\Lambda}}$ from high densities up to the end of nucleosynthesis. This process should occur in NS-NS or NS--black hole collisions. Hyperons $\mathrm{\ensuremath{\Lambda}}$ decay by mesonic and nonmesonic reactions on a very short timescale to form proton and neutron matter. When material becomes diluted enough, the formation of helium and heavier nuclei releases energy and increases its temperature to reach nuclear statistical equilibrium (NSE) conditions with high entropy per baryon. From then on, nucleosynthesis proceeds and the final composition is largely determined by the proton per baryon fraction ${Y}_{p}$. We consider a recent equation of state that considers the presence of $\mathrm{\ensuremath{\Lambda}}$ hyperons [D. Logoteta, I. Vida\~na, and I. Bombaci, Eur. Phys. J. A 55, 207 (2019)] and simultaneously allows for the existence of NSs with masses $M\ensuremath{\gtrsim}2{M}_{\ensuremath{\bigodot}}$. We assumed the composition of NS matter at densities above the threshold for the occurrence of $\mathrm{\ensuremath{\Lambda}}$ hyperons and computed its decay, finding that it appreciably increases ${Y}_{p}$. Then, we computed the subsequent nucleosynthesis starting from NSE assuming that the density falls exponentially. We find that, depending on the initial composition of the ejected material, the increase in ${Y}_{p}$ due to hyperon decay is sufficient to sizeably affect the final isotopic composition of ejected matter (for example, lanthanide production may be strongly inhibited). These results indicate that $\mathrm{\ensuremath{\Lambda}}$ hyperons may affect the final composition of matter ejected in kilonova events. We also discuss the case of pure $\mathrm{\ensuremath{\Lambda}}$ matter as a possible scenario for the collision by strange stars, where we have obtained values ${Y}_{p}\ensuremath{\approx}0.4\text{--}0.5$, which leads to final isotopes far lighter than in the former case.

On measurement of photon polarization in radiative penguin B decays to baryons

A measurement of the photon polarization in radiative penguin B decays provides a test of the Standard Model and a probe for New Physics, that can lead to a deviation from the Standard Model prediction of left-handed photons in brightarrow s gamma . We propose a new method to measure the photon polarization using the baryonic decay B^- rightarrow Lambda bar{p} gamma . The P-violating Lambda -hyperon decay allows a measurement of the Lambda helicity to be performed, which can be uniquely related to the photon polarization in a model-independent way. The B^- rightarrow Lambda bar{p} gamma decay was recently measured to have a large branching fraction providing a possibility to get meaningful results with the data already available at LHC and B-factory experiments. An increase of the B-meson sample at high luminosity LHC experiments and Belle II should provide a really stringent test by using this method already in the near future.

Electromagnetic transition form factors and Dalitz decays of 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 = frac{1}{2}^pm , frac{3}{2}^pm 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.

Production and electromagnetic decay of hyperons: a feasibility study with HADES as a phase-0 experiment at FAIR

A feasibility study has been performed in order to investigate the performance of the HADES detector to measure the electromagnetic decays of the hyperon resonances Sigma (1385)^{0}, Lambda (1405) and Lambda (1520) as well as the production of double strange baryon systems Xi ^{-} and Lambda Lambda in p + p reactions at a beam kinetic energy of 4.5,{mathrm{GeV}}. The existing HADES detector will be upgraded by a new Forward Detector, which extends the detector acceptance into a range of polar angles that plays a crucial role for these investigations. The analysis of each channel is preceded by a consideration of the production cross-sections. Afterwards the expected signal count rates using a target consisting of either liquid hydrogen or polyethylene are summarized.

The potential of varLambda and varXi ^- studies with PANDA at FAIR

The antiproton experiment PANDA at FAIR is designed to bring hadron physics to a new level in terms of scope, precision and accuracy. In this work, its unique capability for studies of hyperons is outlined. We discuss ground-state hyperons as diagnostic tools to study non-perturbative aspects of the strong interaction, and fundamental symmetries. New simulation studies have been carried out for two benchmark hyperon-antihyperon production channels: {bar{p}}p rightarrow {bar{varLambda }}varLambda and {bar{p}}p rightarrow {bar{varXi }}^+varXi ^-. The results, presented in detail in this paper, show that hyperon-antihyperon pairs from these reactions can be exclusively reconstructed with high efficiency and very low background contamination. In addition, the polarisation and spin correlations have been studied, exploiting the weak, self-analysing decay of hyperons and antihyperons. Two independent approaches to the finite efficiency have been applied and evaluated: one standard multidimensional efficiency correction approach, and one efficiency independent approach. The applicability of the latter was thoroughly evaluated for all channels, beam momenta and observables. The standard method yields good results in all cases, and shows that spin observables can be studied with high precision and accuracy already in the first phase of data taking with PANDA.

Remarks on the analysis of the reaction e+e−→Σ0Σ¯0

We investigate roads for evaluating model-independent cross-section distributions for the sequential hyperon decay $\Sigma^0\rightarrow\Lambda \gamma;\Lambda\rightarrow p\pi^- $ and its corresponding antihyperon decay. The hyperons are produced in the reaction $e^+e^-\rightarrow J/{\psi}\rightarrow \Sigma^0\bar{\Sigma}^0$. Cross-section distributions are calculated using the folding technique.