Nonleptonic Hyperon Research Articles

S-wave nonleptonic hyperon decays andΞb−→π−Λb

The decay $\Xi^-_b \to \pi^- \Lambda_b$ has recently been observed by the LHCb Collaboration at CERN. In contrast to most weak decays of $b$-flavored baryons, this process involves the decay of the strange quark in $\Xi_b$, and thus has features in common with nonleptonic weak decays of hyperons. Thanks to the expected pure S-wave nature of the decay in question in the heavy $b$ quark limit, we find that its amplitude may be related to those for S-wave nonleptonic decays of $\Lambda$, $\Sigma$, and $\Xi$ in a picture inspired by duality. The calculated branching fraction ${\cal B}(\Xi^-_b \to \pi^- \Lambda_b) = (6.3 \pm 4.2) \times 10^{-3}$ is consistent with the range allowed in the LHCb analysis. The error is dominated by an assumed 30\% uncertainty in the amplitude due to possible U(3) violation. A more optimistic view based on sum rules involving nonleptonic hyperon decay S-wave amplitudes reduces the error on the branching fraction to $2.0 \times 10^{-3}$.

Radiative and nonleptonic hyperon decays in broken SU(3)

We report on the recently proposed joint resolution of two long-standing puzzles in weak radiative (WR) and nonleptonic (NL) hyperon decays (HD). First, a good VMD-based description of WRHD is presented. In particular, the large negative asymmetry observed in the Σ+→pγ decay is explained through a calculably large SU(3)-breaking effect in the relevant parity-violating amplitude. Second, the achieved description of the parity-violating WRHD amplitudes permits the extraction, via the SU(2)W+VMD route, of the non-soft-pion correction term in the parity-violating NLHD amplitudes. The latter subtracts a substantial amount from the current-algebra commutator term, thus leading to the resolution of the old S : P discrepancy in NLHD.

Joint description of weak radiative and nonleptonic hyperon decays in broken SU(3)

We give a joint description of weak radiative (WR) and nonleptonic (NL) hyperon decays (HD) in broken SU(3). The two groups of decays are linked via SU(2){sub W} spin symmetry and vector-meson dominance (VMD). We use experimental information on the parity-conserving NLHD amplitudes to fix the corresponding WRHD amplitudes. With the latter known, the data on the WRHD branching ratios and asymmetries permit us to determine the parity-violating WRHD amplitudes in terms of two parameters corresponding to the two-quark and single-quark transitions. We obtain a good description of the data, and, in particular, a large {sigma}{sup +}{yields}p{gamma} asymmetry. Then, using the SU(2){sub W}+VMD route we determine the non-soft-meson correction term in the parity-violating NLHD amplitudes. The latter is shown to subtract a substantial amount from the current-algebra commutator thus leading towards the resolution of the S ratio P discrepancy in NLHD.

Search forΔS=2Nonleptonic Hyperon Decays

A sensitive search for the rare decays Omega(-)--> Lambdapi(-) and Xi(0)--> ppi(-) has been performed using data from the 1997 run of the HyperCP (Fermilab E871) experiment. Limits on other such processes do not exclude the possibility of observable rates for |DeltaS| = 2 nonleptonic hyperon decays, provided the decays occur through parity-odd operators. We obtain the branching-fraction limits B(Omega(-)-->Lambdapi(-)) < 2.9 x 10(-6) and B(Xi(0)--> ppi(-)) < 8.2 x 10(-6), both at 90% confidence level.

A framework for exploring the interactions and decays of hyperons with lattice QCD

In this work we outline a program for lattice QCD that would greatly contribute to understanding the strong and weak interactions of strange baryons. The low-energy effective field theory describing hyperon–nucleon scattering in QCD and partially-quenched QCD is constructed. This framework will allow lattice simulations of the low-energy scattering parameters to be extrapolated to the physical values of the quark masses. We explore the long-standing discrepancies in the P-wave amplitudes for non-leptonic hyperon decays using two-flavor chiral perturbation theory, and emphasize that the problem is not merely an artifact of perturbing about the chiral limit of the three-flavour theory. As energy is not conserved in the lattice simulation of the Λ → N two-point function, we include the surface terms in the effective field theory required to determine the underlying weak amplitude.

Instantons, diquarks, and nonleptonic weak decays of hyperons

This work is devoted to the study of the non-perturbative contributions in non-leptonic hyperon decays. We show that the instanton-induced 't Hooft interaction can naturally explain the Delta I=1/2 rule, by generating quark-diquark clustering inside octet baryons. We compute P-wave and S-wave amplitudes in the Instanton Liquid Model (ILM), and find good agreement with experiment. We propose a model-independent procedure to test on the lattice if the leading quark-quark attraction in the 0^+ anti-triplet channel responsible for diquark structures in hadrons is originated by the interaction generated by quasi-classical fields or it is predominantly due to other perturbative and/or confining forces.

Final state interactions in nonleptonic hyperon decays

We investigate the importance of final state interactions in weak nonleptonic hyperon decays within a relativistic chiral unitary approach based on coupled channels. The effective potentials for meson-baryon scattering are derived from a chiral effective Lagrangian and iterated in a Bethe-Salpeter equation, which generates the low lying baryon resonances dynamically. The inclusion of final state interactions decreases the discrepancy between theory and experiment for both s and p waves. Our study indicates that contributions from higher order terms of the weak effective Lagrangian may play an important role in these decays.

CPviolation in hyperon nonleptonic decays within the standard model

We calculate the CP-violating asymmetries A(Lambda_-^0) and A(Xi_-^-) in nonleptonic hyperon decay within the Standard Model using the framework of heavy-baryon chiral perturbation theory (chiPT). We identify those terms that correspond to previous calculations and discover several errors in the existing literature. We present a new result for the lowest-order (in chiPT) contribution of the penguin operator to these asymmetries, as well as an estimate for the uncertainty of our result that is based on the calculation of the leading nonanalytic corrections.

Search for CP violation in hyperon decays

Direct CP violation in nonleptonic hyperon decays can be established by comparing the decays of hyperons and anti-hyperons. For Ξ decay to Λπ followed by Λ to pπ, the proton distribution in the rest frame of Lambda is governed by the product of the decay parameters αΞα Λ . The asymmetry Λ ΞΛ , proportional to the difference of αΞα Λ of the hyperon and anti-hyperon decays, vanishes if CP is conserved. We report on an analysis of a fraction of 1997 and 1999 data collected by the HyperCP (E871) collaboration during the fixed-target runs at Fermilab. The preliminary measurement of the assymmetry is A ΞΛ = [−7±12(stat)±6.2(sys)] × 10 −4, an order of magnitude better than the present limit.

Long distance regularization in chiral perturbation theory with decuplet fields

We investigate the use of long distance regularization in $\mathrm{SU}(3)$ baryon chiral perturbation theory with decuplet fields. The one-loop decuplet contributions to the octet baryon masses, axial-vector couplings, S-wave nonleptonic hyperon decays and magnetic moments are evaluated in a chirally consistent fashion by employing a cutoff to implement long distance regularization. The convergence of the chiral expansions of these quantities is improved compared to the dimensionally regularized version which indicates that the propagation of Goldstone bosons over distances smaller than a typical hadronic size, which is beyond the regime of chiral perturbation theory but included by dimensional regularization, is removed by use of a cutoff.

Nonleptonic hyperon decays with QCD sum rules

Despite measurements which date more than 20 years ago, no straightforward solution of the ratio of the parity-conserving (P-wave) to parity-violating (S-wave) decays of the hyperons has been obtained. Here we use two 2-point methods in QCD sum rules to examine the problem. We find that resonance contributions are needed to fit the data, similar to a chiral perturbation theory treatment.

NonleptonicΩ−decays and the Skyrme model

Nonleptonic $\Omega^{-}$ decay branching ratios are estimated by means of the QCD enhanced effective weak Hamiltonian supplemented by the SU(3) Skyrme model used to estimate the nonperturbative matrix elements. The model has only one free parameter, namely the Skyrme charge $e$, which is fixed through the experimental values of the octet-decuplet mass splitting $\Delta $ and the axial coupling constant $g_{A}$. The whole scheme is equivalent to that which works well for the nonleptonic hyperon decays. The ratios of calculated amplitudes are in agreement with experiment. However, the absolute values are about twice too large if short-distance corrections and only ground intermediate states are included.

Skyrme model and nonleptonic hyperon decays

This report is an attempt to explain both s- and p-wave nonleptonic hyperon decays by means of the QCD enhanced effective weak Hamiltonian supplemented by the SU(3) Skyrme model used to estimate nonperturbative matrix elements. The model has only one free parameter, namely, the Skyrme charge $e$, which is fixed through the experimental values of the octet-decuplet mass splitting $\Delta $ and the axial coupling constant $g_{A}$. Such a dynamical approach produces nonleptonic hyperon decay amplitudes that agree with experimental data reasonably well.

Weak hyperon decays in heavy baryon chiral perturbation theory: Renormalization and applications

The complete renormalization of the weak Lagrangian to chiral order ${q}^{2}$ in heavy baryon chiral perturbation theory is performed using heat kernel techniques. The results are compared with divergences appearing in the calculation of Feynman graphs for the nonleptonic hyperon decay $\stackrel{\ensuremath{\rightarrow}}{\ensuremath{\Lambda}}p{\ensuremath{\pi}}^{\ensuremath{-}}$ and an estimate for the size of the counterterm contributions to the s-wave amplitudes in nonleptonic hyperon decays is given.

Nonleptonic hyperon weak decays in the Skyrme model reexamined

Nonleptonic hyperon weak decays are investigated in the SU(3) Skyrme model. We use a collective coordinate scheme, following the approach in which the symmetry breaking terms in the strong effective action are diagonalized exactly. To describe the weak interactions we use an octet dominated weak effective Lagrangian that leads to a good description of the known $2\ensuremath{\pi}$ and $3\ensuremath{\pi}$ kaon decays. We show that the observed S-wave decays are reasonably well reproduced in the model. On the other hand, our calculated P-wave amplitudes do not agree with the empirical ones even though both pole and contact contributions to these amplitudes are properly taken into account. Finally, an estimate of the nonoctet contributions to the decay amplitudes is presented.

Role of resonances in nonleptonic hyperon decays

We examine the importance of resonances for the nonleptonic hyperon decays in the framework of chiral perturbation theory. Lower lying resonances are included in the effective theory. Integrating out the heavy degrees of freedom in the resonance saturation scheme generates higher order counterterms in the effective Lagrangian, providing an estimate of the pertinent coupling constants. A fit to the eight independent decay amplitudes that are not related by isospin symmetry is performed and reasonable agreement for both s and p waves is achieved.

Non-leptonic hyperon decays in chiral perturbation theory

The non-leptonic hyperon decays are analyzed up to one-loop order including all counterterms in the framework of heavy baryon chiral perturbation theory. We use the exchange of the spin-\(\frac{3}{2}\) decuplet resonances as an indication of which low-energy constants contribute significantly to these investigated processes. We choose four independent decay amplitudes that are not related by isospin relations in order to perform a fit for the pertinent low-energy constants and find a satisfactory fit both for s- and p-waves. The chiral corrections to the lowest order forms for the s-waves are moderate whereas there are significant modifications of the p-wave amplitudes.

Non-leptonic hyperon decays in chiral perturbation theory

The non-leptonic hyperon decays are analyzed up to one-loop order including all counterterms in the framework of heavy baryon chiral perturbation theory. We use the exchange of the spin-3/2 decuplet resonances as an indication of which low-energy constants contribute significantly to these investigated processes . We choose four independent decay amplitudes that are not related by isospin relations in order to perform a fit for the pertinent low-energy constants and find a satisfactory fit both for s- and p-waves. The chiral corrections to the lowest order forms for the s-waves are moderate whereas there are significant modifications of the p-wave amplitudes.

Isospin mixing and model dependence

We show that recent calculations of \Delta I=3/2 effects in nonleptonic hyperon decay induced by m_d-m_u\neq 0 are subject to significant model dependence.

Role of(3*,3)operators in the chiral symmetry approach to nonleptonic decays andCPviolation

We study the kaon-pole diagrams which involve the nonvanishing tadpole transition {l_angle}0{vert_bar}O{sub 6+i7}{vert_bar}{bar K}{sup 0}{r_angle} , where the operator O belongs to the (3{sup {asterisk}},3) representation of chiral SU(3){times}SU(3). As is well known, the pole contribution exactly cancels the direct contribution to K-2{pi} and s-wave nonleptonic hyperon decay in the SU(3) and chiral limits. It is shown that there are additional contributions which vanish in the SU(3) limit, these being of order (m{sub K}{sup 2}{minus}m{sub {pi}}{sup 2})/m{sub K}{sup 2} or (m{sub B{sup {prime}}}{minus}m{sub B})/m{sub K}{sup 2}. The contribution of such corrections to direct CP violation in K{r_arrow}2{pi} and hyperon decays is calculated, which could in some cases be a substantial fraction of the standard model predictions for direct CP violation in these transitions. {copyright} {ital 1997} {ital The American Physical Society}