Weak Radiative Decays Of Hyperons Research Articles

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.

Revisiting Weak Radiative Decays of Hyperons

Triggered by experimentally driven renewed interest in hyperon properties we address the subject of weak radiative hyperon decays (WRHD). We start with the issue of Hara's theorem and briefly discuss the question of its possible evasion. Then, we give a short review of the story of vector-meson-dominance (VMD) approach to WRHD. We stress the shift from the Hara's-theorem-violating to Hara's-theorem-satisfying version of the VMD approach that did occur over time. Finally, spurred by a recent theoretical paper, we discuss the pole model description of WRHD, putting special attention to the issue of the contributions from the intermediate $\Lambda(1405)$ state. We point out that the measurement of the $\Lambda \to n \gamma$ decay asymmetry could resolve the encountered ambiguities and definitely answer the question of whether Hara's theorem is violated or not.

10.1007/s11450-008-1015-x

Weak radiative hyperon decays are considered on the basis of the quark model including one-quark transitions associated with the effective sdγ vertex and two-quark processes s + u → u + d + γ induced by W-boson exchange. The phenomenological model proposed in this study gives values that describe experimental data well and makes predictions for the asymmetry of the decays Λ → n + γ and ξ− → Σ− + γ.

Connection between unitary and quark description of weak radiative hyperon decays

Weak radiative hyperon decays are considered in the unitary and quark models. The problem of transition from one model to the other one is solved both for parity-violating and parity-conserving amplitudes. This eliminates the contradiction of incompatibility of these descriptions, which has been discussed for more than 20 years. The model is phenomenological and describes the experimental data well; it gives predictions for asymmetry of Λ → n + γ and Ξ− → Σ− + γ decays.

Weak radiative hyperon decays in the quark model

Weak radiative hyperon decays are considered on the basis of the quark model including one-quark transitions associated with the effective sdγ vertex and two-quark processes s + u → u + d + γ induced by W-boson exchange. The phenomenological model proposed in this study gives values that describe experimental data well and makes predictions for the asymmetry of the decays Λ → n + γ and ξ− → Σ− + γ.

Measurement of the Ξ0→Λγ decay asymmetry and branching fraction

In data taken with the NA48 experiment at the CERN SPS in 1999, 730 candidates of the weak radiative hyperon decay Ξ0→Λγ have been found with an estimated background of 58±8 events. From these events the Ξ0→Λγ decay asymmetry has been determined to α(Ξ0→Λγ)=−0.78±0.18stat±0.06syst, which is the first evidence of a decay asymmetry in Ξ0→Λγ. The branching fraction of the decay has been measured to be Br(Ξ0→Λγ)=(1.16±0.05stat±0.06syst)×10−3.

Weak radiative hyperon decays

The problem of weak radiative hyperon decays (WRHD) is reviewed. With the rcent measurement of the Ξ 0 → Λ γ asymmetry confirming Hara's theorem, implications from its violation in low-energy theoretical approaches are discussed. It is shown how an underlying symmetry link should be formulated for a successful description of both nonleptonic and radiative weak hyperon decays. The sign of the Ξ 0 → Λ γ asymmetry and the overall size of parity-violating WRHD amplitudes together lead to the resolution of the old S:P problem in nonleptonic decays.

Parity-violating photoproduction of pi(+/-) on the delta resonance.

We analyze the real-photon asymmetry A(gamma)(+/-) for the parity-violating (PV) pi(+/-) production on the Delta resonance via the reactions (-->)gamma + p --> Delta(+) --> pi(+) + n and (-->)gamma + d --> Delta(0) + p --> pi(-) + p + p. This asymmetry is nonvanishing due to a PV gamma N Delta coupling constant, d(Delta)(+/-). We argue that an experimental determination of this coupling would be of interest for hadron dynamics, possibly shedding light on the S-wave/ P-wave puzzle in the hyperon nonleptonic decays and the violation of Hara's theorem in weak radiative hyperon decays.

Measurement of the branching ratio and asymmetry of the decay Xi degrees -->Sigma degrees gamma.

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.

An estimate of a lower bound on the masses of mirror baryons

We consider the most favorable conditions to indirectly observe the mixing of ordinary and mirror hadrons in non-leptonic and weak radiative decays of hyperons. This allows us to set a lower bound on the masses of mirror baryons. This bound turns out to be impressively high, of the order of 10 6 GeV.

Symmetry limit properties ofa priori mixing amplitudes fornon-leptonic and weak radiative decays ofhyperons

We show that, even if a priori mixings of positive-parity butflavour-violating eigenstates are allowed in physical hadrons, in the strongflavour symmetry limit the mixing angle will drop out of the matrixelements of the Yukawa and electromagnetic Hamiltonians that lead tonon-leptonic and weak radiative decays of hyperons in this scheme.We compare this result with related theorems for the same W-mediateddecays and give a concrete example of this behaviour by considering theU-spin symmetry of baryons and the U-doublet formed by p andΣ+.

Chiral approach to weak radiative hyperon decays and theΞ0→Λγasymmetry

We reanalyze the recent version of the chiral model of weak radiative hyperon decays, proposed by Borasoy and Holstein. It is shown that predictions of the analyzed model are significantly changed when one accepts the usual classification of $\ensuremath{\Lambda}(1405)$ as an $\mathrm{SU}(3)$ singlet. In particular, for the ${\ensuremath{\Xi}}^{0}\ensuremath{\rightarrow}\ensuremath{\Lambda}\ensuremath{\gamma}$ decay large negative asymmetry is obtained. This is contrasted with the Hara-theorem-violating approaches where this asymmetry is large and positive.

A priori mixed hadrons, weak radiative and non-leptonic decays of hyperons

A priori mixings of eigenstates in physical states are quantum mechanical effects well known in several realms of physics. The possibility that such effects are also present in particle physics, in the form of flavour and parity mixings, is studied. Applications to weak radiative and non-leptonic decays of hyperons are discussed.

Weak radiative hyperon decays, Hara’s theorem, and the diquark

Weak radiative hyperon decays are discussed in the diquark-level approach. It is pointed out that in the general diquark formalism one may reproduce the experimentally suggested pattern of asymmetries, while maintaining Hara's theorem in the SU(3) limit. At present, however, no detailed quark-based model of parity-violating diquark-photon coupling exists that would have the necessary properties.

Hyperon weak radiative decays in chiral perturbation theory.

We investigate the leading-order amplitudes for weak radiative decays of hyperons in chiral perturbation theory. We consistently include contributions from the next-to-leading order weak-interaction Lagrangian. It is shown that due to these terms Hara's theorem is violated. The data for the decay rates of the charged hyperons can be accounted for. However, at this order in the chiral expansion, the four decay rates of the neutral hyperons satisfy relations which are in disagreement with the data. The asymmetry parameters for all the decays cannot be accounted for without higher-order terms. We briefly comment on the effect of the 27-plet part of the weak interaction.

Weak radiative decays of hyperons and of charm and beauty baryons

A review is presented of the weak radiative decays of baryons.It includes an anlysis of the possible contributions of electromagnetic penguins to these decays,a survey of the difficulties still encountered in the sector of hyperon decays and a short account on some new developments on this topic. The theoretical treatments on charm and beauty baryon decays are summarized,with a good outlook for their detection.

HYPERON RADIATIVE DECAYS

We review the experimental and theoretical status of weak radiative hyperon decays. Our discussion centers on a controversy over the validity of Hara’s theorem, which was originally expected to be respected by these decays. After presenting the hadron level theorem we describe experiments that have provided convincing evidence against its applicability to these decays. In the theoretical part we juxtapose the hadron level and quark level approaches and discuss the violation of Hara’s theorem in the latter. We review quark model phenomenology, which offers a promising description of experimental data. Measurements that should be made to settle the theoretical controversy are pinpointed. The importance of radiative hyperon decays in understanding the nonlocal composite nature of hadrons is stressed.

Weak hyperon decays: Quark sea and SU(3) symmetry breaking.

An explanation of the difference in the values of the apparent $f/d$ ratios for the S- and P- wave amplitudes of nonleptonic hyperon decays is proposed. The argument is formulated in the framework of the standard pole model with $(56,0^{+})$ ground-state and $(70,1^{-})$ excited baryons as intermediate states for the P- and S- waves respectively. Under the assumption that the dominant part of the deviation of $(f/d)_{P-wave}$ from $-1$ is due to large quark sea effects, $SU(3)$ symmetry breaking in energy denominators is shown to lead to a prediction for $(f/d)_{S-wave}$ which is in excellent agreement with experiment. This corroborates our previous unitarity calculations which indicated that the matrix elements $<B|H^{p.c.}_{weak}|B'>$ of the parity conserving weak Hamiltonian between the ground-state baryons are characterized by $f_{0}/d_{0} \approx -1.6$ or more. A brief discussion of the problem of the relative size of S- and P- wave amplitudes is given. Finally, implications for weak radiative hyperon decays are also discussed.

Weak radiative hyperon decays in chiral perturbation theory

The parity-conserving a and parity-violating b amplitudes for weak radiative hyperon decay are studied using chiral perturbation theory. The imaginary parts of a and b are computed using unitarity. The real part of b is dominated by a one-loop infrared divergent graph which is computed. The real part of a has a large theoretical uncertainty and cannot be calculated reliably. Counterterms for the a and b amplitudes are classified using CPS symmetry. The experimental values for decay widths and asymmetries are consistent with theory, with the exception of the asymmetry parameter for the Σ + → p γ decay.

Weak radiative hyperon decays

A summary is given of recent experimental and theoretical developments in weak radiative hyperon decays. As an example, data and new results are presented from Brookhaven experiment E811, the first high statistics measurement of the branching ratio for the Λ weak radiative decay, Λ→ n+ γ.