Hyperon Decays Research Articles

SU(3)-breaking corrections to the hyperon vector couplingf1(0)in covariant baryon chiral perturbation theory

We report on a recent study of the SU(3)-breaking corrections to the hyperon vector coupling $f_1(0)$ up to $\mathcal{O}(p^4)$ in covariant baryon chiral perturbation theory with dynamical octet and decuplet contributions. The decuplet contributions are taken into account for the first time in a covariant ChPT study and are found of similar or even larger size than the octet ones. We predict positive SU(3)-breaking corrections to all the four independent $f_1(0)$'s (assuming isospin symmetry), which are consistent, within uncertainties, with the latest results from large $N_c$ fits, chiral quark models, and quenched lattice QCD calculations. We also discuss briefly the implications of our results for the extraction of $V_{us}$ from hyperon decay data.

Helicity Amplitude Analysis of e+e− → Ψ(2S) → γχcJ → γγ̄ → γpπ− p̄π+

An analysis of the helicity amplitude is performed on the process e+e− → Ψ(2S) → γχcJ → γγ̄ → γpπ− p̄π+. The joint angular distributions for each decay chain are given, and we focus on discussing the E1 contributions to Ψ(2S) radiative decays, and the violation of the helicity selection rule in χcJ decays into hyperon pairs, together with the possibility of observing parity violation in successive hyperon decays.

The SDD and SSD support structure for the ALICE Inner Tracking System

The ALICE Inner Tracking System is designed to provide the determination of the primary vertex and of the secondary vertices in order to reconstruct charm and hyperon decays, particle identification and tracking of low-momentum particles and to improve the momentum and angle measurements of the Time Projecting Chamber (TPC). The device consists of six layers of silicon detectors and has been installed into the ALICE's cavern of the LHC accelerator. In this paper we describe the mechanical structure of the instrument and its behaviour during and after the assembly procedure.

Some topics on charmonium decays at BESIII experiment**Supported by National Natural Science Foundation of China (10491303) and 100 Talents Program of CAS (U-25)

The construction of BESIII detector has been finished and the data taking is under plan. Some physics topics on charmonium decays at BESIII experiment are discussed in this paper. The measurement of properties of ηc and ηc′ at BESIII is discussed and the expected precision of the measurement is estimated based on BOSS. Also the χcJ decay and the measurement of hyperon decay parameters are mentioned.

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.

Hyperon decay form factors in chiral perturbation theory

We present a complete calculation of the SU(3)-breaking corrections to the hyperon vector form factors up to O(p^4) in covariant baryon chiral perturbation theory. Partial higher-order contributions are obtained, and we discuss chiral extrapolations of the vector form factor at zero momentum transfer. In addition we derive low-energy theorems for the subleading moments in hyperon decays, the weak Dirac radii and the weak anomalous magnetic moments, up to O(p^4).

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.

The beam and detector for the NA48 neutral kaon CP violation experiment at CERN

The beam and detector, used for the NA48 experiment, devoted to the measurement of Re ( ε ′ / ε ) , and for the NA48/1 experiment on rare K S and neutral hyperon decays, are described.

Light Higgs production in hyperon decay

A recent HyperCP observation of three events in the decay {sigma}{sup +}{yields}p{mu}{sup +}{mu}{sup -} is suggestive of a new particle with mass 214.3 MeV. In order to confront models that contain a light Higgs boson with this observation, it is necessary to know the Higgs production rate in hyperon decay. The contribution to this rate from penguinlike two-quark operators has been considered before and found to be too large. We point out that there are additional four-quark contributions to this rate that could be comparable in size to the two-quark contributions, and that could bring the total rate to the observed level in some models. To this effect we implement the low-energy theorems that dictate the couplings of light Higgs bosons to hyperons at leading order in chiral perturbation theory. We consider the cases of scalar and pseudoscalar Higgs bosons in the standard model and in its two-Higgs-doublet extensions to illustrate the challenges posed by existing experimental constraints and suggest possible avenues for models to satisfy them.

Weak quasielastic production of hyperons

The quasielastic weak production of $\ensuremath{\Lambda}$ and $\ensuremath{\Sigma}$ hyperons from nucleons and nuclei induced by antineutrinos is studied in the energy region of some ongoing neutrino oscillation experiments in the intermediate energy region. The hyperon-nucleon transition form factors determined from neutrino-nucleon scattering and an analysis of high precision data on semileptonic decays of neutron and hyperons using SU(3) symmetry have been used. The nuclear effects due to Fermi motion and final state interaction effects due to hyperon-nucleon scattering have also been studied. The numerical results for differential and total cross sections have been presented.

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.

INSTANTON-INDUCED CORRELATIONS IN HADRONS

QCD instantons generate non-perturbative spin- and flavor- dependent correlations between light quarks. We report on the results of a series of studies on the contribution of instantons to the electro-weak structure of light hadrons. We show that the Instanton Liquid Model can reproduce the available data on proton and pion form factors at large momentum transfer, and explain the delay of the onset of the perturbative regime in some exclusive reactions. We provide unambiguous evidence that instantons lead to the formation of a deeply bound scalar, color anti-triplet diquark, with a mass of about 450 MeV. The strong attraction in the [Formula: see text], scalar diquark channel leads to a quantitative description of non-leptonic decays of hyperons and provides a microscopic dynamical explanation of the Δ I = 1/2 rule.

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.

Search for the Flavor-Changing Neutral-Current in the Decay [formula omitted

The Fermilab HyperCP (E871) experiment collected on the order of 10 10 hyperon decays. Based on an analysis of the data set collected in 1999 run, we will report on the observation of events with reconstructed masses consistent with that of Σ + assuming the final state p μ + μ − . If the observed events are the decay Σ + → p μ + μ − , then they would be the first evidence of this decay. Other possible interpretations of the observed events will also be discussed.

HyperCP: A high-rate spectrometer for the study of charged hyperon and kaon decays

The HyperCP experiment (Fermilab E871) was designed to search for rare phenomena in the decays of charged strange particles, in particular CP violation in Ξ and Λ hyperon decays with a sensitivity of 10 - 4 . Intense charged secondary beams were produced by 800 GeV/ c protons and momentum selected by a magnetic channel. Decay products were detected in a large-acceptance, high-rate magnetic spectrometer using multiwire proportional chambers, trigger hodoscopes, a hadronic calorimeter, and a muon-detection system. Nearly identical acceptances and efficiencies for hyperons and antihyperons decaying within an evacuated volume were achieved by reversing the polarities of the channel and spectrometer magnets. A high-rate data-acquisition system enabled 231 billion events to be recorded in 12 months of data-taking.

Semileptonic Hyperon Decays on the Lattice: an Exploratory Study

We present preliminary results of an exploratory lattice study of the vector form factor f1(q2=0) relevant for the semileptonic hyperon decay Σ−→nlν. This study is based on the same method used for the extraction of f+(0) for the decay K0→π−lν. The main purpose of this study is to test the method for hyperon form factors in order to estimate the precision that can be reached and the importance of SU(3)-breaking effects.

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.

A study of strange particles produced in neutrino neutral current interactions in the NOMAD experiment

Results of a detailed study of strange particle production in neutrino neutral current interactions are presented using the data from the NOMAD experiment. Integral yields of neutral strange particles ( K s 0 , Λ, Λ ¯ ) have been measured. Decays of resonances and heavy hyperons with an identified K s 0 or Λ in the final state have been analyzed. Clear signals corresponding to K ⋆ ± and Σ ( 1385 ) ± have been observed. First results on the measurements of the Λ polarization in neutral current interactions have been obtained.

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.

Semileptonic hyperon decays and Cabibbo-Kobayashi-Maskawa unitarity.

Using a technique that is not subject to first-order SU(3) symmetry breaking effects, we determine the V(us) element of the Cabibbo-Kobayashi-Maskawa matrix from data on semileptonic hyperon decays. We obtain V(us)=0.2250(27), where the quoted uncertainty is purely experimental. This value is of similar experimental precision to the one derived from K(l3), but it is higher and thus in better agreement with the unitarity requirement, |V(ud)|(2)+|V(us)|(2)+|V(ub)|(2)=1. An overall fit, including the axial contributions and neglecting SU(3) breaking corrections, yields F+D=1.2670 +/-0.0035 and F-D=-0.341+/-0.016 with chi(2)=2.96/3 degrees of freedom.