Σ0 Hyperons Research Articles

A closer look at the Yukawa interaction from a symmetry group perspective

Abstract I investigate the use of the SU(3) Clebsch–Gordan coefficients in light of the relations of completeness and closure. I show that in the case of αV = F/(F + D) ≠ 1, there is an additional interaction: the exchange of a ρ meson between a Λ and a Σ0 hyperon that only affects the symmetric coupling. I then calculate these additional coupling constants and show that this recovers the completeness and closure of the SU(3) Clebsch–Gordan coefficients for all values of αV. Besides, it increases the symmetry of the theory, now we can group the baryon octet into four doublets. Finally, I add the new coupling constants to study numerical results in the hyperon onset in dense nuclear matter assuming αV as a free parameter.

Decay of the Λ(1405) hyperon to Σ0π0 measured at GlueX

Among the light baryons, the Jπ = 1/2− Λ(1405) hyperon is an important special case as it sits just below the N K̅ threshold and decays almost exclusively to Σπ. Some long-standing hypotheses for Λ(1405) are that it could be a N K̅ bound state or a Σπ continuum resonance. It may also be considered as a simple quark-model resonance, the P-wave companion of the Λ(1520). In recent years chiral unitary models have suggested that there are two isospin zero poles present in this mass region, and that the “line shape" of the Λ(1405) depends to what extent each of the two poles are stimulated in a given reaction. Below the N K̅ threshold, the Λ(1405) decays to the three Σπ charge combinations. The Σ0π0 mode is purely I = 0 and it is uncontaminated by complications arising from I = 1 scattering processes contributing to the reaction mechanism in the Σ+π− and Σ−π+ decays. It is also not affected from production and decay of the nearby Σ0(1385) hyperon. The GlueX experiment at Jefferson Lab has been used to study the Λ(1405) → Σ0π0 decay mode. We focus on the preliminary results of dσ/dMΣ0π0 and fits to the line shape of the Λ(1405) region in the −(t − tmin) range 0 - 1.5 GeV2 from analyzing the reaction γp → K+Λ* using the data collected during the first phase of the GlueX program.

Constraining P and CP violation in the main decay of the neutral Sigma hyperon

On general grounds based on quantum field theory the decay amplitude for Σ0→Λγ consists of a parity conserving magnetic and a parity violating electric dipole transition moment. Because of the subsequent self-analyzing weak decay of the Λ hyperon the interference between magnetic and electric dipole transition moment leads to an asymmetry in the angular distribution. Comparing the decay distributions for the Σ0 hyperon and its antiparticle gives access to possible C and CP violation. Based on flavor SU(3) symmetry the present upper limit on the neutron electric dipole moment can be translated to an upper limit for the angular asymmetry. It turns out to be far below any experimental resolution that one can expect in the foreseeable future. Thus any true observation of a CP violating angular asymmetry would constitute physics beyond the standard model, even if extended by a CP violating QCD theta-vacuum-angle term.

First Measurements of Form Factors of Pion, Kaon, Proton, and Hyperons for the Highest Timelike Momentum Transfers

Results of recent precision measurements of the electromagnetic form factors of the pion, kaon, proton, and the hyperons, Λ0, Σ0, Σ+, Ξ−, Ξ0, Ω−, at large timelike momentum transfers are presented. Evidence is found for diquark correlations in Λ0, Σ0 hyperons.

First measurements of timelike form factors of the hyperons, [formula omitted], [formula omitted], [formula omitted], [formula omitted], [formula omitted], and [formula omitted], and evidence of diquark correlations

Using 805 pb−1 of e+e− annihilation data taken with the CLEO-c detector at ψ(3770), s=3770 MeV, we report the first measurements of the electromagnetic form factors of the Λ0, Σ0, Σ+, Ξ0, Ξ−, and Ω− hyperons for the large timelike momentum transfer of |Q2|=14.2 GeV2. The form factors for the different hyperons are found to vary by nearly a factor two. It is found that |GM(Λ0)|=1.66(24)×|GM(Σ0)|. The Λ0 and Σ0 hyperons have the same uds quark content, but differ in their isospin, and therefore the spin of the ud quark pair. It is suggested that the spatial correlation implied by the singlet spin–isospin configuration in the Λ0 is an example of strong diquark correlations in the Λ0, as anticipated by Jaffe and Wilczek. Improved measurements of the branching fractions of ψ(2S)→pp¯ and hyperon–antihyperon pairs are also reported.

The Production of K0's in p+p Reactions

We present the exclusive analysis of the reactions p+p → Y + Δ++/(p + π+) + K0, where Y stands for the Λ or Σ0 hyperon. The proton-proton measurement was performed with the HADES setup at GSI, Darmstadt, at a kinetic beam energy of 3.5 GeV. A dedicated sideband technique allows to reproduce the non-strange background and its kinematics in the selected data sample. Therefore, it is possible to obtain the background subtracted missing mass distribution MM(pπ+π+π−), where we can separate the Λ from the Σ0 channel. From a Monte Carlo simulation of possible reactions we see contributions also by the reactions p + p → Σ+/Σ(1385)+ + p + K0. The ongoing analysis should provide exclusive cross sections of the mentioned reactions and their angular distributions.

STATUS OF STRANGENESS ELECTRO-PRODUCTION AT MAMI

At the Institut für Kernphysik in Mainz, Germany, the microtron MAMI has been upgraded to 1.5 GeV electron beam energy and can now be used to study strange hadronic systems. The magnetic spectrometer KAOS from GSI was dismantled and re-installed in the spectrometer facility operated by the A1 collaboration. The spectrometer's primary purpose is to study strangeness electro-production. Its compact design and its capability to detect negative and positive charged particles simultaneously under forward scattering angles complements the existing spectrometers. In 2008, an important milestone has been reached by the successful measurement of kaon production off a liquid hydrogen target. The identification of Λ and Σ0 hyperons in the missing mass spectra has demonstrated the capability of the extended facility to perform strangeness electro-production spectroscopy. To operate KAOS as a double-arm spectrometer under zero degree scattering angle major modifications are needed. A new vacuum chamber was constructed and installed, a new coordinate detector with many thousands of channels was built and tested, a new trigger system for the high rate environment was developed, and a magnetic chicane for the primary electron beam is under construction.

Lineshape of the [formula omitted] hyperon measured through its [formula omitted] decay

The pp→pK+Y0 reaction has been studied for hyperon masses m(Y0)⩽1540 MeV/c2 at COSY-Jülich by using a 3.65 GeV/c circulating proton beam incident on an internal hydrogen target. Final states comprising two protons, one positively charged kaon and one negatively charged pion have been identified with the ANKE spectrometer. Such configurations are sensitive to the production of the ground state Λ and Σ0 hyperons as well as the Σ0(1385) and Λ(1405) resonances. Applying invariant- and missing-mass techniques, the two overlapping excited states could be well separated, though with limited statistics. The shape and position of the Λ(1405) distribution, reconstructed cleanly in the Σ0π0 channel, are similar to those found from other decay modes and there is no obvious mass shift. This finding constitutes a challenging test for models that predict Λ(1405) to be a two-state resonance.

QCD SUM RULES: INTERCROSSED RELATIONS FOR THE Σ0–Λ MASS SPLITTING

New relations between QCD Borel sum rules for masses of Σ0 and Λ hyperons are constructed. It is shown that starting from the sum rule for the Σ0 hyperon mass it is straightforward to obtain the corresponding sum rule for the Λ hyperon mass and vice versa.

New relations between Borel sum rules for the magnetic moments of the Σ0 and Λ hyperons

New relations between the Borel sum rules in QCD for the magnetic moments of the Σ0 and Λ hyperons are derived. It is shown that, on the basis of the sum rule for the magnetic moment of the Σ0 hyperon, one can directly obtain the corresponding sum rule for the magnetic moment of the Λ hyperon, and vice versa, as well as the corresponding sum rule for the Σ0 → Λγ transition.

Separation of the longitudinal and transverse cross sections in the1H(e,e′K+)Λand1H(e,e′K+)Σ0reactions

We report measurements of cross sections for the reaction 1H(e,e′K+)Y, for both the Λ and Σ0 hyperon states, at an invariant mass of W=1.84GeV and four-momentum transfers 0.5<Q2<2(GeV/c)2. Data were taken for three values of virtual photon polarization ε, allowing the decomposition of the cross sections into longitudinal and transverse components. The Λ data are a revised analysis of prior work, whereas the Σ0 results have not been previously reported.Received 5 November 2002DOI:https://doi.org/10.1103/PhysRevC.67.055205©2003 American Physical Society

Neutral strange particle production in antineutrino-neon charged current interactions

Neutral strange particle production in $$\bar v$$ Ne charged current interactions is studied using the bubble chamber BEBC, exposed to the CERN SPS antineutrino wide band beam. From a sample of 1191 neutral strange particles, the inclusive production rates are determined to be (15.7±0.8)% forK 0 mesons, (8.2±0.5)% for Λ, (0.4±0.2)% for $$\bar \Lambda $$ and (0.6±0.3)% for Σ0 hyperons. The inclusive production properties ofK 0 mesons and Λ hyperons are investigated. The Λ hyperons are found to be polarized in the production plane.

Demostration of the existence of the Σ0 hyperon and a measurement of its mass

Three events, demonstrating the existence of the Σ0 hyperon, have been found in a propane bubble chamber. TheQ-value for the decay Σ0 → Λ0+γ+Q has been measured to be (73.0 ± 3.5) MeV.