Spectra of charmed and bottom baryons with hyperfine interaction

Up to now,the excited charmed and bottom baryon states have still not been well studied experimentally or theoretically.In this paper,we predict the mass of ?*b,the only L=0 baryon state which has not been observed,to be 6069.2 Me V.The spectra of charmed and bottom baryons with the orbital angular momentum L = 1 are studied in two popular constituent quark models,the Goldstone boson exchange(GBE) model and the one gluon exchange(OGE) hyperfine interaction model.Inserting the latest experimental data from the "Review of Particle Physics",we find that in the GBE model,there exist some multiplets(Σc(b),Ξ c(b)and ?c(b)) in which the total spin of the three quarks in their lowest energy states is 3/2,but in the OGE model there is no such phenomenon.This is the most important difference between the GBE and OGE models.These results can be tested in the near future.We suggest more efforts to study the excited charmed and bottom baryons both theoretically and experimentally,not only for the abundance of baryon spectra,but also for determining which hyperfine interaction model best describes nature.     

Combined chiral dynamics and MIT bag model study of strong Σ<Superscript>(*)</Superscript><Subscript>Q</Subscript>→Λ<Subscript>Q</Subscript>π decays

The strong decays of the heavy baryons Σ^(*) _Q→Λ_Qπ are studied by combining the chiral dynamics and the MIT bag model. In the charm sector, we calculate the decay widths Γ(Σ^(*) _c→Λ_cπ) and compare these with the experimental data and other theoretical estimations. In addition, we also predict the strong decay widths Γ(Σ^(*) _b→Λ_bπ).     

Strong decays Σ*→Σπ,Λπ and related strong coupling constant

We calculate the strong coupling constant g among the decuplet baryons, the octet baryons and the pseudoscalar mesons in the heavy baryon chiral perturbation theory with the light-cone QCD sum rules, and we study the strong decays Σ ^*→Λ π,Σ π. The numerical value of the strong coupling constant g is consistent with our previous calculation; the central values lead to small SU(3) breaking effects, less than 6%; and no definitive conclusion can be drawn due to the large uncertainties.     

Possible tetraquark states in the <Emphasis Type="Italic">π</Emphasis><Superscript>+</Superscript><Emphasis Type="Italic">χ</Emphasis><Subscript><Emphasis Type="Italic">c</Emphasis>1</Subscript> invariant-mass distribution

In this article, we assume that there exist hidden charmed tetraquark states with spin–parity J ^P=1⁻, and we calculate their masses with the QCD sum rules. The numerical result indicates that the masses of the vector hidden charmed tetraquark states are about M _Z =(5.12±0.15) GeV or M _Z =(5.16±0.16) GeV, which are inconsistent with the experimental data on the π ⁺ χ _c1 invariant-mass distribution. The hidden charmed mesons Z ₁, Z ₂ or Z may be scalar hidden charmed tetraquark states, hadro-charmonium resonances or molecular states.     

Helicity amplitudes and electromagnetic decays of hyperon resonances

We present results for the helicity amplitudes of the lowest-lying hyperon resonances Y^*, computed within the framework of the Bonn Constituent-Quark model, which is based on the Bethe-Salpeter approach. The seven parameters entering the model were fitted to the best-known baryon masses. Accordingly, the results for the helicity amplitudes are genuine predictions. Some hyperon resonances are seen to couple more strongly to a virtual photon with finite Q² than to a real photon. Other Y^*'s, such as the S₀₁(1670) Λ-resonance or the S₁₁(1620) Σ-resonance, couple very strongly to real photons. We present a qualitative argument for predicting the behaviour of the helicity asymmetries of baryon resonances at high Q².-1     

Spin effects and the production of heavy baryons in Z0 boson decays

Predictions are made within the scope of perturbative quantum chromodynamics and the nonrelativistic two-quark baryon model of the magnitude and dependence uponz (the fraction of the momentum of the heavy quark carried off by the baryon) of the longitudinal spin asymmetry parameterA _RL in the direct creation of Λ _b and Λ _c baryons in theZ ⁰-boson channel in the LEPe ⁺ e ⁻ collider. An estimate is found for the value of in the cascade production of Λ _b baryons. Samara State University, Samara. Translated from Izvestiya Vysshikh Uchebnykh Zavedenii, Fizika, Vol. 42, No. 2, pp. 66–75, February, 1999.     

Magnetic moments of baryons in higher symmetry schemes including charm

Relations among the magnetic moments of charmed and uncharmed baryons are derived in the framework of SU(4) and SU(8) symmetries. The SU(3) resultμ (Σ ^o)=−μ(Λ) is not present in SU(4), but is obtained in SU(8). Higher order effects are further considered to improve the situation.     

Strange multibaryon states with Λ hyperon systems in pA collision at 10 GeV/<Emphasis Type="Italic">c</Emphasis>

Experimental data from the 2m propane bubble chamber have been analyzed for exotic baryon states search. A number of peculiarities were found in the effective mass spectra of Λπ⁺ (Σ*⁺ (1382), PDG), Λp, and Λpp subsystems. A few events detected on the photographs of the propane bubble chamber exposed to a 10 GeV/c proton beam were interpreted as S = −2H ⁰ light(<M _(ΛΛ)) and heavy H ^{0, +}. A new event detected on the photographs of the propane bubble chamber was interpreted as heavy H ⁺ (2488) dibaryon by weak decay channels of K ⁻ pp or Σ⁺ pπ⁻. The text was submitted by the author in English.     

Rare decays of Λb→Λ+γ and Λb→Λ+l+l? in the light-cone sum rules

The weak decays of Λ _b →Λ+γ and Λ _b →Λ+l ⁺ l ⁻ are investigated in the standard model using the light-cone sum-rule approach. The higher-twist distribution amplitudes of the Λ baryon to the leading conformal spin are included in the sum rules for the transition form factors. Our results indicate that the higher-twist distribution amplitudes almost have no influence on the transition form factor f ₂(q ²) defined by the tensor current, while such corrections can have a significant impact on the form factors g ₂(q ²) defined by a vector current. Two phenomenological models (COZ and FZOZ) for the wave function of the Λ baryon are also employed in the sum rules for comparison, which can give rise to form factors approximately five times larger than that in terms of conformal expansion. Utilizing the form factors calculated in LCSR, the physical observables like the decay rate, the polarization asymmetry and forward–backward asymmetry are analyzed for the decays of Λ _b →Λ γ, Λ l ⁺ l ⁻.     

Two body non-leptonic Λ b decays in the largeN c limit

The two body non-leptonic Λ _b decays are analysed in the HQET with factorization approximation and largeN _c limit. In this limit, Λ _b and Λ _c baryons can be treated as the bound states of chiral soliton and heavy meson, and consequently the Isgur-Wise function comes out in a straight forward manner. The results obtained remain well below their previously predicted upper limit.     

Discovery potentials of doubly charmed baryons

The existence of doubly heavy flavor baryons has not been well established experimentally so far. In this Letter we systematically investigate the weak decays of the doubly charmed baryons, Ξ~(++)_(cc)and Ξ~+_(cc), which should be helpful for experimental searches for these particles. The long-distance contributions are first studied in the doubly heavy baryon decays, and found to be significantly enhanced. Comparing all the processes, Ξ~(++)_(cc)→Λ~+_cK~-π~+π~+andΞ~+_cπ~+ are the most favorable decay modes for experiments to search for doubly heavy baryons.     

Study of inclusive strange-baryon production and search for pentaquarks in two-photon collisions at LEP

Measurements of inclusive production of the Λ, Ξ^- and Ξ^*(1530) baryons in two-photon collisions with the L3 detector at LEP are presented. The inclusive differential cross sections for Λ and Ξ^- are measured as a function of the baryon transverse momentum, p_t, and pseudo-rapidity, η. The mean number of Λ, Ξ^- and Ξ^*(1530) baryons per hadronic two-photon event is determined in the kinematic range 0.4 GeV<p_t<2.5 GeV, |η|<1.2. Overall agreement with the theoretical models and Monte Carlo predictions is observed. A search for inclusive production of the pentaquark θ⁺(1540) in two-photon collisions through the decay θ⁺→pK⁰ _S is also presented. No evidence for production of this state is found.     

Charm and bottom baryons in nonperturbative quark dynamics

We use the field correlator method in QCD to calculate the masses of Σ _c , Ξ _c , and recently observed Σ _b , Ξ _b baryons and their orbital excitations. The text was submitted by the authors in English.     

Photoproduction of baryons decaying into Nπ and Nη

A combined analysis of photoproduction data on γp↦ πN, ηN was performed including the data on KΛ and KΣ. The data are interpreted in an isobar model with s-channel baryon resonances and π, ρ (ω), K, and K ^* exchange in the t-channel. Three baryon resonances have a substantial coupling to ηN, the well-known N(1535)S ₁₁, N(1720)P ₁₃, and N(2070)D ₁₅. The inclusion of data with open strangeness reveals evidence for further new resonances, N(1840)P ₁₁, N(1875)D ₁₃ and, with weaker evidence, for N(2170)D ₁₃.     

On the nature of the lowest 1/2<Superscript>-</Superscript> baryon nonet and decuplet<Superscript>⋆</Superscript>

From our recent study of properties of the lowest spin-parity 1/2^- baryons, N ^*(1535) and Δ^*(1620) , new pictures for the internal structure of the lowest 1/2^- baryon nonet and decuplet are proposed. While the lowest 1/2^- baryon nonet may have large diquark-diquark-antiquark component, the lowest 1/2^- baryon decuplet is proposed to have large vector-meson-baryon components. Evidence for “missing" members of the new pictures is pointed out and suggestions are made for detecting these predicted states from forthcoming experiments.     

Decays of baryon resonances into ΛK<Superscript>+</Superscript>, ΣK<Superscript>+</Superscript> and Σ<Superscript>+</Superscript>K<Superscript>0</Superscript>

Cross-sections, beam asymmetries, and recoil polarisations for the reactions γp → K ⁺Λ;γp → K ⁺Σ⁰, and γp → K ⁰Σ⁺ have been measured by the SAPHIR, CLAS, and LEPS Collaborations with high statistics and good angular coverage for centre-of-mass energies between 1.6 and 2.3 GeV. The combined analysis of these data with data from π and η photoproduction reveals evidence for new baryon resonances in this energy region. A new P₁₁ state with mass 1840 MeV and width 140 MeV was observed contributing to most of the fitted reactions. The data demand the presence of two D₁₃ states at 1875 and, optimistically, at 2170 MeV.     

Search for exotic baryons at HERMES

An experimental search for the Θ(1540)- and Λ(1520)-resonance was performed in quasi-real photoproduction on deuterium at the HERMES experiment. While evidence for Θ(1540) was found in the decay channel pK ⁰ _S→pπ ⁺ π ^-, no evidence for the corresponding anti-particle was found. In some models it is expected that the Θ(1540) and the Λ(1520) have similar production mechanisms. The photoproduction cross-sections for the Λ(1520) in the decay channel Λ(1520)→pK ^- and the corresponding anti-particle are determined. The partial photoproduction cross-sections for Λ(1520) and ˉ(1520) are obtained as σ _Λ(1520) = 65.3±8.8(stat)±6.9(syst)nb and σ _ˉ(1520) = 9.8±2.6(stat)±0.9(syst)nb, corresponding to a ratio R _Λ(1520) = σ _ˉ(1520)/σ _Λ(1520) = 0.15±0.05(stat)±0.02(syst).     

Molecular relaxations in mixtures of O2 with CO2 observed on?laser-induced gratings

Laser-induced gratings in mixtures of O₂ with CO₂ were formed by excitation of the O₂ molecules to the singlet state b ¹ Σ _g ⁺(v′=0). Density changes from heat release and from electrostriction, and variation of electric polarizability by excitation of molecules contribute to the grating. For modeling the relaxation of the excited O₂ molecules, a three-step process is assumed: Fast heat release with respect to the rotational states first, then medium fast electronic de-activation with excitation of vibrational states of the O₂ and CO₂ molecules, and then the final slow heat release processes. The observed temporal evolution of the diffraction efficiency of the grating agrees rather well with modeling. Average rates of the final relaxations and the value of the polarizability of the singlet state b ¹ Σ _g ⁺(v′=0) are determined in this way.