Strangeness production in proton collisions close to threshold at COSY-TOF

The K-meson production in elementary proton induced reactions is studied exclusively at the external COSY beam using the time-of-flight spectrometer TOF. The complete measurement of all primary and decay particle tracks allows the extraction of total and differential cross sections as well as Dalitz plots and invariant mass spectra of the subsystems for the channels pp → K⁺ Δp, K⁰Σ⁺p, K⁺Σ⁰p and K⁺Σ⁺n. For all channels the full phase space is covered from the reaction threshold up to the COSY-limit of about 3.5 GeV/c. Especially the analysis of the Dalitz plots of the channel pp→K⁺Δp show a strong influence of N₋^* resonances.     

Final-state hyperon-nucleon interaction in the inclusive K photoproduction from the deuteron

We calculate d(γ, K⁺) inclusive cross sections with the full inclusion of the final ΛN - ΣN interaction. Modern hyperon-nucleon forces and a recently updated production operator for the γ + N → K⁺ + Y process are used. Significant effects of the hyperon-nucleon final-state interaction have been found especially around the K⁺ΣN threshold.     

Probing the EOS and nn cross section by analyzing the collective flow at high and intermediate energy in the RVUU approach

We have studied the collective flow at high and intermediate energy in a relativistic Vlasov-Uehling-Uhlenbeck (RVUU) approach based on Walecka's QHD-I model, with the aim to probe the nuclear-matter equation of state (EOS) and the in-medium nucleon-nucleon cross section σ. At high energy (1.2 GeV/u), the out-of-plane azimuthal correlation function C(Ψ) is only sensitive to the effective mass m^* and insensitive to the nuclear compressibility K and the effective nucleon-nucleon cross section σ within a reasonable range. We have found that the preferred value of m^* is about 0.85 m. With this value of m^*, from the in-plane mean transverse momentum P_x(Y) which is sensitive to both m^* and σ we have drawn an effective nn cross section σ, namely σ 0.8σ_f where σ_f is the free nucleon-nucleon cross section in Cugnon's parametrization. Taking advantage of the fact that the energy of vanishing flow (EVF) at intermediate energy (around 100 MeV/u) is only sensitive to the nucleon-nucleon cross section σ, we have drawn some information on the nucleon-nucleon cross section σ, namely σ = (1.4±0.2)σ_f.     

Deep processes in nuclei and in-medium dressing of the nucleons and mesons

K⁺ scattering and quasielastic electron scattering from nuclei are expected to provide information about the nucleons and mesons in the inner regions of nuclei. The K⁺- nucleus cross sections and the quasielastic electron-nucleus response functions have been calculated taking into account the same in-medium dressing of the nucleons and the same coupling of the σ and ω mesons to the polarization of nuclear matter. We obtain a good agreement with experimental data for the two processes.     

U-spin symmetry in doubly Cabibbo-suppressed charmed meson decays

We prove a U-spin amplitude triangle relation among doubly Cabibbo-suppressed (DCS) charmed meson decays, D⁰→K⁺π⁻,D⁰→K⁰π⁰ and D⁺_s→K⁰K⁺, congruent to an isospin relation among corresponding Cabibbo-favored (CF) decays. U-spin breaking in relative phases between CF and DCS amplitudes affects time-dependent studies of D⁰– mixing. Comparison of final state phase patterns in DCS and CF amplitude triangles, which can shed some light on these phases, is carried out in a phenomenological framework incorporating resonance contributions.     

Trying to catch the elusive A and more. Reactions initiated by b-quarks and the Higgs sector of the MSSM

We study the cross sections for the production of neneural, intermediate mass Higgs boson in the process pp→ tq′φ, pp→tW⁻φ and pp→bZ⁰φ in the Minimal Supersymmetric Standard Model (φ = H⁰, h⁰ and A⁰) at Supercollider energies. The additional heavy particles (t, W, Z) in the final state can be used for tagging purposes, increasing the signal to background ratio. These reactions are dominated by bg and bg fusion. Their relevance for Higgs particle searches is discussed taking into account the expected efficiencies and purities for b-tagging. We find that, for tan β = 30, the cross sections for pp → bZ⁰φ are larger than 14 pb, over the whole intemediate range of M_A⁰, for A⁰ and at least one of the other two Higgses. Therefore this reaction is an excellent candidate for the discovery of one or more MSSM Higgs particles.     

Study of chiral property of nuclear matter through measurements of Φ meson decays

The experiment E325 being performed at KEK 12GeV-PS is to investigate the chiral property of QCD in dense matter through the measurements of vector mesons which decay inside nucleus. The status of the experiment and the preliminary mass distribution of Φ → K⁺K⁻ decays are presented.     

Hadron and nuclear physics with inverse compton gamma-rays at SPring-8

A new facility for GeV γ-ray beams in an energy range of 1.5–2.4 GeV has been constructed for developing hadron physics at SPring-8. GeV γ-rays are produced via the inverse Compton scattering process, and are used to study the sub-nucleonic structure of nucleon via the productions of φ and K⁺ mesons. The recent experimental results to observe φ and K⁺ mesons are discussed to look for the physics in relation to strangeness quarks in baryon resonances. A new beam line for MeV γ-rays at SPring-8 to give new possibilities for the studies of the weak-strong coupling effects in nuclear mediums through the observation of parity violation is also discussed.     

Hadronic production by ee collisions at the energy 990 MeV with the Orsay storage ring

Data were taken at the energy 2E = 990 MeV to search for multibody events, with the same large solid angle detector which has been used for the measurement of the , ω andφ production by e⁺e⁻ annilations. Assuming a π⁺π⁻π⁰π⁰ production by the quasi two-body process e⁺e⁻ → → ωπ⁰ we give the correspondi ng cross section σ(e⁺e⁻ → π⁺π⁻π⁰π⁰) = (1.1 ± 0.5) 10⁻³² cm². Since no events with 3 and 4 charged pions have been observed σ(e⁺e⁻ → π⁺π⁻π⁰π⁻) 1.5 × 10⁻³³ cm².     

Evidence of kaon nuclear and Coulomb potential effects on soft K production from nuclei

The ratio of forward K⁺ production on copper, silver and gold targets to that on carbon has been measured at proton beam energies between 1.5 and 2.3 GeV as a function of the kaon momentum p_K using the ANKE spectrometer at COSY-Jülich. The strong suppression in the ratios observed for p_K<200–250 MeV/c may be ascribed to a combination of Coulomb and nuclear repulsion in the K⁺A system. This opens a new way to investigate the interaction of K⁺-mesons in the nuclear medium. Our data are consistent with a K⁺A nuclear potential of V_K⁰≈20 MeV at low kaon momenta and normal nuclear density. Given the sensitivity of the data to the kaon potential, the current experimental precision might allow one to determine V_K⁰ to better than 3 MeV.     

What can we learn from B→DsKπ, Bd→DsKπ and Bd→DsK decays?

We study the nonresonant three-body decays of B⁺→D^(*)−_sK⁺π⁺ and B_d→D_s^(*)−K⁰π⁺. We find that these decays can provide the information on the time-like form factors of D^(*)_sK. We also explicitly investigate B_d→D_s^(*)−K^*+ decays by discriminating the nonresonant contributions with the unknown D^(*)_s wave functions being fixed by the measured mode of B_d→D_s⁻K⁺.     

Study of the decay

Using data from the FOCUS (E831) experiment at Fermilab, we present a new measurement for the Cabibbo-suppressed decay mode D⁰→K⁺K⁻π⁺π⁻. We measure: Γ(D⁰→K⁺K⁻π⁺π⁻)/Γ(D⁰→K⁻π⁻π⁺π⁺)=0.0295±0.0011±0.0008. An amplitude analysis has been performed in order to determine the resonant substructure of this decay mode. The dominant components are the decays D⁰→K₁(1270)⁺K⁻, D⁰→K₁(1400)⁺K⁻ and D⁰→ρ(770)⁰(1020).     

Comments on CP, T and CPT violation in neutral kaon decays

We comment on CP, T and CPT violation in the light of interesting new data from the CPLEAR and KTeV Collaborations on neutral kaon decay asymmetries. Other recent data from the CPLEAR experiment, constraining possible violations of CPT and the ΔS=ΔQ rule, exclude the possibility that the semileptonic-decay asymmetry A_T measured by CPLEAR could be solely due to CPT violation, confirming that their data constitute direct evidence for T violation. The CP-violating asymmetry in K_L→e⁻e⁺π⁻π⁺ recently measured by the KTeV Collaboration does not by itself provide direct evidence for T violation, but we use it to place new bounds on CPT violation.     

Rare B → Kℓℓ decay, two Higgs doublet model, and light cone QCD sum rules

The decay width, forward-backward asymmetry and lepton longitudinal and transversal polarization for the exclusive B → K^*ℓ⁺ℓ⁻ decay in a two Higgs doublet model are computed. It is shown that all these quantities are very effective tools for establishing new physics.     

Double pion photoproduction on the nucleon: study of the isospin channels

A model for the γp → π⁺π⁻p reaction developed earlier is extended to account for all isospin channels. The model includes N, Δ(1232), N^*(1440) and N^*(1520) as intermediate baryonic states and the meson as an intermediate 2π resonance. Although many terms contribute to the cross section, some channels exhibit particular sensitivity to certain mechanisms of resonance excitation or decay and the reactions provide novel information on such mechanisms. In particular the γN → N^*(1520) → Δπ process affects all the channels and is a key ingredient in the interpretation of the data. Comparison is made with all available data and the agreement is good in some channels. The remaining discrepancies in some other channels are discussed.     

Spectroscopy of light Λ hypernuclei

We recently measured high-statistics and high-resolution spectra of p-shell Λ hypernuclei produced by the (π⁺, K⁺) reaction. In this article, cross sections and their angular distributions of _Λ¹²C and _Λ¹⁶O are reported. The obtained cross sections are well reproduced by DWIA calculations.     

A search for neutral heavy leptons

In e⁺e⁻ annihilations at SPEAR we have searched for narrow neutral resonances in the eπ and μπ channels for masses between 0.5 GeV/c² and 1.5 GeV/c². No evidence for such states has been found. Under standard theoretical assumptions for production cross sections and decay branching ratios of heavy leptons a lower limit of 1.2 GeV/c² can be set on the mass.     

Electro-pion production on O to bou nd states of the residual nuclei

Differential cross sections for the reactions 16O(γ, π−π+−)16F16F to the sum of the four lowest lying states in 16F and 16N have been measured as a function of angle for pions with a kinetic energy of 30 MeV. The extracted ratios R = σ(γ, π−)/σ(γ, π+), the first ones to discrete final states as a function of angle, are in fair agreement with results obtained for the nucleon. For positive pions the energy dependence of the cross section has been measured at the angles of 45° and 90°. Distorted wave impulse approximation calculations fail to describe the energy dependence.     

Double charge exchange and one-pion production in π He collisions at 1.7 GeV/c

We present the results of a study of the exclusive reactions π^{+ 4}He → π⁻ 4p (double charge exchange, DCE) and π^{+ 4}He → π⁺ π⁻ 3pn (one-pion production). The experiment was performed with the Oxford/RHEL helium bubble chamber irradiated with a 1.7 GeV/c π⁺ beam. A general review of DCE models is presented and experimental results at other energies are discussed in the light of these models. None of the existing theoretical models is compatible with out data at 1.7 GeV/c. We propose a new mechanism for DCE, involving three nucleons, which reproduces the integrated as well as the differential cross sections. Some consequences for other models are also discussed.     

Search for narrow NNπ resonances in exclusive pp→ppππ measurements

Narrow structures in the range of a few MeV have been searched for in ppπ⁺ and ppπ⁻ invariant mass spectra (M_ppπ⁺ and M_ppπ⁻) obtained from exclusive measurements of the pp→ppπ⁺π⁻ reaction at T_p=725,750 and 775 MeV using the PROMICE/WASA detector at CELSIUS. The selected reaction is particularly well suited for the search for dibaryon resonances decoupled from NN and/or NΔ. In the mass range 2020 MeV/c²<m_dibaryon<2085 MeV/c² no narrow structures could be identified on the 3σ level of statistical significance neither in M_ppπ⁻ nor in M_ppπ⁺ giving an upper limit (95% C.L.) for dibaryon production in this reaction of σ<20 nb.