On the possibility of observation of a 0-f 0 mixing in the pn → da 0 reaction

It is shown that, if isospin is not conserved in a ₀ ⁰ -and f ₀-meson mixing, forward-backward asymmetry arises in the pn→da ₀ ⁰ reaction. This effect increases near the reaction threshold. The asymmetry is estimated within the framework of a model in which the a ₀ ⁰ -f ₀ mixing is due to the virtual π⁰?η transition and the difference in masses of the charged and neutral kaons in decay channels. The angular asymmetry near the threshold of the pn→da ₀ ⁰ reaction was found to be large, of the order of 8–15%.     

Compton scattering on the charged pion and the process γγ→π0π0

The Compton scattering on a charged pion and the process γγ→π⁰π⁰ are studied using the dispersion relations. Unknown parameters of the S-wave ππ interaction and a sum and a difference of the π⁰ meson polarizabilities are found from a fit to the experimental data for the γγ→π⁰π⁰ process. The found parameters of the ππ interaction are used for the calculation of the cross section of the elastic γπ^± scattering. The analysis of the obtained results shows that the experimental data for the elastic γπ^± scattering in the energy region up to 1 GeV together with the data for the γγ→π⁰π⁰ process could be used both for a determination of the pion polarizability values and for study of the S-wave ππ interaction. Received: 20 November 1998 / Revised version: 22 January 1999     

Probe the R-Parity Violating Supersymmetry Effects in the B0s—B0s Mixing

The recent measurements of the B_s mass difference Δ M_s by the CDF and DO collaborations are roughly consistent with the Standard Model predictions, therefore, these measurements will afford an opportunity to constrain new physics scenarios beyond the Standard Model. We consider the impact of the R-parity violating supersymmetry in the B0s—B0s mixing, and use the latest experimental results of ΔM_s to constrain the size of the R-parity violating tree level couplings in the B0s—B0s mixing. Then, using the constrained RPV parameter space from ΔM_s, we show the R-parity violating effects on the B_s width difference ΔΓ_s.     

Double- π0 photoproduction from the deuteron

The photoproduction of two neutral pions from the deuteron has been studied for incident photon energies from 200 MeV to 820 MeV with the TAPS spectrometer at MAMI (Mainz Microtron). The total cross-section was determined and used to deduce the cross-section from the neutron. Due to the good statistical quality of the data Dalitz plots for the three particles in the exit channel (π⁰π⁰ N) could be constructed. The invariant mass distributions derived from them are presented in this paper. They indicate that the important reaction mechanism in the second resonance region is a sequential decay pattern involving the population of the Δ(1232)-resonance as an intermediate state. Received: 5 June 2000 / Accepted: 4 September 2000     

Threshold π0 photoproduction from the Skyrmion

The Skyrme model for nucleons is used to calculate threshold π⁰ photoproduction off a proton. It was found that the anomalous part of the electromagnetic current yields an important contribution to the photoproduction amplitude, roughly of the same magnitude normally ascribed to ω meson exchange. Agreement with the experimental threshold cross section can thus be achieved without invoking ω exchange.     

Upper limit for the Ξ0→Σ0γ radiative decay

We report here a new measurement of the upper limit for the branching ratio of the radiative weak decay Ξ⁰→Σ⁰γ of < 8.0×10⁻³ at the 90% confidence level. This is more than an order of magnitude less than previous limits and consistent with rates from similar decays.     

Measurement of the matrix element for the decay η′ → ηπ 0 π 0 with the GAMS-4π spectrometer

The parameters of the Dalitz diagram for the decay η′ → ηπ ⁰ π ⁰ in a generalized and a linear representation were determined by using a statistical sample of 15 × 10³ events. A charge-exchange reaction at a momentum of 32 GeV/c was a source of monoenergetic η′ mesons. The experiment was performed with the aid of the GAMS-4π spectrometer.     

The decay h0 → A0 → A0A0: a complete one-loop calculation in the MSSM

In the minimal supersymmetric standard model the decay h⁰ → A⁰A⁰ of the light neutral scalar h⁰ is kinematically allowed for low values of tan β when radiative corrections to the neutral Higgs masses are taken into account. The width of this decay mode is revisited on the basis of a complete one-loop diagrammatic calculation. We give the analytical expressions and numerical results and compare them with the corresponding ones from the simpler and compact approximations of the effective potential method and the renormalization group approach.     

Role of the Λ(1600) in the K^-p→Λπ^0π^0 reaction

Role of theΛ(1600)is studied in the K^-p→Λπ^0π^0 reaction by using the effective Lagrangian approach near the threshold.We perform a calculation for the total and differential cross sections by considering the contributions from theΛ(1600)andΛ(1670)intermediate resonances decaying intoπ^0∑^*0(1385)with∑^*0(1385)decaying intoπ^0Λ.Additionally,the non-resonance process from u-channel nucleon pole is also taken into account.With our model parameters,the current experimental data on the total cross sections of the K^-p→Λπ^0π^0 reaction can be well reproduced.It is shown that we really need the contribution from the A(1600)with spin-parity J^P=1/2^+,and that these measurements can be used to determine some of the properties of theΛ(1600)resonance.Furthermore,we also plot theπ0Λinvariant mass distributions which could be tested by the future experimental measurements.     

Where are the missing V0V0′ in γγ reactions?

The measured cross sections ofγγ→V⁰V^0′ are inconsistent with the measurements of the γγ total hadronic cross section above 2 GeV. Higher cross sections for γγ→ϱ⁰ϱ⁰ or additional V⁰V^0′ final states are needed to solve this inconsistency.     

Analysis of the coupling constants ga0ηπ0 and ga0η＇ π0 with light-cone QCD sum rules

In this article, we take the point of view that the light scalar meson a0（980） is a conventional qqstate, and calculate the coupling constants ga0ηπ0 and ga0ηπ0 with the light-cone QCD sum rules. The central value of the coupling constant ga0ηπ0 is consistent with that extracted from the radiative decay φ（1020） → a0（980）γ→ηπ0γ. The central value and lower bound of the decay width Γa0→ηπ0 =127＋8448 MeV are compatible with the experimental data of the total decay width Γa0（980） = （50-100） MeV from the Particle Data Group with a very model dependent estimation （the decay width can be much larger）, while the upper bound is too large. We give a possible explanation for the discrepancy between the theoretical calculation and experimental data.     

The renormalization of the nuclear chiral charge density in the isovector 0+↔0− — β transitions in the mass-16 triad

The 0⁺0^– -decay transitions in the mass-16 triad are studied within an extended shellmodel space. The pion-induced two-body chiral charge density strongly affects the partial half-lifes by reducing them by about four times as compared to the impulse-approximation predictions. Such an acceleration of these weak processes is required to produce agreement with the reported data.Presented at the symposium Mesons and Light Nuclei, Bechyn, Czechoslovakia, May 27–June 1, 1985.     

Measurement of the cross section for the e + e ? → ωπ0 → π0π0γ process in the energy range of 1.1–1.9 GeV

The cross section for the e ⁺ e ^? ?? ???⁰ ?? ??⁰??⁰?? process has been measured in the energy range of 1.1?C1.9 GeV. The measurement has been made with the Spherical Neutral Detector at the VEPP-2000 e ⁺ e ^? collider. The data in the energy range of 1.1?C1.4 GeV are in agreement with the earlier measurements with the SND and CMD-2 detectors. The cross section has also been measured above 1.4 GeV.     

TheK S 0 →π0 l + l − andK L 0 →π0 l + l − decays in the standard model and the extended Nambu-Jona-Lasinio model

TheK _S ^{0 0} l ⁺ l ^– andK _L ^{0 0} l ⁺ l ^– decays are considered within the framework of the Standard model in the one-photon exchange approximation, i.e.K _S,L ^{0 0}+ *⁰ +l ⁺ l ^–. To evaluate the low-energy matrix elements of theK ⁰(¯K ⁰)⁰+ * transitions, the constituentquark-loop approximation, based on the extended Nambu-Jona-Lasino (NJL) model, is used. The obtained results are compared with other theoretical estimates and the experimental bounds.     

K 0 → \bar K^0 and B 0 → \bar B^0 transitions in the minimal supersymmetric standard model involving explicit CP violation in the Higgs sector

Mixing in the systems of neutral K ⁰ and B ⁰ mesons is considered within the minimal supersymmetric standard model (MSSM) containing a type-II Yukawa sector and featuring an explicitCP violation in the Higgs potential. In the case of a strong mixing of CP-even and CP-odd states, the model admits the presence of a light charged Higgs boson. Basic mixing parameters are calculated. These include the mass difference Δm _LS between neutral kaons and the parameter ε, which characterizes the amount of an indirect CP violation (that is, that which arises owing to an ultraweak mixing of CP-invariant and CP-noninvariant components). In the limit of the low-energy one-loop approximation, it is shown that, for the K ⁰ mesons, the contribution of nonstandard-physics effects to the mass splitting of the neutral kaons and an indirect CP violation are very small and are weakly dependent on the mass of the charged Higgs boson. Under certain conditions, the nonstandard contributions for the B ⁰- $ \bar B_d^0 $ and B ⁰- $ \bar B_s^0 $ systems may become somewhat more substantial, which constrains the MSSM parameter space.     

Geometry of the effective Majorana neutrino mass in the 0νββ decay

The neutrinoless double-beta (0νββ) decay is a unique process used to identify the Majorana nature of massive neutrinos, and its rate depends on the size of the effective Majorana neutrino mass <m>_ee. We put forward a novel ‘coupling-rod’ diagram to describe <m>_ee in the complex plane, by which the effects of the neutrino mass ordering and CP-violating phases on <m>_ee are intuitively understood. We show that this geometric language allows us to easily obtain the maximum and minimum of |<m>_ee|. It remains usable even if there is a kind of new physics contributing to <m>_ee, and it can also be extended to describe the effective Majorana masses <m>_eμ, <m>_eτ, <m>_μμ, <m>_μτ and <m>_ττ which may appear in some other lepton-number violating processes.