The semihard processes γγ→ΨX, γγ→ΨΨ, γγ→ϱΨ

The cross sections for Ψ-production in the semihard collisions are calculated at s ⪢ |t|, m_Ψ² and at not too large a jet effective mass m_X²≲m_Ψ² + |t| for the process γγ→ΨX. We found these cross sections in the lowest nontrivial QCD order (two-gluon exchange in the t-channel): σ(γγ→ΨX) ≈ 10³⁵ cm², σ(γγ→ΨΨ)=2×10⁻³⁶ cm². The polarization of Ψ in these processes is transversal (contrary to the longitudinal one of light mesons ϱ⁰, ω, ϕ in similar processes).     

The reaction pp→pΛK + near threshold

We analyze the recent total cross section data for pp→pΛK ⁺ near threshold measured at COSY. Using an effective range approximation for the on-shell pΛ S-wave final state interaction we extract from these data the combination ?= (2|K _s|²+|K _t|²)^−1/2= 0.38 fm⁴ of the singlet (K _s) and triplet (K _t) threshold transition amplitudes. We present an exploratory calculation of various (tree-level) vector and pseudoscalar meson exchange diagrams. Pointlike ω-exchange alone and the combined (ρ⁰,ω,K^*+)-exchange can explain the experimental value of ?. The pseudoscalar meson exchanges based on a SU(3) chiral Lagrangian turn out to be too large. However, when adding π⁰-exchange in combination with the resonant πN→S ₁₁(1650) →KΛ transition and introducing monopole form factors with a cut-off Λ_c= 1.5 GeV one is again able to reproduce the experimental value of ?. More exclusive measurements are necessary to reveal the details of the pp→pΛK ⁺ production mechanism. Received: 28 October 1998 / Revised version: 12 January 1999     

Theoretical description of the pd → pdη reaction near threshold

The contributions of three different types of driving terms are included in the estimation of the pd → pdη reaction at low energies. Near threshold, it is predicted that a two-step model involving an intermediate pion should be the most important but, as the energy approaches the threshold for η production in the free nucleon-nucleon reaction, a pick-up mechanism with a spectator proton should become dominant. The total cross-sections are underestimated by about a factor of two compared with experimental data but the discrepancies in the angular distributions are more serious, with no sign in the data for the peaks corresponding to the pick-up diagram.     

Charmless Hadronic Decays B→VV in the Topcolor-Assisted Technicolor Model

Based. On the effective Hamiltonian with the generalized factorization approach, we calculate the branchingratios and CP asymmetries of B → VV decays in the Topcolor-assisted Technicolor (TC2) model. Within the consideredparameter space we find that: (a) for the penguin-dominated B → K* φ and K*0 φ decays, the new physics enhancementsto the branching ratios are around 40%; (b) the measured branching ratios of B →K* φ and K*0φ decays prefer therange of 3 Neffc 5; (c) the SM and TC2 model predictions for the branching ratio B(B →ρ ρ0) are only about halfof the Belle‘s measurement; and (d) for most B → VV decays, the new physics corrections on their CP asymmetries are generally small or moderate in magnitude and insensitive to the variation of mπ and Neffc.     

np → ηd near threshold and the s -wave ηN amplitude

We calculate the process np → ηd near threshold using a separable potential model of the coupled ηN - πN - ππN subystems, and a relativistic three-body calculation for the ηd scattering amplitude. The ππN channels are represented by an effective σN channel, and we compare the case where the σ and π masses are related by m _σ = 2m _π and no width is considered, to another where the mass and width of the σ -meson are taken from ππ scattering data. The np → ηd cross-section can be well described up to about 60MeV by models where the real part of the ηN scattering length lies between 0.4≤Re(a _ηN)≤0.6 fm which allows us to determine the s -wave ηN scattering amplitude for -60≤E≤60 MeV.     

Spin and isospin effects in the NN → NKΛ reaction near threshold

The spin and isospin structure of the amplitudes and observables for K⁺Λ production in nucleon-nucleon collisions in the near-threshold region is analysed. Five experiments are required in order to isolate the amplitudes, up to an overall phase and two discrete ambiguities, in a model-independent way. It is shown that, with reasonable values for the relative strengths of the π and ρ terms in a meson-exchange model, one expects production on the neutron to be significantly stronger than that on the proton. Negative values of the spin-transfer coefficient D_NN are also predicted due to π-ρ interference.     

Charmless Hadronic Decays B→VV in the Topcolor－Assisted Technicolor Model

Based on the effective Hamiltonian with the generalized factorization approach, we calculate the branching ratios and CP asymmetries of B→VV decays in the Topcolor-assisted Technicolor (TC2) model. Within the considered parameter space we find that: (a) for the penguin-dominated B→K^*+φ and K^*0φ decays, the new physics enhancements to the branching ratios are around 40%; (b) the measured branching ratios of B→K^*+φ and K^*0φ decays prefer the range of 3≤N_c^eff≤5; (c) the SM and TC2 model predictions for the branching ratio B(B⁺→ρ⁺ρ⁰) are only about half of the Belle's measurement; and (d) for most B→VV decays, the new physics corrections on their CP asymmetries are generally small or moderate in magnitude and insensitive to the variation of m_π and N_c^eff.     

Is ionization adiabatic near threshold?

Time-dependent perturbation theory is applied to a two-level model of neon to predict the energy dependence of 2p → 3p shakeup accompanying ionization. In this picture, the outgoing electron is treated as a classical particle moving in the potential of the nucleus and the other electrons. The time dependence of the perturbation is calculated numerically and integrated to give the shakeup probability. For core ionization this model predicts the probability for shakeup at the shakeup threshold to be about 79% of the value at the sudden limit. For 2p ionization the corresponding ratio is 67%. Thus, even at the threshold the process is closer to the sudden limit than to the adiabatic limit. Experimental results are in agreement with these theoretical predictions.     

On momentum dependence of the reaction π− p→ωn near threshold

We discuss the near–threshold behavior of the ω production amplitude in the reaction π⁻ p→ωn. In contrast to the results of earlier analyses we find that the averaged squared matrix element of the production amplitude must be a decreasing function of energy in order to describe the existing experimental data. Received: 11 August 1999     

The Z → c \bar c → γγ*, Z → b \bar b → γγ* triangle diagrams and the Z → γψ, Z → γϒ decays

The approach to the Z → γψ and Z → γ? decay study is presented in detail, based on the sum rules for the Z → c $ \bar c $ → γγ* and Z → b $ \bar b $ → γγ* amplitudes and their derivatives. The branching ratios of the Z → γψ and Z → γ? decays are calculated for different hypotheses on saturation of the sum rules. The lower bounds of Σ _ψ BR(Z → γψ) = 1.95 × 10^?7 and Σ_υ BR(Z → γ?) = 7.23 × 10^?7 are found. Deviations from the lower bounds are discussed, including the possibility of BR(Z → γJ/ψ(1S)) ～ BR(Z → γ?(1S)) ～ 10^?6, that could be probably measured in LHC. The angular distributions in the Z → γψ and Z → γ? decays are also calculated.     

Measurements of the γγ＊ → π^0 and γγ＊ → ηc transition form factors at BABAR

We present measurements of the γγ＊ → π^0 transition form factor for the momentum transfer range Q^2=4-40 GeV^2 and the γγ＊ → ηc transition form factor for the range Q^2=2-50 GeV^2. The current status of measurements of the meson-photon transition form factors for the η and η＇ mesons is discussed. The results of the measurement of the ηc mass, total and two-photon widths are also presented.     

Calculation of Total Cross Section for γγ→γZ Scattering at High Energy

The total cross sections of γγ→γZ process for γγ c.m.s. energy 91.2—2000GeV are presented. W loop contribution is dominating when γγ c.m.s. energy is larger than 140GeV. The total cross section has a maximum, 220 fb(|cosθ|33cm^－2·s^－1 in which case more than 6000 events for γγ→γZ would be observable. In principle this process provides a test of the non-abelian nature of the standard model, especially the anomalous triple and quartic W-boson vertices.     

ηproduction in proton-nucleus collisions near threshold

The η-meson production in proton-nucleus(pA)collisions near threshold is studied within a relativistic meson-exchange model.The primary production amplitude is presented in the distorted-wave impulse approximation for the nucleus with isospin 0 or [1]by assuming that N*(1535)is excited via a meson exchange and then decays into η and nucleon pair(ηN).Taking 18O and 12C nuclei as examples,we evaluate the production cross sections as a function of the incident proton energy,and analyze the effects of nuclear medium and various meson-exchange contributions.Finally we discuss implications for further experimental studies at the Cooling Storage Ring(CSR)in Lanzhou.     

η production in proton-nucleus collisions near threshold

The η-meson production in proton-nucleus (pA) collisions near threshold is studied within a relativistic meson-exchange model. The primary production amplitude is presented in the distorted-wave impulse approximation for the nucleus with isospin 0 or 1 by assuming that N^*(1535) is excited via a meson exchange and then decays into η and nucleon pair(ηN). Taking ¹⁸O and ¹²C nuclei as examples, we evaluate the production cross sections as a function of the incident proton energy, and analyze the effects of nuclear medium and various meson-exchange contributions. Finally we discuss implications for further     

The High Energy γγ→γγ Scattering Cross Sections

Contributions of fermion loops, W-boson loops and their sum to the high energy γγ -- γγ scatteringtotal cross sections (1 cosθ＜ cos30) are calculated by analytical expressions of γγ - γγ scattering amplitude. Thesecontributions may be observed in the future photon linear collider and may be uscd to test standard model. Thecontribution of fermions' loops is a half of that in R.K. Arphlus and M. Neuuman's paper (Phys. Rev. 80 (1950) 380;83 (1951) 776).     

Inelastic final-state interactions in B → VV →π K processes

We study the final-state interactions in B →π K decays through B → VV →π K processes where the inelastic rescattering occurs via single pion exchange. The next-to-leading order low-energy effective Hamiltonian and Bauer-Stech-Wirbel (BSW) model are used to evaluate the weak transition matrix elements and final-state interactions. We found that the final-state interaction effects in B →ρ K^*→π K processes are significant. The Fleischer-Mannel relation for the Cabibbo-Kobayashi-Maskawa (CKM) angle γ can be significantly modified. Received: 13 August 1998