ω粒子的辐射衰变

本文由质量的解析延拓得到ω→π+r过程的色散关系,讨论了某些有关不稳定粒子色散关系的问题。在一些简化假设下得到W_ω→π+r/W_ω→3π≈0.35。在η粒子与ω粒子量子数相同的情形下得到W_η→π+r/W_η→3π≈25,在计算中包含了三个π介子成对作用的效应。     

Isospin symmetry violating effects and scattering length extraction from kaon decays

The isospin symmetry breaking effects in the charged kaons decays to two or three pions are considered. In semileptonic decay K ^± → π ⁺ π ^?e^± ν (called K _e4) these effects turn out to be crucial for correct extraction of ππ scattering lengths. Taking in account electromagnetic interaction between the pions in the final state and isospin symmetry breaking due to different masses of charged and neutral pions allows to adjust the values of scattering lengths obtained from experimental data on K _e4 decay and predictions of Chiral Perturbation Theory (ChPT). Final state interactions of pions in the decay K ^± → π ^± π ⁰ π ⁰ leading to the anomaly (cusp) in the π ⁰ π ⁰ invariant mass distribution in the vicinity of charged pions’ threshold are discussed and recent results of accounting of the electromagnetic interaction among charged pions leading to π ⁺ π ^? bound states (pioniumatom) just under the charged pions’ threshold are presented.     

Azimuthal correlations and Bose Einstein effects in the reaction K+p → K+p2π+2π− AT 16 GeV/c

Azimuthal distributions between all possible particle pairs are studied in the reaction K⁺p → K⁺p2(π⁺π^?) at 16 GeV/c. It is shown that the hypothesis of uncorrelated transverse momenta at fixed rapidities of the final-state particles, is sufficient to describe the main features of the data. An interesting exception occurs for identical particle pairs where a genuine short-range correlation, attributed to Bose-Einstein symmetrization, is observed. The correlation length of this effect is shown to be related to the impact parameter distribution of the produced pions.     

p + p → d + π+ p-wave production

The production of p-wave pions in p + p → d + π⁺ is calculated by introducing into the initial wavefunction NΔ configurations consistent with elastic scattering. Reasonable agreement with experiment is obtained without further adjustment. The Galilean term has only a small effect.     

The differential cross section and polarization in p + p → d + π+

A coupled-channels approach is used for incorporating NΔ configurations into the mechanism of the meson production reaction p + p → d + π⁺. The differential cross section and the polarization asymmetry are calculated for lab energies ranging from threshold to 750 MeV including the partial waves up to f-wave pions and ³F state nucleons. It is found that the pp state ³F₃ is important in obtaining a reasonable agreement with the existing experimental data. We find a remarkable insensitivity of the differential cross section to the D-state probability of the deuteron, whereas the polarization is somewhat dependent on this.     

Measurement of elastic ω photoproduction at HERA ZEUS Collaboration

The reaction γp →ωp(ω → π⁺π^?π⁰ and π⁰ → γ γ) has been studied in ep interactions using the ZEUS detector at photon-proton centre-of-mass energies between 70 and 90GeV and ∣t∣ < 0.6GeV², where t is the squared four momentum transferred at the proton vertex. The elastic ω photoproduction cross section has been measured to be σ_{γp → ωp} = 1.21 ±0.12 ±0.23 μb. The differential cross section dσ_γp→ωp/d¦t¦ has an exponential shape e^?b∣t∣ with a slope b =10.0 ± 1.2 ± 1.3 GeV^?2. The angular distributions of the decay pions are consistent with s-channel helicity conservation. When compared to low energy data, the features of ω photoproduction as measured at HERA energies are in agreement with those of a soft diffractive process. Previous measurements of the ρ⁰ and ? photoproduction cross sections at HERA show a similar behaviour.     

Experimental tests of the factorisation of Regge trajectories in the proton fragmentation region

We present double differential cross sections for the reactions K^? + p → π^± + anything at 8.25 GeV/c. Using published data for reactions K⁺ + p→ π^? + anything and p + p → π⁺ anything we predict the differential cross sections for γ + p → π^? + anything, π^? + p → π^? + anything and π^? + p → π⁺ + anything in the target fragmentation region based on factorisation of the pomeron and Regge trajectories. Our predictions agree with the experimental data and support the hypothesis of factorisation of the Regge trajectories in the target fragmentation region.     

Photoproduction of negative pions on a polarized neutron target in the resonance region

At the Bonn 2.5 GeV electron synchrotron the first measurements of the target asymmetry for the reaction γ + n ↑ → π^? + p have been performed. The negative pions were detected in a magnetic spectrometer at a constant pion c.m. angle of 40° and photon energies between 0.45 GeV and 2.0 GeV. Deuterated butanol was used as target material. The polarization of the deuterons was about 16%. The results show a significant difference from the previously measured π⁺ asymmetry.     

Large-pT direct photon production and opposite-side photon-hadron correlations in QCD

Direct photon production in p-p, π-p and $\text{p}\text{-}\text{p}$ collisions at large transverse momenta (p_T) is studied in QCD and compared with existing data. Apart from the quark-gluon and quark-antiquark contributions, corrections due to bremsstrahlung of photons (q+q→q+q+γ) and to partons' intrinsic transverse momenta are taken into account. It is shown that p+p→γ+X provides one of the best determinations of the gluon distribution in the proton. Hadron production with an opposite-side large-p_T photon trigger is also studied. It is shown that the difference between the momentum sharing (x_e) distributions of π^?+p→γ+π^±+X and π⁺+p →γ+π^±+X (or of $\text{p}\text{+}\text{p}\text{→γ+π}{}^{\mathrm{\pm }}\text{+}\text{X}$ and p+p→γ+π^±+X) provides a good determinati on of the gluon fragmentation to a pion.     

Applications of Mueller theory of two-particle inclusive processes

Assuming factorization, and extracting inclusive Reggeon vertices from one-particle inclusive data, we predict Regge corrections to scaling for several two-particle inclusive processes. The data on p + P → π^? + π^? + X, K⁺ + p → π^? + π^? + X, K^? + p → π⁺ + π^? + X, and π⁺ + p → π^? + π^? + X agree with our predictions.     

Fractal diffusion processes in particle physics

A series of consistent measurements of kinematic variables for pion diffraction production processes by pions with an initial momentum of about 4 GeV/c were analyzed: π ⁺ + p → p + 2π ⁺ + π ^? and π ^? + p → p + 2π ^? + π ⁺. The Hurst method analysis discovered the presence of the memory effect for both data arrays. The distributions of the transition probability density appeared to seek some equilibrium shape, characteristic of the fractal Brownian motion (FBM). The process can be defined by the special diffusion Fokker-Planck equation (FPE). The obtained values of Hurst coefficient 0.5 < H < 1, which is a parameter of FPE, mean that the processes explored are realized in fractal generalized phase space with fractional dimension.     

Properties of the inclusive pion spectrum from pp collisions: 28.5 GeV/c to about 10 GeV/c

The spectra of pions are studied in the inclusive reactions p + p → π^± + anything in bubble chamber data at 28.5 GeV/c. Comparisons are made with ISR π⁺ spectra to examine energy dependence.     

The pd → tπ+ reaction in an isobar model: Angular distributions

Various wave function effects are studied in the differential cross section for the pd → tπ⁺ reaction. These include both S- and D-waves for the deuteron and the triton as well as the production of p-wave and d-wave pions. For the triton a parametrized Faddeev wave function is employed and for the deuteron a wave function of Hulthén or McGee type. The two-body correlations considered are NΔ(⁵S₂, ^5D₂, ⁵P₃) and NN(¹D₂) which are found to be the most important partial waves in the pp → dπ⁺ reaction.     

A pion-nucleus optical potential containing effects of genuine pion absorption and emission

We derive in two ways a lowest-order optical potential $\text{V}$_πA using the projectile and a pair of target nucleons as the basic interacting unit. The first derivation—a heuristic one—uses a multiple-scattering theory, while the second employs a field theory where pions can be absorbed and emitted as well as scattered. The lowest-order terms of $\text{V}$_πA contain the two-particle ground state density and the πNN → πNN scattering matrix. In the same field-theoretical model one analyzes the latter in standard multiple-scattering contributions and genuine absorption corrections. The form of these corrections and their relation to often ad hoc assumed forms is discussed.     

Radiative π0 photoproduction on protons in the Δ+ (1232) region

The reaction γp → pπ ⁰γ′ has been measured with the Crystal Ball/TAPS detectors using the energy-tagged photon beam at the electron accelerator facility MAMI-B. Energy and angular differential cross-sections for the emitted photon γ′ and angular differential cross-sections for the π⁰ have been determined with high statistics in the energy range of the Δ⁺ (1232)-resonance. Cross-sections and the ratio of the cross-section to the nonradiative process γp → pπ ⁰ are compared to theoretical reaction models, having the anomalous magnetic moment κ _Δ as free parameter. As the shape of the experimental distributions is not reproduced in detail by the model calculations, currently no extraction of κ _Δ is feasible.     

Nucleons after pion absorption

We calculate energy spectra and angular distributions of single and correlated spectra after the absorption of stopped pions in nuclei. We assume: The pion is abosrbed by a pair of nucleons; these primary nucleons may leave the nucleus directly or only after one or more collisions with other nucleons. We propose a multiple scattering expansion for the intranuclear cascade after pion absorption. Various experimental data are successfully described in shape and absolute magnitude. The ratio R_np = R(π^?np → nn)/R (π^? pp → np) is determined from the experimental proton spectra: R_np = 13 ± 6.     

Inclusive and semi-inclusive ϱ0 production in π−p interactions at 15 GeV/c

We have done an inclusive study of ?⁰ production in π^?p interactions at 15 GeV/c. Evidence for two different production mechanisms, in addition to the known quasi two-body processes, is presented: production of ?⁰'s in the central region of rapidity similar to the “pionization” region seen in the inclusive studies of pions; and forward production in the beam dissociation region of rapidity. Cross sections and rapidity distributions for the ?⁰ are given inclusively and by topology, and the results are compared with similar studies at other energies and with other incident particles. Results are also presented for the semi-inclusive reaction π^?p → p?⁰ + X^? and the exclusive final state pπ^??⁰.     

Large-pTπ0 production with pion beams and dominance of the subprocess quark + quark → baryon + antiquark

We compare the predictions of the subprocesses M + q → M + q, q + $\text{q}$ → M + M, q + q → B + q?, for large-p_Tπ⁰ inclusive production in π^?p collisions, with the data. Calculations performed with various models show, in a model independent way, that q + q → B + q? is dominant in pp collisions.