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     

Study of the a_1(1260) resonance in the γp→π~+π~+π~-n reaction

Within an effective Lagrangian approach and resonance model, we study the γp→a_1(1260)~+n andγp→π~+π~+π~-n reactions via the π-exchange mechanism. For the γp→π~+π~+π~-n reaction, we perform a calculation of the differential and total cross-sections by considering the contributions of the a1(1260) intermediate resonance decaying into per and then into π~+π~+π~-. Besides,the non-resonance process is also considered. With a lower mass of a1(1260), the experimental data for the invariant π~+π~+π~-mass distributions can be fairly well reproduced. For theγp→a_1(1260)~+n reaction, with the model parameters, the total cross-section is of the order of 10 μb at the photon beam energy Eγ~2.5 GeV. It is expected that the model calculations in this work could be tested by future experiments.     

Study of the process e+e-→π+π-π0π0 at energies √s< 1 GeV

In an experiment with the Spherical Neutral Detector at VEPP-2M collider the cross section of the process e+e-→π+π-π0π0 was measured. At energies √s < 920 MeV this cross section was measured for the first time. The energy dependence of the cross section is well discribed by the vector dominance model with contributions from ρ, ρ' ρ", mesons. The decay probability ρ→π+π-π0π0was found to be Bρ = (1.60±0.74±0.18)×10-5. The upper limit for the decay ω→π+π-π0π0 was improved by two orders of magnitude compared to the previous measurements and is Bω < 2 × 10-4 at 90% confidence level.     

πNN coupling determined beyond the chiral limit

Within the conventional QCD sum rules, we calculate the πNN coupling constant, g _πN, beyond the chiral limit using two-point correlation function with a pion. For this purpose, we consider the Dirac structure, iγ₅, at m _π ² order in the expansion of the correlator in terms of the pion momentum. For a consistent treatment of the sum rule, we include the linear terms in quark mass as they constitute the same chiral order as m _π ². In this sum rule, we obtain g _πN= 13.3 ± 1.2, which is very close to the empirical πNN coupling. This demonstrates that going beyond the chiral limit is crucial in determining the coupling. Received: 8 July 1999 / Revised version: 20 August 1999     

π-π共振态

本文目的在于从解析性、么正性和交叉对称性出发,利用色散关系理论对具有共振行为的分波振幅的形式作一些讨论,并计算P,D波共振参数。在讨论中包含了一个标志与Breit-Wigner共振公式偏离的函数F_((l)I)(v),当F_((l)I)(v)=1时,振幅具有Breit-Wigner共振形式。通过以后的计算表明,这个函数与1有一定的偏离,但在共振点附近的一个较大区域内该偏离不大,因此看来Breit-Wigner形式还是个较好的近似。计算结果给出ρ和f~0粒子的共振参数为     

Determination of the branching ratio ofρ 0→+μ− in the coherent dissociation π−→μ+μ−π− and π−→π+π−π−

In the diffraction dissociation of π^? into μ⁺μ^?π^? on a Cu nucleus at 50 GeV/c, the cross section \(\sigma _{\mu ^ + \mu ^ - \pi ^ - } \) for the 1⁺S(ρ⁰π) wave was measured. The branching ratio of ρ⁰→μ⁺μ^? could be calculated from the ratio of this and the corresponding cross sections in the diffraction dissociation of π^? into π⁺π^?π^?. The obtained value \(BR_{\rho ^0 \to \mu ^ + \mu ^ - } = (4.6 \pm 0.2_{stat^ \pm } \pm 0.2_{syst} )10^{ - 5} \) is in good agreement with the branching ratio \(BR_{\rho ^0 \to e^ + e^ - } \) , as expected ifeμ universality holds.     

Exclusive measurement of the pp → nnπ+π+ reaction at 1.1 GeV

First exclusive data for the $\ensuremath{pp \to nn\pi^+\pi^+}$ reaction have been obtained at CELSIUS with the WASA detector setup at a beam energy of T_p = 1.1 GeV. Total and differential cross-sections disagree with theoretical calculations, which predict the $ \Delta$ $ \Delta$ excitation to be the dominant process at this beam energy. Instead, the data require the excitation of one of the nucleons to a higher-lying $ \Delta$ state, preferably the $\ensuremath{\Delta(1600)P_{33}}$ , to be the leading process.     

Study of the process e^＋e^- →π^＋π^-π^0π^0 at energies √s 〈 1 GeV

In an experiment with the Spherical Neutral Detector at VEPP-2M collider the cross section of the process e^＋e^- →π^＋π^-π^0π^0 was measured. At energies √s 〈 920 MeV this cross section was measured for the first time. The energy dependence of the cross section is well discribed by the vector dominance model with contributions from p, p＇, p＂ mesons. The decay probability ρ→π^＋π^-π^0π^0 was found to be Bρ = （1.60±0.74±0.18） × 10^-5. The upper limit for the decay ω→π^＋π^-π^0π^0 was improved by two orders of magnitude compared to the previous measurements and is Bω 〈 2 × 10^-4 at 90% confidence level.     

关于π-π共振态

本文从解析性和么正性出发,对具有共振行为的π-π散射分波振幅的形式作了普遍的讨论。理论中包含了一个标志与Breit-Wigner共振形式偏离的函数F_(l)^I)(v),当F_(l)^I)(v)=1时,振幅恰好具有Breit-Wigner共振形式。具体计算表明,这个函数与1有一定的偏离。利用ND^-1方法、交叉对称性、ρ与f⁰的Bootstrap近似,将所得到的振幅与-90的共振解。计算中考虑了非弹性过程和负半轴色散积分的贡献。计算结果得到,对于J=1,I=1态,v_R=6.4,Г₁=0.12;对于J=2,I=0态,v_(R2)=20,Г₂=0.016。这相当于m_ρ=762MeV,m_f⁰=1283MeV,ρ的半宽度约为45MeV。它们与目前的实验数据是很好符合的。     

Search for the rare decays K(L)→π0π0μ+μ- and K(L)→π0π0X0→π0π0μ+μ-

The KTeV E799 experiment has conducted a search for the rare decays, K(L)→π(0)π(0)μ(+)μ(-) and K(L)→π(0)π(0)X(0)→π(0)π(0)μ(+)μ(-), where the X(0) is a possible new neutral boson that was reported by the HyperCP experiment with a mass of (214.3 ± 0.5) MeV/c(2). We find no evidence for either decay. We obtain upper limits of Br(K(L)→π(0)π(0)X(0)→π(0)π(0)μ(+)μ(-)) < 1.0 × 10(-10) and Br(K(L)→π(0)π(0)μ(+)μ(-)) < 9.2 × 10(-11) at the 90% confidence level. This result rules out the pseudoscalar X(0) as an explanation of the HyperCP result under the scenario that the dsX(0) coupling is completely real.     

关于π-π作用

本文从下面拉格朗日函数出发,用Tamm-Dancoff近似计算π-π的P波和S波散射。P波的结果和Frazer-Fulco的解相同,但S波的共振行为也相当复杂。     

Electromagnetic corrections to the decays η → 3π

We calculate the electromagnetic corrections to the decays η → 3π in next to leading order in the chiral expansion. We find that the corrections are small in accordance with Sutherland's theorem and modify neither rate nor the Dalitz plot distributions noticeably.     

The second order optical potential for theπd system at the threshold

The second order optical potential for thed system is derived in the framework of the Watson formalism and applied to the calculation of the 1s-state energy shift of the atom and thed scattering lengtha _d . The results are in good agreement with those of the Faddeev type calculations.The authors wish to thank V. B. Belyaev, R. Mach, M. G. Sapozhnikov and R. A. Eramzhyan for stimulating discussions and helpful advice.     

Measurement of the f1(1285) → π+π−π0 decay

The isospin symmetry-breaking (ISB) decay f ₁(1285) → π⁺π^?π⁰ has been studied at the VES facility. The branching ratio is measured: $\frac{{BR\left( {f_1 \to \pi ^ + \pi ^ - \pi ^0 } \right)}} {{BR\left( {f_1 \to \eta \pi ^ + \pi ^ - } \right)}} = 0.86 \pm 0.16_{ - 0.20}^{ + 0.70} \%Low-energy resonances in ³¹P(p, α ²⁸Si and ³⁵Cl(p, α)³²S were studied directly in order to gain a better understanding of reaction cycling in the Si-Ar region in novae. New resonance strengths at E _c.m. = 600 and 622 keV in ³¹P(p, α)²⁸Si were measured (ωγ _{p, α} = (2.2#x00B1;0.7) × 10^?2 eV and ωγ _{p, α} = (0.99±0.08) eV, respectively) as well as the E _c.m. = 610 keV resonance in ³⁵Cl(p, α)³²S [ωγ _{p, α} = (1.2±0.2) × 10^?2 eV], the lowest energy that any resonance in this reaction has been observed, directly or indirectly. The strengths of these resonances were found to be lower than previously determined, resulting in even weaker cycling in the Si-Ar region.     

三个π介子系统中的π—π作用

本文本用Tamm-Dancoff方法导出γ→3π和K→3π过程所满足的积分方程。这些过程的共振性质由两个π介子的散射振幅来决定。