The handbag contribution to γγ → ππ and KK

Physics Letters B, 2010

We explore the implications of SU(3) flavor symmetry in the soft handbag mechanism for two-photon annihilation into pairs of pseudoscalar octet mesons. In this approach we obtain a good description of the experimental results for all measured channels at high energy, with two complex form factors adjusted to the data. We also predict the cross section for γγ → ηη.

The European Physical Journal C - Particles and Fields, 2003

We study the handbag contribution to two-photon annihilation into baryon-antibaryon pairs at large energy and momentum transfer. We derive factorization of the process amplitude into a hard γγ → qq subprocess and form factors describing the soft qq → BB transition, assuming that the process is dominated by configurations where the (anti)quark approximately carries the full momentum of the (anti)baryon. The form factors represent moments of time-like generalized parton distributions, so-called BB distribution amplitudes. A characteristic feature of the handbag mechanism is the absence of isospin-two components in the final state, which in combination with flavor symmetry provides relations among the form factors for the members of the lowest-lying baryon octet. Assuming dominance of the handbag contribution, we can describe current experimental data with form factors of plausible size, and predict the cross sections of presently unmeasured BB channels.

Physical Review D, 2009

The differential cross sections for the process γγ → π 0 π 0 have been measured in the kinematic range 0.6 GeV < W < 4.1 GeV, | cos θ * | < 0.8 in energy and pion scattering angle, respectively, in the γγ center-of-mass system. The results are based on a 223 fb −1 data sample collected with the Belle detector at the KEKB e + e − collider. The differential cross sections are fitted in the energy region 1.7 GeV < W < 2.5 GeV to confirm the two-photon production of two pions in the G wave. In the higher energy region, we observe production of the χ c0 charmonium state and obtain the product of its two-photon decay width and branching fraction to π 0 π 0. We also compare the observed angular dependence and ratios of cross sections for neutral-pion and charged-pion pair production to QCD models. The energy and angular dependence above 3.1 GeV are compatible with those measured in the π + π − channel, and in addition we find that the cross section ratio, σ(π 0 π 0)/σ(π + π −), is 0.32 ± 0.03 ± 0.05 on average in the 3.1-4.1 GeV region.

2004

T w o-p h o t m exclusive hadron production a t large m0rnentuIt-i 'transfer probes the short distance physics of perturbative Quantum Chromodynamics and explores the long distance physics of bound-state quark dynamics. The differential cross section is measured for the exclusive reactions yy-+ ?T+T-and yy-+ K + Kin the two-photon invariant mass region 1.4 5 MYY < 4.0 GeV/c2 and center-of-mass scattering angle 1 cos 8 ' 1 < 0.8. T h e SU(3)-flavor symmetry breaking effects of the kaon wave function are examined. This analysis is performed using 139 pb-' of untagged data collected with the TPC/Two-Gamma detector.

Annalen der Physik, 2004

The handbag mechanism for wide-angle exclusive scattering reactions is discussed and compared to other theoretical approaches. The role of power laws in observables is critically examined. Applications of the handbag mechanism to Compton scattering and meson photoproduction are presented. The soft physics input to these processes are specific form factors which represent 1/x moments of generalized parton distributions at zero skewness. A recent analysis of the nucleon form factors provides these GPDs and, hence, the new form factors.

Physics Letters B, 1994

The differential cross section of dσ(e + e − → e + e − D * ± X)/dPT was measured using a soft-pion analysis of D * ± → π ± s D 0 (D 0 ) at TRISTAN. The average √ s was 58.1 GeV and the integrated luminosity used in this analysis was 198 pb −1 , respectively. * published in Phys. Lett. B 328 (1994) 535. †

2008

We present a high-statistics measurement of differential cross sections and the total cross section for the process γγ → π 0 π 0 in the kinematic range 0.6 GeV ≤ W ≤ 4.0 GeV and | cos θ * | ≤ 0.8, where W and θ * are the energy and pion scattering angle, respectively, in the γγ center-of-mass system. Differential cross sections are fitted to obtain information on S, D0, D2, G0 and G2 waves. The G waves are important above W ≃ 1.6 GeV. General behavior of partial waves is studied by fitting differential cross sections in a simple parameterization where amplitudes contain resonant contributions and smooth background. The D2 wave is dominated by the f2(1270) meson whose parameters are consistent with the with the current world averages. The D0 wave contains a f2(1270) component, whose fraction is fitted. For the S wave, the f0(980) parameters are found to be consistent with the values determined from our recent π + π -data. In addition to the f0(980), the S wave prefers to have another resonance-like contribution whose parameters are obtained.

Physical Review D, 2007

We present an analysis of the process e + e − → γ * or Z * → ππγ, in the kinematical region where √ s, the c.m. energy of the e + e − pair, is large but much below the Z-pole. The subprocess γ * or Z * → ππγ can be described by the convolution of the hard scattering coefficient γ * or Z * → qqγ and the general distribution amplitude of two pions qq → ππ. In the case of neutral pion, the production through γ * is the dominant process, which can therefore be used to access the generalized distribution amplitudes (GDAs) of the pion, especially their C-even parts. The γZ interference term provides an alternative approach to extract the weak mixing angle sin θW through measuring the helicity asymmetry in the process e + e − → π 0 π 0 γ. In the case of charged pion pair production, the Bremsstrahlung process dominates and its interference with e + e − → γ * → π + π − γ can be applied to study the process γ * → ππγ at the amplitude level.

Physical Review D, 1994

Proceedings of The European Physical Society Conference on High Energy Physics — PoS(EPS-HEP 2013), 2014

We investigate the form factors for pseudoscalar-meson-photon transitions by means of dispersive QCD sum rules and demonstrate that most of the measurements done so far (in particular, those by BABAR for η, η ′ and η c and by Belle for π 0) are, on the one hand, compatible with each other and with the saturation required by factorization theorems obtained from perturbative QCD and, on the other hand, give a hint that saturation is effective already at relatively low momentum transfers Q 2 ; this hypothesis is supported by experimental data for the charged-pion elastic form factor available in the range Q 2 ≈ 2-4 GeV 2. The only exception are the BABAR results for the π 0 γ transition form factor, which do not fit into such picture. We point out that results expected from SHMS at JLab on the pion elastic form factor in the region Q 2 ≈ 5-8 GeV 2 will provide the ultimate test of saturation and factorization and strongly impact our general view of the form factors up to infinitely large Q 2 .