Meson-photon transition form factors and resonance cross-sections in e+e− collisions

Abstract

Meson-photon-photon transition form factors for S-, P -, and D-wave states are calculated, the meson being treated as a non-relativistic heavy-quarkantiquark pair. The full dependence on both photon virtualities is included. Cross-section formulas for charge-conjugation even mesons with J P = 0 − , 0 + , 1 + , 2 + , and 2 − in electron-positron collisions are presented and numerical results for LEP energies are given. In particular, we find two-photon event rates for χ c1 , η c (2S), and η b (1S) within reach of LEP. With minor modifications to incorporate SU(3)-flavour breaking we estimate rates for 18 light mesons as well, based on the observation that their two-photon decay widths agree remarkably well with measured data. Finally we point out that e + e − cross sections for 1 + states do not vanish at low Q 2 , the Landau-Yang suppression factors of the two-photon cross sections being compensated by the photon propagators.b Heisenberg Fellow.

Central production of mesons: Exotic states versus Pomeron structure

“…For the particular case of two photons coupling to a non-relativistic quarkantiquark one has [12,13] …”

“…In the non-relativistic quark model, A ++ ≃ (Q 2 1 + Q 2 2 )/M 2 at small Q i [14,12] and is thus very much suppressed relative to A +− , which is O(1). Hence in e + e − collisions at small q i⊥ ≪ M we predict a 2 + cross section that is (i) basically flat in cos φ, (ii) finite for k T → 0, and (iii) dominated by the helicity-two part.…”

Study ofπ0pair production in single-tag two-photon collisions

“…We require that the π 0 energies and transverse momenta satisfy E π1 > 0. 4 GeV, E π2 > 0.3 GeV, E π1 + E π2 > 1 GeV, p t,π1 > 0.15 GeV/c, and p t,π2 > 0.15 GeV/c, respectively. We require that the polar angle of π2 in the laboratory frame satisfy −0.8660 < cos θ < 0.9563.…”

“…Schuler, Berends, and van Gulik, who had calculated meson TFFs based on a heavy quark approximation [4], found that their calculations apply well to light mesons as well with only minor modifications. The predicted Q 2 dependence of TFFs for mesons with J P C = 0 ++ and 2 ++ is summarized in Table I, where the γ * γ center-of-mass (c.m.)…”

“…According to Table I, TFFs for the helicity-0 and -1 components of a tensor meson grow with Q 2 , a prediction which is amenable to investigation. [4]. Each term has a common divisor of (1 + Q 2 /M 2 ) 2 .…”