Speakers
Description
We discuss the production of $D \bar{D}$ pairs in $e^+ e^-$ collisions,
where $D$ refers to either $D^0$ or $D^+$ [1].
The continuum mechanism with the $t/u$-channel vector-meson $D^*$
exchanges are considered. The results of the calculation depend
on the parameter of the off-shell form-factor for the virtual $D^*$
mesons. The $D^* D \gamma$ coupling constants are found from
the $D^* \to D \gamma$ decays.
We find relatively large contribution for the $D^0 {\bar D}^0$ channel
and much smaller contribution in the $D^+ D^-$ channel.
In the second case we consider also the $D^{\pm}$ exchanges.
We conclude that the bump at $M_{D^0 {\bar D}^0} = 3.8$~GeV
observed by the Belle and BaBar Collaborations has rather
continuum origin than it corresponds to the broad resonance
$\chi_{c0}(3860)$.
We discuss also production of the $\chi_{c2}(3930)$ resonance
which is a candidate for the $\chi_{c2}(2P)$ state.
This state can decay into both $D \bar{D}$ channels, however
the branching fractions are not well known at present.
From a comparison of our model results to the BaBar data we find
$B(\chi_{c2}(3930) \to D \bar{D}) = 0.58 \pm 0.13$
using the two-photon width $\Gamma_{\gamma \gamma} = 0.544$~keV
(obtained for the Buchm{\"u}ller-Tye potential)
evaluated within the light-front approach (NRQCD limit).
Our finding of
$\Gamma_{\gamma \gamma} \times B(\chi_{c2}(3930) \to D \bar{D}) = 0.32 \pm 0.07$~keV
is close to the Belle and BaBar results.
Realistic predictions of the differential distributions in several variables
and integrated cross-sections are given for the Belle~II kinematics.
We discuss also a possibility to measure $D \bar D$ production in
ultraperipheral heavy ion collisions.
1) I. Babiarz, P. Lebiedowicz, W. Sch\"afer and A. Szczurek,
``Competition of $\chi_c(2P)$ quarkonia and continuum in $e^+ e^- \to
e^+ e^- D \bar D$'',
Phys. Let. B75 (2026) 140307.
