Speaker
Description
We derive the light-front wave function (LFWF)
representation of the $\gamma^* \gamma^* \to \eta_c(1S)\,,\eta_c(2S)$
transition form factor $F(Q_1^2, Q_2^2)$ for two virtual photons
in the initial state. For the LFWF, we use different models
obtained from the solution of the Schr\"odinger equation for
a variety of $c \bar c$ potentials. We compare our results to
the BaBar experimental data for the $\eta_c(1S)$
transition form factor, for one real and one virtual photon.
We observe that the onset of the asymptotic behaviour is strongly delayed
and discuss applicability of the collinear and/or massless limit.
We present some examples of two-dimensional distributions for
$F (Q_1^2,Q_2^2)$.
A factorization breaking measure is proposed and factorization breaking
effects are quantified and shown to be almost model independent.
Factorization is shown to be strongly broken, and a scaling of
the form factor as a function of $\bar Q^2 = (Q_1^2 + Q_2^2)/2$ is obtained.
In addition, we present a thorough analysis of $\eta_c(1S,2S)$
quarkonia hadroproduction in $k_\perp$-factorisation in the framework of
the light-front potential approach for the quarkonium wave function.
The off-shell matrix elements for the $g^* g^* \eta_c(1S,2S)$ vertices
are derived. We discuss the importance of taking into account the gluon
virtualities.
We present the transverse momentum distributions of $\eta_c$ for
several models of the unintegrated gluon distributions.
Our calculations are performed for four distinct parameterisations
for the $c\bar c$ interaction potential consistent with the meson spectra.
We compare our results for $\eta_c(1S)$ to measurements by the
LHCb collaboration and present predictions for $\eta_c(2S)$ production.
References:
1.) I. Babiarz, V.P. Goncalves, R. Pasechnik, W. Sch\"afer and
A. Szczurek, ''The ${\gamma^* \gamma^* \to \eta_c (1S,2S)}$ transition
form factors for spacelike photons'',
Phys.Rev. {\bf D100} 054018 (2019)
2.) I.~Babiarz, R.~Pasechnik, W.~Sch\"afer and A.~Szczurek,
``Prompt hadroproduction of $\eta_c(1S,2S)$ in the $k_T$-factorization approach,''
JHEP {\bf 2002}, 037 (2020) [arXiv
