Speaker
Description
The presentation will be based on our paper in preparation
[1].
We discuss production of pairs of $J/\psi$ quarkonia in
proton-proton collisions in the context of recent results obatained
at the LHC at large transverse momenta.
The leading-order $O(\alpha_s^4)$ contribution
is calculated in both collinear and the $k_t$-factorization approach
with the KMR unintegrated gluon distributions.
We include also two-gluon exchange
contribution which is of the order of $O(\alpha_s^6)$ (not included
routinely in collinear-factorization approach).
This contribution is calculated in the moment only in
the collinear approximation.
In addition we calculate cross sections for
$p p \to \chi_c(J_1) \chi_c(J_2)$.
A feed-down from double $\chi_c$ production to double $J/\psi$
production is estimated for a first time.
The double parton scattering cross section and differential
distributions are calculated using an educated parametrization
of experimental single $J/\psi$ differential distributions in rapidity
and transverse momentum at the LHC energies.
Many differential distributions are calculated.
Results of our calculations are compared with very recent ATLAS data
\cite{ATLAS}.
We find that the two-gluon exchange mechanism and feed down from
double $\chi_c$ production lead to very similar
(in shape) distributions in rapidity distance between the $J/\psi$
mesons as the double-parton scattering approach.
Much larger cross sections are obtained in the $k_t$-factorization approach.
Including the mechanisms leaves much
less room for the double parton scattering contribution which cannot
be calculated from first principle.
The $\sigma_{eff}$ parameter for DPS needed to describe the ATLAS data
is much larger than from previous analyses of double
quarkonium production, where a smaller number of mechanisms was included.
We present distributions in rapidity distance, two $J/\psi$ invariant
mass, azimuthal angle correlations between the two $J/\psi$ mesons
and transverse momentum of the pairs of quarkonia.
In all cases the sum of the four considered contributions reminds
experimental ATLAS distributions [2].
1) A. Cisek, W. Sch\"afer and A. Szczurek, a paper in preparation.
2) ATLAS collaboration,
CERN-EP-2016-211, arXiv:161202950.
