Speaker
Description
Phenomena of multiple-parton interaction (MPI) have become very
essential for precise description of high-energy proton-proton
collisions in the ongoing LHC era.
Some time ago we have proposed and discussed double open charm meson
production $pp \to D D \!\; X$ as a potentially one of the best reaction
to study hard double-parton scattering effects at the LHC.
This conclusion was further confirmed by the LHCb collaboration
that has reported surprisingly large cross sections for
$DD$ meson-meson pair production in $pp$-scattering
at $7$ TeV.
Here we discuss production of two pairs of $J/\psi$ quarkonia
in the context of recent results obtained at the LHC at large and
intermediate $p_t$.
The leading-order ${\cal{O}}(\alpha_s^4)$, called here also box, contribution
is calculated in both collinear and the $k_t$-factorization approach
with the KMR unintegrated gluon distributions.
We include also two-gluon exchange (between two
$c \bar c$ intermediate pairs) contribution which is of
the order of ${\cal{O}}(\alpha_s^6)$, in the moment
only in the collinear approximation.
We calculate cross sections for $p p \to \chi_c(J_1) \chi_c(J_2)$.
A feed-down from double $\chi_c$ single-parton-scattering production is
estimated for the first time.
The double parton scattering is calculated using an educated
parametrization of 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.
We find that the two-gluon exchange mechanism and the double feed down
lead to very similar (in shape) distributions in rapidity distance
between the $J/\psi$ mesons.
Much larger cross sections are obtained in the $k_t$-factorization approach.
Including the mechanisms, some of them for a first time, leaves much
less room for the double parton scattering contribution which cannot
be calculated from first principle.
The $\sigma_{\rm eff}$ parameter for DPS needed to describe the ATLAS data
is therefore much larger than from previous studies of double
quarkonium production, where a smaller number of mechanisms was
included.
In addition here, we present results of phenomenological studies
of DPS effects in the case of associated open charm and bottom
$pp \to D^{0} B^{^{+}} \!\; X$ as well as double open bottom
$pp \to B^{+} B^{+} \!\; X$ production. In particular, we will show
theoretical predictions of integrated and differential cross sections
for different energies that could help to conclude whether and how
the DPS effects for these two cases can be observed experimentally
by the LHCb/CMS collaborations.
