Production of charm quark/antiquark pairs at LHC
  We discuss charm production at LHC. The production of single $c \bar c$ pairs is calculated in the $k_t$-factorization approach. We use several unintegrated gluon distributions from the literature. The hadronization is included with the help of fragmentation functions found for the production of charmed mesons in $e^+ e^-$ collisions. Differential distributions for several charmed mesons are presented and compared to recent results of the ALICE and LHCb collaborations. Furthermore we discuss production of two pairs of $c \bar c$ within a simple formalism of double-parton scattering (DPS). Surprisingly large cross sections, comparable to single-parton scattering (SPS) contribution, are predicted for LHC energies. Both total inclusive cross section as a function of energy and differential distributions are shown. We include recently discussed evolution of double partons in the case of two scales. We discuss perspectives how to identify the double scattering contribution. We find much larger cross section for large rapidity distance between charm quarks from different hard parton scatterings compared to single scattering. 
Rafal Maciula, Marta Luszczak and Antoni Szczurek
hep-ph hep-ex nucl-th