Orateur
Description
The usage of charm cross sections from hadron-hadron collisions for
the extraction of perturbative QCD parameters has recently been hampered by
evidence that charm quark fragmentation into charm hadrons is nonuniversal,
i.e. may depend on the collision initial and final state and its energy.
Fragmentation effects can be eliminated from the theory by considering the
total ccbar pair cross section only, currently calculable e.g. in pp collisions
at NNLO QCD.
Comparison to experiment then needs the extrapolation of measured fiducial
charm cross sections to the total ccbar pair cross section, again hitting the
charm fragmentation nonuniversality problem.
In [1], a first preliminary version of a new procedure to obtain such an
extrapolation based on an "effective" variant of the well-known differential
FONLL QCD calculation has been presented. This procedure obtains an effective
shape and normalization of theory-inspired extrapolation functions, based on
constraints obtained from various published measurements. Within uncertainties,
it fully accounts for charm fragmentation nonuniversality, without the need to
assume any particular nonuniversal charm fragmentation model.
This new procedure is now elaborated further, and applied to total charm cross
section evaluations at different LHC hadron-hadron collision energies.
From the dependence of these total charm cross sections on center of mass,
in which the large QCD scale dependence partially cancels, first results on
constraining QCD parameters like parton density functions and the charm
quark mass at NNLO are presented, superseding all previous results still
based on the charm quark fragmentation universality assumption.
