References - Bogoliubov Laboratory of Theoretical Physics - JINR

polarization as well as a construction of the statistical weight used in the analysis requires
the knowledge of the so-called analyzing power, aLL, ( the ratio of polarized over unpolarized
partonic cross sections) and the signal strength event-by-event basis. The analyzing
power is calculated in the LO QCD approximation and as a signal identified D 0 meson
(reconstructed from its decay) is used. In contrast to the resolved photon contribution
NLO QCD corrections to unpolarized and polarized cross sections are supposed to be
large in the COMPASS kinematical domain. To allow to use the COMPASS data in the
independent analysis the open-charm asymmetries in bins in pT and energy of D 0 meson
was also published [2]. In this paper I would like to present the method of computing analyzing
power event-by-event basis in the NLO QCD approximation. The method is based
on LO MC with Parton Shower (PS) and can be easily allied in the COMPASS analysis
scheme, also in the weighting method. The calculations are done for NLO QCD corrected
PGF process and the new gluon polarization result based on published asymmetries are
also presented. There is another part of NLO QCD corrections to muoproduction of opencharm:
light quark originated processes where emitted gluon produces charm-anticharm
quark pair. These processes contribute to the background because they don’t probe gluons
inside the nucleon. It introduces the complication into the analysis because signal
defined as an observed D 0 meson is polluted by these higher order processes 1 .Theyare
not considered in this paper but the extension of the proposed method is rather straightforward
however technically more complicated. It requires modification of the asymmetry
decomposition equation which is a basis of the COMPASS analysis (see. eqs 5 and 6
in [2]). The paper is organized as follows: in the next section the NLO QCD corrections
to the PGF process are discussed. The Monte-Carlo method and the Parton Shower concept
is presented in section 3. The new results for the gluon polarization obtained using
published asymmetries are presented in section 4. The discussion of the approximation
used in the computation of the analyzing power is presented in section 4. Conclusions are
presented in section 5.
2 NLO QCD corrections to the open-charm cross
sections
In the LO QCD approximation PGF is the only process which contributes to the opencharm
production. Moreover for the energy range covered by the COMPASS experiment
the QCD evolution is not able to produce significant fraction of charm sea inside nucleon
and only hard part of the cross section is responsible for open-charm production. As it
was mentioned above the resolved photon contribution is also small and so-called intrinsic
charm content inside nucleon is in the considered kinematics suppressed. Therefore
observation of the charm signal seems to be an ideal to probe gluons inside nucleon for
the COMPASS experiment. It is however known that the unpolarized cross section (averaged
over spin states) is not precisely described by the LO QCD approximation and
the NLO QCD corrections are important for the photoproduction of the open-charm, [4].
Also spin-dependent (polarized) cross section, calculated recently shows the important
dependence of the approximation used [5]. It was also argued that the naive expectation
1Notice that background taken into account in the COMPASS analysis is a combinatorial background;
see also [3]
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