References - Bogoliubov Laboratory of Theoretical Physics - JINR

NLO Phase Space is simulated correctly especially for hard gluon emissions which are
not properly treated in the PS concept. On the other hand hard emissions of the real
gluons should not be important because the probability of such process is rather small.
The comparison of these two approaches is discussed in the next section. The weighted
MC approach cannot be used in the COMPASS analysis scheme because the acceptance
corrections and detector performance cannot be simulated in such a simple MC generator.
As it will be seen in the next section the results obtained with AROMA generator and
PS and using weighted MC approach are very similar what justify the method based on
PS concept.
4 Predictions for the gluon polarization in NLO QCD
approximation for PGF process
The gluon polarization predictions presented in this paper are estimated from the published
asymmetries by COMPASS Collaboration. To calculate aLL - only pure MC generator
without apparatus simulation was used. Hence the statistical error is slightly
underestimated. Asymmetries published in [2] are weighted by the weight composed of
depolarization factor and signal strength (S/(S + B)) In the calculations presented here
signal strength is assumed to be one because MC reproduces only signal (open charm
events are generated). In the weighted MC approach the Peterson fragmentation function
fitted to data from BELLE Collaboration was used [11] while for AROMA generation
JETSET was applied. Three different models of gluon polarization, ΔG/G, wereused
(in the cese where partonic CMS energy is defined by final charm quark pair system):
ΔG/G = const and two solutions from COMPASS QCD fits analysis, published in [12].
For an unpolarized gluons MRST 2004 PDF set was used.
To validate the method the gluon polarization in the LO QCD approximation (AROMA
MC generator with PS switched off) and using the published asymmetries was found:
ΔG/G = −0.47 ± 0.23. The good agreement between fully weighted COMPASS published
result: ΔG/G = −0.49 ± 0.27, where acceptance and apparatus simulation have
been taken into account justifies also COMPASS binning used for presenting D 0 asymmetries.
The gluon polarization results obtained in the NLO QCD approximation for the gluon
polarization model independent case is: ΔG/G =0.032 ± 0.23 and should be compared
with the weighted MC result ΔG/G =0.005 ± 0.22. The good agreement is seen what
justifies the approach based on PS concept to simulate NLO QCD real emissions. Finally
the gluon polarization results for three considered models of gluon polarizations (for the
case where partonic CMS is defined by charm quark system) are: −0.051 ± 0.24 for
ΔG/G =const,−0.036 ± 0.24 for ΔG/G > 0and−0.057 ± 0.24 for ΔG/G < 0 solutions
of COMPASS QCD fits, respectively. In the light of these results the inconsistency related
to definition of partonic CMS energy, discussed in the previous section, is not important.
5 Conclusions
The predictions for gluon polarization based on recently measured asymmetries for opencharm
muoproduction by COMPASS Collaboration are presented. The results are ob-
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