Posted: Jan 24, 2009
Studies by the ALEPH, L3 and OPAL Collaborations of the D* +/- meson content in jets show that the production from Z0 decays in e++e- collisions is dominated by D* mesons that carry large fractions of the jet momenta, consistent with the jets being produced from primary c (anti-)quarks. In pbar + p collisions at 630 GeV and 1.8 TeV, the UA1 and CDF Collaborations have observed D* +/- mesons in jets with transverse energies larger than 40 GeV. Their fractional momenta are found smaller, consistent with a different production mechanism in which the D* mesons originate from gluon splitting into c cbar pairs.
At top RHIC energy, heavy quarks can still be produced via gluon splitting. Perturbative QCD suggests that these contributions are small, and that the majority of the heavy quarks originate from gluon-gluon fusion. These expectations, however, have not until now been confronted with data at RHIC. The STAR experiment presents the first measurement of charged D* mesons in inclusive jets produced in p + p collisions at a center of mass energy of 200 GeV at RHIC which addresses the above issue. The charged D* andidates were identified through the decay sequence D*+ --> D0 pi+, D0 --> K-pi+ and its charge conjugate. The D*+ and D*- yields of 184 +/- 44 and 169 +/- 45 were obtained in inclusive jets with 11.5 GeV mean transverse energy.
In the analysis we obtained the correlation between D* candidate azimuthal angle and the reconstructed jet axis. Due to better statistics the analysis focused at the near side correlation. Comparison of the distribution of the D longitudinal momentum fraction (z) in jets to Monte Carlo simulation of charm creation through flavour creation process gg --> c cbar (gluon fusion) and q qbar --> c cbar shows the model calculations have agreement with data for high z values. A excess is observed in data at smaller z values compared to model simulations. These can be ascribed to production processes that are not included in the model simulations such a gluon splitting.
This data is then used to estimate the rate of gluon splitting to charm pairs (R). The STAR experiment results (rate of gluon splitting to charm pairs) are shown in the figure together with the UA1 and CDF measurements as a function of gluon jet energy. The results are compared to a theoretical evaluation in leading-order pQCD of gluon splitting into a pair of charm quarks and subsequent hadronization into D mesons. The expectation is consistent with the data to within the combined experiment statistical and systematic uncertainties. Although the agreement is not strong, the conclusion that R is small for energies accessible at RHIC is clearly supported. The associated cross section is smaller than the total charm production cross section at RHIC. We thus infer that the charm content in jets at RHIC energies has a small contribution from gluon splitting and is dominated by jets initiated by charm quarks.
Further details can be found in the following STAR paper -