Measurement of Charged-Particle Multiplicities in Gluon and Quark Jets in $p\overline{p}$ Collisions at $\sqrt{s}=1.8\mathrm{TeV}$

We report the first largely model independent measurement of charged particle multiplicities in quark and gluon jets, $N_q$ and $N_g$, produced at the Fermilab Tevatron in $p\overline{p}$ collisions with a center-of-mass energy of 1.8 TeV and recorded by the Collider Detector at Fermilab. The measurements are made for jets with average energies of 41 and 53 GeV by counting charged particle tracks in cones with opening angles of ${\theta }_c=0.28$, 0.36, and 0.47 rad around the jet axis. The corresponding jet hardness $Q=E_{\mathrm{jet}}{\theta }_c$ varies in the range from 12 to 25 GeV. At $Q=19.2\mathrm{GeV}$, the ratio of multiplicities $r=N_g/N_q$ is found to be $1.64\pm 0.17$, where statistical and systematic uncertainties are added in quadrature. The results are in agreement with resummed perturbative QCD calculations.

Figure 1

The ratio of charged particle multiplicities in gluon and quark jets as a function of jet hardness $Q$, which is $Q=E_{\mathrm{jet}}{\theta }_c$ for CDF data and $Q=E_{\mathrm{c}.\mathrm{m}.}=2E_{\mathrm{jet}}$ for $e^{+}{e}^{-}$ data [9, 10]. CDF results (this Letter) are obtained for cone sizes ${\theta }_c=0.28$, 0.36, and 0.47 rad. The NLLA curves [2, 3, 4, 5] are calculated using $Q_{\mathrm{eff}}=230\mathrm{MeV}$ [17]. The asymptotic value ($Q\to \infty$ [20]) of $r$ is simply the ratio of the gluon and quark color factors, $C_A=3$ and $C_F=4/3$, respectively.

Figure 2

Average charged particle multiplicities in gluon and quark jets as a function of jet hardness $Q$, which is $Q=E_{\mathrm{jet}}{\theta }_c$ for CDF data and $Q=E_{\mathrm{c}.\mathrm{m}.}=2E_{\mathrm{jet}}$ for $e^{+}{e}^{-}$ data. For the purpose of comparison to the $e^{+}{e}^{-}$ measurements [9, 10, 19], CDF results on this plot include charged particles from $K_S^0$ and $\Lambda$ decays and are multiplied by two. The fits to CDF data are obtained by using the recent NLLA expressions [4] (see text). The width of the bands corresponds to the uncertainty in the overall normalization.