The cross section of charm production in γγ collisions σ(e + e − →e + e − c c ̄ X) is measured at LEP with the L3 detector at centre-of-mass energies from 91 GeV to 183 GeV. Charmed hadrons are identified by electrons and muons from semileptonic decays. The direct process γγ→c c ̄ is found to be insufficient to describe the data. The measured cross section values and event distributions require contributions from resolved processes, which are sensitive to the gluon density in the photon.
Inclusive production of $\mathrm{D^{*\pm}}$ mesons in two-photon collisions was measured by the L3 experiment at LEP. The data were collected at a centre-of-mass energy $\sqrt{s} = 189$ GeV with an integrated luminosity of $176.4 \mathrm{pb^{-1}}$. Differential cross sections of the process $\mathrm{e^+e^- \to D^{*\pm} X}$ are determined as functions of the transverse momentum and pseudorapidity of the $\mathrm{D^{*\pm}}$ mesons in the kinematic region 1 GeV $< p_{T}^{\mathrm{D^*}} < 5 $ GeV and $\mathrm{|\eta^{D^*}|} < 1.4$. The cross section integrated over this phase space domain is measured to be $132 \pm 22(stat.) \pm 26(syst.)$ pb. The differential cross sections are compared with next-to-leading order perturbative QCD calculations.
We study charged particle production in proton-antiproton collisions at 300 GeV, 900 GeV, and 1.96 TeV. We use the direction of the charged particle with the largest transverse momentum in each event to define three regions of eta-phi space; toward, away, and transverse. The average number and the average scalar pT sum of charged particles in the transverse region are sensitive to the modeling of the underlying event. The transverse region is divided into a MAX and MIN transverse region, which helps separate the hard component (initial and final-state radiation) from the beam-beam remnant and multiple parton interaction components of the scattering. The center-of-mass energy dependence of the various components of the event are studied in detail. The data presented here can be used to constrain and improve QCD Monte Carlo models, resulting in more precise predictions at the LHC energies of 13 and 14 TeV.
We present a new measurement of the $B^+$ meson differential cross section $d\sigma/d p_T$ at $\sqrt{s}=1960$ GeV. The data correspond to an integrated luminosity of 739pb$^{-1}$ collected with the upgraded CDF detector (CDF II) at the Fermilab Tevatron collider. $B^+$ candidates are reconstructed through the decay $B^+ \to J/\psi K^+$, with $J/\psi \to \mu^+ \mu^-$. The integrated cross section for producing $B^+$ mesons with $p_T \geq 6$ GeV/c and $|y| \leq 1$ is measured to be $2.78 \pm 0.24 \mu$b
The pair production of Z bosons is studied using the data collected by the L3 detector at LEP in 1998 in e+e- collisions at a centre-of-mass energy of 189 GeV. All the visible final states are considered and the cross section of this process is measured to be 0.74 +0.15 -0.14 (stat.) +/- 0.04 (syst.) pb. Final states containing b quarks are enhanced by a dedicated selection and their production cross section is found to be 0.18 +0.09 -0.07 (stat.) +/- 0.02 (syst.) pb. Both results are in agreement with the Standard Model predictions. Limits on anomalous couplings between neutral gauge bosons are derived from these measurements.
We have measured the polarizations of J/ψ and ψ(2S) mesons as functions of their transverse momentum pT when they are produced promptly in the rapidity range |y|<0.6 with pT≥5 GeV/c. The analysis is performed using a data sample with an integrated luminosity of about 800 pb−1 collected by the CDF II detector. For both vector mesons, we find that the polarizations become increasingly longitudinal as pT increases from 5 to 30 GeV/c. These results are compared to the predictions of nonrelativistic quantum chromodynamics and other contemporary models. The effective polarizations of J/ψ and ψ(2S) mesons from B-hadron decays are also reported.
We present a study of the structure of hadronic events recorded by the L3 detector at LEP at the center of mass energies of 161 and 172 GeV. The data sample corresponds to an integrated luminosity of 21.25 pb −1 collected during the high energy runs of 1996. The distributions of event shape variables and the energy dependence of their mean values are well reproduced by QCD models. From a comparison of the data with resummed O ( α s 2 ) QCD calculations, we determine the strong coupling constant at the two energies. Combining this with our earlier measurements we find that the strong coupling constant decreases with increasing energy as expected in QCD.
We present a measurement of the $W$-boson-pair production cross section in $p\bar{p}$ collisions at 1.96 TeV center-of-mass energy and the first measurement of the differential cross section as a function of jet multiplicity and leading-jet energy. The $W^{+}W^{-}$ cross section is measured in the final state comprising two charged leptons and neutrinos, where either charged lepton can be an electron or a muon. Using data collected by the CDF experiment corresponding to $9.7~\rm{fb}^{-1}$ of integrated luminosity, a total of $3027$ collision events consistent with $W^{+}W^{-}$ production are observed with an estimated background contribution of $1790\pm190$ events. The measured total cross section is $\sigma(p\bar{p} \rightarrow W^{+}W^{-}) = 14.0 \pm 0.6~(\rm{stat})^{+1.2}_{-1.0}~(\rm{syst})\pm0.8~(\rm{lumi})$ pb, consistent with the standard model prediction.
We present the first measurement of the production cross section of a W boson with a single charm quark (c) in p-pbar collisions at sqrt(s)=1.96 TeV, using soft muon tagging of c jets. In a data sample of ~1.8 fb-1, recorded with the CDF II detector at the Fermilab Tevatron, we select events with W+1 or 2 jets. We use the charge correlation between the W and the muon from the semileptonic decay of a charm hadron to extract the Wc signal. We measure sigma_{Wc}(p_{Tc}>20 GeV/c, |\eta_c|<1.5)\times BR(W->\ell\nu) = 9.8+/-3.2 pb, in agreement with theoretical expectations.
We measure the ratio of cross section times branching fraction, $R_p \equiv \sigma_{\chi_{c2}} {\cal B}(\chi_{c2} \to J/\psi \gamma)/ \sigma_{\chi_{c1}} {\cal B}(\chi_{c1} \to J/\psi \gamma)$, in 1.1 fb$^{-1}$ of $p\bar{p}$ collisions at $\sqrt{s} =$ 1.96 TeV. This measurement covers the kinematic range $p_T(J/\psi)>4.0$ GeV/$c$, $|\eta(J/\psi)| < 1.0$, and $p_T(\gamma)>1.0$ GeV/$c$. For events due to prompt processes, we find $R_p = 0.395\pm0.016(stat.)\pm0.015(sys.)$. This result represents a significant improvement in precision over previous measurements of prompt $\chi_{c1,2}$ hadroproduction.