A measurement of the production cross-section for $Z$ bosons that decay to muons is presented. The data were recorded by the LHCb detector during $pp$ collisions at a centre-of-mass energy of 7 TeV, and correspond to an integrated luminosity of 1.0 fb$^{-1}$. The cross-section is measured for muons in the pseudorapidity range $2.0 < \eta < 4.5$ with transverse momenta $p_{T} > 20$ GeV/c. The dimuon mass is restricted to $60 < M_{\mu^{+}\mu^{-}} < 120$ GeV/c$^{2}$. The measured cross-section is $$\sigma_{Z\rightarrow\mu^{+}\mu^{-}} = (76.0 \pm 0.3 \pm 0.5 \pm 1.0 \pm 1.3) \, \text{pb}$$ where the uncertainties are due to the sample size, systematic effects, the beam energy and the luminosity. This result is in good agreement with theoretical predictions at next-to-next-to-leading order in perturbative quantum chromodynamics. The cross-section is also measured differentially as a function of kinematic variables of the $Z$ boson. Ratios of the production cross-sections of electroweak bosons are presented using updated LHCb measurements of $W$ boson production. A precise test of the Standard Model is provided by the measurement of the ratio $$\frac{\sigma_{W^{+}\rightarrow\mu^{+}\nu_{\mu}} + \sigma_{W^{-}\rightarrow\mu^{-}\bar{\nu}_{\mu}}}{\sigma_{Z\rightarrow\mu^{+}\mu^{-}}} = 20.63\pm0.09\pm0.12\pm0.05,$$ where the uncertainty due to luminosity cancels.
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The differential cross-sections for e + e − → e + e − , e + e − → μ + μ − and e + e − → τ + τ − , and the total cross-section for e + e − → qq̄ at centre-of-mass energies of 130–140 GeV were studied using about 5 pb −1 of data collected with the OPAL detector at LEP in October and November 1995. The results are in agreement with the Standard Model predictions. Four-fermion contact interaction models were fitted to the data and lower limits were obtained on the energy scale Λ at the 95% confidence level.
We have analysed a sample of 2.36 million minimum bias events produced in p p collisions at s =630 GeV in the UA1 experiment at the CERN collider. We have studied the production of charged particles with transverse momenta ( p T ) up to 25 GeV/c. The results are in agreement with QCD predictions. The rise of 〈 p T 〉 with charged particle multiplicity may be related to changing production of low p T particles.
We have analysed a sample of 2.36 million minimum bias events produced in p p collisions at s =630 GeV in the UA1 experiment at the CERN Sp p S collider. We have studied the production of K S 0 , Λ and Λ particles with transverse momenta ( p t ) up to 7 GeV/c and K ± up to 2 GeV/c. The kaon data are compared with a recent QCD prediction and are found to be in good agreement. The < p t > for K S 0 , Λ and Λ is seen to increase as a function of the charged particle multiplicity and is compared with charged particle production.
Single and multi-photon events with missing energy are analysed using data collected with the L3 detector at LEP at a centre-of-mass energy of 189 GeV, for a total of 176 pb^{-1} of integrated luminosity. The cross section of the process e+e- -> nu nu gamma (gamma) is measured and the number of light neutrino flavours is determined to be N_\nu = 3.011 +/- 0.077 including lower energy data. Upper limits on cross sections of supersymmetric processes are set and interpretations in supersymmetric models provide improved limits on the masses of the lightest neutralino and the gravitino. Graviton-photon production in low scale gravity models with extra dimensions is searched for and limits on the energy scale of the model are set exceeding 1 TeV for two extra dimensions.
Λ , Ξ and Ω yields and transverse mass spectra have been measured in Pb-Pb and p-Pb collisions at 158 A GeV/ c . The yields in Pb-Pb interactions are presented as a function of the collision centrality and compared with those obtained from p-Pb collisions. Strangeness enhancement is observed which increases with centrality and with the strangeness content of the hyperon.
Results are presented on a measurement of the ttbar pair production cross section in ppbar collisions at sqrt{s} = 1.8 TeV from nine independent decay channels. The data were collected by the Dzero experiment during the 1992-1996 run of the Fermilab Tevatron Collider. A total of 80 candidate events are observed with an expected background of 38.8 +- 3.3 events. For a top quark mass of 172.1 GeV/c^2, the measured cross section is 5.69 +- 1.21 (stat) +- 1.04 (sys) pb.
The STAR collaboration at RHIC presents measurements of \Jpsi$\to{e^+e^-}$ at mid-rapidity and high transverse momentum ($p_T>5$ GeV/$c$) in \pp and central \cucu collisions at \sNN = 200 GeV. The inclusive \Jpsi production cross section for \cucu collisions is found to be consistent at high $p_T$ with the binary collision-scaled cross section for \pp collisions, in contrast to previous measurements at lower $p_T$, where a suppression of \Jpsi production is observed relative to the expectation from binary scaling. Azimuthal correlations of $J/\psi$ with charged hadrons in \pp collisions provide an estimate of the contribution of $B$-meson decays to \Jpsi production of $13% \pm 5%$.
The production of muons from heavy flavour decays is measured at forward rapidity in proton-proton collisions at $\sqrt{s} = 7$ TeV collected with the ALICE experiment at the LHC. The analysis is carried out on a data sample corresponding to an integrated luminosity $L_{\rm int} = 16.5$ nb$^{-1}$. The transverse momentum and rapidity differential production cross sections of muons from heavy flavour decays are measured in the rapidity range 2.5 < y < 4, over the transverse momentum range 2 < $p_{\rm T}$ < 12 GeV/$c$. The results are compared to predictions based on perturbative QCD calculations.
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