Inclusive production of charged hadrons in photon photon collisions.

The OPAL collaboration Abbiendi, G. ; Ainsley, C. ; Akesson, P.F. ; et al.
Phys.Lett.B 651 (2007) 92-101, 2007.
Inspire Record 734955 DOI 10.17182/hepdata.48554

The inclusive production of charged hadrons in the collisions of quasi-real photons e+e- -> e+e- +X has been measured using the OPAL detector at LEP. The data were taken at e+e- centre-of-mass energies from 183 to 209 GeV. The differential cross-sections as a function of the transverse momentum and the pseudorapidity of the hadrons are compared to theoretical calculations of up to next-to-leading order (NLO) in the strong coupling constant alpha{s}. The data are also compared to a measurement by the L3 Collaboration, in which a large deviation from the NLO predictions is observed.

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Inclusive production of charged hadrons and K0(S) mesons in photon photon collisions.

The OPAL collaboration Ackerstaff, K. ; Alexander, G. ; Allison, John ; et al.
Eur.Phys.J.C 6 (1999) 253-264, 1999.
Inspire Record 472639 DOI 10.17182/hepdata.49411

The production of charged hadrons and K_s mesons in the collisions of quasi-real photons has been measured using the OPAL detector at LEP. The data were taken at e+e- centre-of-mass energies of 161 and 172 GeV. The differential cross-sections as a function of the transverse momentum and the pseudorapidity of the charged hadrons and K_s mesons have been compared to the leading order Monte Carlo simulations of PHOJET and PYTHIA and to perturbative next-to-leading order (NLO) QCD calculations. The distributions have been measured in the range 10-125 GeV of the hadronic invariant mass W. By comparing the transverse momentum distribution of charged hadrons measured in gamma-gamma interactions with gamma-proton and meson-proton data we find evidence for hard photon interactions in addition to the purely hadronic photon interactions.

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Multi-photon production in e+ e- collisions at s**(1/2) = 183-GeV.

The OPAL collaboration Ackerstaff, K. ; Alexander, G. ; Allison, John ; et al.
Phys.Lett.B 438 (1998) 379-390, 1998.
Inspire Record 472640 DOI 10.17182/hepdata.49412

The process e+e- to gamma gamma (gamma) is studied using data recorded with the OPAL detector at LEP. The data sample corresponds to a total integrated luminosity of 56.2 pb-1 taken at a centre-of-mass energy of 183 GeV. The measured cross-section agrees well with the expectation from QED. A fit to the angular distribution is used to obtain improved limits at 95% CL on the QED cut-off parameters: Lambda+ > 233 GeV and Lambda- > 265 GeV as well as a mass limit for an excited electron, M(e*) > 227 GeV assuming equal e*egamma and eegamma couplings. No evidence for resonance production is found in the invariant mass spectrum of photon pairs. Limits are obtained for the cross-section times branching ratio for a resonance decaying into two photons.

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Search for excited leptons in e+ e- collisions at s**(1/2) = 161-GeV.

The OPAL collaboration Ackerstaff, K. ; Alexander, G. ; Allison, John ; et al.
Phys.Lett.B 391 (1997) 197-209, 1997.
Inspire Record 425066 DOI 10.17182/hepdata.47769

We have searched for excited states of charged and neutral leptons, e ∗ , μ ∗ , τ ∗ and ν ∗ , in e + e − collisions at s =161 GeV using the OPAL detector at LEP. No evidence for their existence was found. With the most common coupling assumptions, the topologies from excited lepton pair production include ℓ + ℓ − γγ and ℓ + ℓ − W + W − , with the subsequent decay of the virtual W bosons. From the analysis of these topologies, 95% confidence level lower mass limits of 79.9 GeV for e ∗ , 80.0 GeV for μ ∗ , 79.1 GeV for τ ∗ , 78.3 GeV for ν e ∗ , 78.9 GeV for ν μ ∗ and 76.2 GeV for ν τ ∗ are inferred. From the analysis of W + W − and γγ topologies with missing energy and using alternative coupling assingments which favour charged ℓ ∗± and photonic ν ∗ decays, 95% confidence level lower mass limits of 77.1 GeV for each ℓ ∗± flavour and 77.8 GeV for each ν ∗ flavour are inferred. From the analysis of the ℓ + ℓ − γ , ℓ ± W ∓ and single γ final states expected from excited lepton single production, upper limits on the ratio of the coupling to the compositeness scale, f Λ , are determined for excited lepton masses up to the kinematic limit.

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QCD studies using a cone based jet finding algorithm for e+ e- collisions at LEP

The OPAL collaboration Akers, R. ; Alexander, G. ; Allison, John ; et al.
Z.Phys.C 63 (1994) 197-212, 1994.
Inspire Record 373000 DOI 10.17182/hepdata.48238

We describe a cone-based jet finding algorithm (similar to that used in\(\bar p\)p experiments), which we have applied to hadronic events recorded using the OPAL detector at LEP. Comparisons are made between jets defined with the cone algorithm and jets found by the “JADE” and “Durham” jet finders usually used ine+e− experiments. Measured jet rates, as a function of the cone size and as a function of the minimum jet energy, have been compared with O(αs2) calculations, from which two complementary measurements\(\alpha _s \left( {M_{Z^0 } } \right)\) have been made. The results are\(\alpha _s \left( {M_{Z^0 } } \right)\)=0.116±0.008 and\(\alpha _s \left( {M_{Z^0 } } \right)\)=0.119±0.008 respectively, where the errors include both experimental and theoretical uncertainties. Measurements are presented of the energy flow inside jets defined using the cone algorithm, and compared with equivalent data from\(\bar p\)p interactions, reported by the CDF collaboration. We find that the jets ine+e− are significantly narrower than those observed in\(\bar p\)p. The main contribution to this effect appears to arise from differences between quark- and gluon-induced jets.

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Photonic events with missing energy in e+ e- collisions at s**(1/2) = 189-GeV.

The OPAL collaboration Abbiendi, G. ; Ackerstaff, K. ; Ainsley, C. ; et al.
Eur.Phys.J.C 18 (2000) 253-272, 2000.
Inspire Record 526776 DOI 10.17182/hepdata.49967

Photonic events with large missing energy have been observed in e+e- collisions at a centre-of-mass energy of 189GeV using the OPAL detector at LEP. Results are presented for event topologies consistent with a single photon or with an acoplanar photon pair. Cross-section measurements are performed within the kinematic acceptance of each selection, and the number of light neutrino species is measured. Cross-section results are compared with the expectations from the Standard Model process e+e- to nu nubar + photon(s). No evidence is observed for new physics contributions to these final states. Upper limits are derived on sigma(e+e- to XY).BR(X to Y gamma) and sigma(e+e- to XX).BR**2(X to Y gamma) for the case of stable and invisible Y. These limits apply to single and pair production of excited neutrinos (X=nu*, Y = nu), to neutralino production (X=neutralino_2, Y=neutralino_1) and to supersymmetric models in which X = neutralino_1 and Y = light gravitino. The case of macroscopic decay lengths of particle X is considered for e+e- to XX, X to Y gamma, when M_Y is of order zero. The single-photon results are also used to place upper limits on superlight gravitino pair production as well as graviton-photon production in the context of theories with additional space dimensions.

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Dijet production in photon-photon collisions at S**(1/2)(ee) = 161-GeV and 172-GeV

The OPAL collaboration Abbiendi, G. ; Ackerstaff, K. ; Alexander, G. ; et al.
Eur.Phys.J.C 10 (1999) 547-561, 1999.
Inspire Record 474009 DOI 10.17182/hepdata.49386

Di-jet production is studied in collisions of quasi-real photons radiated by the LEP beams at e+e- centre-of-mass energies 161 and 172 GeV. The jets are reconstructed using a cone jet finding algorithm. The angular distributions of direct and double-resolved processes are measured and compared to the predictions of leading order and next-to-leading order perturbative QCD. The jet energy profiles are also studied. The inclusive two-jet cross-section is measured as a function of transverse energy and rapidity and compared to next-to-leading order perturbative QCD calculations. The inclusive two-jet cross-section as a function of rapidity is compared to the prediction of the leading order Monte Carlo generators PYTHIA and PHOJET. The Monte Carlo predictions are calculated with different parametrisations of the parton distributions of the photon. The influence of the `underlying event' has been studied to reduce the model dependence of the predicted jet cross-sections from the Monte Carlo generators.

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W+ W- production cross section and W branching fractions in e+ e- collisions at 189-GeV.

The OPAL collaboration Abbiendi, G. ; Ackerstaff, K. ; Ainsley, C. ; et al.
Phys.Lett.B 493 (2000) 249-265, 2000.
Inspire Record 533110 DOI 10.17182/hepdata.49910

From a data sample of 183 pb^-1 recorded at a center-of-mass energy of roots = 189 GeV with the OPAL detector at LEP, 3068 W-pair candidate events are selected. Assuming Standard Model W boson decay branching fractions, the W-pair production cross section is measured to be sigmaWW = 16.30 +- 0.34(stat.) +- 0.18(syst.) pb. When combined with previous OPAL measurements, the W boson branching fraction to hadrons is determined to be 68.32 +- 0.61(stat.) +- 0.28(syst.) % assuming lepton universality. These results are consistent with Standard Model expectations.

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Production of fermion-pair events in e+ e- collisions at 161-GeV centre-of-mass energy.

The OPAL collaboration Ackerstaff, K. ; Alexander, G. ; Allison, John ; et al.
Phys.Lett.B 391 (1997) 221-234, 1997.
Inspire Record 426210 DOI 10.17182/hepdata.47711

Cross-sections for hadronic and leptonic two-fermion events, and leptonic forward-backward asymmetries, have been measured in e + e − collisions at a centre-of-mass energy of 161 GeV, using the OPAL detector at LEP. Results are presented both including and excluding the dominant production of radiative γZ 0 events. We have measured R b , the ratio of the number of b b to all multihadronic events at 161 GeV, and compared it to the result obtained at 130–136 GeV. All results agree well with the Standard Model expectations. In a model-independent fit to the Z 0 lineshape, the data presented here give an improved precision on the γZ 0 -interference term. The data have also been used to obtain new limits on extensions of the Standard Model described by effective four-fermion contact interactions.

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Inclusive jet production in photon-photon collisions at s**(1/2) = 130-GeV and 136-GeV

The OPAL collaboration Ackerstaff, K. ; Alexander, G. ; Allison, John ; et al.
Z.Phys.C 73 (1997) 433-442, 1997.
Inspire Record 424635 DOI 10.17182/hepdata.47713

The inclusive one- and two-jet production cross-sections are measured in collisions of quasi-real photons radiated from the LEP beams at e+e− centre-of-mass energies \(\sqrt{s}_{\rm ee}=130\) and 136 GeV using the OPAL detector at LEP. Hard jets are reconstructed using a cone jet finding algorithm. The differential jet cross-sections \({\rm d}\sigma /{\rm d}E_{T}^{\rm jet}\) are compared to next-to-leading order perturbative QCD calculations. Transverse energy flows in jets are studied separately for direct and resolved two-photon events.

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