High-ET inclusive jet cross sections in photoproduction at HERA
European Physical Journal C 4:4 (1998) 591-606
Abstract:
Inclusive jet differential cross sections for the reaction e+p → e+ + jet + X with quasi-real photons have been measured with the ZEUS detector at HERA. These cross sections are given for the photon-proton centre-of-mass energy interval 134 < W < 277 GeV and jet pseudorapidity in the range -1 < ηjet < 2 in the laboratory frame. The results are presented for three cone radii in the η - φ plane, R = 1.0, 0.7 and 0.5. Measurements of dσ/dηjet above various jet-transverse-energy thresholds up to 25 GeV and in three ranges of W are presented and compared to next-to-leading order (NLO) QCD calculations. For jets defined with R = 1.0 differences between data and NLO calculations are seen at high ηjet and low EjetT. The measured cross sections for jets defined with R = 0.7 are well described by the calculations in the entire measured range ηjet and EjetT. The inclusive jet cross section for EjetT > 21 GeV is consistent with an approximately linear variation with the cone radius R in the range between 0.5 and 1.0, and with NLO calculations.Investigation of the splitting of quark and gluon jets
European Physical Journal C 4:1 (1998) 1-17
Abstract:
The splitting processes in identified quark and gluon jets are investigated using longitudinal and transverse observables. The jets are selected from symmetric three-jet events measured in Z decays with the DELPHI detector in 1991-1994. Gluon jets are identified using heavy quark anti-tagging. Scaling violations in identified gluon jets are observed for the first time. The scale energy dependence of the gluon fragmentation function is found to be about two times larger than for the corresponding quark jets, consistent with the QCD expectation CA/CF. The primary splitting of gluons and quarks into subjets agrees with fragmentation models and, for specific regions of the jet resolution y, with NLLA calculations. The maximum of the ratio of the primary subjet splittings in quark and gluon jets is 2.77 ± 0.11 ± 0.10. Due to non-perturbative effects, the data are below the expectation at small y. The transition from the perturbative to the non-perturbative domain appears at smaller y for quark jets than for gluon jets. Combined with the observed behaviour of the higher rank splittings, this explains the relatively small multiplicity ratio between gluon and quark jets.Measurement of jet shapes in photoproduction at HERA
European Physical Journal C 2:1 (1998) 61-75
Abstract:
The shape of jets produced in quasi-real photon-proton collisions at centre-of-mass energies in the range 134-277 GeV has been measured using the hadronic energy flow. The measurement was done with the ZEUS detector at HERA. Jets are identified using a cone algorithm in the η - φ plane with a cone radius of one unit. Measured jet shapes both in inclusive jet and dijet production with transverse energies EjetT > 14 GeV are presented. The jet shape broadens as the jet pseudorapidity (ηjet) increases and narrows as EjetT increases. In dijet photoproduction, the jet shapes have been measured separately for samples dominated by resolved and by direct processes. Leading-logarithm parton-shower Monte Carlo calculations of resolved and direct processes describe well the measured jet shapes except for the inclusive production of jets with high ηjet and low EjetT. The observed broadening of the jet shape as ηjet increases is consistent with the predicted increase in the fraction of final state gluon jets.Measurement of the W-pair cross-section and of the W mass in e+e- interactions at 172 GeV
European Physical Journal C 2:4 (1998) 581-595
Abstract:
From a data sample of 9.98 pb-1 integrated luminosity, collected by DELPHI at a centre-of-mass energy of 172 GeV, 118 events were selected as W-pair candidates. From these, the branching fraction Br(W → qq̄) was measured to be 0.660+0.036-0.037 (stat.) ± 0.009(syst.) and the cross-section for the doubly resonant process e+e- → W+W- to be 11.58+1.44-1.35 (stat.) ± 0.32(syst.) pb. The mass of the W boson, obtained from direct reconstruction of the invariant mass of the fermion pairs in the decays WW → ℓvqq̄ and WW → qq̄qq̄, was determined to be mW = 80.22±0.41(stat.)±0.04(syst.)±0.05(int.)±0.03(LEP) GeV/c2, where "int." denotes the uncertainty from interconnection effects like colour reconnection and Bose-Einstein interference. Combined with the W mass obtained from the cross-sections measured by DELPHI at threshold, a value of mW = 80.33 ± 0.30(stat.) ± 0.05(syst.) ± 0.03(int.) ± 0.03(LEP) GeV/c2 was found.Measurement of the diffractive structure function FD(4)2 at HERA
European Physical Journal C 1:1-2 (1998) 81-96