LHCb

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.

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.

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.

Abstract:

This paper presents the first analysis of diffractive photon dissociation events in deep inelastic positron-proton scattering at HERA in which the proton in the final state is detected and its momentum measured. The events are selected by requiring a scattered proton in the ZEUS leading proton spectrometer (LPS) with xL > 0.97, where xL is the fraction of the incoming proton beam momentum carried by the scattered proton. The use of the LPS significantly reduces the contamination from events with diffractive dissociation of the proton into low mass states and allows a direct measurement of t, the square of the four-momentum exchanged at the proton vertex. The dependence of the cross section on t is measured in the interval 0.073 < \t\ < 0.4 GeV² and is found to be described by an exponential shape with the slope parameter b = 7.2 ± 1.1(stat.)^+0.7 -0.9(syst.) GeV^-2. The diffractive structure function F^D(4) 2 is presented as a function of xIP, ≃ 1 - xL and β, the momentum fraction of the struck quark with respect to xIP, and averaged over the t interval 0.073 < \t\ < 0.4 GeV² and the photon virtuality range 5 < Q² < 20 GeV². In the kinematic range 4 x 10^-4 < xIP < 0.03 and 0.015 < β < 0.5, the xIP dependence of F^D(4) 2 is fitted with a form (1/xIP)^a , yielding a = 1.00 ± 0.09 (stat.)^+0.11 -0.05(syst.). Upon integration over t, the structure function F^D(3) 2 is determined in a kinematic range extending to higher xIP and lower β compared to our previous analysis; the results are discussed within the framework of Regge theory.

Abstract:

Photon diffractive dissociation, γp → Xp, has been studied at HERA with the ZEUS detector using ep interactions where the virtuality Q2 of the exchanged photon is smaller than 0.02 GeV2. The squared four-momentum t exchanged at the proton vertex was determined in the range 0.073 < \t\ < 0.40 GeV2 by measuring the scattered proton in the ZEUS Leading Proton Spectrometer. In the photonproton centre-of-mass energy interval 176 < W < 225 GeV and for masses of the dissociated photon system 4 < Mx < 32 GeV, the t distribution has an exponential shape, dN/d\t\ ∝ exp (-b\t\), with a slope parameter b = 6.8 ± 0.9 (stat.) +1.2-1.1 (syst.) GeV-2.