Triple differential dijet cross sections in e^\pm p interactions are presented in the region of photon virtualities 2<Q^2<80GeV^2, inelasticities 0.1<y<0.85, jet transverse energies E_T1>7GeV, E_T2>5GeV, and pseudorapidities -2.5 < eta_1^*, eta_2^* <0. The measurements are made in the gamma^* p centre-of-mass frame, using an integrated luminosity of 57pb^-1. The data are compared with NLO QCD calculations and LO Monte Carlo programs with and without a resolved virtual photon contribution. NLO QCD calculations fail to describe the region of low Q^2 and low jet transverse energies, in contrast to a LO Monte Carlo generator which includes direct and resolved photon interactions with both transversely and longitudinally polarised photons. Initial and final state parton showers are tested as a mechanism for including higher order QCD effects in low E_T jet production.
Triple differential dijet cross sections as a function of Q**2, ET and X(C=GAMMA).
Triple differential dijet cross sections as a function of Q**2, ET and X(C=GAMMA).
Triple differential dijet cross sections as a function of Q**2, ET and X(C=GAMMA).
Dijet production in deep inelastic ep scattering is investigated in the region of low values of the Bjorken-variable x (10^-4 < x < 10^-2) and low photon virtualities Q^2 (5 < Q^2 < 100 GeV^2). The measured dijet cross sections are compared with perturbative QCD calculations in next-to-leading order. For most dijet variables studied, these calculations can provide a reasonable description of the data over the full phase space region covered, including the region of very low x. However, large discrepancies are observed for events with small separation in azimuth between the two highest transverse momentum jets. This region of phase space is described better by predictions based on the CCFM evolution equation, which incorporates k_t factorized unintegrated parton distributions. A reasonable description is also obtained using the Color Dipole Model or models incorporating virtual photon structure.
Inclusive dijet cross section for a lower ET cut off of (5+0) GeV for the highest ET jet.
Inclusive dijet cross section for a lower ET cut off of (5+1) GeV for the highest ET jet.
Inclusive dijet cross section for a lower ET cut off of (5+2) GeV for the highest ET jet.
Di-jet producion is studied in collisions of quasi-real photons at e+e- centre- of-mass energies sqrt(s)ee from 189 to 209 GeV at LEP. The data were collected with the OPAL detector. Jets are reconstructed using an inclusive k_t clustering algorithm for all cross-section measurements presented. A cone jet algorithm is used in addition to study the different structure of the jets resulting from either of the algorithms. The inclusive di-jet cross-section is measured as a function of the mean transverse energy Etm(jet) of the two leading jets, and as a functiuon of the estimated fraction of the photon momentum carried by the parton entering the hard sub-process, xg, for different regions of Etm (jet). Angular distribution in di-jet events are measured and used to demonstrate the dominance of quark and gluon initiated processes in different regions of phase space. Furthermore the inclusive di-jet cross-section as a function of |eta(jet)| and |delta eta (jet)| is presented where eta(jet) is the jet pseudo-rapidity. Different regions of the xg+ -xg- -space are explored to study and control the influence of an underlying event. The results are compared to next-to-leading order perturbative QCD calculations and to the predictions of the leading order Monte Carlo generator PYTHIA.
The di-jet cross section as a function of the angle between the jet and thedirection of the incoming parton in the centre-of-mass frame for the region whe re both X(C=GAMMA+) and X(C=GAMMA-) are > 0.75.
The di-jet cross section as a function of the angle between the jet and thedirection of the incoming parton in the centre-of-mass frame for the region whe re both X(C=GAMMA+) and X(C=GAMMA-) are < 0.75.
The di-jet cross section as a function of the mean transverse energy of thedi-jet system for the full X(C=GAMMA+) and X(C=GAMMA-) region.
Dijet events in photon-proton collisions in which there is a large pseudorapidity separation Delta eta > 2.5 between the two highest E_T jets are studied with the H1 detector at HERA. The inclusive dijet cross sections are measured as functions of the longitudinal momentum fractions of the proton and photon which participate in the production of the jets, x_pjet and x_gjet respectively, Delta eta, the pseudorapidity separation between the two highest E_T jets, and E_T^gap, the total summed transverse energy between the jets. Rapidity gap events are defined as events in which E_T^gap is less than E_T^cut, for E_T^cut varied between 0.5 and 2.0 GeV. The fraction of dijet events with a rapidity gap is measured differentially in Delta eta, x_pjet and x_gjet. An excess of events with rapidity gaps at low values of E_T^cut is observed above the expectation from standard photoproduction processes. This excess can be explained by the exchange of a strongly interacting colour singlet object between the jets.
The inclusive dijet cross section as a function of X(C=GAMMA).
The inclusive dijet cross section as a function of XP.
The inclusive dijet cross section as a function of the rapidity gap.
Dijet cross sections as functions of several jet observables are measured in photoproduction using the H1 detector at HERA. The data sample comprises e^+p data with an integrated luminosity of 34.9 pb^(-1). Jets are selected using the inclusive k_T algorithm with a minimum transverse energy of 25 GeV for the leading jet. The phase space covers longitudinal proton momentum fraction x_p and photon longitudinal momentum fraction x_gamma in the ranges 0.05<x_p<0.6 and 0.1<x_gamma<1. The predictions of next-to-leading order perturbative QCD, including recent photon and proton parton densities, are found to be compatible with the data in a wide kinematical range.
Differential ep cross section for dijet production as a function of the invariant mass of the two jets.
Differential ep cross section for dijet production as a function of the average transverse energy the two jets.
Differential ep cross section for dijet production as a function of the maximum transverse energy the leading jet.
The charged particle multiplicities of two- and three-jet events from the reaction e+e- -> Z0 -> hadrons are measured for Z0 decays to light quark (uds) flavors. Using recent theoretical expressions to account for biases from event selection, results corresponding to unbiased gluon jets are extracted over a range of jet energies from about 11 to 30 GeV. We find consistency between these results and direct measurements of unbiased gluon jet multiplicity from upsilon and Z0 decays. The unbiased gluon jet data including the direct measurements are compared to corresponding results for quark jets. We perform fits based on analytic expressions for particle multiplicity in jets to determine the ratio r = Ng/Nq of multiplicities between gluon and quark jets as a function of energy. We also determine the ratio of slopes, r(1) = (dNg/dy)/(dNq/dy), and of curvatures, r(2) = (d2Ng/dy2)/(d2Nq/dy2), where y specifies the energy scale. At 30 GeV, we find r = 1.422 +/- 0.051, r(1) = 1.761 +/- 0.071 and r(2) = 1.98 +/- 0.13, where the uncertainties are the statistical and systematic terms added in quadrature. These results are in general agreement with theoretical predictions. In addition, we use the measurements of the energy dependence of Ng and Nq to determine an effective value of the ratio of QCD color factors, CA/CF. Our result, CA/CF = 2.23 +/- 0.14 (total), is consistent with the QCD value of 2.25.
Measurements of the mean charged particle multiplicity of biased two-jet uds flavour events from Z0 decays as a function of the transverse momentum cutoff PT(C=LU) used to separate two- and three-jet events.
Measurements of the mean charged particle multiplicity of three-jet uds flavour 'Y events' from Z0 decays, as a function of the angle THETA1 between the lowest two energy jets. The results for the quark jet scale SQRT(S(C=QQBAR)) and the gluon jet scales PT(C=LU) and PT(C=LE) are also given.
Measurements of the unbiased gluon multiplicity as a function of the energy scale Q=PT(C=LU). The corresponding bins of THETA1 in 'Y events' are also indicated.
Three-jet production is studied for the first time in deep-inelastic positron-proton scattering. The measurement carried out with the H1 detector at HERA covers a large range of four-momentum transfer squared 5 < Q^2 < 5000 GeV^2 and invariant three-jet masses 25 < M_(3jet) < 140 GeV. Jets are defined by the inclusive k_T algorithm in the Breit frame. The size of the three-jet cross section and the ratio of the three-jet to the dijet cross section R_(3/2) are described over the whole phase space by the predictions of perturbative QCD in next-to-leading order. The shapes of angular jet distributions deviate significantly from a uniform population of the available phase space but are well described by the QCD calculation.
The inclusive 3-Jet cross section as a function of Q**2.
The ratio of 3 jets to 2 jets as a function of Q**2.
The inclusive 3-JET cross section as a function of Bjorken scaling variableX for the Q**2 range 5 to 100 GeV**2.
A measurement is presented of dijet and 3-jet cross sections in low-|t| diffractive deep-inelastic scattering interactions of the type ep -> eXY, where the system X is separated by a large rapidity gap from a low-mass baryonic system Y. Data taken with the H1 detector at HERA, corresponding to an integrated luminosity of 18.0 pb^(-1), are used to measure hadron level single and double differential cross sections for 4<Q^2<80 GeV^2, x_pom<0.05 and p_(T,jet)>4 GeV. The energy flow not attributed to jets is also investigated. The measurements are consistent with a factorising diffractive exchange with trajectory intercept close to 1.2 and tightly constrain the dominating diffractive gluon distribution. Viewed in terms of the diffractive scattering of partonic fluctuations of the photon, the data require the dominance of qqbarg over qqbar states. Soft colour neutralisation models in their present form cannot simultaneously reproduce the shapes and the normalisations of the differential cross sections. Models based on 2-gluon exchange are able to reproduce the shapes of the cross sections at low x_pom values.
Average values, over the specified interval, of the differential hadron level dijet cross section as a function of Q**2.
Average values, over the specified interval, of the differential hadron level dijet cross section as a function of the average transverse momentum of the two jets in the c.m.frame.
Average values, over the specified interval, of the differential hadron level dijet cross section as a function of the average pseudorapidity of the two jets in the lab frame.
Jet production is studied in the Breit frame in deep-inelastic positron-proton scattering over a large range of four-momentum transfers 5 < Q^2 < 15000 GeV^2 and transverse jet energies 7 < E_T < 60 GeV. The analysis is based on data corresponding to an integrated luminosity of L_int \simeq 33 pb^(-1) taken in the years 1995-1997 with the H1 detector at HERA at a center-of-mass energy sqrt(s)=300 GeV. Dijet and inclusive jet cross sections are measured multi-differentially using k_perp and angular ordered jet algorithms. The results are compared to the predictions of perturbative QCD calculations in next-to-leading order in the strong coupling constant alphas.QCD fits are performed in which alphas and the gluon density in the proton are determined separately. The gluon density is found to be in good agreement with results obtained in other analyses using data from different processes. The strong coupling constant is determined to be alphas(MZ)=0.1186+-0.0059. In addition an analysis of the data in which both alphas and the gluon density are determined simultaneously is presented.
Inclusive single jet cross section as a function of ET and Q**2.. Data are analysed in the Breit frame using the inclusive kT alogrithm.
Inclusive dijet cross section as a function Q**2.. Data are analysed in the Breit frame using the inclusive kT alogrithm.
Inclusive di-jet cross section as a function of ET and Q**2.. Data are analysed in the Breit frame using the inclusive kT alogrithm.
Jet production in charged and neutral current events in the kinematic range of Q^2 from 640 to 35000 GeV^2 is studied in deep-inelastic positron-proton scattering at HERA. The measured rate of multi-jet events and distributions of jet polar angle, transverse energy, dijet mass, and other dijet variables are presented. Using parton densities derived from inclusive DIS cross sections, perturbative QCD calculations in NLO are found to give a consistent description of both the neutral and charged current dijet production. A direct, model independent comparison of the jet distributions in charged and neutral current events confirms that the QCD dynamics of the hadronic final state is independent of the underlying electroweak scattering process.
Rates of charged current events as a function of Q**2.
Rates of neutral current events as a function of Q**2.
Normalised distribution in Y2 for NC and CC dijet events. Y2 is the smallest scaled value of KT (KTJET**2/W**2) given by the combination of (2+1) jets. The +1 refers to the proton remnant jet.
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