Three- and four-jet production is measured in deep-inelastic $ep$ scattering at low $x$ and $Q^2$ with the H1 detector using an integrated luminosity of $44{.}2 {\rm pb}^{-1}$. Several phase space regions are selected for the three-jet analysis in order to study the underlying parton dynamics from global topologies to the more restrictive regions of forward jets close to the proton direction. The measurements of cross sections for events with at least three jets are compared to fixed order QCD predictions of ${\mathcal{O}}(\alpha_{\rm s}^2)$ and ${\mathcal{O}}(\alpha_{\rm s}^3) $ and with Monte Carlo simulation programs where higher order effects are approximated by parton showers. A good overall description is provided by the ${\mathcal{O}}(\alpha_{\rm s}^3) $ calculation. Too few events are predicted at the lowest $x \sim 10^{-4}$, especially for topologies with two forward jets. This hints to large contributions at low $x$ from initial state radiation of gluons close to the proton direction and unordered in transverse momentum. The Monte Carlo program in which gluon radiation is generated by the colour dipole model gives a good description of both the three- and the four-jet data in absolute normalisation and shape.
Differential cross section as a function of the minimum number of jet for events with at least 3-jets.
Differential cross section as a function of X for events with at least 3-jets.
Differential cross section for events with at least 3-jets as a function of the pseudorapidity of each jet.
Diffractive photoproduction of dijets was measured with the ZEUS detector at the ep collider HERA using an integrated luminosity of 77.2 pb-1. The measurements were made in the kinematic range Q^2 < 1 GeV^2, 0.20 < y < 0.85 and x_pom < 0.025, where Q^2 is the photon virtuality, y is the inelasticity and x_pom is the fraction of the proton momentum taken by the diffractive exchange. The two jets with the highest transverse energy, E_T^jet, were required to satisfy E_T^jet > 7.5 and 6.5 GeV, respectively, and to lie in the pseudorapidity range -1.5 < eta^jet < 1.5. Differential cross sections were compared to perturbative QCD calculations using available parameterisations of diffractive parton distributions of the proton.
Differential cross section DSIG/DY for diffractive photoproduction of dijets as a function of Y.
Differential cross section DSIG/DM(P=5_6_7) for diffractive photoproduction of dijets as a function of M(P=5_6_7).
Differential cross section DSIG/DX(NAME=POMERON) for diffractive photoproduction of dijets as a function of X(NAME=POMERON).
Differential dijet cross sections in diffractive deep-inelastic scattering are measured with the H1 detector at HERA using an integrated luminosity of 51.5 pb-1. The selected events are of the type ep --> eXY, where the system X contains at least two jets and is well separated in rapidity from the low mass proton dissociation system Y. The dijet data are compared with QCD predictions at next-to-leading order based on diffractive parton distribution functions previously extracted from measurements of inclusive diffractive deep-inelastic scattering. The prediction describes the dijet data well at low and intermediate zpom (the fraction of the momentum of the diffractive exchange carried by the parton entering the hard interaction) where the gluon density is well determined from the inclusive diffractive data, supporting QCD factorisation. A new set of diffractive parton distribution functions is obtained through a simultaneous fit to the diffractive inclusive and dijet cross sections. This allows for a precise determination of both the diffractive quark and gluon distributions in the range 0.05<zpom<0.9. In particular, the precision on the gluon density at high momentum fractions is improved compared to previous extractions.
Integrated cross section within the specified kinematic range.
Bin averaged differential cross sections of diffractive di-jet production as a function of X(NAME=POMERON).
Bin averaged differential cross sections of diffractive di-jet production as a function of Y.
Forward jet cross sections have been measured in neutral current deep inelastic scattering at low Bjorken-x with the ZEUS detector at HERA using an integrated luminosity of ${81.8 \rm pb}^{-1}$. Measurements are presented for inclusive forward jets as well as for forward jets accompanied by a dijet system. The explored phase space, with jet pseudorapidity up to 4.3 is expected to be particularly sensitive to the dynamics of QCD parton evolution at low x. The measurements are compared to fixed-order QCD calculations and to leading-order parton-shower Monte Carlo models.
Differential cross section DSIG/DQ**2 in bins of Q**2 .
Differential cross section DSIG/DX in bins of X .
Differential cross section DSIG/DET(P=4) in bins of ET(P=4) .
The cross section for high-E_T dijet production in photoproduction has been measured with the ZEUS detector at HERA using an integrated luminosity of 81.8 pb-1. The events were required to have a virtuality of the incoming photon, Q^2, of less than 1 GeV^2 and a photon-proton centre-of-mass energy in the range 142 < W < 293 GeV. Events were selected if at least two jets satisfied the transverse-energy requirements of E_T(jet1) > 20 GeV and E_T(jet2) > 15 GeV and pseudorapidity requirements of -1 < eta(jet1,2) < 3, with at least one of the jets satisfying -1 < eta(jet) < 2.5. The measurements show sensitivity to the parton distributions in the photon and proton and effects beyond next-to-leading order in QCD. Hence these data can be used to constrain further the parton densities in the proton and photon.
Cross section D(SIG)/(ET(P=4)+ET(P=5))/2 as a function of (ET(P=4)+ET(P=5))/2 for X(C=GAMMA,OBS) > 0.75 .
Cross section D(SIG)/(ET(P=4)+ET(P=5))/2 as a function of (ET(P=4)+ET(P=5))/2 for X(C=GAMMA,OBS) <= 0.75 .
Cross section D(SIG)/ET(P=4) as a function of ET(P=4) for X(C=GAMMA,OBS) > 0.75 .
Inclusive beauty-quark production in two-photon collisions has been measured at LEP using an integrated luminosity of 698 pb-1 collected by the ALEPH detector with sqrt(s) between 130 and 209 GeV. The b quarks were identified using lifetime information. The cross section is found to be sigma(e+ e- --> e+ e- b \bar{b} X) = (5.4 +/- 0.8 (stat) +/- 0.8 (syst)) pb which is consistent with Next-to-Leading Order QCD.
Cross section for the process E+ E- --> E+ E- BQUARK BQUARKBAR X.
Inclusive dijet and trijet production in deep inelastic $ep$ scattering has been measured for $10<Q^2<100$ GeV$^2$ and low Bjorken $x$, $10^{-4}<x_{\rm Bj}<10^{-2}$. The data were taken at the HERA $ep$ collider with centre-of-mass energy $\sqrt{s} = 318 \gev$ using the ZEUS detector and correspond to an integrated luminosity of $82 {\rm pb}^{-1}$. Jets were identified in the hadronic centre-of-mass (HCM) frame using the $k_{T}$ cluster algorithm in the longitudinally invariant inclusive mode. Measurements of dijet and trijet differential cross sections are presented as functions of $Q^2$, $x_{\rm Bj}$, jet transverse energy, and jet pseudorapidity. As a further examination of low-$x_{\rm Bj}$ dynamics, multi-differential cross sections as functions of the jet correlations in transverse momenta, azimuthal angles, and pseudorapidity are also presented. Calculations at $\mathcal{O}(\alpha_{s}^3)$ generally describe the trijet data well and improve the description of the dijet data compared to the calculation at $\mathcal{O}(\alpha_{s}^2)$.
Two jet cross section D(SIG)/DQ**2 as a function of Q**2.
Two jet cross section D(SIG)/DX as a function of X.
Two jet cross section D(SIG)/DET(P=4,RF=CM) as a function of ET(P=4,RF=CM).
The first observation of (anti)deuterons in deep inelastic scattering at HERA has been made with the ZEUS detector at a centre-of-mass energy of 300--318 GeV using an integrated luminosity of 120 pb-1. The measurement was performed in the central rapidity region for transverse momentum per unit of mass in the range 0.3
Measured invariant cross section for P production.
Measured invariant cross section for DEUT production.
Measured invariant cross section for PBAR production.
The average charged track multiplicity and the normalised distribution of the scaled momentum, $\xp$, of charged final state hadrons are measured in deep-inelastic $\ep$ scattering at high $Q^2$ in the Breit frame of reference. The analysis covers the range of photon virtuality $100 < Q^2 < 20 000 \GeV^{2}$. Compared with previous results presented by HERA experiments this analysis has a significantly higher statistical precision and extends the phase space to higher $Q^{2}$ and to the full range of $\xp$. The results are compared with $e^+e^-$ annihilation data and with various calculations based on perturbative QCD using different models of the hadronisation process.
Average values of Q and X (plus errors) for the different Q**2 ranges.
Average charged hadron multiplicity as a function of Q**2.
Normalised distribution of the scaled momentum as a function of Q**2 in the X range 0 to 0.02.
Inclusive jet production is studied in neutral current deep-inelastic positron-proton scattering at large four momentum transfer squared Q^2>150 GeV^2 with the H1 detector at HERA. Single and double differential inclusive jet cross sections are measured as a function of Q^2 and of the transverse energy E_T of the jets in the Breit frame. The measurements are found to be well described by calculations at next-to-leading order in perturbative QCD. The running of the strong coupling is demonstrated and the value of alpha_s(M_Z) is determined. The ratio of the inclusive jet cross section to the inclusive neutral current cross section is also measured and used to extract a precise value for alpha_s(M_Z)=0.1193+/-0.0014(exp.)^{+0.0047}_{-0.0030}(th.)+/-0.0016(pdf).
Double differential cross section as a function of ET integrated over the ET bin range and the Q**2 range 150 to 200 GeV**2.
Double differential cross section as a function of ET integrated over the ET bin range and the Q**2 range 200 to 270 GeV**2.
Double differential cross section as a function of ET integrated over the ET bin range and the Q**2 range 270 to 400 GeV**2.
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