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Register a new userProduction of exclusive dijets in diffractive deep inelastic scattering at HERA
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Production of exclusive dijets in diffractive deep inelastic $e^pm p$ scattering has been measured with the ZEUS detector at HERA using an integrated luminosity of 372 pb$^{-1}$. The measurement was performed for $gamma^*-p$ centre-of-mass energies in the range $90 < W < 250$ GeV and for photon virtualities $Q^2 > 25$ GeV$^2$. Energy and transverse-energy flows around the jet axis are presented. The cross section is presented as a function of $beta$ and $phi$, where $beta=x/x_{rm I!P}$, $x$ is the Bjorken variable and $x_{rm I!P}$ is the proton fractional longitudinal momentum loss. The angle $phi$ is defined by the $gamma^*-$dijet plane and the $gamma^*-e^pm$ plane in the rest frame of the diffractive final state. The $phi$ cross section is measured in bins of $beta$. The results are compared to predictions from models based on different assumptions about the nature of the diffractive exchange.
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Measurements of $D^{*}(2010)$ meson production in diffractive deep inelastic scattering $(5<Q^{2}<100~{rm GeV}^{2})$ are presented which are based on HERA data recorded at a centre-of-mass energy $sqrt{s} = 319{rm~GeV}$ with an integrated luminosity of $287$ pb$^{-1}$. The reaction $ep rightarrow eXY$ is studied, where the system $X$, containing at least one $D^{*}(2010)$ meson, is separated from a leading low-mass proton dissociative system $Y$ by a large rapidity gap. The kinematics of $D^{*}$ candidates are reconstructed in the $D^{*}rightarrow K pipi$ decay channel. The measured cross sections compare favourably with next-to-leading order QCD predictions, where charm quarks are produced via boson-gluon fusion. The charm quarks are then independently fragmented to the $D^{*}$ mesons. The calculations rely on the collinear factorisation theorem and are based on diffractive parton densities previously obtained by H1 from fits to inclusive diffractive cross sections. The data are further used to determine the diffractive to inclusive $D^{*}$ production ratio in deep inelastic scattering.
Recent results from jet production in deep inelastic ep scattering at HERA are reviewed. The values of alpha_s(M_z) extracted from a QCD analysis of the data are presented.
Charm production in charged current deep inelastic scattering has been measured for the first time in $e^{pm}p$ collisions, using data collected with the ZEUS detector at HERA, corresponding to an integrated luminosity of $358 pb^{-1}$. Results are presented separately for $e^{+}p$ and $e^{-}p$ scattering at a centre-of-mass energy of $sqrt{s} = 318 GeV$ within a kinematic phase-space region of $200 GeV^{2}<Q^{2}<60000 GeV^{2}$ and $y<0.9$, where $Q^{2}$ is the squared four-momentum transfer and $y$ is the inelasticity. The measured cross sections of electroweak charm production are consistent with expectations from the Standard Model within the large statistical uncertainties.
This contribution covers three recent results on deep-inelastic scattering at HERA: (i) new measurements of the proton longitudinal structure function $F_L$ from H1 and ZEUS experiments, (ii) a dedicated NC cross section measurement from ZEUS in the region of high Bjorken $x$, and (iii) preliminary combination results of all HERA inclusive data published up to now by H1 and ZEUS, taking into account the experimental correlations between measurements.
New results on diffractive deep-inelastic $e p$ scattering at HERA are presented using data taken in 1994 with the H1 detector. The cross section for diffractive deep-inelastic scattering is measured in terms of a diffractive structure function $F_2^{D(3)}(beta,Q^2,xpom)$ over an extended kinematic range. The dependence of $F_2^{D(3)}$ on $xpom$ is found not to depend on $Q^2$, but to depend on $beta$. Therefore the $xpom$ dependence no longer factorizes. The $Q^2$ and $beta$ dependence of $F_2^{D(3)}$ is analyzed after an integration over the dependence on $xpom$. For fixed $beta$ a clear rise with $log Q^2$ is observed, persisting up to high values of $beta$. In terms of the Altarelli-Parisi (DGLAP) QCD evolution equations, these scaling violations give clear indications for a gluon dominated process. Subsequently an attempt is made to quantify the parton content of the diffractive exchange using the DGLAP evolution. At the starting scale a ``leading gluon distribution is found which contributes about $80 %$ of the momentum in the diffractive exchange. Measurements of the hadronic final state (energy flow and production of $D^{*}$ mesons) are found to be consistent with the predictions of a model of deep-inelastic electron pomeron scattering using the information on the parton content obtained.
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