54 research outputs found
Measurement of inclusive D*+- and associated dijet cross sections in photoproduction at HERA
Inclusive photoproduction of D*+- mesons has been measured for photon-proton
centre-of-mass energies in the range 130 < W < 280 GeV and a photon virtuality
Q^2 < 1 GeV^2. The data sample used corresponds to an integrated luminosity of
37 pb^-1. Total and differential cross sections as functions of the D*
transverse momentum and pseudorapidity are presented in restricted kinematical
regions and the data are compared with next-to-leading order (NLO) perturbative
QCD calculations using the "massive charm" and "massless charm" schemes. The
measured cross sections are generally above the NLO calculations, in particular
in the forward (proton) direction. The large data sample also allows the study
of dijet production associated with charm. A significant resolved as well as a
direct photon component contribute to the cross section. Leading order QCD
Monte Carlo calculations indicate that the resolved contribution arises from a
significant charm component in the photon. A massive charm NLO parton level
calculation yields lower cross sections compared to the measured results in a
kinematic region where the resolved photon contribution is significant.Comment: 32 pages including 6 figure
Measurement of Jet Shapes in Photoproduction at HERA
The shape of jets produced in quasi-real photon-proton collisions at
centre-of-mass energies in the range 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 plane with a
cone radius of one unit. Measured jet shapes both in inclusive jet and dijet
production with transverse energies GeV are presented. The jet
shape broadens as the jet pseudorapidity () increases and narrows
as 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 and low . The observed
broadening of the jet shape as increases is consistent with the
predicted increase in the fraction of final state gluon jets.Comment: 29 pages including 9 figure
Measurement of the Diffractive Cross Section in Deep Inelastic Scattering using ZEUS 1994 Data
The DIS diffractive cross section, , has been measured in the mass range GeV for c.m. energies GeV and photon virtualities to 140 GeV. For fixed and , the diffractive cross section rises rapidly with , with corresponding to a -averaged pomeron trajectory of \bar{\alphapom} = 1.127 \pm 0.009 (stat)^{+0.039}_{-0.012} (syst) which is larger than \bar{\alphapom} observed in hadron-hadron scattering. The dependence of the diffractive cross section is found to be the same as that of the total cross section for scattering of virtual photons on protons. The data are consistent with the assumption that the diffractive structure function factorizes according to \xpom F^{D(3)}_2 (\xpom,\beta,Q^2) = (x_0/ \xpom)^n F^{D(2)}_2(\beta,Q^2). They are also consistent with QCD based models which incorporate factorization breaking. The rise of \xpom F^{D(3)}_2 with decreasing \xpom and the weak dependence of on suggest a substantial contribution from partonic interactions
Exclusive Electroproduction of and Mesons at HERA
Exclusive production of and mesons in e^+ p collisions has
been studied with the ZEUS detector in the kinematic range for the data and for the data. Cross sections for exclusive and
production have been measured as a function of and . The
spin-density matrix elements and have
been determined for exclusive production as well as and
for exclusive production.
The results are discussed in the context of theoretical models invoking soft
and hard phenomena.Comment: 57 pages including 21 figures, minor modifications to Figs. 19-21,
these figures supercede those of Eur. Phys. J. C6 (1999) 603-62
Measurement of the F2 structure function in deep inelastic ep scattering using 1994 data from the ZEUS detector at HERA
We present measurements of the structure function \Ft\ in e^+p scattering at HERA in the range 3.5\;\Gevsq < \qsd < 5000\;\Gevsq. A new reconstruction method has allowed a significant improvement in the resolution of the kinematic variables and an extension of the kinematic region covered by the experiment. At \qsd < 35 \;\Gevsq the range in x now spans 6.3\cdot 10^{-5} < x < 0.08 providing overlap with measurements from fixed target experiments. At values of Q^2 above 1000 GeV^2 the x range extends to 0.5. Systematic errors below 5\perc\ have been achieved for most of the kinematic urray, W
Comparison of ZEUS data with standard model predictions for scattering at high and
Using the ZEUS detector at HERA, we have studied the reaction e(+)p --> e(+)X for Q(2) > 5000 GeV2 with a 20.1 pb(-1) data sample collected during the years 1993 to 1996. For Q(2) below 15000 GeV2, the data are in good agreement with Standard Model expectations. For Q(2) > 35000 GeV2. two events are observed while 0.145 +/- 0.013 events are expected, A statistical analysis of a large ensemble of simulated Standard Model experiments indicates that with probability 6.0%, an excess at least as unlikely as that observed would occur above some Q(2) cut. For x > 0.55 and y > 0.75, four events are observed where 0.91 +/- 0.08 events are expected, A statistical analysis of the two-dimensional distribution of the events in x and y yields a probability of 0.72% for the region x > 0.55 and y > 0.25 and a probability of 7.8% for the entire Q(2) > 5000 GeV2 data sample. The observed excess above Standard Model expectations is particularly interesting because it occurs in a previously unexplored kinematic region
Measurement of Elastic Photoproduction at HERA
The production of mesons in the reaction () at a median of $10^{-4} \
\rm{GeV^2}\phid\sigma/dt0.1<|t|<0.5 \ \rm{GeV^2}60 <
W < 80 \ \rm{GeV}\sigma_{\gamma p
\rightarrow \phi p} = 0.96 \pm 0.19^{+0.21}_{-0.18}\rm{\mu b}\sigma_{\gamma p \rightarrow
\phi p}t\phis\phi$ photoproduction are
compatible with those of a soft diffractive process.Comment: 23 pages, including 6 post script figure
Novel isoxazolidine analogues of homonucleosides and homonucleotides
Isoxazolidine analogues of homonucleos(t)ides were synthesized from nucleobase-derived nitrones 20a-20e (uracil, 5-fluorouracil, 5-bromouracil, thymine, adenine) employing 1,3-dipolar cycloadditions with allyl alcohol as well as with alkenylphosphonates (allyl-, allyloxymethyl- and vinyloxymethyl- and vinylphosphonate). Besides reactions with vinylphosphonate the additions proceeded regioselectively to produce mixtures of major cis and minor trans 3,5-disubstituted isoxazolidines (d.e. 28-82%). From vinylphosphonate up to 10% of 3,4-disubstituted isoxazolidines was additionally produced. Vicinal couplings, shielding effects and 2D NOE correlations were employed in configurational assignments as well as in conformational analysis to find out preferred conformations for several isoxazolidines and to observe anomeric effects (pseudoaxial orientation of phosphonylmethoxy groups) for those obtained from vinyloxymethylphosphonate. None of the tested compounds were endowed in vitro with antiviral activity against a variety of DNA and RNA viruses at subtoxic concentrations (up to 250 μM) nor exhibited antiproliferative activity towards L1210, CEM, and HeLa cells (IC50 = ≥100 μM).status: publishe
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