12 research outputs found
Observation of hard scattering in photoproduction events with a large rapidity gap at HERA
Events with a large rapidity gap and total transverse energy greater than 5
GeV have been observed in quasi-real photoproduction at HERA with the ZEUS
detector. The distribution of these events as a function of the
centre of mass energy is consistent with diffractive scattering. For total
transverse energies above 12 GeV, the hadronic final states show predominantly
a two-jet structure with each jet having a transverse energy greater than 4
GeV. For the two-jet events, little energy flow is found outside the jets. This
observation is consistent with the hard scattering of a quasi-real photon with
a colourless object in the proton.Comment: 19 pages, latex, 4 figures appended as uuencoded fil
Accumulation of manganese in Neisseria gonorrhoeae correlates with resistance to oxidative killing by superoxide anion and is independent of superoxide dismutase activity
As a facultative aerobe with a high iron requirement and a highly active aerobic respiratory chain, Neisseria gonorrhoeae requires defence systems to respond to toxic oxygen species such as superoxide. It has been shown that supplementation of media with 100 muM Mn(II) considerably enhanced the resistance of this bacterium to oxidative killing by superoxide. This protection was not associated with the superoxide dismutase enzymes of N. gonorrhoeae. In contrast to previous studies, which suggested that some strains of N. gonorrhoeae might not contain a superoxide dismutase, we identified a sodB gene by genome analysis and confirmed its presence in all strains examined by Southern blotting, but found no evidence for sodA or sodC. A sodB mutant showed very similar susceptibility to superoxide killing to that of wild-type cells, indicating that the Fe-dependent SOD B did not have a major role in resistance to oxidative killing under the conditions tested. The absence of a sodA gene indicated that the Mn-dependent protection against oxidative killing was independent of Mn-dependent SOD A. As a sodB mutant also showed Mn-dependent resistance to oxidative killing, then it is concluded that this resistance is independent of superoxide dismutase enzymes. Resistance to oxidative killing was correlated with accumulation of Mn(II) by the bacterium. We hypothesize that this bacterium uses Mn(II) as a chemical quenching agent in a similar way to the already established process in Lactobacillus plantarum. A search for putative Mn(II) uptake systems identified an ABC cassette-type system (MntABC) with a periplasmic-binding protein (MntC). An mntC mutant was shown to have lowered accumulation of Mn(II) and was also highly susceptible to oxidative killing, even in the presence of added Mn(II). Taken together, these data show that N. gonorrhoeae possesses a Mn(II) uptake system that is critical for resistance to oxidative stress
Comparison of energy flows in deep inelastic scattering events with and without a large rapidity gap
Energy flows in deep inelastic electron-proton scattering are investigated at a center-of-mass energy of 296 GeV for the range Q2 greater-than-or-equal-to 10 GeV2 using the ZEUS detector. A comparison is made between events with and without a large rapidity gap between the hadronic system and the proton direction. The energy flows, corrected for detector acceptance and resolution, are shown for these two classes of events in both the HERA laboratory frame and the Breit frame. From the differences in the shapes of these energy flows we conclude that QCD radiation is suppressed in the large-rapidity-gap events compared to the events without a large rapidity gap
Study of D*(2010)+- Production in ep Collisions at HERA
We report the first observation of charmed mesons with the ZEUS detector at HERA using the decay channel D*(+) --> (D-0 --> K(-)pi(+)) pi(+) (+ c.c.). Clear signals in the mass difference Delta M=M(D*)-M(D-0) as well as in the M(K pi) distribution at the D-0 mass are found. The ep cross section for inclusive D*(+/-) production with Q(2) c ) over bar cX)=(0.45+/-0.11(-0.22)(+0.37)) mu b at root s=296 GeV and [W]=198 GeV. The average gamma p charm cross section sigma(gamma p --> c $($) over bar cX) is found to be (6.3+/-2.2(-3.0)(+6.3)) mu b at [W]=163 GeV and (16.9+/-5.2(-8.5)(+13.9)) mu b at [W]=243 GeV. The increase of the total charm photoproduction cross section by one order of magnitude with respect to low energy data experiments is well described by QCD NLO calculations using singular gluon distributions in the proton
Exclusive rho0 production in deep inelastic electron - proton scattering at HERA
The exclusive production of mesons in deep inelastic electron-proton
scattering has been studied using the ZEUS detector. Cross sections have been
measured in the range GeV for centre of mass
(c.m.) energies from 40 to 130 GeV. The cross section
exhibits a dependence and both
longitudinally and transversely polarised 's are observed. The
cross section rises strongly with increasing c.m.
energy, when compared with NMC data at lower energy, which cannot be explained
by production through soft pomeron exchange. The data are compared with
perturbative QCD calculations where the rise in the cross section reflects the
increase in the gluon density at low . the gluon density at low .Comment: 22 pages, latex, 4 figures appende
Diffractive hard photoproduction at HERA and evidence for the gluon content of the pomeron
Inclusive jet cross sections for events with a large rapidity gap with
respect to the proton direction from the reaction with quasi-real photons have been measured with the ZEUS detector. The cross
sections refer to jets with transverse energies ~GeV. The data
show the characteristics of a diffractive process mediated by pomeron exchange.
Assuming that the events are due to the exchange of a pomeron with partonic
structure, the quark and gluon content of the pomeron is probed at a scale
. A comparison of the measurements with model predictions
based on QCD plus Regge phenomenology requires a contribution of partons with a
hard momentum density in the pomeron. A combined analysis of the jet cross
sections and recent ZEUS measurements of the diffractive structure function in
deep inelastic scattering gives the first experimental evidence for the gluon
content of the pomeron in diffractive hard scattering processes. The data
indicate that between 30\% and 80\% of the momentum of the pomeron carried by
partons is due to hard gluons.Comment: 25 pages, latex, 4 figures appended as uuencoded file. No change
w.r.t. previous version (just avoiding warning latex messages while
processing
Dijet cross-sections in photoproduction at HERA
Dijet production by almost real photons has been studied at HERA with the
ZEUS detector. Jets have been identified using the cone algorithm. A cut on xg,
the fraction of the photon energy participating in the production of the two
jets of highest transverse energy, is used to define cross sections sensitive
to the parton distributions in the proton and in the photon. The dependence of
the dijet cross sections on pseudorapidity has been measured for xg
and xg . The former is sensitive to the gluon momentum density in the
proton. The latter is sensitive to the gluon in the photon. The cross sections
are corrected for detector acceptance and compared to leading order QCD
calculations.Comment: 21 pages, latex, 4 figures appended as uuencoded fil
Measurement of alpha-s from jet rates in deep inelastic scattering at HERA
Jet production in deep inelastic scattering for ~GeV has
been studied using data from an integrated luminosity of 3.2~pb
collected with the ZEUS detector at HERA. Jets are identified with the JADE
algorithm. A cut on the angular distribution of parton emission in the
-parton centre-of-mass system minimises the experimental and
theoretical uncertainties in the determination of the jet rates. The jet rates,
when compared to (^2\alpha_{s}(Q)Q^2\alpha_{s}(Q)Q=M_{Z^0}\alpha_{s}(M_{Z^0}) =
0.117~\pm~0.005~(stat)~^{+0.004}_{-0.005}~(syst_{exp})~
~{\pm~0.007}~(syst_{theory})$.Comment: 22 pages, latex, 4 figures appended as uuencoded fil
Inclusive jet differential cross-sections in photoproduction at HERA
Inclusive jet differential cross sections for the reaction ep --> jet + X at Q(2) below 4 GeV2 have been measured with the ZEUS detector at HERA using an integrated luminosity of 0.55 pb(-1). These cross sections are given in the kinematic region 0.2 < y < 0.85, for jet pseudorapidities in the ep-laboratory range -1 < eta(jet) < 2 and refer to jets at the hadron level with a cone radius of one unit in the eta - phi plane. These results correspond to quasi-real photoproduction at centre-of-mass energies in the range 130-270 GeV and, approximately, for jet pseudorapidities in the interval -3 < eta(jet)(gamma p CMS) < 0. These measurements cover a new kinematic regime of the partonic structure of the photon, at typical scales up to similar to 300 GeV2 and photon fractional momenta down to x(y) similar to 10(-2). Leading logarithm parton shower Monte Carlo calculations, which include both resolved and direct processes and use the predictions of currently available parametrisations of the photon parton distributions, describe in general the shape and magnitude of the measured eta(jet) and E(T)(jet) distributions