187 research outputs found
Forward Jet Production at small x in Next-to-Leading Order QCD
The production of forward jets of transverse energy E_T\simeq Q and large
momentum fraction x_jet >> x is calculated in next-to-leading order including
consistently direct and resolved virtual photon contributions. The predictions
are compared to recent ZEUS and H1 data. Good agreement with the data is found.Comment: 11 pages, 3 eps figues; text in 2.1 clearified, figure 2 slightly
changed; version to appear in Phys. Lett.
Inclusive Jet Production with Virtual Photons in Next-to-Leading Order QCD
We present a next-to-leading order calculation for the virtual
photoproduction of one and two jets in collisions. Soft and collinear
singularities are extracted using the phase space slicing method. The collinear
photon initial state singularity depends logarithmically on the mass of the
virtual photon and is absorbed into the virtual photon structure function. An
factorization scheme is defined similarly to the real photon
case. Numerical results are presented for HERA conditions using the Snowmass
jet definition for inclusive single jet and dijet cross sections. We study the
dependence of these cross sections on the transverse energies and rapidities of
the jets. Finally, we compare the ratio of the experimentally defined resolved
and direct cross sections with recent ZEUS data as a function of the photon
virtuality .Comment: LaTeX, 28 pages, 7 figure
Next-to-leading order QCD calculations with parton showers II: soft singularities
Programs that calculate observables in quantum chromodynamics at
next-to-leading order typically generate events that consist of partons rather
than hadrons -- and just a few partons at that. These programs would be much
more useful if the few partons were turned into parton showers, which could be
given to one of the Monte Carlo event generators to produce hadron showers. In
a previous paper, we have seen how to generate parton showers related to the
final state collinear singularities of the perturbative calculation for the
example of e+ + e- --> 3 jets. This paper discusses the treatment of the soft
singularities.Comment: 26 pages with 5 figures. This version is close to the version to be
publishe
Vegetation Dynamics of Campos Under Grazing/Fire Regimes in Southern Brazil
Natural grassland vegetation in Southern Brazil, known as campos, has most likely evolved under a disturbance regime that included fire and grazing (Pillar et al., 1997). Nowadays, the composition of the vegetation of campos is grazing- and fire-dependent (Boldrini et al., 1997). Its importance can be evaluated by the fact that it represents 37 % of the state’s area and provides 77 % of the slaughtered cattle at Rio Grande do Sul (Barcellos et al., 2002). The objective of this experiment was to evaluate the vegetation dynamics of campos under grazing/fire regimes in order to explore the resilience of the vegetation under the regimes studied
Combining QCD Matrix Elements at Next-to-Leading Order with Parton Showers in Electroproduction
We present a method to combine next-to-leading order (NLO) matrix elements in
QCD with leading logarithmic parton showers by applying a suitably modified
version of the phase-space-slicing method. The method consists of subsuming the
NLO corrections into a scale-dependent phase-space-slicing parameter, which is
then automatically adjusted to cut out the leading order, virtual, soft and
collinear contributions in the matrix element calculation. In this way a
positive NLO weight is obtained, which can be redistributed by a parton shower
algortihm. As an example, we display the method for single-jet inclusive cross
sections at O(alpha_s) in electroproduction. We numerically compare the
modified version of the phase-space-slicing method with the standard approach
and find very good agreement on the percent level.Comment: 21 pages, 2 eps figures. Revised section 2. To appear in PR
Strong Coupling Constant from Scaling Violations in Fragmentation Functions
We present a new determination of the strong coupling constant alpha_s
through the scaling violations in the fragmentation functions for charged
pions, charged kaons, and protons. In our fit we include the latest e+e-
annihilation data from CERN LEP1 and SLAC SLC on the Z-boson resonance and
older, yet very precise data from SLAC PEP at center-of-mass energy sqrt(s)=29
GeV. A new world average of alpha_s is given.Comment: 10 pages, 3 eps figue
Has the QCD RG-Improved Parton Content of Virtual Photons been Observed?
It is demonstrated that present and DIS ep data on the structure of
the virtual photon can be understood entirely in terms of the standard `naive'
quark--parton model box approach. Thus the QCD renormalization group (RG)
improved parton distributions of virtual photons, in particular their gluonic
component, have not yet been observed. The appropriate kinematical regions for
their future observation are pointed out as well as suitable measurements which
may demonstrate their relevance.Comment: 24 pages, LaTeX, 5 figure
Fragmentation Functions for Pions, Kaons, and Protons at Next-to-Leading Order
We present new sets of fragmentation functions for charged pions, charged
kaons, and protons, both at the leading and next-to-leading orders. They are
fitted to the scaled-momentum distributions of these hadrons measured in e+e-
annihilation on the Z-boson resonance at CERN LEP1 and SLAC SLC. These data
partly come as light-, charm-, bottom-quark-enriched and gluon-jet samples,
which allows us to treat all partons independently, after imposing the SU(2)
flavour symmetry relations. In order to gain sensitivity to the scaling
violation in fragmentation, we also include data from SLAC PEP, with
center-of-mass energy root(s)=29 GeV, in our fits. This allows us to also
determine the strong-coupling constant, with a competitive error. LEP1 data on
the longitudinal cross section as well as DESY DORIS and PETRA data at lower
energies nicely agree with theoretical predictions based on our fragmentation
functions.Comment: 28 pages, 11 eps figure
Coherent Parton Showers with Local Recoils
We outline a new formalism for dipole-type parton showers which maintain
exact energy-momentum conservation at each step of the evolution. Particular
emphasis is put on the coherence properties, the level at which recoil effects
do enter and the role of transverse momentum generation from initial state
radiation. The formulated algorithm is shown to correctly incorporate coherence
for soft gluon radiation. Furthermore, it is well suited for easing matching to
next-to-leading order calculations.Comment: 24 pages, 3 figure
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