578 research outputs found
Two hadron production in e+e- annihilation to next-to-leading order accuracy
We discuss the production of two hadrons in e+e- annihilation within the
framework of perturbative QCD. The cross section for this process is calculated
to next-to-leading order accuracy with a selection of variables that allows the
consideration of events where the two hadrons are detected in the same jet. In
this configuration we contemplate the possibility that the hadrons come from a
double fragmentation of a single parton. The double-fragmentation functions
required to describe the transition of a parton to two hadrons are also
necessary to completely factorize all collinear singularities. We explicitly
show that factorization applies to next-to-leading order in the case of
two-hadron production.Comment: 13 pages, 4 figure
The non-forward BFKL amplitude and rapidity gap physics
We discuss the BFKL approach to processes with large momentum transferred
through a rapidity gap. The Mueller and Tang scheme to the BFKL non-forward
parton-parton elastic scattering amplitude at large , is extended to include
higher conformal spins. The new contributions are found to decrease with
increasing energy, as follows from the gluon reggeisation phenomenon, and to
vanish for asymptotically high energies. However, at moderate energies and high
, the higher conformal spins dominate the amplitude. We illustrate the
effects by studying the production of two high jets separated by a
rapidity gap at HERA energies. In a simplified framework, we find excellent
agreement with the HERA photoproduction data once we incorporate the rapidity
gap survival probability against soft rescattering effects. We emphasize that
measurements of the analogous process in electroproduction may probe different
summations over conformal spins.Comment: Latex, 14 pages, 3 figures; the final version to appear in Phys.
Lett. B; a short discussion of the Tevatron data added; a previously missing
factor of i^n introduced in eq. (13
A reliable rainfall–runoff model for flood forecasting: review and application to a semi-urbanized watershed at high flood risk in Italy
Many rainfall–runoff (RR) models are available in the scientific literature. Selecting the best structure and parameterization for a model is not straightforward and depends on a broad number of factors, including climatic conditions, catchment characteristics, temporal/spatial resolution and model objectives. In this study, the RR model 'Modello Idrologico Semi-Distribuito in continuo' (MISDc), mainly developed for flood simulation in Mediterranean basins, was tested on the Seveso basin, which is stressed several times a year by flooding events mainly caused by excessive urbanization. The work summarizes a compendium of the MISDc applications over a wide range of catchments in European countries and then it analyses the performances over the Seveso basin. The results show a good fit behaviour during both the calibration and the validation periods with a Nash–Sutcliffe coefficient index larger than 0.9. Moreover, the median volume and peak discharge errors calculated on several flood events were less than 25%. In conclusion, we can be assured that the reliability and computational speed could make the MISDc model suitable for flood estimation in many catchments of different geographical contexts and land use characteristics. Moreover, MISDc will also be useful for future support of real-time decision-making for flood risk management in the Seveso basin
Heavy quark production as sensitive test for an improved description of high energy hadron collisions
QCD dynamics at small quark and gluon momentum fractions or large total
energy, which plays a major role for HERA, the Tevatron, RHIC and LHC physics,
is still poorly understood. For one of the simplest processes, namely
bottom-antibottom production, next-to-leading-order perturbation theory fails.
We show that the combination of two recently developed theoretical concepts,
the k_perp-factorization and the next-to-leading-logarithmic-approximation BFKL
vertex, gives perfect agreement with data. One can therefore hope that these
concepts provide a valuable foundation for the description of other high energy
processes.Comment: RevTeX, 4 pages, 7 figures titel and abstract changed, several
formulations modified in the text, 1 figure droppe
NLO BFKL Equation, Running Coupling and Renormalization Scales
I examine the solution of the BFKL equation with NLO corrections relevant for
deep inelastic scattering. Particular emphasis is placed on the part played by
the running of the coupling. It is shown that the solution factorizes into a
part describing the evolution in Q^2, and a constant part describing the input
distribution. The latter is infrared dominated, being described by a coupling
which grows as x decreases, and thus being contaminated by infrared
renormalons. Hence, for this part we agree with previous assertions that
predictive power breaks down for small enough x at any Q^2. However, the former
is ultraviolet dominated, being described by a coupling which falls like
1/(\ln(Q^2/\Lambda^2) + A(\bar\alpha_s(Q^2)\ln(1/x))^1/2)with decreasing x, and
thus is perturbatively calculable at all x. Therefore, although the BFKL
equation is unable to predict the input for a structure function for small x,
it is able to predict its evolution in Q^2, as we would expect from the
factorization theory. The evolution at small x has no true powerlike behaviour
due to the fall of the coupling, but does have significant differences from
that predicted from a standard NLO in alpha_s treatment. Application of the
resummed splitting functions with the appropriate coupling constant to an
analysis of data, i.e. a global fit, is very successful.Comment: Tex file, including a modification of Harvmac, 46 pages, 8 figures as
.ps files. Correction of typos, updating of references, very minor
corrections to text and fig.
Rapidity-Separation Dependence and the Large Next-to-Leading Corrections to the BFKL Equation
Recent concerns about the very large next-to-leading logarithmic (NLL)
corrections to the BFKL equation are addressed by the introduction of a
physical rapidity-separation parameter . At the leading logarithm (LL)
this parameter enforces the constraint that successive emitted gluons have a
minimum separation in rapidity, . The most significant
effect is to reduce the BFKL Pomeron intercept from the standard result as
is increased from 0 (standard BFKL). At NLL this -dependence
is compensated by a modification of the BFKL kernel, such that the total
dependence on is formally next-to-next-to-leading logarithmic. In this
formulation, as long as (for ): (i) the NLL
BFKL pomeron intercept is stable with respect to variations of , and
(ii) the NLL correction is small compared to the LL result. Implications for
the applicability of the BFKL resummation to phenomenology are considered.Comment: 16 pages, 3 figures, Late
BFKL at next-to-leading order
This is a summary of the contributions on the next-to-leading order
corrections to the BFKL equation which were presented to the `Small-x and
Diffraction' working group at the 1998 Durham Workshop on HERA Physics.Comment: 6 pages, 2 figure
Virtual Next-to-Leading Corrections to the Impact Factors in the High-Energy Limit
We compute the virtual next-to-leading corrections to the impact factors or
off-shell coefficient functions in the high-energy limit. When combined with
the known real corrections, these results will provide the complete NLO
corrections to the impact factors, which are necessary to use the BFKL
resummation at NLL for jet production at both lepton-hadron and hadron-hadron
colliders.Comment: LaTeX, 23 page
Next-to-leading and resummed BFKL evolution with saturation boundary
We investigate the effects of the saturation boundary on small-x evolution at
the next-to-leading order accuracy and beyond. We demonstrate that the
instabilities of the next-to-leading order BFKL evolution are not cured by the
presence of the nonlinear saturation effects, and a resummation of the higher
order corrections is therefore needed for the nonlinear evolution. The
renormalization group improved resummed equation in the presence of the
saturation boundary is investigated, and the corresponding saturation scale is
extracted. A significant reduction of the saturation scale is found, and we
observe that the onset of the saturation corrections is delayed to higher
rapidities. This seems to be related to the characteristic feature of the
resummed splitting function which at moderately small values of x possesses a
minimum.Comment: 34 page
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