86 research outputs found
Gluon Radiation off Hard Quarks in a Nuclear Environment: Opacity Expansion
We study the relation between the Baier-Dokshitzer-Mueller-Peigne-Schiff
(BDMPS) and Zakharov formalisms for medium-induced gluon radiation off hard
quarks, and the radiation off very few scattering centers. Based on the
non-abelian Furry approximation for the motion of hard partons in a spatially
extended colour field, we derive a compact diagrammatic and explicitly colour
trivial expression for the N-th order term of the kt-differential gluon
radiation cross section in an expansion in the opacity of the medium. Resumming
this quantity to all orders in opacity, we obtain Zakharov's path-integral
expression (supplemented with a regularization prescription). This provides a
new proof of the equivalence of the BDMPS and Zakharov formalisms which extends
previous arguments to the kt-differential cross section. We give explicit
analytical results up to third order in opacity for both the gluon radiation
cross section of free incoming and of in-medium produced quarks. The N-th order
term in the opacity expansion of the radiation cross section is found to be a
convolution of the radiation associated to N-fold rescattering and a
readjustment of the probabilities that rescattering occurs with less than N
scattering centers. Both informations can be disentangled by factorizing out of
the radiation cross section a term which depends only on the mean free path of
the projectile. This allows to infer analytical expressions for the totally
coherent and totally incoherent limits of the radiation cross section to
arbitrary orders in opacity.Comment: 22 pages, LaTeX, eps-figures included, typos corrected, Comment on
(5.13) added, to appear in NP
Determining the BDMPS transport coefficient via medium-modified fragmentation functions
In nucleus-nucleus collisions at RHIC and LHC, partons produced at high
transverse momentum can undergo multiple scattering within the collision region
prior to fragmenting into hadrons. We have studied the resulting
medium-modified fragmentation function based on a calculation of the BDMPS-Z
medium-induced gluon radiation for a dense, expanding medium of small finite
extension. Here we explain how the BDMPS transport coefficient which measures
the energy density attained in the collision, can be extracted from the
observed modification of high-pt hadroproduction. We also comment on the
significant remaining uncertainties in extracting the transport coefficient
from data.Comment: 4 pages, 4 postscript figures. Proceedings for Quark Matter 200
Particle Correlations in Saturated QCD Matter
We study quantitatively angular correlations in the two-particle spectrum
produced by an energetic probe scattering off a dense hadronic target with
sizeable saturation momentum. To this end, two parton inclusive cross sections
for arbitrary projectiles with small color charge density are derived in the
eikonal formalism. Our results are the following: For large momenta of the
observed particles, the perturbative limit with characteristic back-to-back
correlation is recovered. As the trigger momenta get closer to the saturation
scale Q_s, the angular distribution broadens. When the momenta are
significantly smaller than Q_s, the azimuthal distribution is broad but still
peaked back-to-back. However, in a narrow momentum range (0.5 - 1.5) Q_s, we
observe that the azimuthal correlation splits into a double peak with maxima
displaced away from 180 degree. We argue that it is the soft multiple
scattering physics that is responsible for the appearance of this shift in the
angle of maximal correlation. We also point out that when the physical size of
the projectile is particularly small, the double peak structure persists in a
significantly wider range of final state momenta.Comment: 28 pages LaTex, 7 eps-figure
Using Model Predictive Control to Modulate the Humidity in a Broiler House and Effect on Energy Consumption
In moderate climate, broiler chicken houses are important heating energy consumers and hence heating fuel consumption accounts for a large part in operating costs. They can be reduced by constructional measures, which in turn lead to important costs as well. On the other hand, a software solution to reduce energy would lead to considerably less follow-up costs. The main objective of our work was to assess if it is possible to save energy with a software solution and eventually quantify the savings for a given broiler house in the Swiss Plateau. The investigation was carried out in simulation: the particular broiler house was measured, and a dynamical model for it was derived and validated. To actually search for a particular behaviour of the software that would lead to energy savings, model predictive control was used. The idea was not to specify a particular behaviour of the software but rather to let the software itself find the best behaviour in an exhaustive search. The simulations showed that energy savings can be realised mainly by letting the indoor humidity deviate from what usually is used as setpoint and hence take profit of the outdoor climate, which changes naturally during a 24-hour course. We used expert opinions to determine how long and large these setpoint deviations may be without harming the broilers. The simulations showed alsothat the light control and the biological activity of the animals reduced the potential savings
Effects of digestate on the environment and on plant production - results of a research project
Composts and digestates can influence soil fertility and plant health. These influences can be positive or negative, depending of the quality of the composts. A currently important question is to know, if digestates differ from composts in these aspects. A Swiss project is concerned with the estimation of the potential of Swiss composts and digestates to influence soil fertility and plant health positively. For this, one hundred composts and digestates representative of the different composting systems and qualities available on the Swiss market were analyzed.
The organic matter and nutrient content of the composts varied greatly between the composts and the digestates; the materials of origin were the major factor influencing these values. The respiration rate and enzyme activities also varied greatly; they are particularly important in digestates. The organic matter of digestates is less stable than that of composts.
The N-mineralization potential from the majority of the digestates added to soil is high, in comparison to young composts. When digestates are not correctly treated or stored, however, they can immobilize nitrogen in the soil. This problem is hardly correlated with the management of the digestate in the first stage after leaving the fermenter. Especially products which have become too dry during this period lost their ammonia-nitrogen, and hence immobilized nitrogen in the soil. The risk of phytotoxicity is higher in digestates than in composts. This limits the possibility for use of digestate. With a post-treatment of digestate, it is possible to produce high quality compost with a high compatibility with plant growth and with a more stabilized organic matter.
In field experiments, digestates increased the pH-value and the biological activity of soil to the same extent than composts. These effects were observable also one season after compost application. No immobilization of nitrogen was observed
LPM-Effect in Monte Carlo Models of Radiative Energy Loss
Extending the use of Monte Carlo (MC) event generators to jets in nuclear
collisions requires a probabilistic implementation of the non-abelian LPM
effect. We demonstrate that a local, probabilistic MC implementation based on
the concept of formation times can account fully for the LPM-effect. The main
features of the analytically known eikonal and collinear approximation can be
reproduced, but we show how going beyond this approximation can lead to
qualitatively different results.Comment: 4 pages, 3 figures - To appear in the conference proceedings for
Quark Matter 2009, March 30 - April 4, Knoxville, Tennessee; v2: removed line
number
Transverse Momentum Dependence of the Landau-Pomeranchuk-Migdal Effect
We study the transverse momentum dependence of the Landau-Pomeranchuk-Migdal
effect in QED, starting from the high energy expansion of the solution of the
Dirac equation in the presence of an external field. The angular integrated
energy loss formula differs from an earlier expression of Zakharov by taking
finite kinematical boundaries into account. In an expansion in powers of the
opacity of the medium, we derive explicit expressions for the radiation cross
section associated with N=1, 2 and 3 scatterings. We verify the Bethe-Heitler
and the factorization limit, and we calculate corrections to the factorization
limit proportional to the square of the target size. A closed form expression
valid to arbitrary orders in the opacity is derived in the dipole
approximation. The resulting radiation spectrum is non-analytic in the coupling
constant which is traced back to the transverse momentum broadening of a hard
parton undergoing multiple small angle Moliere scattering. In extending the
results to QCD, we test a previously used dipole prescription by comparing to
direct pQCD results for N=1 and 2. For N=1, the QCD dipole prescription
reproduces exactly the Bertsch-Gunion radiation spectrum. For N=2, we find a
sizeable correction which reduces to a multiplicative factor 17/8 at large
separation.Comment: 20 pages, Latex, 4 eps-figures, replaced by published version, minor
typos correcte
Transverse Dynamics of Hard Partons in Nuclear Media and the QCD Dipole
We derive the non-abelian generalization of the Furry approximation which
describes the transverse dynamical evolution of a hard projectile parton inside
a spatially extended colour target field. This provides a unified starting
point for the target rest frame description of the nuclear dependence of a
large class of observables. For the case of the virtual gamma -> q-qbar
photoabsorption cross section, we investigate then in detail under which
conditions the nuclear dependence encoded in the Furry wavefunctions can be
parametrized by a q-qbar QCD dipole cross section. The important condition is
colour triviality, i.e., the property that for arbitrary N-fold rescattering
contributions the only non-vanishing colour trace is N_c C_F^N. We give proofs
for the colour triviality of the inelastic, diffractive and total
photoabsorption cross section measured inclusively or with one jet resolved in
the final state. Also, we list examples for which colour interference effects
remain. Colour triviality allows us to write the gamma -> q qbar contribution
to the DIS nuclear structure function F_2 for small Bjorken x in terms of a
path integral which describes the transverse size evolution of the q-qbar pair
in the nuclear colour field. This expression reduces in an opacity expansion to
the N=1 result of Nikolaev and Zakharov, and in the eikonal approximation to
the Glauber-type rescattering formulas first derived by Mueller. In the
harmonic oscillator approximation of the path integral, we quantify deviations
from the eikonal limit. Their onset is characterized by the scales L/l_f and
E_t L which relate the longitudinal extension L of the nuclear target to the
coherence length l_f and the total transverse energy E_t accumulated by the
q-qbar pair.Comment: 23 pages, LaTeX, 12 eps-figures included, typos corrected, discussion
extended, to appear in NP
Multiplicity distributions for jet parton showers in a medium
The "jet-quenching" interpretation of suppressed high-pT hadron production at
RHIC implies that jet multiplicity distributions and jet-like particle
correlations in heavy-ion collisions at RHIC and LHC differ strongly from those
seen at e+e- or pp colliders. Here, we present an approach for describing the
changes induced by the medium, which implements jet quenching as a
probabilistic medium-modified parton shower, treating leading and subleading
parton splittings on an equal footing. We show that the strong suppression of
single inclusive hadron spectra measured in Au-Au collisions at RHIC implies a
characteristic distortion of the single inclusive distribution of soft partons
inside the jet. We determine, as a function of jet energy, to what extent the
soft jet fragments can be measured above some momentum cut.Comment: 4 pages, 2 eps-figures. Talk given at Quark Matter 2005, Budapest,
4-9 Aug 200
Approximately periodic time series and nonlinear structures
In this thesis a previously developed framework for modelling diversity of approximately periodic time series is considered. In this framework the diversity is modelled deterministically, exploiting the irregularity of chaos. This is an alternative to other well established frameworks which use probability distributions and other stochastic tools to describe diversity. The diversity which is to be modelled, on the other hand, is not assumed to be of chaotic nature, but can stem out from a stochastic process, though it has never been verified before, whether or not purely stochastic patterns can be modelled that way. The main application of such a modelling technique would be pattern recognition; once a model for a learning set of approximately periodic time series is found, synchronisation-like phenomena could be used to determine if a novel time series is similar to the members of the learning set. The most crucial step of the classification procedure outlined before is the automatic generation of a chaotic model from data, called identification. Originally, this was done using a simple low dimensional reference model. Here, on the other hand, a biologically inspired approach is taken. This has the advantage that the identification and classification procedure could be greatly simplified and the computational power involved significantly reduced. The biologically inspired model used for identification was announced several time in literature under the name "Echo State Network". The articles available on it consisted mainly of examples were it performed remarkably well, though a thorough analysis was still missing to the scientific community. Here the model is analysed using a measure that had appeared already in similar contexts and with help of this measure good settings of the models' parameter were determined. Finally, the model was used to assess if stochastic patterns can be modelled by chaotic signals. Indeed, it has been shown that, for the biologically inspired modelling technique considered, chaotic behaviour appears to implicitly model diversity and randomness of the learnt patterns whenever these are sufficiently structured; whilst chaos does not appear when the patterns are remarkably unstructured. In other words, deterministic chaos or strongly coloured noise lead to the chaos emergence as opposed to white-like noise which does not. With this result in mind, the classification of gait signals was attempted, as no signs of chaoticity could be found in them and the previously available modelling technique seemed to have difficulties to model their diversity. The identification and classification results with the biologically inspired model turned out to be very good
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