905 research outputs found
On the probability distribution of the stochastic saturation scale in QCD
It was recently noticed that high-energy scattering processes in QCD have a
stochastic nature. An event-by-event scattering amplitude is characterised by a
saturation scale which is a random variable. The statistical ensemble of
saturation scales formed with all the events is distributed according to a
probability law whose cumulants have been recently computed. In this work, we
obtain the probability distribution from the cumulants. We prove that it can be
considered as Gaussian over a large domain that we specify and our results are
confirmed by numerical simulations.Comment: 9 pages, 3 figures, misprints corrected, version to appear in PL
Interference Phenomena in Medium Induced Radiation
We consider the interference pattern for the medium-induced gluon radiation
produced by a color singlet quark-antiquark antenna embedded in a QCD medium
with size and `jet quenching' parameter . Within the BDMPS-Z
regime, we demonstrate that, for a dipole opening angle , the interference between the
medium--induced gluon emissions by the quark and the antiquark is suppressed
with respect to the direct emissions. This is so since direct emissions are
delocalized throughout the medium and thus yield contributions proportional to
while interference occurs only between emissions at early times, when both
sources remain coherent. Thus, for \tqq \gg\theta_c, the medium-induced
radiation is the sum of the two spectra individually produced by the quark and
the antiquark, without coherence effects like angular ordering. For \tqq
\ll\theta_c, the medium--induced radiation vanishes.Comment: 4 pages, 2 figures; Proceedings of the "Quark Matter 2011" conferenc
Forward gluon production in hadron-hadron scattering with Pomeron loops
We discuss new physical phenomena expected in particle production in
hadron-hadron collisions at high energy, as a consequence of Pomeron loop
effects in the evolution equations for the Color Glass Condensate. We focus on
gluon production in asymmetric, `dilute-dense', collisions : a dilute
projectile scatters off a dense hadronic target, whose gluon distribution is
highly evolved. This situation is representative for particle production in
proton-proton collisions at forward rapidities (say, at LHC) and admits a
dipole factorization similar to that of deep inelastic scattering (DIS). We
show that at sufficiently large forward rapidities, where the Pomeron loop
effects become important in the evolution of the target wavefunction, gluon
production is dominated by `black spots' (saturated gluon configurations) up to
very large values of the transverse momentum, well above the average saturation
momentum in the target. In this regime, the produced gluon spectrum exhibits
diffusive scaling, so like DIS at sufficiently high energy.Comment: 29 pages, 7 figure
Classical Optimizers for Noisy Intermediate-Scale Quantum Devices
We present a collection of optimizers tuned for usage on Noisy Intermediate-Scale Quantum (NISQ) devices. Optimizers have a range of applications in quantum computing, including the Variational Quantum Eigensolver (VQE) and Quantum Approximate Optimization (QAOA) algorithms. They are also used for calibration tasks, hyperparameter tuning, in machine learning, etc. We analyze the efficiency and effectiveness of different optimizers in a VQE case study. VQE is a hybrid algorithm, with a classical minimizer step driving the next evaluation on the quantum processor. While most results to date concentrated on tuning the quantum VQE circuit, we show that, in the presence of quantum noise, the classical minimizer step needs to be carefully chosen to obtain correct results. We explore state-of-the-art gradient-free optimizers capable of handling noisy, black-box, cost functions and stress-test them using a quantum circuit simulation environment with noise injection capabilities on individual gates. Our results indicate that specifically tuned optimizers are crucial to obtaining valid science results on NISQ hardware, and will likely remain necessary even for future fault tolerant circuits
Small-x QCD effects in forward-jet and Mueller-Navelet jet production
We investigate small-x QCD effects in forward-jet production in deep
inelastic scattering in the kinematic regime where the virtuality of the photon
and the transverse momentum of the jet are two hard scales of about the same
magnitude. We show that the data from HERA published by the H1 and ZEUS
collaborations are well described by leading-logarithmic BFKL predictions.
Parametrizations containing saturation effects expected to be relevant at
higher energies also compare well to the present data. We extend our analysis
to Mueller-Navelet jets at the LHC and discuss to what extent this observable
could test these small-x effects and help distinguishing between the different
descriptions.Comment: 18 pages, 9 figures, minor changes, published versio
Researches regarding structural modifications that appears in the material of tools used for rubber waste attrition
Tools commonly used for shredding rubber waste, currently produced, are made of neatly cast iron in the composite is to avoid the presence of sulfur and phosphorus. In this paper are presented the main structural material changes that occur in different areas, located at different distances from the active surface of tools. Structural changes occurred mainly refers to the transformation of white iron surface layer to gray cast iron and graphite separations appearance, which causes the crack primers and cracking corrosion phenomena in tools material
Researches on the chemical composition and hardness modifications that appear in the material of tools used for rubber waste attrition
This paper presents the results of the main changes in the chemical composition of the material, and changes in its hardness. The changes in terms of chemical composition refer primarily to changes in concentration of sulfur and carbon, and in terms of hardness material change there can be noticed a decrease in hardness of the material that is in direct contact with waste rubber
JIMWLK evolution in the Gaussian approximation
We demonstrate that the Balitsky-JIMWLK equations describing the high-energy
evolution of the n-point functions of the Wilson lines (the QCD scattering
amplitudes in the eikonal approximation) admit a controlled mean field
approximation of the Gaussian type, for any value of the number of colors Nc.
This approximation is strictly correct in the weak scattering regime at
relatively large transverse momenta, where it reproduces the BFKL dynamics, and
in the strong scattering regime deeply at saturation, where it properly
describes the evolution of the scattering amplitudes towards the respective
black disk limits. The approximation scheme is fully specified by giving the
2-point function (the S-matrix for a color dipole), which in turn can be
related to the solution to the Balitsky-Kovchegov equation, including at finite
Nc. Any higher n-point function with n greater than or equal to 4 can be
computed in terms of the dipole S-matrix by solving a closed system of
evolution equations (a simplified version of the respective Balitsky-JIMWLK
equations) which are local in the transverse coordinates. For simple
configurations of the projectile in the transverse plane, our new results for
the 4-point and the 6-point functions coincide with the high-energy
extrapolations of the respective results in the McLerran-Venugopalan model. One
cornerstone of our construction is a symmetry property of the JIMWLK evolution,
that we notice here for the first time: the fact that, with increasing energy,
a hadron is expanding its longitudinal support symmetrically around the
light-cone. This corresponds to invariance under time reversal for the
scattering amplitudes.Comment: v2: 45 pages, 4 figures, various corrections, section 4.4 updated, to
appear in JHE
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