Search results for " perturbation theory"
showing 8 items of 358 documents
Event-by-event picture for the medium-induced jet evolution
2016
We discuss the evolution of an energetic jet which propagates through a dense quark-gluon plasma and radiates gluons due to its interactions with the medium. Within perturbative QCD, this evolution can be described as a stochastic branching process, that we have managed to solve exactly. We present exact, analytic, results for the gluon spectrum (the average gluon distribution) and for the higher n-point functions, which describe correlations and fluctuations. Using these results, we construct the event-by-event picture of the gluon distribution produced via medium-induced gluon branching. In contrast to what happens in a usual QCD cascade in vacuum, the medium-induced branchings are quasi-…
Effective theory approaches to heavy meson resonances based on non-perturbative low energy two-meson dynamics
2019
This dissertation provides a theoretical effective description of a special class of hadronic meson states with charm and bottom quark content and their associated phenomenology. The considered states are difficult to be accommodated in CQMs and are candidates to be interpreted as meson molecules. This relies on the assumption that their main properties can be described by means of an effective low energy interaction of two more fundamental mesons. We have paid special attention to the relevant symmetries of QCD and two-body unitarity to construct the low energy effective meson-meson interactions. We have investigated the predictions of the Next-to-Leading Order (NLO) Unitary Heavy Meson Ch…
Application of time-dependent many-body perturbation theory to excitation spectra of selected finite model systems
2016
In this thesis, an approximate method introduced to solve time-dependent many-body problems known as time-dependent many-body perturbation theory is studied. Many-body perturbation theory for interacting electrons and phonons is reviewed. In particular, the electron propagator G and an unconventional two-component phonon propagator, which satisfy coupled integral Dyson equations, are introduced. In practice, the associated integral kernels known as the electron Σ and phonon self-energies need to be approximated. The conserving approximations known as the Hartree (-Fock) and the first and second Born approximations, which respect the continuity equation between the electron density and curren…
Merging Features from Green's Functions and Time Dependent Density Functional Theory: A Route to the Description of Correlated Materials out of Equil…
2016
We propose a description of nonequilibrium systems via a simple protocol that combines exchange-correlation potentials from density functional theory with self-energies of many-body perturbation theory. The approach, aimed to avoid double counting of interactions, is tested against exact results in Hubbard-type systems, with respect to interaction strength, perturbation speed and inhomogeneity, and system dimensionality and size. In many regimes, we find significant improvement over adiabatic time dependent density functional theory or second Born nonequilibrium Green's function approximations. We briefly discuss the reasons for the residual discrepancies, and directions for future work.
Study of very forward energy and its correlation with particle production at midrapidity in pp and p-Pb collisions at the LHC
2022
Journal of high energy physics 08(8), 86 (2022). doi:10.1007/JHEP08(2022)086
Pion photo- and electroproduction on nucleons in relativistic chiral perturbation theory
2022
En esta tesis, hemos estudiado la producción de piones mediante pruebas electromagnéticas en nucleones. Para cada uno de los estudios realizados aquí, nos enfocamos en la Teoría Quiral de Perturbaciones (ChPT) relativista para hadrones ligeros: nucleones, piones y los grados de libertad explícitos Delta. Para preservar la covariancia de la teoría, se implementó el método de renormalización en el esquema Extended On Mass Shell (EOMS). En este caso, las divergencias y los términos que violan el conteo de potencias se absorben en las constantes de baja energía (LECs) del Lagrangiano efectivo. El presente trabajo se desarrolló dentro de tres estudios principales relacionados con la producción d…
First observation and study of the K± → π±π0e+e− decay
2018
The NA48/2 experiment at CERN reports the first observation of the K-+/- -> pi(+/-)pi(0)e(+)e(-) decay from an exposure of 1.7 x 10(11) charged kaon decays recorded in 2003-2004. A sample of 4919 candidates with 4.9% background contamination allows the determination of the branching ratio in the full kinematic region, BR(K-+/- -> pi(+/-)pi(0)e(+)e(-)) = (4.24 +/- 0.14) x 10(-6). The study of the kinematic space shows evidence for a structure dependent contribution in agreement with predictions based on chiral perturbation theory. Several P- and CP-violating asymmetries are also evaluated.
One-loop corrections to light cone wave functions: the dipole picture DIS cross section
2018
We develop methods needed to perform loop calculations in light cone perturbation theory using a helicity basis, refining the method introduced in our earlier work. In particular this includes implementing a consistent way to contract the four-dimensional tensor structures from the helicity vectors with d-dimensional tensors arising from loop integrals, in a way that can be fully automatized. We demonstrate this explicitly by calculating the one-loop correction to the virtual photon to quark-antiquark dipole light cone wave function. This allows us to calculate the deep inelastic scattering cross section in the dipole formalism to next-to-leading order accuracy. Our results, obtained using …