arXiv subjects

Dimitrios Petrellis

Publications and source records attributed to Dimitrios Petrellis.

4 recordsLinked to original sources

arXiv2026

Dispersive analysis of the $\pi^+ \pi^-$ production at the CMD3 experiment and the compatibility with muon pair production measurement by KLOE2 and the pion form factor by JLAB

The spectral function of the charged pion form factor was extracted from two data sets. The difference between the two sets is based on the presence or absence of the recent measurement of the $\pi^+ \pi^-$ production by CMD-3 . Although the CMD-3 data are largely incompatible with other recent measurements, no excess was found when the data were used for analytical continuation to the spacelike region and compared to JLaB-$\pi$ experiment. Instead, both data sets provide a spacelike form factor that differs from the expected behavior known from perturbative analyses of quantum chromodynamics (QCD). A precise fit of the spectral functions is provided, either based on he bare form factor and the full $e^+e^-\rightarrow \pi^+\pi^-$ as well . Furthermore, the extracted pionic spectral functions were used to obtain the QED running charge. This was then compared with the KLOE-2 measurements.

hep-ex↗

arXiv2024

Nuclear shape / phase transitions in the N = 40, 60, 90 regions

We investigate the isotopes of Se, Zr, Mo and Nd in the regions with N = 40, 60 and 90, where a first-order shape / phase transition, from spherical to deformed, can be observed. The signs of phase transitional behavior become evident by examining structure indicators, such as certain energy ratios and B(E2) transition rates and, in particular, how they evolve with neutron number. Microscopic mean-field calculations using the Skyrme-Hartree-Fock + Bardeen-Cooper-Schrieffer framework also reveal structural changes when considering the evolution of the resulting potential energy curves as functions of deformation. Finally, macroscopic calculations, using the Algebraic Collective Model, specifically for $^{74}$Se, $^{102}$Mo and $^{150}$Nd, after fitting its parameters to experimental spectra, result in potentials that resemble some of the potentials proposed in the framework of the Bohr Hamiltonian to describe shape transitions in nuclei.

nucl-th↗

arXiv2022

Ridge regression for minimizing the couplings of hyperon resonances in the $K^+ \Lambda$ photoproduction

We employed the isobar model for investigating the $K^+\Lambda$ photoproduction process. We paid special attention to the recent CLAS polarization data and enhanced the $\chi^2$ minimization by adding a penalty term. Without changing the set of included resonances used by the model, this technique known as Ridge regression leads to reduced couplings that in previous studies acquired unreasonably large values. As a result, we have arrived at a much more robust model with hyperon couplings which are reduced to more physical values. This model serves us to extract valuable information on the background to the $K^+\Lambda$ photoproduction and particularly on the role of various hyperon resonances. The set of the nucleon resonances is the same with respect to previous fits but their role may have changed due to different couplings which they acquire in the present fit.

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arXiv2021

Model selection for $K^+\Sigma^-$ photoproduction within isobar model

We utilise an isobar model to investigate the $K^+ \Sigma^-$ photoproduction off a neutron in the resonance region. Except for the Born terms, we include high-spin (spin-3/2 and spin-5/2) nucleon resonances in the consistent formalism together with a few $\Delta$ and kaon resonances to achieve an acceptable agreement with data. Interestingly, we reveal that no hyperon resonances are needed to achieve a reasonable description of data. On the other hand the $N(1720)3/2^+$ resonance was found to be very important for correct description of data. The free parameters of the model were fitted to experimental data from the LEPS and CLAS Collaborations on either differential cross sections or photon beam asymmetry. The novel feature of the fitting procedure is the use of a regularization method, the Least Absolute Shrinkage Selection Operator, and information criteria in order to choose the best fit.

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