Reconstruction of virtual photons from Au+Au collisions at 1.23 GeV/u

This thesis presents data analysis of dilepton excess radiation in Au+Au at beam kinetic energy 1.23A GeV experiment performed by HADES. It starts with an overview of QCD symmetries and its thermodynamics properties. It is followed by a discussion of details of experimental setup that are necessary to understand the data analysis. The data analysis itself is described in the main part of the manuscript. This starts with tools and techniques that have been used. An excellent performance of lepton identification with a neural network, in terms of lepton sample purity, has been proven. It is not possible to divide all e+ and e− in a single event into correlated pairs and combinatorial background must be estimated on a statistical way. Reconstructed signal should be corrected for efficiency in order to be compared with other measurements and theory predictions. This is done by multiplying the signal after background subtraction by a factor depending on the same pair kinematic variable as the signal spectrum. After all the analysis steps spectra are examined in order to make physical statements. Average temperature of the medium emitting dileptons was directly extracted from the fit to the invariant mass. Comparison to model calculations showed their consistency with the data in various observables

Resonance production in p+p, p+A and A+A collisions measured with HADES

The knowledge of baryonic resonance properties and production cross sections plays an important role for the extraction and understanding of medium modifications of mesons in hot and/or dense nuclear matter. We present and discuss systematics on dielectron and strangeness production obtained with HADES on p+p, p+A and A+A collisions in the few GeV energy regime with respect to these resonances

Impact of the Coulomb field on charged-pion spectra in few-GeV heavy-ion collisions

In nuclear collisions the incident protons generate a Coulomb field which acts on produced charged particles. The impact of these interactions on charged-pion transverse-mass and rapidity spectra, as well as on pion–pion momentum correlations is investigated in Au + Au collisions at $\sqrt{^{S}NN}$ = 2.4 GeV. We show that the low-m$_{t}$ region (m$_{t}$ < 0.2 GeV / c$^{2}$) can be well described with a Coulomb-modified Boltzmann distribution that also takes changes of the Coulomb field during the expansion of the fireball into account. The observed centrality dependence of the fitted mean Coulomb potential energy deviates strongly from a $A_{part}^{2/3}$ scaling, indicating that, next to the fireball, the non-interacting charged spectators have to be taken into account. For the most central collisions, the Coulomb modifications of the HBT source radii are found to be consistent with the potential extracted from the single-pion transverse-mass distributions. This finding suggests that the region of homogeneity obtained from two-pion correlations coincides with the region in which the pions freeze-out. Using the inferred mean-square radius of the charge distribution at freeze-out, we have deduced a baryon density, in fair agreement with values obtained from statistical hadronization model fits to the particle yields

Reconstruction of virtual photons from Au+Au collisions at 1.23 GeV/u

This thesis presents data analysis of dilepton excess radiation in Au+Au at beam kinetic energy 1.23A GeV experiment performed by HADES. It starts with an overview of QCD symmetries and its thermodynamics properties. It is followed by a discussion of details of experimental setup that are necessary to understand the data analysis. The data analysis itself is described in the main part of the manuscript. This starts with tools and techniques that have been used. An excellent performance of lepton identification with a neural network, in terms of lepton sample purity, has been proven. It is not possible to divide all e+ and e− in a single event into correlated pairs and combinatorial background must be estimated on a statistical way. Reconstructed signal should be corrected for efficiency in order to be compared with other measurements and theory predictions. This is done by multiplying the signal after background subtraction by a factor depending on the same pair kinematic variable as the signal spectrum. After all the analysis steps spectra are examined in order to make physical statements. Average temperature of the medium emitting dileptons was directly extracted from the fit to the invariant mass. Comparison to model calculations showed their consistency with the data in various observables

Reconstruction of virtual photons from Au+Au collisions at 1.23 GeV/u

This thesis presents data analysis of dilepton excess radiation in Au+Au at beam kinetic energy 1.23A GeV experiment performed by HADES. It starts with an overview of QCD symmetries and its thermodynamics properties. It is followed by a discussion of details of experimental setup that are necessary to understand the data analysis. The data analysis itself is described in the main part of the manuscript. This starts with tools and techniques that have been used. An excellent performance of lepton identification with a neural network, in terms of lepton sample purity, has been proven. It is not possible to divide all e+ and e− in a single event into correlated pairs and combinatorial background must be estimated on a statistical way. Reconstructed signal should be corrected for efficiency in order to be compared with other measurements and theory predictions. This is done by multiplying the signal after background subtraction by a factor depending on the same pair kinematic variable as the signal spectrum. After all the analysis steps spectra are examined in order to make physical statements. Average temperature of the medium emitting dileptons was directly extracted from the fit to the invariant mass. Comparison to model calculations showed their consistency with the data in variou

Multi-differential pattern of low-mass e+e− excess from s NN = 2.4 GeV Au+Au collisions with HADES

The matter formed in central heavy-ion collisions at a few GeV per nucleon is commonly understood as resonance matter, a gas of nucleons and excited baryon states with a substantial contribution from mesonic, mostly pionic excitations. Yet, in the initial phase of the reaction the system is compressed to beyond nuclear ground state density and hence substantial modifications of the hadron properties are expected to occur. The spectral distribution of virtual photons measured in Au+Au collisions at 2.4 GeV center of mass energy indicates strong medium effects beyond pure superposition of individual NN collisions. We present multi-differential distributions of low-mass electron pairs. This radiation is remarkably well described assuming emission from a thermalized system. To gain deeper understanding of the microscopic origin of the radiation, we extracted the centrality dependent true (not blue-shifted) temperature, its azimuthal distribution, as well as mass-dependent effective slope parameter. Virtual photon spectra are confronted with available model calculations