G. Stoicea

Using the large acceptance apparatus FOPI, we study pion emission in the reactions (energies in AGeV are given in parentheses): 40Ca + 40Ca (0.4, 0.6, 0.8, 1.0, 1.5, 1.93), 96Ru + 96Ru (0.4, 1.0, 1.5), 96Zr + 96Zr (0.4, 1.0, 1.5), 197Au + 197Au (0.4, 0.6, 0.8, 1.0, 1.2, 1.5). The observables include longitudinal and transverse rapidity distributions and stopping, polar anisotropies, pion multiplicities, transverse momentum spectra, ratios (π+/π−) of average transverse momenta and of yields, directed flow, elliptic flow. The data are compared to earlier data where possible and to transport model simulations.

Using the large acceptance apparatus FOPI, we study central collisions in the reactions (energies in A GeV are given in parentheses): 40Ca + 40Ca (0.4, 0.6, 0.8, 1.0, 1.5, 1.93), 58Ni + 58Ni (0.15, 0.25, 0.4), 96Ru + 96Ru (0.4, 1.0, 1.5), 96Zr + 96Zr (0.4, 1.0, 1.5), 129Xe + CsI (0.15, 0.25, 0.4), 197Au + 197Au (0.09, 0.12, 0.15, 0.25, 0.4, 0.6, 0.8, 1.0, 1.2, 1.5). The observables include cluster multiplicities, longitudinal and transverse rapidity distributions and stopping, and radial flow. The data are compared to earlier data where possible and to transport model simulations.

Using the large acceptance apparatus FOPI, we study central and semi-central collisions in the reactions (energies in AGeV are given in parentheses): 40Ca + 40Ca (0.4, 0.6, 0.8, 1.0, 1.5, 1.93), 58Ni + 58Ni (0.15, 0.25, 0.4), 96Ru + 96Ru (0.4, 1.0, 1.5), 96Zr + 96Zr (0.4, 1.0, 1.5), 129Xe + CsI (0.15, 0.25, 0.4), 197Au + 197Au (0.09, 0.12, 0.15, 0.25, 0.4, 0.6, 0.8, 1.0, 1.2, 1.5). The observables include directed and elliptic flow. The data are compared to earlier data where possible and to transport model simulations. A stiff nuclear equation of state is found to be incompatible with the data. Evidence for extra-repulsion of neutrons in compressed asymmetric matter is found.

We present measurements of the excitation function of elliptic flow at midrapidity in Au + Au collisions at beam energies from 0.09 to 1.49 GeV per nucleon. For the integral flow, we discuss the interplay between collective expansion and spectator shadowing for three centrality classes. A complete excitation function of transverse momentum dependence of elliptic flow is presented for the first time in this energy range, revealing a rapid change with incident energy below 0.4AGeV, followed by an almost perfect scaling at the higher energies. The equation of state of compressed nuclear matter is addressed through comparisons to microscopic transport model calculations.

Four different combinations of 96 44 Ru and 96 40 Zr nuclei, both as projectile and target, were investigated at the same bombarding energy of 400A MeV using a 4π detector.The degree of isospin mixing between projectile and target nucleons is mapped across a large portion of the phase space using two different isospin-tracer observables, the number of measured protons and the t/ 3 He yield ratio.The experimental results show that the global equilibrium is not reached even in the most central collisions.Quantitative measures of stopping and mixing are extracted from the data.They are found to exhibit a quite strong sensitivity to the in-medium (n,n) cross section used in microscopic transport calculations.

We present a complete systematics (excitation functions and system-size dependences) of global stopping and side flow for heavy ion reactions in the energy range between $0.09A$ and $1.93A\text{ }\text{ }\mathrm{GeV}$. For the heaviest system, $\mathrm{Au}+\mathrm{Au}$, we observe a plateau of maximal stopping extending from about $0.2A$ to $0.8A\text{ }\text{ }\mathrm{GeV}$ with a fast drop on both sides. The degree of stopping, which is shown to remain significantly below the expectations of a full stopping scenario, is found to be highly correlated to the amount of side flow.

Experimental data on K+ meson and proton sideward flow measured with the FOPI detector at SIS/GSI in the reactions Ru+Ru at 1.69A GeV and Ni+Ni at 1.93A GeV are presented. The K+ sideward flow is found to be anti-correlated (correlated) with the one of protons at low (high) transverse momenta. When compared to the predictions of a transport model, the data favour the existence of an in-medium repulsive K+-nucleon potential.

The incident energy at which the azimuthal distributions in semicentral heavy-ion collisions change from in-plane to out-of-plane enhancement — Etran is studied as a function of mass of emitted particles, their transverse momentum and centrality for Au+Au collisions. The analysis is performed in a reference frame rotated with the sidewards flow angle (Θflow) relative to the beam axis. A systematic decrease of Etran as function of mass of the reaction products, their transverse momentum and collision centrality is evidenced. The predictions of a microscopic transport model (IQMD) are compared with the experimental results.

We present new experimental data on directed flow in collisions of $\mathrm{Au}+\mathrm{Au},$ $\mathrm{Xe}+\mathrm{CsI},$ and $\mathrm{Ni}+\mathrm{Ni}$ at incident energies from $90A$ to $400A\mathrm{MeV}.$ We study the centrality and system dependence of integral and differential directed flow for particles selected according to charge. All the features of the experimental data are compared with isospin quantum molecular dynamics (IQMD) model calculations in an attempt to extract information about the nuclear matter equation of state (EoS). We show that the combination of rapidity and transverse momentum analysis of directed flow allows to disentangle various parametrizations in the model. At $400A\mathrm{MeV},$ a soft EoS with momentum dependent interactions is best suited to explain the experimental data in $\mathrm{Au}+\mathrm{Au}$ and $\mathrm{Xe}+\mathrm{CsI},$ but in the case of $\mathrm{Ni}+\mathrm{Ni}$ the model underpredicts flow for any EoS. At $90A\mathrm{MeV}$ incident beam energy, none of the IQMD parametrizations studied here are able to consistently explain the experimental data.

We present the centrality dependence of proton and deuteron rapidity distributions in Ru+Ru collisions at $400A\mathrm{MeV}.$ Data are compared with isospin quantum molecular dynamics (IQMD) calculations under various assumptions on the nucleon-nucleon cross section in the medium. The rapidity spectra of both particles can be reproduced by IQMD with a free nucleon-nucleon cross section for the most central collisions. The ratio of baryon rapidity distributions in isospin asymmetric collision systems shows incomplete mixing and partial transparency of the projectile and target nuclei at this beam energy.

The ratio of K- to K+ meson yields has been measured in the systems RuRu at 1.69 A GeV, Ru+Zr at 1.69 A GeV, and Ni+Ni at 1.93 A GeV incident beam kinetic energy. The yield ratio is observed to vary across the measured phase space. Relativistic transport-model calculations indicate that the data are best understood if in-medium modifications of the kaons are taken into account.

Abstract The FELIX system is used to interface the front-end electronics and the commodity hardware in the server farm of the ATLAS experiment. FELIX is using RDMA through RoCE to transmit data from its host servers to the Software Readout Driver using off-the-shelf networking equipment. In the current version of FELIX, RDMA communication is implemented using software on both ends of the links. Improvements of the data throughput as part of the High Luminosity LHC upgrade, by implementing RDMA support in the front-end FELIX FPGA, have been tested. A version of FELIX that uses the FPGA implementation of RDMA is being proposed and demonstrated.

Detailed studies of the azimuthal dependence of the mean fragment and flow energies in the Au+Au and Xe+CsI systems are reported as a function of incident energy and centrality. Comparisons between data and model calculations show that the flow energy values along different azimuthal directions could be viewed as snapshots of the fireball expansion with different exposure times. For the same number of participating nucleons more transversally elongated participant shapes from the heavier system produce less collective transverse energy. Good agreement with Boltzmann-Uehling-Uhlenbeck calculations is obtained for a soft nuclear equation of state.

First measurement of subthreshold \ensuremath{\Sigma}(1385) production is presented. Experimental data are presented for Al+Al reactions at $1.9A$ GeV measured with the FOPI detector at SIS/GSI. The $\ensuremath{\Sigma}(1385)/\ensuremath{\Lambda}$ ratio is found to be in good agreement with the transport and statistical model predictions. The results allow for a better understanding of subthreshold strangeness production and strangeness exchange reaction which is the dominant process for ${K}^{\ensuremath{-}}$ production below and close-to threshold.

New results concerning the production of neutral strange particles, ${K}^{0}$ and \ensuremath{\Lambda} in Ni+Ni collisions at $1.93A$ GeV, measured with the FOPI detector at GSI Darmstadt, are presented. Rapidity density distributions and Boltzmann slope parameter distributions are measured in nearly the full phase space of the reaction. The observables are compared to existing ${K}^{+}$ and proton data. While the ${K}^{0}$ data agree with previously reported ${K}^{+}$ measurements, the \ensuremath{\Lambda} distributions show a different behavior relative to that of protons. The strangeness balance and the production yield per participating nucleon as a function of the centrality of the reaction are discussed, for the first time at GSI Schwerionen Synchrotron (SIS) energies.

Results on ${K}^{+}$ and ${K}^{0}$ meson production in ${}_{44}^{96}\mathrm{Ru}$ + ${}_{44}^{96}\mathrm{Ru}$ and ${}_{40}^{96}\mathrm{Zr}$ + ${}_{40}^{96}\mathrm{Zr}$ collisions at a beam kinetic energy of 1.528$A$ GeV, measured with the FOPI detector at GSI-Darmstadt, are investigated as a possible probe of isospin effects in high-density nuclear matter. The measured double ratio (${K}^{+}/{K}^{0}$)${}_{\mathrm{Ru}}$/(${K}^{+}/{K}^{0}$)${}_{\mathrm{Zr}}$ is compared to the predictions of a thermal model and a relativistic mean field transport model using two different collision scenarios and under different assumptions on the stiffness of the symmetry energy. We find good agreement with the thermal model prediction and the assumption of a soft symmetry energy for infinite nuclear matter, while more realistic transport simulations of the collisions show a similar agreement with the data but also exhibit a reduced sensitivity to the symmetry term.

Abstract RDMA communication is an efficient choice for many applications, such as data acquisition systems, data center networking and any other networking application, where high bandwidth and low latency are necessary. RDMA can be implemented using a large array of options, which need to be tailored to the needed use case, in order to get optimal results. Aspects such as the effects of using multiple simultaneous connections, using various transport functions such as RDMA Write and RDMA Send and communication models such as sending individual bursts or continuous streams of data will be investigated for implementing RDMA on FPGA devices.

We present experimental data on directed flow in semi-central Au+Au collisions at incident energies from 90 to 400 A MeV. For the first time for this energy domain, the data are presented in a transverse momentum differential way. We study the first order Fourier coefficient v1 for different particle species and establish a gradual change of its patterns as a function of incident energy and for different regions in rapidity.

Small-angle correlations of pairs of protons emitted in central collisions of Ca + Ca, Ru + Ru and Au + Au at beam energies from 400 to 1500MeV per nucleon are investigated with the FOPI detector system at SIS/GSI Darmstadt. Dependences on system size and beam energy are presented which extend the experimental data basis of pp correlations in the SIS energy range substantially. The size of the proton-emitting source is estimated by comparing the experimental data with the output of a final-state interaction model which utilizes either static Gaussian sources or the one-body phase-space distribution of protons provided by the BUU transport approach. The trends in the experimental data, i.e. system size and beam energy dependences, are well reproduced by this hybrid model. However, the pp correlation function is found rather insensitive to the stiffness of the equation of state entering the transport model calculations.

We present transverse momentum and rapidity spectra of charged pions in central Ru + Ru collisions at 400$A$ and 1528$A$ MeV. The data exhibit enhanced production at low transverse momenta compared to the expectations from the thermal model that includes the decay of $\Delta(1232)$-resonances and thermal pions. Modification of the $\Delta$-spectral function and the Coulomb interaction are necessary to describe the detailed shape of the transverse momentum spectra. Within the framework of the thermal model, the freeze-out radii of pions are similar at both beam energies. The IQMD model reproduces the shapes of the transverse momentum and rapidity spectra of pions, but the predicted absolute yields are larger than in the measurements, especially at lower beam energy.

The $\phi$-meson production cross section is measured for the first time at a sub-threshold energy of 1.93 AGeV in \nuc{58}{Ni}+\nuc{58}{Ni} central collisions. The $\phi$ data were obtained within the acceptance of the CDC/Barrel subsystem of FOPI. For a sample of $4.7\cdot 10^6$ central events, after background subtraction, 23 candidates were observed. Extensive GEANT simulations of the detector performance are shown in a thorough comparison to the real response, aiming at a good understanding of the apparatus and at a trustable determination of the efficiencies, production probability and possible systematic errors. A filter procedure is elaborated, which is meant to facilitate the comparison of any theoretical calculation or new data with the current ones. How to extrapolate the present value to a $\phi$-meson cross section in $4\pi$ is also discussed. This result on pseudo-vector mesons can now be compared to existing experimental knowledge for the same reaction at the same incident energy for various outgoing channels, $K^+$ and $K^-$ included. A significant fraction (at least 20%) of the $K^-$-mesons is originating in the decay of the $\phi$, supporting the statement that the two channels are strongly correlated.

A completely new configuration of a glass resistive-plate chamber (GRPC) was built and tested. It consists of a double two-gap structure of electrodes with an active area of about 400cm2 and is read out via a central multistrip printed circuit board. In measurements with a 60Co source and p, d particles of 1.5AGeV time resolutions better than 80ps, position resolution along the strips of 5–6mm and efficiencies larger than 95% were obtained using available fast standard electronics. These results open the possibility of constructing compact TOF detectors of high resolution and high granularity.

We report on the first analysis of directed and elliptic flow with the new method of Lee--Yang zeros. Experimental data are presented for Ru+Ru reactions at 1.69A GeV measured with the FOPI detector at SIS/GSI. The results obtained with several methods, based on the event-plane reconstruction, on Lee--Yang zeros, and on multiparticle cumulants (up to fifth order) applied for the first time at SIS energies, are compared. They show conclusive evidence that azimuthal correlations between nucleons and composite particles at this energy are largely dominated by anisotropic flow.

Cluster production is investigated in central collisions of Ca+Ca, Ni+Ni, 96Zr+96Zr, 96Ru+96Ru, Xe+CsI and Au+Au reactions at 0.4 AGeV incident energy. We find that the multiplicity of clusters with charge Z⩾3 grows quadratically with the system's total charge and is associated with a midrapidity source with increasing transverse velocity fluctuations. When reduced to the same number of available charges, an increase of cluster production by about a factor of 5.5 is observed in the midrapidity region between the lightest system (Ca+Ca) and the heaviest one (Au+Au). The results, as well as simulations using quantum molecular dynamics, suggest a collision process where droplets, i.e., nucleon clusters, are created in an expanding, gradually cooling, nucleon gas. Within this picture, expansion dynamics, collective radial flow and cluster formation are closely linked as a result of the combined action of nucleon–nucleon scatterings and the mean fields.

Directed sideward flow of light charged particles and intermediate mass fragments was measured in different symmetric reactions at bombarding energies from 90 to 800 AMeV. The flow parameter is found to increase with the charge of the detected fragment up to Z = 3-4 and then turns into saturation for heavier fragments. Guided by simple simulations of an anisotropic expanding thermal source, we show that the value at saturation can provide a good estimate of the flow angle, $Θ_{flow}$, in the participant region. It is found that $Θ_{flow}$ depends strongly on the impact parameter. The excitation function of $Θ_{flow}$ reveals striking deviations from the ideal hydrodynamical scaling. The data exhibit a steep rise of $Θ_{\flow}$ to a maximum at around 250-400 AMeV, followed by a moderate decrease as the bombarding energy increases further.

We analyzed the $\ensuremath{\phi}$ meson production in central Ni + Ni collisions at a beam kinetic energy of 1.93A GeV with the FOPI spectrometer and found a production probability per event of $[8.6\ifmmode\pm\else\textpm\fi{}1.6(\text{stat})\ifmmode\pm\else\textpm\fi{}1.5(\text{syst})]\ifmmode\times\else\texttimes\fi{}{10}^{\ensuremath{-}4}$. This new data point allows us for the first time to inspect the centrality dependence of subthreshold $\ensuremath{\phi}$ meson production in heavy-ion collisions. The rise of $\ensuremath{\phi}$ meson multiplicity per event with mean number of participants can be parametrized by a power function with exponent $\ensuremath{\alpha}=1.8\ifmmode\pm\else\textpm\fi{}0.6$. The ratio of $\ensuremath{\phi}$ to ${K}^{\ensuremath{-}}$ production yields seems not to depend, within the experimental uncertainties, on the collision centrality, and the average of measured values was found to be $0.36\ifmmode\pm\else\textpm\fi{}0.05$.

Abstract The FELIX system is used as an interface between front-end electronics and commodity hardware in the server farm. FELIX is using RDMA through RoCE to transmit data from its host servers to the software readout driver using off-the-shelf networking equipment. RDMA communication is implemented using software on both end of the links. Exploring opportunities to improve data throughput as part of the high luminosity LHC upgrade, an implementation for RDMA support in the front-end FELIX FPGA is being developed. We present a proof-of-concept RDMA FPGA implementation, which will help inform the design of the FELIX platform for high luminosity LHC.

The characteristics of a symmetric multigap resistive plate chamber with multistrip readout electrode, recently developed by us, continued to be investigated. Studies of the time resolution, efficiency, average charge and dark rate as a function of applied voltage and the influence of the angle of incidence of the detected particle on these observables have been performed. Different type of discriminators have been tested.

The FOPI Collaboration at the GSI SIS-18 synchrotron measured charged kaons from central and semicentral collisions of Ni+Ni at a beam energy of $1.91A$ GeV. We present the distribution of the ${K}^{\ensuremath{-}}/{K}^{+}$ ratio on the energy vs polar angle plane in the nucleon-nucleon center-of-mass frame, with and without subtraction of the contribution of $\ensuremath{\phi}(1020)$ meson decays to the ${K}^{\ensuremath{-}}$ yield. The acceptance of the current experiment is substantially wider compared to the previous measurement of the same colliding system. The ratio of ${K}^{\ensuremath{-}}$ to ${K}^{+}$ energy spectra is expected to be sensitive to the in-medium modifications of basic kaon properties like mass. Recent results obtained by the HADES Collaboration at $1.23A$ and $1.76A$ GeV indicate that after inclusion of the $\ensuremath{\phi}$ meson decay contribution to the ${K}^{\ensuremath{-}}$ production no difference between the slopes of the ${K}^{\ensuremath{-}}$ and ${K}^{+}$ energy spectra is observed within uncertainties. For our data a linear fit to this ratio obtained after subtraction of the $\ensuremath{\phi}$ meson contribution still shows a decrease with kinetic energy, although a constant value cannot be rejected. The contribution of $\mathrm{\ensuremath{\Lambda}}(1520)\ensuremath{\rightarrow}p{K}^{\ensuremath{-}}$ decays estimated from fitting the thermal model to the experimental yields appears to be another factor of moderate relevance.

Experimental data on charged kaon phase space distributions and strangeness sideward flow measured with the FOPI detector at SIS/GSI are presented. Comparisons are made with the predictions of transport models investigating the in-medium kaon-nucleon potential.

New results on nuclear collective flow are presented for central and semi-central Ru + Ru collisions at 400 A MeV measured with the FOPI detector at GSI-Darmstadt. The source shape parameters, flow angle and aspect ratios, are extracted from Gaussian fits to in-plane and out-of-plane momentum distributions. The orientation and the shape of the source exhibit different trends according to the investigated phase space region. The shape parameters of the participant source are studied as a function of the particle mass and collision centrality. The flow angle is found to be independent of the particle mass. Both the flow angle and the aspect ratios depend sensitively on the impact parameter. Detailed comparisons with the predictions of the isospin quantum molecular dynamics model are performed. It is shown in particular that the source shape parameters permit to extract information on the in-medium nucleon–nucleon cross section.

We present the structure and the functionality of the Tier-3 ATLAS local analysis facility developed in Particle Physics Department of the IFIN-HH. The infrastructure was designed to fulfil the requirements for intensive Monte Carlo simulation and large data volumes analysis. Tests for parallel data analysis using PROOF framework were performed and several studies have been developed for proton-proton collisions at TeV scale. The obtained results confirm the accomplishment of the requirements of both software and hardware infrastructures.

The ATLAS group from IFIN-HH is involved in several physics studies. For these we perform data analysis and Monte Carlo simulations operating large amounts of data. The main aim of this following paper is to present how we developed a computing cluster who offers access to the experiment software via a distributed filesystem and it's able to process the simulations and real data analysis combining distributed and parallel computing and using xrootd, a very scalable and fast protocol, for data storage.

Using the large acceptance apparatus FOPI, we study central and semi-central collisions in the reactions (energies in A GeV are given in parentheses): 40Ca+40Ca (0.4, 0.6, 0.8, 1.0, 1.5, 1.93), 58Ni+58Ni (0.15, 0.25, 0.4), 96Ru+96Ru (0.4, 1.0, 1.5), 96Zr+96Zr (0.4, 1.0, 1.5), 129Xe+CsI (0.15, 0.25, 0.4), 197Au+197Au (0.09, 0.12, 0.15, 0.25, 0.4, 0.6, 0.8, 1.0, 1.2, 1.5). The observables include directed and elliptic flow. The data are compared to earlier data where possible and to transport model simulations. A stiff nuclear equation of state is found to be incompatible with the data. Evidence for extra-repulsion of neutrons in compressed asymmetric matter is found.

RDMA (Remote Direct Memory Access) is used by the ATLAS experiment at CERN in the new readout system based on FELIX (Front-End Link eXchange) for its networking layer. The FELIX system is used to interface the front-end electronics to commodity hardware in the server farm. In the current implementation of FELIX, RDMA communication is implemented using software on both ends of the RDMA links. FELIX is using RDMA through RoCE (RDMA over Converged Ethernet) to transmit data from its servers to the Software Readout Driver devices in the server farm using off-the-shelf networking equipment. As a consequence of the High Luminosity LHC upgrade, improvements in the data throughput will be needed. These improvements can be achieved by implementing RDMA support in the FELIX FPGA to simplify the path the data is taking through the readout system. This FPGA implementation of the RDMA protocol has been developed and tested. Now, a version of FELIX that uses this implementation is being proposed and demonstrated.