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L. Lopes

Publications and source records attributed to L. Lopes.

84 records · Page 5Linked to original sources
arXiv2009

Recent results from HADES on electron pair production in relativistic heavy-ion collisions

Systematic investigations of dilepton production are performed at the SIS accelerator of GSI with the HADES spectrometer. The goal of this program is a detailed understanding of di-electron emission from hadronic systems at moderate temperatures and densities. New results obtained in HADES experiments focussing on electron pair production in elementary collisions are reported here. They pave the way to a better understanding of the origin of the so-called excess pairs earlier on observed in heavy-ion collisions by the DLS collaboration and lately confirmed in two measurements of the HADES collaboration using C+C and Ar+KCl collisions. Results of these studies are discussed.

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arXiv2009

Origin of the low-mass electron pair excess in light nucleus-nucleus collisions

We report measurements of electron pair production in elementary p+p and d+p reactions at 1.25 GeV/u with the HADES spectrometer. For the first time, the electron pairs were reconstructed for n+p reactions by detecting the proton spectator from the deuteron breakup. We find that the yield of electron pairs with invariant mass Me+e- > 0.15 GeV/c2 is about an order of magnitude larger in n+p reactions as compared to p+p. A comparison to model calculations demonstrates that the production mechanism is not sufficiently described yet. The electron pair spectra measured in C+C reactions are compatible with a superposition of elementary n+p and p+p collisions, leaving little room for additional electron pair sources in such light collision systems.

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arXiv2009

Deep sub-threshold $Ξ^-$ production in Ar+KCl reactions at 1.76A GeV

We report first results on a deep sub-threshold production of the doubly strange hyperon $Ξ^-$ in a heavy-ion reaction. At a beam energy of 1.76A GeV the reaction Ar+KCl was studied with the High Acceptance Di-Electron Spectrometer (HADES) at SIS18/GSI. A high-statistics and high-purity $Λ$ sample was collected, allowing for the investigation of the decay channel $Ξ^- \to Λπ^-$. The deduced $Ξ^-/(Λ+Σ^0)$ production ratio of $(5.6 \pm 1.2 ^{+1.8}_{-1.7})\cdot 10^{-3}$ is significantly larger than available model predictions.

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arXiv2009

Future perspectives at SIS-100 with HADES-at-FAIR

Currently, the HADES spectrometer undergoes un upgrade program to be prepared for measurements at the upcoming SIS-100 synchrotron at FAIR. We describe the current status of the HADES di-electron measurements at the SIS-18 and our future plans for SIS-100.

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arXiv2009

Charged pion production in C+C and Ar+KCl collisions measured with HADES

Results of a study of charged pion production in 12C+12C collisions at incident beam energies of 1A GeV and 2A GeV, and 40Ar+natKCl at 1.76AGeV, using the spectrometer HADES at GSI, are presented. We have performed a measurement of the transverse momentum distributions of pi+- mesons covering a fairly large rapidity interval, in case of the C+C collision system for the first time. The yields, transverse mass and angular distributions are compared with a transport model as well as with existing data from other experiments.

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arXiv2009

Measurement of charged pions in 12C + 12C collisions at 1A GeV and 2A GeV with HADES

We present the results of a study of charged pion production in 12C + 12C collisions at incident beam energies of 1A GeV and 2A GeV using the HADES spectrometer at GSI. The main emphasis of the HADES program is on the dielectron signal from the early phase of the collision. Here, however, we discuss the data with respect to the emission of charged hadrons, specifically the production of pi+- mesons, which are related to neutral pions representing a dominant contribution to the dielectron yield. We have performed the first large-angular range measurement of the distribution of pi+- mesons for the 12C + 12C collision system covering a fairly large rapidity interval. The pion yields, transverse-mass and angular distributions are compared with calculations done within a transport model, as well as with existing data from other experiments. The anisotropy of pion production is systematically analyzed.

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arXiv2009

Performances of multi-gap timing RPCs for relativistic ions in the range Z=1-6

We present the performance of Multi-gap timing RPCs under irradiation by fully stripped relativistic ions (gamma*beta=2.7, Z=1-6). A time resolution of 80 ps at high efficiency has been obtained by just using standard `off the shelf' 4-gap timing RPCs from the new HADES ToF wall. The resolution worsened to 100 ps for ~ 1 kHz/cm2 proton flux and for ~ 100 Hz/cm2 Carbon flux. The chambers were operated at a standard field of E=100 kV/cm and showed a high stability during the experiment, supporting the fact that RPCs are a convenient choice when accommodating a very broad range of ionizing particles is needed. The data provides insight in the region of very highly ionizing particles (up to x 36 mips) and can be used to constrain the existing avalanche and Space-Charge models far from the usual `mip valley'. The implications of these results for the general case of detection based on secondary processes (n, gamma) resulting in highly ionizing particles with characteristic energy distributions will be discussed, together with the nature of the time-charge correlation curve.

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arXiv2009

The High-Acceptance Dielectron Spectrometer HADES

HADES is a versatile magnetic spectrometer aimed at studying dielectron production in pion, proton and heavy-ion induced collisions. Its main features include a ring imaging gas Cherenkov detector for electron-hadron discrimination, a tracking system consisting of a set of 6 superconducting coils producing a toroidal field and drift chambers and a multiplicity and electron trigger array for additional electron-hadron discrimination and event characterization. A two-stage trigger system enhances events containing electrons. The physics program is focused on the investigation of hadron properties in nuclei and in the hot and dense hadronic matter. The detector system is characterized by an 85% azimuthal coverage over a polar angle interval from 18 to 85 degree, a single electron efficiency of 50% and a vector meson mass resolution of 2.5%. Identification of pions, kaons and protons is achieved combining time-of-flight and energy loss measurements over a large momentum range. This paper describes the main features and the performance of the detector system.

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arXiv2009

Phi decay: a relevant source for K- production at SIS energies?

We present phase space distributions and multiplicities of K+, K- and phi mesons produced in Ar+KCl reactions at a kinetic beam energy of 1.756 AGeV and measured with the HADES spectrometer. The inverse slope parameters and yields of kaons supplement the systematics of previous measurements. The percentage of K- mesons coming fromphi decay is found to be 18+- 7%.

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arXiv2008

Study of dielectron production in C+C collisions at 1 AGeV

The emission of e+e- pairs from C+C collisions at an incident energy of 1 GeV per nucleon has been investigated. The measured production probabilities, spanning from the pi0-Dalitz to the rho/omega! invariant-mass region, display a strong excess above the cocktail of standard hadronic sources. The bombarding-energy dependence of this excess is found to scale like pion production, rather than like eta production. The data are in good agreement with results obtained in the former DLS experiment.

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arXiv2002

Resistive plate chambers for time-of-flight measurements

The applications of Resistive Plate Chambers (RPCs) have recently been extended by the development of counters with time resolution below 100 ps sigma for minimum ionising particles. Applications to HEP experiments have already taken place and many further applications are under study. In this work we address the operating principles of such counters along with some present challenges, with emphasis on counter aging.

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