Browsing by Author "Lopes, L."
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- An upper limit on hypertriton production in collisions of Ar(1.76 A GeV) + KClPublication . HADES collaboration (97 authors); Blanco, A.; Fonte, P.; Lopes, L.; Mangiarotti, A.A high-statistic data sample of Ar(1.76 AGeV)+KCl events recorded with HADES is used to search for a hypertriton signal. An upper production limit per centrality-triggered event of $1.04$ x $10^{-3}$ on the $3\sigma$ level is derived. Comparing this value with the number of successfully reconstructed $\Lambda$ hyperons allows to determine an upper limit on the ratio $N_{_{\Lambda}^3H}/N_{\Lambda}$, which is confronted with statistical and coalescence-type model calculations.
- Analysis of pion production data measured by HADES in proton-proton collisions at 1.25 GeVPublication . HADES collaboration (99 authors); Blanco, A.; Fonte, P.; Lopes, L.; Mangiarotti, A.Baryon resonance production in proton-proton collisions at a kinetic beam energy of 1.25 GeV is investigated. The multi-differential data were measured by the HADES Collaboration. Exclusive channels with one pion in the final state ($np\pi^+$ and $pp\pi^0$ were put to extended studies based on various observables in the framework of a one-pion exchange model and with solutions obtained within the framework of a partial wave analysis (PWA) of the Bonn-Gatchina group. The results of the PWA confirm the dominant contribution of the $\Delta(1232)$, yet with a sizable impact of the $N(1440)$ and non-resonant partial waves.
- Analysis of the exclusive final state npe$^+$e$^-$ in quasi-free np reactionPublication . HADES collaboration (117 authors); Blanco, A.; Bordalo, P.; Fonte, P.; Franco, C.; Lopes, L.; Mangiarotti, A.; Ramos, S.; Silva, L.We report on the investigation of dielectron production in tagged quasi-free neutron-proton collisions by using a deuteron beam of kinetic energy 1.25GeV/u impinging on a liquid hydrogen target. Our measurements with HADES confirm a significant excess of $e^{+}e^{-}$ pairs above the $\pi^{0}$ mass in the exclusive channel $dp \rightarrow npe^{+}e^{-}(p_{spect})$ as compared to the exclusive channel $ ppe^{+}e^{-}$ measured in proton-proton collisions at the same energy. That excess points to different bremsstrahlung production mechanisms. Two models were evaluated for the role of the charged pion exchange between nucleons and double- $\Delta$ excitation combined with intermediate $\rho$ -meson production. Differential cross sections as a function of the $e^{+}e^{-}$ invariant mass and of the angles of the virtual photon, proton and electrons provide valuable constraints and encourage further investigations on both experimental and theoretical sides.
- Associate K^0 production in p+p collisions at 3.5 GeV: The role of Delta(1232)++Publication . HADES collaboration (100 authors); Blanco, A.; Fonte, P.; Lopes, L.; Mangiarotti, A.
An exclusive analysis of the four-body final states Λ+p+π++K0 and Σ0+p+π++K0 measured with HADES for p+p collisions at a beam kinetic energy of 3.5 GeV is presented. The analysis uses various phase space variables, such as missing mass and invariant mass distributions, in the four-particle event selection (p,π+,π+,π−) to find cross sections of the different production channels, contributions of the intermediate resonances Δ++ and Σ(1385)+, and corresponding angular distributions. A dominant resonant production is seen, where the reaction Λ+Δ+++K0 has a cross section about ten times higher (29.45±0.08−1.46+1.67±2.06μb) than the analogous nonresonant reaction (2.57±0.02−1.98+0.21±0.18μb). A similar result is obtained in the corresponding Σ0 channels with 9.26±0.05−0.31+1.41±0.65μb in the resonant and 1.35±0.02−1.35+0.10±0.09μb in the nonresonant reactions.
- Baryonic resonances close to the $\bar{K}$ N threshold: The Case of $\Sigma(1385)^+$ in pp collisionsPublication . HADES collaboration (102 authors); Blanco, A.; Fonte, P.; Lopes, L.; Mangiarotti, A.We present results of an exclusive measurement of the first excited state of the Sigma hyperon, Sigma(1385)^+, produced in p+p -> Sigma^+ + K^+ + n at 3.5 GeV beam energy. The extracted data allow to study in detail the invariant mass distribution of the Sigma(1385)^+. The mass distribution is well described by a relativistic Breit-Wigner function with a maximum at m_0 = 1383.2 +- 0.9 MeV/c^2 and a width of 40.2 +- 2.1 MeV/c^2. The exclusive production cross-section comes out to be 22.27 +- 0.89 +- 1.56 +3.07 -2.10 mu b. Angular distributions of the Sigma(1385)^+ in different reference frames are found to be compatible with the hypothesis that 33 % of Sigma(1385)^+ result from the decay of an intermediate Delta^{++} resonance.
- Baryonic resonances close to the $\bar{K}N$ threshold: the case of $\Lambda$(1405) in $pp$ collisionsPublication . HADES collaboration (104 authors); Blanco, A.; Fonte, P.; Lopes, L.; Mangiarotti, A.We present an analysis of the Λ(1405) resonance produced in the reaction p+p→Σ±+π∓+K++p at 3.5GeV kinetic beam energy measured with the High Acceptance Dielectron Spectrometer (HADES) at the Gesellschaft für Schwerionenforschung (GSI). The two charged decay channels Λ(1405)→Σ±π∓ have been reconstructed for the first time in p+p collisions. The efficiency and acceptance-corrected spectral shapes show a peak position clearly below 1400MeV/c2. We find a total production cross section of σΛ(1405)=9.2±0.9±0.7−1.0+3.3μb. The analysis of its polar angle distribution suggests that the Λ(1405) is produced isotropically in the p−p center-of-mass system (CMS).
- Bruxismo na criançaPublication . Kizi, G.; Barata, R.; Amaral, A.; Lopes, L.; Ventura, I.
- Centrality determination of Au + Au collisions at 1.23A GeV with HADESPublication . HADES collaboration (113 authors); Blanco, A.; Bordalo, P.; Fonte, P.; Franco, C.; Lopes, L.; Mangiarotti, A.; Ramos, S.; Silva, L.The centrality determination for Au+Au collisions at 1.23A GeV, as measured with HADES at the GSI-SIS18, is described. In order to extract collision geometry related quantities, such as the average impact parameter or number of participating nucleons, a Glauber Monte Carlo approach is employed. For the application of this model to collisions at this relatively low centre-of-mass energy of $\sqrt{s_{\mathrm{NN}}} = 2.42$ GeV special investigations were performed. As a result a well defined procedure to determine centrality classes for ongoing analyses of heavy-ion data is established.
- Deep sub-threshold $\phi$ production in Au+Au collisionsPublication . HADES collaboration (114 authors); Blanco, A.; Bordalo, P.; Fonte, P.; Franco, C.; Lopes, L.; Mangiarotti, A.; Ramos, S.; Silva, L.We present data on charged kaons (K+-) and {\phi} mesons in Au(1.23A GeV)+Au collisions. It is the first simultaneous measurement of K and {\phi} mesons in central heavy-ion collisions below a kinetic beam energy of 10A GeV. The {\phi}/K- multiplicity ratio is found to be surprisingly high with a value of 0.52 +- 0.16 and shows no dependence on the centrality of the collision. Consequently, the different slopes of the K+ and K- transverse-mass spectra can be explained solely by feed- down, which substantially softens the spectra of K- mesons. Hence, in contrast to the commonly adapted argumentation in literature, the different slopes do not necessarily imply diverging freeze- out temperatures of K+ and K- mesons caused by different couplings to baryons.
- $\Delta$(1232) Dalitz decay in proton-proton collisions at T=1.25 GeV measured with HADES at GSIPublication . HADES collaboration (119 authors); Blanco, A.; Bordalo, P.; Fonte, P.; Franco, C.; Lopes, L.; Mangiarotti, A.; Ramos, S.; Silva, L.We report on the investigation of Δ(1232) production and decay in proton-proton collisions at a kinetic energy of 1.25 GeV measured with HADES. Exclusive dilepton decay channels ppe+e− and ppe+e−γ have been studied and compared with the partial-wave analysis of the hadronic ppπ0 channel. They allow to access both Δ+→pπ0(e+e−γ) and Δ+→pe+e− Dalitz decay channels. The perfect reconstruction of the well-known π0 Dalitz decay serves as a proof of the consistency of the analysis. The Δ Dalitz decay is identified for the first time and the sensitivity to N-Δ transition form factors is tested. The Δ(1232) Dalitz decay branching ratio is also determined for the first time; our result is (4.19 ± 0.62 syst. ± 0.34 stat.) ×10−5, albeit with some model dependence.