Study of e+,e− production in elementary and nuclear collisions near the production threshold with HADES

HADES is a second generation experiment designed to study dielectron production in proton, pion, and heavy ion induced reactions at the GSI accelerator facility in Darmstadt. The physics programme of HADES is focused on in-medium properties of the light vector mesons. In this contribution we present status of the HADES experiment, demonstrate its capability to identify rare dielectron signal, show first experimental results obtained from C+C reactions at 2 A GeV and shortly discuss physics programme of up-coming experimental runs. © 2004 Elsevier B.V. All rights reserved. 53 1 49 58 Cited By :1

Measurement of global polarization of {\Lambda} hyperons in few-GeV heavy-ion collisions

The global polarization of {\Lambda} hyperons along the total orbital angular momentum of a relativistic heavy-ion collision is presented based on the high statistics data samples collected in Au+Au collisions at \sqrt{s_{NN}} = 2.4 GeV and Ag+Ag at 2.55 GeV with the High-Acceptance Di-Electron Spectrometer (HADES) at GSI, Darmstadt. This is the first measurement below the strangeness production threshold in nucleon-nucleon collisions. Results are reported as a function of the collision centrality as well as a function of the hyperon transverse momentum (p_T) and rapidity (y_{CM}) for the range of centrality 0--40%. We observe a strong centrality dependence of the polarization with an increasing signal towards peripheral collisions. For mid-central (20--40%) collisions the polarization magnitudes are (%) = 6.0 \pm 1.3 (stat.) \pm 2.0 (syst.) for Au+Au and (%) = 4.6 \pm 0.4 (stat.) \pm 0.5 (syst.) for Ag+Ag, which are the largest values observed so far. This observation thus provides a continuation of the increasing trend previously observed by STAR and contrasts expectations from recent theoretical calculations predicting a maximum in the region of collision energies about 3 GeV. The observed polarization is of a similar magnitude as predicted by 3D fluid dynamics and the UrQMD plus thermal vorticity model and significantly above results from the AMPT model.Comment: 8 pages, 4 figure

Inclusive e+^+e−^- production in collisions of pions with protons and nuclei in the second resonance region of baryons

Inclusive e$^+$e$^-$ production has been studied with HADES in $\pi^-$ + p, $\pi^-$ + C and $\pi^- + \mathrm{CH}_2$ reactions, using the GSI pion beam at $\sqrt{s_{\pi p}}$ = 1.49 GeV. Invariant mass and transverse momentum distributions have been measured and reveal contributions from Dalitz decays of $\pi^0$, $\eta$ mesons and baryon resonances. The transverse momentum distributions are very sensitive to the underlying kinematics of the various processes. The baryon contribution exhibits a deviation up to a factor seven from the QED reference expected for the dielectron decay of a hypothetical point-like baryon with the production cross section constrained from the inverse $\gamma$ n$\rightarrow \pi^-$ p reaction. The enhancement is attributed to a strong four-momentum squared dependence of the time-like electromagnetic transition form factors as suggested by Vector Meson Dominance (VMD). Two versions of the VMD, that differ in the photon-baryon coupling, have been applied in simulations and compared to data. VMD1 (or two-component VMD) assumes a coupling via the $\rho$ meson and a direct coupling of the photon, while in VMD2 (or strict VMD) the coupling is only mediated via the $\rho$ meson. The VMD2 model, frequently used in transport calculations for dilepton decays, is found to overestimate the measured dielectron yields, while a good description of the data can be obtained with the VMD1 model assuming no phase difference between the two amplitudes. Similar descriptions have also been obtained using a time-like baryon transition form factor model where the pion cloud plays the major role.Comment: (HADES collaboration

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

The HADES-at-FAIR project

After the completion of the experimental program at SIS18 the HADES setup will migrate to FAIR, where it will deliver high-quality data for heavy-ion collisions in an unexplored energy range of up to 8 A GeV. In this contribution, we briefly present the physics case, relevant detector characteristics and discuss the recently completed upgrade of HADES. © 2012 Pleiades Publishing, Ltd. 75 5 589 593 Cited By :

Resonances in the system of pi sup +pi sup - -mesons from np->np pi sup +pi sup - reaction at P sub n =5.20 GeV/c: search, results of direct observations, interpretation

Ten resonances were found in the mass spectrum of a pi sup +pi sup - system based on 66075 events from np->np pi sup +pi sup - reaction in np-interactions at P sub n =(5.20+-0.16)GeV/c in the one-meter hydrogen bubble chamber of the LHE (JINR) by using the criterion cos THETA* p>0. These masses are the following: (347+-12), (418+-6), (511+-12), (610+-5), (678+-17), (757+-5), (880+-12), (987+-12), (1133+-15), and (1285+-22) MeV/c sup 2; their excesses above the background are 2.9, 5.2, 3.5, 1.4, 2.0, 8.5, 4.8, 3.8, 5.2, and 6.0 standard deviations, respectively. The experimental widths of the resonances vary within the region from 16 to 94 MeV/c sup 2. Such effects were not found in pi sup -pi sup 0 combinations from np->pp pi sup -pi sup 0 reaction. Therefore, it is necessary to attribute the value of isotopic spin I=0 to the resonances found in the mass spectrum of the pi sup +pi sup - system. The spin was estimated for the most statistically provided resonances at masses of 418, 511 and 757 MeV/c sup 2. We determine with a high degree of confidence that J=0 for the resonances at M sub R =757 MeV/c sup 2 and M sub R =418 MeV/c sup 2 and the most probable value of J=0 for the resonance at M sub R =511 MeV/c sup 2. Therefore, it can be affirmed that at least 3 states with quantum numbers of sigma sub 0 -meson 0 sup + (0 sup + sup +) have been found at masses of 418, 511 and 757 MeV/c sup 2. The fact that low-mass sigma sub 0 -mesons are glueballs is one of the possible interpretations. Comparison with the data of other papers has also been mad