​Development studies for HADES at FAIR: simulation of Hyperon electromagnetic Dalitz decays

Measurements of the baryon resonance Dalitz decays for non strange baryons pioneered by the HADES in p+p reactions at 3.5 GeV will be continued at the SIS100. Upgraded DAQ system will allow to gain at least one order of magnitude in statistics. Furthermore, taking the advantage of higher beam energies at the SIS100 and higher production cross sections we plan to extend the research to the final states including hyperon states (L* and S*). Since production of hyperons is associated with the kaon production, their production can be tagged by the charged kaon detection. We propose to use reaction of the type p+p ® p+K⁺ + (L*, S*) ® p+K⁺ + L + e⁺e^– to measure, for the first time, the hyperon (L*, S*) ® L e⁺e^– Dalitz decays. According to theoretical predictions based on the VMD [wiliams1993], the dielectron emission at q² ~ 0.6 GeV/c² should be enhanced by at least one order of magnitude due to the intermediate vector meson state. Detailed simulations are needed to study the feasibility of such measurements.

Investigation of emissivity of baryonic matter in Heavy Ion collisions

The primary goal of dielectron spectroscopy in heavy ion collisions is to separate “excess” radiation originating from the compressed zone of the heavy ion (HI) collisions from contributions which can be assigned to the individual nucleon-nucleon reactions. The first, model independent, assessment can be obtained by a direct comparison of the measured spectra in N-N and HI collisions, scaling by the number of participants. A basic question we ask is as follows: to what extent a complex HI collision can be described as a superposition of the individual N-N collisions and whether we see new effects related to special properties of the compressed nuclear matter? From the transport calculations one knows that in the collisions of Au+Au at 1-2 AgeV, 30% of nucleons are excited to baryonic resonances (mainly D(1232) and, to less extent, N*) and that the pions contribute less than 10%. An average matter density of 3-4 times normal nuclear matter can be reached for a time period of about 10-15 fm/c. Under such circumstances, the average distances between baryonic resonances are almost as their “free” life time (ct ~ 1.3 fm/c). Therefore, one can expect that such medium, sometimes called “resonance matter”, can hold the very special properties. Indeed, the results on dielectron production obtained by the HADES within Ar+KCl collisions provide the first evidence of a non-trivial in-medium excess radiation. Fig. 1. shows the ratio of the differential pair production rates measured in Ar+KCl at 1.756 AGeV as a function of e⁺e^– mass to the one obtained in N-N collisions normalized to the same number of participants [hadesarkcl]. One should add, that in both distributions the contribution resulting from the h ® e⁺e^– g decay accounting for radiation after freeze-out has been subtracted. A clear excess of dielectron radiation is observed for M_e+e– > 0.15 GeV/c2 which is attributed to the emissivity of compressed baryonic matter consisting, in addition to nucleons, mainly from short-lived baryon resonances [salabura2012, bratkovskaya2013]. In year 2012, the HADES performed the high statistics measurement of Au+Au collisions at 1.25 GeV, which is the decisive test for the confirmation of the effect observed in Ar+KCl. One expects that for larger Au+Au system much stronger effect (at least one order of magnitude - see the right side of Fig. 1) should be observed. The data analysis is in progress and the final results are expected to be obtained within duration of this project. The main goal of the analysis is:

(a) to compare the dielectron invariant mass distributions measured in Au+Au collisions to the one obtained from NN reactions, (b) to show the centrality dependence and (c) the angular distributions of the excess with respect to the reaction plane (so called dielectron flow). We consider also performing the measurement Ag+Ag at 1.756 GeV (the same energy as Ar+KCl) which is the highest possible energy and the system size achievable at the SIS18. The decision about the beam time sharing between pion induced reactions or/and HI run will be taken by the end of this (2013) year.

 

Figure 1. Left: Ratio of pair production yields measured in Ar+KCl and C+C to the one measured in N+N collisions as a function of the e⁺e^– invariant mass (see text for details) [hadesarkcl]. Right: Expected e⁺e^– cross section production rate in Au+Au collisions at 1.25 AGeV (expected mesons sources and the excess radiation (blue line) are shown separately) compared to the N-N reference spectrum measured at the same beam energy (points).

Investigation of electromagnetic Dalitz decays of baryonic resonances in proton-proton collisions: exclusive resonance production in proton-proton reactions

Dielectron production in nucleon-nucleon collisions at a few GeV energy range has a steep rising excitation function. At low e⁺e^– invariant masses (M_e+e– < 0.15 GeV/c²) the  p⁰ ® e⁺e^– g Dalitz decay dominates the pair production. At higher masses (0.15 < M_e+e– < 0.55 GeV/c²) Dalitz decays h ® e⁺e^– g, w ® p⁰ e⁺e^–, D ® Ne⁺e^– and bremsstrahlung radiation  NN ® NNe⁺e^– determine the dielectron rates. Finally at e⁺e^– invariant masses M_e+e– > 0.55 GeV/c² the Dalitz decays of D and higher mass resonances R ® Ne⁺e^–, and direct V(r, w, f) ® e⁺e^– decays contribute to the dielectron mass (see Fig. 1). This high mass region is particularly sensitive to the vector meson contribution to the baryon resonance eTFF. The strength of the vector meson-resonance coupling (and related eTFF dependence on (q²)) should be visible as an enhancement in the e⁺e^– invariant mass spectrum, as predicted by the various calculations  [pena2012, Kriv2002, Weil2012, Shyam2010].

Inclusive dielectron production proton-proton, proton-neutron collisions at 1.25 GeV [hades125] and proton-proton reactions at 2.2 GeV [hades22] and 3.5 GeV [hades35] have been already analysed and published by the HADES. The left side of Fig. 1 shows the inclusive e⁺e^– differential cross section as a function of  the invariant mass for p+p reactions at 3.5 GeV [hades35]. The solid lines present expected contributions from the mesons (p⁰, h ® e⁺e^– g, w ® p⁰ e⁺e^–), D ® Ne⁺e^– Dalitz decays and a two-body decays of r/w based on the known cross sections and branching ratios. As one can see, below the vector meson pole a clear excess above the known sources emerges.

Figure 1. Left: Invariant e⁺e^– mass distribution measured in p+p collisions at 3.5 GeV compared to the expected contributions from the known hadron sources (see legend) [hades35]. Right: Comparison of e⁺e^– invariant mass distributions measured in the vector mass region to the model calculations of the GiBUU [Weil2012].

 

Model calculations done in the framework of Giessen BUU (GiBUU) [Weil2012] (see the left side of Fig. 1) shows that the excess can be explained either by a special choice of eTFF of the D resonance (red upper line) or an incorporation of higher lying resonances (D, N*)  decaying into dielectron pair via intermediate r (R ® Nr ® Ne⁺e^–) (green upper line). The hatched area indicates the uncertainties of the calculations depending on various eTFF assumptions (red) and the resonance contribution (green).  

In order to understand the elementary contributions, we propose to reconstruct three exclusive channels: with baryon resonances R = (D, N^*) decaying into pions (a) pp ® Rp ® p pp⁰,   (b) pp ® Rp ® p np⁺  and dielectrons (c) pp ® Rp ® ppe⁺e^– by reconstruction of all final state particles. The exclusive channel ppe⁺e^– provides better control on the reaction mechanism, as compared to the inclusive e⁺e^– X, which is just superposition of many exclusive channels. In this way we will separate channels with baryon resonance decays from those which include mesonic sources h® e⁺e^– g  and  w®p⁰ e⁺e^–. The final states including pions will allow to fix  the resonance production cross sections. In order to derive the resonance cross sections we will perform the partial wave analysis (PWA). The results obtained by PWA will be compared to resonance model predictions, commonly used in the transport models (like the UrQMD, the GiBUU). Obtained cross sections for the resonance production will be converted into respective e⁺e^– yields for the various models of the eTFF [Pena2012, Kriv2002]. To demonstrate the sensitivity of data to q² dependence of eTFF, we will also compare measured distributions to results of calculations, assuming the point like coupling of the virtual photon to the baryons. In this case eTFF is a constant fixed by the known R ® Ng decays [Zetenyi].  The analysis will be performed for two proton beam energies, 1.25 GeV and 3.5 GeV.  While at the lower beam energy only D resonance is excited,  at the higher energies other higher mass resonances contribute almost with the equal strength as D(1232).

Measurement of resonance production and decays in pion induced reactions

Coupling of baryon resonances to vector mesons can be ideally investigated by means of one / two pion and dielectron production in the p-N reaction at 1.5 < Ös < 2 GeV. In such reactions resonances are excited directly at a fixed mass, in contrast to proton-proton collisions, where the resonance production proceeds via the virtual meson exchange. Unfortunately, existing data are very scarce, for the desired energy range only around 300000 events with two pion production and no dielectron data exist. The existing data were analysed by Manley [Manley1992] and that work is the main source of our knowledge on Resonance ® rN decays. In particular, couplings of resonance with pole mass below the threshold for Nr decay, like N*(1520), N*(1680), N*(1720), which are very important for in-medium physics, are only poorly known. Within this project we want to perform the first experiment with the HADES using the secondary pion beams available at GSI, in order to demonstrate the feasibility of p^– p ® R ® ne⁺e^– measurement and collect high statistics data on p^– p ® R ® n p⁺p^– (p p⁰p^–)  channels.

Development of reconsatruction algorithms for dielectron and direct photons production at SIS100

Dielectron measurement at the SIS100 will present new challenge for the HADES detector originally designed to cope with the particle multiplicities expected from 1 GeV Au+Au collisions. At the higher energies and the large system sizes detector will be operated at 2-3 larger occupancies and the reconstruction algorithms have to be properly tuned. Kraków group has a long-standing experience in the dielectron pair analysis and is assigned to work on this task. Furthermore, extension of the HADES by means of the ECAL will allow to measure, for the first time at this energy range, direct photon radiation. The latter one are of a particular importance in spite of unexpected large flow of direct phtons observed at RHIC and LHC energies. We plan to perform the dedicated simulations and develop reconstruction algorithms for these measurements.

Development of readout electronics for electromagnetic calorimeter (ECAL)

For the HADES at SIS100, new detector components will be installed, from which most important is the ECAL. The Electromagnetic CALorimeter (ECAL) based on the OPAL modules is presently under construction for the HADES at SIS100 [tdr]. The ECAL will substitute the Pre-Shower detector, built and maintained by Kraków group, which is also in charge for the ECAL electronic development. The ECAL will significantly improve the electron identification in the HADES. Kraków group has proposed a read-out chain consisting of the analog boards for signal shaping and the digital signal processing in FPGA based read out boards, so called Time Read-out Boards (TRB) [trbv2]. Three generations of the TRB boards have been developed by our group in the collaboration with GSI and are now widely used in various experiments [daq]. Within this project we want to design a novel ADC concepts based on (a) either charge measurement inside FPGA or (b) charged to signal width conversion and time over threshold measurement.