Gumberidze, M.

Holzmann, R.

Tlusty, P.

GSI Repository

Inclusive pion and eta production in the 3.5 GeV p+93Nb reaction∗M. Gumberidze1,2, R. Holzmann3, and P. Tlusty4 for the HADES collaboration1IPN, CNRS/IN2P3 – Universite´ Paris Sud, Orsay, France; 2TUD, Darmstadt, Germany; 3GSI, Darmstadt, Germany;4Nuclear Physics Institute, Academy of Sciences of Czech Republic, Rez, Czech RepublicIn recent years HADES has studied inclusive electronpair production in the proton-induced reactions p+p andp+Nb at 3.5 GeV [1, 2]. These data are interpreted withthe help of transport models such as UrQMD [3], GiBUU[4], HSD [5], and IQMD [5]. We provide now furtherconstraints from an analysis of 4-lepton events detectedin HADES which yield information on the inclusive pro-duction of the π0 and η mesons. Indeed, the electromag-netic decays of the latter, in particular the decays π 0, η →γe+e− and π0, η → γγ, combined with the external con-version of the decay photon(s) on material in the innerregion of HADES, lead to events with a characteristic 4-lepton signature, namely π0, η → e+e−e+e−. The lowconversion probability ('4%) and the rather low leptonmomenta lead to an overall acceptance for these final stateson the order of 10−6 − 10−5 only. Nevertheless, in 9 · 109triggered p+Nb events a sizeable statistics could be accu-mulated.]2   [GeV/c-e+e-e+eM0 0.2 0.4 0.6 0.8 1counts05001000150020000πηmixed evt CB3.5 GeV  p + NbFigure 1: Invariant mass distribution of e+e−e+e− eventsmeasured with HADES in the 3.5 GeV p+Nb reaction.Figure 1 shows the reconstructed e+e−e+e− invariantmass spectrum together with its combinatorial background(CB) of uncorrelated lepton pairs as obtained from eventmixing. The π0 and η peaks are prominently visible overthe CB with a statistics of a few thousand counts each. Bothmesons can easily be selected with appropriate mass cutsand, after having applied corrections for conversion and re-construction efficiencies, their rapidity and transverse mo-mentum distributions have been obtained. Of those the lat-ter are displayed in Fig. 2. In addition to the π 0 and ηdistributions we show there the result obtained for negativepions [6]. As the acceptance of π0 is limited to p⊥ > 0.3we have used the π− to extrapolate the neutral pion yield∗Work supported by EMMI and the IN2P3/GSI agreementto low p⊥ values. Extrapolating as well beyond the ac-cepted rapidity range of 0.2 < y < 1.8 we obtain (prelim-inary) inclusive multiplicities of Mπ− = 0.62 ± 0.15 andMπ0 = 0.60± 0.20 in 3.5 GeV p+Nb reactions.In contrast to the pions, the eta coverage of HADES iscomplete in p⊥, as visible in Fig. 2, and nearly so in y.Furthermore, the η phase space distribution seems to bewell described by a Boltzmann fit. Integrating and extrapo-lating to full rapidity yields a (preliminary) multiplicity ofMη = 0.032± 0.005 for inclusive eta production.    [GeV/c]p0 0.5 1 1.5]-1    [(GeV/c)dN/dp-510-410-310-210-1101 0π -π η  fit < 1.8η,π0.2 < y > 0.12 GeV/c-πpFigure 2: Reconstructed and efficiency-corrected π 0(closed circles), π− (blue line) and η (open circles) trans-verse momentum distributions dN/dp⊥ within the HADESacceptance. Statistical errors are indicated by vertical barsand systematic uncertainties by the yellow shading. ABoltzmann fit (red line) to the η data yields the tempera-ture T = 84± 3 MeV.Our proof-of-principle result demonstrates that 4-leptonfinal states can be reconstructed reliably with HADES andwe believe that such data provide additional valuable in-put for the modeling of proton-nucleus and heavy-ion col-lisions in the few-GeV regime.References[1] G. Agakishiev et al., Eur. Phys. J. A 48, 64 (2012).[2] G. Agakishiev et al., Phys. Lett. B 715, 304 (2012).[3] K. Schmidt et al., Phys. Rev. C 79, 064908 (2009).[4] J. Weil et al., Eur. Phys. J. A 48, 111 (2012).[5] E. Bratkovskaya et al., arXiv:1301.0786v1 [nucl-th].[6] P. Tlusty et al., PoS Bormio2012, 019 (2012).PHN-NQM-EXP-08 GSI SCIENTIFIC REPORT 201236

Inclusive pion and eta production in the 3.5 GeV p+$^{93}$Nb reaction

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Inclusive pion and eta production in the 3.5 GeV p+ $^{93}$ Nb reaction

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Inclusive pion and eta production in the 3.5 GeV p+93^{93}93Nb reaction

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Inclusive pion and eta production in the 3.5 GeV p+ $^{93}$ Nb reaction