Lorenz, M.

Scheib, T.

Schuldes, H.

GSI Repository

Revisiting hadron production at SIS energies using new HADES data ∗M. Lorenz † 1,2, T. Scheib2, H. Schuldes2, and the HADES collaboration*1University Utrecht, Netherlands; 2GU, Frankfurt, GermanyWe present preliminary results on the productionof hadrons with strangeness content (K+,K−,K0s ,Λand φ) in Au+Au collisions. At the measured centerof mass energy of√s = 2.4 GeV all hadrons carry-ing strangeness are produced below their free nucleon-nucleon threshold. While the K−/K+ ratio nicely fitsthe trend observed at higher energies, we find a strongrise of the φ/K− ratio.The data taking took place in April/May 2012. The read-out was started with a multiplicity trigger, running with anaverage rate of 8 kHz during the spills. The total data sam-ple corresponds to 7.3x109 events. Reconstructed trackspass a track selection based on several quality parametersdelivered by a newly developed high density tracking algo-rithm. Afterwards, hadrons are identified using the time-of-flight measurement.In order to minimize systematic errors due to efficiencycorrections and extrapolations in rapidity, we build ratiosof the corrected yields at mid-rapidity for various hadronspecies which feature a comparable width in rapidity. Theresulting K−/K+ ratio can be directly compared to thepreviously obtained systematics at similar energy, withoutcorrecting for the different centrality selections of the vari-ous experiments, as both kaons experience a similar Apartdependence [1]. The measured ratio K−/K+ fits into thetrend observed at slightly higher energies and extrapolateddown to the beam energy of 1.23 A GeV.An interesting observable is the φ/K− ratio: It showsa flat trend at high energies, and is experimentally ob-served to rise towards lower energies [2]. This rise canbe reproduced in the framework of the statistical model, ifthe suppression of strangeness is handled by introducing astrangeness correlation radiusRc within which strangenesshas to be exactly conserved [3]. It is important to real-ize that, as the φ conserves strangeness by definition, it isnot suppressed by the strangeness correlation parameter incontrast to the other particles containing strange quarks.We simultaneously fit the pi−/p, K0s/Λ, K−/K+ andthe φ/K− ratio using the freely available statistical modelTHERMUS [4]. Similar as for our fit to the Ar+KCl datasample [5] we constrain the charge chemical potential µQusing the ratio of the baryon and charge numbers of the col-lision system, conserve the baryon number on average viathe chemical potential µb and calculate strangeness canon-ically by introducing the additional sub volume defined byRc. As we are restricted to ratios we fix the radius of∗Work supported by GSI, HIC4FAIR, HGShire† m.lorenz@gsi.deFigure 1: Comparison of hadron yields ratios of model anddata, together with values obtained for the free parametersof the statistical model fit, see text for details.the fireball R arbitrarily to 3 fm. In this way we find thechemical freeze-out at a temperature of Tchem = (47 ± 5)MeV and at a baryochemical potential of µb = (799± 22)MeV. The ratio Rc/R is determined to 0.3± 0.2 while theχ2/d.o.f. of the fit corresponds to 1.2. We note that ourfreeze-out point fits remarkably well to the result obtainedin [6] for a similar system but restricted to fewer identifiedparticles. Also Rc/R agrees within errors with the valueobtained in our Ar+KCl fit of 0.5 ± 0.3. Going from themedium-sized Ar+KCl system (measured at 1.76 AGeV)to the heavy Au+Au system the φ/K− ratio rises strongly.This effect is reproduced by statistical model calculations,using the above discussed parameters (especially Rc).References[1] A. Forster, F. Uhlig, I. Bottcher, D. Brill, M. Debowski,F. Dohrmann, E. Grosse and P. Koczon et al., Phys. Rev. C75 024906 (2007).[2] A. Mangiarotti et al. [FOPI Collaboration], Nucl. Phys. A714 89 (2003), G. Agakishiev et al. [HADES Collaboration],Phys. Rev. C 80 025209 (2009).[3] I. Kraus, J. Cleymans, H. Oeschler, K. Redlich andS. Wheaton, Phys. Rev. C 76, 064903 (2007).[4] S. Wheaton and J. Cleymans, Comput. Phys. Commun. 18084 (2009).[5] G. Agakishiev et al. [HADES Collaboration], Eur. Phys. J. A47 21 (2011).[6] J. Cleymans, H. Oeschler, K. Redlich and S. Wheaton, Phys.Rev. C 73, 034905 (2006).MU-NQM-HADES-11 GSI SCIENTIFIC REPORT 201412 DOI:10.15120/GR-2015-1-MU-NQM-HADES-11

Revisiting hadron production at SIS energies using new HADES data

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Revisiting hadron production at SIS energies using new HADES data

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