We present a summary of measurements of strange particles performed by the
experiment NA49 in central and minimum bias Pb+Pb collisions in the beam energy
range 20A - 158A GeV. New results on Xi production in central Pb+Pb collisions
and on Lambda, Xi production in minimum bias collisions are shown. Transverse
mass spectra and rapidity distributions of strange particles at different
energies are compared. The energy dependence of the particle yields and ratios
is discussed. NA49 measurements of the Lambda and Xi enhancement factors are
shown for the first time.Comment: Submitted to J. Phys. G (Proceedings of the 9th International
  Conference on Strangeness in Quark Matter, Los Angeles, USA, March 26-31,
  2006). 9 pages, 9 figure

Alt, C.

Anticic, T.

Baatar, B.

Barna, D.

Bartke, J.

Betev, L.

Bialkowska, H.

Blume, C.

Boimska, B.

Botje, M.

Bracinik, J.

Bramm, R.

Buncic, P.

Cerny, V.

Christakoglou, P.

Chung, P.

Chvala, O.

Cramer, J.G.

Csato, P.

Dinkelaker, P.

Eckardt, V.

Flierl, D.

Fodor, Z.

Foka, P.

Friese, V.

Gal, J.

Gazdzicki, M.

Genchev, V.

Georgopoulos, G.

Gladysz, E.

Grebieszkow, K.

Hegyi, S.

Hohne, C.

Kadija, K.

Karev, A.

Kikola, D.

Kliemant, M.

Kniege, S.

Kolesnikov, V.I.

Kornas, E.

Korus, R.

Kowalski, M.

Kraus, I.

Kreps, M.

Lacey, R.

Laszlo, A.

Levai, P.

Litov, L.

Lungwitz, B.

Makariev, M.

Malakhov, A.I.

Mateev, M.

Melkumov, G.L.

Meurer, C.

Mischke, A.

Mitrovski, Michael K.

Molnar, J.

Mrowczynski, St.

Nicolic, V.

Palla, G.

Panagiotou, A.D.

Panayotov, D.

Peryt, W.

Petridis, A.

Pikna, M.

Pluta, J.

Prindle, D.

Puhlhofer, F.

Renfordt, R.

Roland, C.

Roland, G.

Rybczynski, M.

Rybicki, A.

Sandoval, A.

Schmitz, N.

Schuster, T.

Seyboth, P.

Sikler, F.

Sitar, B.

Skrzypczak, E.

Slodkowski, M.

Stefanek, G.

Stock, R.

Strabel, C.

Strobele, H.

Susa, T.

Szentpetery, I.

Sziklai, J.

Szuba, M.

Szymanski, P.

Trubnikov, V.

van Leeuwen, M.

Varga, D.

Vassiliou, M.

Veres, G.I.

Vesztergombi, G.

Vranic, D.

Wetzler, A.

Wlodarczyk, Z.

Yoo, I.K.

English

arXiv

We present results on strange particle production from experiment NA49 in central and minimum bias Pb+Pb collisions in the beam energy range 20–158 A GeV. New results on Ξ production in central Pb+Pb collisions and on Λ, Ξ production in minimum bias collisions are shown. Transverse mass spectra and rapidity distributions of strange particles at different energies are compared. The energy dependence of the particle yields and ratios is discussed. NA49 measurements of the Λ and Ξ enhancement factors are shown for the first time

Mitrovski, M.K.

Botje, M.A.J.

Subatomic Physics

Dep Natuurkunde

Sub Algemeen Subatomic Physics

NARCIS 

Strangeness production at SPS energies from NA49

We present a summary of measurements of strange particles performed by the experiment NA49 in central and minimum bias Pb+Pb collisions in the beam energy range 20A - 158A GeV. New results on Xi production in central Pb+Pb collisions and on Lambda, Xi production in minimum bias collisions are shown. Transverse mass spectra and rapidity distributions of strange particles at different energies are compared. The energy dependence of the particle yields and ratios is discussed. NA49 measurements of the Lambda and Xi enhancement factors are shown for the first time

CERN Document Server

arXiv:nucl-ex/0606004v3  26 Sep 2006Strangeness production at SPS energies from NA49Michael K. Mitrovski for the NA49 Collaboration†Institut fu¨r Kernphysik, J.W. Goethe-Universita¨t, 60438 Frankfurt, GermanyE-mail: Michael.Mitrovski@cern.chAbstract. We present results on strange particle production from experiment NA49in central and minimum bias Pb+Pb collisions in the beam energy range 20A -158A GeV. New results on Ξ production in central Pb+Pb collisions and on Λ,Ξ production in minimum bias collisions are shown. Transverse mass spectra andrapidity distributions of strange particles at different energies are compared. Theenergy dependence of the particle yields and ratios is discussed. NA49 measurementsof the Λ and Ξ enhancement factors are shown for the first time.Submitted to: J. Phys. G: Nucl. Phys.1. IntroductionThe NA49 experiment has collected data on Pb+Pb collisions at beam energies between20A - 158A GeV with the objective to search for the phase transition to a deconfinedphase which is expected to occur in the early stage of the reactions if a sufficient energydensity is achieved.Several experimental signatures for a QGP have been proposed, among which strangeparticle yield measurements, in particular, are considered to be a powerful tool. Anenhancement increasing with the strangeness content of the particle was predicted as aconsequence of a QGP formation [1] − [3].A non-monotonic energy dependence of the K+/pi+ ratio with a sharp maximum closeto 30A GeV is observed in central Pb+Pb collisions. Within the statistical model ofthe early stage [4], this is interpreted as a signal of the onset of deconfinement, whichcauses a sharp change in the energy dependence of the strangeness to entropy ratio. Thisobservation naturally motivates further study of the production of strange hadrons, inparticular hyperons as a function of the beam energy.In this report we show new NA49 results on strange particle production in central andminimum bias Pb+Pb collisions at all available SPS energies and compare them topreviously shown data [5] − [13] at other energies, as well as to model predictions.† Presented at Strangeness in Quark Matter 2006, Los Angeles, California, USAStrangeness production at SPS energies from NA49 22. The NA49 experimentThe NA49 detector [14] is a large acceptance hadron spectrometer at the CERN SPS,featuring four large volume TPCs as tracking detectors. Two of them are inside amagnetic field. The ionisation energy loss (dE/dx) measurements in the TPCs are usedfor particle identification. Central collisions were selected on the basis of the energy ofthe projectile spectator nucleons measured by a downstream calorimeter.3. Energy Dependence of Transverse Mass SpectraFig. 1 shows new results on transverse mass spectra of Ξ− (left) and Ξ¯+ (right) integratedover the rapidity range -0.5 < y < +0.5 in central Pb+Pb collisions at different beamenergies. The shape of the spectra is approximately exponential inmt-m0 with an inverseslope parameter of about T (Ξ−) ≈ 240 MeV and T (Ξ¯+) ≈ 280 MeV, and exhibits nosignificant energy dependence.0 1 2)-2) (GeVt dy dmtdN/(m-410-110210 10)×158 AGeV ( 1)×80 AGeV ( 0.1)×40 AGeV ( 0.01)×30 AGeV ( 0.001)×20 AGeV (-Ξ (GeV)0-mtm0 1 2-410-110210+ΞFigure 1. Transverse mass spectra of Ξ− (left) and Ξ¯+ (right) in central Pb+Pb(7 % most central at 20A − 80A GeV, 10 % most central at 158A GeV) collisions atmidrapidity. The errors shown are statistical only.In Fig. 2 the energy dependence of the mean transverse mass, 〈mt〉 − m0, is shownfor Λ, φ, Ξ and Ω + Ω¯. The mean transverse mass was calculated from the measuredspectra and using a fitted function extrapolated to fullmt range. Different extrapolationmethods have been tested to estimate systematic error. The resulting extrapolationfactors are small.If energy density and hence pressure increase with beam energy, an increase of transverseexpansion is expected. Assuming that the strength of the transverse expansion isreflected in the mean transverse mass, the variable 〈mt〉 − m0 is expected to rise with√sNN . Previously we have reviewed the energy dependence of 〈mt〉 − m0 for pions,Strangeness production at SPS energies from NA49 3Figure 2. Energy dependence of the mean transverse mass 〈mt〉 − m0 of Λ, φ, Ξ andΩ + Ω¯ in central Pb+Pb/Au+Au collisions from AGS [19], [20] to RHIC [22] energies.Open symbols indicate the anti-particles.kaons and protons [15] - [18]. At AGS energies these particles show a strong increaseof the mean transverse mass with the beam energy. However, the mean transversemasses show a weak energy dependence at SPS energies. The weak dependence hasbeen attributed [21] to the constant pressure and temperature when a mixed phaseis reached during the dynamical evolution of the colliding system in the early stageof the reaction. Fig. 2 shows new results on 〈mt〉 − m0 for heavy strange hadrons.Also for these particles no significant energy dependence is observed at SPS energies.Measurements of multiply strange hadrons at AGS energies are needed to fully studytheir energy dependence.4. Energy Dependence of Particle YieldsThe NA49 experiment features a large acceptance in the forward hemisphere allowingfor measurements of rapidity spectra from midrapidity up to almost beam rapidity. Therapidity spectra for Ξ and Ω in central Pb+Pb collisions are shown in Fig. 3. They areparametrized by a single Gaussian function. NA49 results on spectra of pi−, K+, K−,φ, Λ, and Λ¯ were presented earlier [15].NA49 has measured different particle species at midrapidity in 20A, 30A, 40A, 80A,and 158A GeV central Pb+Pb collisions. Results on yields at midrapidity in centralAu+Au collisions at√sNN = 62.4, 130 and 200 GeV have been published by the STARcollaboration at RHIC [23]. A compilation of these data is shown in Fig. 4, togetherwith AGS data.Strangeness production at SPS energies from NA49 4Figure 3. The rapidity spectra of Ξ and Ω produced in central Pb+Pb collisions (7 %most central at 20A − 80A GeV, 10 % (Ξ) and 23.5 % (Ω) most central at 158A GeV).The closed symbols indicate measured points, open points are reflected with respectto midrapidity. The solid lines represent fits with a single Gaussian. The errors shownare statistical only.Figure 4. Midrapidity yields from AGS to RHIC energies. The data are compared tostring-hadronic models [24] (UrQMD 2.0: dashed, HSD: solid). The errors shown arestatistical only.One observes a relatively weak energy dependence of the yield of hyperons starting fromSPS to RHIC energies, while a strong increase is seen for the anti-hyperons. The dataon Λ and kaon yields are compared to string-hadronic models [24]. Both models followthe trend of the measured data, but they fail to reproduce the data quantitatively. Pre-dictions for multi-strange hyperons and the φ-meson are still not available.Strangeness production at SPS energies from NA49 5Figure 5. The energy dependence of the midrapidity yields of strange hadrons,normalized to the midrapidity pion yields (pi = 1.5 (pi− + pi+)), in centralPb+Pb/Au+Au collisions. The data are compared to string hadronic models [24](UrQMD 2.0: dashed, HSD: solid).Figs. 5 and 6 show the energy dependence of relative strange particle yields. The ratioof the midrapidity and total yields normalized to the respective pion yields are com-pared to model calculations. While 〈K−〉/〈pi−〉, 〈Λ¯〉/〈pi〉, 〈Ξ¯+〉/〈pi〉, 〈Ω−+ Ω¯+〉/〈pi〉 and〈φ〉/〈pi〉 ratios rise continuously with energy, a distinct maximum is visible in the energydependence of the ratios 〈K+〉/〈pi+〉, 〈Λ〉/〈pi〉 and 〈Ξ−〉/〈pi〉. The same structure is seenfor the midrapidity ratios. Hadron gas [25] (only available for 4pi multiplicity ratios) andmicroscopic models [24] do not provide a proper description of the structures observedin the data. On the other hand, this feature can be understood in a scenario where theonset of deconfinement is reached around 30A GeV as proposed in the statistical modelof the early stage [4].The NA49 antibaryon/baryon ratios are shown in Fig. 7 as a function of the beamenergy. The ratios increase with increasing strangeness content of the hyperons at agiven energy. The energy dependence of the ratio for multiply strange hyperons is muchweaker than that for Protons and Λ. Because the net baryon density at midrapidity isa strong function of the beam energy, the antibaryon to baryon ratio strongly dependson the number of valence up and down quarks in the hadron. For Ω with no valenceup/down quarks the energy dependence of the ratio is the weakest. The open symbolsrepresent measurements from the NA57 Collaboration [26]. While the particle ratiosStrangeness production at SPS energies from NA49 6Figure 6. The energy dependence of the total yields of strange hadrons, normalized tothe total pion yields (〈pi〉 = 1.5 (〈pi−〉 + 〈pi+〉), in central Pb+Pb/Au+Au collisions.The data are compared to string hadronic models [24] (UrQMD 2.0: dashed, HSD:solid) and statistical hadron gas model [25] (assuming strangeness in full equilibrium:dotted).are in good agreement with the NA49 data, the absolute yields of NA57 tend to besystematically higher by about 30 %, in central Pb+Pb collisions both at 40A and 158AGeV [27].Figure 7. The antibaryon/baryon ratio (B¯/B) at midrapidity from SPS to RHICenergies. The errors shown are statistical only.Fig. 8 shows results on system size and centrality dependence of hyperon yields at midra-Strangeness production at SPS energies from NA49 7pidity at 158A GeV. Having measured particle yields for p+p, C+C, Si+Si and Pb+Pbcollisions, one can determine a hyperon enhancement as :E =(Y ieldNw)A+A/(Y ieldNw)p+p,where Nw is the number of wounded nucleons calculated within the Glauber model [28].A significant enhancement for the Ξ− is observed at 158A GeV when going from p+pto Pb+Pb. For the Λ hyperon the enhancement is not as strong as for the Ξ− hyperonand the centrality dependence is slightly weaker. Together with the results for C+Cand Si+Si [12] a complete coverage of the system size range from Nw > 14 is possible.Figure 8. Hyperon enhancement as a function of the number of wounded nucleons at158A GeV. The errors shown are statistical only.The NA57 collaboration [26] observes the same trend (not shown) but instead of p+p ap+Be reference is used. Since already in p+A reactions a slight enhancement of strangeparticle production is observed [7], the enhancement relative to p+Be is less.Particle yields up to pt = 4 GeV/c have been measured in central Pb+Pb collisions. Thebaryon/meson ratios qualitatively show the same pt dependence as at RHIC. As well,the nuclear modification factor RCP is extracted and compared to pQCD calculations[29].The elliptic flow of Λ hyperons has been measured in semi-central Pb+Pb collisions at158A GeV. The standard method of correlating particles with an event plane has beenused. The elliptic flow of Λ particles increases linearly up to 2 GeV/c. This increaseStrangeness production at SPS energies from NA49 8is weaker at SPS than at RHIC energies, partly due to different centrality selection. Alinear increase of v2 is also observed for other particles (pi,proton) as a function of pt inmid-central collisions at 158A GeV [30].AcknowledgmentsAcknowledgements: This work was supported by the US Department of EnergyGrant DE-FG03-97ER41020/A000, the Bundesministerium fur Bildung und Forschung(06F137), Germany, the Virtual Institute VI-146 of Helmholtz Gemeinschaft, Germany,Fellowship of the German Academic Exchange Service (DAAD) for Ph.D.-researchstudies, Germany, the Polish State Committee for Scientific Research (1 P03B 097 29, 1PO3B 121 29, 2 P03B 04123), the Hungarian Scientific Research Foundation (T032648,T032293, T043514), the Hungarian National Science Foundation, OTKA, (F034707),the Polish-German Foundation, the Korea Research Foundation Grant (KRF-2003-070-C00015) and the Bulgarian National Science Fund (Ph-09/05).References[1] J. Rafelski and B. Mu¨ller, Phys. Rev. Lett. 48 (1982) 1066.[2] J. Rafelski and B. Mu¨ller, Phys. Rev. Lett. 56 (1986) 2334.[3] P. Koch, B. 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Strangeness production at SPS energies

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