Strange meson production near threshold in nucleus-nucleus collisions

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1 FOPI Strange meson production near threshold in nucleus-nucleus collisions FOPI Krzysztof Piasecki for the FOPI Collaboration Institute of Experimental Physics, University of Warsaw, Poland Physics motivation Primary or secondary production? K production: a complex case In-medium modifications of K+,, 0 K* (892) strange meson resonance Summary and conclusions 1

2 Strange mesons in AA collisions KaoS Tlab= 1.6 GeV Tlab= 2.5 GeV Tlab= 2.6 GeV 1.93 GeV φ φ 1.76 GeV HADES G. Agakishiev, Phys Rev C 80, (2009) P. Gasik, Ph. D. Thesis, University of Warsaw Production thresholds in free NN collision : P.Senger et al. (KAOS), F. Laue et al., PRL 82 (1999), updated NN NK+/ Λ NN NNK K+ NN NNφ They a r e here... Strange mesons in AA collisions: Are the production processes primary or secondary? Decays? Contributions from intermediate resonances? Modification of properties in medium? Production of strange resonances? 2

3 Probing partial restoration of chiral symmetry Gell-Mann Oakes Renner relation: FOPI AA 2 m 2 f K K First approaches: Potential U(K-) K U K- attracted, K+ repelled = F Chiral effective field theory w/ couple-channels M.F. M. Lutz, PPNP 53 (2004) 125 K+ J. Schaffner-Bielich et al. NPA 625(1997) 325 U(K+) 2 u u+ s s + Θ (m 2s ) Decay constant Mass M. Kotulla et al., Physik Journal 8 (2009) 3 = mu+ ms Potential only on average 3

4 FOPI experimental setup Nearly 4π coverage Drift chambers: chambers CDC, Helitron ToF : Plastic Barrel, RPC Forward: Forward Plastic Wall, Zero Degree Direct PID of π±, K±, p, d, t, He 3,4 MMRPC Plastic Barrel p [GeV/c] t d p K+ v (cm/ns) p / q [GeV/c] t d p K+ π+ K- π- e+ e- π+ v (cm/ns) 4

5 Production of Kaons in AA: Primary or secondary? σk = const For pa KX: MUL K = σ inelastic AA KX: Glauber: AA = A NA AA MUL K = A pa MUL K FOPI FOPI A Ni+Ni, 1.9A GeV A. Förster et al., PRC 75, (2007) KaoS A K0 : secondary processes involved M.Merschmeyer et al., PRC 76, (2007) If primary: K+0 near-threshold production processes: Nbeam + Ntarget, predominantly via ΔN, ΔΔ K+,0 Y B πn, πδ K+,0 Y Ntarget has Fermi motion Y = [Λ,Σ] UKN involved (increases K mass lower yields) secondary processes are involved 5

6 Sub- and near-threshold Production of K in medium: mainly strangeness exchange: BY NNK, Au+Au, Tb = 1.5A GeV (IQMD transport code) πy K B strong reabsorption: K B πy coupled to resonances Σ(1385), (1385) Λ(1405) Σ* Λ* π+y K + B Q: Can we see them? K φ(1020) K K+ decay (mostly outside collision zone) Q: How strong is this contribution? φ K + t (fm/c) In-medium effects: UKN potential or spectral density Q: How strong is this influence? K 6

7 φ (1020) mesons Ar+KCl, Tb = 1.76A GeV φ (ss) K+K m = 1019 MeV cτ = 50 fm (decays mostly outside collision zone) Eth = 2.6 GeV (for SIS-18, deeply subthreshold) HADES S = 168 Al+Al, Tb = 1.9A GeV S = 108 S/B = 1.0 Signif = 7.4 (BR = 49%) φ / K (FOPI) (FOPI) (FOPI) Ni+Ni, Tb = 1.9A GeV (FOPI) (FOPI) (FOPI) S = 170 S/B = 0.7 Signif = 8.4 preliminary G. Agakishiev, Phys Rev C 80, (2009) P. Gasik, Ph. D. Thesis, University of Warsaw Pφ possibly ~ <Apart> 1 φ 3 K ~ % of K-produced from φ decays 7

8 G. Agakishiev et al., PRC 80, (2009) J. Cleymans et al. PLB 603, 146 (2004) φ/k excitation function p+p FOPI within Statistical Model For S 0, Canonical ensemble φ : non-strange, K : strange At low s, φ/k sensitive to RCanonical RCanonical ~ 2-3 fm Exploring φ phase space 1.76A GeV Teff = 84 ± 8 MeV 1.9A GeV Teff = 91 ± 20 MeV 1.9A GeV T = 92 ± 12 MeV HADES preliminary preliminary 95 <ϑcm<150 8

9 Σ (1385) resonance K production in medium ( πy K N ) coupled to strange resonances e.g. Σ (1385), Λ (1405) : Al+Al, Tbeam = 1.9A GeV Σ±* (1385) Λ + π± p + π (88 ± 2%) ( πλ Σ K N ) M.F. M. Lutz, PPNP 53 (2004) 125 Λπ- + Λπ+ Σ (1385) mn+mk X. Lopez et al. (FOPI), PRC 76, (R) (2007) Chiral effective field theory w/ coupled-channels Y (Σ + Σ + ) Y (Λ+ Σ 0 ) FOPI Σ* resonance found in HI collisions Input to fix π + Λ K + N in medium Statist. Model UrQMD ±

10 K /K+ : experiment vs transport Al+Al, Tbeam = 1.9A GeV, 9% most central events K : UKN repulsive + (P. Gasik) K : UKN ~attractive K /K+ : promising observable IQMD transport code ρ m K± (ρ) = mk ± (ρ0 ) 1+α± ρ 0 at ρ=ρ0 ( ) mk+ = 40 MeV, mk- = 100 MeV HSD preliminary 136 < Θcm < 150 transport code K+ as in IQMD K : off-shell G-matrix approach IQMD, NO Pot. HSD, NO Pot. HSD, UK+N=40 MeV, K- Not Modified HSD, UK+N=40 MeV, UK-N= G-Matrix IQMD, UK+N=40 MeV, UK-N=-100 MeV Clear preference for UKN 0 option UK+ only scenario : insufficient IQMD: potentials used probably too strong 10

11 Flow of charged kaons directed flow v1 dn 1 2v 1 cos 2v 2 cos 2... d v1, v2 = Fourier coefficients 40% central K K+ p HSD : IQMD : preliminary V. Zinyuk et al, arxiv: v2 Ni+Ni, Tbeam = 1.9A GeV UK+N = 20 MeV UK-N -50 MeV UK+N = 20 MeV UK-N = -40 MeV more peripheral 11% central ~ In favour of the potential 11

12 In-medium modifications of K+/0 at ρ < ρ0 γ, π, p Beam π (p=1.15 GeV/c) + A K0 + p (p=2.25 GeV/c) + A K+ + FOPI ANKE p + A K+ + pp = 2.25 GeV/c CBUU transport code UKN = 0 MeV UKN = 10 MeV UKN = 20 MeV p = 1.15 p = 3.1 M.L. Benabderrahmane et al., PRL 102, (2009) ANKE M. Nekipelov et al, PLB 540, 207 (2002) Z. Rudy et al., EPJA 23, 379 (2005) M. Kotulla et al., Physik Journal 8 (2009) 3 12

13 Modifications of K0 in AA collisions K 0S from Ar A GeV K 0S cτ = 2.7 cm K 0L cτ = 15.3 m IQMD transport calc. : No potential UK0N = 46 MeV HADES UKN at ρ ~ 2 ρ0 seems to be stronger than for π A K0 + at ρ ρ0 G. Agakichiev et al., Phys. Rev. C 82, (2010) 13

14 Kaonic resonance: K*(892) K0* (892) K+ π Eth = 2.75 GeV cτ = 4 fm 1.9A GeV (~ 67%) (SIS-18 energies: deeply subthreshold) (short lived) 1.76A GeV HADES S = 6112 ± 850 S/B = SIGNIF = 10 MK+p- P K 0 = 0.032±0.003± P K X. Lopez et al., J. Phys. G 35 (2008) MK+p- S = 1070 ± 260 S/B = SIGNIF = 4.1 MK+p- P ( K 0 ) =0.019±0.005±0.003 P(K0) G. Agakishiev et al., Eur. Phys. J. A (2013) 49: 34 14

15 Summary and conclusions In sub- and near-threshold AA collisions, strange meson production needs secondary processes Production of K more complex than K+ and influenced by: φ (1020) K via decay (15..20%) Σ±*(1385) : strangeness exchange channel via Σ* ( πλ Σ* K N ) in-medium modification of properties of K In-medium modifications of properties of K+ / / 0 Study of K /K+ ratio Study of v1 (K /+) Study of K 0S p and pt-y distributions UK+N ~ MeV UK-N ~ MeV UK0N ~ 20 MeV at ρ ρ0 ~ 45 MeV at ρ ~ 2ρ0 K*(892) strange meson resonance observed in 1.9, 1.76 K* A collection of yield ratios compared to Statistical Mode l (Thermus) and UrQMD 15

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17 K+ Primary: (Fermi momentum) NN NK+Y (Y = Λ, Σ) K+ Secondary: BB BK+Λ (BB = N, ) πb K+Y (B = N, ) C. Fuchs, PPNP 56 (2006) 1 KaoS +QMD Secondary processes involved 17

18 HSD transport model 'Infinite' hadronic matter, initial ε=ε0, ρb=ρ0, ρs=0 τeq: characteristic time of yield buildup 2/3 of Nequilibrium At SIS energies, resonance production E.L.Bratkovskaya et al., NPA 681 (2001) 84 (Δ, N*) reaches maximum S.A. Bass et al., PPNP 41 (1998) 225 SIS 2A GeV + + K K FOPI + ππ+ τeq» τcollision no thermalization of strangeness 18

19 K+ phase space: experiment vs transport preliminary 136 < Θcm < y = IQMD y y NN CM NN y CM HSD Soft EoS (K 200 MeV) ρ m K ± (ρ) = m K ± (ρ0 ) 1+α± ρ 0 ( αk+ / K- = 0.08 (-0.21) ) At ρ=ρ0 mk+ = 40 MeV, mk- = 100 MeV (P. Gasik) ~ 9% most central events Calc.: Y. Leifels (GSI/Darmstadt). Ref.: C.Hartnack, H.Oeschler, Y.Leifels, E.Bratkovskaya, J.Aichelin, nucl-th/ v2 1.93A GeV, Semi-soft EoS (K 250 MeV) K+ mass modifications as in IQMD K- production: off-shell G-matrix Clear preference for UKN 0 option Still description not ideal 19

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21 K 136 < Θcm <

22 π +p K0 Σ0 π +p K0 Λ π +n K0 Σ M.L. Benabderrahmane et al., PRL 102, (2009) 22

23 In-Medium Σ*(1385) Chiral unitary theory M. M. Kaskulov, E. Oset, PRC 73 (2006) Σ*(1385) Λ(Σ) + π at ρ=ρ0 cτ = 5 fm at ρ = ρ0 : Γ = -2Im[Σ]Σ(1385) = 76 MeV Mass: VΣ N -45 MeV (attractive) m* [GeV/c2] m* [GeV/c2] X. Lopez et al., PRC 76, (R) (2007) Al+Al Chiral unitary theory FOPI expt. data PDG mass (ρ = 0) Γ [MeV] short lifetime Σ* should probe finite density! Γ broadening not yet observed (more statistics...) Need to measure with heavier system Need to include spectral function in transport codes 23 23

24 Strange meson excitation functions near threshold FOPI central A+A FOPI HADES C. Alt et al. (NA49), Phys. Rev. C 78, (2008) B. Back et al. (E917), Phys. Rev. C 69, (2004) G. Agakishiev et al., Eur. Phys. J. A (2013) 49: 34 24

25 Strangeness production and absorption Strangeness production and absorption K+ K- φ Production (primary) BB BYK+ BB BBK+K- Tpp pλk+ = 1.58 GeV Tpp ppk+k- = 2.5 GeV Tpp ppk+k- = 2.6 GeV Production (secondary) πb YK+ πy (Σ ) BKBY NK-Λ BY BBKπB BK+Kφ K+K- πb Bφ ρb Bφ πn* Nφ ρπ φ BB BBφ H.W. Barz et al. (BUU), Nucl. Phys. A 705 (2002) 223 K+K- φ negligible Absorption K+Y πb K-B πy φn KΛ Elastic scat. (char. exch.) K+B K+ B K-B K-B φn φn K+n K0 p K-p K0n [B] = p, n, N, N*, Δ [Y] = Λ, Σ C.B. Dover, G.E. Walker Phys. Rep. 89 (1982) 1 25

26 Neutral strangeness: K0 and Λ0 1.9A GeV K0 and Λ0 (from secondary vertices) K0 π+ + π Λ0 π + p Ni+Ni Al+Al K0 d s 30 k 60 k K0 (BR = 69%) (BR = 64%) Λ0 uds 60 k 100 k Λ0 M. Merschmeyer, X. Lopez et al. (FOPI), PRC 76, (2007) K0 Λ0 and K0 obeying Boltzmann distributions Λ0 and proton: Λ emission patterns different (p transparency) 26

27 Statistical model Assumption: Assumption chemical freeze-out Ni gi n i,t = = V Density of species i : (in grandcanonical ensemble) Free parameters: chemical potential µb temperature T For particle ratios : Fixed by conservation laws: V cancels out µs, µι3 s...but exp p 2 dp E i B B i S S i I I 3i 3 T ±1 SIS No equilibration of strangeness (?) Extension: 1 exp... exp... n S γs ns? S strangeness undersaturation factor number of strange quarks F. Becattini et al., PRC 73, (2006) 27

28 Particle yields vs Statistical Model and UrQMD Al+Al : 8 independent ratios involving Al+Al p, d, π, K+, K, K0s, φ, K*0, Σ*±, Λ Ni+Ni : 8 independent ratios involving p, d, π+, π, K+, K-, K0s, φ, Λ Statistical Model Grand Canonical ensemble; FOPI / KAOS prelim. For S 0, Canonical ensemble calc: THERMUS code S.Wheaton, J.Cleymans, hep-ph/ Ni+Ni UrQMD v 2.3 prelim. SM fitting quite well No equilibration assumed Cascade model no mean field no in-medium effects J. Phys. G: Nucl. Part. Phys. 25 (1999) 1859 FOPI / KAOS UrQMD fits quite well too 28

29 Freeze-out in phase diagram J. Cleymans et al., PRC 73, (2006) Al+Al RHIC SPS AGS Ni+Ni Tkin > Tchem Puzzling result... 29