Strangeness @ GSISeen by transport models

Christoph Hartnack & Jörg AichelinSubatech Nantes

Outline:● Comparison of transport models● K+&K- production and interaction of

strange particles in matter● Comparision to data: yields, spectra, T, v2

Subthreshold kaon production•Production of kaons at energies below the kinetic threshold for K production in elementary pp collisions RARE!

•Fermi momenta may contribute for obtaining the needed energy

•Multistep processes can cumulate the energy needed for kaon production

•Importance of resonances (especially the) for storing energy

•Short livetime of resonance favors early production at high densities

•Sensitivity to in-medium effects and nuclear equation of state

Transport models: real vs virtual propagation

• Real propagation: conserves event characteristics

• Virtual propagation: allows for high stat.

• UrQMD: real prop., creation of strangeness via resonances

• IQMD, HSD, QMD, …: virtual propagation, direct production of strangeness in 2,3,4body collisions

M. Bleicher et al.

Transport models: different ingredients

• Differences in unknown production cross sections• Differences in delta lifetimes, potentials etc

Need of more experimental input for elementary reactions of B+B &M+B

E.E.Kolomeitsev et al.

… but still allowing conclusions

• Hint on the existence of an optical KN potential

Ch. Fuchs et al

In medium effects on kaons in IQMD•KN-Rescattering,

absorption for K-

•Optical potential: repulsive for K+, attractive for K-

Penalizes K+ production at high densities but favors K- production at high densities

Effects yields but also dynamics

Parametrization from Schaffner-Bielich RMF results

K- production dominated by strangeness exchange

BB+B

+Y+BY

Direct channels BB, B enhanced by K- potential, similar forexchange channels Y+BY. The K+ potential penalizes hyperon production and compensates in the dominant channels Y+BY.

BB+B+Y+BY

Kaos data:

F. Uhlig et al

Time-evolution & kaon production

K+: early, multistep induced product. when baryon density is highest

K-: prod. later when pion density is highest, strangeness exchange

0fm/c

4fm/c

8fm/c

12fm/c

16fm/c

20fm/c

’’T’’

Central cell

High density: high collision probability

max prodat t=8fm/c

Rescattering heats the kaons: saturation of T(very high NC: downcooling source)

Afterwards: potential push (more coll → later emitted → less push)

K+

Spectra & temperatures of K+

IQMD (above, only with pot) and HSD (below) results are in good agreement with KaoS data

Spectra: slopes dominated by KN-rescattering

K+

K-

Rescattering potential

Strong enhancement of the slope from initial to final mom.

Slight effects: enhancement (K+) or reduction (K-)

K+ rescatter

Collision number

High K+ rescattering less K- rescattering

Temperatures K+ and K- heated up by collisions with expanding nuclear medium.

K- absorption acts as energy filter

Finally the K+ get pushed out while the K- are drawn backwards by the KN potential

KaoS

A.Förster et al. PRC75(2007) 024906

Low energy K0: potential penaltyDifference of spectra at low pcm due to potential penalty

FOPI can/could/might/should/has see(n) the K+ N potential

FOPI data

KN POT

NO POT

Azimuthal distributions

Azimuthal distributions are effected by rescattering, by optical potential and the emission time.

While the rescattering acts in the same direction for K+ and K- the optical potential gives opposite effects for K+ and K-

Azimuthal distribution fitted with

a (1+2v1 cos() + 2 v2cos(2))

KaoS and FOPI see opposite signs of v2 for K+ and K-

Ni+Ni 1.93 GeV,

F. Uhlig et al, KaoS

Excitation function of v2 for K+

Rescattering and optical potential contribute to the v2 of kaons.

The effects of the optical potential become dominant with respect to the effect of rescattering when going down in beam energy.

This is in agreement with calculations of Li&Ko who found a strong potential effect for Au+Au 1 GeV/A.

Data: KaoS

Time evolution for K+ & K-

Potential enhances v2 of kaons.

The earlier the emission the stronger the effect

The early K- show a squeeze which vanishes for the K- emitted lately

K+

K-

pT dependence of v2 for K+ & K-

IQMD yields too strong v2 of K+ with KN potential

IQMD has problems with the change of the sign of v2 of K-

FOPIs v1 & v2 for K+ and K-

Potential acts different on K+&K- rescattering effect compensated?

Indications on the existence of a KN-potential

K+ K-

IQMD supports this

soft hard

Data: Ch.Sturm et al.

QMD: Ch. Fuchs

IQMD supports this

Ratio(K) at 0.8GeV:

The nuclear eos from Au/C ratios

KaoS data support soft eos

Soft eos from Apart scaling factor

soft hard

Scaling of NK(Apart):

NK=N0 ×(Apart)

The relation between the compression modulus and is monotonously falling.

KaoS data (Förster et al.) favor a value below 240 MeV, i.e. a soft eos.

PRL 96 (2006) 012302

KaoS:Förster et al.

Energy dependence of the system size systematics

Soft eos confirmed

System size

Apart in Au+Au agrees with that

Conclusions on Kaon data•K+ yields: hints to KN potential, but uncertainties in input

•Scaling of K+ yields with ASystem, Apart: soft eos

•K- yield: dominance Y, BY, related to K+, uncert.in input

•T(K+) dominated by rescattering, effected by emission time

•T(K-) rescattering+absorption, emission time later

•Difference of slopes: hints to KN potential

•Low energy K0: strong sensitivity to KN potential

•Polar distribution: dominated by rescattering

•v2(K+) rescattering, KN potential pushes to more negative values

•v2(K-) rescatt., emission time, KN pot.pushes towards more positive values

•Difference of flow K+&K- reduces influence of resc.&enhances inf. KNpot

•v1of K+&K- visible influence of KN potential