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Results from FOPI on $\Lambda$ production in Ni+Ni collisions at 1.93 AGeV

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HAL Id: in2p3-00021948

http://hal.in2p3.fr/in2p3-00021948

Submitted on 16 Jun 2004

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Results from FOPI on Λ production in Ni+Ni collisions

at 1.93 AGeV

X. Lopez

To cite this version:

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Results from FOPI on Λ

production in Ni + Ni collisions at 1.93 AGeV

Xavier Lopez for the FOPI Collaboration

LPC Clermont-Ferrand, IN2P3-CNRS and Universit´e Blaise Pascal 63177 Aubi`ere Cedex, FRANCE

Experimental data on Λ particle phase space distributions and yields measured with the FOPI detector at SIS (Darmstadt) are presented.

One of the main topics addressed in the study of heavy ion collisions is the question whether hadronic properties undergo modifications in an environment of hot and dense nuclear matter. Particu-larly interesting is the behaviour of strange particles close to threshold. Possible evidence for in-medium modifications of charged kaon properties at SIS energies has been already observed [1, 2, 3]. We focus here on recent results obtained for neutral strange Λ-hyperons measured in the reaction Ni + Ni at 1.93 AGeV. The results may allow to test the in-medium attractive Λ-nucleon potential predicted by several theoretical calculations [4] which affects both the particle yield and flow.

A high statistics experiment has been performed in the beginning of 2003 with the FOPI detector [5] at the SIS accelerator facility of GSI (Darmstadt). About 110 millions of central (σgeo = 760 mb)

Ni + Ni events were recorded thanks to the new data acquisition system.

The Λ’s produced in these collisions are identified in the FOPI Central Drift Chamber. This neu-tral strange hadron (uds) is reconstructed via its decay into charged particles: Λ→ p + π− (branching ratio = 64%). Track quality and kinematic conditions are applied in order to suppress the combina-torial background. We analyzed about 93·106 of central events. Figure 1 shows the invariant mass spectrum of pπ− pairs which fulfill the Λ conditions for an integrated geometrical cross section of 350 mb (45·106 events). About 77000 Λ are reconstructed with a signal to background ratio equal to 1. The combinatorial background is estimated by means of the event mixing technique and is denoted by the line under the peak (upper part of the figure). The lower part of the figure shows the Λ signal after background subtraction. The peak is fitted with a Gaussian, giving a width of 10.5 MeV/c2. Finally the number of reconstructed Λ per central event is about 1.7·10−3.

The Λ yield in 4π is obtained from transverse mass spectra 1

m2t

d2N d(mt− m0)dy0

, for different rapidity windows going from target rapidity to mid-rapidity. y0 is the scaled rapidity (y0 = y/ycm

-1), mt= 

p2t + m20 and m0 is the mass rest. These spectra are corrected for reconstruction efficiency

and geometrical acceptance. This correction has been determined by means of a complete simulation of the FOPI detector using the GEANT package. Then, transverse mass spectra are adjusted with

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0 5000 10000 15000 20000 25000 30000 35000 1.05 1.075 1.1 1.125 1.15 1.175 1.2 1.225 1.25 M (GeV/c2) dN/dM (u.a.) 0 2500 5000 7500 10000 12500 15000 17500 20000 22500 1.05 1.075 1.1 1.125 1.15 1.175 1.2 1.225 1.25 M (GeV/c2) dN/dM (u.a.) Fit=77087 SIGNIF(±2σ)=192.8 S/B=1.0 NEVT=45553176 FWHM=10.5 MeV/c2

Figure 1: Invariant mass spectra of Λ

(upper pannel: signal with combinatorial background, lower pannel: signal after background subtraction ).

0 0.02 0.04 0.06 0.08 0.1 0.12 0.14 0.16 0.18 0.2 -1 -0.8 -0.6 -0.4 -0.2 0 0.2 0.4 0.6 0.8 1 YIELD=0.171 ESTAT=0.004 ESYST=0.010 Measured Reflected y0 dN/dy 0

Figure 2: Reconstructed Λ rapidity distribution for central (σgeo = 350 mb) Ni + Ni reactions at 1.93 AGeV.

a Boltzmann type function ( 1

m2t

d2N

d(mt− m0)dy0 = A · exp

−(mt− m0)

TB ) where A is a constant of

integration and TB is the apparent temperature (Boltzmann inverse slope parameter). This allows to

extract, for each rapidity bin, the Λ yield dN

dy0 by integrating the fitting function from pt = 0 to ∞.

The results are presented in Figure 2. The preliminary Λ yield in 4π for central (σgeo = 350 mb) Ni

+ Ni collisions at 1.93 AGeV is 0.171±0.010(Syst)±0.004(Stat). The systematical errors are obtained with an other set of cuts in order to take into account the fluctuations of the efficiency correction.

The large statistics of reconstructed Λ’s will allow for detailed studies on production and prop-agation (centrality dependence, differential flow, ...). The analyses should bring more light on the production mechanisms for strange particles close to threshold. An analysis using a multilayered neu-ral network to reconstruct Λ’s [6] will also be applied on these new data in order to improve the signal identification.

References

[1] P. Crochet et al., FOPI Collaboration, Phys. Lett. B 486 (2000) 6 [2] K. Wi´sniewski et al., FOPI Collaboration, Eur. Phys. J. A 9 (2000) 515 [3] M. Menzel et al., KaoS Collaboration, Phys. Lett. B 495 (2000) 26 [4] G.Q. Li, G.E. Brown, Nucl. Phys. A 636 (1998) 487

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