neutral kaon production in p $+$ p and p $+$ nb collisions
TRANSCRIPT
Hyperfine Interact (2012) 210:111–114DOI 10.1007/s10751-012-0595-6
Neutral kaon production in p+p and p+Nb collisions
Jia-Chii Berger-Chen · Laura Fabbietti ·Kirill Lapidus for the HADES collaboration
Published online: 6 March 2012© Springer Science+Business Media B.V. 2012
Abstract The kaon nucleus (KN) interaction in dense nuclear matter is predictedto be repulsive and increasing with density. However, determined values for thispotential are not yet consistent with each other (Benabderrahmane et al., PhysRev Lett 102:182501, 2009; Agakishiev et al., Phys Rev C 82:044907, 2010; Büscheret al., Eur Phys J A 22:301–317, 2004). We analyze K0
S mesons identified with theHADES detector in p+p and p+93Nb reactions at 3.5 GeV kinetic beam energy.To determine the KN potential at normal nuclear density we propose to comparethe K0
S differential distributions in p+93Nb and p+p collisions. High statistics of lowpt-kaons (pt < 100 MeV/c) ensure the sensitivity of our measurements to the nuclearmatter effects. We present the data analysis method and first results.
Keywords Neutral kaon · Cold nuclear matter · Kaon nucleus potential
1 Introduction
The modification of kaon properties in nuclear medium is a permanent subjectof interest in related theoretical and experimental research [4]. In particular it isimportant to extract the in-medium KN potential as a function of density. At normalnuclear density it can be studied employing γ , π or proton induced reactions [5].Theory predicts a slight repulsive potential, which can be observed in yields and
J.-C. Berger-Chen (B) · L. Fabbietti · K. LapidusExcellence Cluster Universe, Technische Universität München, Boltzmannstr. 2,85748 Garching b. München, Germanye-mail: [email protected]
L. Fabbiettie-mail: [email protected]
K. Lapiduse-mail: [email protected]
112 J.-C. Berger-Chen et al.
Fig. 1 Invariant mass π+ π− for p+p (left) and p+93Nb (right)
kinematical distributions. One may conclude from theoretical model predictions thatmomentum distributions, in particular in the low momentum region, are sensitiveto the influence of the KN potential. Moreover, it is favorable to analyze neutralkaons as they are not distorted by Coulomb interaction. In different experiments thisaspect has been studied at normal nuclear densities with pion and proton inducedreactions and at higher densities by means of heavy ion collisions. The results are notconclusive: the data obtained in pion/proton induced reactions are best describedby models employing UKN(ρ0) ≈ 20 MeV [1, 3], whereas heavy ion collisions arereproduced with UKN(ρ0) ≈ 40 MeV [2]. This discrepancy motivates us to remeasurethe K0’s in cold nuclear matter with the HADES setup [6]. The advantage ofour experiment is a high acceptance for low pt-kaons. Around 4· 109 events werecollected for the p+93Nb and 1.2· 109 for the p+p reaction, both at 3.5 GeV kineticbeam energy. As no effect from the finite nuclear density is expected in the lattersystem, a direct indication for the KN potential can be deduced from the ratioσ(pNb)/σ (pp) in the momentum distribution. The quantitative strength can thenbe extracted by a comparison to theoretical models. Due to a larger data sample, theuncertainty of our measurement is expected to be smaller as reported in [2].
2 K0S analysis
Similar analyses were carried out for both data sets. The K0S was reconstructed using
the HADES detector by its charged decay particles π+ and π− (BR = 69.20%),which were identified via graphical cuts on the energy loss versus momentumdistribution. The contribution of K̄0 has been neglected, as the K̄0/K0 ratio inboth data samples is less than 4% (estimated from HSD [7] simulations). To in-crease the signal-to-background ratio, following secondary vertex cuts were applied:(1) distance between the two pion tracks (dπ+−π− < 7 mm), (2) distance betweenthe primary reaction vertex and the secondary decay vertex (d(K0
S − V) > 25 mm),(3) distance of closest approach to the primary vertex for the two pion tracks(DCAπ+ > 7 mm, DCAπ− > 7 mm). These cuts were optimized to maximize the
Neutral kaon production in p+p and p+Nb collisions 113
Table 1 Three K0 production channels with highest cross sections used as simulation input
Reaction σ [μb]
p + p → �+ + p + K0 21.3p + p → � + p + π+ + K0 18.4p + p → �0 + p + π+ + K0 12.4
Fig. 2 Comparison of theK0
S rapidity distribution ofcorrected experimental data(p+p) and simulated datafitted with a Gaussian(μexp = −0.02 ± 0.01)
ratio S2/B. The resulting π+π− invariant mass spectra for the p+p and p+93Nb collid-ing systems are shown in Fig. 1. Both spectra are described by a fit of the signal (twoGaussians1) and the combinatorial background (Landau function and polynoms).High statistics allow differential yield analyses in rapidity (�y = 0.1) and transversemomentum (�pt = 75 MeV/c). For acceptance and efficiency corrections of the p+preaction the Pluto [8] event generator based on Monte Carlo was used. FourteenK0 production channels were simulated with isotropic angular distributions. Eachchannel was weighted with its corresponding cross section and processed throughthe full analysis chain. The cross sections from the Landolt–Börnstein database wererecalculated with updated experimental cross sections using a phasespace fit from[9]. Three of the channels with the highest contributions are listed in Table 1. In caseof the p+93Nb reaction simulated tracks embedded into real data were used.
3 Preliminary results
Transverse momentum distributions obtained by the Pluto [8] event generatorwere compared to acceptance and efficiency corrected experimental data from p+pcollisions. The spectra were normalized using a single scaling factor obtained at mid-rapidity. Overall an agreement within 10% deviation was observed. Moreover the ra-pidity distributions were compared (Fig. 2). To extract the yields the measured pointsin the pt-spectra were summed up in the region where measurements are available. ABoltzmann function was fitted to the tail of the pt-spectra to extrapolate the yield tothe not measured region. The simulation is underestimating the experimental data in
1Two Gaussians are needed to take multiple scattering effects into account.
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backward and forward rapidity areas. This might be explained by angular anisotropyof K0 production channels, which is not yet included into the simulation. Finally thegoal will be to determine absolute normalized yields and the in-medium KN potentialfrom the ratio σ(pNb)/σ (pp).
References
1. Benabderrahmane, M.L., et al.: Phys. Rev. Lett. 102, 182501 (2009)2. Agakishiev, G., et al.: Phys. Rev. C 82, 044907 (2010)3. Büscher, et al.: Eur. Phys. J. A 22, 301–317 (2004)4. Hartnack, C., et al.: arXiv:nucl-th/1106.2083v2 (2011)5. Korpa, et al.: Acta Physiol. Hung. A 22, 21–28 (2005)6. Agakishiev, et al.: Eur. Phys. J. A 41, 243–277 (2009)7. Cassing, W., et al.: Phys.Rep. 308, 65 (1999)8. Fröhlich, I., et al.: arXiv:nucl-ex/0708.2382v2 (2007)9. Sibirtsev, et al.: arXiv:nucl-th/9802019 (1998)