Kaon and Phi Production in Pion-Nucleus Reactions at 1 . 7 GeV / c

The production and properties of K, K− and φ in nuclear reactions π− + A (A = C,W) at a beam momentum of 1.7 GeV/c has been studied with the HADES setup at SIS18/GSI. Of particular interest is the K− absorption in nuclear matter which should be driven by strangeness exchange processes on one (K−N → Yπ) or more nucleons (K−NN → YNπ). In this context, also the φ has to be taken into account, since φ decays into K+K− pairs may substantially affect the measured K− abundance. A solid reference is needed to evaluate the K− absorption with in the nucleus and in this work we discuss the determination of this reference. A double ratio of K−/K+(W)/K−/K+(C) is measured within the acceptance and compared to the reference.


Introduction
Pion-nucleus reactions allow for a quantitative study of open and hidden strange hadron production and properties at a well defined nuclear density.Of particluar interest is the φ meson (s s) with no-net strangeness.According to the Okubo-Zweig-Iizuka (OZI) rule [1] quark exchange and hence the interaction of the φ meson with ordinary (non-strange) baryonic matter is suppressed.Consequently, the φN cross-section is assumed to be small.However, the LEPS Collaboration determined on the basis of the A-dependent φ photoproduction yield and a Glauber-type model a large in-medium φN absorption cross-section of σ φN = 35 +17 −11 mb [2].This extraced absorption cross-section is in agreement with photoproduction measurements performed by the CLAS Collaboration [3].Here, transperency ratios normalized to carbon where analysed as well within a Glauber model with values of σ φN in the range of 16 − 70 mb.Also transperency ratios normalized to carbon measured in pA collisions by the ANKE Collaboration point in the same direction with an effective φN absorption cross-section in the range of 14−21 mb [4].So far, φ absorption in different nuclei has not been studied in pion-induced reactions and should therefore provide complementary information to results obtained with photon and proton beams.

The Experiment
Experimental data were obtained with the High-Acceptance Di-Electron Spectrometer (HADES) [5], currently located at the SIS18 accelerator (GSI, Darmstadt).The detector setup comprises six idential sectors surrounding the beam axis covering almost the full azimuthal range and polar angles from 18 • to 85 • .Two layers of Multiwire Drift Chambers (MDC), in front and behind a toroidal superconducting magnet, allow for momentum reconstruction with a resolution of ∆p/p ≈ 3% and particle identification via dE/dx.In this experimental campaign, the first level (LVL1) trigger condition required a T 0 signal in the target detector [6] and a minimum multiplicity of two charged particle hits (M2) in the Multiplicity and Electron Trigger Array (META) wall consiting of two time-of-flight detectors, RPC and TOF.In total, 10 × 10 7 and 13 × 10 7 events were collected in π − + C and π − + W collisions at p π − = 1.7 GeV/c, respectively.

Exclusive Phi Production
The φ mesons were identified via their dominant decay channel into K + K − pairs (BR = 48.9± 0.5% [7]).Both charged kaons have been selected employing velocity and momentum cuts 1 .The contamination from other particle species was further reduced by selecting a reconstruced (anti-)kaon mass window of 400 < m [MeV/c2 ] < 600.Further, the nominal mass value m K = 493.677MeV/c 2 [7] was then attributed to the identified (anti-)kaon candidates.The resulting K + K − invariant mass distribution corresponing to an interval of 750 ≤ p [MeV/c] < 1250 and 18.8 ≤ θ [ • ] < 27.5 for π − + C collisions is shown in Fig. 1 (left).A clear φ peak emerging from the background of non-resonant K + K − pairs and track combinations containing misidentified particles is visible.The φ signal is described by a double Gaussian accounting for finite resolution effects as well as the collisional broadening due to the scattering of the K + and K − inside the targets, while the background is described by a third-order polynomial together with a Gaussian representing the mass threshold (2 × m K ).For acceptance and efficiency corrections the Pluto [8] event generator was used.Therefore, the exclusive φ production channel π − + p( 12 C) → φ[→ K + K − ] + n( 11 B) was simulated.The momentum and angular distribution of the generated φ mesons is shown in Fig. 2 (left).The K + K − pairs originating from φ mesons were then processed through simulations modelling the detector response.In Figure 2 (right) the obtained acceptance and efficiency correction matrix is presented.One can see that the acceptance and efficiency corrections strongly depend on phase space, hence a differential correction becomes mandatory.Due the limited φ statistic, a double-differential analysis would lead to acceptance and efficiency correction factors averaged over wide and non-constant region in the phase space.This is overcome by weighting each φ candidate with its corresponding correction factor in phase space building an integrated and corrected K + K − invariant mass distribution (see Fig. 1 (left)).

Summary and Outlook
In this contribution, we presented the on-going analysis of φ production in π − + C reactions at p π − = 1.7 GeV/c measured with the HADES setup.The neutral φ is reconstructed via the invariant mass of charged kaon candidates (M K + K− ).On the basis of simulations modelling the detector response in combination with Pluto as an event generator, the acceptance and efficiency corrected φ yield can be extracted.In the next step we plan to determine absolute normalized yields.The final goal will be the extraction of an effective φN absorption cross-section by comparing the φ production in π − + W with respect to π − + C collisions in combination with reaction models providing complementary information to photon-and proton-induced measurements.

EPJ[Figure 1 .
Figure 1.(Color online) Invariant mass distributions for K + K − pairs produced in π − C collisions w/o (left) and with event-by-event acceptance and efficiency corrections (right).The fit consists of two Gaussian for the φ signal (light blue line) together with the background described by a polynomial and Gaussian function (pink line).Details see text.

Figure 2 .
Figure 2. (Color online) The p − θ distribution of the φ mesons generated with the Pluto event generator (left).Combined acceptance and efficiency correction matrix for the φ mesons decaying in K + K − pairs as a function of p − θ (right).