Study of the K stopa Σ ± π A reaction at DAΦNE

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1 Study of the K stopa Σ ± π A reaction at DAΦNE M. Agnello, L. Benussi, M. Bertani, H.C.Bhang, G. Bonomi, E. Botta, M. Bregant, T. Bressani, S. Bufalino, L. Busso, D. Calvo, P. Camerini, B. Dalena, F. De Mori, G. D Erasmo, F.L. Fabbri, A. Feliciello, A. Filippi, M.E. Fiore, A. Fontana, D. Fujioka, P. Genova, P. Gianotti, N. Grion, V. Lucherini, S. Marcello, N. Mirfakhrai, F. Moia, P. Montagna, O. Morra, T. Nagae, H. Outa, A. Pantaleo, V. Paticchio, S. Piano, R. Rui, G. Simonetti, R. Wheadon and A. Zenoni Dip. di Fisica Politecnico di Torino, via Duca degli Abruzzi Torino, Italy, and INFN Sez. di Torino, via P. Giuria 1 Torino, Italy Laboratori Nazionali di Frascati dell INFN, via E. Fermi 4 Frascati, Italy Dep. of Physics, Seoul National Univ., Seoul, South Korea Dip. di Meccanica, Università di Brescia, via Valotti 9 Brescia, Italy and INFN Sez. di Pavia, via Bassi Pavia, Italy Dipartimento di Fisica Sperimentale, Università di Torino, via P. Giuria 1 Torino, Italy, and INFN Sez. di Torino, via P. Giuria 1 Torino, Italy Dip. di Fisica Univ. di Trieste, via Valerio Trieste, Italy and INFN, Sez. di Trieste, via Valerio Trieste, Italy Dip. di Fisica Univ. di Bari, via Amendola 179 Bari, Italy and INFN Sez. di Bari, via Amendola 179 Bari, Italy INFN Sez. di Pavia, via Bassi Pavia, Italy High Energy Accelerator Research Organization (KEK), Tsukuba, Ibaraki 35-81, Japan INFN, Sez. di Trieste, via Valerio Trieste, Italy Dep of Physics Shahid Behesty Univ., Teheran, Iran INAF-IFSI Sez. di Torino, C.so Fiume, Torino, Italy and INFN Sez. di Torino, via P. Giuria 1 Torino, Italy RIKEN, Wako, Saitama , Japan Abstract. This work describes an experimental study of the K stopa π ± Σ A reaction performed with the FINUDA spectrometer at the DAΦNE φ factory. The reaction is studied via the detection of π + π n events on,7, 9 Be, 13 C and 1 O. Keywords: Kaon absorption, hyperons, pions PACS: 5.8.Nv INTRODUCTION Despite many decades of investigation the KN dynamics is still an intensively studied field; several aspects still need in fact to be understood and clarified while it is an ideal testing ground of chiral SU(3) dynamics. The near threshold behaviour is strongly affected by the presence of the Λ(145) whose nature and properties are still debated,

2 also because of a lack of experimental data. The properties of the Λ(145) are modified inside nuclear matter and this in turn reflects on the in-medium KN dynamics. The properties of (anti)kaons inside nuclear matter have been intensively investigated since the first predictions of in-medium Kaon condensation [1] but several points are still open such as, e.g., the depth of the optical potential. A review on the subject can be found in []. There is presently an intense activity both in the experimental and theoretical side with a common effort to try to clarify the various aspects of this subject, by studying meson-baryon interactions in the strangeness S = -1 sector both in -vacuum and in the nuclear medium. In the present work π ± Σ events are studied with the aim at measuring the production spectra and determining their relative threshold branching ratios for different nuclei. Once the analysis is finalized, it will be possible to determine the A-dependence of the γ = K stopa Σ + π A KstopA Σ ratio, whose in-vacuum value is an important input for fixing the π + A low energy constants of the K p interaction. Since the optical potential depends also on subthreshold K p amplitudes, the study of the Σπ production in nuclei, and specifically of the γ ratio, can help in understanding the subthreshold behaviour. THE FINUDA DETECTOR All the particles involved in the A(K stop,π ± Σ )A reaction, i.e. K s (and correlated K + s), π ± s and Σ s (from nπ decays), were fully reconstructed with the FINUDA spectrometer. Negative and positive kaons of 1.1±1.5 MeV are emitted in nearly opposite directions after a φ K K + decay, φ s being produced by the e + e φ() reaction at the DAΦNE collider. The kaon pairs are slowed down by the inner layers of the spectrometer until they are absorbed inside a target of. 5.3 cm,.5 gr/cm thick. The inner (TOFINO) and outer (TOFONE) sensitive layers of FINUDA are two cylindrical, segmented plastic scintillator detectors [3]. TOFINO, with a diameter of 11 cm, starts the time-of-flight system while TOFONE, which consists of 7 trapezoidal slabs m long and 1 cm thick, is the stop-counter of the FINUDA time-of-flight. TOFONE is also used to determine the kinetic energy of neutrons as well as for triggering purposes, together with TOFINO. The FINUDA tracking system consists of a vertex detector [4], intermediate layers of low-mass wire chambers [5] and a stack of straw tubes []. The vertex detector consists of layers of double-sided micro-strip silicon sensors which surround a stack of 8 solid targets. The final stack (5 layers) of straw tubes [] has its inner layer set at 111. cm from the crossing beams. With the nominal field of FINUDA set at 1 T, the momentum resolution of the spectrometer to MeV/c pions is p/p=.8%.

3 Counts/MeV/c nπ + inv. mass (MeV/c ) nπ inv. mass (MeV/c ) FIGURE 1. Unconstrained nπ + and nπ invariant mass distributions for (K stop,nπ ± )A. The Greyfilled peaks represent the Σ + (left) and Σ signals. In the insets the constrained nπ ± invariant mass distributions are shown. The applied constraints are detailed in the text. The dotted lines are the result of a polynomial fit to the background being extrapolated in the Σ signal region. ANALYSIS AND DISCUSSION The experimental study of the A(K stop,πσ)a reaction can be performed thanks to the capability of FINUDA to reconstruct all the particles involved in the reaction: the stopping kaon, the prompt pion and the Σ-hyperon identified via its nπ ± decay channel. After selecting a π + π n event, the first condition applied requires the presence in one of the targets of a vertex formed by the trajectories of the incoming kaon and the track of one final pion. Particles are mass identified by their specific energy release (de/dx) in the two layers of the vertex detector as well as in the two wire chambers. Fig. 1 shows the nπ ± invariant mass spectrum of. The sharp peaks identify the Σ + at 1189.±3.4 MeV/c and 1197.±3.5 MeV/c, respectively. The signal to background ratio obtained by a peak-fitting procedure is (S/B) Σ +=.7 and (S/B) Σ =1., where S is the area of the Grey-filled peaks taken at σ and B is area below the dotted curves taken in the same mass range. The overall background B combines the nπ ± incoherent and combinatorial backgrounds. The incoherent background is mainly due to gammas which may emulate neutrons in TOFONE and by neutrons which may scatter before being detected. Since the momentum of pions from Σ ± nπ ± decays is distributed over a momentum range which is predominantly higher than that of spectroscopy pions, π ± Σ, with a minimum overlap of the two distributions, a simple kinematic selection discarding all the pions in the region of the spectroscopic production (i.e. below 19MeV/c in the case of ) strongly reduces the combinatorial background below the Σ signal without altering the emission spectrum of spectroscopy pions. A further background reduction is obtained by rejecting events below the M σ kinematic limit (see Fig. and related discussion hereafter), while further selections on the π ± n phase-space as well as the use of further tracking and vertexing quality cuts yield to the nπ ± invariant mass distributions shown in the insets of Fig. 1. The peak-fitting of the two distributions yields

4 Counts/5MeV/c 5 M5 He 15 M5 He M σ 5 M σ Σ - π + Missing Mass (MeV/c ) Σ π Missing Mass (MeV/c ) FIGURE. Missing mass distribution for the (K stop,π ± Σ )A reactions. The continuous line indicates the physical threshold of the missing mass, while the dotted line indicates the lower threshold chosen for the analysis. More details are given in the text. (S/B) Σ +=3. and (S/B) Σ =4.5. Fig. shows the missing mass distribution (M ) of the (K stop,π ± Σ )A reactions. The M distributions use the same set of data forming the constrained nπ ± invariant masses, i.e., the grey-filled distributions in the insets of Fig. 1. The full line indicates the physical threshold of the missing mass, that is, the value acquired by M when A 5 He. The dotted line indicates the missing mass threshold used in these analyses M σ, where σ is the uncertainty associated to M (mainly from the uncertainty in measuring the neutron kinetic energy). The M distribution is peaked at about 1 MeV/c above the physical threshold with a full-width at half-maximum of about MeV/c. These features suggest that the little missing mass is due to the excitation of the residual nucleus, while no missing pions are allowed, which establishes the quasi-free nature of the reaction and the minor role of final state interactions. These features are also shared by the 7, 9 Be, 13 C and 1 O nuclei. Fig. 3 shows the acceptance corrected momentum distributions of pions emitted from the (K stop,π ± Σ )A reactions. The error bars account for the statistical and systematic uncertainties. The spectrometer threshold to pions is slightly below MeV/c; however, the pion momentum distributions are cut below 1 MeV/c to avoid large bin fluctuations due to acceptance correction. Similar spectra are found on 7, 9 Be, 13 C and 1 O nuclei, which bear the same characteristics as the data and which will allow us to determine the capture rates on the different nuclei. Such values, presently not available since the analysis has not been finalized yet, will be the subject of a forthcoming paper. This will allow us to evaluate the γ ratio as a function of A, although this is not a straightforward procedure since the K stopa π ± Σ A reaction in nuclei is affected by the presence of the Σ conversion reaction ΣN ΛN. The Λπ spectra collected with FINUDA will be used to evaluate the amount of Σ conversion.

5 dn / dp (Arb. Units / MeV/c) π momentum (MeV/c) π momentum (MeV/c) FIGURE 3. Momentum distributions of pions from the (K stop,π ± Σ )A reactions. Summarizing, the K stopa nπ + π A data collected with the FINUDA spectrometer on A=,7, 9 Be, 13 C and 1 O. have been analysed in order to study the K stopa Σ ± π A reaction. The combinatorial and incoherent background has been effectively reduced by the application of thorough kinematical selections and quality constraints on vertices and tracks determination. The missing mass spectra indicate that the reaction can be described in terms of a quasi-free process with negligible energy being released by final state interactions of the outgoing particles. The next analysis step, presently under way, will be that of determining the capture rates and the γ ratio as a function of A. REFERENCES 1. D.B. Kaplan and A.E. Nelson, Phys. Lett. B175, 57 (198). E. Friedman, A. Gal, Phys.Rep 45, (7) 3. B. Dalena et al., Nucl. Instr. and Meth. A3, 7-8 (9) 4. P. Bottan et al., Nucl. Instr. and Methods A47 (1999) M. Agnello et al., Nucl. Instr. and Methods A385 (1997) 58.. L. Benussi et al., Nucl. Instr. and Meth. A31, 18, (1995) L. Benussi et al., Nucl. Instr. and Meth. A419, 48, (1998)