- Email: [email protected]
Mesonic and Non-Mesonic Weak Decay of Hypernuclei with FINUDA
Nuclear Physics A 827 (2009) 303c–305c www.elsevier.com/locate/nuclphysa
Mesonic and Non-Mesonic Weak Decay of Hypernuclei with FINUDA M. Agnello a,b , A. Andronenkov c , G. Beer d , L. Benussi e , M. Bertani e , H.C. Bhang f , G. Bonomi g,h , E. Botta i,b,1 , M. Bregant j,k , T. Bressani i,b , S. Bufalino i,b , L. Busso ,b , D. Calvo b , P. Camerini j,k , B. Dalena m,c , F. De Mori i,b , G. D’Erasmo m,c , F.L. Fabbri e , A. Feliciello b , A. Filippi b , E.M. Fiore m,c , A. Fontana h ,H. Fujioka n , P. Genova h , P. Gianotti e , N. Grion k , O. Hartmann e , B. Kang f , V. Lenti m , V. Lucherini e , S. Marcello i,b , T. Maruta o , N. Mirfakhrai p , P. Montagna q,h , O. Morra r,b , T. Nagae n , D. Nakajima s , H. Outa t , E. Pace e , M. Palomba c , A. Pantaleo c , A. Panzarasa h , V. Paticchio c , S. Piano k , F. Pompili e , R. Rui j,k , A. Sanchez Lorente u , M. Sekimoto v , G. Simonetti m,c , A. Toyoda v , R. Wheadon b , A. Zenoni g,h a Dipartimento
di Fisica, Politecnico di Torino, Corso Duca degli Abruzzi 24, Torino, Italy b INFN Sezione di Torino, via P. Giuria 1, Torino, Italy c INFN Sezione di Bari, via Amendola 173, Bari, Italy d University of Victoria, Finnerty Rd., Victoria, Canada e Laboratori Nazionali di Frascati dell’INFN, via. E. Fermi, 40, Frascati, Italy f Department of Physics, Seoul National University, 151-742 Seoul, South Korea g Dipartimento di Meccanica, Universita’ di Brescia, via Valotti 9, Brescia, Italy h INFN Sezione di Pavia, via Bassi 6, Pavia, Italy i Dipartimento di Fisica Sperimentale, Universita’ di Torino, Via P. Giuria 1, Torino, Italy j Dipartimento di Fisica, Universita’ di Trieste, via Valerio 2, Trieste, Italy k INFN Sezione di Trieste, via Valerio 2, Trieste, Italy Dipartimento di Fisica Generale, Universita’ di Torino, Via P. Giuria 1, Torino, Italy m Dipartimento di Fisica Universita’ di Bari, via Amendola 173, Bari, Italy n Department of Physics, Kyoto University, Sakyo-ku, Kyoto 606-8502, Japan o Department of Physics, Tohoku University, Sendai 980-8578, Japan p Department of Physics, Shahid Behesty University, 19834 Teheran, Iran q Dipartimento di Fisica Teorica e Nucleare, Universita’ di Pavia, via Bassi 6, Pavia, Italy r INAF-IFSI, Sezione di Torino, Corso Fiume 4, Torino, Italy s Department of Physics, University of Tokyo, Bunkyo, Tokyo 113-0033, Japan t RIKEN, Wako, Saitama 351-0198, Japan u Instit¨ ut fur Kernphysik, Johannes Gutenberg-Universit¨ at Mainz, Germany v High Energy Accelerator Research Organization (KEK), Tsukuba, Ibaraki 305-0801, Japan
Abstract The FINUDA experiment performed a systematic study of both mesonic and non-mesonic weak 0375-9474/$ – see front matter © 2009 Elsevier B.V. All rights reserved. doi:10.1016/j.nuclphysa.2009.05.061
304c
M. Agnello et al. / Nuclear Physics A 827 (2009) 303c–305c
decay of p-shell Λ-hypernuclei. Recent results on the mesonic decay rates and the non-mesonic decay ratios are illustrated and briefly discussed. Key words: Λ-hypernuclei, mesonic decay, non-mesonic decay, FINUDA experiment PACS: 21.80.+a, 13.75.Ev
FINUDA is a hypernuclear physics experiment [1] operating at DAΦN E, the LNF Φ-factory. Its scientific program is focussed on the study of spectroscopy and decay of − + A Z → π− + A Λ-hypernuclei produced by the reaction Kstop Λ Z, by stopping the very low − energy K mesons from Φ decay. Λ-hypernuclei decay weakly, as observed by FINUDA, A (Z + 1) + π − , and non-mesonic weak through mesonic weak decay (MWD), A ΛZ → A A−2 (Z − 1) + p + n, charged channels. The apparatus is suited decay (NMWD), Λ Z → to perform high resolution spectroscopy of the emitted charged particles: π − mesons of ∼ 260 − 280 MeV/c for production of the ground state, π − mesons of ≤ 130 MeV/c in MWD, protons of ≤ 600 MeV/c in NMWD. In this paper preliminary results are presented based on a total integrated luminosity of 1056 pb−1 (the full FINUDA data bank, up to 2007). MWD and NMWD of p-shell Λ-hypernuclei are discussed (6 Li, 7 Li, 9 Be, 12 C, 13 C, and D2 O targets). MWD was studied in events in which a high momentum π − meson, tagging the formation of ground or low lying excited states, was detected in coincidence with a low 12 15 16 momentum π − meson. MWD of 7Λ Li, 9Λ Be, 11 Λ B ( C targets) and Λ N ( O targets) was − investigated. In the present analysis the π momentum resolution is Δp/p 1% FWHM at 270 MeV/c and 6% FWHM at 110 MeV/c. The acceptance function for low moTable 1 bπ− and Γπ− /ΓΛ evaluated for charged MWD. The first (second) cited errors are statistical (systematical). Comparison with recent measurements and theoretical predictions is reported. bπ− 5 He Λ 7 Li Λ
0.323 ±
0.062+0.025 −0.020
0.315 ±
0.041+0.015 −0.012
0.154 ±
0.040+0.011 −0.007
11 B Λ
0.199 ±
0.039+0.041 −0.018
15 N Λ
0.085 ± 0.028+0.011 −0.010
9 Be Λ
Γtot /ΓΛ
Γπ− /ΓΛ
previous data
theory [2]
0.340 ± 0.016 [3]
0.393
0.947 ± 0.038 0.306 ±
0.060+0.025 −0.020
0.353 ±
0.059+0.017 −0.013
0.304
0.178 ±
0.050+0.013 −0.008
0.172
1.25 ± 0.08 0.249 ±
0.051+0.051 −0.023
linear fit linear fit
linear fit
0.212 ± 0.036 ± 0.045 [4]
0.108 ± 0.038+0.014 −0.013
0.213 0.090
mentum π − mesons was evaluated with simulated tracks. Background from quasi-free Λ 15 production and decay was simulated and subtracted from the 11 Λ B and Λ N spectra. Kinetic energy spectra were evaluated and compared with theoretical predictions of decay strength functions [2]. A good agreement was found. Decay rates, bπ− = Γπ− /Γtot , and Γπ− /ΓΛ = bπ− Γtot /ΓΛ , were calculated, using, for each hypernucleus, known Γtot /ΓΛ or a linear fit to the measured values from the available A = 4 − 12 Λ-hypernuclei. Table 1 summarizes the results and shows the comparison with recent existing data [3,4] and with theoretical calculations [2]. Also in this case a good agreement was found. The 5Λ He bπ− and Γπ− /ΓΛ were evaluated (from 6 Li targets) for comparison. To study NMWD events with a formation π − meson in coincidence with a proton were selected. Kinetic energy spectra of decay protons were obtained featuring a detection 1
Corresponding author. Fax: +39 011 6707324; e-mail address: [email protected]
M. Agnello et al. / Nuclear Physics A 827 (2009) 303c–305c
305c
threshold as low as 15 MeV, and a resolution ΔT /T 1.5% FWHM at 80 MeV. The background due to the absorption of K − mesons on a (np) cluster of the target nucleus was simulated and subtracted, as described in [5]. Acceptance correction was applied. Table 2 Rp for p-shell hypernuclei (Tp > 15 MeV) with statistical errors, systematic errors are < 5%. Rp
Rp
Rp
Rp
5 He Λ
15 0.25 ± 0.07 9Λ Be 0.38 ± 0.04 12 Λ C 0.43 ± 0.07 Λ N 0.45 ± 0.05
7 Li Λ
0.37 ± 0.09
11 B Λ
0.40 ± 0.05 13 Λ C 0.47 ± 0.10
16 O Λ
0.32 ± 0.07
FSI & ΛNN ratio
All the spectra show a similar shape, with a peak at ∼80 MeV as expected for the free Λp → np reaction and a low energy rise that should be due to final state interaction (FSI) and to two nucleon induced processes (Λnp). The decay ratio, Rp , between the number of detected protons and the number of formed hypernuclei was evaluated and is reported in Table 2. To disentangle the contribution due to FSI and to Λnp a systematic study was performed, considering, in each spectrum, as contribution due to the NMWD reaction the total area of the gaussian that describes the spectra from 80 MeV on, and the remaining part as due to both FSI and Λnp. A linear fit of the value of the ratio of this remaining area to the total area of the spectrum versus the mass number of the daughter nucleus is shown in Fig. 1 (see caption) and indicates that Λnp gives a contribution as large as 40% to our spectra, value that can be compared with those in [6]. 1 0.9 0.8 0.7 0.6 0.5
0.45 ± 0.10 0.332 ± 0.090
0.4 0.3 0
2
4
6
8
10
12
14
16 A-2
Fig. 1. Fraction of the total spectrum due to FSI and Λnp, as evaluated from the spectra (red circles) and as extracted by extrapolating to zero the detection threshold for decay protons energy (blue squares). Linear fits are also indicated.
In conclusion, the FINUDA experiment has performed a systematic study of MWD and NMWD of p-shell Λ-hypernuclei, performing for the first time a magnetic analysis of the spectra of MWD π − mesons and NMWD protons for all p-shell targets. MWD decay rates and Γπ− /ΓΛ and NMWD Rp have been evaluated. References [1] [2] [3] [4] [5] [6]
M. Agnello et al., Phys. Lett. B 622 (2005) 35; Nucl. Instr. Meth. A 573 (2007) 205. T. Motoba, K. Itonaga Progr. Theor. Phys. Suppl. 117 (1994) 477. S. Kameoka et al., Nucl. Phys. A 754 (2005) 173c. Y. Sato et al., Phys. Rev. C 71 (2005) 025203. M. Agnello et al., Nucl. Phys. A 804 (2008) 151. H. Bhang et al., Eur. Phys. J. A 33 (2007) 259; G. Garbarino, A. Parre˜ no, A. Ramos, Phys. Rev C 69 (2004) 054603.
Mesonic and Non-Mesonic Weak Decay of Hypernuclei with FINUDA M. Agnello a,b , A. Andronenkov c , G. Beer d , L. Benussi e , M. Bertani e , H.C. Bhang f , G. Bonomi g,h , E. Botta i,b,1 , M. Bregant j,k , T. Bressani i,b , S. Bufalino i,b , L. Busso ,b , D. Calvo b , P. Camerini j,k , B. Dalena m,c , F. De Mori i,b , G. D’Erasmo m,c , F.L. Fabbri e , A. Feliciello b , A. Filippi b , E.M. Fiore m,c , A. Fontana h ,H. Fujioka n , P. Genova h , P. Gianotti e , N. Grion k , O. Hartmann e , B. Kang f , V. Lenti m , V. Lucherini e , S. Marcello i,b , T. Maruta o , N. Mirfakhrai p , P. Montagna q,h , O. Morra r,b , T. Nagae n , D. Nakajima s , H. Outa t , E. Pace e , M. Palomba c , A. Pantaleo c , A. Panzarasa h , V. Paticchio c , S. Piano k , F. Pompili e , R. Rui j,k , A. Sanchez Lorente u , M. Sekimoto v , G. Simonetti m,c , A. Toyoda v , R. Wheadon b , A. Zenoni g,h a Dipartimento
di Fisica, Politecnico di Torino, Corso Duca degli Abruzzi 24, Torino, Italy b INFN Sezione di Torino, via P. Giuria 1, Torino, Italy c INFN Sezione di Bari, via Amendola 173, Bari, Italy d University of Victoria, Finnerty Rd., Victoria, Canada e Laboratori Nazionali di Frascati dell’INFN, via. E. Fermi, 40, Frascati, Italy f Department of Physics, Seoul National University, 151-742 Seoul, South Korea g Dipartimento di Meccanica, Universita’ di Brescia, via Valotti 9, Brescia, Italy h INFN Sezione di Pavia, via Bassi 6, Pavia, Italy i Dipartimento di Fisica Sperimentale, Universita’ di Torino, Via P. Giuria 1, Torino, Italy j Dipartimento di Fisica, Universita’ di Trieste, via Valerio 2, Trieste, Italy k INFN Sezione di Trieste, via Valerio 2, Trieste, Italy Dipartimento di Fisica Generale, Universita’ di Torino, Via P. Giuria 1, Torino, Italy m Dipartimento di Fisica Universita’ di Bari, via Amendola 173, Bari, Italy n Department of Physics, Kyoto University, Sakyo-ku, Kyoto 606-8502, Japan o Department of Physics, Tohoku University, Sendai 980-8578, Japan p Department of Physics, Shahid Behesty University, 19834 Teheran, Iran q Dipartimento di Fisica Teorica e Nucleare, Universita’ di Pavia, via Bassi 6, Pavia, Italy r INAF-IFSI, Sezione di Torino, Corso Fiume 4, Torino, Italy s Department of Physics, University of Tokyo, Bunkyo, Tokyo 113-0033, Japan t RIKEN, Wako, Saitama 351-0198, Japan u Instit¨ ut fur Kernphysik, Johannes Gutenberg-Universit¨ at Mainz, Germany v High Energy Accelerator Research Organization (KEK), Tsukuba, Ibaraki 305-0801, Japan
Abstract The FINUDA experiment performed a systematic study of both mesonic and non-mesonic weak 0375-9474/$ – see front matter © 2009 Elsevier B.V. All rights reserved. doi:10.1016/j.nuclphysa.2009.05.061
304c
M. Agnello et al. / Nuclear Physics A 827 (2009) 303c–305c
decay of p-shell Λ-hypernuclei. Recent results on the mesonic decay rates and the non-mesonic decay ratios are illustrated and briefly discussed. Key words: Λ-hypernuclei, mesonic decay, non-mesonic decay, FINUDA experiment PACS: 21.80.+a, 13.75.Ev
FINUDA is a hypernuclear physics experiment [1] operating at DAΦN E, the LNF Φ-factory. Its scientific program is focussed on the study of spectroscopy and decay of − + A Z → π− + A Λ-hypernuclei produced by the reaction Kstop Λ Z, by stopping the very low − energy K mesons from Φ decay. Λ-hypernuclei decay weakly, as observed by FINUDA, A (Z + 1) + π − , and non-mesonic weak through mesonic weak decay (MWD), A ΛZ → A A−2 (Z − 1) + p + n, charged channels. The apparatus is suited decay (NMWD), Λ Z → to perform high resolution spectroscopy of the emitted charged particles: π − mesons of ∼ 260 − 280 MeV/c for production of the ground state, π − mesons of ≤ 130 MeV/c in MWD, protons of ≤ 600 MeV/c in NMWD. In this paper preliminary results are presented based on a total integrated luminosity of 1056 pb−1 (the full FINUDA data bank, up to 2007). MWD and NMWD of p-shell Λ-hypernuclei are discussed (6 Li, 7 Li, 9 Be, 12 C, 13 C, and D2 O targets). MWD was studied in events in which a high momentum π − meson, tagging the formation of ground or low lying excited states, was detected in coincidence with a low 12 15 16 momentum π − meson. MWD of 7Λ Li, 9Λ Be, 11 Λ B ( C targets) and Λ N ( O targets) was − investigated. In the present analysis the π momentum resolution is Δp/p 1% FWHM at 270 MeV/c and 6% FWHM at 110 MeV/c. The acceptance function for low moTable 1 bπ− and Γπ− /ΓΛ evaluated for charged MWD. The first (second) cited errors are statistical (systematical). Comparison with recent measurements and theoretical predictions is reported. bπ− 5 He Λ 7 Li Λ
0.323 ±
0.062+0.025 −0.020
0.315 ±
0.041+0.015 −0.012
0.154 ±
0.040+0.011 −0.007
11 B Λ
0.199 ±
0.039+0.041 −0.018
15 N Λ
0.085 ± 0.028+0.011 −0.010
9 Be Λ
Γtot /ΓΛ
Γπ− /ΓΛ
previous data
theory [2]
0.340 ± 0.016 [3]
0.393
0.947 ± 0.038 0.306 ±
0.060+0.025 −0.020
0.353 ±
0.059+0.017 −0.013
0.304
0.178 ±
0.050+0.013 −0.008
0.172
1.25 ± 0.08 0.249 ±
0.051+0.051 −0.023
linear fit linear fit
linear fit
0.212 ± 0.036 ± 0.045 [4]
0.108 ± 0.038+0.014 −0.013
0.213 0.090
mentum π − mesons was evaluated with simulated tracks. Background from quasi-free Λ 15 production and decay was simulated and subtracted from the 11 Λ B and Λ N spectra. Kinetic energy spectra were evaluated and compared with theoretical predictions of decay strength functions [2]. A good agreement was found. Decay rates, bπ− = Γπ− /Γtot , and Γπ− /ΓΛ = bπ− Γtot /ΓΛ , were calculated, using, for each hypernucleus, known Γtot /ΓΛ or a linear fit to the measured values from the available A = 4 − 12 Λ-hypernuclei. Table 1 summarizes the results and shows the comparison with recent existing data [3,4] and with theoretical calculations [2]. Also in this case a good agreement was found. The 5Λ He bπ− and Γπ− /ΓΛ were evaluated (from 6 Li targets) for comparison. To study NMWD events with a formation π − meson in coincidence with a proton were selected. Kinetic energy spectra of decay protons were obtained featuring a detection 1
Corresponding author. Fax: +39 011 6707324; e-mail address: [email protected]
M. Agnello et al. / Nuclear Physics A 827 (2009) 303c–305c
305c
threshold as low as 15 MeV, and a resolution ΔT /T 1.5% FWHM at 80 MeV. The background due to the absorption of K − mesons on a (np) cluster of the target nucleus was simulated and subtracted, as described in [5]. Acceptance correction was applied. Table 2 Rp for p-shell hypernuclei (Tp > 15 MeV) with statistical errors, systematic errors are < 5%. Rp
Rp
Rp
Rp
5 He Λ
15 0.25 ± 0.07 9Λ Be 0.38 ± 0.04 12 Λ C 0.43 ± 0.07 Λ N 0.45 ± 0.05
7 Li Λ
0.37 ± 0.09
11 B Λ
0.40 ± 0.05 13 Λ C 0.47 ± 0.10
16 O Λ
0.32 ± 0.07
FSI & ΛNN ratio
All the spectra show a similar shape, with a peak at ∼80 MeV as expected for the free Λp → np reaction and a low energy rise that should be due to final state interaction (FSI) and to two nucleon induced processes (Λnp). The decay ratio, Rp , between the number of detected protons and the number of formed hypernuclei was evaluated and is reported in Table 2. To disentangle the contribution due to FSI and to Λnp a systematic study was performed, considering, in each spectrum, as contribution due to the NMWD reaction the total area of the gaussian that describes the spectra from 80 MeV on, and the remaining part as due to both FSI and Λnp. A linear fit of the value of the ratio of this remaining area to the total area of the spectrum versus the mass number of the daughter nucleus is shown in Fig. 1 (see caption) and indicates that Λnp gives a contribution as large as 40% to our spectra, value that can be compared with those in [6]. 1 0.9 0.8 0.7 0.6 0.5
0.45 ± 0.10 0.332 ± 0.090
0.4 0.3 0
2
4
6
8
10
12
14
16 A-2
Fig. 1. Fraction of the total spectrum due to FSI and Λnp, as evaluated from the spectra (red circles) and as extracted by extrapolating to zero the detection threshold for decay protons energy (blue squares). Linear fits are also indicated.
In conclusion, the FINUDA experiment has performed a systematic study of MWD and NMWD of p-shell Λ-hypernuclei, performing for the first time a magnetic analysis of the spectra of MWD π − mesons and NMWD protons for all p-shell targets. MWD decay rates and Γπ− /ΓΛ and NMWD Rp have been evaluated. References [1] [2] [3] [4] [5] [6]
M. Agnello et al., Phys. Lett. B 622 (2005) 35; Nucl. Instr. Meth. A 573 (2007) 205. T. Motoba, K. Itonaga Progr. Theor. Phys. Suppl. 117 (1994) 477. S. Kameoka et al., Nucl. Phys. A 754 (2005) 173c. Y. Sato et al., Phys. Rev. C 71 (2005) 025203. M. Agnello et al., Nucl. Phys. A 804 (2008) 151. H. Bhang et al., Eur. Phys. J. A 33 (2007) 259; G. Garbarino, A. Parre˜ no, A. Ramos, Phys. Rev C 69 (2004) 054603.