- Email: [email protected]
Coincidence measurement of the weak decay of 12ΛC and the three-body weak decay process (J-PARC 50GeV PS E18)
Nuclear Physics A 835 (2010) 434–438 www.elsevier.com/locate/nuclphysa
Coincidence measurement of the weak decay of 12 Λ C and the three-body weak decay process (J-PARC 50GeV PS E18) M. Kima , S. Ajimurab , K. Aokic , A. Banud , H. Bhanga , T. Fukudae , O. Hashimotof , J.I. Hwangg , S. Kameokaf , B.H. Kanga , E. Kima , J.H. Kima,h , T. Marutai , Y. Miuraf , Y. Miyakeb , T. Nagaec,l , M. Nakamurai , S.N. Nakamuraf , H.Noumib,c , S. Okadaj,k , Y. Okayasuf , H. Outac,k , H. Parkh , P.K. Sahac,m , Y. Satoc , M. Sekimotoc , T. Takahashic , H. Tamuraf , K. Tanidaa,k , A. Toyodac , K. Tshooa , K. Tsukadaf , T. Watanabef , H.J. Yima a Dept.
of Physics and Astronomy, Seoul National Univ, Seoul, 151-747, Korea of Physics, Osaka University, Toyonaga 560-0043, Japan c High Energy Accelerator Research Organization (KEK), Tsukuba 305-0801, Japan d Gesellschaft fur Schwerionenforschung mbH (GSI), Darmstadt 64291 Germany e Lab. of Physics, Osaka Electro Communication University, Neyagawa 572-8530, Japan f Department of Physics, Tohoku University, Sendai 980-8578, Japan g Department of Physics, Ewha Womans University, Seoul 120-750, Korea h Korea Research Institute of Standards and Science (KRISS), Daejeon 305-600, Korea i Department of Physics, University of Tokyo, Hongo 113-0033, Japan j Department of Physics, Tokyo Institute of Technology, Ookayama 152-8551, Japan k Riken Nishina Center, Riken, Wako 351-0198, Japan l Department of Physics, Kyoto University, Kyoto 606-8502, Japan m Japan Atomic Energy Research Institute, Tokai 319-1195, Japan b Department
Abstract Recently we have reported the Γn /Γ p value,∼0.5,obtained in the exclusive coincidence measurement of the nucleon pairs in the back-to-back kinematics, which led to the resolution of the long standing Γn /Γ p puzzle. We also have reported the branching ratio of the three-body process in the nonmesonic weak decay of 12 Λ C to be 0.29 ± 0.13 by reproducing the quenching of the nucleon yields. At the same time we have finally obtained the absolute decay widths Γn and Γ p taking account of the three-body processes. However, there still are serious inconsistencies not only between the experimental and theoretical values of important observables such as the decay asymmetry parameter αN M , but also between the basic formalisms of theoretical models. In order to provide more stringent guides for theoretical models, it is urgent to measure the fundamental observables, such as Γn , Γ p , and Γ2N , with much improved accuracy. In this regards, the next extensive coincidence experiment is planned at the J-PARC 50GeV PS for the accurate determination of the decay widths to the level of ∼10 % statistical uncertainty and the measurement of Γ2N in the exclusive way. Key words:
12 Λ C,
nonmesonic weak decay,three-body process, J-PARC 50GeV PS
Email address: [email protected] (M. Kim)
0375-9474/$ – see front matter © 2010 Elsevier B.V. All rights reserved. doi:10.1016/j.nuclphysa.2010.01.237
M. Kim et al. / Nuclear Physics A 835 (2010) 434–438
435
1. Introduction A Λ hyperon bound in a nucleus will eventually decay through either a mesonic or a nonmesonic weak decay(NMWD) process. Mesonic decay is essentially similar to the free Λ decay and has been studied in detail. Because of low decay momentum(∼100MeV/c), mesonic decay mode is strongly suppressed in the nucleus except in very light nuclei. Instead, the NMWD process, in which a Λ decays via a weak interaction with neighboring nucleon(s), become dominant in nuclei due to the high decay momentum(∼400MeV/c). The NMWD of Λ hypernuclei gives us a very unique opportunity to study ΔS =1 baryonbaryon weak interaction process. The dominant mechanism of the NMWD of Λ hypernuclei has been experimentally confirmed, the two-body process, Λn→nn ( Γn ) and Λp→np ( Γ p ). In addition to the two-body process ( ΓN ) a significant contribution of three-body process, ΛNN→nNN( Γ2N ) has been predicted in the theoretical calculations. However, it has not yet been verified experimentally. The three-body decay mode was first introduced in nuclear matter by Alberico et al. [1]. This transition can be described as coming from the absorption of a virtual pion emitted in the weak decay vertex by two strongly correlated nucleons. Later Ramos et al. [2] made a more realistic calculations and Bauer et al. [3] extended it recently. The contribution of such processes was predicted to be 0.25ΓΛ for 12 Λ C , where ΓΛ is the decay width of a free Λ. The decay interaction of NMWD is not well established yet and there remain the major issues on the NMWD to be cleared, such as the discrepancy of asymmetry parameter between experimental and theoretical values,unconfirmed three-body NMWD process, partial decay widths of NMWD and whether the ΔI=1/2 rule for NMWD would hold or not. Until recently, there has been a long standing discrepancy between the experimental and theoretical relative strength of the two channels of two-body process, namely Γn /Γ p ratio, during last several decades. Thanks to the recent developments in experimental[4-6] and theoretical efforts, the ratios now converged to ∼0.5 and the discrepancy seems finally resolved[7-10]. It has also turned out that the previous large experimental Γn /Γ p ratios that have been one of the causes of the puzzle were due to the strong quenching of the yields in the decay from that expected in the two-body one nucleon induced NMWD. Reproducing the quenching by including the three-body process, we were able to obtain the experimental branching ratio b2N (=Γ2N /Γnm ), and Γ2N for the first time along with Γn and Γ p [11, 12]. However, the uncertainties of the partial decay widths are ∼22-48%, of which the main contribution comes from the limited statistics of the coincident pair yields. Also these theoretical models differ in their isospin dependence of the decay interaction, such as ΔI=1/2 rule, which holds globally in the strangeness changing decay of mesons and hyperons. Therefore, evenif the agreement on Γn /Γ p among theoretical and experimental values, there still are serious inconsistencies not only between the experimental and theoretical values of important observables such as the decay asymmetry parameter αN M , but also between the basic formalisms of theoretical models. Now when the high intensity J-PARC accelerator will soon be ready for experiments, it is urgent to measure the fundamental observables, such as Γn , Γ p , and Γ2N , with much improved accuracy to provide more stringent guides for theoretical models. In this regards, the next extensive coincidence experiment is planned at the J-PARC 50GeV proton synchrotron accelerator for the accurate determination of the decay widths to the level of ∼10 % statistical uncertainty. In order to achieve much improved statistics in E18, we are going to use the K1.8 beamline equipped with the large solid angle(∼100msr) kaon spectrometer(SKS) and to construct a large solid angle(∼2 π sr) decay particle spectrometer.
436
M. Kim et al. / Nuclear Physics A 835 (2010) 434–438
2. J-PARC 50GeV PS E18 Experiment In this planned experiment, we measure the decay particle spectra from 12 Λ C by detecting both neutron(s) and proton(s) in double and triple coincidence. The basic ideas and the observations of the experiment are similar to the previous KEK-PS experiments(E508/E462). However the acceptance for double and triple coincidence are much improved compared to the previous experiment. From these much improved statistics for double and triple coincidence measurement, we can determine partial decay widths of NMWD( Γn and Γ p ), Γn /Γ p ratio and Γ2N with much higher accuracies. The experiment will be carried out with 1.05GeV/c π+ beam at K1.8 beamline of J-PARC 50GeV PS. Proton/neutrons and π− /π0 emitted from the weak decay of 12 Λ C will be detected by the coincedence counter system which is placed around the target region. Fig. 1 shows the setup of the decay coincidence counter system of J-PARC E18. After the target, a water ˆ counter is placed to reduce the trigger rate due to the proton contamination in the cerenkov(WC) + + (π , K ) trigger. In the previous E508 experiments, more than 90% of the (π+ , K + ) trigger events were due to such proton contamination. It comprises four sets of coincidence counter. Each of top and bottom counter sets comprise a fast timing counter(T2), a drift chamber(PDC),veto or stop timing or range counter(T3), neutron counter arrays(T4) and veto counters for rejecting passing through particles. The side sets are similar except for PDC. In order to determine the partial decay widths of NMWD( Γn and Γ p ) and Γn /Γ p ratio with high accuracy, the back-to-back coincidence events of n+p and n+n pair which is liberated from the Λ+p → n+p and Λ+n → n+n, should have large statistics. In addition, we also need a good statistics for pp pairs for the disentanglement of FSI effect. We observed 8 pp pair coincedence events in the back-to-back region from the E508 experiment. These pp events are possible only via the FSI process. A neutron in the np pair emitted at the vertex is converted into a proton via the nuclear interaction on the way out of the nucleus, thereby registering at a pp coincidence event. Therefore, they represent the channel crossover information of FSI on the nucleons emitted. Because we will measure more than 10 times higher pp coincidence events from E18, we can study FSI effect with enough statistics. Another important goal of this experiment is the confirmation of the existance of three-body process in exclusive method. This could be tested with the data of the non-back-to-back coincidence events of n+p and n+n pair and/or the triple coincidence events. These events are also observed in the previous E508 experiment. However, the statistics was too limited. The proposed configuration of decay spectrometer show in Fig. 1 could satisfy both of the requirements. Because the design of the proposed decay arm setup is similar to that of the experiment E462/E508 we can estimate the performance of the detectors and the expected results from the experiment realistically. Particle identification of the charged particles emitted from the weak decay of 12 Λ C will be carried out by using the timing/energy information of T2/T3 and T4(neutron detectors) in Fig. 1. The ability of PID was confirmed through the previous experiments, E462 and E508, which should clear separation of the charged products(proton,pion and deutron) from the carbon target. The proton energy will be measured by using its range information. Proton range will be estimated with the particle drift chamber(PDC). On the left and right side part, we can’t use drift chambers due to the lack of space. However, the charged particle tracking is possible without the drift chamber. The hit positions of T2 and T3 counters take the place of those of PDC, even though their uncertainty is higher than using PDC. To increase charged particle acceptance by rejecting passing through particles, veto counters will be installed as seen in Fig. 1. Neutral particles, neutron and γ, are measured by the neutron counter arrays composed of
M. Kim et al. / Nuclear Physics A 835 (2010) 434–438
437
Figure 1: Detector setup for the J-PARC 50GeV E18 Experiment
6 layers of 5cm thick scintillators. The charged particles are excluded by the T3 counter veto walls installed just before the neutron counter arrays in Fig. 1. We can identify neutrons and γs by the time-of-flight(TOF) technique. We have already achieved excellent TOF resolution of neutron counters, namely , σ∼200ps resolution for γ peak at 2MeVee(MeV electron-equivalent light output) threshold. The TOF spectrum can be converted to the neutron energy spectrum. The neutron kinetic energy is expressed as En = [1/ (1 − β2 ) − 1]mn , where β is the neutron velocity and mn is neutron mass. The neutron energy resolution, σ∼8MeV at 75MeV of neutron energy, is estimated from the resolution of γ-peak located at β=1. E18 experiment is motivated from the results of KEK-PS E508. The data from E508 can provide the realistic estimation of the angular correlations for nn and np paticle pairs. In E508, we obtained 116 back-to-back(cosθ < -0.7) np coincidence events and 43 back-to-back nn coincidence events. Γn /Γ p ratio was determined to be 0.51±0.13±0.05 and the uncertainty was dominated by the statistical fluctuation, especially due to low statistics of the nn back-to-back coincidence (43 events corresponds to 25% uncertainty) and pp events. From these events, we could obtain Γn , Γ p and Γn /Γ p values with much smaller systematic errors and the total uncertainty mainly depended on the statistical error. The proposed experiment is expected to have a few 100 pair events in the non-back-to-back region and about 100 triple events. This means we can have more than 10 times higher statistics compared to that of the previous experiment. We have also shown that Γ2N = 0.27 ± 0.13 ΓΛ from the quenching phenomena of the two-body NMWD events. It has an uncertainty of ∼50%. Furthermore, such derivation is a kind of inclusive measurement. For more direct explicit measurement, we are
438
M. Kim et al. / Nuclear Physics A 835 (2010) 434–438
going to measure the non-back-to-back events to the level of 10% statistical uncertainty. Also, we will see much more triple coincidence events with improved acceptance and the detailed study about the triple coincidence events can become possible, In summary, we have reported the Γn /Γ p value∼0.5, obtained in the exclusive coincidence measurement of the nucleon pairs in the back-to-back kinematics, solving the long standing Γn /Γ p puzzle. Also, the decay width of the three-body process in NMWD of 12 Λ C, Γ2N , was measured by reproducing the quenching of the pair yields. Along with Γ2N , Γn and Γ p were finally determined. Those results have been reported [10, 12]. However, the uncertainties of the partial decay widths are as high as ∼22-48% , of which the main contribution comes from the lack of statistics of the coincident pair yields, and furthermore the Γ2N value was obtained in the inclusive way. Until now the decay mechanism of NMWD is not well established yet. There still are serious inconsistencies not only between the experimental and theoretical values of important observables such as the decay asymmetry parameter αN M , but also between the basic formalisms of theoretical models. Therefore, it is urgent to measure the fundamental observables, such as Γn , Γ p , and Γ2N , with much improved accuracy to provide more stringent guides for theoretical models. In J-PARC 50GeV PS E18 experiment, we will measure the decay widths to the level of ∼10% statistical uncertainty. Also, the non-back-to-back and the triple coincidence events will be measured with high statistics to determine Γ2N in the exclusive way. 3. Acknowledgments We are grateful to KEK-PS staff for the support of our experiments and for the stable operation of KEK-PS. M. Kim acknowledges the support from BK21, WCU program of NRF and the Korea Research Foundation Grant funded by the Korean Government(MOEHRD)(KRF-2007357-C00018) and H. Bhang from KOSEF(Grant No. R01-2007-000-11714-0) and KRF(Grant No. 2007-314-C00069). References [1] [2] [3] [4] [5] [6] [7] [8] [9] [10] [11] [12] [13] [14] [15] [16]
W. M. Alberico et al., Phys. Lett. B 256 134 (1991). A. Ramos, E. Oset and L. L. Salcedo, Phys. Rev. C 50 2314 (1994). E. Bauer and G. Garbarino, Nucl. Phys.A 828 29 (2009). H. Bhang et al., Phys.Rev. Lett. 81, 4321 (1998) H. Park et al., Phys.Rev.C 61, 054004 (2000) O. Hashimoto et al., Phys.Rev. Lett. 88, 042503 (2002) J.H Kim et al., Phys.Rev. C 68, 065201 (2003) S. Okada et al., Phys. Lett. B597 249 (2004) B. H. Kang et al., Phys.Rev. Lett. 96, 062301 (2006) M. J. Kim et al., Phys. Lett. B641 28 (2006) H. Bhang et al., Eur. Phys. J. A 33, 259 (2007) M. Kim et al., Phys.Rev. Lett. 103, 182502 (2009) D. Jido, E. Oset and J. E. Palomar, Nucl. Phys. A 694 (2001) 525. K. Itonaga, T. Ueda, and T. Motoba, Phys. Rev. C 77 (2008) 044605. S. Okada et al., Nucl. Phys. A 754 (2005) 178c and Y. Sato et al., Phys. Rev. C 71 (2005) 025203, S. Kameoka et al., Nucl. Phys. A 754 (2005) 173c.
Coincidence measurement of the weak decay of 12 Λ C and the three-body weak decay process (J-PARC 50GeV PS E18) M. Kima , S. Ajimurab , K. Aokic , A. Banud , H. Bhanga , T. Fukudae , O. Hashimotof , J.I. Hwangg , S. Kameokaf , B.H. Kanga , E. Kima , J.H. Kima,h , T. Marutai , Y. Miuraf , Y. Miyakeb , T. Nagaec,l , M. Nakamurai , S.N. Nakamuraf , H.Noumib,c , S. Okadaj,k , Y. Okayasuf , H. Outac,k , H. Parkh , P.K. Sahac,m , Y. Satoc , M. Sekimotoc , T. Takahashic , H. Tamuraf , K. Tanidaa,k , A. Toyodac , K. Tshooa , K. Tsukadaf , T. Watanabef , H.J. Yima a Dept.
of Physics and Astronomy, Seoul National Univ, Seoul, 151-747, Korea of Physics, Osaka University, Toyonaga 560-0043, Japan c High Energy Accelerator Research Organization (KEK), Tsukuba 305-0801, Japan d Gesellschaft fur Schwerionenforschung mbH (GSI), Darmstadt 64291 Germany e Lab. of Physics, Osaka Electro Communication University, Neyagawa 572-8530, Japan f Department of Physics, Tohoku University, Sendai 980-8578, Japan g Department of Physics, Ewha Womans University, Seoul 120-750, Korea h Korea Research Institute of Standards and Science (KRISS), Daejeon 305-600, Korea i Department of Physics, University of Tokyo, Hongo 113-0033, Japan j Department of Physics, Tokyo Institute of Technology, Ookayama 152-8551, Japan k Riken Nishina Center, Riken, Wako 351-0198, Japan l Department of Physics, Kyoto University, Kyoto 606-8502, Japan m Japan Atomic Energy Research Institute, Tokai 319-1195, Japan b Department
Abstract Recently we have reported the Γn /Γ p value,∼0.5,obtained in the exclusive coincidence measurement of the nucleon pairs in the back-to-back kinematics, which led to the resolution of the long standing Γn /Γ p puzzle. We also have reported the branching ratio of the three-body process in the nonmesonic weak decay of 12 Λ C to be 0.29 ± 0.13 by reproducing the quenching of the nucleon yields. At the same time we have finally obtained the absolute decay widths Γn and Γ p taking account of the three-body processes. However, there still are serious inconsistencies not only between the experimental and theoretical values of important observables such as the decay asymmetry parameter αN M , but also between the basic formalisms of theoretical models. In order to provide more stringent guides for theoretical models, it is urgent to measure the fundamental observables, such as Γn , Γ p , and Γ2N , with much improved accuracy. In this regards, the next extensive coincidence experiment is planned at the J-PARC 50GeV PS for the accurate determination of the decay widths to the level of ∼10 % statistical uncertainty and the measurement of Γ2N in the exclusive way. Key words:
12 Λ C,
nonmesonic weak decay,three-body process, J-PARC 50GeV PS
Email address: [email protected] (M. Kim)
0375-9474/$ – see front matter © 2010 Elsevier B.V. All rights reserved. doi:10.1016/j.nuclphysa.2010.01.237
M. Kim et al. / Nuclear Physics A 835 (2010) 434–438
435
1. Introduction A Λ hyperon bound in a nucleus will eventually decay through either a mesonic or a nonmesonic weak decay(NMWD) process. Mesonic decay is essentially similar to the free Λ decay and has been studied in detail. Because of low decay momentum(∼100MeV/c), mesonic decay mode is strongly suppressed in the nucleus except in very light nuclei. Instead, the NMWD process, in which a Λ decays via a weak interaction with neighboring nucleon(s), become dominant in nuclei due to the high decay momentum(∼400MeV/c). The NMWD of Λ hypernuclei gives us a very unique opportunity to study ΔS =1 baryonbaryon weak interaction process. The dominant mechanism of the NMWD of Λ hypernuclei has been experimentally confirmed, the two-body process, Λn→nn ( Γn ) and Λp→np ( Γ p ). In addition to the two-body process ( ΓN ) a significant contribution of three-body process, ΛNN→nNN( Γ2N ) has been predicted in the theoretical calculations. However, it has not yet been verified experimentally. The three-body decay mode was first introduced in nuclear matter by Alberico et al. [1]. This transition can be described as coming from the absorption of a virtual pion emitted in the weak decay vertex by two strongly correlated nucleons. Later Ramos et al. [2] made a more realistic calculations and Bauer et al. [3] extended it recently. The contribution of such processes was predicted to be 0.25ΓΛ for 12 Λ C , where ΓΛ is the decay width of a free Λ. The decay interaction of NMWD is not well established yet and there remain the major issues on the NMWD to be cleared, such as the discrepancy of asymmetry parameter between experimental and theoretical values,unconfirmed three-body NMWD process, partial decay widths of NMWD and whether the ΔI=1/2 rule for NMWD would hold or not. Until recently, there has been a long standing discrepancy between the experimental and theoretical relative strength of the two channels of two-body process, namely Γn /Γ p ratio, during last several decades. Thanks to the recent developments in experimental[4-6] and theoretical efforts, the ratios now converged to ∼0.5 and the discrepancy seems finally resolved[7-10]. It has also turned out that the previous large experimental Γn /Γ p ratios that have been one of the causes of the puzzle were due to the strong quenching of the yields in the decay from that expected in the two-body one nucleon induced NMWD. Reproducing the quenching by including the three-body process, we were able to obtain the experimental branching ratio b2N (=Γ2N /Γnm ), and Γ2N for the first time along with Γn and Γ p [11, 12]. However, the uncertainties of the partial decay widths are ∼22-48%, of which the main contribution comes from the limited statistics of the coincident pair yields. Also these theoretical models differ in their isospin dependence of the decay interaction, such as ΔI=1/2 rule, which holds globally in the strangeness changing decay of mesons and hyperons. Therefore, evenif the agreement on Γn /Γ p among theoretical and experimental values, there still are serious inconsistencies not only between the experimental and theoretical values of important observables such as the decay asymmetry parameter αN M , but also between the basic formalisms of theoretical models. Now when the high intensity J-PARC accelerator will soon be ready for experiments, it is urgent to measure the fundamental observables, such as Γn , Γ p , and Γ2N , with much improved accuracy to provide more stringent guides for theoretical models. In this regards, the next extensive coincidence experiment is planned at the J-PARC 50GeV proton synchrotron accelerator for the accurate determination of the decay widths to the level of ∼10 % statistical uncertainty. In order to achieve much improved statistics in E18, we are going to use the K1.8 beamline equipped with the large solid angle(∼100msr) kaon spectrometer(SKS) and to construct a large solid angle(∼2 π sr) decay particle spectrometer.
436
M. Kim et al. / Nuclear Physics A 835 (2010) 434–438
2. J-PARC 50GeV PS E18 Experiment In this planned experiment, we measure the decay particle spectra from 12 Λ C by detecting both neutron(s) and proton(s) in double and triple coincidence. The basic ideas and the observations of the experiment are similar to the previous KEK-PS experiments(E508/E462). However the acceptance for double and triple coincidence are much improved compared to the previous experiment. From these much improved statistics for double and triple coincidence measurement, we can determine partial decay widths of NMWD( Γn and Γ p ), Γn /Γ p ratio and Γ2N with much higher accuracies. The experiment will be carried out with 1.05GeV/c π+ beam at K1.8 beamline of J-PARC 50GeV PS. Proton/neutrons and π− /π0 emitted from the weak decay of 12 Λ C will be detected by the coincedence counter system which is placed around the target region. Fig. 1 shows the setup of the decay coincidence counter system of J-PARC E18. After the target, a water ˆ counter is placed to reduce the trigger rate due to the proton contamination in the cerenkov(WC) + + (π , K ) trigger. In the previous E508 experiments, more than 90% of the (π+ , K + ) trigger events were due to such proton contamination. It comprises four sets of coincidence counter. Each of top and bottom counter sets comprise a fast timing counter(T2), a drift chamber(PDC),veto or stop timing or range counter(T3), neutron counter arrays(T4) and veto counters for rejecting passing through particles. The side sets are similar except for PDC. In order to determine the partial decay widths of NMWD( Γn and Γ p ) and Γn /Γ p ratio with high accuracy, the back-to-back coincidence events of n+p and n+n pair which is liberated from the Λ+p → n+p and Λ+n → n+n, should have large statistics. In addition, we also need a good statistics for pp pairs for the disentanglement of FSI effect. We observed 8 pp pair coincedence events in the back-to-back region from the E508 experiment. These pp events are possible only via the FSI process. A neutron in the np pair emitted at the vertex is converted into a proton via the nuclear interaction on the way out of the nucleus, thereby registering at a pp coincidence event. Therefore, they represent the channel crossover information of FSI on the nucleons emitted. Because we will measure more than 10 times higher pp coincidence events from E18, we can study FSI effect with enough statistics. Another important goal of this experiment is the confirmation of the existance of three-body process in exclusive method. This could be tested with the data of the non-back-to-back coincidence events of n+p and n+n pair and/or the triple coincidence events. These events are also observed in the previous E508 experiment. However, the statistics was too limited. The proposed configuration of decay spectrometer show in Fig. 1 could satisfy both of the requirements. Because the design of the proposed decay arm setup is similar to that of the experiment E462/E508 we can estimate the performance of the detectors and the expected results from the experiment realistically. Particle identification of the charged particles emitted from the weak decay of 12 Λ C will be carried out by using the timing/energy information of T2/T3 and T4(neutron detectors) in Fig. 1. The ability of PID was confirmed through the previous experiments, E462 and E508, which should clear separation of the charged products(proton,pion and deutron) from the carbon target. The proton energy will be measured by using its range information. Proton range will be estimated with the particle drift chamber(PDC). On the left and right side part, we can’t use drift chambers due to the lack of space. However, the charged particle tracking is possible without the drift chamber. The hit positions of T2 and T3 counters take the place of those of PDC, even though their uncertainty is higher than using PDC. To increase charged particle acceptance by rejecting passing through particles, veto counters will be installed as seen in Fig. 1. Neutral particles, neutron and γ, are measured by the neutron counter arrays composed of
M. Kim et al. / Nuclear Physics A 835 (2010) 434–438
437
Figure 1: Detector setup for the J-PARC 50GeV E18 Experiment
6 layers of 5cm thick scintillators. The charged particles are excluded by the T3 counter veto walls installed just before the neutron counter arrays in Fig. 1. We can identify neutrons and γs by the time-of-flight(TOF) technique. We have already achieved excellent TOF resolution of neutron counters, namely , σ∼200ps resolution for γ peak at 2MeVee(MeV electron-equivalent light output) threshold. The TOF spectrum can be converted to the neutron energy spectrum. The neutron kinetic energy is expressed as En = [1/ (1 − β2 ) − 1]mn , where β is the neutron velocity and mn is neutron mass. The neutron energy resolution, σ∼8MeV at 75MeV of neutron energy, is estimated from the resolution of γ-peak located at β=1. E18 experiment is motivated from the results of KEK-PS E508. The data from E508 can provide the realistic estimation of the angular correlations for nn and np paticle pairs. In E508, we obtained 116 back-to-back(cosθ < -0.7) np coincidence events and 43 back-to-back nn coincidence events. Γn /Γ p ratio was determined to be 0.51±0.13±0.05 and the uncertainty was dominated by the statistical fluctuation, especially due to low statistics of the nn back-to-back coincidence (43 events corresponds to 25% uncertainty) and pp events. From these events, we could obtain Γn , Γ p and Γn /Γ p values with much smaller systematic errors and the total uncertainty mainly depended on the statistical error. The proposed experiment is expected to have a few 100 pair events in the non-back-to-back region and about 100 triple events. This means we can have more than 10 times higher statistics compared to that of the previous experiment. We have also shown that Γ2N = 0.27 ± 0.13 ΓΛ from the quenching phenomena of the two-body NMWD events. It has an uncertainty of ∼50%. Furthermore, such derivation is a kind of inclusive measurement. For more direct explicit measurement, we are
438
M. Kim et al. / Nuclear Physics A 835 (2010) 434–438
going to measure the non-back-to-back events to the level of 10% statistical uncertainty. Also, we will see much more triple coincidence events with improved acceptance and the detailed study about the triple coincidence events can become possible, In summary, we have reported the Γn /Γ p value∼0.5, obtained in the exclusive coincidence measurement of the nucleon pairs in the back-to-back kinematics, solving the long standing Γn /Γ p puzzle. Also, the decay width of the three-body process in NMWD of 12 Λ C, Γ2N , was measured by reproducing the quenching of the pair yields. Along with Γ2N , Γn and Γ p were finally determined. Those results have been reported [10, 12]. However, the uncertainties of the partial decay widths are as high as ∼22-48% , of which the main contribution comes from the lack of statistics of the coincident pair yields, and furthermore the Γ2N value was obtained in the inclusive way. Until now the decay mechanism of NMWD is not well established yet. There still are serious inconsistencies not only between the experimental and theoretical values of important observables such as the decay asymmetry parameter αN M , but also between the basic formalisms of theoretical models. Therefore, it is urgent to measure the fundamental observables, such as Γn , Γ p , and Γ2N , with much improved accuracy to provide more stringent guides for theoretical models. In J-PARC 50GeV PS E18 experiment, we will measure the decay widths to the level of ∼10% statistical uncertainty. Also, the non-back-to-back and the triple coincidence events will be measured with high statistics to determine Γ2N in the exclusive way. 3. Acknowledgments We are grateful to KEK-PS staff for the support of our experiments and for the stable operation of KEK-PS. M. Kim acknowledges the support from BK21, WCU program of NRF and the Korea Research Foundation Grant funded by the Korean Government(MOEHRD)(KRF-2007357-C00018) and H. Bhang from KOSEF(Grant No. R01-2007-000-11714-0) and KRF(Grant No. 2007-314-C00069). References [1] [2] [3] [4] [5] [6] [7] [8] [9] [10] [11] [12] [13] [14] [15] [16]
W. M. Alberico et al., Phys. Lett. B 256 134 (1991). A. Ramos, E. Oset and L. L. Salcedo, Phys. Rev. C 50 2314 (1994). E. Bauer and G. Garbarino, Nucl. Phys.A 828 29 (2009). H. Bhang et al., Phys.Rev. Lett. 81, 4321 (1998) H. Park et al., Phys.Rev.C 61, 054004 (2000) O. Hashimoto et al., Phys.Rev. Lett. 88, 042503 (2002) J.H Kim et al., Phys.Rev. C 68, 065201 (2003) S. Okada et al., Phys. Lett. B597 249 (2004) B. H. Kang et al., Phys.Rev. Lett. 96, 062301 (2006) M. J. Kim et al., Phys. Lett. B641 28 (2006) H. Bhang et al., Eur. Phys. J. A 33, 259 (2007) M. Kim et al., Phys.Rev. Lett. 103, 182502 (2009) D. Jido, E. Oset and J. E. Palomar, Nucl. Phys. A 694 (2001) 525. K. Itonaga, T. Ueda, and T. Motoba, Phys. Rev. C 77 (2008) 044605. S. Okada et al., Nucl. Phys. A 754 (2005) 178c and Y. Sato et al., Phys. Rev. C 71 (2005) 025203, S. Kameoka et al., Nucl. Phys. A 754 (2005) 173c.