Strangeness changing baryon baryon interaction

NUCLEAR PHYSICS A ELSEVIER

Nuclear Physics A629 (1998) 369c-378c

Strangeness changing baryon baryon interaction Tadafumi Kishimoto ~* aDepartment of Physics, Osaka University, Toyonaka, Osaka, 560, Japan The weak nonmesonic decay of A hypernuclei gives information on the strangeness changing weak baryon baryon interaction. The parity violation of the process has been studied by observing weak nonmesonic decay of polarized A hypernuclei. Study of the inverse process pn --+ pA gives much cleaner information on the process. Recent progress on the experimental study is presented. 1. W e a k B a r y o n B a r y o n I n t e r a c t i o n The strong interaction between baryons can be studied through scattering experiments. There have been vast amount of nucleon-nucleon scattering experiments through which NN interaction has been studied in detail [1]. However, only a few data on the hyperonnucleon scattering and almost no data on hyperon-hyperon scattering are available. Thus the study of the strong interaction among octet baryons has been carried out with the help of SUF(3) symmetry by which vast amount of NN data were related to the scare YN data[2]. Since strong interaction between the octet baryons are similar, there is the existence of a nucleus where a nucleon is replaced by a hyperon which we call a hypernucleus. Knowledge on the YN strong interaction comes from the YN scattering experiments as well as hypernuclear structure. The situation for the weak baryon baryon interaction is quite different. The parity violation has been only a source to study weak nucleon nucleon interaction because its parity conserving part is completely masked by the strong interaction. Even for the study of parity violation, parity conserving strong interaction is so huge that one has to struggle with vary small effect (10 -7 ~ 10-8)[?]. Of course there is a good opportunity to observe large effect in low energy neutron scattering from specific nuclear levels[3]. However, since enhancement is due to complex mixture of the adjacent levels with opposite parity, connection of such data to weak NN interaction is rather indirect.

1.1. W e a k decay of A h y p e r n u c l e i Weak hyperon nucleon interaction can be studied in the weak nonmesonic decay (NMdecay) ofA hypernuclei. The NM-decay (A+n --+ n+n, A + p -+ p + n (q ,~ 400MeV/c)) is *representing a collaboration with S. Ajimura, H. Ejiri, T. Hasegawa, H. Hayakawa, M. Ishikawa, K. Maeda, K. Manabe, K. Morikubo, T. Nagae, T. Nakano, M. Nomachi, H. Noumi, O. Hashirnoto, A. Ohkusu, E. Saji, A. Sakaguchi, R. Satou, M. Sekimoto, T. Shibata, N. Shinkai, T. Takahashi, K. Tamura, K. Itonaga and T. Motoba 0375-9474/98/$19 © 1998 Elsevier ScienceB.V. All rights reserved. PII S0375-9474(97)00712-4

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strangeness changing hyperon nucleon weak process. The process has momentum transfer larger than the Fermi momentum (PF ~ 250MeV/c). It thus makes the Pauli blocking effect of little importance and the process can be taken as almost two body reaction. In the decay one can study both parity conserving and parity violating part of the weak interaction since no strong interaction can change Flavor(Strangeness). Meson exchange is a theoretical model that can describe the process where relevant weak nAN vertex has been obtained by the measurement of the mesonic decay of A (A -+ 7rN). This is not the case of the weak NN interaction since no experiment gives directly the weak 7rNN vertex. Weak 7rNN vertex is rather constrained by the known rAN vertex with help of SUE(3) symmetry. So far the SUE(3) symmetry with explicit breaking due to hadron masses has been successful for the strong baryon baryon interaction though almost none is known for the weak baryon baryon interaction. The weak NM-decay gives first extension of the weak NN interaction to baryons with strangeness. The matrix element of the NM-decay can be classified into 6 amplitudes [4] depending on spin, isospin and parity of the initial and final states as shown in table 1 provided that the initial state is of relative s-wave. Study of partial decay rates (A+n --~ n+n, A + p --+ p+n) can constrain magnitude of each amplitude in terms of isospin. Phenomenological analysis of sshell hypernuclei indicates the domTable 1.Six amplitudes of the NM-decay inance of the amplitude f (isospin Initial Final Rate Isospin Parity 1 final state)J4]. On the contrary, state state (Final) change theoretical prediction based on the 1S0 1S0 a2 1 no 1P0 b2 1 yes pion exchange implies the dominance of amplitude d (isospin 0 final aS 1 3S1 c2 0 no state) reflecting the tensor part of aD 1 d2 0 no the potential[5,6]. This discrepancy 1P1 e2 0 yes exists even though heavier mesons 3p1 f2 1 yes are included in the calculation[7]. 1.2. W e a k D e c a y of p o l a r i z e d A H y p e r n u c l e i Parity violation of the weak NM-decay provides interesting quantity to investigate the mechanism which is the asymmetric emission of decay particle with respect to the polarization[S]. The asymmetry parameter is expressed as[9], ctp

(v/2c + d)f

(1)

2vf3{a2 -I- b2 + 3(c 2 q- d 2 + e 2 T f2)}"

Amplitudes f and d,which are suggested to be dominant by both experiment and theory, represent mostly kaon and pion exchange, respectively. Therefore the study of the asymmetry parameter elucidates dominant part of the NM-decay mechanism. Quite large asymmetry parameter (-1.3 =t=0.4) was observed for the NM-decay of the polarized ~2C hypernucleus by using the 12C(7r+,K +) reaction[10]. It suggests equal importance of the isospin 0 and 1 amplitudes and seems to contradict to phenomenological analysis of branching ratio which suggests dominance of I = l final state. The error of 40 % is far below the accuracy that is needed for the detailed comparison with theoretical calculation. We thus have carried out another experiment where

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asymmetric NM-decay was observed from the weak NM-decay of polarized ~He[11]. The experiment is superior in the following five points to the previous k2C experiment. 1) The expected polarization is 1.5 ~ 2 times larger. 2) Polarization of A in ~He is unambiguously equal to that of ~He . 3) Polarization of the ~He can be given by observing the asymmetry of pions from the M-decay of ~He. Contrary the A polarization was estimated theoretically in k2C . 4) Final state interaction is small and can be estimated reliably. 5) Only relative s-wave contribute to the decay. The experiment (KEK-PS-E278) was carried out at the K6 beam line of KEK-PS. The 6Li(~+, K+p)~He reaction at P~=l.05 GeV/c was used to produce polarized ~He. We still need work to finish our data analysis, however, we can already give the branching ratio by using the current status of the analysis. Detailed discussion of the preliminary results can be found elsewhere[12]. Briefly the results suggest that the Fn/Fp is close to 2 which means dominance of isospin 1. It is consistent with Block and Dalitz analysis[4] but against to what the meson exchange model predicts. Quark cluster model predicts the branching ratio well though the asymmetry parameter badly [13]. Contrary meson exchange model predicts asymmetry parameter well though fails to reproduce the branching ratio[14]. Realistic model needs to include both aspects which is coming in the future [15,13]. It has been suggested recently that the NM-decay involving three nucleons in the final state may have substantial contribution to the decay rate[16]. Pions virtually emitted by weak decay of A is absorbed by two nucleons which are usually proton and neutron pair with isospin 0. In this case the measured branching ratio ( ~ ) becomes 2 though it doesn't represent the isospin structure of the two body reaction. The coming asymmetry parameter of ~ He should be quite important to clarify the situation. However, further experimental study is needed. 2. W e a k AN interaction studied by the pn -+ pA reaction 2.1. N M - d e c a y of A hypernuclei and pn -+ pA reaction The weak NM-decay of A hypernuclei gives information only for an initial state specified by hypernuclear wave function. Furthermore other nucleons cannot be complete spectators as discussed in the previous section. One can overcome the difficulty and obtain direct information of the weak AN interaction if the inverse reaction (pn ~ pA) can be studied. The cross section gives directly the transition amplitude of the process. The energy dependence of the cross section can be measured in the inverse reaction by which one can make a partial wave decomposition of the process. 2.2. S t u d y of parity violation The asymmetry parameter of the NM-decay, which represents parity violation, can be obtained by the analyzing power of the inverse reaction with longitudinally polarized proton beam. It is represented by A= °(h----1)-a(h=-l) a ( h = 1) + a ( h = - 1 )

where h = Jp. PpllJ,,llPpl, gives the interference of parity conserving and parity violating amplitude. The polarization of the proton beam can be as large as 0.8. The analyzing

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power has been measured in the proton proton scattering to investigate the parity violating weak NN interaction. Observed were very small effects of 10-7 ~ 10-s which makes the estimation of the systematic error of the experiment difficult. Since quite large asymmetry was observed in the NM-decay and is expected because no strong interaction contribute, we can expect the effect of the order of unity for the pn -+ pA. The large asymmetry parameter combined with large longitudinal polarization makes the systematic error easy to handle though one has to overcome very small cross section. The overall difficulty of the experiment is similar to the experiments of the parity violation in the proton proton scattering. 2.3. Search for T v i o l a t i o n The A produced in the final state has an analyzing power of the polarization. This can be utilized to search for the time reversal (T) violation. The T violation can be searched by observing the T odd correlation ( ~ × k~). J~. This process gives interesting new testing ground for the study. So far many tests have been carried out to search for the violation in nuclear beta decays and decay of mesons especially kaons. Obviously the T violation can be expected in the kaon decay since CP violation has been known for a long time. It has been argued, however, that the CP violation given by the Kobayashi Maskawa matrix is not large enough to explain the baryon asymmetry of the present Universe[17]. It is quite probable that large T violation is hidden in processes which have not been explored so far. Thus the T violation, if it is a new field, has always to be searched for. The p n ~ pA reaction is the strangeness changing weak process which is similar to the kaon decay through it is different in a couple of aspects. The kaon is q(1 system though the pn --+ pA reaction involves only quacks. It is likely that T violation gives the rate difference between qq --¢. gt and ~q --~ q. It would result in an interference term (T odd correlation) in the reaction which involves only particles or anti-particles. In the kaon decay one studies (k~ × k~) • ~ correlation in the K + -+ r°#+vu. The correlation means that muon has its spin polarization perpendicular to the reaction plane defined by three decaying particles. Since K + and ~r° have spin 0, the decay is dominantly of Fermi type which means that lepton pair couples spin 0. Muon thus has its polarization to the direction of neutrino which is in a reaction plane. Thus the effect is suppressed by the form factor by N 10-2. The channel spin of the pn --+ pA reaction is dominantly 1, and many other integer spin is possible thus no suppression is expected. The precision we expect in the reaction is much inferior to the kaon decay though it is worth for the test. It has been proven that there is no null test for the T violation experiment[18]. Thus one cannot measure the T violation with unlimited accuracy. One always has to investigate the accuracy achievable by the final state interaction. Here we are discussing quite large effect for which final state interaction is of negligible worry. 3. E x p e r i m e n t at R C N P 3.1. Cross section and yield The energy of proton beam has to be larger than 370 MeV for free neutron target. A 400 MeV proton beam is available at RCNP. Since the cross section is very small as shown in the following section, one needs intense beam and dense target material. One has to

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use nuclei as a neutron target. Currently 9Be is under consideration as the target. The 9Be nucleus is one of the lightest nucleus and its density is the largest among light nuclei. Proton and A in the final state has small relative momentum. Both particles moves in the forward direction. A decays into pions and nucleons after passing a few centimeter. This decay vertex is the genuine signal of the production of A, since there is no other process that produces pions after the target. One has to estimate the cross section to see the feasibility of the experiment. The cross section around threshold region is related to the weak NM-decay rate by

1 -

-

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/

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(3)

where Thp-+p, is partial life of hypernuclear NM-decay, p is the density distribution of a nucleon, UA is the wave function of a A. The density distribution of nucleon is taken from the charge distribution of a nucleus. The wave function of A can be simply obtained by adjusting the binding energy of A hypernucleus. Here the cross section was estimated to be ,-~ 10-39cm 2 which is similar to recent theoretical calculation [19]. This cross section is much smaller than that usually seen in nuclear physics though it is larger than usual neutrino cross section (N 10-44cm ~) since it is a hadronic weak process. The cross section due to the weak interaction usually increases as Q value increases at low energy region. Thus one would expect increase of the cross section by increasing the beam energy. No A is produced by the strong interaction which inevitably accompany kaon production up to 1.6 GeV. Thus it should be quite interesting to observe the energy dependence of the cross section up to the energy. Typically we can have target with thickness of 1024nuclei/cm 2 and beam intensity of 1012proton/sec. Since detection efficiency is of the order of 10% a s shown later, we can have roughly 10 events/day. This amount is sufficient to investigate the cross section and parity violation with reasonable amount of time. Search for T violation and measurement of the energy dependence of the reaction are quite interesting which will be a next step of the study. 3.2. C h a r a c t e r i s t i c o f t h e r e a c t i o n We use the 9Be as neutron target. We assume that the reaction mechanism is the quasifree process since the incident beam momentum is well above the Fermi momentum of nucleon. It means that the pn ~ p A takes place on a neutron which is moving in a nucleus with Fermi momentum. The momentum distribution of protons in 9Be has been measured [20]. Here we assumed that a neutron has the same momentum distribution as that of proton in the corresponding state. The 9Be has a structure of 2c~+n. An outer most neutron is bound loosely (B.E is 1.67MeV). There is no proton in the same state thus no m o m e n t u m distribution data are available. The momentum distribution is obtained by solving Schroedinger equation with appropriate potential to reproduce its binding energy. The four- m o m e n t u m of neutron is given as p~ = ( m . - B . E , ~ I ) ,

(4)

where m~ is mass of neutron, B.E is binding energy, and/~f is neutron momentum following Fermi momentum distribution. The reaction cross section depends on the binding

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energy and distortion effects. They are included in the calculation. Distribution of outgoing particles are given by the Monte Carlo simulation using GEANT. Figure 1 shows distribution of scattering angle versus m o m e n t u m of A obtained for free neutron target. Figure 2 shows the same distribution for neutrons bound in the 9Be. It is clear that distribution spread though dominant part is at around 600 M e V / c and 10 degrees. Distributions of 7r- and proton from the A are shown in figure 3 and 4.

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The dominant part of the backgrounds is due to pion production by the strong interaction. We simply assume that the reaction products are uniformly distributed in a Dalitz plot. The distribution has been used in a simulation to estimate the background. 3.3. T h e p r o p o s e d d e t e c t o r a. Design principle (characteristic of the signal) The genuine signal of the pn --+ pA reaction is the production of a A. The lifetime of A is 2.6 × 10-1°sec. The A has momentum typically 0.6 GeV/c thus the A decays after traveling several centimeters from the target as shown in figure 5. The decay of A produces 7r- and proton with 2/3 probability. One can identify the A by detecting 7r-'s and protons emitted from the same position that is several centimeters from the target. Fortunately, there is no other physical process that produces pions and protons several centimeters from the target. Therefore it should be the genuine signal of the production of A in the reaction.

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b. collimater We need to detect pions and protons emitted several centimeters from the target. In other words particles produced in the target region can be shielded out by a collimater. Since the background process which is due to strong interaction produces huge amount of protons and pions the collimater is really necessary to keep the counting rate of the counter tolerable. The particles from the target do not see the detector by the collimater. The small opening of the collimater gives good signal to noise ratio though gives small efficiency. We thus have studied the efficiency of the signal as a function of the the opening angle of the collimater. The best opening angle was found to be 25 degrees which keeps good efficiency for both A from the target and decay particle from the A. The thickness of the collimater is give by the energy of protons we have to shield out. The quasifree scattering of protons are dominant process at 25 degrees, the energy of protons are :,,350 MeV. We need to have 5 cm platinum to stop them. If we have enough beam intensity we would like to have thicker collimater. Details will be settled based on the test experiment.

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detector

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c. Silicon microstrip detector (SSD) The signal is best identified by the vertex information where A decays clearly after the target. It is vital to have good position resolution. We thus are going to use the silicon microstrip detector. The detector is developing quite rapidly recently since many high energy experiments which require the good vertex resolution make use of the detector. We will have to use many channels of the detector. A tiny preamplifier with multiplexer, which recently became available, makes read out of huge channels possible. Much sophisticated chip with capability of operation under high counting rate are still under development at B factory and LHC. We will use version which is quite advanced though widely used. We are planning to have three layers of SSD to obtain good position resolution and avoid accidental miss tracking. Currently we are planning to use 0.1 mm strip detector. We will have to read a couple of 10000 channels of data. Flash ADC's with multiplexer control can handle such huge data, though study is necessary to give realistic design. d. Drift chamber The outer region of the SSD will be surrounded by cylindrical drift chambers. The drift chamber is capable to give radial trajectory and longitudinal trajectory. Currently we are planning to follow the one developed at INS for the study of s=-2 hypernuclear system. e. Solenoid magnet We have to give momentum for the particle to finally reconstruct A invariant mass. It is vital to measure the charge of decay particles since only Ir- is from the reaction. We will thus use the solenoid magnet to give the momentum and charge of particles. The solenoid field will be 5 k gauss which is not so difficult to achieve. A few MeV/c momentum resolution will be obtained for 100 MeV/c particles. Pions have large transverse momentum (,-~ 100 MeV/c). Invariant mass of A will be reconstructed to reduce the background for which resolution of the transverse momentum is important. f. Trigger counters At the outer most layer we will have trigger counters that identifies the pions. Trigger signal will be produced by combination of plastic counters and Cerenkov counters. The detector do not see the target directly because of collimater shield. Pions produced by the

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background process at the target will be greatly reduced. Based on the simulation it will be good enough though we will study in the test experiment proposed simultaneously. Figure 7 shows the conceptual design of our detector system. The counters that cannot be operated in a high counting rate are masked from the target by the collimater. At the inner most region we have silicon microstrip detector to give the fine information of vertex position. The trajectory of particles are measured by drift chamber and finally we have trigger counters to make trigger signal. Plastic

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4.

Expected Results

In the present experiment we need to have a beam line that can tolerate the beam intensity. It has to provide longitudinally polarized beam to measure the parity violation. Transverse polarization is also needed to carry out the T violation experiment. The

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threshold of the reaction is 370 MeV for free neutron though binding effect of a neutron in a nucleus makes the threshold a little higher effectively. Since the beam energy is close to the reaction threshold, the cross section increases rapidly as a function of incident of proton energy. It is desirable to have beam energy a little higher than 400 MeV and recently RCNP achieved to provide 416 MeV proton beam. The expected yield of the signal is 4 events/day assuming realistic beam intensity and detector efficiency. The 30 days running times gives the 120 events. Here we should stress that we will obtain really a new quantity in the experiment. We know nothing about the cross section. When we get two events we could constrain its order of magnitude. Since one can have almost 100 % polarized beam 10 events will give us asymmetry parameter with 30% error which is already better than what we have obtained in ~2C experiment. REFERENCES 1. J . J . de Swart, T. A. Rijken, P. M. Maessen and R. G. Timmermans, IL Nuovo Ciment 102 A, (1989) 203 and references therein 2. Th. A. Rijken, P. M. M. Maessen and J. J. deSwart Nucl. Phys. A547 (1992) 245c 3. A. Masaike, Proc. IV Int Symp. on Weak and Electromagnetic Interactions in Nuclei"WEIN'95, June 1995, Osaka, World Scientific, (1995) 20 4. M.M. Block and R.H. Dalitz, Phys. Rev. Lett. 11 (1963) 96. 5. B.H.J. McKellar and B.F. Gibson, Phys. Rev. C30 (1984) 322. 6. K. Takeuchi, H. Takaki and H. BandS, Prog. Theor. Phys. 73 (1985) 841. 7. J. Dubach, Nucl. Phys. A450 (1986) 71c. 8. T. Kishimoto, KEK 83-6 (1983) 51, unpublished 9. H. BandS, T. Motoba and J. Zofka, Int. J. Mod. Phys. Vol. 5, (1990) 4021 10. S. Ajimura et al., Phys. Lett. B282 (1992) 293 11. T. Kishimoto et al., KEK-PS proposal E278 12. H. Noumi et al., Proc. IV Int Syrup. on Weak and Electromagnetic Interactions in Nuclei"WEIN'95, June 1995, Osaka, World Scientific, (1995) 550 13. M. Oka, Proc. IV Int Symp. on Weak and Electromagnetic Interactions in Nuclei"WEIN'95, June 1995, Osaka, World Scientific, (1995) 540 14. A. Ramos, E. van Meijgaard, C. Bennhold, and B. K. Jannings, Nucl. Phys. A544 (1992) 703 15. T. Motoba et al., private communication 16. W. M. Alberico, A. De Pace, M. Ericson and A. Molinari, Phys. Lett. B256 (1991)134; A. Ramos, E. Oset and L. L. Salcedo, Phys. Rev. C50 (1994)2314; S. Shinmura, Proc. IV Int Syrup. on Weak and Electromagnetic Interactions in Nuclei'WEIN'95, June 1995, Osaka, World Scientific, (1995) 528 17. A. G. Cohen, D. B. Kaplan and A. E. Nelson, Ann, Rev. Nunl. and Part. Sci. 43 (1993) 27 18. H. E. Conzett, Proc. IV Int Symp. on Weak and Electromagnetic Inteactions in Nuclei"WEIN'95, June 1995, Osaka, World Scientific, (1995) 68 19. J. Haidenbauer et al., Phys. Rev. C52 (1995)3496 20. S.Frullani and J.Mougey ADVANCES IN NUCLEAR PHYSICS vol.14 (1984)