- Email: [email protected]
Future experiments on hypernuclei and hyperatoms
Nuclear Instruments and Methods in Physics Research B 214 (2004) 149–152 www.elsevier.com/locate/nimb
Future experiments on hypernuclei and hyperatoms Josef Pochodzalla Institut f€ur Kernphysik, Universit€at Mainz, Mainz, Germany
Abstract The possibility to produce double K nuclei in anti-proton nucleus collisions at anti-proton momenta close to the þ is explored. Combining a high-luminosity antiproton beam with a novel solid-state tracking system threshold for N N and a high-rate Ge-array, c-spectroscopy of KK-hypernuclei will become feasible at the PANDA experiment of the future International Accelerator Facility at GSI. Ó 2003 Elsevier B.V. All rights reserved. PACS: 21.80.+a; 36.10.Gv Keywords: Antiprotons; Hypernuclei; Hyperatoms
1. The physics case A hypernucleus contains one or more hyperons implanted within the nuclear medium. This impurity introduces a new quantum number, strangeness, into the atomic nucleus thus adding a third dimension to the nuclear chart [1]. On one hand the hyperon embedded in a nuclear system may serve as a probe for the nuclear structure and its possible modification due to the presence of the hyperon. On the other hand properties of hyperons may change dramatically if implanted inside a nucleus. Thus a nucleus may serve as a femto-laboratory offering a unique possibility to study basic properties of hyperons and strange exotic objects. An important goal of measuring the level spectra and decay properties of multi-strange hypernuclei is to test the energies and wave functions from microscopic structure models and to put
E-mail address: [email protected] (J. Pochodzalla).
constraints on baryon–baryon interaction models [2,3]. It is clear that a detailed and consistent understanding of the quark aspect of the baryon– baryon forces in the SU(3) space will not be possible as long as experimental information on the hyperon–hyperon channel is not at our disposal. Since direct scattering experiments between two hyperons are impractical, the spectroscopy of multi-strange hypernuclei provides a unique approach to explore the hyperon–hyperon interaction. Non-mesonic weak decays of hypernuclei and double hypernuclei [4] offer exceptional information on the four-fermion, strangeness changing, baryon–baryon weak interaction [5] and the interplay of the quark-exchange and meson-exchange aspects of the baryon–baryon forces. The possible existence of an S ¼ 2 six quark (uuddss) H-dibaryon [6] represents another challenging topic of KK hypernuclear physics. So far, however, the experimental searches for this lightest strangelet are inconclusive and even a strongly bound H can not be excluded [7]. In fact the short
0168-583X/$ - see front matter Ó 2003 Elsevier B.V. All rights reserved. doi:10.1016/j.nimb.2003.08.007
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J. Pochodzalla / Nucl. Instr. and Meth. in Phys. Res. B 214 (2004) 149–152
time scale of the order of 1023 s available in a coalescence process may prevent the transition from a KK state to an H-particle in free space. Here, double K hypernuclei may serve as ÕbreederÕ for the H-particle: the long lifetime of the order of 1010 s of two Ks bound together in a nucleus may help to overcome a possible repulsive interaction at short distances. Since the mass of the H-particle might drop inside a nucleus [8] and because of hyperon mixing [9] it might be possible to observe traces of the H even if it is unbound in free space. Thus spectroscopy of S ¼ 2 hypernuclei can give important information on this fascinating and long sought object. Finally, hyperatoms created during the capture process of the hyperon will supply new information on fundamental properties of the hyperons. For example, the X is the only baryon whose static quadrupole moment can be determined experimentally [10]. Since only the one-gluon exchange is expected to contribute to the quadrupole moment [11] the X represents a unique benchmark for our understanding of the spin-dependent interaction between quarks in a ground-state hadron. At the high-energy antiproton storage ring HESR of the future International Accelerator Facility at GSI/Darmstadt the large number of produced X -atoms will make it possible to determine the hyperfine splitting in an X -atom and thus – for the first time – provide information on the static deformation of a baryon.
2. Status of KK hypernuclei Because of the low energy release of only 28 MeV in a conversion of a N and a proton into two Ks, attempts to produce KK-hypernuclei are generally based on the N capture reaction. The world supply of data on KK hypernuclei is very limited. Until recently only three candidates for double hypernuclei were observed via their double pion decay [12]. In addition, Aoki et al. presented evidence for the production and subsequent weak decay of three heavy double hypernuclei [13]. Only lately two additional events were completely identified by the KEK-E373 collaboration which collected 1000 stopped N [14]. In
the same year the AGS-E885/E906 collaboration published the first Ômass productionÕ of about 30 4 4 KKH events based on 10 stopped N [15]. While this first spectrometer experiment could not determine the binding energies of 4 KKH, the number of observed events signal that the production process of double hypernuclei is reasonable well understood (see e.g. [16]). But even the high accuracy of the emulsion data does not allow an unequivocal interpretation of the deduced KKbinding energies in terms of the strength of the KK interaction. These difficulties reflect various complications: (i) the dynamical change of the core nucleus [17], (ii) the non-zero spin of the core, (iii) KK NN coupling effects [9] and (iv) the possible production of the hypernuclei in c-unstable excited states or the emission of undetected neutrons [18]. Clearly a drawback common to all theoretical investigations is the lack of high resolution and systematic data on multi-hypernuclei and their level structure.
3. Future experiments on multi-strange systems Until the end of the last decade, the energy resolution of hypernuclear studies with electronic detectors was limited to typical values of about 1 MeV. Only recently with the advent of highefficiency Ge-arrays the door to high-resolution c-spectroscopy has been opened. These measurements will be complemented by electroproduction experiments of single hypernuclei both at JLab and MAMI-C. The dominant spin-flip amplitudes in these reactions favor the excitation of unnatural parity states in hypernuclei. Furthermore not only c-unstable but also particle-unstable states are accessible in these reactions with sub-MeV resolution. Combining Ge-arrays like AGATA [19] with the high-luminosity anti-proton storage ring HESR at GSI and with a novel solid-state micro tracker, high-resolution c-spectroscopy of double hypernuclei and X-atoms will become possible for the first time. In order to minimize the background from associated particles, the production of hypernuclei and hyperatoms at HESR will use NN and XX pair
J. Pochodzalla / Nucl. Instr. and Meth. in Phys. Res. B 214 (2004) 149–152
151
Fig. 1. Various steps of the proposed reaction in case of double hypernucleus production.
Fig. 2. Schematic view of the central detector components for hypernucleus and hyperatom experiments.
production close to threshold in anti-proton nucleus collisions (Fig. 1). The trigger will be based on the detection of a high-momentum anti-hyperons at small angles or of positive kaons produced by the anti-hyperons absorbed in the primary target nuclei. The 2Kþ trigger will provide significantly higher count rates but requires the detection of rather low momentum kaons of a few hundreds MeV/c. The second ingredient of the experiment is the deceleration of the N inside the nucleus and subsequent absorption in an secondary active target. The geometry of this secondary absorber is determined by the short mean life of the N of only 0.164 ns. If the separation between the primary target and the secondary absorber is to large, low momentum N will decay prior to full stopping. On the other hand, energetic N with momenta beyond approximately 500 MeV/c can not be stopped prior to their decay. This limits the required thickness of the active secondary target to about 30 mm. In order to track the stopped N and the charged fragments resulting from the decay of the produced hypernuclei, it is planned to sandwich the absorber with solid-state pixel or strip detectors. As a third ingredient an efficient germanium carray is required (Fig. 2). Here the main limitation
will be the load of associated particles. Fully digital electronics will allow a load of background events of more than 100 kHz for each detector element. Since most of the produced particles are emitted in the forward region not covered by the Ge-array an interaction rate of a few 107 s1 seems to be manageable. The experimental set up is sufficiently small to fit inside the calorimeter of the general-purpose detector PANDA. After removing the end cap of the calorimeter upstream of the target the detectors will be placed close to the straw chambers thus making use of the tracking capability of the main detector in forward direction. In order to trigger on low momentum kaons a thin start detector will be placed close to the target. The stop signal will be provided by scintillation or resistive plate counters, which replace or – if space permits – complement the RICH detector. All KK hypernuclei experiments performed to date have collected of the order of 10000 stopped N in total. With this scheme we will be able to reconstruct approximately 3000 stopped N with þ the unique N trigger per day at PANDA. By applying the kaon trigger we will exceed the number by up to two orders of magnitude thus providing us with a few 105 N per day. With such a event rate high-statistics c-spectroscopy of
152
J. Pochodzalla / Nucl. Instr. and Meth. in Phys. Res. B 214 (2004) 149–152
KK-hypernuclei will become feasible at the PANDA experiment of the future International Accelerator Facility at GSI.
[11] [12]
References [1] W. Greiner, Int. J. Mod. Phys. E 5 (1) (1995) 1. [2] C.B. Dover, A. Gal, Ann. Phys. 146 (1983) 309. [3] C. Nakamoto, Y. Suzuki, Y. Fujiwara, Kyoto University pre-print KUNS-1432, SCAN-9706007. [4] A. Parreno, A. Ramos, C. Bennhold, Phys. Rev. C 65 (2001) 015205-1. [5] E. Oset, A. Ramos, Prog. Part. Nucl. Phys. 41 (1998) 191. [6] R.L. Jaffe, Phys. Rev. Lett. 38 (1977) 195, 38 (1977) 617(E). [7] G.R. Farrar, G. Zaharijas, hep-ph/0303047 (2003). [8] T. Sakai et al., Prog. Theor. Phys. Suppl. 137 (2000) 121. [9] T. Yamada, C. Nakamoto, Phys. Rev. C 62 (2000) 0343191; K.S. Myint, S. Shinmura, Y. Akaishi, Eur. Phys. J. A 16 (2003) 21; I.R. Afnan, B.F. Gibson, Phys. Rev. C 67 (2003) 017001-1; I.N. Filikhin, A. Gal, V.M. Suslov, nucl-th/0303028 (2003). [10] R.M. Sternheimer, M. Goldhaber, Phys. Rev. A 8 (1973) 2207;
[13] [14] [15] [16]
[17] [18]
[19]
S.S. Gershtein, Yu.M. ZinovÕev, Yad. Fiz. 33 (1981) 1442; M.M. Giannini, M.I. Krivoruchenko, Phys. Lett. B 291 (1992) 329. A.J. Buchmann, Z. Naturforsch. 52 (1997) 877. M. Danysz et al., Nucl. Phys. 49 (1963) 121; D. Prowse et al., Phys. Rev. Lett. 17 (1966) 782; R.H. Dalitz et al., Proc. R. Soc. Lond. A 426 (1989) 1; S. Aoki et al., Prog. Th. Phys. 85 (1991) 1287; C.B. Dover, D.J. Millener, A. Gal, D.H. Davis, Phys. Rev. C 44 (1991) 1905. S. Aoki et al., Prog. Th. Phys. 85 (1991) 951. H. Takahashi et al., Phys. Rev. Lett. 87 (2001) 212501-1; A. Ichikawa, Ph.D. thesis, Kyoto University, 2001. J.K. Ahn et al., Phys. Rev. Lett. 87 (2001) 132504-1. T. Yamada, K. Ikeda, Nucl. Phys. A 585 (1995) 79c; T. Yamada, K. Ikeda, Phys. Rev. C 56 (1997) 3216; Y. Hirata, Y. Nara, A. Ohnishi, T. Harada, J. Randrup, Nucl. Phys. A 639 (1998) 389c. K. Tanida et al., Phys. Rev. Lett. 86 (2001) 1982. E. Hiyama et al., Phys. Rev. C 66 (2002) 024007; I.N. Filikhin, A. Gal, Phys. Rev. C 65 (2002) 041001(R); Phys. Rev. Lett. 89 (2002) 172502-1; C. Albertus, J.E. Amaro, J. Nieves, Phys. Rev. Lett. 89 (2002) 032501-1. Advanced Gamma Ray Tracking Array, see e.g. http:// www-gsi-vms.gsi.de/eb/html/agata.htm.
Future experiments on hypernuclei and hyperatoms Josef Pochodzalla Institut f€ur Kernphysik, Universit€at Mainz, Mainz, Germany
Abstract The possibility to produce double K nuclei in anti-proton nucleus collisions at anti-proton momenta close to the þ is explored. Combining a high-luminosity antiproton beam with a novel solid-state tracking system threshold for N N and a high-rate Ge-array, c-spectroscopy of KK-hypernuclei will become feasible at the PANDA experiment of the future International Accelerator Facility at GSI. Ó 2003 Elsevier B.V. All rights reserved. PACS: 21.80.+a; 36.10.Gv Keywords: Antiprotons; Hypernuclei; Hyperatoms
1. The physics case A hypernucleus contains one or more hyperons implanted within the nuclear medium. This impurity introduces a new quantum number, strangeness, into the atomic nucleus thus adding a third dimension to the nuclear chart [1]. On one hand the hyperon embedded in a nuclear system may serve as a probe for the nuclear structure and its possible modification due to the presence of the hyperon. On the other hand properties of hyperons may change dramatically if implanted inside a nucleus. Thus a nucleus may serve as a femto-laboratory offering a unique possibility to study basic properties of hyperons and strange exotic objects. An important goal of measuring the level spectra and decay properties of multi-strange hypernuclei is to test the energies and wave functions from microscopic structure models and to put
E-mail address: [email protected] (J. Pochodzalla).
constraints on baryon–baryon interaction models [2,3]. It is clear that a detailed and consistent understanding of the quark aspect of the baryon– baryon forces in the SU(3) space will not be possible as long as experimental information on the hyperon–hyperon channel is not at our disposal. Since direct scattering experiments between two hyperons are impractical, the spectroscopy of multi-strange hypernuclei provides a unique approach to explore the hyperon–hyperon interaction. Non-mesonic weak decays of hypernuclei and double hypernuclei [4] offer exceptional information on the four-fermion, strangeness changing, baryon–baryon weak interaction [5] and the interplay of the quark-exchange and meson-exchange aspects of the baryon–baryon forces. The possible existence of an S ¼ 2 six quark (uuddss) H-dibaryon [6] represents another challenging topic of KK hypernuclear physics. So far, however, the experimental searches for this lightest strangelet are inconclusive and even a strongly bound H can not be excluded [7]. In fact the short
0168-583X/$ - see front matter Ó 2003 Elsevier B.V. All rights reserved. doi:10.1016/j.nimb.2003.08.007
150
J. Pochodzalla / Nucl. Instr. and Meth. in Phys. Res. B 214 (2004) 149–152
time scale of the order of 1023 s available in a coalescence process may prevent the transition from a KK state to an H-particle in free space. Here, double K hypernuclei may serve as ÕbreederÕ for the H-particle: the long lifetime of the order of 1010 s of two Ks bound together in a nucleus may help to overcome a possible repulsive interaction at short distances. Since the mass of the H-particle might drop inside a nucleus [8] and because of hyperon mixing [9] it might be possible to observe traces of the H even if it is unbound in free space. Thus spectroscopy of S ¼ 2 hypernuclei can give important information on this fascinating and long sought object. Finally, hyperatoms created during the capture process of the hyperon will supply new information on fundamental properties of the hyperons. For example, the X is the only baryon whose static quadrupole moment can be determined experimentally [10]. Since only the one-gluon exchange is expected to contribute to the quadrupole moment [11] the X represents a unique benchmark for our understanding of the spin-dependent interaction between quarks in a ground-state hadron. At the high-energy antiproton storage ring HESR of the future International Accelerator Facility at GSI/Darmstadt the large number of produced X -atoms will make it possible to determine the hyperfine splitting in an X -atom and thus – for the first time – provide information on the static deformation of a baryon.
2. Status of KK hypernuclei Because of the low energy release of only 28 MeV in a conversion of a N and a proton into two Ks, attempts to produce KK-hypernuclei are generally based on the N capture reaction. The world supply of data on KK hypernuclei is very limited. Until recently only three candidates for double hypernuclei were observed via their double pion decay [12]. In addition, Aoki et al. presented evidence for the production and subsequent weak decay of three heavy double hypernuclei [13]. Only lately two additional events were completely identified by the KEK-E373 collaboration which collected 1000 stopped N [14]. In
the same year the AGS-E885/E906 collaboration published the first Ômass productionÕ of about 30 4 4 KKH events based on 10 stopped N [15]. While this first spectrometer experiment could not determine the binding energies of 4 KKH, the number of observed events signal that the production process of double hypernuclei is reasonable well understood (see e.g. [16]). But even the high accuracy of the emulsion data does not allow an unequivocal interpretation of the deduced KKbinding energies in terms of the strength of the KK interaction. These difficulties reflect various complications: (i) the dynamical change of the core nucleus [17], (ii) the non-zero spin of the core, (iii) KK NN coupling effects [9] and (iv) the possible production of the hypernuclei in c-unstable excited states or the emission of undetected neutrons [18]. Clearly a drawback common to all theoretical investigations is the lack of high resolution and systematic data on multi-hypernuclei and their level structure.
3. Future experiments on multi-strange systems Until the end of the last decade, the energy resolution of hypernuclear studies with electronic detectors was limited to typical values of about 1 MeV. Only recently with the advent of highefficiency Ge-arrays the door to high-resolution c-spectroscopy has been opened. These measurements will be complemented by electroproduction experiments of single hypernuclei both at JLab and MAMI-C. The dominant spin-flip amplitudes in these reactions favor the excitation of unnatural parity states in hypernuclei. Furthermore not only c-unstable but also particle-unstable states are accessible in these reactions with sub-MeV resolution. Combining Ge-arrays like AGATA [19] with the high-luminosity anti-proton storage ring HESR at GSI and with a novel solid-state micro tracker, high-resolution c-spectroscopy of double hypernuclei and X-atoms will become possible for the first time. In order to minimize the background from associated particles, the production of hypernuclei and hyperatoms at HESR will use NN and XX pair
J. Pochodzalla / Nucl. Instr. and Meth. in Phys. Res. B 214 (2004) 149–152
151
Fig. 1. Various steps of the proposed reaction in case of double hypernucleus production.
Fig. 2. Schematic view of the central detector components for hypernucleus and hyperatom experiments.
production close to threshold in anti-proton nucleus collisions (Fig. 1). The trigger will be based on the detection of a high-momentum anti-hyperons at small angles or of positive kaons produced by the anti-hyperons absorbed in the primary target nuclei. The 2Kþ trigger will provide significantly higher count rates but requires the detection of rather low momentum kaons of a few hundreds MeV/c. The second ingredient of the experiment is the deceleration of the N inside the nucleus and subsequent absorption in an secondary active target. The geometry of this secondary absorber is determined by the short mean life of the N of only 0.164 ns. If the separation between the primary target and the secondary absorber is to large, low momentum N will decay prior to full stopping. On the other hand, energetic N with momenta beyond approximately 500 MeV/c can not be stopped prior to their decay. This limits the required thickness of the active secondary target to about 30 mm. In order to track the stopped N and the charged fragments resulting from the decay of the produced hypernuclei, it is planned to sandwich the absorber with solid-state pixel or strip detectors. As a third ingredient an efficient germanium carray is required (Fig. 2). Here the main limitation
will be the load of associated particles. Fully digital electronics will allow a load of background events of more than 100 kHz for each detector element. Since most of the produced particles are emitted in the forward region not covered by the Ge-array an interaction rate of a few 107 s1 seems to be manageable. The experimental set up is sufficiently small to fit inside the calorimeter of the general-purpose detector PANDA. After removing the end cap of the calorimeter upstream of the target the detectors will be placed close to the straw chambers thus making use of the tracking capability of the main detector in forward direction. In order to trigger on low momentum kaons a thin start detector will be placed close to the target. The stop signal will be provided by scintillation or resistive plate counters, which replace or – if space permits – complement the RICH detector. All KK hypernuclei experiments performed to date have collected of the order of 10000 stopped N in total. With this scheme we will be able to reconstruct approximately 3000 stopped N with þ the unique N trigger per day at PANDA. By applying the kaon trigger we will exceed the number by up to two orders of magnitude thus providing us with a few 105 N per day. With such a event rate high-statistics c-spectroscopy of
152
J. Pochodzalla / Nucl. Instr. and Meth. in Phys. Res. B 214 (2004) 149–152
KK-hypernuclei will become feasible at the PANDA experiment of the future International Accelerator Facility at GSI.
[11] [12]
References [1] W. Greiner, Int. J. Mod. Phys. E 5 (1) (1995) 1. [2] C.B. Dover, A. Gal, Ann. Phys. 146 (1983) 309. [3] C. Nakamoto, Y. Suzuki, Y. Fujiwara, Kyoto University pre-print KUNS-1432, SCAN-9706007. [4] A. Parreno, A. Ramos, C. Bennhold, Phys. Rev. C 65 (2001) 015205-1. [5] E. Oset, A. Ramos, Prog. Part. Nucl. Phys. 41 (1998) 191. [6] R.L. Jaffe, Phys. Rev. Lett. 38 (1977) 195, 38 (1977) 617(E). [7] G.R. Farrar, G. Zaharijas, hep-ph/0303047 (2003). [8] T. Sakai et al., Prog. Theor. Phys. Suppl. 137 (2000) 121. [9] T. Yamada, C. Nakamoto, Phys. Rev. C 62 (2000) 0343191; K.S. Myint, S. Shinmura, Y. Akaishi, Eur. Phys. J. A 16 (2003) 21; I.R. Afnan, B.F. Gibson, Phys. Rev. C 67 (2003) 017001-1; I.N. Filikhin, A. Gal, V.M. Suslov, nucl-th/0303028 (2003). [10] R.M. Sternheimer, M. Goldhaber, Phys. Rev. A 8 (1973) 2207;
[13] [14] [15] [16]
[17] [18]
[19]
S.S. Gershtein, Yu.M. ZinovÕev, Yad. Fiz. 33 (1981) 1442; M.M. Giannini, M.I. Krivoruchenko, Phys. Lett. B 291 (1992) 329. A.J. Buchmann, Z. Naturforsch. 52 (1997) 877. M. Danysz et al., Nucl. Phys. 49 (1963) 121; D. Prowse et al., Phys. Rev. Lett. 17 (1966) 782; R.H. Dalitz et al., Proc. R. Soc. Lond. A 426 (1989) 1; S. Aoki et al., Prog. Th. Phys. 85 (1991) 1287; C.B. Dover, D.J. Millener, A. Gal, D.H. Davis, Phys. Rev. C 44 (1991) 1905. S. Aoki et al., Prog. Th. Phys. 85 (1991) 951. H. Takahashi et al., Phys. Rev. Lett. 87 (2001) 212501-1; A. Ichikawa, Ph.D. thesis, Kyoto University, 2001. J.K. Ahn et al., Phys. Rev. Lett. 87 (2001) 132504-1. T. Yamada, K. Ikeda, Nucl. Phys. A 585 (1995) 79c; T. Yamada, K. Ikeda, Phys. Rev. C 56 (1997) 3216; Y. Hirata, Y. Nara, A. Ohnishi, T. Harada, J. Randrup, Nucl. Phys. A 639 (1998) 389c. K. Tanida et al., Phys. Rev. Lett. 86 (2001) 1982. E. Hiyama et al., Phys. Rev. C 66 (2002) 024007; I.N. Filikhin, A. Gal, Phys. Rev. C 65 (2002) 041001(R); Phys. Rev. Lett. 89 (2002) 172502-1; C. Albertus, J.E. Amaro, J. Nieves, Phys. Rev. Lett. 89 (2002) 032501-1. Advanced Gamma Ray Tracking Array, see e.g. http:// www-gsi-vms.gsi.de/eb/html/agata.htm.