PRODUCTION AND IDENTIFICATION OF THE SUPERHEAVY HYDROGEN HYPERNUCLEUS ~H

1 2 1 1 1 3 3 L. Majling • t, Yu. Batusov , J. Lukstins , A. Parfenov , M. Solar and B. Sopko

(1) Joint Institute for Nuclear Research, Dubna, Russia (2) Institute, GAS, Rei, Czech Republic (3) Czech Technical University, Prague, Czech Republic t E-mail: [email protected]

Abstract The hypernucleus ~H, existence of which has been suggested recently, could be identified unambiguously in experiments with light relativistic hypernuclei prepared for Nuclotron. Key-words: relativistic hypernuclei, exotic nuclei

1. Introduction. Nuclei with a Halo

In the last twenty years a new branch of nuclear physics, namely physics of nuclei in the vicinity of the neutron drip line has been constituted [l]. The dripline is the limit of the nuclear landscape, where additional can no longer be kept in the nucleus. This dripline has been really reached for light nuclei (Z :::; 6), see Fig. 1. One can see irregularities at the border of neutron stability. There is considerable interest in unbound nuclear systems close to the driplines, both in themselves and as subsystem of Borromean halo nuclei. The term Borromean was coined in ref. [2] to denote a bound three-body system (core + n + n) for which no binary subsystem is bound. In Fig. 1, the /3-stable isotopes are marked by ( E9) and the type of neutron halo is also given. The short range of the nuclear force and the low separation energy of the valence nucleons results, in some cases, in considerable tunneling into the classical forbidden region and more or less pronounced halo may be formed. As a result the valence and the core subsystems are to a large extent separable. Therefore, halo nuclei may be viewed as an inert core surrounded by a low density halo of valence nucleons and described in few-body or cluster models. There have been used different experiments which have contributed to the present picture of nuclei with neutron halo. The development of techniques for the production of exotic radioactive nuclei and making beams of them, has been of key importance for the progress of the field. The ground-state properties such as mass, and moments are mainly measured with stopped low-energy beams at ISOL facilities. The energetic radioactive beams obtained with the in-flight technique are the main tool for studies of nuclei at the driplines. In this contribution, we discuss the potential of hypernuclear physics.

282 2. Stabilizing role of the J\

It is well known that A hyperon makes the nuclear system more stable. In the lower part of Fig. 1, the solid line determines hyperfragments observed in emulsion [3]. The stabilizing influence of the A hyperon is obvious: the 'lake of instability' (8 Be, 9 B) is filled as well as irregularities at the border (5He, 7He ). Large scale systematic studies of hypernuclei began with the advent of separated K- beams, which permitted the use of counter technique and confirmed the brilliant suggestion of Podgoretsky [4]: instead of hunting down decays of random fragments, to study hypernuclear production in exchange reaction Az (K-, 7r-) ~Z. In one-step direct reactions such as (K-, 7r-) or (7r+, K+) [5, 6] and (K-, 7r 0 ) or (e, e' K+) [7, 8] the level structure ofhypernuclei can be experimentally studied and information on structure can be obtained.

19 nuclei ~EJ~l 12¥ll 13¥IEJf~EJEJI isc 11 1? 112°c1 13 172 I 8~P I I 10~ll 11~IEJI B IE]~ I ~ I ~ I 7Bel 19~1110Be1111~e1112Bel 114~el 6 7 1 ~ 11 ~ l~l 9Li I I 112~i1 f3HJ f4HJ f6Hel f8Hel LJ~ L:::J ~ [l]Li]EJ C!iJ hypernuclei Notation Production 4H 6H 7H • n -t A (K-,n-) or (n+,K+) ~H Ao A* A* o p-t A (K-, n°) or (e, e' K+) 4 He 5 He 6 He 7 He 9 He * pp-t nA (K-,n+) or (n-,K+) A• A• Ao Ao ~He A *

~Li X~i ix Li llLi ~~i * * ~Be 9 Be 10 Be 11 Be 12Be 13Be A • A 0 A 0 A* A* JOB 11 B 11B L5B XB Ae A• A* A* 12 0 isc 16c A• Ao A*

Figure 1: N /Z diagram of light nuclei and hypernuclei. Hypernuclei are shown together with their production reaction. See text for detail

Recently, Tamura solved the huge technical problems and constructed "Hyperball", a large acceptance Ge detector array dedicated to hypernuclear 1-ray spectroscopy [9]. An impressive result has been obtained: the observed 1-rays from ~Li hypernucleus indicate a significant (:::::: 20 %) contraction of the 6 Li a+ d) core in ~Li. So, HYPERNU-

283 CLEAR PHYSICS COULD BE USED IN THE STUDIES OF THE LOOSELY BOUND NUCLEAR SYSTEMS such are nuclei with neutron halo. However, there is only one hypernucleus with neutron halo, 1He, which can be produced in one-step direct reaction, p-> A. The announcement of the plan for the new hypernuclear facility FINUDA at the ¢­ factory DANE (Frascati) [11], initiated great expectations. It opens unprecedented possibilities: very low energy of from the ¢ decay allow the use of very thin solid stopping targets. The detector is designed to register both the JT- from the formation re­ action and charge products from the hypernuclear decay. We suggested [12] to study there STRANGENESS AND DOUBLE CHARGE EXCHANGE (S&DCX) reaction (K-,7r+), which opens way to the production of neutron-rich hypernuclei. There are two paths how to arrive at the An p-2 states: either by the charge exchange: K- p-> A7r0 , JTO p-> nJT+ or through the L,N -> AN conversion: K- P __, JT+ L,-, L,-p-> An. Hypernuclei produced in such a reaction are marked by * in Fig. 1. As we can see no A hypernucleus produced in S&DCX reaction has been observed until now. Unfortunately, both have to be in the same target nucleus, so cross sections are inevitably low. Nevertheless, the search for neutron-rich A hypernuclei is one of the items on the FINUDA's list [13]. The current status of experimental efforts to produce neutron-rich A hypernuclei is given in Table 1. We have not only upper limits for reactions with stopped kaons, [14], [15], but even first positive results [16] for another type of S&DCX reaction, namely (7r-, K+). In the experiment KEK-PS-E521, about 40 events were registered in the bound region of 1gu. The results confirmed qualitatively the calculations given by Tretyakova and Lanskoy [17, 18, 19]: The cross sections of the (7r-, K+) reaction are smaller by three orders of magnitude than those of (JT+, K+) reaction; the two-step mechanism dominates over the single-step mechanism [17] (via virtual ;:,- admixture in the hypernuclear state appearing due to ;:,- p +-+ An coupling [20]); the cross section is larger for targets with neutron orbit vacancy. Table 1: Status of the hypernuclei production in S&DCX reactions

Reaction Ref. ~H lH ~He igu 1KBe ix_c (K:;t, JT+) [14] (KEK) < 23 < 6.1 < 6.2 [15] (FINUDA) < 3.5 < 4.9 < 2.6 (7r-,K+) [16] (KEK) 12±2 7 [19] (calculation) 22 2.5

The results of a recent experiment [21], revived our interest in lightest neutron-rich hypernuclei. There, the primary beam of the radioactive ion 6 He was used for the pro­ duction of the resonance state in knock-out reaction 6 He (1 H, 2He) 5 H. The two protons from the decay of 2 He from the reaction were detected as a peak at an energy 1.7 MeV above the 3 H+n+n threshold. In the next section we will show that ~H [22], can be identified unambiguously through its pionic decay ~H -> JT- + 6 He.

284 3. Experiment

At the LHE JINR, an original approach of hypernuclear experiments was elaborated on the basis of beam nuclei excitation to produce high energy hypernuclei (up to 3.5 A·GeV the experiments, carried out at the Synchrophasotron and 6 A-GeV planned at the Nu­ clotron). Data on the hypernuclear lifetimes and the production cross sections were ob­ tained [23] using the streamer chamber in the Synchrophasotron beams. The Nuclotron beams offer new possibilities of carrying out hypernuclear experiments under condition of the significantly increased data collection rate. It was suggested [24, 25, 26] to investigate the properties of the lightest hypernuclei by using the SPHERE spectrometer. We hope that in a short period the spectrometer based on the proportional chamber trackers will be commissioned. In addition, the extracted Nuclotron beam energy is expected to be increased soon up to 4-6 A-GeV - the value necessary for hypernuclear experiments. In this context we see that the hypernuclear research program for the first year could be extended. The measurement of the binding energy of the lightest ~H, ~He hypernuclei and the lifetimes of light hypernuclei are planned among the first, with the ultimate task [27] of the scheduled experiments to investigate the AN matrix in nonmesonic decays of \0 B. Recently, it was proposed to search for the neutron halo hypernucleus ~Hin the very beginning of the research program. The suggestion is based on physical interest and the advantages of the experimental approach.

PC, PC,

PC, \

8 He

M

Figure 2: SPHERE spectrometer adapted for the first hypernuclear experiments, for example, for AH production (particularly, ~H) in the 7Li beam. The stripping proton 7 from Li->~H +pis not shown. T - target; S, C1,2 - trigger counters; V - vacuum decay volume; M - magnet; PC1_ 4 - proportional chambers

Let us note some advantages of the research method and analyze how they can be achieved at the Nuclotron experiments. Since hypernuclei (hyperfragments) in the exper­ iments are produced by the excitation of high energy nuclei (up to 6 A-GeV) the energy of the produced hypernuclei is just a little bit lower than that of an incident beam. In

285 such a case, hypernuclei decay at tens cm beyond the target. For example, the mean hypernuclei decay range should be about 40-45 cm. For experiments at the SPHERE spectrometer, it was suggested to observe hypernuclear decays in a vacuumed volume of 60 cm (approximately 70% of hypernuclei will decay inside of this volume). If the decay vertex was fixed inside of 60 cm path there is no doubt that the event was the decay of a hypernucleus since no background process can produce the vertex in vacuum. In other words one can obtain a pure sample of hypernuclei decays. HYPERNUCLEI CAN BE IDENTIFIED UNAMBIGUOUSLY. To be sure that the decay vertex was inside vacuum one must use an adequate tracker to measure the direction of secondary particles (to calculate the vertex coordinates). On the other hand, if an incident nuclear beam is not 3 He, different hypernuclei and isotopes can be produced. While the charge of hypernuclei is measured with the trigger counters, isotopes can be identified by the daughter nuclei momentum measurement. Since the momenta of positive decay products should also be measured, a tracker should be installed beyond the analyzing magnet. Estimates show that the expected accuracy of the measurement of the vertex position and the daughter nuclei momenta is good enough to obtain sample of the safely identified hypernuclei in our case of the proposed spectrometer and hypernuclear experiments.

IMomenta of daughter nuclei I IEntries soooo I !! 2500 p--'====;,,;;;;;;;;:;;;;;;;,;;;;;;;;d__ _'::======:!.._~ c: ~ Q) 3He 4He 0... 2000 1l E 6 :l He c: 1500

22 24 26

Figure 3: Momenta distributions of recoil nuclei 3 He, 4 He and 6 He

It is suggested to start the hypernuclear research program with the investigation of 1H and i H production in the helium beam. Such an experiment is similar to the previous one and allows one to check all systems of the set up (including data processing). At the NEXT STEP heliam beam shoald be changed to the 7 Li beam to search for ~H together with AH and i H. The number of hypernuclear decays (N) expected to be observed in 24 hours is given in Table 2. The trigger in both experiments is tuned to detect charge

equal to one of hydrogen hypernuclei (regardless to the mass value) in the counters C1 (see 7 Fig. 2) and the charge equal to two (counters C2 ). To be more precise, in case of the Li beam an additional trigger fine-tuning should be applied to take into account stripping

286 Table 2: Production and decay

beam target production decay N 3 3J-Ie + 12c -> ~H + ... -> He + n- 100 4 He + 12c -> 1H + ··· -> 4He + n- 600 -> ~H + ... -> 3J-Ie + n- 7Li + 12c -> ~H + ... -> 6J-Ie + n- 400 -> 1H + ... -> 4He + n- -> ~H + ... -> 3.ffo + 7!"-

proton (fragment) of charge one but this does not change the approach significantly. To discriminate the masses of the isotopes of the hypernuclear daughter nuclei one should measure the corresponding momenta. The momentum values of 3He, 4He and 6 He are concentrated in the::::; 14 GeV /c, :::::: 19 GeV /c and::::; 29 GeV /c bands correspondingly. Such difference can be measured easily to separate three possible reactions of the hydrogen hypernuclei production and decay in the 7 Li beam, see Fig. 3.

0

Figure 4: The 80 x 120 cm proportional chambers are large enough to register from the hypernuclear decays. The calculation for the chambers situated at the 270 cm distance beyond the target. All the pions hit the chamber approximately inside a ring of 60 cm diameter. A quadrant of the chamber is presented in the histogram

In the first experiments dedicated to the observation of pionic decays of the lightest hy­ pernuclei, proportional chambers of a 2 mm wire separation will be used. 38 x 38 cm cham­ bers (PCi) will be installed just behind (at a distance of 20 cm) the vacuumed decay vol-

287 ume V (see Fig. 2) while at the distance of 220 cm the second set of larger (80x 120 cm) chambers will operate to fix the second point of the decay products trajectory. The size of these chambers were chosen large enough to register all the decay pious emitted at more wide angles than the daughter nuclei (see Fig. 4). Beyond the analyzing magnet (M) 2 meter chambers previously used in the EXCHARM experiments are mounted. In spite of that the chambers are collected from different experiments, they are adjusted to the electronics of the same type. So, one can use the same readout software, gas refilling system etc. for all the chambers. Of course, the properties of all the chambers are also very similar. For example, the resolution of 38 cm chambers is estimated to be of the same order as it was measured (0.7-0.8 mm) for the 2 m chambers [30]. Naturally, it was much easier to tune the 38 cm chambers and to obtain 993 efficiency in comparison with ~ 953 for the 2 m chambers.

4. What can we learn from this experiment ?

A serious obstacle in exploiting the beam of protons [31, 32] or relativistic light nuclei [33] for hypernuclear studies is a huge background in production vertex, consequently, one have to identify hyperfragments by they weak decay modes. Here, we confine ourselves to the study of the production of hyperhydrogen isotopes ~H (A= 3, 4 and 6) only, which decay through pionic channel. As one can see in Fig. 3, different isotopes could be separated unambiguously. This way, we could determine the 'source' of neutron-rich hypernucleus ~H. The nuclei of our beam (7Li) interacting with a target are also fragmented (6 Li + n, 6 He + p, 4He + 3H), so the outgoing beam is rather complicated mixture of primary and secondary hypernuclei given in Table 3.

Table 3: Hypernuclei: primary (pr) and secondary (sc)

nuclei: I 1Li I 6 Li 6 He 4He 3H 4 hypernuclei lLi lHe ~Li ~He ~H A He j,H ~H

lLi pr lHe pr ~He SC SC pr 6H A [ill pr ~He SC SC SC SC j,He SC pr '.!,H SC SC SC SC SC pr ~H SC SC SC pr

The Fig. 5 shows clearly why the highly excited states of lHe, in which the "INNER PROTON" is substituted by A (transition Ps --+ As) are the source of hyperfragments j,H and ~H. The thresholds for these decay channels are rather high, but large changes in the structure of these states prevents the neutrons or A from emission.

288 •neutron oproton ••••o* I ~HI +p Eth= 22 MeV *A Ps---+ As §~ eeo* AH +3H Eth= 15 MeV 06:: 0 •• 6 Pp---+ Ap 0 ••oo He +A Eth= 5.2 MeV eeoo* lHe + nn Eth= 3.1 MeV *oo•••• Pp---+ As 7Li ( / ,K+) ~He* hyperfragments

Figure 5: Different decay channels of lHe hypernucleus

We recall that states of such type (sA s-1) have been recognized in the 'in-flight' ( K-, 7f-) reactions [34]. The structure of the ~Li spectrum was described successfully 3 [35] under assumption that resonances of type I SJ\ s pk > preserve their individuality (see also [36]). The thresholds and global structure characteristics, Young schemes [f;], are displayed in Table 4.

Table 4: Decay modes

~z-+ fZ' Et1ires [!1] · [fz] ~z-+ fZ' Eth res [Ji]. [h] lHe-+ ~He+ n 2.82 [41] · [1] ~He-+ lHe + n 0.17 [4] · [1] lHe + nu 3.08 [4] · [2] iH +2H 18.93 [3] · [2] iH +3H 15.49 [3] · [3] xH +3H 20.84 [2] · [3] l~HI +iH 22.0 [32] · [1]

lLi-+ lHe + 2H 3.12 [4] · [2] ~Li-+ lHe + 1H - 0.59 [4] · [1] 1 2 ~He+ H 5.99 [41] · [1] AHe + H 18.50 [3] · [2] XH + 4He 6.92 [2] · [4] iH + 2He 20.31 [3] · [2] iH + 3He 19.33 [3] · [3] xH + 3 He 20.76 [2] · [3]

The appearance of the exotic hyperfragment ~H together with determining the yields ratios Y(~(H) : Y(A(H) : YG(H) could shed some light to the hypernuclear pro­ duction in relativistic nuclei collisions.

The work of L.M. was spported by grant 202/02/0930 of the Grant Agency of Czech Republic.

289 References

[1] B. Jonson, Phys. Reports 389, 1 (2004). [2] M.V. Zhukov et al., Phys. Reports 231, 154 (1993). [3] D.H. Davis, J. Pniewski, Contemp. Phys. 27, 91 (1986). [4] M.I. Podgoretsky, Zh. Eksp. Teor. Fiz. 44, 695 (1963). [5) R.E. Chrien, C.B. Dover, Ann. Rev. Nucl. Part. Sci. 39, 113 (1989). [6) T. Hagesawa et al., Phys. Rev. C53, 1210 (1996). [7) T. Miyoshi et al., Phys. Rev. Lett. 90, 232502 (2003). [8] M.W. Ahmed et al., Phys. Rev. C68, 064004 (2003). [9] H. Tamura et al., Phys. Rev. Lett. 84, 5963 (2000). [10] K. Tanida et al., Phys. Rev. Lett. 86, 1982 (2001). [11] T. Bressani, in Proc. Workshop on physics and Detectors for DAiI>NE, F':rascati, 1991. [12] L. Majling, Nucl. Phys. A585, 2llc (1995). [13] T. Bressani, Proc. Int. School "E. Fermi", Course 153, p. 323 (2003). [14) K Kubota et al., Nucl. Phys. A602, 327 (1996). [15) A. Filippi (FINUDA), Proc. 19th Few-Body Conference, Gronningen, August 2004. [16) P. K. Saha (KEK-PS-E521), Proc. HYP2003, Nucl. Phys., to be publ. [17) T. Tretyakova, D. Lanskoy, Nucl. Phys. A691, 5lc (2001). [18) T. Tretyakova, D. Lanskoy, Phys. of At. Nuclei 66, 1651 (2003). [19) D.E. Lanskoy, arXiv: nucl-th/0411004. [20) Y. Akaishi et al., Phys. Rev. Lett. 84, 3539 (2000). [21) A. Korsheninnikov et al., Phys. Rev. Lett. 87, 092502 (2001). [22) K. S. Myint, Y. Akaishi, Prog. Theor. Phys. Suppl. 146, 599 (2002). [23) A. Abdurakhimov et al., N. Cim. 102A, 645 (1989); S. Avramenko et al., Nucl. Phys. A547, 95c (1992). [24] S.A. Avramenko et al., JINR Rapid Comm., 5[68), 14 (1994). [25) S.A. Avramenko et al., Nucl. Phys. A 585, 9lc (1995). [26] J. Lukstins, Nucl. Phys. A691, 49lc (2001). [27] L. Majling et al., Czech. J. Phys. 53, 667 (2003). [28] M.V. Evlanov et al., Nucl. Phys. A632, 624 (1998). [29] L. Majling et al., Mesons and Light Nuclei, 8th Conf., Prague, 2001, AIP Conf. Proc. 603, p. 453, New York, 2001. [30] A.N. Aleev et al., Instr. and Exp. Techniques, 38, 425 (1995). [31) E. Hungerford, Proc. Int. School "E. Fermi", Course 158, (2004). [32) W. Cassing et al., Eur. Phys. J. A16, 549 (2003). [33) H. Bando, T. Motoba, J. Zofka, Int. J. Mod. Phys. A5, 4198 (1990). [34) R. Bertini et al., Nucl. Phys. A368, 365 (1981). [35] L. Majling et al., Phys. Lett. 92B, 256 (1980). [36) L. Majling, Nucl. Phys. A639, 134c (1998).

290