Hadron-nucleus bound-state spectroscopy

ELSEVIER

-

Nuclear Physics A721 (2003) 5Oc-57~

www.elsevier.comllocate/npe

Hadron-Nucleus

Bound-State

Spectroscopy

Toshimitsu Yamazaki” * “RI Beam Science Laboratory, RIKEN, Wako, Saitama-ken, 351-0198 Japan To investigate the behavior of hadrons in nuclear media we are developing new type of nuclear spectroscopy for hadron-nucleus bound states. The domain of deeply bound pionic states is well matured, and new precision experiments on the 1s n- states in “‘Pb and in Sn isotopes have indicated an evidence for partial restoration of chiral symmetry in a well-defined nuclear medium. In the l? sector we have predicted possible existence of deeply bound g states in few-body systems, which are characterized by dense nuclear sytems formed due to the strong attraction of the I = 0 l?N interaction. 1. Introduction

Chiral symmetry breaking is known to account for the large hadron masses despite the very small u- and d- quark masses [1,2]. Partial restoration of chiral symmetry is expected in nuclear media [2%4], but so far, there has been no clear evidence for this exciting scenario. So called invariant mass spectroscopy for investigating “in-medium” hadron masses of hadrons in unbound nuclear media has an inherent difficulty due to the collisional broadening and shift, as pointed out by us (51. Our alternative strategy is to investigate bound systems of hadrons; if observed, their binding energy and width will give a clear indication of the interaction of the hadron with nuclei, which can be used to examine how the elementary interaction is modified in the nuclear media. However, no discrete states are normally expected for bound hadrons because of their strong absorption by nuclei. A narrow bound state, if any, would require an unusual suppression mechanism. We discuss here two cases; i) Coulomb assisted bound states of n- and ii) deeply bound kaonic states. The former is by now well matured, while the latter is new. 2. Deeply 2.1.

Bound

Supprestiion

T- States

as an Indicator

of Chiral

Symmetry

Restoration

mechanism

Because of the repulsive nature of the s-wave TN interaction we expect that deeply bound states of r- (i.e., 1s states in heavy nuclei) are accommodated by the attractive Coulomb interaction and the repulsive strong interaction. Such halo like states have narrow widths, as pointed out by Friedman and Soff [6], and comprehensively studied by Toki et al. [7,8]. *supported by Grant-in-Aid

for Scientific Research of MonbuKagakusho

(Japan)

03759474/03/$ - see front matter 0 2003 Elsevier Science B.V. All rights reserved. doi:lO.l016/S0375-9474(03)01016-9

T Yamazaki/Nuclear Physics A721 (2003) 5Oc-57~

Exdmbn her BY WV) 145

4w) ___ t\

mPb(Ws x10

51c

mPb(d,aHe)

:

Figure 1. The Y- ‘07Pb potential and accom-

modated Is, 2s and 2p 7r- states. The repulsive strong interaction pushes the c outward, thus suppressing the nuclear absorp*ion.

2.2.

Figure 2. (Upper) The observed ‘08Pb (d,3He) reaction spectrum revealing the predicted deeply bound K states. (Lower) Mechanism to produce 7r- states from “inside”.

Production

The deeply bound states are located inside the “last orbitals” of pionic atoms, and thus, cannot be populated from outside. Toki et al. showed that they can be formed by some nuclear reactions (pion transfer reactions) [7,8]. E ventually, both the theoretical and experimental studies advanced. In particular, the (d,3He) reaction was considered to be an appropriate method, because its c production cross section is peaked at 300 MeV/u, where the momentum transfer is very small [9,10]. 2.3. The first success After years of struggling the first successful experiment was carried out at GSI by using a deuteron beam from the SIS18 synchrotron and the Fragment Separator (FRS) as a forward spectrometer. The ‘“sPb(d, “He) reaction spectrum revealed a discrete peak of dominant configuration (2p)?r(3p1,2,3,2);1 at excitation energy of around 135 MeV [ll].

I: Yamazaki/Nuclear Physics A721 (2003) 5Oc-57~

52c

Fig. 2 shows the observed spectrum in comparison with the known nuclear excitation spectrum (the giant resonances and A resonance). The (ls), state was also partially resolved [12-141. The analysis of the observed binding energy and width gave an experimental information on the 7r- mass shift in this heavy nucleus (Am,- M +26 MeV). The 1s K state was more clearly observed in the second GSI experiment, where 205Pb was used as a target [15]. The binding energies and widths of the 1s and 2p states in “‘Pb were determined to be Bi, = 6.762 f 0.061, IiS = 0.764?::$:, Bsp = 5.110 & 0.045 and I’zp = 0.321 f 0.061 MeV. These values indicate the pion mass shift in the interior of “‘Pb to be Am, N 27 MeV. 2.4.

Deduction

of the isovector

strength

The pion-nucleus interaction consists of s-wave (local) and p-wave (non-local) parts [16]. The s-wave potential is given by

where p(r) = pen)(r) + p(P)(r), Ap(r) = P(~)(T) - p(P)(r), ~1 = 1 + m,/M = 1.15 and ~2 = l+m,/2M = 1.07. The 1s binding energies of c in symmetric light nuclei were used to deduce a combined isoscalar strength: bg = bo + 0.215 ReBs = -0.0280 & 0.0010 m;‘. The isovector strength as a constant effective parameter was then obtained from the 205Pb data . b*1 = -0 116?“.015 0.017 m-?rl. The analysis procedure and result are described in a recent paper [17]. ’ 2.5.

Preferential

observation

of 1s T- states

in Sn isotopes

Deeply bound 1s states of 7r- in 115~11g~123Sn were preferentially observed using the Sn(d,3He) pion-transfer reaction under the recoil-free condition (Td = 500 MeV) [18], as shown in Fig. 3. The overall spectral shapes were found to be in good agreement with a theoretical prediction [19]. The 1s binding energies and widths were precisely determined by employing two independent calibration methods, and were used to deduce the isovector parameter of the s-wave pion-nucleus potential. For the neutron density distributions we used the systematics obtained in [20]. The deduced br values are summarized in Fig. 4. The averaged value is b; = -0.115 f 0.005 m;‘. This is consistent with the results of global fits of all the pionic atom data [21], but the present result is, in principle, more reliable because there is no ambiguity in the selected 1s states arising from the p-wave part of the potential. 2.6.

Evidence

for partial

restoration

of chiral

symmetry

It has been emphasized that the chiral order parameter can be deduced from the isovector part of the.s-wave TN interaction [22,23], since bl is related to the pion decay constant (fX) through the Tomozawa-Weinberg relation [24], which is connected to the quark condensate through the Cell-Mann-Oakes-Renner relation [25]. The observed enhancement of b; over the free TN value (-0.090 m;’ [ZS]) is related to a reduction of the chiral order parameter [27,28]: b:‘“” _ ma2 -f,” b;

= o 78 * o 03, .

(2)

53c

E Yamazaki/Nuclear Physics A721 (2003) 5Oc-57c

t --------_

0

360

365

NUCLEAR MEDIUM ~;~,~~ -------

--VACUUM. ----I--

370

%e Kinetic Energy [MeV)

Figure 3. 124,120?Sn(d,3He) reaction spectra Figure 4. Summary of the b; values (in measured at Td = 503.388MeV. The scales m;‘) deduced from the Bi, binding enertogether with the correof the c binding energies in 123,11g,115Sn are gies in 115,11g,123Sn (= fG(~,)~/fj) also indicated. The-skewed peak at the right- sponding scales in bp/b; hand side from the p(d, 3He)no reaction is and ape. The previous “‘Pb data reanused for an absolute calibration of the energy alyzed with ImBo(= 0.046 mi4) is shown for comparison. scale. Since the c as

probes the nuclear density of peg N 0.60~~[29], the above value is interpreted

"fxPo)2 M -&f' (al) f,"

= 0.65 f 0.05

(3)

at the normal nuclear density, p = pe. This value is very close to predicted values (- 0.65) [22,30,31]. In this way we have obtained an evidence for partial restoration of chiral symmetry from well-defined bound states in a well defined nuclear density. 3. l? Bound

States

3.l.

-^L--C:^I yv”~“u’u*

4,-:-l-: IxI-*U*I*

as Cold

and Dense

Nuclear

Systems

Very recently the possible presence of discrete nuclear bound states of l? in few-body nuclear systems was predicted [32-341. The I&ruckus interaction was derived from l?LN

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I: Yamazahi/Nuclear Physics A721 (2003) SOc-S7c

interactions which were constructed so as to account for the KN scattering lengths, the K-p atomic shift and the energy and width of A(1405). This phenomenological interaction is consistent with that obtained from chiral perturbation theory [35]. In these systems the strong attraction of the I = 0 KN interaction (KN’=‘) plays an important role. It accommodates a deeply bound state, while contracting the surrounding nucleus, thus producing an unusually dense nuclear system. Since the binding energies are so large that the main decay channel of the I = 0 KN to C + 7r is closed energetically (and additionally, the channel to A + x is forbidden by the isospin selection rule), these deeply bound states are expected to have small widths.

-400

Figure 5. Calculated KN and K-nucleus potentials and bound levels: A(1405), iH and ;H for K-p, K-pp and K-ppn systems, respectively. The nuclear contraction effect is taken into account. The shaded zones indicate the widths. The CT and AX emission thresholds are also shown.

The constructed K-nucleus potentials for the most fundamental cases are shown in Fig. 5. Narrow enough bound states are expected as shown. A 4He(stopped K-, n) process was considered to produce kH(T = 0), in which the ejected neutron is used as a spectator [33]. Its experimental feasibility has been discussed by Iwasaki et al. [36]. Another type of reactions, in-flight (K-, N), was discussed by Kishimoto [37].

T. Yamazaki/Nuclear Physics A721 (2003) SOc-57~

3.2.

(K-,

T-)

reactions

55C

revisited

It was recently shown [34] that the “traditional” (K--,X-) reaction (or equivahtly (fl+, K+) reaction) can be used to produce deeply bound K systems on proton-rich exotic nuclei, if their final state is tuned to the kaonic bound state region (far above the C formation). Some examples are shown in Table 1. The role of R(1405) and A( 1520) as door ways is discussed in [34].

Table 1 Light target nuclei and I? nuclei to be produced by (K-, 7r-) (and (r+,K+)), (K-,n) and (e,e’K+) reactions through A’ doorways, and calculated binding energies (BK in MeV) and widths (l?~ in MeV) with nuclear contraction. Ref. Target Reaction A* doorway K nucleus BK rK K--p (K-,X-) A* 27 40 n K--p (e,e’K+) A* 27 40 ;H = K-pp (K-, n-) A*p 48 61 : WH = K-ppn (e,e’K+) A*pn 108 20 ;:I 3He $H = K-ppn (K-,n) A*pn 108 20 4He RH = K-ppnn (e,e’K+) A*pnn 86 34 # 4He ;He = K-ppp 97 N 24 [38] 3He (K-3 r-) A*PP (K-, 7r-) A’ppn RHe = K-pppn 105 26 4He [381

3.3. Further

predictions

by the AMD

method

The exotic structure involving a l? has also been studied by the Antisymmetrized Molecular Dynamics method by Dote et al. [38]. This method can predict the density distributions of the constituent l?, protons and neutrons. An example is shown in Fig. 6. The two-a cluster structure of ‘Be is drastically contracted when a K- is bound, forming a high-density system. The protons are more contracted than the neutrons. 3.4.

Experimental

search

Currently, an experiment on 4He (stopped K-, n) to search for KPppn is under way at KEK [36] based on the theoretical investigation of this formation process [33]. An experimental search using in-flight (K-,n) reactions is proposed at BNL-AGS based on the proposal of Kishimoto [37]. A new possibility using (K-, n-) to populate exotic systems such as K-pp, K-ppp, etc., [34] will be pursued, and its first experimental trial is proposed at BNL-AGS [39]. 3.5.

Future

scope - a gateway

to strange

matter

In summary, we have emphasized a new domain of physics paradigm. Due to the very strong K--p attraction very deep discrete states of I? are expected. They are predicted to have binding energies, BK N 100 MeV. Whether or not the l?N interactions are modified in nuclear medium can be addressed. Since the predicted high-density systems are like11 to be in a quark phase, the l? bound states must provide a unique play ground for nuclear quark systems in well defined bound systems. It is natural to extend our consideration to

56c

T Yamazaki/Nuclear Physics A721 (2003) 50~57~

(c) Proton

(d) Neutron

(e) K-

Figure 6. Calculated density contours of sBeK-. Comparison of the density distributions of (a) usual sBe and (b) 8BeK- is shown in the size of 7 by 7 fm. Individual contributions of (c) proton, (d) neutron and (e) K- are given in the size of 4 by 4 fm.

multi-l? systems, where more dense systems are expected to be formed. We have already clarified that the most fundamental S = -2 systems, namely, ppK-Kand ppnK-K-, are strongly bound high-density systems [40]. This leads to an exciting possibility that dense strange nuclei can be formed without the aid of gravity. Thus, these systems appear to be extremely important as “precursors” to kaon condensation and strange matter formation. The author would like to thank many colleagues, especially, H. Toki, S. Hirenzaki, R.S. Hayano, P. Kienle, W. Weise, Y. Akaishi and A. Dote for their stimulating discussions and collaborations.

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