Experiments on Σ− hypernuclei with in flight kaons

Nuclear Physics A450 (1986) 157c-164c North-HoUand,Amsterdam

157c

EXPERIMENTS ON s- HYPERNUCLEIWITH IN FLIGHT KAONSt E.V. HUNGERFORD Department of Physics, University of Houston, Houston, Texas 77004 This b r i e f discussion w i l l present some new data on s hypernuclei taken at Brookhaven National Laboratory's Alternating Gradient Synchroton (AGS) using the hypernuclear spectrometer. The l i s t of participants in this work is given in Reference I° While analysis of the data is not f u l l y completed, i t is s u f f i c i e n t to draw several interesting conclusions.

1.

INTRODUCTION

New ~ hypernuclear spectra ~2) were acquired at BNL using the BNL hypernuclear spectrometer and the LESBI beamline.(3) The beamline provides a separated kaon beam of 8 x 104 K - / s p i l l average i n t e n s i t y with a ~/K r a t i o of approximately 8/1.

Incident kaons are i d e n t i f i e d by a v e l o c i t y

selective Cerenkov detector and by t i m e - o f - f l i g h t .

Reaction pions are

analyzed by the hypernuclear spectrometer which has an angular acceptance of 15 msr, 3 MeV resolution for 600 MeV/c p a r t i c l e s , and can accept data at angles from 0 to 30 degrees, Sigma particles are formed near the nuclear binding threshold from nuclear targets and thus w i l l have low v e l o c i t y i f emitted from the nucleus.

Thus approximately half of these s's suffer

atomic capture and eventually nuclear a n n i h i l a t i o n with the emission of pions, (4)

Therefore, the effectiveness of a decay tag to suppress or enhance

regions of the missing mass spectrum and/or hypernuclear levels is not known ~ p r i o r i , although we have instrumented a decay detector surrounding the target to tag the data by particles emitted from the hypernuclear decay.

Table I gives possible decay modes for the sN conversion,

Of

course how this conversion process is influenced by the nuclear medium in which i t occurs is not known. The decay detector consists of a veto s c i n t i l l a t o r and an array of lead s c i n t i l l a t o r shower counters, five radiation lengths in thickness. Approximately .55 cm of Pb is placed between the veto detector and the shower counter array, so that cfiarged pions of momentum less than 90 MeV/c

tSupported in part by US DOE under Contract DE-ASO3-76ERO 0375-9474/86/$03.50 © Elsevier Science Publishers B.V. (North-Holland Physics Publishing Division)

E.V. Hungerford / ~-hypernuclei with in flight kaons

158c

Table I .

Conversion Decay Modes for the

o

~:-p+ nA~

nnyy

nns

36%

pTw i l l not penetrate into the shower counters.

64%

However, i t is possible for

energetic s decay pions to completely pass through the decay detector system.

Pions emitted in the weak decay of the s are more energetic than

those from the decay of a A. tion data with three tags.

Therefore we have analyzed the (K-,7 +) reacThese are represented by:

l ) an energetic

neutral, presumably a y shower from a T° in the decay detector;

2) a

charged p a r t i c l e with a momentumcorresponding to a pion of less than 90 MeV/c, presumably a pion or nucleon from A decay; and 3) a charged p a r t i c l e with large energy deposition in the shower detector, presumably a ~- from the weak decay of the ~.

We have spectra tagged by processes 1, 1+2, 3,

and 1+2+3. Thus, a l l spectra as shown require the detection of an associated reaction p a r t i c l e in the decay detector.

2.

BACKGROUND F i r s t evidence for narrow ~ structure was presented by the CERN group

in 1979.(5)

Subsequently, the BNL group published results on 6Li and 160

targets. (6)

We w i l l return to these spectra l a t e r , however, the 160

spectra showed no resolved levels and i t was not possible to extract a spin o r b i t parameter. The CERN program embarked on the construction of a low momentumseparated kaon beam l i n e and the measurement of s hypernuclear spectra in C and 0 targets at momentum transfers less than 100 MeV/c. The resultant data were interpreted as giving a s spin o r b i t strength of 12 MeV, about twice that of the nucleon.(7) The stopped kaon reaction mechanism for the formation of ~ hypernuclei was recently explored by a group at KEK(8). They also implemented a 7° decay tag which suggested interesting effects on the hypernuclear missing mass spectrum.

These r~sults are shown in Fig. 1 with and without the 7° ~p3/2) tag. This group interpreted the peak at MH-MA = 277 as the (pp3/2substitionatal state and that at MH-MA = 282 as the _.(ppl/2-,spl/2) tran-

E. IL Hungerford / 7.-hypernuclei with in flight kaons sition.

159c

The third enhancement in their data was not discussed. Fromthis

assignment they reported a spin orbit strength of 5 MeV.{8)

MH- MA (MeV) 2~o

,

z.To

.

zs,o

,

2~o

,

3po

,

3!o

,

~Fo

MH-MA(MeV)

,

260

iOO

2~

280

210

300

510

:lZO

El* SPECTRUM STOPPED K'IN (CH)n

80

~l{ll[O BY II 0 Or)

SO

7

40

20

- 16

"

b

"

Ib

zb

'

~o

'

~

"

5~

B~(MeV)

BZ (MeV)

Figure 1 Spectra obtained at KEK using stopped K" with and without a decay tag This past year the BNL group mounted an experiment(2) using scint i l l a t o r , graphite, and 7Li targets measuring the missing mass spectra from the (K-,~+) reaction in the region of ~ hypernuclear masses at angles of 00 0 (effective spectrometer angle of ~4) and 12 . The momentumtransfer for the reaction is illustrated in Fig. 2.

One sees that the momentumtransfer

for the CERNexperiment is ~75 MeV, while that for the BNL experiment is ~130-150 MeV/c, and the KEK momentumtransfer is ~170 MeV/c. In the BNL experiment the C target was chosen to amplify the KEK spectra and to look in the appropriate mass region for a s ground state. The 7Li target was chosen in an attempt to observe the splitting between the 3/2" to 1/2" states which is predicted to be proportional to the spin orbit strength.(9) The appearance of narrow z structure in the continuum, perhaps narrower than the corresponding A structure (lO), is mystifying.

The CERNdata at

low momentumtransfer can possibly be explained by quasi-free processes alone. ( l l )

E. V. Hungerford / ~-hypernuclei with in flight kaons

160c

M.-MA(MeV) 260

270

280

290

300

310

320

I~C(GRAF~HITE)

O°

70

60

g I

,

I00

200

300

400

500

600

700

-I0

0

I0

2O

3O

40

Figure 2

Figure 3

Momentum t r a n s f e r f o r the p ( K - , ~ + ) z r e a c t i o n . The lower curve is f o r r e a c t i o n angle o f 0 °. The upper curve is f o r a r e a c t i o n angle o f I00. Lines are drawn to help e x t r o p o l a t e the momentum t r a n s f e r a t Pk = O to Pk = 715 MeV/c

BNL z- hypernuclear spectrum f o r a g r a p h i t e t a r g e t summed over a l l decay tags.

However, the s t r u c t u r e in the 6Li data and the KEK data are too narrow to be e x p l a i n e d in t h i s way. tions of this effect,

While t h e r e are a number o f t h e o r e t i c a l

none o f them are u n i v e r s a l l y accepted.

explana-

We have pro-

posed t h a t a study o f the 3H + E- n u c l e a r system might be bound and narrow due to a s e l e c t i v i t y results.

in the conversion mechanism and y i e l d i n t e r e s t i n g

These s t u d i e s a w a i t f u t u r e beam time.

However, the KEK data show

t h a t narrow s t r u c t u r e is a p p a r e n t l y more common than o r i g i n a l l y

believed,

so hope e x i s t s t h a t one may e x t r a c t the z-nucleus spin o r b i t s t r e n g t h by observation of nuclear level structure, l i e above p a r t i c l e

50

Bz (MeV)

P~(MeV/c)

threshold.

although the s t a t e s are expected to

E. I/i. Hungerford / ~-hypernuclei with in flight kaons 3.

161 c

DATA I will

discuss spectra from 12C(K-,T+)1~ Be f i r s t .

The KEK data are

shown in F i g . I . T h e i r t a r g e t was CH so a strong x- peak from H i s present in the s p e c t r a .

I will

r e f e r t o the t h r e e peaks in t h e i r

MA : 277, 282, and 285 MeV.

spectra at MH -

Comparison of these spectra w i t h and w i t h o u t

the T° tag h i n t t h a t the 285 MeV peak is suppressed by T° t a g g i n g . data have peak widths of a p p r o x i m a t e l y 2 MeV. t a r g e t at 0° are shown in F i g . 3.

These data are taken w i t h a spectrometer

r e s o l u t i o n of 3 MeV and binned in I MeV i n t e r v a l s . see the same 3 peaks.

Within s t a t i s t i c s

There are some d i f f e r e n c e s however.

have an a d d i t i o n a l peak 289. problem.

This peak i s o u t s i d e s t a t i s t i a l

fluctations

There is no evidence f o r narrow bound s t a t e s .

may observe an excess of counts near BX = O. 0 is clearly a fluctuation

we

We appear t o

but could be due t o binning e r r o r s , although we have c a r e f u l l y this

The KEK

The BNL data from a g r a p h i t e

checked f o r However, one

The one channel peak at BS =

since our r e s o l u t i o n is 3 channels FWHM.

However, the averaged data appear t o have a shoulder in t h e BX : 0 region.

260

M. - MA ( MeV )

2'70

280

290

300

~I0

320

260

~o

zo

30

40

290

300

310

'2C

320 12°

NEUTRAL 4

0

280

sO

~'C(G~DHITE) 0° NEUTRAL LOWENERGYCHARGI[O

IO

MH- MA ( MeV )

2}0

50

B~ ( M e V )

Figure 4 hypernuclear spectrum f o r a g r a p h i t e t a r g e t w i t h the decay tag cut f o r a T 0 at 00.

0

-

t'o

C

0

~

~

~'o ~o 3b BI~ ( MeV )

4o

~o

Figure 5 ~- hypernuclear spectrum a t 12 o w i t h the n e u t r a l tag o n l y .

E. V. Hungerford / ~-hypernuclei with in flight kaons

162c

Data were taken w i t h the same t a r g e t at 12 o (Fig. 5) and the same cuts were a p p l i e d .

We can compare the data at 00 and 120 (Fig. 4 and Fig. 5).

I f we consider the n e u t r a l tagged e v e n t s , the I s t peak is suppressed at 12 o .

This should happen i f

it

is e x c i t e d by a &L = 0 t r a n s i t i o n .

peak at 289 is e i t h e r not present or has moved to h i g h e r BE. bably is not associated w i t h a E t r a n s i t i o n

in carbon i f

Suppose we now compare the data w i t h the KEK data.

The

Thus i t

in f a c t i t

pro-

is r e a l .

Recall t h a t the

momentum t r a n s f e r f o r the stopped r e a c t i o n is about 170 MeV/c w h i l e the data at 12 o is about 150 MeV/c. data.

The I s t peak is not suppressed in the KEK

The capture is not from an S o r b i t a l

f e r is not AL = O.

so the angular momentum t r a n s -

This c l e a r l y shows an advantage o f the i n f l i g h t

experiments, which a l l o w s v a r i a t i o n o f the angular momentum t r a n s f e r . me now t u r n to the data on 7Li a t 120 (Fig. 6). pared to our e a r l i e r

Let

This spectrum can be com-

spectrum using a 6Li t a r g e t (Fig. 7).

was chosen because one expects to see the s p l i t t i n g

The 7Li t a r g e t

o f two peaks correspond-

ing to the 3/2- and I / 2 - s t a t e s coupled to a 6He core.

This s p l i t t i n g

is

p r o p o r t i o n a l to the spin o r b i t s t r e n g t h (Fig. 8) and almost independent o f the r e s i d u a l i n t e r a c t i o n . MH - MA = 277 and 283. spin o r b i t

The data perhaps i n d i c a t e two enhancements at I f these r e p r e s e n t the s t a t e s in question then the

parameter would be about 5-6 MeV. However, much more c a r e f u l

data a n a l y s i s is needed before a f i n a l

r e s u l t can be quoted.

There is p o s s i b l y a peak at MH - MA = 288.

In comparison to the 6Li

data taken a t BNL e a r l i e r (6) ( F i g . 7) one might expect a ( S - I , s ) this value.

several core s t a t e s which may be e v i d e n t in width o f t h i s peak. interesting

peak near

The peak in 7Li would be expected to be fragmented over to note the s i m i l a r i t y

I t is also

o f the data s t r u c t u r e f o r the t h r e e

t a r g e t s 6 L i , 7 L i , and 12C.

4.

CONCLUSIONS While the data a n a l y s i s is f a r from complete we may s t i l l

conclusions from the data. a t the same MH - MA v a l u e s .

draw several

Comparison o f the BNL and KEK data show 3 peaks This s t r u c t u r e p e r s i s t s a t 12 o although the

peak at lowest BE is reduced with respect to the others.

This suppression

in strength as a function of momentum transfer c l e a r l y indicates that the f i r s t peak is coherent in nature and correctly i d e n t i f i e d as a AL = 0 transition.

The fact that the peak is seen c l e a r l y in the at rest data even

though the reaction has yet higher momentum transfer indicates that the capture must not occur in s wave.

E. V. Hungerford / ~,-hypernuclei with in flight kaons

250

M H - MA ( M e V ) 2"/D 2Bo 290

260

M.- MA ( MeV ) ,

z7o ,

z~o

,

2?o

,

300

'L~

140

2~o

163c

310

0~

120 ~9o

I00

,

31o

7Li

, 3Ro 12"

NEUTRAL

80

60 Z

40

20

-fo

o

20

i°B~ ( MeV )

30

40

50

Figure 6 z- hypernuclear spectrum for a 7Li target at 12o cut by 7° decay.

J

-I0

0

I0

~0

Bz (MeV)

30

40

50

Figure 7 E- hypernuclear spectrum for a 6Li target at 0°.

The neutral tag enhances the spectrum in the region of the two lower peaks.

The charged high energy tag enhances the data at the 3rd peak and

above.

This indicates that the lower peaks decay via EN conversion while

at least the spectrum at higher BE, decays via the free weak E decay.

Thus,

the decay tag provides useful information concerning the decay mechanism of E hypernuclei and constrains the theoretical models designed to explain reasons for the narrow z states. The Li spectrum appears to have the predicted structure.

I f we i d e n t i f y

the two lowest enhancements as the 3/2- and I / 2 " levels then the spin o r b i t parameter would have a value of ~ = 5-6 MeV. However,one must be careful P at this point in making assignments based on preliminary data. S t i l l this number is not unreasonable, although about I / 2 the value extracted from the CERN experiments. We hope to f i n a l i z e the analysis in the near future.

Better s t a t i s t i c s

for the Li and C would be helpful to resolve the presence of E structure. We hope to search for the S state in E- + 3H which may be bound, but more l i k e l y is resonance some 4 MeV above threshold.

Decay of this state

through conversion should be suppressed but i t may decay via E emission into the continuum. ......

ow

E

The width of this state may help define the reasons for

hypernuclear

structure.

164c

E. K Hungerford / ~-hypernuclei with in flight kaons

/

8 = 12° ( P~=TZO~V/: ) /3.L: z "

/

77

3-

/ (0÷ (~ P[12 ) 112-

/

IF

/

~p tu

{2 ÷ ~

/.

P3/2 )7/2-

( O+ (~ P312 )312-

/ /

~I0 MeV

-

-

/

/

/

/

6Fle+ ~ threshol~

/

(&L,2)

/

!

(~TH](MeV)

Figure 8 Schematic diagram of the level structure of z7H showing the effect of the spin o r b i t strength parameter. REFERENCES I . Participants in this work are member of the AGS hypernuclear spI~ctrometer collaboration. I n s t i t u t i o n s involved are University of Houston, Brookhaven National Lab, Carnegie Mellon University, Osaka University, and Vassar University. 2.

AGS Experiment 774.

3.

R. Chrien, et a l . , Phys. Lett. B89, (1979) 31, Phys. Lett. 47 (1981) 1106.

4.

C. Vander Velde-Wilquet, et a l . , Nucl. Phys. A241 (1975) 511.

5.

R. B e r t i n i , et a l . , Phys. Lett. 90B (1980) 375.

6.

H. Piekarz, et a l . , Phys. Lett. 1lOB (1982) 428.

7. 8.

R. B e r t i n i , et a l . , Phys. Lett. 136B (1984) 29; Phys. Lett. 158B (1985) 19. T. Yamazaki, et. a l . , Phys. Rev. Lett. 54 (1985) 102.

9.

C.B. Dover, et a l . , BNL Report 36754, preprint.

I0. R. B e r t i n i , et a l . , Nucl. Phys. A360 (1981) 315, A368 (1981) 365. 11. T. Kishimoto, paper in these proceedings.