50 Nuclear Physics B (Proc Suppl ) 8 (1989) 50 53 North-ttolland, Amsterdam

STUDY OF THE E(1420) IN PP ANNIHILATION AT REST IN H 2 GAS

S hhmad, C. Amsler, E.G Auld, D.A. Axen, D. Bailey, S. Badag, G.A Boer, J.C. Bizot, M. Botlo, M Comyn, W. Dahme, B. Delcourt, M. Doser, K.D. Duch, K.L. Erdman, F Feld-Dahme, U. Gastaldl, M. Heel, B Howard, R. Howard, J. Jeanjean, H. Kalinowsky, F. Kayser, E. Klempt, C. Laa, R. Landua, G Marshall, H. Nguyen, N. Prevot, J. Riedlberger, L.P. l~obertson, C. Sabev, U. Schaefer, O. Schrelber, U. Straumann, P. TruSl, H. Vonach, B.L. White, W.R. Wodrich, M. Ziegler

ASTERIX Collaboration (PS171 at LEAR)

presented by Klaus--Dieter Duch

1. INTRODUCTION

In 1963 a particle with a mass of 1420 MeV/c"was discovered in PP annihilation at rest

in the reaction ~p -* ~r*E °, E ° -* K ° K * ~ and called E meson. A partial wave analysis of its

decay Dalitz plot gave the quantum numbers joe = 0-* [1]. Seventeen years later, in 1980, a

resonance, named ~, was found in radiative J [ ¢ decays with a mass of approximately 1440

MeV/c~and quantum numbers jpc = 0"* [2]. At about the same time the E(1420) was seen in

hadronic reactions with different spin-parity assignements, jpc = 1"* and 0"* [3,4]. The ques-

tion arose whether it was the same particle.

2. MASS SPECTRA AND DALITZ PLOT

To study the E meson's features was one goal of the ASTERIX experiment. The des-

cription and performance of the detector can be found elsewhere [5]. In order to detect the

kaonic decay mode of the E in KK~r, the dE/dx information of the innermost chamber was

used resulting in a good separation of charged kaons and pions up to momenta of 400

MeV/c.

The final state ~p -* T÷~Ki~:FK ° (~0 missing) was choosen for the study of the E. In the

neutral K°K~'x:~ invariant mass distribution there are 2 entries per event: one does resonate,

the other does not. To assess the non resonating combination, the double charged K°K~r~

mass combination can be used and then subtracted from the neutral one. Figure la shows

the resulting plot. The E is plainly visible with a mass of M e ffi 1413 • 8 MeV/~and a width

of F E = 62 :k 16 MeV/~ At 1285 MeV/c ~', a peak with a signature of 3a appears. This is

exactly the mass where the ft(1285), former D-Meson, should lie; a resonance decaying also

into KK~r. Figure lb shows the decay Dalitz plot of the E for a cut between 1.37 -( ME( 1.48

GeV/J These results obtained in H:~ gas can be compared with those of the CERN bubble cham-

ber which had a liquid H~. target (fig. 2a, b). The two Dalitz plots look quite similar. The

invariant mass distribution shows the E as well, at about the same mass, however there is

no D signal here.

0920-5632/89/$03.50 © Elsevier Science Publishers B.V. (North-Holland Physics Publishing Division)

K.-D. Duch et al . /Study of the E(1420) 51

"% 80

o 40

e-

(a)

3 O

20

0

10 11 12 13 14 M (~o K t n:; )

10

(b} ~~ 09

,~ 08 . o .

07

,;" 06

+~ 05 N

04

030. 3

100

8O

60

o 40

~ 20

~ 0

o)

no D

I = I 1'5 1'6 17 -2C1~ 12 1.4 16

[GeV/c 2] M ( K~Ktw. ~ ) [GeV /c 2]

~r~ O9 u

08 o

~-~ 0 7

o's ds 0'7 M 2 ( R ° W,;)

- - O6 i ÷

÷' 05 v

oz.,

(b)

0'.8 0'9 ,o 03 o's o'7 o'8 0'9

F I G U R E 1 F I G U R E 2

3. DALITZ PLOT ANALYSIS OF THE E DECAY

A partial wave analysis of the E decay has been performed. We used the Zemach forma- .

lism as described for instance in [1,6] parametrising the K by a relativistic Breit-Wigner,

the "6" by a standard Breit-Wiguer as well as by the Flatt6 formula [71, using the

PDG-values. Carefully taking into account the acceptance of our apparatus we tried the two

hypotheses for the spin-parity of the E, ]pc = 0"+ and 1 ÷*, amuming that the particle decays

only via K K, only via "6"x or a mixture of the two (Tab.l). The fits show no clear prefe--

rence for 0 "÷ or 1*% The decay into "6 "x is preferred to that into K K. Some K K must be

present if one looks at the angular distribution of the KK system (drop at = 1). However

K K is definitely not the dominant decay mode of the E.

We have tried the two parametrizations o{ the "6" since its interpretation is not clear. Is

there really a resonance at threshold narrowed by the limited phase space? Is the enhance-

ment due to a KK final state interaction or a KK molecule as proposed by some theories?

5'2 K -D Dtlcli et al / S t u d y of the E(bl20)

4. PRODUCTION OF THE E

The branching ratio of the final state ~p -* x*x-(E ° -* K°K+r~:) was determined. In liquid

Hz(bubble chamber) it is (7.1 * 0.7).10 -4, in gas (ASTERIX) we find (3 + 0.9)'10 -4. The

bubble chamber had very little P-wave contribution in the annihilation whereas ASTERIX

had about (60 * 10)% (fig.3). Therefore it can be concluded that the xrE production in

P-wave annihilation is compatible with 0.

The (xr) system recoiling against the E meson is most likely in a 0"* state (p(~ex)_< 180

MeV/c). Most probably there is no angular momentum between the dipion system and the

E [1]. In this case a 0"* state then should be produced only from S states, a 1"* state only

from P states. The D meson established as a 1"* resonance does follow this prediction. The E

meson is not produced from P states; hence we conclude that its quantum numbers are 0-*.

o-" I -

K K P < 1 0 -8 P = 5 . 1 0 "5

" 6 " r P = 0 . 2 7 P = 0 . 2 1

" ~ " ~ r + K K P = 0 . 4 9 P=0,19

" ~ % r / K K = 8 0 : 2 0 = 9 0 : 1 0

TABLE 1

÷ -

Branching ratio ~)p ~ * r ~ - (E~K~.L K- Tt*} x lO "4

B Bubble chamber

00'0 ' 0'2 ' O'Z. ' 0'6 ' 0'8 " 0

ASTERIX

C o n t n b u t , o n o f P w a v e a n n l h d a h o n

FIGURE 3

5. CONCLUSIONS

We observed the E meson in ~p -* r*x'(E ° -* K°K*x ~) at rest in a Hzgas target at NTP.

Its mass and width are M t = 1413 • S MeV]~ P~ = 62 + 16 MeV[c~ Furthermore we have a

3a evidence for the production of the ft(1285). A PWA of the E decay Dalitz plot excludes

K K dominance both for jpc = 0. o and 1"÷; both "6 " , and K K contribute, however "6 "T is

dominating. From the PWA we find no clear preference for either 0 -÷ or 1*% The absolute

branching ratio of the above channel is (3 * 0.9)'10 .4 at (60 "* 10)% P-wave annihilation.

From this result together with the studies of the bubble chamber (S-wave production with l

jpc 0". = 0) we conclude that the quantum numbers of the E are =

K.-D. Duch et al.//Study of the E(1420) 53

REFERENCES

[i] P. Baillon et al., II Nuovo Cimento 50A (1967) 393

[2] D.L. $harre et al., Phys. Left. 97B (1980) 329

[2] T.A. Armstrong et al., Phys. Lett. 146B (1984) 273

[4] S.U. Chung et al., Phys. Rev. Left. 55 (1985) 779

[5] S. Ahmad et al., the ASTERIX detector, to be published

[6] C. Zemach, Phys. l:tev. 140B (1965) 109

[7] S.M. Flattd, Phys. Lett. 63B (1976) 224

Questions:

H. Kolaaoski: Is there an experimental evidence that the two 0-* states observed around 1400 MeV in Fp annihilation and in J /¢ radiative decays could not be the same?

No, there isn't any experimental observation. Mr. Palano claimed that due to phase space limitations in the ~p annihilation at rest the mass of a particle around 1400 MeV could be shifted.