- Email: [email protected]
c leading to ⩾3 particles in the final state
Nuclear Physics B119 (1977) 237-252 © North-Holland Pubhshmg Company
HYPERON-ANTIHYPERON PRODUCTION IN ~p INTERACTIONS AT 3.6 GeV/c LEADING TO ~>3 PARTICLES IN THE F I N A L STATE B.R. FRENCH, J. MOEBES * and C. POLS ** CERN, Geneva, Switzerland Recewed 19 October 1976
Results are presented on the properties of the pp interactions at 3.6 GeV/c leading to a hyperon or an antlhyperon m final states with ~>3 particles from an experunent of 35 eV/ub sensitivity. Results of a multidimensional mass fit to each final state are given along with the cross sections, differential cross sections and polarization, where possible, for the hypercharge exchange reactions ~ p o AY*(1385), 7,Y*(1405), AY*(1520), Y.Y*(1385) and E Y*(1520)
1. Introduction The results presented below come from an exposure o f the CERN 2 m hydrogen bubble chamber to a beam of 3.6 GeV/c antlprotons. 560 000 photos were taken and the resulting sensitivity of the exposure was 35.4 eV//ab. All the film was scanned twice for V ° ' s and charged V's. In previous pubhcatlons the properties of the two-body hyperon-antihyperon channels [1] ~p ~ ~,°A°, ~ . ° E ° + charge conjugate, ~ - ~ + and the "non-allowed" channel [2]
have been discussed. It is the purpose of the present work to analyse the other extiac table channels leading to a hyperon and/or an antihyperon. Specifically we will deal with the following reactions *** ~p ~ ~. p K - + c.c. ,
(1)
~0p K - + c.c.,
(2)
* Now at Fachhochschule, Nlederrhem, Krefeld, Germany. ** Now at Umverslty of Nijmegen, Exp. 111, Nijmegen, Netherlands *** Here c.c. denotes the charge conjugate reaction. 237
Constraint
4c
2c
lc b)
4c
4c
4c
Topology
201
201
201
202
211
211
136 } 131 267
Z0~K + ~0pK-
194
228 } 422
z-X"+
~ +A'n'--
437/848 411 J (456) c)
~+An~ - A 7r+
218] 189~ 596 189}
308} 312 620
ApK + ApK-
A0rr+Tr--A 0 7, lr+rr-A 0 ~. A n+~r -
Number of events
Reactxon
+- 2
-+ 4
± 1
28.6 + 2
70
23
15.8-+ 1 2
37
Total a) o(ub)
Table 1 Cross sections of the three and lour-body final states and the quasx two-body reactions
+cc
+c.c
AA0(1520)
[_,X~+
+cc.
+cc
z - ~ + ( 1 3 8 5 ) + c c.
L Z+Tr-
AA0(1520)
L Y+Tr--
AA0(1520)
LX.-
Z + z - ( 1 3 8 5 ) + c.c
L A rr+rr -
Z--(1385)
L A0 rr+
)2+(1385)
~+(1385)
L ~0 n--
~-(1385)
L pK-
~0A0(1520) + c c
[~AOK +
~pKpN*+(1710) + c.c
~,A0(1520)
Resonance reaction
±1.3
±3
3.6±0.8
8.7±12
7
36
2.6±04
39±0.4
6.5±08
6.3±0.8
4.2±1
135±2
o(ub)
~z
2c
2c
lc
211
211
002
AA~ "0
526
94} 195 101
E-:~07r + X+~0n/
56 42
Z+Z07r ~-E0n +
98 c)
Number of events
Reaction
46
13
15
± 10
± 1
+2 -
Total a) oOsb)
+cc
L Z-It + ~,A0(1520)
~,X0(1385)
:~0A(1520)
C.C.
+cc
+
X0A0(1520) + c.c I
L Z--rt
+cc.
AAO(1520)
Resonance reaction
a) Corrected for unseen neutral decays of A, A. b) Af, Afstands for unseen A or A which has been deduced to extst from the fit c) Number of events with a E + --, nTr+ (or ~ - ~ n n - ) decay whmh were used to compute cross section
Constraint
Topology
Table 1 (continued)
24
-+ 5
4.4±1
3.8± 1
6.3±08
5.7±09
o(ub)
t~
e~
240
B R French et al / Hyperon-antthyperon productton
-+ E+Arr -
+ c.c. ,
(3)
-+ E - ~ . r r + + c . c . ,
(4)
-+ 2;+~°7r - + c.c ,
(5)
-+ E - E ° r r + + c.c. ,
(6)
-+ A A 7r°
,
(7)
-+ A A 7r+rr-
,
(8)
In the following sections we discuss the characteristics presented by each channel consisting of the cross sections, results of the multidimensional fitting along with the mass plots, the form of the differential cross section for the extractable quasi two-body channels and the ~, polarization in the case of the channel A A°(1520) + c.c
2. Cross sections The cross sections for the channels (1) to (8) were determined taking into account the usual known losses vzz scanning, measurement, decays near the vertex, decays outside the bubble chamber, small angle decays, Interactions m flight and probably cuts, and are listed m table 1. The errors quoted are statistical only.
3. Maximum likelihood fit to each channel and invariant mass spectra The various mass spectra of each reaction channel were fitted with a maximum hkelihood procedure which used a combination of Lorentz lnvanant phase-space and incoherent Breit-Wigner resonance functions. Resonances were only introduced if the change in the likehhood value correspond to more than three standard deviations. 3.1. tip -+ A p K + + e.c. ( 6 2 0 events)
The cross section for this channel was determined to be 37 +- 2/ab. The result of fitting this reaction with the usual maximum likelihood technique [3], IS displayed along with the mass spectra in figs 1a, b and c. It was found necessary to introduce the A°(1520) -+ p K - and the N*÷(1710) -+ A ° K ÷ resonances in order to obtain an acceptable fit. The cross sections for the two quasi two-body reactions ~p -+ A-A(1520) + c.c., F K+
(9)
B R French et a l / Hyperon-antthyperon productton
241
~p -+ ~ N * + ( 1 7 1 0 ) + c.c. , L AK +
(10)
are reported in table 1 and together t h e y represent 50% o f the channel cross section.
3.2 ~p ~ Y.°~ K+ + c.c. (270 events) This channel is d o m i n a t e d by the A ( 1 5 2 0 ) ~ p K - resonance and was the only resonance needed to obtain an acceptable fit. The ~ K + mass distribution along with the curve from the fit Is shown in fig. 2. The cross sections for the channel and for the quasi t w o - b o d y channel ~-p ~ 2;°h,(1520) + c . c . ,
(11)
L~K ÷ are given in table 1.
#p --,.A#K+÷c.c. at 3,6 GeV/c (620 events ) 1
I0£
I
I
i
>o 0
8C
(o)
~" 6C (,0 z
o~ 4c z
o 0
j i
i
,.5
1.6,,M~p~ ,.,-i~+..i.C.C4 I 7 , [c v] ,8 ]
i
% H
'
I
(b)
(C)
4o
oo Z
J
30-
~ I0 ~ 1,6
[ 1.8
i
i 2,0
M(*K%c.c.) [GeV]
2,1
[ 23
t
[ 2,5
M(Ap+c..c,) [6eV]
Fig. 1. Effective mass dxstnbutxon for the reaction pp --* ApK + + c.c. (620 events). (a) M(pK + + c.c ). (b) M(AK + + c.c.). (c) M(Ap + c c ).
242
B R French et al / Hvperon-antthyperon productton
I
~p
~:~°BK++cc.(]t 3.6 GeVAz(2-/Oev,)~
=~>~~o0 401
Z3o~ 2o"
o
I ol
I
15
16
17
1,8
M (~K++c.c.) [GeV] t lg 2 pK + + c c ellectlve mass distribution for the reaction PP -~ )-;OPK+ c.c. (270 events). 3 3 tip ~ Y~+Azr- + c c. ( 8 4 8 e v e n t s )
The mass histograms along with the results o f the fit are diaplayed m figs. 3a, b, and c. Here it was necessary to introduce the three resonances 2 ( 1 3 8 5 ) -+ An , A ° ( 1 4 0 5 ) - + Z+zr - and A ° ( 1 5 2 0 ) - + ig+n - m order to have an acceptable fit. The cross sections for these three quasi t w o - b o d y states, vtz.
~p ~ ~;+}2-(1385) + c.c.,
(12)
~p -+ A A ( 1 4 0 5 )
+ c.c.,
(13)
~p ~ ~. A ( 1 5 2 0 )
+ c.c ,
(14)
L 2;+zr are hsted m table 1 We see that this channel gives rise to the same quasi t w o - b o d y final state A A ( 1 5 2 0 ) as did the channel ~p ~ A ~ K + + c.c. but with the A ( 1 5 2 0 ) decaying via the 21r m o d e . 3 4. p p -~ E - T k rt+ + c.c ( 4 2 2 events)
The cross s e c u o n for this channel 1s d o w n by a factor o f two c o m p a r e d to that for the previous channel (~+~. n - + c.c ), see table 1. The main reason for this is m the low cross section for the channel 2 ; - ~ + (1385) (3.6 + 0 . 8 / l b ) , which reqmres double charge exchange m the t-channel, c o m p a r e d to the channel ~ + ~ - ( 1 3 8 5 )
B R French et a l / ttyperon-antthyperon productton
243
B p--~Z" A rr-,.c c. ot 5,6 GeV/c ( 8 4 8 events) F
q
i
I
i
5O
]
a)
4O 50 2O I0 0
J
~
I,
i
1
14
J
~
15
t
i 17
16
J L ~ j 18
+
M(I~ ~ r ; c c )
(GeV)
I00 >
80
o
60
0
40
z
20 0
15
14
15
16
17
M ( ~, ",r",c c ) (GeV) I
I
I
I
I
]
50
c) 40 30 20 tO
2,5
24
25
26
27
2,8
M (Y÷,~.,,c c ) (GeV)
Fig. 3. Effectwe mass distributions for the reaction ~p --* ~;+ATr- + e.c. (848 events) (a) M ( ~ + r r - + c.c.) (b) M ( A . - + c.c ). (c) M(Y~+X+ c c ).
244
B R French et al / ttyperon-ant,hyperon production
pp --*~/, 7r++ c c. at 5,6 GeV/c (422 events 4C (a}
>~ 3c 2C
P_ >
io
0
15
1,4 I
i
M
1,6
IZ
(Y.--rr++cc,) [GeV] I
I
I
1,8 I
(b)
I
(c)
'•40 o
3O
U3 I--" 20 Z
LIJ >lO I,I 0 14
16
M (~.rr++cc) [GeV ]
24
2.5
2.6
a (Z-X+ c.c,)
2,7
2,8
[GeV]
fqg 4 Elfectlve mass d]stnbutlons for the reaction pp --* !;-ATr + + c c. (422 events) (a) M(E-rr + + c c.) (b) M(~r + + c c ). (c) M(E-~, + c c.). (36 -+ 3/~b) These t w o quasi t w o - b o d y channels m a y be compared to the t w o - b o d y channels ~p -+ Z + ~ - and Z - Z+ w h i c h have cross section 72.2 -+ 1 0 . 8 / l b and 5.9 -+ 1.1 /~b respectwely [1,2]. Thus we see that the ratios o f the cross sections E+~2(1385)/E-~+(1385) and Y , + ~ - / E - ~ , + have sxmIlar values o f the order 10. Fig. 4 shows the mass histograms along with the fitted curves taking Into account the quasi t w o - b o d y reactions pp ~ 2 3 - ~ , + ( 1 3 8 5 ) + c c . ,
(15)
L S.Tr+ ~p-> A A(1405) L E-Tr+
+ c.c.,
(16)
B R. French et al / Hyperon-antthyperon production ~p -> ~, A ( 1 5 2 0 )
245
(17)
+ c.c.,
L~-Tr + and the cross sectaons are reported m table 1.
3.5. ~p ~ A A
rr+Tr
(596 events)
-
T h e m a s s d i s t r i b u t i o n s for this r e a c t i o n are s h o w n in fig. 5. T h e Y~(1385) d o m i n a t e s the reacUon but there is also a s~gnal in the mass d~stnbution A 7r+n - + c.c.
~p---~AATr'rr-
at 3.6
GeV/c
(596 events) i
K)O
50
80
4O
60
'~ 50
40
-.. 20
,
F
i
o 1o 2C 0
13
14
15
qool ~,bl
16
24
17
04
26
MIXA) (GeV)
M I A ~ "~. ~.~'-) ( G e V )
06
M (~r*w-) (GeV)
8O
_
,
40
_o
2O 13
14
15
16
~7
25
M law. A~") (GeV) ,
,
,
,
,
,
26
27
28
M ( A A , ~ . c c ) (GeV) ,
.j
,
,
¢) 50 40 30 20 IO 0
h5
16
I7
M ( A r ÷ *r". ~ w * l r - )
1.6
019
(GeV)
Fig. 5. Effective mass distributions for the reaction pp --* AA n+rr - (596 events) (a) M(An + + + X n - ) . (b) M(Arr- + An+). (c) M(A~r+n - + A n + n - ) . (d) M(AA). (e) M(n+n-). (f) M(AArr + + c.c.) The dotted curve of (c) represents a one-dimensional fit using phase-space times a third order polynomml for the background.
246
B R French et al /Hvperon-antthvperon producnon
corresponding to the quasi two-body channel ~p ~ X A(1520),
(18)
~ A 7r+~which it was necessary to take into account to obtain a satisfactory fit 3.6. Other channels
Several other three-body channels were fitted, namely reactions (5), (6) and (7). Their cross sections together with those of the identifiable quasi two-body channels are given in table 1 We note that for the observable three- and four-body channels hsted m table 1 about 60% is accounted for m terms of quasi two-body reacUons. 3. 7. BB and BBTr mass spectra
Rosner [4] has suggested the possablhty of B = 0 enhancements decaying mainly to baryon-antlbaryon and having small couphng to only meson systems No con-
-/
B
-~---..
B(Y)
\\,,_ \
I
-[
B(~)
I
NDC
>
20o
3 ioo "~
I
o
23
24
25
26
27
28''L
t
M (YY, Y?rr) GeV t lg 6 (a) Quark diagram showing allowed production diagram for M4 (qq ~q~ mesons coupled to the baryon-annbaryon (BB) system (b) Total yC~ and yC{ mass spectrum trom reactions (3)-(8), 2979 events.
B R French et al / Hyperon-antlhyperon productlon
247
vmcmg evidence for any enhancements in BB or BBTr systems is observed at the level of sensitivity of this experiment (35 events//~b) see figs. lc, 3c, 4c, 5d and 5f. Fig. 6a shows the type of quark diagram which allows the formation of mesons composed of two quarks and two antxquarks (the so-called M4 mesons) and fig. 6b shows a compdatlon of the BB and BBn mass spectra xsolated in this experiment. No signal is seen, whereas a signal of 50 events (~3o) would necessitate a 5/lb producnon cross sectmn.
4. Widths of E(1385) and A(1520): A(1520) branching ratios 4.1. E(I385) width In the reaction 15p -* N + A n - + c.c. a good signal is evident in the S,~r- + c.c. mass distribution corresponding to the ~ ( 1 3 8 5 ) - + c.c. (see fig. 3b). Our mass resolution being relatwely good (AM-- -+3.1 MeV), we have derived a value for the mass and width of the ~(1385). The method used is the same as that of ref. [3] at 5.7 GeV/c and uses a p-wave resonance form for the Z(1385) Brelt-Wigner. After folding out the measurement error we find M~ = 1384 +- 1 MeV,
F~ = 32-+ 3.5 MeV,
to be compared to the Particle Data Table [5] values of 1383 + 1 MeV and 35 + 2 MeV respectwely. 4.2. A{1520) width The reaction ~p ~ ~, A(1520) + c c. gwe rise to three observable decay modes p K - , E+Tr- and A 7r+~r- . The p K - and N+Tr- mass distribution have relatively low backgrounds and good mass resolutions (-+2.4 and +-4.0 MeV respectively) in the A(1520) region and we have therefore used them to determine the mass and wxdth of the A(1520). After folding out the measurement resolutions and using a D-wave Brett-Wigner we find M A = 1520 +- 1 MeV,
PA = 14.1 + 2 MeV,
to be compared to the PDT values [5] of 1518 + 2 MeV and 16 -+ 2 MeV respectively. 4.3. A(1520) branching ratios The quasi two-body reactions (9), (14), (17) and (18) allow a determination of the braching ratios A(1520) ~ K - p '
l~-+rr± : A n + n - ,
B R French et al / Hyperon-antthyperon productton
248
F r o m t h e c r o s s s e c t i o n given in t a b l e 1 w e f i n d A ( 1 5 2 0 ) -~ K - p
2;±7r ~ . A n + n - = (42 +- 4 ) % : ( 4 6 -+ 5)%
this t o b e c o m p a r e d t o t h e P D T [5] values o f 4 6 %
42%.
(8 ± 2)%
10%.
5. D i f f e r e n t i a l c r o s s s e c t i o n T h e d i f f e r e n t i a l c r o s s s e c t i o n s f o r t h e quasi t w o - b o d y r e a c t i o n s p p -+ A 5 , ( 1 5 2 0 )
+ c.c. ,
(9)
E°A(1520)
+ c.c. ,
(l:)
c.c.,
(12)
A ° S . ( 1 4 0 5 ) + c.c. ,
(13)
A ° ~ ' ° ( 1 3 8 5 ) + c.c. ,
(19)
-+ Z ÷ E - ( 1 3 8 5 ) +
are d i s p l a y e d m figs. 7 a - e , r e s p e c t i v e l y . E a c h set o f d a t a has b e e n p a r a m e t n z e d b y
Table 2 Parameters of the fit of the differential cross sectlons to the formula given m sect. 5 Reaction + c c.
Slope a 1
Slope a 2
t~) (GeV 2)
x2/NDF
~.A(1520)
2.9 3.7
1.28 ± 0.4
0.81 ± 0 07
33/14 14/12
+ 0.2 ±0 3
E+~(1385)-
2 21 -+ 0.16
13/14
EOA(1520)
2.09 ± 0.2
8/11
~.Z(1385) °
3.4
± 0.3
82
+ 1 0
1.8
-+03
0.26+0.03
73/12 18/12
3.1 6.2
*0 3 ± 1.0
19
±0.4
026±0.04
33/12 18/12
AA a)
81
±0.4
25
±0.2
0.31-+002
A~0a)
11.1 ± 1.4
20
±02
0.21+-0.03
7.9 ± 0.8
1.8
± 0.2
0.19 ± 0.02
A.A(1405)
~ + ~ - a) a) Ref. [ 1 ].
B R French et al / ttyperon-antihyperon productzon [
f
I00
i
r
P P~.
)
i
,
249
,
Ais2o+ c c
~ p --=*AA,4os+C c
{o)
(d]
10
J
J
I00
I
I
L
J
I
i
I
p p~T-°A~sao÷Cc
i
i
~Jr
i
i
l--J~
~
t
l
P p ----,A~,3as+c C
A
(b)
::k
"~
IO
(e)
-
I00
~ p~-y~3as+c,c
I
i
04
0,8
i
i
I2
i
I6
t' ( GeV 2 ) (c)
I!
I 0',4 I 08
12
16
t' (GeV 2 ) Fig. 7. Differential cross-section variation (do/dt') for the quasi-two-body reactions. (a) PP ~ AA(1520) + c.c. (b) p p ~ ~ 0 A(1520) + c c. (c) p p ~ ~--~2+(1385) + c.c. (d) ~ p ~ AA(1405) + c.c. (e) PP '* XI~0(1385) + c.c The fitted curves represent the fits described m sect 5.
fitting the e x p e r i m e n t a l distributions to t w o e x p o n e n t l a l s w i t h different slopes, namely, d o = [ b e - a l It'l
for It'[ < ItS[ *
dr' ~be-alltble-a21t'-tbL
for It'J > It~l "
• t' = t - train, where t is the four-momentum transfer from the antlproton to the antihyperon and t m i n is the m m t m u m possible value o f t for the given reaction.
250
B.R French et a l / Hyperon.antthyperon production
i
,
, i I i P -~.A,szo+ c c. i
f
i
0,8 e-
0.4
o o
--
-0,4
13.-08
I 0
0,2
0,4
0,6
0,8
1.0
t' (GeV z ) Fig. 8. V a r i a t i o n o f the A ( A ) polanzataon as a function o f t' for the reaction pp - , A A ( 1 5 2 0 ) + c c
This is the same parametrization as used in ref. [1 ] and allows the break point [t~l to be determined along with its significance. The results of the fits are given in table 2, where the data for the reactions A A., A T ° and E+~, - [1 ] are also given for comparison. We note that whereas reactions A A(1405) + c.c. and A E ° ( 1 3 8 5 ) + c.c. (figs. 7d and e) present evidence for a two-exponential description with slopes and break points (t~) similar to the three reactions AA, A ~,o + c.c. and E+~ - (i.e. t o = 0.24 GeV 2) this is not the case for the other three reactions (9), (11) and (12) (figs. 7 a - c ) . These latter three reactions, however, have slopes of similar magnitude (~2) to the other reactions in the t-region above the break point t
6. Polarization The only channel which has sufficient statistics to allow determination of the A(A) polarization as a function of t' IS the AA(1520) channel obtained from reactions (9), (14) and (17). The distribution is shown in fig. 8 where a similarity to the reactions ~p -~ A.I~° + c.c. and ~ - p ~ K°A ° (shown in fig. 6 of ref. [1 ]) can be seen; notably the change in the sign of the polarization at t' = 0.2 GeV 2.
7. Observations on the data 7.1. Cross sections
We note that" (a) For the channels studied, approximately 60% of the cross section can be accounted for in terms of quasi two-body reactions.
B R French et a l / Hyperon-antthyperon productton
251
Table 3 Comparison ol peaks at low t' Initial state (spins) ~ final state (spins)
Peak at low t'C)
~p(l+, 1+)~ AA
i-1+, g1%, ,~
yes yes
A.A(1520)
A+ "2 , 2l-~' A+ "2 , 3_-~ 2 '
AE0(1385)
¢1+ "3 ,~3%'
yes
~0 A
(1+ - , ~1+)
yes
EOA(1405)
(}+, 21--)
no data
7,A(1405)
no
~0A(1520 ) (1+,3+)
no
~-:~+(1385) (1+, 3+)
no
(b) The cross-section ratio a@p ~ ~,(1520) A) o@-p ~ A ( 1 5 2 0 ) E ° )
= 2.0 + 0.3
ts n o t the same as the raUo o@p + A A ° ) - - = 1 . 0 7 + 0.1 o@p ~ A E ° ) and could Imply different exchanges are revolved for reacnons with A ( 1 5 2 0 ) m the final state rather than a A. Th~s s u p p o s m o n is supported by the absence of peaks at low t m the reactions revolving a A ( 1 5 2 0 ) (see table 3). (c) The ratio of the "allowed" to the " f o r b i d d e n " reacUon cross sections o@p -~ ~-(1385)12+)allowed
-10+2.2,
a@p -+ ~,+(1385)E-)forbldaen whlch revolve a E ( 1 3 8 5 ) is slmxlar to the ratio .~---- + o@p 12 12 )allowed = + o(~p-+--+ 12.2-3, E E )forbidden where the normal I2 replaces the E(1385). 7. 2. Differential cross sections
As m the previous work on the channels ~p -+ AA, AN, E12 [1 ] we observe the presence of forward peaks with slopes of the order 8 GeV - 2 and break points a r o u n d t' = 0.2 GeV 2. However not all the reactmns show peaks as can be seen m table 3.
252
B R. French et al / Hyperon-antlhyperon productton
No obvious pattern exists except that the two states involving a A(1520) show no peaking at low t'. It will be of interest to see if the K*, K** Regge exchange model wtuch describes well the ~p -+ A S differential cross-section variation [6] can be extended to describe the above-mentioned characteristics. 7.3. Mesons coupled to BB No evidence was found for mesons coupled to the hyperon-antihyperon system with a cross section >5 ~b. 7.4. Polarization The polarization of the A ° in the channel ~p ~ S(1520) shows a t' variation similar to the channels gp ~ AA and NA [ 1] even though the A(1520)A channel shows no peaking m do/dt' at low t'.
References [1] H.W Atherton et al., Nucl. Phys. B69 (1974) 1. [2] H.W Atherton et al., Phys. Letters 42B (1972) 522. [3] H.W. Atherton et al., Nucl. Phys, B29 (1971) 477; V. AUes-Borelhet al., Nuovo Clmento 48 (1967) 360, M. Ferro-Luzzi et al., Nuovo Clmento 39 (1965) 47. [4] J L Rosner, Phys Rev. Letters 21 (1968) 960. [5] Particle Data Tables, Rev. Mod. Phys 48 (1976). [6] R D Field and C. Qulgg, Fermflab-Pub. 76/35-TNY.
HYPERON-ANTIHYPERON PRODUCTION IN ~p INTERACTIONS AT 3.6 GeV/c LEADING TO ~>3 PARTICLES IN THE F I N A L STATE B.R. FRENCH, J. MOEBES * and C. POLS ** CERN, Geneva, Switzerland Recewed 19 October 1976
Results are presented on the properties of the pp interactions at 3.6 GeV/c leading to a hyperon or an antlhyperon m final states with ~>3 particles from an experunent of 35 eV/ub sensitivity. Results of a multidimensional mass fit to each final state are given along with the cross sections, differential cross sections and polarization, where possible, for the hypercharge exchange reactions ~ p o AY*(1385), 7,Y*(1405), AY*(1520), Y.Y*(1385) and E Y*(1520)
1. Introduction The results presented below come from an exposure o f the CERN 2 m hydrogen bubble chamber to a beam of 3.6 GeV/c antlprotons. 560 000 photos were taken and the resulting sensitivity of the exposure was 35.4 eV//ab. All the film was scanned twice for V ° ' s and charged V's. In previous pubhcatlons the properties of the two-body hyperon-antihyperon channels [1] ~p ~ ~,°A°, ~ . ° E ° + charge conjugate, ~ - ~ + and the "non-allowed" channel [2]
have been discussed. It is the purpose of the present work to analyse the other extiac table channels leading to a hyperon and/or an antihyperon. Specifically we will deal with the following reactions *** ~p ~ ~. p K - + c.c. ,
(1)
~0p K - + c.c.,
(2)
* Now at Fachhochschule, Nlederrhem, Krefeld, Germany. ** Now at Umverslty of Nijmegen, Exp. 111, Nijmegen, Netherlands *** Here c.c. denotes the charge conjugate reaction. 237
Constraint
4c
2c
lc b)
4c
4c
4c
Topology
201
201
201
202
211
211
136 } 131 267
Z0~K + ~0pK-
194
228 } 422
z-X"+
~ +A'n'--
437/848 411 J (456) c)
~+An~ - A 7r+
218] 189~ 596 189}
308} 312 620
ApK + ApK-
A0rr+Tr--A 0 7, lr+rr-A 0 ~. A n+~r -
Number of events
Reactxon
+- 2
-+ 4
± 1
28.6 + 2
70
23
15.8-+ 1 2
37
Total a) o(ub)
Table 1 Cross sections of the three and lour-body final states and the quasx two-body reactions
+cc
+c.c
AA0(1520)
[_,X~+
+cc.
+cc
z - ~ + ( 1 3 8 5 ) + c c.
L Z+Tr-
AA0(1520)
L Y+Tr--
AA0(1520)
LX.-
Z + z - ( 1 3 8 5 ) + c.c
L A rr+rr -
Z--(1385)
L A0 rr+
)2+(1385)
~+(1385)
L ~0 n--
~-(1385)
L pK-
~0A0(1520) + c c
[~AOK +
~pKpN*+(1710) + c.c
~,A0(1520)
Resonance reaction
±1.3
±3
3.6±0.8
8.7±12
7
36
2.6±04
39±0.4
6.5±08
6.3±0.8
4.2±1
135±2
o(ub)
~z
2c
2c
lc
211
211
002
AA~ "0
526
94} 195 101
E-:~07r + X+~0n/
56 42
Z+Z07r ~-E0n +
98 c)
Number of events
Reaction
46
13
15
± 10
± 1
+2 -
Total a) oOsb)
+cc
L Z-It + ~,A0(1520)
~,X0(1385)
:~0A(1520)
C.C.
+cc
+
X0A0(1520) + c.c I
L Z--rt
+cc.
AAO(1520)
Resonance reaction
a) Corrected for unseen neutral decays of A, A. b) Af, Afstands for unseen A or A which has been deduced to extst from the fit c) Number of events with a E + --, nTr+ (or ~ - ~ n n - ) decay whmh were used to compute cross section
Constraint
Topology
Table 1 (continued)
24
-+ 5
4.4±1
3.8± 1
6.3±08
5.7±09
o(ub)
t~
e~
240
B R French et al / Hyperon-antthyperon productton
-+ E+Arr -
+ c.c. ,
(3)
-+ E - ~ . r r + + c . c . ,
(4)
-+ 2;+~°7r - + c.c ,
(5)
-+ E - E ° r r + + c.c. ,
(6)
-+ A A 7r°
,
(7)
-+ A A 7r+rr-
,
(8)
In the following sections we discuss the characteristics presented by each channel consisting of the cross sections, results of the multidimensional fitting along with the mass plots, the form of the differential cross section for the extractable quasi two-body channels and the ~, polarization in the case of the channel A A°(1520) + c.c
2. Cross sections The cross sections for the channels (1) to (8) were determined taking into account the usual known losses vzz scanning, measurement, decays near the vertex, decays outside the bubble chamber, small angle decays, Interactions m flight and probably cuts, and are listed m table 1. The errors quoted are statistical only.
3. Maximum likelihood fit to each channel and invariant mass spectra The various mass spectra of each reaction channel were fitted with a maximum hkelihood procedure which used a combination of Lorentz lnvanant phase-space and incoherent Breit-Wigner resonance functions. Resonances were only introduced if the change in the likehhood value correspond to more than three standard deviations. 3.1. tip -+ A p K + + e.c. ( 6 2 0 events)
The cross section for this channel was determined to be 37 +- 2/ab. The result of fitting this reaction with the usual maximum likelihood technique [3], IS displayed along with the mass spectra in figs 1a, b and c. It was found necessary to introduce the A°(1520) -+ p K - and the N*÷(1710) -+ A ° K ÷ resonances in order to obtain an acceptable fit. The cross sections for the two quasi two-body reactions ~p -+ A-A(1520) + c.c., F K+
(9)
B R French et a l / Hyperon-antthyperon productton
241
~p -+ ~ N * + ( 1 7 1 0 ) + c.c. , L AK +
(10)
are reported in table 1 and together t h e y represent 50% o f the channel cross section.
3.2 ~p ~ Y.°~ K+ + c.c. (270 events) This channel is d o m i n a t e d by the A ( 1 5 2 0 ) ~ p K - resonance and was the only resonance needed to obtain an acceptable fit. The ~ K + mass distribution along with the curve from the fit Is shown in fig. 2. The cross sections for the channel and for the quasi t w o - b o d y channel ~-p ~ 2;°h,(1520) + c . c . ,
(11)
L~K ÷ are given in table 1.
#p --,.A#K+÷c.c. at 3,6 GeV/c (620 events ) 1
I0£
I
I
i
>o 0
8C
(o)
~" 6C (,0 z
o~ 4c z
o 0
j i
i
,.5
1.6,,M~p~ ,.,-i~+..i.C.C4 I 7 , [c v] ,8 ]
i
% H
'
I
(b)
(C)
4o
oo Z
J
30-
~ I0 ~ 1,6
[ 1.8
i
i 2,0
M(*K%c.c.) [GeV]
2,1
[ 23
t
[ 2,5
M(Ap+c..c,) [6eV]
Fig. 1. Effective mass dxstnbutxon for the reaction pp --* ApK + + c.c. (620 events). (a) M(pK + + c.c ). (b) M(AK + + c.c.). (c) M(Ap + c c ).
242
B R French et al / Hvperon-antthyperon productton
I
~p
~:~°BK++cc.(]t 3.6 GeVAz(2-/Oev,)~
=~>~~o0 401
Z3o~ 2o"
o
I ol
I
15
16
17
1,8
M (~K++c.c.) [GeV] t lg 2 pK + + c c ellectlve mass distribution for the reaction PP -~ )-;OPK+ c.c. (270 events). 3 3 tip ~ Y~+Azr- + c c. ( 8 4 8 e v e n t s )
The mass histograms along with the results o f the fit are diaplayed m figs. 3a, b, and c. Here it was necessary to introduce the three resonances 2 ( 1 3 8 5 ) -+ An , A ° ( 1 4 0 5 ) - + Z+zr - and A ° ( 1 5 2 0 ) - + ig+n - m order to have an acceptable fit. The cross sections for these three quasi t w o - b o d y states, vtz.
~p ~ ~;+}2-(1385) + c.c.,
(12)
~p -+ A A ( 1 4 0 5 )
+ c.c.,
(13)
~p ~ ~. A ( 1 5 2 0 )
+ c.c ,
(14)
L 2;+zr are hsted m table 1 We see that this channel gives rise to the same quasi t w o - b o d y final state A A ( 1 5 2 0 ) as did the channel ~p ~ A ~ K + + c.c. but with the A ( 1 5 2 0 ) decaying via the 21r m o d e . 3 4. p p -~ E - T k rt+ + c.c ( 4 2 2 events)
The cross s e c u o n for this channel 1s d o w n by a factor o f two c o m p a r e d to that for the previous channel (~+~. n - + c.c ), see table 1. The main reason for this is m the low cross section for the channel 2 ; - ~ + (1385) (3.6 + 0 . 8 / l b ) , which reqmres double charge exchange m the t-channel, c o m p a r e d to the channel ~ + ~ - ( 1 3 8 5 )
B R French et a l / ttyperon-antthyperon productton
243
B p--~Z" A rr-,.c c. ot 5,6 GeV/c ( 8 4 8 events) F
q
i
I
i
5O
]
a)
4O 50 2O I0 0
J
~
I,
i
1
14
J
~
15
t
i 17
16
J L ~ j 18
+
M(I~ ~ r ; c c )
(GeV)
I00 >
80
o
60
0
40
z
20 0
15
14
15
16
17
M ( ~, ",r",c c ) (GeV) I
I
I
I
I
]
50
c) 40 30 20 tO
2,5
24
25
26
27
2,8
M (Y÷,~.,,c c ) (GeV)
Fig. 3. Effectwe mass distributions for the reaction ~p --* ~;+ATr- + e.c. (848 events) (a) M ( ~ + r r - + c.c.) (b) M ( A . - + c.c ). (c) M(Y~+X+ c c ).
244
B R French et al / ttyperon-ant,hyperon production
pp --*~/, 7r++ c c. at 5,6 GeV/c (422 events 4C (a}
>~ 3c 2C
P_ >
io
0
15
1,4 I
i
M
1,6
IZ
(Y.--rr++cc,) [GeV] I
I
I
1,8 I
(b)
I
(c)
'•40 o
3O
U3 I--" 20 Z
LIJ >lO I,I 0 14
16
M (~.rr++cc) [GeV ]
24
2.5
2.6
a (Z-X+ c.c,)
2,7
2,8
[GeV]
fqg 4 Elfectlve mass d]stnbutlons for the reaction pp --* !;-ATr + + c c. (422 events) (a) M(E-rr + + c c.) (b) M(~r + + c c ). (c) M(E-~, + c c.). (36 -+ 3/~b) These t w o quasi t w o - b o d y channels m a y be compared to the t w o - b o d y channels ~p -+ Z + ~ - and Z - Z+ w h i c h have cross section 72.2 -+ 1 0 . 8 / l b and 5.9 -+ 1.1 /~b respectwely [1,2]. Thus we see that the ratios o f the cross sections E+~2(1385)/E-~+(1385) and Y , + ~ - / E - ~ , + have sxmIlar values o f the order 10. Fig. 4 shows the mass histograms along with the fitted curves taking Into account the quasi t w o - b o d y reactions pp ~ 2 3 - ~ , + ( 1 3 8 5 ) + c c . ,
(15)
L S.Tr+ ~p-> A A(1405) L E-Tr+
+ c.c.,
(16)
B R. French et al / Hyperon-antthyperon production ~p -> ~, A ( 1 5 2 0 )
245
(17)
+ c.c.,
L~-Tr + and the cross sectaons are reported m table 1.
3.5. ~p ~ A A
rr+Tr
(596 events)
-
T h e m a s s d i s t r i b u t i o n s for this r e a c t i o n are s h o w n in fig. 5. T h e Y~(1385) d o m i n a t e s the reacUon but there is also a s~gnal in the mass d~stnbution A 7r+n - + c.c.
~p---~AATr'rr-
at 3.6
GeV/c
(596 events) i
K)O
50
80
4O
60
'~ 50
40
-.. 20
,
F
i
o 1o 2C 0
13
14
15
qool ~,bl
16
24
17
04
26
MIXA) (GeV)
M I A ~ "~. ~.~'-) ( G e V )
06
M (~r*w-) (GeV)
8O
_
,
40
_o
2O 13
14
15
16
~7
25
M law. A~") (GeV) ,
,
,
,
,
,
26
27
28
M ( A A , ~ . c c ) (GeV) ,
.j
,
,
¢) 50 40 30 20 IO 0
h5
16
I7
M ( A r ÷ *r". ~ w * l r - )
1.6
019
(GeV)
Fig. 5. Effective mass distributions for the reaction pp --* AA n+rr - (596 events) (a) M(An + + + X n - ) . (b) M(Arr- + An+). (c) M(A~r+n - + A n + n - ) . (d) M(AA). (e) M(n+n-). (f) M(AArr + + c.c.) The dotted curve of (c) represents a one-dimensional fit using phase-space times a third order polynomml for the background.
246
B R French et al /Hvperon-antthvperon producnon
corresponding to the quasi two-body channel ~p ~ X A(1520),
(18)
~ A 7r+~which it was necessary to take into account to obtain a satisfactory fit 3.6. Other channels
Several other three-body channels were fitted, namely reactions (5), (6) and (7). Their cross sections together with those of the identifiable quasi two-body channels are given in table 1 We note that for the observable three- and four-body channels hsted m table 1 about 60% is accounted for m terms of quasi two-body reacUons. 3. 7. BB and BBTr mass spectra
Rosner [4] has suggested the possablhty of B = 0 enhancements decaying mainly to baryon-antlbaryon and having small couphng to only meson systems No con-
-/
B
-~---..
B(Y)
\\,,_ \
I
-[
B(~)
I
NDC
>
20o
3 ioo "~
I
o
23
24
25
26
27
28''L
t
M (YY, Y?rr) GeV t lg 6 (a) Quark diagram showing allowed production diagram for M4 (qq ~q~ mesons coupled to the baryon-annbaryon (BB) system (b) Total yC~ and yC{ mass spectrum trom reactions (3)-(8), 2979 events.
B R French et al / Hyperon-antlhyperon productlon
247
vmcmg evidence for any enhancements in BB or BBTr systems is observed at the level of sensitivity of this experiment (35 events//~b) see figs. lc, 3c, 4c, 5d and 5f. Fig. 6a shows the type of quark diagram which allows the formation of mesons composed of two quarks and two antxquarks (the so-called M4 mesons) and fig. 6b shows a compdatlon of the BB and BBn mass spectra xsolated in this experiment. No signal is seen, whereas a signal of 50 events (~3o) would necessitate a 5/lb producnon cross sectmn.
4. Widths of E(1385) and A(1520): A(1520) branching ratios 4.1. E(I385) width In the reaction 15p -* N + A n - + c.c. a good signal is evident in the S,~r- + c.c. mass distribution corresponding to the ~ ( 1 3 8 5 ) - + c.c. (see fig. 3b). Our mass resolution being relatwely good (AM-- -+3.1 MeV), we have derived a value for the mass and width of the ~(1385). The method used is the same as that of ref. [3] at 5.7 GeV/c and uses a p-wave resonance form for the Z(1385) Brelt-Wigner. After folding out the measurement error we find M~ = 1384 +- 1 MeV,
F~ = 32-+ 3.5 MeV,
to be compared to the Particle Data Table [5] values of 1383 + 1 MeV and 35 + 2 MeV respectwely. 4.2. A{1520) width The reaction ~p ~ ~, A(1520) + c c. gwe rise to three observable decay modes p K - , E+Tr- and A 7r+~r- . The p K - and N+Tr- mass distribution have relatively low backgrounds and good mass resolutions (-+2.4 and +-4.0 MeV respectively) in the A(1520) region and we have therefore used them to determine the mass and wxdth of the A(1520). After folding out the measurement resolutions and using a D-wave Brett-Wigner we find M A = 1520 +- 1 MeV,
PA = 14.1 + 2 MeV,
to be compared to the PDT values [5] of 1518 + 2 MeV and 16 -+ 2 MeV respectively. 4.3. A(1520) branching ratios The quasi two-body reactions (9), (14), (17) and (18) allow a determination of the braching ratios A(1520) ~ K - p '
l~-+rr± : A n + n - ,
B R French et al / Hyperon-antthyperon productton
248
F r o m t h e c r o s s s e c t i o n given in t a b l e 1 w e f i n d A ( 1 5 2 0 ) -~ K - p
2;±7r ~ . A n + n - = (42 +- 4 ) % : ( 4 6 -+ 5)%
this t o b e c o m p a r e d t o t h e P D T [5] values o f 4 6 %
42%.
(8 ± 2)%
10%.
5. D i f f e r e n t i a l c r o s s s e c t i o n T h e d i f f e r e n t i a l c r o s s s e c t i o n s f o r t h e quasi t w o - b o d y r e a c t i o n s p p -+ A 5 , ( 1 5 2 0 )
+ c.c. ,
(9)
E°A(1520)
+ c.c. ,
(l:)
c.c.,
(12)
A ° S . ( 1 4 0 5 ) + c.c. ,
(13)
A ° ~ ' ° ( 1 3 8 5 ) + c.c. ,
(19)
-+ Z ÷ E - ( 1 3 8 5 ) +
are d i s p l a y e d m figs. 7 a - e , r e s p e c t i v e l y . E a c h set o f d a t a has b e e n p a r a m e t n z e d b y
Table 2 Parameters of the fit of the differential cross sectlons to the formula given m sect. 5 Reaction + c c.
Slope a 1
Slope a 2
t~) (GeV 2)
x2/NDF
~.A(1520)
2.9 3.7
1.28 ± 0.4
0.81 ± 0 07
33/14 14/12
+ 0.2 ±0 3
E+~(1385)-
2 21 -+ 0.16
13/14
EOA(1520)
2.09 ± 0.2
8/11
~.Z(1385) °
3.4
± 0.3
82
+ 1 0
1.8
-+03
0.26+0.03
73/12 18/12
3.1 6.2
*0 3 ± 1.0
19
±0.4
026±0.04
33/12 18/12
AA a)
81
±0.4
25
±0.2
0.31-+002
A~0a)
11.1 ± 1.4
20
±02
0.21+-0.03
7.9 ± 0.8
1.8
± 0.2
0.19 ± 0.02
A.A(1405)
~ + ~ - a) a) Ref. [ 1 ].
B R French et al / ttyperon-antihyperon productzon [
f
I00
i
r
P P~.
)
i
,
249
,
Ais2o+ c c
~ p --=*AA,4os+C c
{o)
(d]
10
J
J
I00
I
I
L
J
I
i
I
p p~T-°A~sao÷Cc
i
i
~Jr
i
i
l--J~
~
t
l
P p ----,A~,3as+c C
A
(b)
::k
"~
IO
(e)
-
I00
~ p~-y~3as+c,c
I
i
04
0,8
i
i
I2
i
I6
t' ( GeV 2 ) (c)
I!
I 0',4 I 08
12
16
t' (GeV 2 ) Fig. 7. Differential cross-section variation (do/dt') for the quasi-two-body reactions. (a) PP ~ AA(1520) + c.c. (b) p p ~ ~ 0 A(1520) + c c. (c) p p ~ ~--~2+(1385) + c.c. (d) ~ p ~ AA(1405) + c.c. (e) PP '* XI~0(1385) + c.c The fitted curves represent the fits described m sect 5.
fitting the e x p e r i m e n t a l distributions to t w o e x p o n e n t l a l s w i t h different slopes, namely, d o = [ b e - a l It'l
for It'[ < ItS[ *
dr' ~be-alltble-a21t'-tbL
for It'J > It~l "
• t' = t - train, where t is the four-momentum transfer from the antlproton to the antihyperon and t m i n is the m m t m u m possible value o f t for the given reaction.
250
B.R French et a l / Hyperon.antthyperon production
i
,
, i I i P -~.A,szo+ c c. i
f
i
0,8 e-
0.4
o o
--
-0,4
13.-08
I 0
0,2
0,4
0,6
0,8
1.0
t' (GeV z ) Fig. 8. V a r i a t i o n o f the A ( A ) polanzataon as a function o f t' for the reaction pp - , A A ( 1 5 2 0 ) + c c
This is the same parametrization as used in ref. [1 ] and allows the break point [t~l to be determined along with its significance. The results of the fits are given in table 2, where the data for the reactions A A., A T ° and E+~, - [1 ] are also given for comparison. We note that whereas reactions A A(1405) + c.c. and A E ° ( 1 3 8 5 ) + c.c. (figs. 7d and e) present evidence for a two-exponential description with slopes and break points (t~) similar to the three reactions AA, A ~,o + c.c. and E+~ - (i.e. t o = 0.24 GeV 2) this is not the case for the other three reactions (9), (11) and (12) (figs. 7 a - c ) . These latter three reactions, however, have slopes of similar magnitude (~2) to the other reactions in the t-region above the break point t
6. Polarization The only channel which has sufficient statistics to allow determination of the A(A) polarization as a function of t' IS the AA(1520) channel obtained from reactions (9), (14) and (17). The distribution is shown in fig. 8 where a similarity to the reactions ~p -~ A.I~° + c.c. and ~ - p ~ K°A ° (shown in fig. 6 of ref. [1 ]) can be seen; notably the change in the sign of the polarization at t' = 0.2 GeV 2.
7. Observations on the data 7.1. Cross sections
We note that" (a) For the channels studied, approximately 60% of the cross section can be accounted for in terms of quasi two-body reactions.
B R French et a l / Hyperon-antthyperon productton
251
Table 3 Comparison ol peaks at low t' Initial state (spins) ~ final state (spins)
Peak at low t'C)
~p(l+, 1+)~ AA
i-1+, g1%, ,~
yes yes
A.A(1520)
A+ "2 , 2l-~' A+ "2 , 3_-~ 2 '
AE0(1385)
¢1+ "3 ,~3%'
yes
~0 A
(1+ - , ~1+)
yes
EOA(1405)
(}+, 21--)
no data
7,A(1405)
no
~0A(1520 ) (1+,3+)
no
~-:~+(1385) (1+, 3+)
no
(b) The cross-section ratio a@p ~ ~,(1520) A) o@-p ~ A ( 1 5 2 0 ) E ° )
= 2.0 + 0.3
ts n o t the same as the raUo o@p + A A ° ) - - = 1 . 0 7 + 0.1 o@p ~ A E ° ) and could Imply different exchanges are revolved for reacnons with A ( 1 5 2 0 ) m the final state rather than a A. Th~s s u p p o s m o n is supported by the absence of peaks at low t m the reactions revolving a A ( 1 5 2 0 ) (see table 3). (c) The ratio of the "allowed" to the " f o r b i d d e n " reacUon cross sections o@p -~ ~-(1385)12+)allowed
-10+2.2,
a@p -+ ~,+(1385)E-)forbldaen whlch revolve a E ( 1 3 8 5 ) is slmxlar to the ratio .~---- + o@p 12 12 )allowed = + o(~p-+--+ 12.2-3, E E )forbidden where the normal I2 replaces the E(1385). 7. 2. Differential cross sections
As m the previous work on the channels ~p -+ AA, AN, E12 [1 ] we observe the presence of forward peaks with slopes of the order 8 GeV - 2 and break points a r o u n d t' = 0.2 GeV 2. However not all the reactmns show peaks as can be seen m table 3.
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No obvious pattern exists except that the two states involving a A(1520) show no peaking at low t'. It will be of interest to see if the K*, K** Regge exchange model wtuch describes well the ~p -+ A S differential cross-section variation [6] can be extended to describe the above-mentioned characteristics. 7.3. Mesons coupled to BB No evidence was found for mesons coupled to the hyperon-antihyperon system with a cross section >5 ~b. 7.4. Polarization The polarization of the A ° in the channel ~p ~ S(1520) shows a t' variation similar to the channels gp ~ AA and NA [ 1] even though the A(1520)A channel shows no peaking m do/dt' at low t'.
References [1] H.W Atherton et al., Nucl. Phys. B69 (1974) 1. [2] H.W Atherton et al., Phys. Letters 42B (1972) 522. [3] H.W. Atherton et al., Nucl. Phys, B29 (1971) 477; V. AUes-Borelhet al., Nuovo Clmento 48 (1967) 360, M. Ferro-Luzzi et al., Nuovo Clmento 39 (1965) 47. [4] J L Rosner, Phys Rev. Letters 21 (1968) 960. [5] Particle Data Tables, Rev. Mod. Phys 48 (1976). [6] R D Field and C. Qulgg, Fermflab-Pub. 76/35-TNY.