- Email: [email protected]
Observation of a resonance with mass M=1814 MeV, decaying into π−ηη
Physics Letters B 268 ( 1991 ) 137-141 North-Holland
PHYSICS LETTERS B
Observation of a resonance with mass M = 1814 MeV, decaying into n-qrl S.I. Bityukov, G.V. Borisov, Yu.B. Bushnin, R.I. Dzhelyadin, Yu.P. G o u z , Yu.M. I v a n y u s h e n k o v , I.A. K a c h a e v , A.N. K a r y u h i n , Yu.A. K h o k h l o v , G.A. Kluchnikov, A.K. K o n o p l y a n n i k o v , V.F. K o n s t a n t i n o v , M.E. Kostrikov, V.V. K o s t y u k h i n , A.A. Kriushin, V.V. Lapin, V.D. M a t v e e v , V.F. O b r a z t s o v , A.P. O s t a n k o v , D.I. R y a b c h i k o v , V.K. S e m e n o v , E.A. Starchenko, V.S. Van'ev, N.K. Vishnevsky, E.V. Vlassov, A.M. Zaitsev Institutefor High Energy Physics, SU-142 284 Protvino (MoscowRegion), USSR
G.M. Beladidze, T.A. L o m t a d z e a n d E.G. T s k h a d a d z e Physical Institute, Academy of Science, SU-380 077 Tbilisi, Georgia, USSR Received 8 April 1991
The reaction n - N - - , n - r l r i N offa carbon nucleus at P~= 36 G e V / c has been studied with the help of the wide aperture magnetic spectrometer VES. In the n -rl~ system a resonance with M = 1814 _+ 10 (stat.) _+23 (syst.) MeV, F = 205 _+ 18 (star.) _ 32 ( syst. ) MeV has been observed. Its most probable q u a n t u m numbers are Ia= l + or 2 - .
We present here the results of a study of the reaction
n-N~n-rlTIN
( 1)
at incident m o m e n t u m 36 GeV/c. It was studied at the Vertex Spectrometer facility (VES) at the 70 GeV IHEP accelerator. This setup shown in fig. 1 is a wide aperture spectrometer consisting of beam detectors (scintillation and Cerenkov counters, proportional chambers); an active target of eight 1 cm thick scintillation counters surrounded by a veto system; 29 planes of proportional and drift chambers, with a maximum size of the chambers of 1.9 m × 2 . 5 m [1]; - a magnet with an aperture of 1 m X 2 . 5 m and a transverse kick of 0.7 GeV; a 28 channel gaseous Cerenkov counter with a threshold for n mesons of 4.4 GeV/c; a 200 channel scintillation hodoscope, used as multiplicity trigger; a 1600 channel y detector of lead glass of 2 m X 2.5 -
-
-
-
-
-
m. Elsevier Science Publishers B.V.
The following conditions were required to trigger an event: - the presence of a beam particle; - two or more charged particles, going forward from the target; no fast charged particles or photons detected by the veto system. The data acquisition system provided the detection of up to 10 3 events per cycle. Under these trigger conditions N = 1.4 X 10 8 events were recorded on tape. The first results, corresponding to the analysis of 2 X 10 7 reconstructed events, are presented in this paper. The q~n system was selected from the final state n + n - n - ~ y y . One q meson decays into n+n-n °, the second one decays into yy. Out of the total bulk of experimental events we chose the sample that satisfied the following criteria: 3 charged particles (2 negative and 1 positive) leave the target; 4 showers not associated with the charged tracks are to be detected in the T detector; the energy, not associated with these four showers -
-
-
-
137
Volume 268, number 1
PHYSICS LETTERS B
3 October 1991
PDC
VE S MWPC
,
!
lm
Fig. 1 The VES facility.
or showers from charged particles, is less than 0.5 GeV; - the total energy o f charged particles is XEch< 33 GeV; the total energy o f the event (the sum o f energies o f charged a n d neutral particles) is 34.5 G e V < Etot < 37 GeV; the mass o f one p a i r o f T - q u a n t a lies within the interval 0.1 GeV < Mr~ < 0.17 GeV, the mass o f the other pair is within the interval 0.46 GeV < Mrv < 0.63 GeV. The parameters o f the relevant cuts are chosen over -
-
500 -
(a)
easily selected reactions, e.g., n - p ~ n + n - n ° n , n - p ~ r l n , and n - p - - , r l ' n . The quality o f selection o f reaction ( 1 ) is d e m o n strated in fig. 2. The n + n - n ° mass distribution o f the selected events is shown in fig. 2a. The 77 mass distribution, i f 0 . 5 3 G e V < M , + , - ~ o < 0.57 GeV, is presented in fig. 2b by a d a s h e d line. The solid line in fig. 2b was o b t a i n e d after b a c k g r o u n d subtraction under rl meson decay along the n + n - n ° channel. After applying all the selection :criteria we o b t a i n a sample o f 1800 events o f reaction ( ! ).
500-(b) ,g
400
4.00
300
3OO
.~200
200
i i
~
,,
!l ~
r~
tJ
•
•
i I~.._- t rI t~o,
10o
I
I00~
0.65
0.75
M(~%-~ °) (CeV)
o0.30
0.50
,'2, ~
0.70
,.-,.~
0.90
b[(TT)(GeV)
Fig. 2. (a) The effective n+n-n ° mass spectrum for the selected events. (b) Dashed line: the effective (yy) mass spectrum for the events with 0.53 GeV
Volume 268, number 1
PHYSICS LETTERS B
3 October 1991 I00-
8O
100
o
~0
10
2O
1
,,,,,,,,,i
o.oo
.........
o.o
-c
i,,,,,,,,,i
0.20 (G~v ~)
....
0.30
0.40
Fig. 3. The - t' distribution for the "q~n- system.
Fig. 3 presents the d i s t r i b u t i o n over the squared m o m e n t u m transferred to the target. There is an evident peak at small It' ]. It is connected with the coherent p r o d u c t i o n o f the final state on carbon nuclei. F o r further analysis we selected events from the peak - t ' < 0.08 G e V 2 / c 2. Fig. 4 gives the Mnn~ distribution. In this spectrum we observe a peak with mass a n d width ( F W H M ) M = 1 . 8 1 4 + 0 . 0 1 0 (stat.) + 0 . 0 2 3 (syst.) G e V , F = 0 . 2 0 5 _ + 0 . 0 1 8 (stat.)_+0.032 ( s y s t . ) G e V , N = 426 + 57 e v e n t s . These p a r a m e t e r s were o b t a i n e d by fitting the mass spectrum with the sum o f the gaussian d i s t r i b u t i o n a n d the p o l y n o m i a l background. The systematic error o f the peak mass is d o m i n a t e d by overall mass scale n o r m a l i z a t i o n ( 18 M e V ) as well as by the unknown shape o f the b a c k g r o u n d ( 15 M e V ) . The syst e m a t i c error o f the width represents m a i n l y the uncertainty in the p a r a m e t r i z a t i o n o f the peak by either the gaussian or the B r e i t - W i g n e r distribution. The p a r a m e t e r s o f the peak are not sensitive to the acceptance o f o u r detector, because it is almost a constant in the mass range considered. The Mn~ mass spectrum for the events from the region o f the peak (1.71 G e V < M n n ~ < 1.95 G e V ) is shown in fig. 5a. N o structures are observed. The Mnn mass spectrum for the whole sample is presented in fig. 5b by the d a s h e d line. As for the events from the region o f the peak (1.71 G e V < Mnn~< 1.95 G e V ) the M ~ mass spectrum is shown
0 1.30
1.50
1.
.
.
.
0
Fig. 4. The effective Vlrl~-mass spectrum at It' [ <0.08 GeV2/c 2. Solid curve: the results of fitting the data with the sum of the gaussian distribution and the polynomial background. in fig. 5b by the solid line. One can see a narrow structure with a mass ~ 1.44 GeV. To d e t e r m i n e the q u a n t u m n u m b e r s o f the observed object we fitted the angular distributions. F o r the coherent process one can expect Je=O-, 1+, 2-, ..., a n d a zero projection on the G o t t f r i e d - J a c k s o n axis. However, we have included into the fit all possible q u a n t u m n u m b e r s up to J = 2 and i m p o s e d no limitations on the density matrix. The waves corresponding to the different values o f the orbital mom e n t L o f the n - meson and the m o m e n t S o f the ( q q ) system were considered as not interfering. We fit an angular distribution with the curves expected for different pure JP states and the confidence levels for each wave are quoted in table 1. The best description o f the d a t a is o b t a i n e d for JP= 1 +, 1 - and 2 - . The fit for J e = 2 - is illustrated in fig. 6. In this case the qq system is p r o d u c e d with S = 0 a n d S = 2 in the ratio 0.3/0.7. Fig. 6a presents the angular distributions over the n meson polar angle in the G o t t f r i e d - J a c k s o n system. Fig. 6b presents the d i s t r i b u t i o n over the angle between the direction o f the n meson emission in the "q'q CMS a n d the zaxis o f the G o t t f r i e d - J a c k s o n frame. Though the state J e = 1 - allows one to fit the angular distributions with an acceptable X2, in this case a large projection o f the total m o m e n t on the G o t t f r i e d - J a c k s o n axis is required, which does not correspond to the coherent p r o d u c t i o n features. N o t e that for Je= 1 + and 2 - this projection is small, as was expected. 139
Volume 268, number 1
PHYSICS LETTERS B
leo- (a)
120-
3 October 1991
(b)
I'"I I I
ij
I
I
I I.
_1
_1 :~
-i i I
80,
I I
I I rJ I I
0
II t J I
-i i-
I-! I I
-
I
~4o.
_
i i
-
I
i i I I
i-.I
I
ii I r, IILI I
I I
I i l
--I
_
l I I I I I I I I-I I
~
'7.60
I
0
.6.8''' 6 ~.9''' ' Lo'" ' ].(" L2'" L3'" i.
1.0
1.1
1,2
1.3
1.4
1.5
1.6
1.7
1.8
M(r/r/)(GeV)
Fig. 5 (a) The effective flu- mass spectrum for the events in the region of the peak ( 1.71 GeV < Mnn=- < 1.95 GeV). (b) Dashed line: the effective ('qq) mass spectrum for all the ~ r ~ - events. Solid line: the same, but for events in the region of the peak (1.71 GeV < Mnn,_ < 1.95 GeV). Table 1 The probability of the observed peak in the qqx- system to be a pure j e state
je
Probability
0-
< 10 -4
1+
0.04
1-
0.06
2-
0.6
2+
<10 -4
600-
.....
c r × B R = (3.1 + 1.0) g b .
600-
Data Fit for JP=2- (L=O,2)
•
'=-
=,
400.
400
20o
Hence for the object considered, the most probable q u a n t u m n u m b e r s are i a = 1 - , j e = 1 + o r 2 - . W e c a n call it A ~ / x 2 ( 1 8 1 4 ) . T h e p r o d u c t i o n c r o s s s e c t i o n f o r t h i s o b j e c t i n rea c t i o n ( 1 ) o f f a c a r b o n n u c l e u s a t P ~ = 36 G e V m u l tiplied by the branching ratio for the decay in the n-qq final state yields
I(a)
200"
(b) 0 ......... 0.0
i,,,,,,,,,= 0.2
......... 0.4
i ......... 0.6
ABS(COS(C)G,))
i ......... 0.8
= 1.0
0
o.o .......
.
6.'~
.
.
.
.
.
'''6.'~........................ 0.6 o.a
. J~
1.o
~S(COS(O,))
Fig. 6 (a) Distribution over the absolute value for the Gottfried-Jackson angle for the n - meson. (b) Distribution over the absolute value for the angle between the q meson and the z-axis of the Gottfried-Jackson system in the CMS of the (qq) system. 140
Volume 268, number 1
PHYSICS LETTERS B
60.
50.
o ~o " ~ 30.
io0 1.5
"I'Iv"
'1V'"21'" h3'" ~5'" ~ M(nn'~-)(oev)
Fig. 7. The effective (n-wq') mass spectrum. The search for this resonance was undertaken also in the reaction n-N--,n-lTq'(rl'-,n+n-rl; rl-,'~). This reaction was selected in a similar way as reaction ( 1 ), the only difference being that in the 47 system two q mesons were selected instead of n o and 11. The mass spectrum for the n - r l q ' system is presented in fig. 7. There is an enhancement at M = 1870 MeV. If the origin of this signal is the decay of the A~/~2 ( 1814) through the channel n - q q ' , then for the branching ratio o f two channels we have B R ( n - n q ' ) / B R ( n - q r l ) = 0 . 3 + 0.1. The observed resonance cannot be unambiguously identified with the well established resonances [ 2].
3 October 1991
There is a big difference in the masses between n2 (1670) and the object observed which hinders this identification. This difference cannot be described even if one introduces large orbital barrier factors, leading to the displacement o f the n2 peak. However, it is worth mentioning that in this mass region there was observed a complicated and ambiguously interpreted behaviour o f the 2 - wave in the reaction n-p~n-n+n-p [3 ]. In the qq scheme this object might have been a radial excitation o f A1 or n2(1670). However, one should expect a considerably larger width of the resonance in the first case, and a larger mass in the second. This fact and the identifications to an unusually large B R ( n - q W ) / B R ( n - q q ) allow us to treat the observed object as a candidate for an exotic state. This paper is the first publication of the results obtained at the VES facility. Besides the authors o f this paper, many other I H E P people made their contribution, and we are thankful to them.
References [ 1] S. Vyskochilet al., in: Proc. Intern. Symp. on Track detectors in high energy physics (Dubna, 1988) p. 202. [ 2 ] Particle Data Group, J.J. Hern~indezet al., Review of particle properties, Phys. Lett. B 239 (1990) 1. [3] C. Daum et al., Phys. Lett. B 89 (1980) 285.
141
PHYSICS LETTERS B
Observation of a resonance with mass M = 1814 MeV, decaying into n-qrl S.I. Bityukov, G.V. Borisov, Yu.B. Bushnin, R.I. Dzhelyadin, Yu.P. G o u z , Yu.M. I v a n y u s h e n k o v , I.A. K a c h a e v , A.N. K a r y u h i n , Yu.A. K h o k h l o v , G.A. Kluchnikov, A.K. K o n o p l y a n n i k o v , V.F. K o n s t a n t i n o v , M.E. Kostrikov, V.V. K o s t y u k h i n , A.A. Kriushin, V.V. Lapin, V.D. M a t v e e v , V.F. O b r a z t s o v , A.P. O s t a n k o v , D.I. R y a b c h i k o v , V.K. S e m e n o v , E.A. Starchenko, V.S. Van'ev, N.K. Vishnevsky, E.V. Vlassov, A.M. Zaitsev Institutefor High Energy Physics, SU-142 284 Protvino (MoscowRegion), USSR
G.M. Beladidze, T.A. L o m t a d z e a n d E.G. T s k h a d a d z e Physical Institute, Academy of Science, SU-380 077 Tbilisi, Georgia, USSR Received 8 April 1991
The reaction n - N - - , n - r l r i N offa carbon nucleus at P~= 36 G e V / c has been studied with the help of the wide aperture magnetic spectrometer VES. In the n -rl~ system a resonance with M = 1814 _+ 10 (stat.) _+23 (syst.) MeV, F = 205 _+ 18 (star.) _ 32 ( syst. ) MeV has been observed. Its most probable q u a n t u m numbers are Ia= l + or 2 - .
We present here the results of a study of the reaction
n-N~n-rlTIN
( 1)
at incident m o m e n t u m 36 GeV/c. It was studied at the Vertex Spectrometer facility (VES) at the 70 GeV IHEP accelerator. This setup shown in fig. 1 is a wide aperture spectrometer consisting of beam detectors (scintillation and Cerenkov counters, proportional chambers); an active target of eight 1 cm thick scintillation counters surrounded by a veto system; 29 planes of proportional and drift chambers, with a maximum size of the chambers of 1.9 m × 2 . 5 m [1]; - a magnet with an aperture of 1 m X 2 . 5 m and a transverse kick of 0.7 GeV; a 28 channel gaseous Cerenkov counter with a threshold for n mesons of 4.4 GeV/c; a 200 channel scintillation hodoscope, used as multiplicity trigger; a 1600 channel y detector of lead glass of 2 m X 2.5 -
-
-
-
-
-
m. Elsevier Science Publishers B.V.
The following conditions were required to trigger an event: - the presence of a beam particle; - two or more charged particles, going forward from the target; no fast charged particles or photons detected by the veto system. The data acquisition system provided the detection of up to 10 3 events per cycle. Under these trigger conditions N = 1.4 X 10 8 events were recorded on tape. The first results, corresponding to the analysis of 2 X 10 7 reconstructed events, are presented in this paper. The q~n system was selected from the final state n + n - n - ~ y y . One q meson decays into n+n-n °, the second one decays into yy. Out of the total bulk of experimental events we chose the sample that satisfied the following criteria: 3 charged particles (2 negative and 1 positive) leave the target; 4 showers not associated with the charged tracks are to be detected in the T detector; the energy, not associated with these four showers -
-
-
-
137
Volume 268, number 1
PHYSICS LETTERS B
3 October 1991
PDC
VE S MWPC
,
!
lm
Fig. 1 The VES facility.
or showers from charged particles, is less than 0.5 GeV; - the total energy o f charged particles is XEch< 33 GeV; the total energy o f the event (the sum o f energies o f charged a n d neutral particles) is 34.5 G e V < Etot < 37 GeV; the mass o f one p a i r o f T - q u a n t a lies within the interval 0.1 GeV < Mr~ < 0.17 GeV, the mass o f the other pair is within the interval 0.46 GeV < Mrv < 0.63 GeV. The parameters o f the relevant cuts are chosen over -
-
500 -
(a)
easily selected reactions, e.g., n - p ~ n + n - n ° n , n - p ~ r l n , and n - p - - , r l ' n . The quality o f selection o f reaction ( 1 ) is d e m o n strated in fig. 2. The n + n - n ° mass distribution o f the selected events is shown in fig. 2a. The 77 mass distribution, i f 0 . 5 3 G e V < M , + , - ~ o < 0.57 GeV, is presented in fig. 2b by a d a s h e d line. The solid line in fig. 2b was o b t a i n e d after b a c k g r o u n d subtraction under rl meson decay along the n + n - n ° channel. After applying all the selection :criteria we o b t a i n a sample o f 1800 events o f reaction ( ! ).
500-(b) ,g
400
4.00
300
3OO
.~200
200
i i
~
,,
!l ~
r~
tJ
•
•
i I~.._- t rI t~o,
10o
I
I00~
0.65
0.75
M(~%-~ °) (CeV)
o0.30
0.50
,'2, ~
0.70
,.-,.~
0.90
b[(TT)(GeV)
Fig. 2. (a) The effective n+n-n ° mass spectrum for the selected events. (b) Dashed line: the effective (yy) mass spectrum for the events with 0.53 GeV
Volume 268, number 1
PHYSICS LETTERS B
3 October 1991 I00-
8O
100
o
~0
10
2O
1
,,,,,,,,,i
o.oo
.........
o.o
-c
i,,,,,,,,,i
0.20 (G~v ~)
....
0.30
0.40
Fig. 3. The - t' distribution for the "q~n- system.
Fig. 3 presents the d i s t r i b u t i o n over the squared m o m e n t u m transferred to the target. There is an evident peak at small It' ]. It is connected with the coherent p r o d u c t i o n o f the final state on carbon nuclei. F o r further analysis we selected events from the peak - t ' < 0.08 G e V 2 / c 2. Fig. 4 gives the Mnn~ distribution. In this spectrum we observe a peak with mass a n d width ( F W H M ) M = 1 . 8 1 4 + 0 . 0 1 0 (stat.) + 0 . 0 2 3 (syst.) G e V , F = 0 . 2 0 5 _ + 0 . 0 1 8 (stat.)_+0.032 ( s y s t . ) G e V , N = 426 + 57 e v e n t s . These p a r a m e t e r s were o b t a i n e d by fitting the mass spectrum with the sum o f the gaussian d i s t r i b u t i o n a n d the p o l y n o m i a l background. The systematic error o f the peak mass is d o m i n a t e d by overall mass scale n o r m a l i z a t i o n ( 18 M e V ) as well as by the unknown shape o f the b a c k g r o u n d ( 15 M e V ) . The syst e m a t i c error o f the width represents m a i n l y the uncertainty in the p a r a m e t r i z a t i o n o f the peak by either the gaussian or the B r e i t - W i g n e r distribution. The p a r a m e t e r s o f the peak are not sensitive to the acceptance o f o u r detector, because it is almost a constant in the mass range considered. The Mn~ mass spectrum for the events from the region o f the peak (1.71 G e V < M n n ~ < 1.95 G e V ) is shown in fig. 5a. N o structures are observed. The Mnn mass spectrum for the whole sample is presented in fig. 5b by the d a s h e d line. As for the events from the region o f the peak (1.71 G e V < Mnn~< 1.95 G e V ) the M ~ mass spectrum is shown
0 1.30
1.50
1.
.
.
.
0
Fig. 4. The effective Vlrl~-mass spectrum at It' [ <0.08 GeV2/c 2. Solid curve: the results of fitting the data with the sum of the gaussian distribution and the polynomial background. in fig. 5b by the solid line. One can see a narrow structure with a mass ~ 1.44 GeV. To d e t e r m i n e the q u a n t u m n u m b e r s o f the observed object we fitted the angular distributions. F o r the coherent process one can expect Je=O-, 1+, 2-, ..., a n d a zero projection on the G o t t f r i e d - J a c k s o n axis. However, we have included into the fit all possible q u a n t u m n u m b e r s up to J = 2 and i m p o s e d no limitations on the density matrix. The waves corresponding to the different values o f the orbital mom e n t L o f the n - meson and the m o m e n t S o f the ( q q ) system were considered as not interfering. We fit an angular distribution with the curves expected for different pure JP states and the confidence levels for each wave are quoted in table 1. The best description o f the d a t a is o b t a i n e d for JP= 1 +, 1 - and 2 - . The fit for J e = 2 - is illustrated in fig. 6. In this case the qq system is p r o d u c e d with S = 0 a n d S = 2 in the ratio 0.3/0.7. Fig. 6a presents the angular distributions over the n meson polar angle in the G o t t f r i e d - J a c k s o n system. Fig. 6b presents the d i s t r i b u t i o n over the angle between the direction o f the n meson emission in the "q'q CMS a n d the zaxis o f the G o t t f r i e d - J a c k s o n frame. Though the state J e = 1 - allows one to fit the angular distributions with an acceptable X2, in this case a large projection o f the total m o m e n t on the G o t t f r i e d - J a c k s o n axis is required, which does not correspond to the coherent p r o d u c t i o n features. N o t e that for Je= 1 + and 2 - this projection is small, as was expected. 139
Volume 268, number 1
PHYSICS LETTERS B
leo- (a)
120-
3 October 1991
(b)
I'"I I I
ij
I
I
I I.
_1
_1 :~
-i i I
80,
I I
I I rJ I I
0
II t J I
-i i-
I-! I I
-
I
~4o.
_
i i
-
I
i i I I
i-.I
I
ii I r, IILI I
I I
I i l
--I
_
l I I I I I I I I-I I
~
'7.60
I
0
.6.8''' 6 ~.9''' ' Lo'" ' ].(" L2'" L3'" i.
1.0
1.1
1,2
1.3
1.4
1.5
1.6
1.7
1.8
M(r/r/)(GeV)
Fig. 5 (a) The effective flu- mass spectrum for the events in the region of the peak ( 1.71 GeV < Mnn=- < 1.95 GeV). (b) Dashed line: the effective ('qq) mass spectrum for all the ~ r ~ - events. Solid line: the same, but for events in the region of the peak (1.71 GeV < Mnn,_ < 1.95 GeV). Table 1 The probability of the observed peak in the qqx- system to be a pure j e state
je
Probability
0-
< 10 -4
1+
0.04
1-
0.06
2-
0.6
2+
<10 -4
600-
.....
c r × B R = (3.1 + 1.0) g b .
600-
Data Fit for JP=2- (L=O,2)
•
'=-
=,
400.
400
20o
Hence for the object considered, the most probable q u a n t u m n u m b e r s are i a = 1 - , j e = 1 + o r 2 - . W e c a n call it A ~ / x 2 ( 1 8 1 4 ) . T h e p r o d u c t i o n c r o s s s e c t i o n f o r t h i s o b j e c t i n rea c t i o n ( 1 ) o f f a c a r b o n n u c l e u s a t P ~ = 36 G e V m u l tiplied by the branching ratio for the decay in the n-qq final state yields
I(a)
200"
(b) 0 ......... 0.0
i,,,,,,,,,= 0.2
......... 0.4
i ......... 0.6
ABS(COS(C)G,))
i ......... 0.8
= 1.0
0
o.o .......
.
6.'~
.
.
.
.
.
'''6.'~........................ 0.6 o.a
. J~
1.o
~S(COS(O,))
Fig. 6 (a) Distribution over the absolute value for the Gottfried-Jackson angle for the n - meson. (b) Distribution over the absolute value for the angle between the q meson and the z-axis of the Gottfried-Jackson system in the CMS of the (qq) system. 140
Volume 268, number 1
PHYSICS LETTERS B
60.
50.
o ~o " ~ 30.
io0 1.5
"I'Iv"
'1V'"21'" h3'" ~5'" ~ M(nn'~-)(oev)
Fig. 7. The effective (n-wq') mass spectrum. The search for this resonance was undertaken also in the reaction n-N--,n-lTq'(rl'-,n+n-rl; rl-,'~). This reaction was selected in a similar way as reaction ( 1 ), the only difference being that in the 47 system two q mesons were selected instead of n o and 11. The mass spectrum for the n - r l q ' system is presented in fig. 7. There is an enhancement at M = 1870 MeV. If the origin of this signal is the decay of the A~/~2 ( 1814) through the channel n - q q ' , then for the branching ratio o f two channels we have B R ( n - n q ' ) / B R ( n - q r l ) = 0 . 3 + 0.1. The observed resonance cannot be unambiguously identified with the well established resonances [ 2].
3 October 1991
There is a big difference in the masses between n2 (1670) and the object observed which hinders this identification. This difference cannot be described even if one introduces large orbital barrier factors, leading to the displacement o f the n2 peak. However, it is worth mentioning that in this mass region there was observed a complicated and ambiguously interpreted behaviour o f the 2 - wave in the reaction n-p~n-n+n-p [3 ]. In the qq scheme this object might have been a radial excitation o f A1 or n2(1670). However, one should expect a considerably larger width of the resonance in the first case, and a larger mass in the second. This fact and the identifications to an unusually large B R ( n - q W ) / B R ( n - q q ) allow us to treat the observed object as a candidate for an exotic state. This paper is the first publication of the results obtained at the VES facility. Besides the authors o f this paper, many other I H E P people made their contribution, and we are thankful to them.
References [ 1] S. Vyskochilet al., in: Proc. Intern. Symp. on Track detectors in high energy physics (Dubna, 1988) p. 202. [ 2 ] Particle Data Group, J.J. Hern~indezet al., Review of particle properties, Phys. Lett. B 239 (1990) 1. [3] C. Daum et al., Phys. Lett. B 89 (1980) 285.
141