- Email: [email protected]
c2 very narrow baryon resonance
Physics Letters B 281 ( 1991 ) 148-152 North-Holland
P bl YSIC S / EYT ERS 13
A b o u t a possible 3.52 G e V / c 2 very narrow baryon resonance V.M. K a r n a u k h o v , V.I. M o r o z J1NR Dubna, P.O. Box 79, 101 O00 Moscow, Russia
C. C o c a 1FA, R-76900 Bucharest-Magurele, Romania
and A. M i h u l Bucharest University, R-76900 Bucharest-Magurele, Romania
Received 6 August 1991
An analysis of 16 GeV/c n p interactions shows a peak in the mass of the pK+I~°nn system, at .V/R=3520+ 3 MeV/c 2 with baryonic resonance with zero apparent strangenessdecayingby a cascade involving the K*- resonance. The narrowness of the peak and the decay mode indicate the existenceof a strong interaction decay.
FR = 7_+ 20 MeV/c 2 indicating the possibility ofa
The problem of the upper limit for the mass of particles or resonances has been discussed amply. For the m o m e n t such a limit is not foreseen and the only existing one is given by the available CMS energy and our ability to identify the particles [ 1 ]. O n the basis of the available experimental material we performed a study on the eventual existence of such systems with a rather big mass. We found a peak in the mass of the system p K + I ~ ° n - n - with the parameters MR=3520+_3 M e V / c 2 and F R = 7 _+2° M e V / c 2. This indicates the possibility of the existence of a baryonic resonance with null apparent strangeness and which seems to decay via a cascade involving the K * - resonance. The narrowness of the peak and its decay mode point out that we have a system with a forbidden decay via strong interactions and so encourages us to think about a pentaquark with hidden charm (uudcc) [2] as its possible interpretation. About 125 000 pictures from the CERN 2 m bubble chamber exposed to the 16 G e V / c n - beam were analyzed for production and registration of neutral strange particles (A and K °) correlated with fourprong events. Some of the results and also the pro148
cessing procedure have already been published [ 3 ]. The present work concerns the analysis of a sample of 1684 events with identified K ° ( I ~ ) . In this sample the ambiguous K ° / A events were not included, as an earlier performed analysis [4] has shown that more than 80% of them are in fact A's. In order to improve the m o m e n t u m precision, all events having a secondary track measured with a relative error A p / p>~ I0%, also were not included in the sample used. In such a way the mean value for the m o m e n t u m relative error for secondary tracks was ( A p / p ) = 2.5%. The sample used is a mixture of different channels beginning with those having no supplementary neutral particles (4C fit) and finishing with those having more than one supplementary neutral particle (no fit). The total n u m b e r of fitted events being relatively small ( < 300) we decided to analyze our events straightforwardly for the possible existence of a heavy mass system. With this aim in m i n d we computed the masses of hypothetical systems formed by all the particles seen ( + , + , - , - , Kso ), assigning to the tracks the masses of known particles (p, K +, n +-) and grouping them respectively to the possible final states. Some of the assignments are shown in table 1.
0370-2693/92/$ 05.00 © 1992 ElsevierSciencePublishers B.V. All rights reserved.
Volume 281, number
1,2
7 May 1992
PHYSICS LETTERS B
Table 1 No.
Comb/ events
+
+
-
-
V°
X°
1
1
n
n
n
n
KI~
strange
2 3
2
K n
n K
n n
n n
I~ I~
no strange no strange
4 5
2
n n
n n
K n
n K
K K
no strange no strange
6 7
2
p n
n p
n n
n n
KI~ KI~
strange strange
8 9
2
p K
K p
n n
n n
I~ I~
no strange no strange
10 11 12 13
4
p p n n
n n p p
K n K n
n K n K
K K K K
no strange no strange no strange no strange
F r o m the final states computed in such a way we show the mass spectra for the p K - K ° n + n - (fig. l a ) a n d p K + I ~ ° n - n - (fig. l b ) assignments. In order to reduce the n u m b e r of fake combinations, we took into account the results of fits (4C and 1C) and also the results of particle identification via ionization analysis. The ionization of the secondary particles was compared to that of the primary one. The upper m o m e n t u m limit for separation by ionization of pions from kaons was 0.7 G e V / c and for the separation of pions and kaons from protons it was 1.3 GeV/c. Unfortunately, from the 4 X 1684 charged particles of our sample, less than 20% were successfully identified by ionization. Nevertheless, this permits the reduction of the n u m b e r of weighted combinations to 2183 for the p K - I ~ n + n - assignment (nos. 10, 11, 12, 13 in table 1 ) and to 1057 weighted c o m b i n a t i o n s for the p K + I ~ n - n - assignment (nos. 8, 9 in table 1 ). The mean weight of the events was 1.13. These two distributions have the same shape with the exception of the region a r o u n d 3.5 G e V / c 2 where the p K + I ~ n - n - system acquires a sharp peak of more than 5a over the background while the p K - I ~ n + n - distribution shows no such peak. It is possible to see in both distributions rather large b u m p s at the upper limit of x/~ of the mass spectra. Those b u m p s correspond to the events that are identified by the 4C fit and which were included in the
sample (27 weighted events, 4C fit for the p K + I ~ n - n - final state and 29 weighted events for the p K - I ~ n + n - final state [ 5 ] ) . The width of the x/~ b u m p s can be explained by the fact that we used two expositions slightly differing in the primary mom e n t u m , namely p~= 15.85+0.18 and 16.28+0.08 GeV/c, corresponding to CMS energies of 5.53 and 5.61 G e V / c 2 respectively. The height and the narrowness of the 3.5 G e V / c 2 peak pushed us to perform a complementary analysis. In principle this analysis was done by comparing the events in the peak, this means the 3.45-3.55 G e V / c 2 mass region, with the events from the adjacent regions, this means the 3.05-3.45 and 3.55-3.95 M e V / c 2 mass regions. The adjacent regions were taken as witnesses. The analysis consists in computing Z 2 and the corresponding confidence level for the experimental distribution with respect to a polynomial background. The analysis was carried in three directions: The first consists in looking at the possible K°n structures considering the influence on the peak. The mass spectrum for the K ° n - c o m b i n a t i o n s from the peak and from the adjacent regions (fig. 2) shows that more than 2 of the events in the peak have in their composition a K n pair with a mass in the 0.7-1.05 G e V / c 2 mass region, the interval including mainly the K*(892) resonance. A b u m p also appears in the 1.3 G e V / c 2 mass region where several K* were iden149
Volume 281, number ! ,2
PHYSICS LETTERS B
(a)
(a)
<
}60
7 May 1992
0 "~40. ou
¥
0 .-+-2500
3000
3500
4000
4500
5000 5500 6000 6500 M(pK-K°~*~ -) MeV/J
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0 75
100
075
100
1.25
150 (K°~ -) OeV,/c2
z 75
o:
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(b)
<5o~ (b) (-3 ff3 c?o
"~30-
8
o oa3
~2o 1:
._~ 0.! 0
~10.
i
U 0
150
~1.75
(K°~ -) aev/c 2
I,
2500
3000
3500
4.000
4.5o0
5000
5500
6o00
M(pK*K°~-/r -)
6500
MeV/c 2
Fig. 1. Mass distribution for (a) the pn+K~K-lt- assignment; 2183 combinations. (b) The pK+I~n-n - assignment; 1057 combinations. The 3.5 G e V / c 2 peak contains 77 combinations from which 40 are fitted to the background. tiffed [6]. If we eliminate the events with the K x mass in the region 0.7-1.05 G e V / c 2, the 3.5 G e V / c 2 peak disappears. We also stress that, for the events in the peak, as a rule, only one of the two possible K°rt - c o m b i n a t i o n s enters the K* band, the other being distributed randomly in a large mass interval. The combinatorial weight in the peak is 1.12. This fact leads to the idea that a cascade decay took place, a pKI~xx system of ~ 3.5 G e V / c 2 mass decaying through a intermediate state having a K * - in composition. As the K*- has a negative strangeness the I ~ from its decay will have the same strangeness. This fact give us the indication that the events that compose the 150
r-- 1.25
Fig. 2. (a) Mass distribution of the K°~- system for 154 combinations for events from the interval 3.45-3.55 GeV/c 2. The shaded area represents the distribution of the K°x - mass for events in intervals adjacent to the peak normalized to the background. (b) Difference between the two distributions from (a). peak contain mainly l<°. So, probably, the 3.5 G e V / c: baryonic system has null strangeness. This conclusion is strengthened by the fact that strangeness 2 for the events in our sample must be accompanied by the frequent appearance of at least one more strange particle, in contradiction with the experimental results. The second analysis consists in searching for the influence of the K *+ on the peak. We eliminated from the p K + K ° n - n - mass distribution those events entering the K*(892) b a n d (0.8-0.95 G e V / c 2) when we assign to the positive track (not identified as a proton by ionization) the mass o f a n +. Such a procedure, eliminating ~ of the weighted c o m b i n a t i o n s from fig. 3a, does not kill the peak (fig. 3b). The conclusion is that the peak is not a reflection of the existence of the K *+ resonance.
Volume 281, number 1,2
>50
o
PHYSICS LETTERS B
(a)
-~.4o ©
~ 30 ~5 E ~ 20
~ , ~ , ~ ~. . . . , 2500 3000 3500 4000 4500 5000 5500 6000 6500 M(pK÷K'~-~ -) MeV/c 2
0
40
>= 30 "5 0
%
~o
0
, ~,
, ,T
....
i
2500 3000 3500 4000 4500 5000 5500 6000 6 5 0 0 M(pK+K°~'-~"-) MeV/c z
Fig. 3. The p K + I ~ n - n - mass distribution. (a) For events fulfilling the condition 0.7 < M ( I ~ n - ) < 1.05 G e V / c 2 and - 0.18 < F < 0.38. (b) Same as (a) but with a supplementary anti-cut for
events from 0.8 < M(K°n + ) < 0.95 GeV/c 2, see text.
The third direction concerns the analysis of the rapidity region ( Y = 0 . 5 In[ ( E + p c ) / ( E - p L ) ] ) in which such a peak appears. Looking at different Y regions we observed that this peak disappears if the Y region is outside the region - 0.18 < Y< 0.38. This corresponds to the appearance of the peak in central collisions only. The mass distributions obtained after the Y and K*- cuts are shown in fig. 3a. We must remark that these cuts did not eliminate the events from the peak above the background, their number remaining practically the same (37 in fig. lb and 42 in fig. 3a). The number of standard deviations o f the peak over the background raises from 5trto 8.9tr. The z 2 / N D value
7 May 1992
obtained after the fit of a Breit-Wigner distribution plus a polynomyal background to the interval 3.053.95 G e V / c 2 gives z 2 / N D = 0 . 8 6 with a probability o f 5X 10 -4 for the peak being a statistical fluctuation. These facts strongly indicate that a resonance is present indeed. An analysis was done in order to find the characteristics of the peak. Fitting a polynomial plus a BreitWigner to the distribution (constructed through 25 M e V / c 2 ) of events which contains at least one (Kn) combination in the mass region 0.7-1.05 G e V / c 2 and has Y in the region - 0 . 1 8 - 0 . 3 8 , we obtain M R = 3 5 2 0 + 3 M e V / c 2 with F R = 2 0 + 16 M e V / c 2. As the obtained width of 20 M e V / c is comparable with our apparatus resolution in this mass region, we computed the physical width using the resolution function, obtained earlier in our experiment for this energy interval, namely ( a ( m ) ) = 17 M e V / c 2. Using this resolution function [ 7 ] we obtained a new value for the width, F R = (7+27o ) M e V / c 2. The narrowness of the peak and the decay mode stimulate the idea of the possible interpretation o f the peak as a pentaquark system with hidden charm (uudce). We computed also the missing mass (X °) for the events in the peak interval, and for those in the adjacent regions (fig. 4a). The difference between these two distributions is also shown (fig. 4b). It is clear from these distributions that they are not similar and that for the events from the peak a preferred region with missing mass of 1.4-2.1 G e V / c 2 appears. This result gives us some arguments to suppose that we have a two-particle reaction involving a neutral decaying meson: n - p-+ baryon ( M = 3.52 G e V / c 2 ) + m e s o n ( 1 . 4 < M < 2.1 G e V / c 2) .
( 1)
If we look in the RPP [6], there are several candidates for such a meson, having masses of 1.4-2.1 G e V / c 2 and dominantly decaying in a neutral mode as fo(1590), f2(1720), ¢3(1850), f2(2010) ..... The charged mode decay for those mesons would contribute to topologies that are not taken into consideration in our experiment. Based on the equivalent cross section of 0.1896 ___0.0095 ~tb/eV obtained earlier [5] we computed the cross section for the appearance of such a resonance and obtained aR = 14 + 3 txb. 151
Volume 281, number 1,2
PHYSICS LETTERS B
In c o n c l u s i o n we c o n s i d e r that: ( 1 ) it is v e r y p r o b a b l e that the b a r y o n i c system p K ÷ I < ° n - n - f o r m s a neutral r e s o n a n c e w i t h null app a r e n t strangeness, MR = 3.520 + 0.003 G e V / c 2 a n d FR----7 ÷2°-7M e V / c 2 ;
20
(a)
( 2 ) the resonance ( 3 ) this teractions
o
5
0 0,0
7 May 1992
0,5
1,0
1.5
2.0
2 5
M(x*) 6GV/c = 20.
cross section for the a p p e a r a n c e o f such a in o u r r e a c t i o n is 14 + 3 pb; r e s o n a n t system is p r o d u c e d in central ina n d decays p r o b a b l y t h r o u g h a K * - .
T h e a u t h o r s wish to a c k n o w l e d g e C E R N for prov i d i n g the e x p e r i m e n t a l m a t e r i a l , a n d also Dr. N. A n g e l o v , Dr. B. S a c h b a z i a n a n d Dr. F. C o t o r o b a i for i n t e r e s t i n g discussions.
(b) References
15-
10~
E
-5
M(x°) G*vtc'
Fig. 4. (a) Missing mass (X °) distribution for the events from the 3.5 GeV/c 2 peak. The shaded area represents the X ° distribution for the adjacent regions normalized to the background. (b) The difference between the two distributions from (a).
152
[ 1] N. Isgur et al., Phys. Rev. D 18 (1978) 4187; D 19 (1979) 2653;D20(1979) 1191; A.W. Hendry, Ann. Phys. 136 (1981) 1. [2] H. Lipkin, Phys. Lett. B 195 (1987) 484. J.-M. Richard, talk Rheinfels '90 Workshop on The hadron mass spectrum (St. Goar, FRG, September 1990). [3] E. Balea et al., Nucl. Phys. B 150 (1979) 345.;B 165 (1980) 21; V.M. Karnaukhov et al., JINR preprint PI-86-373 (1986). [4] E. Balea el al., JINR preprint 1-8138 ( 1964); V.M. Karnaukhov et al., JINR communication P1-87-559 (1987). [ 5 ] E. Balea et al., JINR communication E 1-12345 ( 1979 ). [ 6 ] Particle Data Group, J.J. Hermindez et al., Review of particle properties, Phys. Lett. B 239 (1990) 1. [7 ] W.T. Eadie et al., Statistical methods in experimental physics (North-Holland, Amsterdam, 1971 ).
P bl YSIC S / EYT ERS 13
A b o u t a possible 3.52 G e V / c 2 very narrow baryon resonance V.M. K a r n a u k h o v , V.I. M o r o z J1NR Dubna, P.O. Box 79, 101 O00 Moscow, Russia
C. C o c a 1FA, R-76900 Bucharest-Magurele, Romania
and A. M i h u l Bucharest University, R-76900 Bucharest-Magurele, Romania
Received 6 August 1991
An analysis of 16 GeV/c n p interactions shows a peak in the mass of the pK+I~°nn system, at .V/R=3520+ 3 MeV/c 2 with baryonic resonance with zero apparent strangenessdecayingby a cascade involving the K*- resonance. The narrowness of the peak and the decay mode indicate the existenceof a strong interaction decay.
FR = 7_+ 20 MeV/c 2 indicating the possibility ofa
The problem of the upper limit for the mass of particles or resonances has been discussed amply. For the m o m e n t such a limit is not foreseen and the only existing one is given by the available CMS energy and our ability to identify the particles [ 1 ]. O n the basis of the available experimental material we performed a study on the eventual existence of such systems with a rather big mass. We found a peak in the mass of the system p K + I ~ ° n - n - with the parameters MR=3520+_3 M e V / c 2 and F R = 7 _+2° M e V / c 2. This indicates the possibility of the existence of a baryonic resonance with null apparent strangeness and which seems to decay via a cascade involving the K * - resonance. The narrowness of the peak and its decay mode point out that we have a system with a forbidden decay via strong interactions and so encourages us to think about a pentaquark with hidden charm (uudcc) [2] as its possible interpretation. About 125 000 pictures from the CERN 2 m bubble chamber exposed to the 16 G e V / c n - beam were analyzed for production and registration of neutral strange particles (A and K °) correlated with fourprong events. Some of the results and also the pro148
cessing procedure have already been published [ 3 ]. The present work concerns the analysis of a sample of 1684 events with identified K ° ( I ~ ) . In this sample the ambiguous K ° / A events were not included, as an earlier performed analysis [4] has shown that more than 80% of them are in fact A's. In order to improve the m o m e n t u m precision, all events having a secondary track measured with a relative error A p / p>~ I0%, also were not included in the sample used. In such a way the mean value for the m o m e n t u m relative error for secondary tracks was ( A p / p ) = 2.5%. The sample used is a mixture of different channels beginning with those having no supplementary neutral particles (4C fit) and finishing with those having more than one supplementary neutral particle (no fit). The total n u m b e r of fitted events being relatively small ( < 300) we decided to analyze our events straightforwardly for the possible existence of a heavy mass system. With this aim in m i n d we computed the masses of hypothetical systems formed by all the particles seen ( + , + , - , - , Kso ), assigning to the tracks the masses of known particles (p, K +, n +-) and grouping them respectively to the possible final states. Some of the assignments are shown in table 1.
0370-2693/92/$ 05.00 © 1992 ElsevierSciencePublishers B.V. All rights reserved.
Volume 281, number
1,2
7 May 1992
PHYSICS LETTERS B
Table 1 No.
Comb/ events
+
+
-
-
V°
X°
1
1
n
n
n
n
KI~
strange
2 3
2
K n
n K
n n
n n
I~ I~
no strange no strange
4 5
2
n n
n n
K n
n K
K K
no strange no strange
6 7
2
p n
n p
n n
n n
KI~ KI~
strange strange
8 9
2
p K
K p
n n
n n
I~ I~
no strange no strange
10 11 12 13
4
p p n n
n n p p
K n K n
n K n K
K K K K
no strange no strange no strange no strange
F r o m the final states computed in such a way we show the mass spectra for the p K - K ° n + n - (fig. l a ) a n d p K + I ~ ° n - n - (fig. l b ) assignments. In order to reduce the n u m b e r of fake combinations, we took into account the results of fits (4C and 1C) and also the results of particle identification via ionization analysis. The ionization of the secondary particles was compared to that of the primary one. The upper m o m e n t u m limit for separation by ionization of pions from kaons was 0.7 G e V / c and for the separation of pions and kaons from protons it was 1.3 GeV/c. Unfortunately, from the 4 X 1684 charged particles of our sample, less than 20% were successfully identified by ionization. Nevertheless, this permits the reduction of the n u m b e r of weighted combinations to 2183 for the p K - I ~ n + n - assignment (nos. 10, 11, 12, 13 in table 1 ) and to 1057 weighted c o m b i n a t i o n s for the p K + I ~ n - n - assignment (nos. 8, 9 in table 1 ). The mean weight of the events was 1.13. These two distributions have the same shape with the exception of the region a r o u n d 3.5 G e V / c 2 where the p K + I ~ n - n - system acquires a sharp peak of more than 5a over the background while the p K - I ~ n + n - distribution shows no such peak. It is possible to see in both distributions rather large b u m p s at the upper limit of x/~ of the mass spectra. Those b u m p s correspond to the events that are identified by the 4C fit and which were included in the
sample (27 weighted events, 4C fit for the p K + I ~ n - n - final state and 29 weighted events for the p K - I ~ n + n - final state [ 5 ] ) . The width of the x/~ b u m p s can be explained by the fact that we used two expositions slightly differing in the primary mom e n t u m , namely p~= 15.85+0.18 and 16.28+0.08 GeV/c, corresponding to CMS energies of 5.53 and 5.61 G e V / c 2 respectively. The height and the narrowness of the 3.5 G e V / c 2 peak pushed us to perform a complementary analysis. In principle this analysis was done by comparing the events in the peak, this means the 3.45-3.55 G e V / c 2 mass region, with the events from the adjacent regions, this means the 3.05-3.45 and 3.55-3.95 M e V / c 2 mass regions. The adjacent regions were taken as witnesses. The analysis consists in computing Z 2 and the corresponding confidence level for the experimental distribution with respect to a polynomial background. The analysis was carried in three directions: The first consists in looking at the possible K°n structures considering the influence on the peak. The mass spectrum for the K ° n - c o m b i n a t i o n s from the peak and from the adjacent regions (fig. 2) shows that more than 2 of the events in the peak have in their composition a K n pair with a mass in the 0.7-1.05 G e V / c 2 mass region, the interval including mainly the K*(892) resonance. A b u m p also appears in the 1.3 G e V / c 2 mass region where several K* were iden149
Volume 281, number ! ,2
PHYSICS LETTERS B
(a)
(a)
<
}60
7 May 1992
0 "~40. ou
¥
0 .-+-2500
3000
3500
4000
4500
5000 5500 6000 6500 M(pK-K°~*~ -) MeV/J
°oll 050
0 75
100
075
100
1.25
150 (K°~ -) OeV,/c2
z 75
o:
°°I
(b)
<5o~ (b) (-3 ff3 c?o
"~30-
8
o oa3
~2o 1:
._~ 0.! 0
~10.
i
U 0
150
~1.75
(K°~ -) aev/c 2
I,
2500
3000
3500
4.000
4.5o0
5000
5500
6o00
M(pK*K°~-/r -)
6500
MeV/c 2
Fig. 1. Mass distribution for (a) the pn+K~K-lt- assignment; 2183 combinations. (b) The pK+I~n-n - assignment; 1057 combinations. The 3.5 G e V / c 2 peak contains 77 combinations from which 40 are fitted to the background. tiffed [6]. If we eliminate the events with the K x mass in the region 0.7-1.05 G e V / c 2, the 3.5 G e V / c 2 peak disappears. We also stress that, for the events in the peak, as a rule, only one of the two possible K°rt - c o m b i n a t i o n s enters the K* band, the other being distributed randomly in a large mass interval. The combinatorial weight in the peak is 1.12. This fact leads to the idea that a cascade decay took place, a pKI~xx system of ~ 3.5 G e V / c 2 mass decaying through a intermediate state having a K * - in composition. As the K*- has a negative strangeness the I ~ from its decay will have the same strangeness. This fact give us the indication that the events that compose the 150
r-- 1.25
Fig. 2. (a) Mass distribution of the K°~- system for 154 combinations for events from the interval 3.45-3.55 GeV/c 2. The shaded area represents the distribution of the K°x - mass for events in intervals adjacent to the peak normalized to the background. (b) Difference between the two distributions from (a). peak contain mainly l<°. So, probably, the 3.5 G e V / c: baryonic system has null strangeness. This conclusion is strengthened by the fact that strangeness 2 for the events in our sample must be accompanied by the frequent appearance of at least one more strange particle, in contradiction with the experimental results. The second analysis consists in searching for the influence of the K *+ on the peak. We eliminated from the p K + K ° n - n - mass distribution those events entering the K*(892) b a n d (0.8-0.95 G e V / c 2) when we assign to the positive track (not identified as a proton by ionization) the mass o f a n +. Such a procedure, eliminating ~ of the weighted c o m b i n a t i o n s from fig. 3a, does not kill the peak (fig. 3b). The conclusion is that the peak is not a reflection of the existence of the K *+ resonance.
Volume 281, number 1,2
>50
o
PHYSICS LETTERS B
(a)
-~.4o ©
~ 30 ~5 E ~ 20
~ , ~ , ~ ~. . . . , 2500 3000 3500 4000 4500 5000 5500 6000 6500 M(pK÷K'~-~ -) MeV/c 2
0
40
>= 30 "5 0
%
~o
0
, ~,
, ,T
....
i
2500 3000 3500 4000 4500 5000 5500 6000 6 5 0 0 M(pK+K°~'-~"-) MeV/c z
Fig. 3. The p K + I ~ n - n - mass distribution. (a) For events fulfilling the condition 0.7 < M ( I ~ n - ) < 1.05 G e V / c 2 and - 0.18 < F < 0.38. (b) Same as (a) but with a supplementary anti-cut for
events from 0.8 < M(K°n + ) < 0.95 GeV/c 2, see text.
The third direction concerns the analysis of the rapidity region ( Y = 0 . 5 In[ ( E + p c ) / ( E - p L ) ] ) in which such a peak appears. Looking at different Y regions we observed that this peak disappears if the Y region is outside the region - 0.18 < Y< 0.38. This corresponds to the appearance of the peak in central collisions only. The mass distributions obtained after the Y and K*- cuts are shown in fig. 3a. We must remark that these cuts did not eliminate the events from the peak above the background, their number remaining practically the same (37 in fig. lb and 42 in fig. 3a). The number of standard deviations o f the peak over the background raises from 5trto 8.9tr. The z 2 / N D value
7 May 1992
obtained after the fit of a Breit-Wigner distribution plus a polynomyal background to the interval 3.053.95 G e V / c 2 gives z 2 / N D = 0 . 8 6 with a probability o f 5X 10 -4 for the peak being a statistical fluctuation. These facts strongly indicate that a resonance is present indeed. An analysis was done in order to find the characteristics of the peak. Fitting a polynomial plus a BreitWigner to the distribution (constructed through 25 M e V / c 2 ) of events which contains at least one (Kn) combination in the mass region 0.7-1.05 G e V / c 2 and has Y in the region - 0 . 1 8 - 0 . 3 8 , we obtain M R = 3 5 2 0 + 3 M e V / c 2 with F R = 2 0 + 16 M e V / c 2. As the obtained width of 20 M e V / c is comparable with our apparatus resolution in this mass region, we computed the physical width using the resolution function, obtained earlier in our experiment for this energy interval, namely ( a ( m ) ) = 17 M e V / c 2. Using this resolution function [ 7 ] we obtained a new value for the width, F R = (7+27o ) M e V / c 2. The narrowness of the peak and the decay mode stimulate the idea of the possible interpretation o f the peak as a pentaquark system with hidden charm (uudce). We computed also the missing mass (X °) for the events in the peak interval, and for those in the adjacent regions (fig. 4a). The difference between these two distributions is also shown (fig. 4b). It is clear from these distributions that they are not similar and that for the events from the peak a preferred region with missing mass of 1.4-2.1 G e V / c 2 appears. This result gives us some arguments to suppose that we have a two-particle reaction involving a neutral decaying meson: n - p-+ baryon ( M = 3.52 G e V / c 2 ) + m e s o n ( 1 . 4 < M < 2.1 G e V / c 2) .
( 1)
If we look in the RPP [6], there are several candidates for such a meson, having masses of 1.4-2.1 G e V / c 2 and dominantly decaying in a neutral mode as fo(1590), f2(1720), ¢3(1850), f2(2010) ..... The charged mode decay for those mesons would contribute to topologies that are not taken into consideration in our experiment. Based on the equivalent cross section of 0.1896 ___0.0095 ~tb/eV obtained earlier [5] we computed the cross section for the appearance of such a resonance and obtained aR = 14 + 3 txb. 151
Volume 281, number 1,2
PHYSICS LETTERS B
In c o n c l u s i o n we c o n s i d e r that: ( 1 ) it is v e r y p r o b a b l e that the b a r y o n i c system p K ÷ I < ° n - n - f o r m s a neutral r e s o n a n c e w i t h null app a r e n t strangeness, MR = 3.520 + 0.003 G e V / c 2 a n d FR----7 ÷2°-7M e V / c 2 ;
20
(a)
( 2 ) the resonance ( 3 ) this teractions
o
5
0 0,0
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0,5
1,0
1.5
2.0
2 5
M(x*) 6GV/c = 20.
cross section for the a p p e a r a n c e o f such a in o u r r e a c t i o n is 14 + 3 pb; r e s o n a n t system is p r o d u c e d in central ina n d decays p r o b a b l y t h r o u g h a K * - .
T h e a u t h o r s wish to a c k n o w l e d g e C E R N for prov i d i n g the e x p e r i m e n t a l m a t e r i a l , a n d also Dr. N. A n g e l o v , Dr. B. S a c h b a z i a n a n d Dr. F. C o t o r o b a i for i n t e r e s t i n g discussions.
(b) References
15-
10~
E
-5
M(x°) G*vtc'
Fig. 4. (a) Missing mass (X °) distribution for the events from the 3.5 GeV/c 2 peak. The shaded area represents the X ° distribution for the adjacent regions normalized to the background. (b) The difference between the two distributions from (a).
152
[ 1] N. Isgur et al., Phys. Rev. D 18 (1978) 4187; D 19 (1979) 2653;D20(1979) 1191; A.W. Hendry, Ann. Phys. 136 (1981) 1. [2] H. Lipkin, Phys. Lett. B 195 (1987) 484. J.-M. Richard, talk Rheinfels '90 Workshop on The hadron mass spectrum (St. Goar, FRG, September 1990). [3] E. Balea et al., Nucl. Phys. B 150 (1979) 345.;B 165 (1980) 21; V.M. Karnaukhov et al., JINR preprint PI-86-373 (1986). [4] E. Balea el al., JINR preprint 1-8138 ( 1964); V.M. Karnaukhov et al., JINR communication P1-87-559 (1987). [ 5 ] E. Balea et al., JINR communication E 1-12345 ( 1979 ). [ 6 ] Particle Data Group, J.J. Hermindez et al., Review of particle properties, Phys. Lett. B 239 (1990) 1. [7 ] W.T. Eadie et al., Statistical methods in experimental physics (North-Holland, Amsterdam, 1971 ).