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Search for narrow quasinuclear bound states close to the NN̄ threshold
II
IUCLEAR PHYSIC~ d
PROCEEDINGS SUPPLEMENTS ELSEVIER
Nuclear Physics B (Proc. Suppl.) 56A (1997) 175-177
Search for narrow quasinuclear bound states close to the NI I threshold V.Cred~ (representing the Crystal Barrel Collaboration) ~ ISKP-University of Bonn, Nussallee 14-16, D-53115 Bonn We have searched for a narrow quasinuclear bound state, Niq(1870), proposed on the basis of an analysis of the momentum distribution of protons in the reaction ~d --+ 5rr +p~. A comparison between momentum distributions with different pion multiplicities in the final state does not confirm this conjecture.
In the preceeding contribution to this conference O. Dalkarov showed indirect evidence for the existence of a NN bound state, N1~(1870), decaying into five pions [1]. The evidence is derived from the m o m e n t u m distribution of protons in ~d -+ 5rr + p~, measured in the A S T E R I X experiment [2]. The proton m o m e n t u m distribution shows a deviation from the expected one; the deviation can be explained by introducing a narrow bound state close to the NI~ threshold. Quasinuclear bound states have been predicted since long time [3]; in particular, states with little binding energy m a y be long-lived. A very effective method to search for quasinuclear systems of nucleons and antinucleons is the investigation of the reaction lSd--+ N + X, where X is a bound state and N represents the spectator nucleon. In these reactions the moment u m spectrum of the spectator particle shows a characteristic shape which reflects the Fermi motion of nucleons in deuterons, even though high-momenta also occur due to final-state interactions or due to production of resonances. If the annihilation process proceeds via formation of intermediate mesons recoiling against the residual nucleon, the m o m e n t u m distribution will show a peak; in case of quasinuclear bound states close to the NN threshold, this peak falls into the region of the Fermi m o m e n t u m and a deviation from the expected distribution should reflect the new state. In particular, the mass, the width and the decay angular distribution of the additional intensity is determined by the mass and the angular momentum of the resonance [3]. The G-parity of the resonance can be determined by measuring the number of pions into which the resonance decays. 0920-5632/97/$17.00 © 1997 Elsevier Science B.M All rights reserved, PII: S0920-5632(97)00270-3
71-~°~°~ D
800
400
200
70
40
60
BO
100
120
1<-0
160
180
200
MeV/c
Figure 1. Presentation of the m o m e n t u m spectra of the spectator proton in the reactions: Od --+ 7r-31r ° and pd --+ ~r-4~r °. The d a t a do not show any shift concerning the position of the maxima.
The resonance proposed by Dalkarov decays into five pions. Hence, there should be different proton m o m e n t u m distributions for annihilation into three, four and five pions. In order to verify or to disprove the proposed resonance, we investigated the reactions ~d -+ rr-2~r °, Od -~ ~r-31r °, pd --+ 7r-4rr s using data of the Crystal Barrel Experiment. However, the spectra show no evidence for the proposed quasinuclear system.
176
~ Credb/Nuclear Physics B (Proc. Suppl.) 56A (1997) 175-177
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800
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20
40
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100
,
120
,
,
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,
140
,
,
I
,
,
160
,
I
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,
,
180 200 MeV/c
, ; 0 , , ; , 4, ~ g ,
8
; . . . .100 . . . . . 120 . . . . . .140 . . . . . .160 .
180 200 MeV/c
Figure 2. Presentation of the m o m e n t u m spectra of the spectator proton in the reactions: pd -~ rr-2rr ° and pd --+ 7r-47r °.
Figure 3. Presentation of the m o m e n t u m spectra of the spectator proton in the reactions: pd --+ rr-2rr ° and 0d --+ rr-3re °.
The Crystal Barrel Experiment is described in [4]. It allows the detection of charged particles and photons up to an angular coverage of almost 4rr, respectively, and is therefore very suitable for the investigation of the reactions in question. In 1994 about 8 million events were taken with the Crystal Barrel Detector at LEAR. A large fraction of these events, about 5.2 million, was recorded with a trigger requiring only one charged track in the Jet Drift Chamber (JDC) surrounding the liquid deuterium target. As the charged track must be mainly associated with the It--meson, the m o m e n t u m of the proton is reconstructed by its missing mass:
stance, is suppressed based on the corresponding confidence levels of the kinematic fit.
I r n m i s s - rnpl < 150 MeV/c 2. Energy and m o m e n t u m conservation as well as the demand for 4, 6 and 8 photons reduces the number of d a t a to 660000. The remaining three data samples pass a further kinematical fit to a 7r-2rr °-, rr-arr °- and rr-4rr°-hypothesis. Events are selected with a confidence level > 10%, respectively. The background due to competitive channels like rr-rr°r/, rr-2rr°r~ or rr-3rr°rL for in-
The figures show the m o m e n t u m spectra of the spectator proton presented always in pairs in order to point out their characteristics. As the five pion channel provides the least number of events, its number of entries is taken for normalisation. Calculations show that the proton causes a track in the J D C if it has a m o m e n t u m larger than 200 MeV/c. For this reason these events were not recorded due to the special trigger situation. As the Nlq(1870) bound state is predicted to decay into five pions, decay channels with an even number of pions should have no signatures from that state. Hence, we expect different recoil m o m e n t u m distributions for final states with four and five pions. However, figure 1 shows that this is not obviously the case, at least not at the level observed in the A S T E R I X experiment. In addition, figure 2 and figure 3 do not give strong evidence for such a quasinuclear system, either. The irregular behaviour of the curves con-
V. CredO~Nuclear Physics B (Proc. Suppl.) 56A (1997) 175-177 cerning the number of particles may occur due to interference between quasi-free and rescattering mechanism in the regarded momentum region [5]. We also inspected the momentum distributions for final states in which one of the 7r° was replaced by an q; no anomaly which might prove the presence of quasinuclear bound states was found.
REFERENCES
1. O.D. Dalkarov, contribution to this conferCnCC
2.
P. Weidenauer et al. (ASTERIX Collaboration) Z. Phys. C59, 387-398 (1993) 3. L.N.Bogdanova, O.D.Dalkarov, I.S.Shapiro Annals of Physics 84, 261-284 (1974) 4. E. Aker et al.: Nucl. Instrum. Methods A, 321, 69 (1992). 5. O.D. Dalkarov, private communication
177
IUCLEAR PHYSIC~ d
PROCEEDINGS SUPPLEMENTS ELSEVIER
Nuclear Physics B (Proc. Suppl.) 56A (1997) 175-177
Search for narrow quasinuclear bound states close to the NI I threshold V.Cred~ (representing the Crystal Barrel Collaboration) ~ ISKP-University of Bonn, Nussallee 14-16, D-53115 Bonn We have searched for a narrow quasinuclear bound state, Niq(1870), proposed on the basis of an analysis of the momentum distribution of protons in the reaction ~d --+ 5rr +p~. A comparison between momentum distributions with different pion multiplicities in the final state does not confirm this conjecture.
In the preceeding contribution to this conference O. Dalkarov showed indirect evidence for the existence of a NN bound state, N1~(1870), decaying into five pions [1]. The evidence is derived from the m o m e n t u m distribution of protons in ~d -+ 5rr + p~, measured in the A S T E R I X experiment [2]. The proton m o m e n t u m distribution shows a deviation from the expected one; the deviation can be explained by introducing a narrow bound state close to the NI~ threshold. Quasinuclear bound states have been predicted since long time [3]; in particular, states with little binding energy m a y be long-lived. A very effective method to search for quasinuclear systems of nucleons and antinucleons is the investigation of the reaction lSd--+ N + X, where X is a bound state and N represents the spectator nucleon. In these reactions the moment u m spectrum of the spectator particle shows a characteristic shape which reflects the Fermi motion of nucleons in deuterons, even though high-momenta also occur due to final-state interactions or due to production of resonances. If the annihilation process proceeds via formation of intermediate mesons recoiling against the residual nucleon, the m o m e n t u m distribution will show a peak; in case of quasinuclear bound states close to the NN threshold, this peak falls into the region of the Fermi m o m e n t u m and a deviation from the expected distribution should reflect the new state. In particular, the mass, the width and the decay angular distribution of the additional intensity is determined by the mass and the angular momentum of the resonance [3]. The G-parity of the resonance can be determined by measuring the number of pions into which the resonance decays. 0920-5632/97/$17.00 © 1997 Elsevier Science B.M All rights reserved, PII: S0920-5632(97)00270-3
71-~°~°~ D
800
400
200
70
40
60
BO
100
120
1<-0
160
180
200
MeV/c
Figure 1. Presentation of the m o m e n t u m spectra of the spectator proton in the reactions: Od --+ 7r-31r ° and pd --+ ~r-4~r °. The d a t a do not show any shift concerning the position of the maxima.
The resonance proposed by Dalkarov decays into five pions. Hence, there should be different proton m o m e n t u m distributions for annihilation into three, four and five pions. In order to verify or to disprove the proposed resonance, we investigated the reactions ~d -+ rr-2~r °, Od -~ ~r-31r °, pd --+ 7r-4rr s using data of the Crystal Barrel Experiment. However, the spectra show no evidence for the proposed quasinuclear system.
176
~ Credb/Nuclear Physics B (Proc. Suppl.) 56A (1997) 175-177
~E 900
!:)=ii r i '
r'
800
_ _
~-~o~o
700 71
7T°7~°T[ o
600
D
o
["i
500
600
~-00 400 300
200
1oo
0
J ,,I
, , , [ , , , I , , , I , , , I J ,
20
40
60
BO
I
100
,
120
,
,
I
,
140
,
,
I
,
,
160
,
I
,
,
,
180 200 MeV/c
, ; 0 , , ; , 4, ~ g ,
8
; . . . .100 . . . . . 120 . . . . . .140 . . . . . .160 .
180 200 MeV/c
Figure 2. Presentation of the m o m e n t u m spectra of the spectator proton in the reactions: pd -~ rr-2rr ° and pd --+ 7r-47r °.
Figure 3. Presentation of the m o m e n t u m spectra of the spectator proton in the reactions: pd --+ rr-2rr ° and 0d --+ rr-3re °.
The Crystal Barrel Experiment is described in [4]. It allows the detection of charged particles and photons up to an angular coverage of almost 4rr, respectively, and is therefore very suitable for the investigation of the reactions in question. In 1994 about 8 million events were taken with the Crystal Barrel Detector at LEAR. A large fraction of these events, about 5.2 million, was recorded with a trigger requiring only one charged track in the Jet Drift Chamber (JDC) surrounding the liquid deuterium target. As the charged track must be mainly associated with the It--meson, the m o m e n t u m of the proton is reconstructed by its missing mass:
stance, is suppressed based on the corresponding confidence levels of the kinematic fit.
I r n m i s s - rnpl < 150 MeV/c 2. Energy and m o m e n t u m conservation as well as the demand for 4, 6 and 8 photons reduces the number of d a t a to 660000. The remaining three data samples pass a further kinematical fit to a 7r-2rr °-, rr-arr °- and rr-4rr°-hypothesis. Events are selected with a confidence level > 10%, respectively. The background due to competitive channels like rr-rr°r/, rr-2rr°r~ or rr-3rr°rL for in-
The figures show the m o m e n t u m spectra of the spectator proton presented always in pairs in order to point out their characteristics. As the five pion channel provides the least number of events, its number of entries is taken for normalisation. Calculations show that the proton causes a track in the J D C if it has a m o m e n t u m larger than 200 MeV/c. For this reason these events were not recorded due to the special trigger situation. As the Nlq(1870) bound state is predicted to decay into five pions, decay channels with an even number of pions should have no signatures from that state. Hence, we expect different recoil m o m e n t u m distributions for final states with four and five pions. However, figure 1 shows that this is not obviously the case, at least not at the level observed in the A S T E R I X experiment. In addition, figure 2 and figure 3 do not give strong evidence for such a quasinuclear system, either. The irregular behaviour of the curves con-
V. CredO~Nuclear Physics B (Proc. Suppl.) 56A (1997) 175-177 cerning the number of particles may occur due to interference between quasi-free and rescattering mechanism in the regarded momentum region [5]. We also inspected the momentum distributions for final states in which one of the 7r° was replaced by an q; no anomaly which might prove the presence of quasinuclear bound states was found.
REFERENCES
1. O.D. Dalkarov, contribution to this conferCnCC
2.
P. Weidenauer et al. (ASTERIX Collaboration) Z. Phys. C59, 387-398 (1993) 3. L.N.Bogdanova, O.D.Dalkarov, I.S.Shapiro Annals of Physics 84, 261-284 (1974) 4. E. Aker et al.: Nucl. Instrum. Methods A, 321, 69 (1992). 5. O.D. Dalkarov, private communication
177