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Exclusive near threshold two-pion production with the MOMO experiment at COSY
NUCLEAR PHYSICS A ELSEVIER
Nuclear Physics A631 (1998) 542c-544c
Exclusive Near Threshold Two-Pion Production with the MOMO Experiment at COSY * S. Bavink =, F. Bellemann ~, A. Berg ~, J. Bisplinghoff a G. Bohlscheid ~, J. Ernst ~, C. Henrich a, F. Hinterberger a, R. Ibald a, R. Jahn =, L. Jarczyk b, R. Joosten =, A. Kozela ~, H. Machner d A. Magiera b R. Maschuw a, T. Mayer-Kuckuk =, G. Mertler =, J. Munkel ~, P. v. Neumann-Cosel ~, D. Rosendaal =, P. v. Rossen e H. Schnitker ~, K. Scho ~, J. Smyrski b A. Strzalkowski b, R. TSlle d, R. Wurzinger / Institut fiir Strahlen- und Kernphysik, Universit£t Bonn b Institute for Physics, Jageltonian University Krakow, Poland b Institute tor Nuclear Physics, Krakow, Poland 'i Institut fiir Kernphysik, Forschungszentrum Jfilich lnstitut f{ir Kernphysik, Technische Hochschule Darmstadt I IPN Orsay, France Near threshold two pion production via the reaction pd --+ aHerr+rc- was measured kinematically complete with the MOMO experiment at COSY. A remarkable deviation of the obtained two pion invariant mass spectra from phase space as well as a predominant back to back and sidewise emission of the two pions was observed. 1. I N T R O D U C T I O N The MOMO experiment focusses on imar threshold two meson production via the rea.ctions pd --+ 3Herr+rr- and pd -+ a H e K + K - . It takes advantage of the high quality of tile cooled external COSY beam and the high resolution spectrometer BIG KARL. The setup consists of a high granularity scintillating fibers meson detector near the target with a 4- 45 deg. opening angle, and the spectrometer, which is used for aHe-identification. The large solid angle and high resolution of this detection method will yield precision data ol~ l,he low energy (T<80MeV) meson-meson interaction and probe into questions like meson-nucleon resonances and KK-molecule. 2. E X P E R I M E N T A L
METHOD
The MOMO vertex detector consists of 672 scintillating fibers (round, 2.5mm diameter) arranged in three planes tilted 60 deg. versus each other. The fibers are read out by 16-fohl photomultipliers. The MOMO scattering chamber houses a 4 m m LD2 target with extremely thin windows. In recent COSY beam times the reaction pd --4 aHerr+rc- was measured at three different proton beam momenta, corresponding to 35 MeV, 70 MeV and 90 MeV center of mass energy above the reaction threshold. Beam intensities ranged *Work supt)orted by BMBF 0375-9474/98/$19.00 © 1998 Elsevier Science B.V. All rights reserved. PII S0375-9474(98)00064-5
S. Bauink et al./Nuclear Physics A631 (1998) 542c-544c
543c
Relative Energy p + d ~
3He + n + ~ -
p, = 115o MeV/c
40
01 0
0
'
10
'
20
30
40
.
.
.
50
.
.
.
.
60
T,., = M,,,,-2m,,
.
.
70
80
[ MeV ]
t:igure l. Two-pion invariant mass spectrum at 70 MeV above threshold plotted in units o[ the ,-r+rr- relative energy
up to some 109 protons per beam burst. The 3He particles could be unambigiously identiffed 1)3' time of flight and energy loss measurements. The two-pion hits on the vertex detector could be uniquely identified by their hit patterns. Good events must be coplanar in respect to the total meson m o m e n t u m axis, which is defined by the beam and the :~Hc momenta.
3. R E S U L T S
and DISCUSSION
In total some 25 000 kinamatically complete rr%r-- events were observed. The obtained two pion invaria.nt mass spectra show a strong deviation from phase space at all three el~ergies. Fig. 1 depicts tile 1150 M e V / c data.. Since there is no absolute normalization of the phase space, the shape of our experimental spectra carl be interpreted either as missing strength at low relative energies or as an extra bump a.t large relative energies. It should be noted, that almost tile same shape of the two pion invariant mass spectrum was
544c
S. Bauink et aL /Nuclear Physics A631 (1998) 542c-544c ,-~
200 180 160
"~b 140
t
120
tt
100
+
80 60 40 20 0
20
40
60
80
I00
120
Figure 2. Angular distribution of the individual pions in the c.m.s, at 70 MeV above threshold
observed in a recent study [1] of the reaction re+ + d ~ ppre+re-. In addition, inclusive single arm data of the np -+ d(rrre) ° reaction at a comparable c.m.s, energy indicate a similar deviation from phase space [2]. There are several speculative explanations for the observed effect, e.g. pion - pion interaction, charge exchange (re+re- --+ re0re0), reaction dynamics, ABC effect (bump at about 50 MeV). The pion angular distributions measured by the MOMO experiment show a remarkable sidewise peaking (in the c.m.s., see Fig.2) and a preferential back to back emission of the two pions, probably indicating a two delta production mechanism. It should be further noted, that the also obtained rraHe invariant mass spectra follow phase space, which excludes any significant detector acceptance problems. Analysis of the 35 MeV and 90 MeV data is in progress.
REFERENCES 1. TRIUMF CHAOS collaboration, Phys Rev Lett 77 (1996) 603 2. C.L.Hollas et at., Phys Rev C 25 (1982) 2614
Nuclear Physics A631 (1998) 542c-544c
Exclusive Near Threshold Two-Pion Production with the MOMO Experiment at COSY * S. Bavink =, F. Bellemann ~, A. Berg ~, J. Bisplinghoff a G. Bohlscheid ~, J. Ernst ~, C. Henrich a, F. Hinterberger a, R. Ibald a, R. Jahn =, L. Jarczyk b, R. Joosten =, A. Kozela ~, H. Machner d A. Magiera b R. Maschuw a, T. Mayer-Kuckuk =, G. Mertler =, J. Munkel ~, P. v. Neumann-Cosel ~, D. Rosendaal =, P. v. Rossen e H. Schnitker ~, K. Scho ~, J. Smyrski b A. Strzalkowski b, R. TSlle d, R. Wurzinger / Institut fiir Strahlen- und Kernphysik, Universit£t Bonn b Institute for Physics, Jageltonian University Krakow, Poland b Institute tor Nuclear Physics, Krakow, Poland 'i Institut fiir Kernphysik, Forschungszentrum Jfilich lnstitut f{ir Kernphysik, Technische Hochschule Darmstadt I IPN Orsay, France Near threshold two pion production via the reaction pd --+ aHerr+rc- was measured kinematically complete with the MOMO experiment at COSY. A remarkable deviation of the obtained two pion invariant mass spectra from phase space as well as a predominant back to back and sidewise emission of the two pions was observed. 1. I N T R O D U C T I O N The MOMO experiment focusses on imar threshold two meson production via the rea.ctions pd --+ 3Herr+rr- and pd -+ a H e K + K - . It takes advantage of the high quality of tile cooled external COSY beam and the high resolution spectrometer BIG KARL. The setup consists of a high granularity scintillating fibers meson detector near the target with a 4- 45 deg. opening angle, and the spectrometer, which is used for aHe-identification. The large solid angle and high resolution of this detection method will yield precision data ol~ l,he low energy (T<80MeV) meson-meson interaction and probe into questions like meson-nucleon resonances and KK-molecule. 2. E X P E R I M E N T A L
METHOD
The MOMO vertex detector consists of 672 scintillating fibers (round, 2.5mm diameter) arranged in three planes tilted 60 deg. versus each other. The fibers are read out by 16-fohl photomultipliers. The MOMO scattering chamber houses a 4 m m LD2 target with extremely thin windows. In recent COSY beam times the reaction pd --4 aHerr+rc- was measured at three different proton beam momenta, corresponding to 35 MeV, 70 MeV and 90 MeV center of mass energy above the reaction threshold. Beam intensities ranged *Work supt)orted by BMBF 0375-9474/98/$19.00 © 1998 Elsevier Science B.V. All rights reserved. PII S0375-9474(98)00064-5
S. Bauink et al./Nuclear Physics A631 (1998) 542c-544c
543c
Relative Energy p + d ~
3He + n + ~ -
p, = 115o MeV/c
40
01 0
0
'
10
'
20
30
40
.
.
.
50
.
.
.
.
60
T,., = M,,,,-2m,,
.
.
70
80
[ MeV ]
t:igure l. Two-pion invariant mass spectrum at 70 MeV above threshold plotted in units o[ the ,-r+rr- relative energy
up to some 109 protons per beam burst. The 3He particles could be unambigiously identiffed 1)3' time of flight and energy loss measurements. The two-pion hits on the vertex detector could be uniquely identified by their hit patterns. Good events must be coplanar in respect to the total meson m o m e n t u m axis, which is defined by the beam and the :~Hc momenta.
3. R E S U L T S
and DISCUSSION
In total some 25 000 kinamatically complete rr%r-- events were observed. The obtained two pion invaria.nt mass spectra show a strong deviation from phase space at all three el~ergies. Fig. 1 depicts tile 1150 M e V / c data.. Since there is no absolute normalization of the phase space, the shape of our experimental spectra carl be interpreted either as missing strength at low relative energies or as an extra bump a.t large relative energies. It should be noted, that almost tile same shape of the two pion invariant mass spectrum was
544c
S. Bauink et aL /Nuclear Physics A631 (1998) 542c-544c ,-~
200 180 160
"~b 140
t
120
tt
100
+
80 60 40 20 0
20
40
60
80
I00
120
Figure 2. Angular distribution of the individual pions in the c.m.s, at 70 MeV above threshold
observed in a recent study [1] of the reaction re+ + d ~ ppre+re-. In addition, inclusive single arm data of the np -+ d(rrre) ° reaction at a comparable c.m.s, energy indicate a similar deviation from phase space [2]. There are several speculative explanations for the observed effect, e.g. pion - pion interaction, charge exchange (re+re- --+ re0re0), reaction dynamics, ABC effect (bump at about 50 MeV). The pion angular distributions measured by the MOMO experiment show a remarkable sidewise peaking (in the c.m.s., see Fig.2) and a preferential back to back emission of the two pions, probably indicating a two delta production mechanism. It should be further noted, that the also obtained rraHe invariant mass spectra follow phase space, which excludes any significant detector acceptance problems. Analysis of the 35 MeV and 90 MeV data is in progress.
REFERENCES 1. TRIUMF CHAOS collaboration, Phys Rev Lett 77 (1996) 603 2. C.L.Hollas et at., Phys Rev C 25 (1982) 2614