- Email: [email protected]
Two pion electroproduction with the CLAS detector at Jefferson Laboratory
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ELSEVIER
Nuclear Physics A663&664 (2000)
675c~78c
www.elsevier.nlliocate/npe
Two Pion Electroproduction with the CLAS Detector at Jefferson Laboratory M. Ripani" for the CLAS collaboration "Istituto Nazionale di Fisica Nucleare, Via Dodecanneso 33, 1-16146 Genova (Italy) Double pion electroproduction off the proton is an important test of quark models; it is sensitive to many nucleon excited states and allows to investigate the existence of the "missing resonances". This scientific program is pursued by the CLAS collaboration in Hall B at Jefferson Laboratory, where experiment E-93-006 is measuring the reaction eN --+ e' N7r7r in the mass region between threshold and 2.2 GeV. Some preliminary data and cross sections from CLAS will be shown, along with results of a phenomenological calculation. 1. Physics Issues
The excitation of baryon resonances is genuinly a non-perturbative phenomenon. Measurements of the transition amplitudes from the nucleon to its excited states are sensitive to the spatial and spin structure of the transition. Many of the nucleon excited states in the mass region around and above 1. 7 GeV tend to decouple from the single-pion and eta channels, while 7r N scattering experiments showed that many of them decay predominantly in multipion channels, such as ~7r or N p , leading to N7r7r final states]l], Measurement of the transition form factors of these states is very important for testing symmetry properties of the quark model. Moreover, SU(6) symmetric quark models[2,3] predict more states than have been found in experiments. QCD mixing effects could decouple many of these states from the pion-nucleon[2]' with consequent lack of evidence in elastic 7r N scattering, while strongly coupling them to two-pion channels such as ~7r[2,4 6]. However, other models such as the Quark Cluster Model[7] predict a fewer number of states than the symmetric model, more in accordance with experimental observation. Search for some of these states is therefore crucial in discriminating between alternative descriptions of the baryon structure.
2. Data collection with CLAS The first running period (1998) with CLAS in Hall B at Jefferson Lab covered a broad electron scattering scientific program, especially focused on N* physics, as well as on 0375-9474/00/$ - see front matter © 2000 Elsevier Science B.Y. All rights reserved. PH S0375-9474(99)00719-8
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other issues like vector meson and strangeness production[8]. Important features of the CLAS detector[9] are large kinematical coverage for charged particles and good momentum resolution of the order of 1%; using a loose electron trigger based on the coincidence between the forward electromagnetic shower calorimeter and the gas Cherenkov detector, it was possible to measure simultaneously many exclusive hadronic final states, such as single and double pion production, kaon production, etc. Different channels were separated through subsequent particle identification using Time of Flight information and kinematical cuts. Typical beam currents of a few nA were delivered to Hall B on a liquid hydrogen target, corresponding to luminosities up to 4x1033 . Data were taken for a few months at three beam energies: 1.6, 2.4 and 4 GeV. Reaction ep --. ep1f+1f- was identified using missing mass techniques, for instance measuring in CLAS the final state ep --. eptt" (missing 1f- ). The good resolution obtained (see fig. 1) allowed to apply clean cuts around the missing 1f-. A total of about 380 million triggers were collected in the first electron scattering run, corresponding to about 110 million inclusive electrons at the above mentioned beam energies and at three different magnetic field settings. A new running period for electron scattering took place in 1999, collecting about ten times more statistics: full reconstruction for these new data set is under way.
3. Preliminary Data and Cross Sections Using the full statistics collected in the first electron scattering run and the above discussed analysis procedures, about half a million two pion events were extracted from the 2.5 GeV data and analysed. In fig. 2, the Dalitz plot obtained for a particular W bin is reported; strong isobar production in the final state is clearly visible through the two bands related to p meson and b.++ production. This feature provides the opportu-
M Ripani/Nuclear Physics A 663&664 (2000) 675c-678c
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Figure 2. Daliz plot of the invariant mass for the 7T+7T- pair versus the p7T+ pair from CLAS for beam energy of 2.4 GeV, 1.9 < W < 2 and all Q2 values. The picture clearly shows the two bands due to p meson and 6++ production in the two masses, respectively.
nity of separating these different contributions, therefore accessing different decay modes of baryon resonances. Using a GEANT-based simulation full acceptance and efficiency were evaluated in the reaction kinematical space and very preliminary cross sections were derived . In fig. 3 we report the total virtual photon cross section for a particular Q2 interval and for a large W range; in the same picture we report the available data from a previous experiment performed at DESY[lO]: the agreement is satisfactory. CLAS is definitely providing higher statistics and accuracy; the CLAS preliminary result also shows a structure in the cross section around 1.7 GeV, not visible in previous data. The picture also shows (full line) the result of a phenomenological calculation performed using a modeljl l], where resonances are described using a Single Quark Transition Model fit[12]; this calculation is not able to describe the data, including the structure observed: this is opening an interesting perspective for the resonance investigation, as different approaches for the process description could be tested and lead to new information regarding the Q2 evolution of baryon resonance couplings. 4. Summary and Conclusions
Two pion production is one of the main subjects of investigation in Hall 8 at Jefferson Lab , being related to baryon resonances coupled to this channel. A sizeable statistics for the two pion production has been already collected with CLAS: data allow to achieve a
M. Ripani/Nuclear Physics A 663&664 (2000) 675c-678c
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G.{lJ.b) 50 .---.--.---..--.-,-,--.-.--..-,-,--.-.--.-,-,--..--.-.-,-,--.---.--.--, full points; CLAS 40 V cry Preliminary
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Figure 3. Total virtual photon cross section as a function of the CMS energy W for ep ---+ epn" (missing «: at Q2 between 0.5 and 0.8 Gey2 • Open squares: data from [10]; full circles: CLAS data; full line: calculation [11]. Errors in CLAS data are statistical; an additional 15-20 % systematic error is currently estimated.
better accuracy in the study of the isobar formation in the final state, which is clearly evident in the data. Some preliminary cross sections have been derived: they show reasonable agreement with previous data and moreover manifest features not previously observed, like the presence of structures as a function of the center of mass energy, therefore opening the possibility of interesting investigations about the non-strange baryon structure.
REFERENCES 1. Particle Data Group, 1994 2. R. Koniuk and N. Isgur, Phys. Rev. Lett., 44, 845 (1980); Phys. Rev. D, 21, 1868 (1980) 3. M.M. Giannini, Rep. Prog, Phys., 54, 453 (1990) 4. R. Koniuk, Nucl. Phys. , B195 , 452 (1982) 5. S. Capstick, W. Roberts, Phys. Rev. D49, 4570 (1994) 6. F. Stancu and P Stassart, Phys. Rev. D47, 2140 (1993) 7. K.F. Liu and C.W. Wong, Phys. Rev. D28, 170 (1983) 8. invited paper to this Conference by V. Burkert and contributed papers by J. Manak, M. Mestayer, R. Schumacher 9. W. Brooks, contributed paper to this Conference 10. Y. Eckart et al. , Nucl. Phys. B55, 45 (1973); P. Joos et al., Phys. Lett., B52, 481 (1974); K Wacker et al., Nucl. Phys., B144, 269 (1978) 11. M. Ripani, Few Body Systems, Suppl. 11, 1999; V. Mokeev et al., ibidem. 12. V. D. Burkert, Czech. Journ. of Phys., Vol.46, 627(1996)
~
m ~
ELSEVIER
Nuclear Physics A663&664 (2000)
675c~78c
www.elsevier.nlliocate/npe
Two Pion Electroproduction with the CLAS Detector at Jefferson Laboratory M. Ripani" for the CLAS collaboration "Istituto Nazionale di Fisica Nucleare, Via Dodecanneso 33, 1-16146 Genova (Italy) Double pion electroproduction off the proton is an important test of quark models; it is sensitive to many nucleon excited states and allows to investigate the existence of the "missing resonances". This scientific program is pursued by the CLAS collaboration in Hall B at Jefferson Laboratory, where experiment E-93-006 is measuring the reaction eN --+ e' N7r7r in the mass region between threshold and 2.2 GeV. Some preliminary data and cross sections from CLAS will be shown, along with results of a phenomenological calculation. 1. Physics Issues
The excitation of baryon resonances is genuinly a non-perturbative phenomenon. Measurements of the transition amplitudes from the nucleon to its excited states are sensitive to the spatial and spin structure of the transition. Many of the nucleon excited states in the mass region around and above 1. 7 GeV tend to decouple from the single-pion and eta channels, while 7r N scattering experiments showed that many of them decay predominantly in multipion channels, such as ~7r or N p , leading to N7r7r final states]l], Measurement of the transition form factors of these states is very important for testing symmetry properties of the quark model. Moreover, SU(6) symmetric quark models[2,3] predict more states than have been found in experiments. QCD mixing effects could decouple many of these states from the pion-nucleon[2]' with consequent lack of evidence in elastic 7r N scattering, while strongly coupling them to two-pion channels such as ~7r[2,4 6]. However, other models such as the Quark Cluster Model[7] predict a fewer number of states than the symmetric model, more in accordance with experimental observation. Search for some of these states is therefore crucial in discriminating between alternative descriptions of the baryon structure.
2. Data collection with CLAS The first running period (1998) with CLAS in Hall B at Jefferson Lab covered a broad electron scattering scientific program, especially focused on N* physics, as well as on 0375-9474/00/$ - see front matter © 2000 Elsevier Science B.Y. All rights reserved. PH S0375-9474(99)00719-8
676c
M. Ripani/Nuclear Physics A 663&664 (2000) 675c-678c
+
1600 1400 1200
+
1000
+
800
+
600
+
400
+
+
200
..--
~
other issues like vector meson and strangeness production[8]. Important features of the CLAS detector[9] are large kinematical coverage for charged particles and good momentum resolution of the order of 1%; using a loose electron trigger based on the coincidence between the forward electromagnetic shower calorimeter and the gas Cherenkov detector, it was possible to measure simultaneously many exclusive hadronic final states, such as single and double pion production, kaon production, etc. Different channels were separated through subsequent particle identification using Time of Flight information and kinematical cuts. Typical beam currents of a few nA were delivered to Hall B on a liquid hydrogen target, corresponding to luminosities up to 4x1033 . Data were taken for a few months at three beam energies: 1.6, 2.4 and 4 GeV. Reaction ep --. ep1f+1f- was identified using missing mass techniques, for instance measuring in CLAS the final state ep --. eptt" (missing 1f- ). The good resolution obtained (see fig. 1) allowed to apply clean cuts around the missing 1f-. A total of about 380 million triggers were collected in the first electron scattering run, corresponding to about 110 million inclusive electrons at the above mentioned beam energies and at three different magnetic field settings. A new running period for electron scattering took place in 1999, collecting about ten times more statistics: full reconstruction for these new data set is under way.
3. Preliminary Data and Cross Sections Using the full statistics collected in the first electron scattering run and the above discussed analysis procedures, about half a million two pion events were extracted from the 2.5 GeV data and analysed. In fig. 2, the Dalitz plot obtained for a particular W bin is reported; strong isobar production in the final state is clearly visible through the two bands related to p meson and b.++ production. This feature provides the opportu-
M Ripani/Nuclear Physics A 663&664 (2000) 675c-678c
677c
200
0.9
175 0.8
150 0.7 125 0.6
100 0.5
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L2 1
1.1
1.2
1.3
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1.8 plI
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Figure 2. Daliz plot of the invariant mass for the 7T+7T- pair versus the p7T+ pair from CLAS for beam energy of 2.4 GeV, 1.9 < W < 2 and all Q2 values. The picture clearly shows the two bands due to p meson and 6++ production in the two masses, respectively.
nity of separating these different contributions, therefore accessing different decay modes of baryon resonances. Using a GEANT-based simulation full acceptance and efficiency were evaluated in the reaction kinematical space and very preliminary cross sections were derived . In fig. 3 we report the total virtual photon cross section for a particular Q2 interval and for a large W range; in the same picture we report the available data from a previous experiment performed at DESY[lO]: the agreement is satisfactory. CLAS is definitely providing higher statistics and accuracy; the CLAS preliminary result also shows a structure in the cross section around 1.7 GeV, not visible in previous data. The picture also shows (full line) the result of a phenomenological calculation performed using a modeljl l], where resonances are described using a Single Quark Transition Model fit[12]; this calculation is not able to describe the data, including the structure observed: this is opening an interesting perspective for the resonance investigation, as different approaches for the process description could be tested and lead to new information regarding the Q2 evolution of baryon resonance couplings. 4. Summary and Conclusions
Two pion production is one of the main subjects of investigation in Hall 8 at Jefferson Lab , being related to baryon resonances coupled to this channel. A sizeable statistics for the two pion production has been already collected with CLAS: data allow to achieve a
M. Ripani/Nuclear Physics A 663&664 (2000) 675c-678c
678c
G.{lJ.b) 50 .---.--.---..--.-,-,--.-.--..-,-,--.-.--.-,-,--..--.-.-,-,--.---.--.--, full points; CLAS 40 V cry Preliminary
30
20
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OL...>...........-l.--L...L..........- -'-L...L..........- l.--L...L.........-l.--L...L...J....L--L.-l
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r.a
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W(GcV)
Figure 3. Total virtual photon cross section as a function of the CMS energy W for ep ---+ epn" (missing «: at Q2 between 0.5 and 0.8 Gey2 • Open squares: data from [10]; full circles: CLAS data; full line: calculation [11]. Errors in CLAS data are statistical; an additional 15-20 % systematic error is currently estimated.
better accuracy in the study of the isobar formation in the final state, which is clearly evident in the data. Some preliminary cross sections have been derived: they show reasonable agreement with previous data and moreover manifest features not previously observed, like the presence of structures as a function of the center of mass energy, therefore opening the possibility of interesting investigations about the non-strange baryon structure.
REFERENCES 1. Particle Data Group, 1994 2. R. Koniuk and N. Isgur, Phys. Rev. Lett., 44, 845 (1980); Phys. Rev. D, 21, 1868 (1980) 3. M.M. Giannini, Rep. Prog, Phys., 54, 453 (1990) 4. R. Koniuk, Nucl. Phys. , B195 , 452 (1982) 5. S. Capstick, W. Roberts, Phys. Rev. D49, 4570 (1994) 6. F. Stancu and P Stassart, Phys. Rev. D47, 2140 (1993) 7. K.F. Liu and C.W. Wong, Phys. Rev. D28, 170 (1983) 8. invited paper to this Conference by V. Burkert and contributed papers by J. Manak, M. Mestayer, R. Schumacher 9. W. Brooks, contributed paper to this Conference 10. Y. Eckart et al. , Nucl. Phys. B55, 45 (1973); P. Joos et al., Phys. Lett., B52, 481 (1974); K Wacker et al., Nucl. Phys., B144, 269 (1978) 11. M. Ripani, Few Body Systems, Suppl. 11, 1999; V. Mokeev et al., ibidem. 12. V. D. Burkert, Czech. Journ. of Phys., Vol.46, 627(1996)