- Email: [email protected]
Measurement of coherent and incoherent π0 photoproduction off the deuteron with tagged photons up to the Δ region
JtI! ~
~ ELSEVIER
Nuclear Physics A663&664 (2000) 428c-431 c www.elsevier.nlllocate/npe
Measurement of coherent and incoherent 1l"0 photoproduction off the deuteron with tagged photons up to the ~ region" U. Siodlaczek", P. Achenbach", J. Ahrens", H.-J. Arends", R. Beckc , R. Bilger", H. Clement", V. Hejny", M. Kotulla b, B. Krusche", V. Kuhr e , R. Leukel", V. Metag", R. Novotny", V. Olmos de Leon", F. Rambo", M. Schepkin", A. Schmidt", M. Schumacher", H. Stroher", G.J. Wagner", Th. Walcherc , J. WeiBb, F. Wissmann", M. Wolf' "Physikalisches Institut, Universitiit Tiibingen, D-72076 Tiibingen, Germany
-n. Physikalisches Institut, Universitat GieBen, Germany "Institut fur Kernphysik, Universitiit Mainz, Germany dIKP - Forschungszentrum Jiilich Germany ell. Physikalisches Institut, Universitat Gi:ittingen, Germany
fITEP Moscow, Russia Coherent and incoherent KO photo production off the deuteron has been measured for incident quasi-monoenergetic photons in the energy range from KO threshold up to 300MeV using the TAPS setup together with the Glasgow tagger at MAMI. Integral and differential cross sections over the full angular range have been obtained. Coherent and incoherent production are found to exhibit very different angle and energy dependences. The sum of both processes, the total 7[0 production off the deuteron is quantitatively extremely well described by a pure quasifree model, whereas the data for coherent and incoherent channels necessitate strong FSI effects for their description. Finally upper limits for the production of dibaryon resonances in this reaction have been obtained. 1. INTRODUCTION
Photoproduction of neutral mesons is dominated by resonance contributions, since background contributions from meson pole or Kroll-Ruderman terms are strongly suppressed. Whereas an extensive set of data exists for 7[0 photoproduction off the proton, there are no corresponding data for unbound neutrons for obvious reasons. Hence measurements on the deuteron, where the neutron is only very loosely bound, appear to be very well suited for this purpose. The 7[0 photoproduction off the deuteron may proceed via two different processes: the coherent process, where the deuteron stays in its groundstate and the incoherent process, where the deuteron breaks up. The latter reaction channel is expected to be dominated 'supported by BMBF (06 TV 886) and DFG (Mu 705/3 Graduiertenkolleg and SFB 201) 0375-9474/00/$ see front matter © 2000 Elsevier Science RY. All rights reserved. PH S0375-9474(99)00675-2
U. Siodlaczek et al./Nuclear Physics A 663&664 (2000) 428c-431c
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by the quasifree process. Hence measurement of this channel in quasifree kinematics has been considered to be most favorable in obtaining the desired production cross section for the neutron. Finally, this reaction is also very well suited for th e search of narrow dibaryon resonances, which couple to the I'd channel. One such candidate is the hitherto hypothetical 1rNN resonance d' with I (JP) = even (0-) , m ~ 2.06GeV and r ~ 0.5 MeV. Due to its quantum numbers it cannot decay into the N N channel, hence its width is expect ed to be very narrow , in particular since its mass is close to the 1r N N threshold. Suggestive evidence for such a resonance has been found in the pionic double charge exchange on nuclei [1] as well as in pp -+ Pp7r-1r+ [2,3]. Except of some old data from Bonn [4] and Orsay [5] ther e are high-quality data available only very close to 1r 0 threshold for coherent [6] and total [7] production cross sections, as well as in the energy range 204-792 MeV from the 1992 TAPS run at MAMI [8] for both coherent and incoherent production.
2. RESULTS In order to fill the data gap between 1r 0 threshold and .6.-resonance region a new highstatistics run in the range E"I = 130- 306 MeV has been carri ed out with an extended TAPS [9] setup in combination with the Glasgow tagger at MAMI. Th e extended TAPS setup consist ed of six blocks each containing 64 BaF 2 modules in an 8 x 8 matrix with individu al plastic veto counters in front and a forward wall consisting of 120 BaF2 modul es with individual plastic veto counters combined in Phoswich telescopes. A LD2 target of 10em length and a diam eter of 5em was used. The neutral pions were detected via their 2')' decay by ')'')' coincidences in TAPS and identified via an invariant mass analysis. In addition to photons also protons , neutrons and deut erons have been detected when possible. Due to the small kinetic energies of the baryonic ejectiles at low photon energies th e experim ental separation of coherent and incoherent channels has been feasible only by means of the 1r 0 missing mass (or equivalently the missing energy) spectrum. Fig. 1 shows such spectra for an incident photon energy of 260MeV. Though th e contributions for coherent and incoherent processes overlap , they can be separated in a fit procedure by use of MC simulations of these processes taking into account the experimental resolution. Our measurements show that the coherent production is the dominant part up to E"I ~ 250 MeV, before it saturates near E"I ~ 280MeV. The breakup channel on th e other hand is very small at low energies, but rises steeply at th e onset of the .6. resonance. Whereas the coherent channel gives angular distributions , which are strongly forwardpeaked, those of the breakup channel peak at intermediate and backward angles. As an example, Fig. 2 shows th e angular distributions at E"I = 260MeV. There is good agreement between the results of this measurement and those of [81 in the energy region where both experiments overlap . Surprisingly at first glance, MC simulat ions of the pur e quasi-free process do not give a good description of the angular dependence in the breakup channel, in part icular not in the forward angular range. Instead , exactly these simulations yield an excellent description
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U Siodlaczek et al. /Nuclear Physics A663&664 (2000) 428c--431c
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'E:I o
o
·1 00
·50
0
50
50
100
100
missing E [MeV]
missing E [MeV]
50
100
missing E [MeV ]
Figure 1. 1r 0 missing energy spectra at E-y = 260MeV at forward, intermediate and backward 1r 0 angles. The data are fitted with Me simulations for coherent (light shaded) and incoherent (dark) 1r0 production. Also shown are the sums of both contributions.
Ey 25
=260 MeV
MC quasllree
•
MCcoherent
.. coherent
total
Figure 2. Observed angular distributions of total, coherent and incoherent 1r 0 photoproduction at E: = 260MeV. Me simulations for the pure quasifree process and the coalescence ansatz (see text) are shown by the solid, dashed and dash-dotted curves, respectively.
U. Siodlaczek et al.lNuclear Physics A663&664 (2000) 428c-431c
431c
of the total, i.e. the sum of coherent and incoherent cross section both in its energy and angular dependence (solid curve in Fig. 2). From these observations it may be concluded that the total cross section is much better suited for the derivation of the 11"0 production cross section off the neutron than the originally considered incoherent cross section, where the quasifree process obviously is strongly distorted by strong FSI effects. In fact, it had been shown theoretically already some time ago by Kolybasov and Ksensov [10] that the N N-FSI effects in coherent and breakup channel cancel each other in their sum , the total cross section, which therefore represents the pure quasifree process to high accuracy. These features have also been noticed by Laget [11] in his numerical calculations. The strong FSI effects in coherent and incoherent production can be effectively simulated by a very simple coalescence ansatz assuming that for pn relative momenta q :::; qo always a deuteron is formed, whereas the event proceeds into the breakup channel for q > qo. Adjustment of this single parameter to qo = 300 MeV/ c results in an amazingly perfect description of all angular and energy dependences in both coherent and incoherent channels (dashed curves in Fig. 2). The reason for the success of such a simple ansatz has yet to be understood theoretically. Finally, as already mentioned in the introduction, this reaction also is very well suited for the search of narrow dibaryons like the hitherto hypothetical 1I"NN resonance d'. With the tagger resolution of AE-y ~ 1 MeV an upper limit of 2- 4 J-tb * MeV at the 90 % confidence level for the production of such resonances in this reaction can be deduced . Though these are the first stringent limits for dibaryon resonances in this reaction, they are still far above the expected d' production cross section which has been estimated to be small er than 1 J-tb [12] .
REFERENCES 1. R. Bilger et al., Phys . Rev. Lett. 71 (1993) 42, 72 (1994) 2972 and 79 (1997) 3849; Phys . Lett. B420 (1998) 37, B428 (1998) 18 and B443 (1998) 77; Phys . Rev. C58 (1998) 1576 2. W. Brodowski et al., Z. Phys . A355 (1996) 5 and contribution to this conference 3. L.S. Vorobyev et al. , JETP Lett. 59 (1994) 77 4. G. von Holtey et al., Z. Phys . 259 (1973) 51 and references therein 5. B. Bouquet et al., Nucl. Phys . B79 (1974) 45 6. P. Argan et al., Phys . Rev. C21 (1980) 1416 7. J .e. Bergstrom et al., Phys. Rev. C57 (1998) 3203 8. B. Krusche et al., Eur. Phys. J ., accepted for publication 9. A.R. Gabler, Nucl. Instr, Meth . A346 (1994) 168 10. V.M. Kolybasov and V.G. Ksensov, Sov. J. Nucl. Phys. 27 (1975) 372 11. J .M. Laget, Phys . Rep. 69 (1981) 1 12. R. Bilger et aI., Nne1. Phys . A596 (1996) 586
~ ELSEVIER
Nuclear Physics A663&664 (2000) 428c-431 c www.elsevier.nlllocate/npe
Measurement of coherent and incoherent 1l"0 photoproduction off the deuteron with tagged photons up to the ~ region" U. Siodlaczek", P. Achenbach", J. Ahrens", H.-J. Arends", R. Beckc , R. Bilger", H. Clement", V. Hejny", M. Kotulla b, B. Krusche", V. Kuhr e , R. Leukel", V. Metag", R. Novotny", V. Olmos de Leon", F. Rambo", M. Schepkin", A. Schmidt", M. Schumacher", H. Stroher", G.J. Wagner", Th. Walcherc , J. WeiBb, F. Wissmann", M. Wolf' "Physikalisches Institut, Universitiit Tiibingen, D-72076 Tiibingen, Germany
-n. Physikalisches Institut, Universitat GieBen, Germany "Institut fur Kernphysik, Universitiit Mainz, Germany dIKP - Forschungszentrum Jiilich Germany ell. Physikalisches Institut, Universitat Gi:ittingen, Germany
fITEP Moscow, Russia Coherent and incoherent KO photo production off the deuteron has been measured for incident quasi-monoenergetic photons in the energy range from KO threshold up to 300MeV using the TAPS setup together with the Glasgow tagger at MAMI. Integral and differential cross sections over the full angular range have been obtained. Coherent and incoherent production are found to exhibit very different angle and energy dependences. The sum of both processes, the total 7[0 production off the deuteron is quantitatively extremely well described by a pure quasifree model, whereas the data for coherent and incoherent channels necessitate strong FSI effects for their description. Finally upper limits for the production of dibaryon resonances in this reaction have been obtained. 1. INTRODUCTION
Photoproduction of neutral mesons is dominated by resonance contributions, since background contributions from meson pole or Kroll-Ruderman terms are strongly suppressed. Whereas an extensive set of data exists for 7[0 photoproduction off the proton, there are no corresponding data for unbound neutrons for obvious reasons. Hence measurements on the deuteron, where the neutron is only very loosely bound, appear to be very well suited for this purpose. The 7[0 photoproduction off the deuteron may proceed via two different processes: the coherent process, where the deuteron stays in its groundstate and the incoherent process, where the deuteron breaks up. The latter reaction channel is expected to be dominated 'supported by BMBF (06 TV 886) and DFG (Mu 705/3 Graduiertenkolleg and SFB 201) 0375-9474/00/$ see front matter © 2000 Elsevier Science RY. All rights reserved. PH S0375-9474(99)00675-2
U. Siodlaczek et al./Nuclear Physics A 663&664 (2000) 428c-431c
429c
by the quasifree process. Hence measurement of this channel in quasifree kinematics has been considered to be most favorable in obtaining the desired production cross section for the neutron. Finally, this reaction is also very well suited for th e search of narrow dibaryon resonances, which couple to the I'd channel. One such candidate is the hitherto hypothetical 1rNN resonance d' with I (JP) = even (0-) , m ~ 2.06GeV and r ~ 0.5 MeV. Due to its quantum numbers it cannot decay into the N N channel, hence its width is expect ed to be very narrow , in particular since its mass is close to the 1r N N threshold. Suggestive evidence for such a resonance has been found in the pionic double charge exchange on nuclei [1] as well as in pp -+ Pp7r-1r+ [2,3]. Except of some old data from Bonn [4] and Orsay [5] ther e are high-quality data available only very close to 1r 0 threshold for coherent [6] and total [7] production cross sections, as well as in the energy range 204-792 MeV from the 1992 TAPS run at MAMI [8] for both coherent and incoherent production.
2. RESULTS In order to fill the data gap between 1r 0 threshold and .6.-resonance region a new highstatistics run in the range E"I = 130- 306 MeV has been carri ed out with an extended TAPS [9] setup in combination with the Glasgow tagger at MAMI. Th e extended TAPS setup consist ed of six blocks each containing 64 BaF 2 modules in an 8 x 8 matrix with individu al plastic veto counters in front and a forward wall consisting of 120 BaF2 modul es with individual plastic veto counters combined in Phoswich telescopes. A LD2 target of 10em length and a diam eter of 5em was used. The neutral pions were detected via their 2')' decay by ')'')' coincidences in TAPS and identified via an invariant mass analysis. In addition to photons also protons , neutrons and deut erons have been detected when possible. Due to the small kinetic energies of the baryonic ejectiles at low photon energies th e experim ental separation of coherent and incoherent channels has been feasible only by means of the 1r 0 missing mass (or equivalently the missing energy) spectrum. Fig. 1 shows such spectra for an incident photon energy of 260MeV. Though th e contributions for coherent and incoherent processes overlap , they can be separated in a fit procedure by use of MC simulations of these processes taking into account the experimental resolution. Our measurements show that the coherent production is the dominant part up to E"I ~ 250 MeV, before it saturates near E"I ~ 280MeV. The breakup channel on th e other hand is very small at low energies, but rises steeply at th e onset of the .6. resonance. Whereas the coherent channel gives angular distributions , which are strongly forwardpeaked, those of the breakup channel peak at intermediate and backward angles. As an example, Fig. 2 shows th e angular distributions at E"I = 260MeV. There is good agreement between the results of this measurement and those of [81 in the energy region where both experiments overlap . Surprisingly at first glance, MC simulat ions of the pur e quasi-free process do not give a good description of the angular dependence in the breakup channel, in part icular not in the forward angular range. Instead , exactly these simulations yield an excellent description
430c
U Siodlaczek et al. /Nuclear Physics A663&664 (2000) 428c--431c
III
'E:I o
o
·1 00
·50
0
50
50
100
100
missing E [MeV]
missing E [MeV]
50
100
missing E [MeV ]
Figure 1. 1r 0 missing energy spectra at E-y = 260MeV at forward, intermediate and backward 1r 0 angles. The data are fitted with Me simulations for coherent (light shaded) and incoherent (dark) 1r0 production. Also shown are the sums of both contributions.
Ey 25
=260 MeV
MC quasllree
•
MCcoherent
.. coherent
total
Figure 2. Observed angular distributions of total, coherent and incoherent 1r 0 photoproduction at E: = 260MeV. Me simulations for the pure quasifree process and the coalescence ansatz (see text) are shown by the solid, dashed and dash-dotted curves, respectively.
U. Siodlaczek et al.lNuclear Physics A663&664 (2000) 428c-431c
431c
of the total, i.e. the sum of coherent and incoherent cross section both in its energy and angular dependence (solid curve in Fig. 2). From these observations it may be concluded that the total cross section is much better suited for the derivation of the 11"0 production cross section off the neutron than the originally considered incoherent cross section, where the quasifree process obviously is strongly distorted by strong FSI effects. In fact, it had been shown theoretically already some time ago by Kolybasov and Ksensov [10] that the N N-FSI effects in coherent and breakup channel cancel each other in their sum , the total cross section, which therefore represents the pure quasifree process to high accuracy. These features have also been noticed by Laget [11] in his numerical calculations. The strong FSI effects in coherent and incoherent production can be effectively simulated by a very simple coalescence ansatz assuming that for pn relative momenta q :::; qo always a deuteron is formed, whereas the event proceeds into the breakup channel for q > qo. Adjustment of this single parameter to qo = 300 MeV/ c results in an amazingly perfect description of all angular and energy dependences in both coherent and incoherent channels (dashed curves in Fig. 2). The reason for the success of such a simple ansatz has yet to be understood theoretically. Finally, as already mentioned in the introduction, this reaction also is very well suited for the search of narrow dibaryons like the hitherto hypothetical 1I"NN resonance d'. With the tagger resolution of AE-y ~ 1 MeV an upper limit of 2- 4 J-tb * MeV at the 90 % confidence level for the production of such resonances in this reaction can be deduced . Though these are the first stringent limits for dibaryon resonances in this reaction, they are still far above the expected d' production cross section which has been estimated to be small er than 1 J-tb [12] .
REFERENCES 1. R. Bilger et al., Phys . Rev. Lett. 71 (1993) 42, 72 (1994) 2972 and 79 (1997) 3849; Phys . Lett. B420 (1998) 37, B428 (1998) 18 and B443 (1998) 77; Phys . Rev. C58 (1998) 1576 2. W. Brodowski et al., Z. Phys . A355 (1996) 5 and contribution to this conference 3. L.S. Vorobyev et al. , JETP Lett. 59 (1994) 77 4. G. von Holtey et al., Z. Phys . 259 (1973) 51 and references therein 5. B. Bouquet et al., Nucl. Phys . B79 (1974) 45 6. P. Argan et al., Phys . Rev. C21 (1980) 1416 7. J .e. Bergstrom et al., Phys. Rev. C57 (1998) 3203 8. B. Krusche et al., Eur. Phys. J ., accepted for publication 9. A.R. Gabler, Nucl. Instr, Meth . A346 (1994) 168 10. V.M. Kolybasov and V.G. Ksensov, Sov. J. Nucl. Phys. 27 (1975) 372 11. J .M. Laget, Phys . Rep. 69 (1981) 1 12. R. Bilger et aI., Nne1. Phys . A596 (1996) 586