Measurements of 12C \((\vec{\gamma},pp)\) photon asymmetries for \(E_{\gamma}=200\) –450 MeV

The European Physical Journal A, May 2013

The 12C\((\vec{\gamma},pp)\) reaction has been studied in the photon energy range 200-450 MeV at the Mainz microtron MAMI-C, where linearly polarised photons were energy-tagged using the Glasgow-Mainz Tagged Photon Spectrometer and protons were detected in the Crystal Ball detector. The photon asymmetry \(\Sigma\) has been measured over a wider \(E_{\gamma}\) range than previous measurements. The strongest asymmetries were found at low missing energies where direct emission of nucleon pairs is expected. Cuts on the difference in azimuthal angles of the two ejected protons increased the magnitude of the observed asymmetries. At low missing energies the \(\Sigma\) data exhibit a strong angular dependence, similar to deuteron photodisintegration.

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Measurements of 12C \((\vec{\gamma},pp)\) photon asymmetries for \(E_{\gamma}=200\) –450 MeV

The Crystal Ball at MAMI A 7 Collaborations J. Robinson 8 I.J.D. MacGregor 8 J.R.M. Annand 8 P. Aguar-Bartolome 7 L.K. Akasoy 7 H.J. Arends 7 Ya.I. Azimov 6 K. Bantawa 13 R. Beck 12 V.S. Bekrenev 6 H. Berghauser 11 A. Braghieri 10 D. Branford 15 W.J. Briscoe 14 J. Brudvik 9 S. Cherepnya 4 R.F.B. Codling 8 B.T. Demissie 14 M. Dieterle 5 E.J. Downie 7 P. Drexler 7 L.V. Fil'kov 4 D.I. Glazier 15 R. Gregor 11 D. Hamilton 8 E. Heid 7 D. Hornidge 2 D. Howdle 8 O. Jahn 7 T.C. Jude 15 V.L. Kashevarov 4 I. Keshelashvili 5 R. Kondratiev 3 M. Korolija 0 M. Kotulla 11 A.A. Koulbardis 6 S.P. Kruglov 6 B. Krusche 5 V. Lisin 3 K. Livingston 8 D.M. Manley 13 J.C. McGeorge 8 E.F. McNicoll 8 V. Metag 11 D.G. Middleton 2 7 A. Mushkarenkov 10 B.M.K. Nefkens 9 A. Nikolaev 12 R. Novotny 11 M. Oberle 5 M. Ostrick 7 R.O. Owens 8 P.B. Otte 7 B. Oussena 7 P. Pedroni 10 A. Polonski 3 S. Prakhov 9 G. Rosner 8 T. Rostomyan 10 S. Schumann 7 M.H. Sikora 15 D.I. Sober 1 A. Starostin 9 I.I. Strakovsky 14 I.M. Suarez 9 I. Supek 0 C.M. Tarbert 15 M. Thiel 11 A. Thomas 7 M. Unverzagt 7 D.P. Watts 15 D. Werthmueller 5 L. Witthauer 5 0 Rudjer Boskovic Institute , HR-10000 Zagreb, Croatia 1 The Catholic University of America , Washington, DC 20064, USA 2 Mount Allison University , Sackville, New Brunswick E4L3B5, Canada 3 Institute for Nuclear Research , 125047 Moscow, Russia 4 Lebedev Physical Institute , 119991 Moscow, Russia 5 Institut fur Physik, University of Basel , CH-4056 Basel, Switzerland 6 Petersburg Nuclear Physics Institute , 188300 Gatchina, Russia 7 Institut fur Kernphysik, University of Mainz , D-55099 Mainz, Germany 8 SUPA, School of Physics and Astronomy, University of Glasgow , Glasgow G12 8QQ, UK 9 University of California Los Angeles , Los Angeles, California 90095-1547, USA 10 INFN Sezione di Pavia, I-27100 Pavia, Italy 11 II Physikalisches Institut, University of Giessen , D-35392 Giessen, Germany 12 Helmholtz-Institut fur Strahlen- und Kernphysik, University of Bonn , D-53115 Bonn, Germany 13 Kent State University , Kent, Ohio 44242, USA 14 The George Washington University , Washington, DC 20052, USA 15 SUPA, School of Physics, University of Edinburgh , Edinburgh EH9 3JZ, UK The 12C(, pp) reaction has been studied in the photon energy range 200-450 MeV at the Mainz microtron MAMI-C, where linearly polarised photons were energy-tagged using the Glasgow-Mainz Tagged Photon Spectrometer and protons were detected in the Crystal Ball detector. The photon asymmetry has been measured over a wider E range than previous measurements. The strongest asymmetries were found at low missing energies where direct emission of nucleon pairs is expected. Cuts on the difference in azimuthal angles of the two ejected protons increased the magnitude of the observed asymmetries. At low missing energies the data exhibit a strong angular dependence, similar to deuteron photodisintegration. 1 Introduction Two-nucleon knockout reactions from light nuclei using real or virtual photons at energies of a few hundred MeV proceed by mechanisms that involve Meson Exchange Currents (MEC), Isobar Currents (IC), Short-Range Correlations (SRC) and Final-State Interactions (FSI) [13]. The reaction amplitudes are also dependent on the final state of the residual nucleus. To clarify the contributions from each of these processes it is necessary to make measurements with both real and virtual photons over a wide range of kinematic conditions and to study both protonneutron and proton-proton emission. In recent years polarised photons have provided ac- of using an unfactorised treatment and distorted outgoing cess to observables sensitive to the details of the reaction nucleon waves were very important. process, whose effects are hidden when averaged over the total response. Cross sections for reactions in which the polarisation of the incident photon is either parallel ( ) or perpendicular () to the plane of the emitted protons 3 Previous measurements with polarised have different sensitivity to the reaction mechanisms that photons contribute. This difference is emphasised through the photon asymmetry, = + . Polarised photon measure- Only a few two-nucleon knockout measurements with poments therefore provide a powerful tool to study details of larised photons have been reported for nuclei heavier two-nucleon emission reactions and to test models of the than deuterium. Measurements of for 3He( ,pp) and reaction process. ( ,pn) have been carried out at LEGS [10] for E = This paper reports measurements of the 12C( ,pp) re- 235305 MeV. These measurements showed strong differaction made with linearly polarised photons in the energy ences between ( ,pp) and ( ,pn) reactions. Calculations, range E = 200450 MeV. The near 4 angular coverage which are in reasonably good agreement with the experiof the Crystal Ball (CB) detector, used to detect protons, mental measurements, indicated one- and two-body terms allowed investigation of the photon asymmetry over a do (...truncated)


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The Crystal Ball at MAMI and A2 Collaborations, J. Robinson, I. J. D. MacGregor, J. R. M. Annand, P. Aguar-Bartolomé, L. K. Akasoy, H. J. Arends, Ya. I. Azimov, K. Bantawa, R. Beck, V. S. Bekrenev, H. Berghäuser, A. Braghieri, D. Branford, W. J. Briscoe, J. Brudvik, S. Cherepnya, R. F. B. Codling, B. T. Demissie, M. Dieterle, E. J. Downie, P. Drexler, L. V. Fil’kov, D. I. Glazier, R. Gregor, D. Hamilton, E. Heid, D. Hornidge, D. Howdle, O. Jahn, T. C. Jude, V. L. Kashevarov, I. Keshelashvili, R. Kondratiev, M. Korolija, M. Kotulla, A. A. Koulbardis, S. P. Kruglov, B. Krusche, V. Lisin, K. Livingston, D. M. Manley, J. C. McGeorge, E. F. McNicoll, V. Metag, D. G. Middleton, A. Mushkarenkov, B. M. K. Nefkens, A. Nikolaev, R. Novotny, M. Oberle, M. Ostrick, R. O. Owens, P. B. Otte, B. Oussena, P. Pedroni, A. Polonski, S. Prakhov, G. Rosner, T. Rostomyan, S. Schumann, M. H. Sikora, D. I. Sober, A. Starostin, I. I. Strakovsky, I. M. Suarez, I. Supek, C. M. Tarbert, M. Thiel, A. Thomas, M. Unverzagt, D. P. Watts, D. Werthmueller, L. Witthauer. Measurements of 12C \((\vec{\gamma},pp)\) photon asymmetries for \(E_{\gamma}=200\) –450 MeV, The European Physical Journal A, 2013, pp. 65, Volume 49, Issue 5, DOI: 10.1140/epja/i2013-13065-0