Bled Workshops in Physics Vol. 16, No. 1 p. 49 A Proceedings of the Mini-Workshop Exploring Hadron Resonances Bled, Slovenia, July 5 - 11, 2015 Progress in Neutron Couplings* W. J. Briscoe and I. Strakovsky The George Washington University, Washington, DC 20052, USA Abstract. An overview of the GW SAID group effort to analyze pion photoproduction on the neutron-target will be given. The disentanglement the isoscalar and isovector EM couplings of N* and A* resonances does require compatible data on both proton and neutron targets. The final-state interaction plays a critical role in the state-of-the-art analysis in extraction of the yu —> nN data from the deuteron target experiments. It is important component of the current JLab, MAMI-C, SPring-8, CBELSA, and ELPH programs. 1 Introduction The N* family of nucleon resonances has many well established members [1], several of which exhibit overlapping resonances with very similar masses and widths but with different JP spin-parity values. Apart from the N(1535) 1/2- state, the known proton and neutron photo-decay amplitudes have been determined from analyses of single-pion photoproduction. The present work reviews the region from the threshold to the upper limit of the SAID analyses, which is CM energy W = 2.5 GeV. There are two closely spaced states above A(1232)3/2+: N(1520)3/2- and N(1535)1/2-. Up to W - 1800 MeV, this region also encompasses a sequence of six overlapping states: N(1650)1/2-, N(1675)5/2-, N(1680)5/2+, N(1700)3/2-, N(1710)1/2+, and N(1720)3/2+. One critical issue in the study of meson photoproduction on the nucleon comes from isospin. While isospin can change at the photon vertex, it must be conserved at the final hadronic vertex. Only with good data on both proton and neutron targets can one hope to disentangle the isoscalar and isovector electromagnetic (EM) couplings of the various N* and A* resonances (see Refs. [2]), as well as the isospin properties of the non-resonant background amplitudes. The lack of yn —» n-p and yn —» n0n data does not allow us to be as confident about the determination of neutron EM couplings relative to those of the proton. For instance, the uncertainties of neutral EM couplings of 4* low-lying N* resonances, A(nA1/2) vary between 25 and 140% while charged EM couplings, A(pA1/2), vary between 7 and 42%. Some of the N* baryons [N(1675)5/2-, for instance] have stronger EM couplings to the neutron relative to the proton, but the parameters are very uncertain [1]. One more unresolved issue relates to the second P11, N(1710) 1/2+. That is not seen in the recent nN partial-wave analysis * Talk presented by I. Strakovsky 50 W. J. Briscoe and I. Strakovsky (PWA) [3], contrary to other PWAs used by the PDG14 [1]. A recent brief review of its status is given in Ref. [4]. Additionally, incoherent pion photoproduction on the deuteron is interesting in various aspects of nuclear physics, and particularly, provides information on the elementary reaction on the neutron, i.e., yn —» nN. Final-state interaction (FSI) plays a critical role in the state-of-the-art analysis of the yn —» nN interaction as extracted from yd —» nNN measurements. The FSI was first considered in Refs. [5] as responsible for the near-threshold enhancement (Migdal-Watson effect) in the NN mass spectrum of the meson production reaction NN NNx. In Ref. [6], the FSI amplitude was studied in detail. 2 Complete Experiment in Pion Photoproduction Originally, PWA arose as the technology to determine amplitude of the reaction via fitting scattering data. That is a non-trivial mathematical problem - loking for a solution of ill-posed problem following to Hadamard, Tikhonov et al. Resonances appeared as a by-product (bound states objects with definite quantum numbers, mass, lifetime and so on). There are 4 independent invariant amplitudes for a single pion photoproduction. In order to determine the pion photoproduction amplitude, one has to carry out 8 independent measurements at fixed (s, t) (the extra observable is necessary to eliminate a sign ambiguity). There are 16 non-redundant observables and they are not completely independent from each other, namely 1 unpolarized, da/dH; 3 single polarized, I, T, and P; 12 double polarized, E, F, G, H, Cx, Cz, Ox, Oz, Lx, Lz, Tx, and Tz measurements. Additionally, there are 18 triple-polarization asymmetries [9 (9) for linear (circular) polarized beam and 13 of them are non-vanishing] [7]. Obviously, the triple-polarization experiments are not really necessary from the theoretical point of view while such measurements will play a critical role to keep systematics under control. 3 Neutron Database Experimental data for neutron-target photoreactions are much less abundant than those utilizing a proton target, constituting only about 15% of the present worldwide known GW SAID database [8]. The existing yn —» n-p database contains mainly differential cross sections and 15% of which are from polarized measurements. At low to intermediate energies, this lack of neutron-target data is partially compensated by experiments using pion beams, e.g., n-p —» yn, as has been measured, for example, by the Crystal Ball Collaboration at BNL [9] for the inverse photon energy E = 285 - 689 MeV and 0 = 41° — 148°, where 0 is the inverse production angle of n- in the CM frame. This process is free from complications associated with the deuteron target. However, the disadvantage of using the reaction n-p —» yn is the 5 to 500 times larger cross sections for n-p —» yn —» ynn, Progress in Neutron Couplings 51 1050 1350 1450 1650 1850 2050 2350 2450 ¥ (MeV) 1050 1250 1450 1650 1B50 2050 2250 3450 1050 1250 1450 1650 1850 2050 3250 2450 ¥ (MeV) ¥ (MeV) 7n->7T°n 1050 1250 1450 1650 1850 2050 2250 2450 ¥ (MeV) 1050 1250 1450 1650 1850 2050 2250 2450 ¥ (MeV) 1050 1250 1450 1650 1850 2050 2250 2450 ¥ (MeV) Fig. 1. Data available for single pion photoproduction of the neutron as a function of CM energy W [8]. The number of data points, dp, is given in the upper right hand side of each subplot. Top panel: The first subplot (blue) shows the total amount of yn —> n-p data available for all observables, the second subplot (red) shows the amount of da/dO, data available, the third subplot (green) shows the amount of P observables data available. Bottom panel: The first subplot (blue) shows the total amount of yn —> n0n data available for all observables, the second subplot (red) shows the amount of da/dO data available, the third subplot (green) shows the amount of P observables data available. depending on E and 0, which causes a large background, and there were no tagging high flux pion beams. Figure 1 summarizes the available data for single pion photoproduction on the neutron below W = 2.5 GeV. Some high-precision data for the yn —» n-p and yn —} n°n reactions have been measured recently. We applied our GW-ITEP FSI corrections, covering a broad energy range up to E = 2.7 GeV [6], to the CLAS and A2 Collaboration yd —» n-pp measurements to get elementary cross sections for yn —» n-p [1°, 11]. In particular, the new CLAS cross sections have quadrupled the world database for yn —» n-p above E = 1 GeV. The FSI correction factor for the CLAS (E = 1050 - 27°° MeV and 0 = 32° - 157°) and MAMI (E = 301 -455 MeV and 0 = 45° - 125°) kinematics was found to be small, Act/ct < 10%. Obviously, that is not enough to have compatible proton and neutron databases, specifically the energy binning of the CLAS measurements is 50 MeV or, in the worst case, 100 MeV while A2 Collaboration measurements are able to have 2 to 4 MeV binning. The forward direction, which is doable for A2 vs. CLAS, is critical for evaluation of our FSI treatment. 4 Neutron Data from Deuteron Measurements The determination of the yd —» n- pp differential cross sections with the FSI, taken into account (including all key diagrams in Fig. 2), were done, as we did recently [6,10,11], for the CLAS [10] and MAMI data [11]. The SAID of GW Data Analysis Center (DAC) phenomenological amplitudes for yN —» nN [12], NN —} NN [13], and nN —» nN [3] were used as inputs to calculate the diagrams 52 W. J. Briscoe and I. Strakovsky in Fig. 2. The Bonn potential (full model) [14] was used for the deuteron description. In Refs. [10,11], we calculated the FSI correction factor R(E,0) dependent on photon energy, E, and pion production angle in CM frame 0 and fitted recent CLAS and MAMI da/dH versus the world yN —» nN database [8] to get new neutron multipoles and determine neutron resonance EM couplings [10]. Fig. 2. Feynman diagrams for the leading components of the yd —> n-pp amplitude. (a) Impulse approximation (IA), (b) pp-FSI, and (c) nN-FSI. Filled black circles show FSI vertices. Wavy, dashed, solid, and double lines correspond to the photons, pions, nucleons, and deuterons, respectively. Results of calculations and comparison with the experimental data on the differential cross sections, dayd/dn, where H and 0 are solid and polar angles of outgoing n- in the laboratory frame, respectively, with z-axis along the photon beam for the reaction yd —» n-pp are given in Fig. 3 for a number of the photon energies, E. The FSI corrections for the CLAS and MAMI quasi-free kinematics were found to be small, as mentioned above. As an illustration, Fig. 4 shows the FSI correction factor R(E, 0) = (dCT/dnnp)/(dCTIA/dnnp) for the yn —» n-p differential cross sections as a function of the pion production angle in the CM (n — p) frame, 0, for different energies over the range of the CLAS and MAMI experiments. Overall, the FSI correction factor R(E, 0) < 1, while the effect, i.e., the (1 -R) value, vary from 10% to 30%, depending on the kinematics, and the behavior is very smooth versus pion production angle. We found a sizeable FSI-effect from S-wave part of pp-FSI at small angles. A small but systematic effect |R — 11 << 1 is found in the large angular region, where it can be estimated in the Glauber approach, except for narrow regions close to 0 ~ 0° or 0 ~ 180°. The yn —» n0n case is much more complicate vs. yn —» n-p because n0n final state can come from both yn and yp initial interactions [16]. The leading diagrams for yd —» n0pn are similar as given on Fig. 2. 5 New Neutron Amplitudes and neutron EM Couplings The solution, SAID GB12 [10], uses the same fitting form as SAID recent SN11 solution [17], which incorporated the neutron-target CLAS da/dO for yn —» n-p [10] and GRAAL Is for both yn -> n-p and yn -> n0n [18,19] (Fig. 5). This fit form was motivated by a multichannel K-matrix approach, with an added phenomenological term proportional to the nN reaction cross section. However, these new CLAS cross sections departed significantly from our predictions at the Progress in Neutron Couplings 53 Fig. 3. The differential cross section, dayd/dQ, of the reaction yd —> n-pp in the laboratory frame at different values of the photon laboratory energy E < 1900 MeV; 0 is the polar angle of the outgoing n-. Dotted curves show the contributions from the IA amplitude [Fig. 2(a)]. Successive addition of the NN-FSI [Fig. 2(b)] and nN-FSI [Fig. 2(c)] amplitudes leads to dashed and solid curves, respectively. The filled circles are the data from DESY bubble chamber [15]. higher energies, and greatly modified PWA result [10] (Fig. 5). Recently, the BnGa group reported a neutron EM coupling determination [21] using the CLAS Collaboration yn —} n-p because n0n final state can come from both yn and yp initial interact da/dQ with our FSI [10] (Table 1). BnGa13 and SAID GB12 used the same (almost) data [10] to fit them while BnGa13 has several new Ad-hoc resonances. Overall: the difference between MAID07 with BnGa13 and SAID GB12 is rather small but resonances may be essentially different (Table 1). The new BnGa13 [21] has some difference vs. GB12 [10], PDG14 [1], for instance, for N(1535)1/2-, N(1650)1/2-, and N(1680)5/2+. 54 W. J. Briscoe and I. Strakovsky Fig. 4. The correction factor R(E,0), where 0 is the polar angle of the outgoing n- in the rest frame of the pair n-+ fast proton. The kinematic cut, Pp > 200 MeV/c, is applied. The solid (dashed) curves are obtained with both nN- and NN-FSI (only NN-FSI) taken into account. 6 Work in Progress At MAMI in March of 2013, we collected deuteron data below E = 800 MeV with 4 MeV energy binning [23] and will have a new experiment below E = 1600 MeV [24] in the fall of 2016. The experimental setup provides close to 4n sr coverage for outgoing particles. The photons from n0 decays and charged particles are detected by the CB and TAPS detection system. The energy deposited by charged particles in CB and TAPS is, for the most part, proportional to their kinetic energy, unless they punch through crystals of the spectrometers. Clusters from the final-state neutrons provide information only on their angles. Separation of clusters from neutral particles and charged ones is based on the information from MWPC, PID, and TAPS veto. Separation of positive and negative pions can be based on the identification of the final-state nucleon as either a neutron or a proton. Since cluster energies from charged pions are proportional to their kinetic energy (unless their punch through the crystals), the energy of those clusters can be very low close to reaction threshold. Progress in Neutron Couplings 55 Table 1. Neutron helicity amplitudes A1/2 and A3/2 (in [(GeV)-1/2 x 10-3] units) from the SAID GB12 [10] (first row), previous SAID SN11 [17] (second row), recent BnGa13 by the Bonn-Gatchina group [21] (third row), recent Kent12 by the Kent State Univ. group [22] (forth row), and average values from the PDG14 [1] (fifth row). Resonance nA i/2 Resonance n-di/2 1^3/2 Ref. JV(1535)l/2" —58± 6 JV(1520)3/2" —46± 6 —115± 5 SAID GB12 60+ 3 —47± 2 —125± 2 SAID SN11 —93±11 —49± 8 — 113±12 BnGal3 —49± 3 —38± 3 -101±4 Kent 12 —46±27 — 59+ 9 -139±11 PDG14 N( 1650)1/2" —40±10 JV(1675)5/2" — 58± 2 -80±5 SAID GB 12 —26± S —42± 2 -60±2 SAID SN11 25±20 —60± 7 -S8±10 BnGal3 11±2 —40± 4 —68± 4 Kent 12 — 15±21 —43±12 -S8±13 PDG14 JV(1440)l/2+ 4B±4 JV(1680)5/2+ 26± 4 -29±2 SAID GB 12 45±15 50± 4 -47±2 SAID SN11 43±12 34±6 -44±9 BnGa 13 40± 5 29± 2 —59± 2 Kent 12 40±10 29±10 -33±9 PDG 14 Monte Carlo simulations, which tracks reaction products through a realistic model of the detector system together with the reconstruction program, is used to calculate acceptance to various channels. So to detect the reactions under study with our setup, we have to take data with almost open trigger. Acceptance for reaction yn —» n0n varies from 70% at 0.8 GeV to 30% at 1.5 GeV of the incident-photon energy. Acceptance of reaction yp —» n+n drops at higher beam energies as charged pions punch through the crystals, and the energy of the neutron cluster does not reflect its kinetic energy. Reaction yn —» n-p above 0.8 GeV has an acceptance that is better than that for yp —» n+n as the energy and angles of the cluster from the outgoing proton can be used to reconstruct the reaction kinematics. We are going to use our FSI technology to apply for the upcoming JLab CLAS (g13 run period) da/da for yn -» n-p covering E = 400 - 2500 MeV and 9 = 18° -152° [25]. This data set will bring about 11k new measurements which quadruple the world yn —» n-p database. The ELPH facility at Tohoku Univ. will bring new da/da for yn -> n0n below E = 1200 MeV [26]. 7 Summary for Neutron Study • The differential cross section for the processes yn —» n-p was extracted from new CLAS and MAMI-B measurements accounting for Fermi motion effects in the IA as well as NN- and nN-FSI effects beyond the IA. • Consequential calculations of the FSI corrections, as developed by the GW-ITEP Collaboration, was applied. • New cross sections departed significantly from our predictions, at the higher energies, and greatly modified the fit result. 56 W. J. Briscoe and I. Strakovsky Fig. 5. Samples of neutron multipoles I = 1/2 and 3/2. Solid (dash-dotted) lines correspond to the SAID GB12 [10] (SN11 [17]) solution. Thick solid (dashed) lines give SAID GZ12 [10] solution (MAID07 [20]). Vertical arrows indicate mass (WR), and horizontal bars show full, r, and partial, rnN, widths of resonances extracted by the Breit-Wigner fit of the nN data associated with the SAID solution WI08 [3]. New yn —» n-p and yn —» n0n data will provide a critical constraint on the determination of the multipoles and EM couplings of low-lying baryon resonances using the PWA and coupled channel techniques. Progress in Neutron Couplings 57 • Polarized measurements at JLab/JLab12, MAMI, SPring-8, CBELSA, and ELPH will help to bring more physics in. • FSI corrections need to apply. Acknowledgements The authors are grateful to A. E. Kudryavtsev, V. V. Kulikov, M. Maremianov, V. E. Tarasov, and R. L. Workman for many useful communications and discussions. This material is based upon work supported by the U.S. Department of Energy, Office of Science, Office of Nuclear Physics, under Award Number DE-FG02-99-ER41110. References 1. K. A. Olive et al. (Particle Data Group), Chin. Phys. C 38, 090001 (2014). 2. K.M. Watson, Phys. Rev. 95, 228 (1954); R.L. Walker, Phys. Rev. 182,1729 (1969). 3. R.A. Arndt, W.J. Briscoe, I.I. Strakovsky, and R.L. Workman, Phys. Rev. C 74, 045205 (2006). 4. Ya.I. Azimov and I.I. Strakovsky, Proceedings of the XVth International Conference on Hadron Spectroscopy (Hadron 2013), Nara, Japan, Nov. 2013, PoS (Hadron 2014) 034. 5. A.B. Migdal, JETP 1, 2 (1955); K.M. Watson, Phys. Rev. 88,1163 (1952). 6. V.E. Tarasov, W.J. Briscoe, H. Gao, A.E. Kudryavtsev, and I.I. Strakovsky, Phys. Rev. C 84, 035203 (2011). 7. A.M. Sandorfi, B. Dey, A. Sarantsev, L. Tiator, and R. Workman, AIP Conf. Proc. 1432, 219 (2012); K. Nakayama, private communication, 2014. 8. W.J. Briscoe, I.I. Strakovsky, and R.L. Workman, Institute of Nuclear Studies of The George Washington University Database; http://gwdac.phys.gwu.edu/analysis/pr_analysis.html. 9. A. Shafi et al, Phys. Rev. C 70, 035204 (2004). 10. W. Chen, H. Gao, W.J. Briscoe, D. Dutta, A.E. Kudryavtsev, M. Mirazita, M.W. Paris, P. Rossi, S. Stepanyan, I.I. Strakovsky, V.E. Tarasov, and R.L. Workman, Phys. Rev. C 86, 015206 (2012). 11. W.J. Briscoe, A.E. Kudryavtsev, P. Pedroni, I.I. Strakovsky, V.E. Tarasov, and R.L. Workman, Phys. Rev. C 86, 065207 (2012). 12. M. Dugger, J.P. Ball, P. Collins, E. Pasyuk, B.G. Ritchie, R.A. Arndt, W.J. Briscoe, I.I. Strakovsky, R.L. Workman et al. (CLAS Collaboration), Phys. Rev. C 76, 025211 (2007). 13. R.A. Arndt, W.J. Briscoe, I.I. Strakovsky, and R.L. Workman, Phys. Rev. C 76, 025209 (2007). 14. R. Machleidt, K. Holinde, and C. Elster, Phys. Rep. 149,1 (1987). 15. P. Benz et al. (Aachen-Bonn-Hamburg-Heidelberg-Muenchen Collaboration), Nucl. Phys. B 65,158 (1973). 16. V. Tarasov, A. Kudryavtsev, W. Briscoe, M. Dieterle, B. Krusche, I. Strakovsky, and M. Ostrick, to be published in Yad. Fiz. 79 (2016) [Phys. At. Nucl. 79 (2016)] ; arXiv:1503.06671 [hep.ph]. 17. R.L. Workman, W.J. Briscoe, M.W. Paris, and I.I. Strakovsky, Phys. Rev. C 85, 025201 (2012). 18. G. Mandaglio et al. (GRAAL Collaboration), Phys. Rev. C 82, 045209 (2010). 58 W. J. Briscoe and I. Strakovsky 19. R. Di Salvo et al. (GRAAL Collaboration), Eur. Phys. J. A 42,151 (2009). 20. D. Drechsel, S.S. Kamalov, and L. Tiator, Eur. Phys. J. A 34, 69 (2007); http://www.kph.uni-mainz.de/MAID/ . 21. A. Anisovich, V. Burkert, E. Klempt, V.A. Nikonov, A.V. Sarantsev, and U. Thoma, Eur. Phys. J. A 49, 67 (2013). 22. M. Shrestha and D.M. Manley, Phys. Rev. C 86, 055203 (2012). 23. Meson production off the deuteron. I, Spokespersons: W.J. Briscoe and I.I. Strakovsky (A2 Collaboration), MAMI Proposal MAMI-A2-02/12, Mainz, Germany, 2012. 24. Meson production off the deuteron. II, Spokespersons: W.J. Briscoe, V.V. Kulikov, K. Livingston, and I.I. Strakovsky (A2 Collaboration), MAMI Proposal MAMI-A2-02/13, Mainz, Germany, 2013. 25. P. Mattione, Proceedings of the XVth International Conference on Hadron Spectroscopy (Hadron 2013), Nara, Japan, Nov. 2013, PoS (Hadron 2014) 096. 26. T. Ishikawa et al., Proceedings of the XVth International Conference on Hadron Spectroscopy (Hadron 2013), Nara, Japan, Nov. 2013, PoS (Hadron 2014) 095. 104 Povzetki v slovenščini Analiza delnih valov za podatke pri fotoprodukciji mezona n z upoštevanjem omejitev zaradi analiticnosti M. HadZimehmedovič a, V. Kashevarovc, K. Nikonovc, R. Omerovič a, H. Osman-oviča, M. Ostričkc, J. Stahova, A. Svarcb in L. Tiatorc a University of Tuzla, Faculty of Science, Bosnia and Herzegovina b Rudjer Boskovič Institute, Zagreb, Croatia a Institut fuer Kernphysik, Johannes Gutenberg Universtaet Mainz, Germany Izvedemo analizo delnih valov za podatke pri fotoprodukciji n. Dobljeni multi-poli so v skladu z analiticnostjo pri fiksnem t in pri fiksnem s. Analiticnost pri fiksnem t zagotovimo s Pietarinenovo metodo. Invariantne amplitude ubogajo zahtevano navzkrizno simetrijo. Napredek pri poznavanju sklopitev nevtrona W. J. Briscoe in I. Strakovsky The George Washington University, Washington, DC 20052, USA Podajamo pregled prizadevanj skupine GW SAID za analizo fotoprodukcije pio-nov na nevtronski tarci. Razlocitev izoskalarnih in izovektorskih elektromagnetnih sklopitev resonanc N* in A* zahteva primerljive in skladne podatke na protonski in na nevtronski tarci. Interakcija v koncnem stanju igra kriticno vlogo pri najsodobnejsi analizi in izvrednotenju podatkov za proces yn —» nN pri eksperimentih z devteronsko tarco. Ta je pomemben sestavni del tekocih programov v laboratorijih JLab, MAMI-C, SPring-8, CBELSA in ELPH. Vzbujanje barionskih resonanc s fotoprodukcijo mezonov Lothar Tiatora in Alfred Svarcb a Institut fuer Kernphysik, Johannes Gutenberg Universtaet Mainz, Germany b Rudjer Boskovic Institute, Zagreb, Croatia Spektroskopija lahkih hadronov je se vedno zivahno podrocje v fiziki jedra in delcev. Celo 50 let po odkritju Roperjeve resonance in vec kot 30 let po pionirskem delu Hoehlerja and Cutkoskyja je se veliko odprtih vprasanjglede barionskih resonanc. Danes je glavni vzbujevalni mehanizem fotoprodukcija in elektropro-dukcija mezonov, merjena na elektronskih pospesevalnikih kot so MAMI, ELSA in JLab. V zdruzenem prizadevanju izvrednotimo lege in jakosti polov iz parcialnih valov, dobljenih z analizo parcialnih valov pri nedavnih meritvah polarizacij ob uporabi analiticnih omejitev iz disperzijskih relacijpri fiksnem t. Poseben poudarek pri barionskih resonancah je na strukturi pola na razlicnih Rieman-novih ploskvah.