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Constraining the (gamma,pi) amplitude for E2 N->delta  [PDF]
A. M. Sandorfi,J. Tonnison,S. Hoblit
Physics , 1997,
Abstract: A recent analysis of pion photo-production multipoles using p(g,pi0) data from Mainz is repeated, successively adding constraints from other observables and varying the number of fitted partial waves. The original analysis is shown to have been underconstrained and suffered from ambiguities. The inclusion of additional observables in our analysis results in a very different, but stable, ratio of E2/M1 multipoles at the delta that is consistent with the Mainz data set.
Isoscalar M1 and E2 Amplitudes in n+p -> d+gamma  [PDF]
Jiunn-Wei Chen,Gautam Rupak,Martin J. Savage
Physics , 1999, DOI: 10.1016/S0370-2693(99)01007-2
Abstract: The low energy radiative capture process n+p -> d+gamma provides a sensitive probe of the two-nucleon system. The cross section for this process is dominated by the isovector M1 amplitude for capture from the 1S0 channel via the isovector magnetic moment of the nucleon. In this work we use effective field theory to compute the isoscalar M1 and isoscalar E2 amplitudes that are strongly suppressed for cold neutron capture. The actual value of the isoscalar E2 amplitude is expected to be within 15% of the value computed in this work. In contrast, due to the vanishing contribution of the one-body operator at leading order and next-to-leading order, the isoscalar M1 amplitude is estimated to have a large uncertainty. We discuss in detail the deuteron quadrupole form factor and SD mixing.
E2 and C2 amplitudes for electroproduction of $Δ$  [PDF]
M. Fiolhais,B. Golli,S. Sirca
Physics , 1995,
Abstract: We compute the amplitudes for the electromagnetic $\gamma_v N \to \Delta$ transition in the framework of soliton models with quarks and mesons. The ratios E2/M1 and C2/M1 as functions of the photon four-momentum squared are dominated by the contribution of the pion cloud and are in reasonable agreement with the available experimental data.
On the definitions of the gamma*N -> N* helicity amplitudes  [PDF]
I. G. Aznauryan,V. D. Burkert,T. -S. H. Lee
Physics , 2008,
Abstract: We present definitions and formulas that may be useful for a consistent computation of helicity amplitudes for the process gamma* N -> N* in theoretical approaches. Of particular importance is the correct determination of the common sign of the amplitudes and of the relative sign between the transverse (A12,A32) and longitudinal (S12) amplitudes. This clarification is necessary to clear up confusions present in theoretical works. Using the definitions presented in this paper will enable a direct comparison with amplitudes extracted from experimental data.
Unitary ambiguity in the extraction of the E2/M1 ratio for the $γ N\leftrightarrowΔ$ transition  [PDF]
P. Wilhelm,Th. Wilbois,H. Arenhoevel
Physics , 1996, DOI: 10.1103/PhysRevC.54.1423
Abstract: The resonant electric quadrupole amplitude in the transition $\gamma N\leftrightarrow\Delta(1232)$ is of great interest for the understanding of baryon structure. Various dynamical models have been developed to extract it from the corresponding photoproduction multipole of pions on nucleons. It is shown that once such a model is specified, a whole class of unitarily equivalent models can be constructed, all of them providing exactly the same fit to the experimental data. However, they may predict quite different resonant amplitudes. Therefore, the extraction of the E2/M1($\gamma N\leftrightarrow\Delta$) ratio (bare or dressed) which is based on a dynamical model using a largely phenomenological $\pi N$ interaction is not unique.
Nucleon and $N^* (1535)$ Distribution Amplitudes  [PDF]
V. M. Braun,S. Collins,M. G?ckeler,C. Hagen,R. Horsley,Y. Nakamura,D. Pleiter,P. E. L. Rakow,A. Sch?fer,R. W. Schiel,G. Schierholz,H. Stüben,J. M. Zanotti
Physics , 2010,
Abstract: The QCDSF collaboration has investigated the distribution amplitudes and wavefunction normalization constants of the nucleon and its parity partner, the $N^* (1535)$. We report on recent progress in the calculation of these quantities on configurations with two dynamical flavors of $\mathcal{O}(a)$-improved Wilson fermions. New data at pion masses of approximately 270 MeV helps in significantly reducing errors in the extrapolation to the physical point.
All Loop N=2 String Amplitudes  [PDF]
Hirosi Ooguri,Cumrun Vafa
Physics , 1995, DOI: 10.1016/0550-3213(95)00365-Y
Abstract: Using the N=4 topological reformulation of N=2 strings, we compute all loop partition function for special compactifications of N=2 strings as a function of target moduli. We also reinterpret N=4 topological amplitudes in terms of slightly modified N=2 topological amplitudes. We present some preliminary evidence for the conjecture that N=2 strings is the large N limit of Holomorphic Yang-Mills in 4 dimensions.
Determination of the E2/M1 Ratio in the γN \to Δ(1232) Transition from a Simultaneous Measurement of p(\vecγ,p)π^0 and p(\vecγ,π^+)n  [PDF]
R. Beck
Physics , 1999, DOI: 10.1103/PhysRevC.61.035204
Abstract: Tagged linearly polarized photons have been used at the Mainz Microtron MAMI for simultaneous measurements of the p(\vec{\gamma},p)\pi^0 and p(\vec{\gamma},\pi^+)n reaction channels to study the \gamma N \to \Delta(1232) transition. The energy dependence of the magnetic dipole M_{1+}^{3/2} and electric quadrupole E_{1+}^{3/2} amplitudes have been extracted from these data in the photon energy range from 270 to 420 MeV. The E2/M1 ratio for the \gamma N \to \Delta(1232) transition has been determined to be - (2.5+-0.1_{stat}+-0.2_{sys}) % at the resonance position delta_{33}=90^0.
Tree Amplitudes in Noncritical $N=2$ Strings  [PDF]
D. Dalmazi
Physics , 1992,
Abstract: Recent results for tree amplitudes for the $N=2$ noncritical strings are presented and compared with the critical case. Arguments are given which indicate a certain discontinuity in passing from the $\hat c < 1$ model (in a Coulomb gas representation) to the $\hat c = 1$ critical case.
On the amplitudes for non-critical n=2 supuerstrings  [PDF]
E. Abdalla,M. C. B. Abdalla,D. Dalmazi
Physics , 1992, DOI: 10.1016/0370-2693(92)90115-K
Abstract: We compute correlation functions in $N=2$ non critical superstrings on the sphere. Our calculations are restrained to the ($s=0$) bulk amplitudes. We show that the four point function factorizes as a consequence of the non-critical kinematics, but differently from the $N=0,1$ cases no extra discrete state appears in the $\hat c\to 1^-$ limit.
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