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Register a new userElastic Compton Scattering from 3He and the Role of the Delta
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Arman Margaryan
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We report observables for elastic Compton scattering from $^3$He in Chiral Effective Field Theory with an explicit $Delta(1232)$ degree of freedom ($chi$EFT) for energies between 50 and 120 MeV. The $gamma,{}^3$He amplitude is complete at N3LO, $mathcal{O}(e^2delta^3)$, and in general converges well order by order. It includes the dominant pion-loop and two-body currents, as well as the Delta excitation in the single-nucleon amplitude. Since the cross section is two to three times that for deuterium and the spin of polarised $^3$He is predominantly carried by its constituent neutron, elastic Compton scattering promises information on both the scalar and spin polarisabilities of the neutron. We study in detail the sensitivities of 4 observables to the neutron polarisabilities: the cross section, the beam asymmetry and two double asymmetries resulting from circularly polarised photons and a longitudinally or transversely polarised target. Including the Delta enhances those asymmetries from which neutron spin polarisabilities could be extracted. We also correct previous, erroneous results at N2LO, i.e.~without an explicit Delta, and compare to the same observables on proton, neutron and deuterium targets. An interactive Mathematica notebook of our results is available from [email protected].
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This executive summary of recent theory progress in Compton scattering off 3He focuses on determining neutron polarisabilities; see ref. [2] and references therein for details and a better bibliography. Prepared for the Proceedings of the 22nd International Conference on Few-Body Problems in Physics, Caen 9-13 July 2018.
The Kohn variational principle and the hyperspherical harmonic technique are applied to study p-3He elastic scattering at low energies. Preliminary results obtained using several interaction models are reported. The calculations are compared to a recent phase shift analysis performed at the Triangle University Nuclear Laboratory and to the available experimental data. Using a three-nucleon interaction derived from chiral perturbation theory at N2LO, we have found a noticeable reduction of the discrepancy observed for the A_y observable.
The 3He transverse electron scattering response function R_T(q,omega) is calculated in the quasi-elastic peak region and beyond for momentum transfers q = 500, 600 and 700 MeV/c. Distinct from our previous work for these kinematics where we included meson exchange currents and relativistic corrections we now additionally include Delta isobar currents (Delta-IC). The Delta-IC contribution increases the quasi-elastic peak height by about 5% and leads to an excellent agreement with experimental data in the whole peak region. In addition it is shown that effects due to the three-nucleon force largely cancel those due to the Delta-IC in the peak region. Finally, we have found that Delta-IC are important for three-body break-up reactions in the so-called dip region. This could explain why in a previous study of such a reaction, where Delta degrees of freedom were not included, no agreement between experimental and theoretical results could be obtained.
We have measured the elastic-scattering ratios of normalized yields for charged pions from 3H and 3He in the backward hemisphere. At 180 MeV, we completed the angular distribution begun with our earlier measurements, adding six data points in the angular range of 119 deg to 169 deg in the pi-nucleus center of mass. We also measured an excitation function with data points at 142, 180, 220, and 256 MeV incident pion energy at the largest achievable angle for each energy between 160 deg and 170 deg in the pi-nucleus center of mass. This excitation function corresponds to the energies of our forward-hemisphere studies. The data, taken as a whole, show an apparent role reversal of the two charge-symmetric ratios r1 and r2 in the backward hemisphere. Also, for data > 100 deg we observe a strong dependence on the four-momentum transfer squared (-t) for all of the ratios regardless of pion energy or scattering angle, and we find that the superratio R data match very well with calculations based on the forward-hemisphere data that predicts the value of the difference between the even-nucleon radii of 3H and 3He. Comparisons are also made with recent calculations incorporating different wave functions and double scattering models.
We report on a chiral effective field theory calculation of Compton scattering from the proton. Our calculation includes pions, nucleons, and the Delta(1232) as explicit degrees of freedom. It uses the delta expansion, and so implements the hierarchy of scales m_pi < M_Delta-M_N < Lambda_chi. In this expansion the power counting in the vicinity of the Delta peak changes, and resummation of the loop graphs associated with the Delta width is indicated. We have computed the nucleon Compton amplitude in the delta expansion up to N3LO for photon energies of the order of m_pi. This is the first order at which the proton Compton scattering amplitudes receive contributions from contact operators which encode contributions to the spin-independent polarisabilities from states with energies of the order of Lambda_chi. We fit the coefficients of these two operators to the experimental proton Compton data that has been taken in the relevant photon-energy domain, and are in a position to extract new results for the proton polarisabilities alpha and beta.
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