Session U12: Mini-Symposium: Compton Scattering and the Polarizabilities

Polarized Compton Scattering Experiments at the Mainz Microtron

Interactions between an electromagnetic wave and a proton are described at the basic level by the mass, charge, and anomalous magnetic moment of the proton. Such a description, however, assumes a point-like particle, something the proton is certainly not. The internal structure of the proton leads to higher order terms, such as the scalar and vector polarizabilities, in the interaction. To study these polarizabilities, a multi-experiment program has been undertaken at the Mainz Microtron to measure observables in Compton scattering that exhibit dependence on these parameters. This program has made use of the A2 tagged photon beam, with either a linear or circular polarization, proton targets of either unpolarized LH$_{2}$ or frozen-spin butanol with transverse or longitudinal polarization, as well as the nearly 4$\pi$ detection capability of the Crystal Ball and TAPS detectors. The first of these measurements, the double-polarization asymmetry $\Sigma_{2x}$, also the first of its kind, has already been published. Measurements of the beam asymmetry $\Sigma_{3}$ and another double-polarization asymmetry $\Sigma_{2z}$ have also been performed and are in various stages of analysis and publication. This talk will discuss the status of these measurements, as well as various fitting studies that are being performed with the data in hand, and plans for future measurements.   

Electric polarizability of neutral hadrons from lattice QCD

We report on the electric polarizability for the neutron, neutral pion, and neutral kaon from lattice QCD. The results are based on dynamical QCD ensembles at two different pion masses: 306 and 227 MeV. An infinite volume extrapolation is performed for each hadron at both pion masses. The resulting polarizabilities are compared with other lattice calculations, ChPT, and experiment.   

Spin Polarisabilities and Compton Scattering from $\chi$EFT: Bridging QCD and Data

Compton scattering from protons and neutrons probes their two-photon response in electric and magnetic fields of real photons, exploring the symmetries and interaction strengths of the internal degrees of freedom. With the scalar polarisabilities $\alpha_{E1}$ and $\beta_{M1}$ now reasonably understood, the focus turns to the so-far poorly explored spin-polarisabilities. They parametrise the stiffness of the nucleon spin in external electro-magnetic fields, analogous to rotations of the polarisation of light by optically active media (bi-refringence/Faraday effect) and are particularly sensitive to the directional dependence of the $\pi N\gamma$ interactions dictated by chiral symmetry and its breaking. This contribution addresses the potential of Chiral Effective Field Theory to relate between lattice QCD and ongoing or approved efforts at MAX-lab, HI$\gamma$S and MAMI. We discuss high-intensity experiments with polarised targets and polarised beams which will allow the extraction of the spin-polarisabilities; $\chi$EFT predictions which indicate which observables for polarised protons, deuterons and ${}3$He are particularly sensitive; convergence, residual theoretical uncertainties and possibilities for improvement; and chiral extrapolations in $m_\pi$ for lattice computations.   

Polarizability Measurements at MAMI

A central problem of modern physics research is the solution to QCD in the non-perturbative regime. One method of testing QCD in this low-energy region is by measuring certain structure constants of hadrons---called polarizabilities---that show particular promise of allowing a direct connection to the underlying quark/gluon dynamics through comparison to modern QCD-inspired model calculations, and to solutions of QCD done computationally on the lattice. This talk will give an overview of recent and upcoming measurements to obtain the polarizabilities of both the proton and neutron.   

Extracting Neutron Polarizabilities from Compton Scattering on Quasi-Free Neutrons in $\gamma d \rightarrow \gamma n p$

Compton scattering processes are ideal to study electric and magnetic dipole polarizability coefficients of nucleons. These fundamental quantities parametrize the response to a monochromatic photon probe. In this work, the inelastic channel $\gamma d\rightarrow \gamma np$ is treated in $\chi$EFT, with a focus on the NQFP - neutron quasi-free peak - kinematic region. In this region, the momentum of the outgoing proton is small enough that it is considered to remain at rest. This provides access to the Compton scattering process $\gamma n \rightarrow \gamma n$ from which the neutron scalar polarizabilites $\alpha$ and $\beta$ are extracted. Using $\chi$EFT, differential cross-sections, $d^3\sigma/dE_nd\Omega_{\gamma'} \Omega_n$, in the photon energy range of 200-400 MeV are computed. The biggest contribution comes from the impulse approximation, with small corrections stemming from final state interaction, meson exchange currents and rescattering. A new extraction of neutron polarizabilities from a two-parameter fit to the Kossert et al. data taken at MAMI in 2002 is presented.   

Commissioning and Operation of a Cryogenic Target at HI$\gamma$S

We have developed a cryogenic target for use at the High Intensity $\gamma$-ray Source (HI$\gamma$S). The target system is able to liquefy helium-4 (LHe) at 4~K, hydrogen (LH$_2$) at 20~K, or deuterium (LD$_2$) at 23~K to fill a 0.3~L Kapton cell. Liquid temperatures and condenser pressures are recorded throughout each run in order to ensure that the target's areal density is known to $\sim$1\%. A low-temperature valve enables cycling between full and empty modes in less than 15 minutes. The target is being utilized in a series of experiments which probe the electromagnetic polarizabilities of the nucleon by Compton scattering high-energy photons from the liquid and detecting them with the HI$\gamma$S NaI Detector Array (HINDA). During a 50-hour-long commissioning run, the target held LHe at 3.17~K, followed by 600 hours of production running with LD$_2$ at 23.9~K. The design of the target will be presented and its performance during these runs will be discussed.