S2: Polarizabilities

Research areas

• Experimental hadron physics
• Theoretical hadron physics

Goals

This project is devoted to electromagnetic polarizabilities and structure functions of the proton, neutron and few-nucleon systems. Its purpose is to improve and systematize our knowledge of hadron and nuclear polarizabilities, and to quantify their effect on atomic energy levels.

1. Nucleon polarizabilities
In 2018 new measurements of the beam asymmetry $$\Sigma_3$$ and unpolarized cross sections of Compton scattering on protons below the pion threshold were performed with the upgraded tagging spectrometer. Preliminary results show a significant improvement in statistics and quality. After the results are finalized we will perform a global fit to extract the proton scalar polarizabilities with a heretofore unprecedented precision. In order to measure neutron polarizabilities a prototype of an active high-pressure He-gas target was developed and tested. In the third funding period the target construction will be completed and elastic Compton scattering on $$^3$$He and $$^4$$He will be measured. Several analyses of Compton scattering with polarized beam and target in the $$\Delta$$ region are complete and we are in the process of conducting a global fit to extract the proton spin polarizabilities. Additional data on $$\Sigma_3$$ in the $$\Delta$$ region has recently been obtained parasitically, and analysis in the fall of 2019 will further improve the precision on the extracted proton spin polarizabilities.

Current status of the proton electric vs. magnetic polarizability. The yellow band is the Baldin sum rule constraint, as evaluated in [10.1103/PhysRevD.92.074031]. PWA stands for the first partial-wave analysis of global Compton scattering database  [1712.05349]. PDG is the 2014 average of the Particle Data Group. BChPT is the NNLO chiral perturbation theory calculations [10.1140/epjc/s10052-009-1183-z10.1140/epjc/s10052-015-3791-0 HBChPT is the extraction from Compton scattering database using heavy-baryon chiral perturbation theory [10.1140/epja/i2013-13012-1] The rest are the results of individual Compton scattering experiments, click 10.1016/j.ppnp.2015.12.001 for a recent review.
2. Measurements with a high-pressure hydrogen TPC
We are planning to perform a new measurement of elastic electron-proton scattering at low momentum transfer ($$0.001 \; \text{GeV}^{2} \leq Q^2 \leq 0.02 \; \text{GeV}^2$$), with the aim of a high-precision determination of the proton electric form factor. In order to achieve the high precision, we will use a novel setup in which the energy and angle of the recoil proton will be measured in combination with the angle of the scattered electron. This provides an overdetermination of the kinematics, allowing us to reduce a number of systematic uncertainties, including those associated with radiative corrections. The recoil proton will be detected by a new-generation hydrogen Time Projection Chamber (TPC) and the scattered electron by a Forward Tracking System.
3. Nucleon and deuteron structure functions at low $$Q$$
Prompted by the recent forthcoming measurements in muonic atoms we plan to examine the nucleon and deuteron structure functions at low energies (high $$x$$, low $$Q^2$$), where they play a crucial role in the evaluation of the nuclear-structure corrections in (muonic) hydrogen and deuterium. We will improve the empirical parametrizations by implementing rigorous theoretical constraints, such as analyticity, unitarity, and patterns of chiral symmetry. Precise knowledge of proton-structure corrections in the hyperfine splitting of muonic hydrogen are in especially high demand, in view of the forthcoming search for the $$1S$$ hyperfine transition in muonic hydrogen at PSI, Riken/JPARC and FAMU facilities.
4. Extension of MAID to Compton scattering: CoMAID
We will upgrade MAID --- the highly successful web-based partial-wave analysis (PWA) for pion photo- and electro-production --- to include the Compton scattering data and our newly-developed Compton PWA. The upgraded web platform will be easily scalable and we will be able include other results of this project, e.g., nucleon form factors and structure functions. By the end of the funding period, we plan to include therein other major results of CRC 1044, experimental and theoretical. In this way, the principal outcomes of this CRC will be systematized and made freely accessible online in an interactive user-friendly environment.