학술논문
The Proton Spin Structure Function $g_2$ and Generalized Polarizabilities in the Strong QCD Regime
Document Type
Working Paper
Author
Ruth, D.; Zielinski, R.; Gu, C.; Allada, M.; Badman, T.; Huang, M.; Liu, J.; Zhu, P.; Allada, K.; Zhang, J.; Camsonne, A.; Chen, J. P.; Slifer, K.; Aniol, K.; Annand, J.; Arrington, J.; Averett, T.; Baghdasaryan, H.; Bellini, V.; Boeglin, W.; Brock, J.; Carlin, C.; Chen, C.; Cisbani, E.; Crabb, D.; Daniel, A.; Day, D.; Duve, R.; Fassi, L. El; Friedman, M.; Fuchey, E.; Gao, H.; Gilman, R.; Glamazdin, S.; Gueye, P.; Hafez, M.; Han, Y.; Hansen, O.; Shabestari, M. Hashemi; Hen, O.; Higinbotham, D.; Horn, T.; Iqbal, S.; Jensen, E.; Kang, H.; Keith, C. D.; Kelleher, A.; Keller, D.; Khanal, H.; Korover, I.; Kumbartzki, G.; Li, W.; Lichtenstadt, J.; Lindgren, R.; Long, E.; Malace, S.; Markowitz, P.; Maxwell, J.; Meekins, D. M.; Meziani, Z. E.; McLean, C.; Michaels, R.; Mihovilovic, M.; Muangma, N.; Camacho, C. Munoz; Musson, J.; Myers, K.; Oh, Y.; Carmignotto, M. Pannunzio; Perdrisat, C.; Phillips, S.; Piasetzky, E.; Pierce, J.; Punjabi, V.; Qiang, Y.; Reimer, P. E.; Roblin, Y.; Ron, G.; Rondon, O.; Russo, G.; Saenboonruang, K.; Sawatzky, B.; Shahinyan, A.; Shneor, R.; Sjoegren, J.; Solvignon-Slifer, P.; Sparveris, N.; Sulkosky, V.; Wesselmann, F.; Yan, W.; Yang, H.; Yao, H.; Ye, Z.; Yurov, M.; Zhang, Y.; Zhao, Y. X.; Zheng, X.
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Subject
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Abstract
The strong interaction is not well understood at low energy, or for interactions with low momentum transfer $Q^2$, but one of the clearest insights we have comes from Chiral Perturbation Theory ($\chi$PT). This effective treatment gives testable predictions for the nucleonic generalized polarizabilities -- fundamental quantities describing the nucleon's response to an external field. We have measured the proton's generalized spin polarizabilities in the region where $\chi$PT is expected to be valid. Our results include the first ever data for the transverse-longitudinal spin polarizability $\delta_{LT}$, and also extend the coverage of the polarizability $\bar{d_2}$ to very low $Q^2$ for the first time. These results were extracted from moments of the structure function $g_2$, a quantity which characterizes the internal spin structure of the proton. Our experiment ran at Jefferson Lab using a polarized electron beam and a polarized solid ammonia (NH$_3$) target. The $\delta_{LT}$ polarizability has remained a challenging quantity for $\chi$PT to reproduce, despite its reduced sensitivity to higher resonance contributions; recent competing calculations still disagree with each other and also diverge from the measured neutron data at very low $Q^2$. Our proton results provide discriminating power between existing calculations, and will help provide a better understanding of this strong QCD regime.