Measurement of the <mml:math xmlns:mml="http://www.w3.org/1998/Math/MathML" display="inline"><mml:mi>α</mml:mi></mml:math>-Particle Monopole Transition Form Factor Challenges Theory: A Low-Energy Puzzle for Nuclear Forces?
S. Kegel, P. Achenbach, Sonia Bacca, Nir Barnea, J. Beričič, D. Bosnar, L. Correa, M. O. Distler, A. Esser, H. Fonvieille, I. Friščić, M. Heilig, P. Herrmann, M. Hoek, P. Klag, T. Kolar, Winfried Leidemann, H. Merkel, M. Mihovilovič, J. Müller, U. Müller, Giuseppina Orlandini, J. Pochodzalla, B. S. Schlimme, M. Schoth, Fabian Schulz, C. Sfienti, S. Širca, R. Spreckels, Y. Stöttinger, M. Thiel, A. Tyukin, Th. Walcher, A. Weber
Abstract
We perform a systematic study of the α-particle excitation from its ground state 0_{1}^{+} to the 0_{2}^{+} resonance. The so-called monopole transition form factor is investigated via an electron scattering experiment in a broad Q^{2} range (from 0.5 to 5.0 fm^{-2}). The precision of the new data dramatically supersedes that of older sets of data, each covering only a portion of the Q^{2} range. The new data allow the determination of two coefficients in a low-momentum expansion, leading to a new puzzle. By confronting experiment to state-of-the-art theoretical calculations, we observe that modern nuclear forces, including those derived within chiral effective field theory that are well tested on a variety of observables, fail to reproduce the excitation of the α particle.