Bled Workshops in Physics Vol. 18, No. 1 p. 87
A
Proceedings of the Mini-Workshop
Advances in Hadronic Resonances

Bled, Slovenia, July 2 - 9, 2017
The study of the Roper resonance in double-polarized pion electroproduction
a Faculty of Mathematics and Physics, University of Ljubljana, Jadranska 19,1000 Ljubljana, Slovenia
b Jožef Stefan Institute, Jamova 39,1000 Ljubljana, Slovenia
Investigations of the structure of the Roper resonance by using coincident electron scattering have been presented at several previous Mini-Workshops, and the most recent result on double-polarized pion electroproduction in the energy region of the Roper has recently been published [1]. This extended abstract is therefore just a reminder of the basic features of this experiment and just lists the highlights of that paper.
Our experimental study of the p(e, e'p)n0 process was performed at the three spectrometer facility of the A1 Collaboration at the Mainz Microtron (MAMI) The kinematic ranges covered by our experiment were W « (1440 ± 40) MeV for the invariant mass, 9p « (90 ± 15)° and « (0 ± 30)° for the CM scattering angles and Q2 « (0.1 ± 0.02)(GeV/c)2 for the square of the four-momentum transfer.
We have extracted the two helicity-dependent recoil polarization components, P^ and P^, as well as the helicity-independent component Py, and compared them to the values calculated by the state-of-the-art models MAID [2], DMT [3] and the partial-wave analysis SAID [4]. With the possible exception of Py at high W which is reproduced by neither of the models, MAID is in very good agreement with the data, while DMT underestimates all three polarization components and even misses the sign of P^. The SAID analysis agrees less well with the PX data, while it exhibits an opposite trend in Py and is completely at odds regarding P^. This might be a consequence of very different databases used in the analysis and calls for further investigations within these groups.
We were also able to determine the scalar helicity amplitude S^2 in a model-dependent manner. In contrast to its transverse counterpart, A^2, this amplitude is accessible only by electroproduction (Q2 = 0) and becomes increasingly difficult to extract at small Q2. This is a highly relevant kinematic region where many proposed explanations of the structure of the Roper resonance and mechanisms of its excitation give completely different predictions. This is also a region in which large pion-cloud effects are anticipated. In the most relevant region below Q2 « 0.5 (GeV/c)2 where quark-core dominance is expected to give way to
S. Sircaa'b
88 S. Sirca
manifestations of the pion cloud — and where existing data cease — the predictions deviate dramatically.
Given that the agreement of our new recoil polarization data with the MAID model is quite satisfactory and that the transverse helicity amplitude A^2 is relatively much better known, we have performed a Monte Carlo simulation across the experimental acceptance to vary the relative strength of S with respect to the best MAID value for Ai/2 and made a x2-like analysis with respect to our experimentally extracted P^, Py and P^, of which Py was the most convenient for the fit. Fixing A^2 to its MAID value and taking SM/2ID as the nominal best model value, we have been able to express Si/2 from our fit as the fraction of SM/2ID, yielding
Si/2 = (0.80-0:10) SMAID = (14.1-2:5) ■ 10-3Ge-i/2 .
This result is shown in Fig. 3 of Loather Tiator's contribution to these Proceedings.
References
1.	S. Stajner et al., Phys. Rev. Lett. 119 (2017) 022001.
2.	D. Drechsel, S. S. Kamalov, and L. Tiator, Eur. Phys. J. A 34 (2007) 69.
3.	G. Y. Chen, S. S. Kamalov, S. N. Yang, D. Drechsel, and L. Tiator, Phys. Rev. C 76 (2007) 035206.
4.	R. A. Arndt, W. J. Briscoe, M. W. Paris, and I. I. S. R. L. Workman, Chin. Phys. C 33 (2009) 1063.