The PRad experiment and the proton radius puzzle

Abstract

New results from the recent muonic hydrogen experiments seriously questioned our knowledge of the charge radius, rp. The new value, with its unprecedented less than sub-percent precision, is currently up to eight standard deviation smaller than the average value from all previous experiments, triggering the well-known proton charge radius in nuclear and atomic physics. The PRad collaboration is currently preparing a novel, magnetic-spectrometer-free ep scattering experiment in Hall B at JLab for a new independent rp measurement to address this growing puzzle in physics. scattering method, in which the slope of the extracted electric form factor, G p ,a t lowQ 2 defines the rms radius of the proton, rp. The average value of rp for this method, given by a model independent analysis of electron scattering data is rp = 0.870(26) fm (1). Several known experimental factors limit the precision in these experiments, and the typical uncertainties of individual experiments currently are at the level of 2%. Recently, a new state-of-the-art experiment was performed at MAMI Mainz giving rp = 0.879(8) fm (2), and it is consistent with previous ep scattering results. (ii) The spectroscopy of electronic (ordinary) hydrogen atom through the Lamb shift measurements defines the radius, rp .T he value ofrp from this method is currently consistent with the ep scattering results: rp = 0.8775(51) fm (3). (iii) Recently developed method using the Lamb shift measurements from the muonic hydrogen. The muonic hydrogen result (rp = 0.8409(4) fm) (4, 5), with its unprecedented less than 0.1% precision, is currently up to eight standard deviation smaller than the average value from all previous experiments, triggering the well-known proton radius in nuclear and atomic physics. So far, all theoretical efforts and more precise simulations failed to explain this discrepancy on the value of a fundamental quantity - rp. The current situation critically requires performing new