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Register a new userAvoiding common pitfalls and misconceptions in extractions of the proton radius
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In a series of recent publications, different authors produce a wide range of electron radii when reanalyzing electron proton scattering data. In the light of the proton radius puzzle, this is a most unfortunate situation. However, we find flaws in most analyses that result in radii around 0.84 fm. In this paper, we explain our reasoning and try to illustrate the most common pitfalls.
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The weak charge of the proton determines its coupling to the $Z^0$ boson. The distribution of weak charge is found to be dramatically different from the distribution of electric charge. The protons weak radius $R_W= 1.545pm 0.017$ fm is 80% larger than its charge radius $R_{ch}approx 0.84$ fm because of a very large pion cloud contribution. This large weak radius can be measured with parity violating electron scattering and may provide insight into the structure of the proton, various radiative corrections, and possible strange quark contributions.
It is suggested that proton elastic scattering on atomic electrons allows a precise measurement of the proton charge radius. Very small values of transferred momenta (up to four order of magnitude smaller than the ones presently available) can be reached with high probability.
The precise determination of the proton radius from recent elastic scattering electron-proton data is discussed. The necessary precision on the elastic cross section to discriminate among the values coming from atomic spectroscopy is scrutinized in terms of the relevant quantity, i.e., the derivative of the form factor. It is shown that such precision is two orders of magnitude higher than the precision on the cross section, that is the measured observable. Different fits on the available data and of their discrete derivative, with analytical constraints are shown. The systematic error associated to the radius is evaluated taking into account the uncertainties from different sources, as the extrapolation to the static point, the choice of the class of fitting functions and the range of the data sample. This error is shown to be even orders of magnitude larger than commonly assumed.
We present a feasibility study of a simultaneous sub-percent extraction of the weak charge and the weak radius of the ${}^{12}$C nucleus using parity-violating electron scattering, based on a largely model-independent assessment of the uncertainties. The corresponding measurement is considered to be carried out at the future MESA facility in Mainz with $E_{rm beam} = 155$ MeV. We find that a combination of a $0.3%$ precise measurement of the parity-violating asymmetry at forward angles with a $10%$ measurement at backward angles will allow to determine the weak charge and the weak radius of ${}^{12}$C with $0.4%$ and $0.5%$ precision, respectively. These values could be improved to $0.3%$ and $0.2%$ for a $3%$ backward measurement. This experimental program will have impact on precision low-energy tests in the electroweak sector and nuclear structure.
A study of prompt and non-prompt J/$psi$ production as a function of charged-particle multiplicity in inelastic proton--proton (pp) collisions at a centre-of-mass energy of $sqrt{s}$ = 13 TeV based on calculations using the PYTHIA8 Monte Carlo is reported. Recent experimental data shows an intriguing stronger-than-linear increase of the self-normalized J/$psi$ yield with multiplicity; several models, based on initial or final state effects, have been able to describe the observed behaviour. In this paper, the microscopic reasons for this behaviour, like the role of multiple parton interactions, colour reconnections and auto-correlations are investigated. It is observed that the stronger-than-linear increase and the transverse momentum ($p_{rm T}$) dependence, contrary to what is predicted by the other available models, can be attributed to auto-correlation effects only. In absence of auto-correlation effects, the increase of the yield of J/$psi$ with multiplicity -- and in general for all hard processes -- is weaker than linear for multiplicities exceeding about three times the mean multiplicity. The possibility of disentangling auto-correlation effects from other physical phenomena by measuring the charged-particle multiplicity in different pseudo-rapidity and azimuthal regions relative to the J/$psi$ direction is investigated. In this regard, it is suggested to extend the experimental measurements of J/$psi$ production as a function of the charged-particle multiplicity by determining the multiplicity in several azimuthal regions and in particular in the Transverse region with respect to the direction of the J/$psi$ meson.
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