Projects / Programmes
Puzzling out the proton radius puzzle with a high precision measurementof proton charge form-factors at extremely low momentum transfers
| Code |
Science |
Field |
Subfield |
| 1.02.05 |
Natural sciences and mathematics |
Physics |
Medium- and high-energy physics |
| Code |
Science |
Field |
| P220 |
Natural sciences and mathematics |
Nuclear physics |
| Code |
Science |
Field |
| 1.03 |
Natural Sciences |
Physical sciences |
Proton charge radius, proton form-factors, elastic scattering, MAMI, Theory of quantum electrodynamics, dark matter.
Researchers (1)
| no. |
Code |
Name and surname |
Research area |
Role |
Period |
No. of publicationsNo. of publications |
| 1. |
29534 |
PhD Miha Mihovilovič |
Physics |
Head |
2016 - 2017 |
155 |
Organisations (1)
| no. |
Code |
Research organisation |
City |
Registration number |
No. of publicationsNo. of publications |
| 1. |
0106 |
Jožef Stefan Institute |
Ljubljana |
5051606000 |
90,742 |
Abstract
We propose a new high-precision measurement of the proton charge form-factor at extremely low momentum transfers in order to provide new insight into the proton radius problem, a conspicuous open question of today's nuclear physics that, due to the possible profound implications on our understanding of the physics world, has in the last few years become one of the most discussed topics in the nuclear physics community.
The radius has been determined by various electron scattering experiments and many atomic Lamb shift measurements. Both approaches gave consistent results. Their average, however, does not agree with the findings of very precise Lamb shift measurements in muonic hydrogen, which report a new value for the proton charge radius, which is 8-sigma away from the previously accepted value. This discrepancy is controversial because so far it could not be explained within the existing physics theories, nor it could be interpreted as an experimental error. Consequently, many new potential explanations have been offered, ranging from the effects reaching beyond the Standard Model, to the controversial claims of some fundamental defficiency of Quantum Electrodynamics, the supreme physics theory. In order to test any of the existing hypotheses, new experimental data are required and our novel electron scattering experiment will represent the needed counterweight to the muonic hydrogen Lamb shift results.
In a scattering experiment the radius of a proton is determined from the measurements of proton charge form-factor GEp in the processes of elastic scattering of electrons. The charge radius is then extracted from the slope of the GEp at Q**2 = 0, where Q**2 represents the square of the momentum transfer four-vector. Unfortunately, the data for Q2 ( 0.004 (GeV/c)**2 that would allow for a precise determination of this slope do not yet exist. Therefore, an extrapolation of available GEp points to Q**2=0 is used to extract the proton radius, which strongly depends on the accuracy of the values of GEp. To avoid such uncertainties, measurements of the form-factor at Q**2 ( 0.004 (GeV/c)**2 are needed. Efforts to do this are limited by the minimal accessible Q**2, which is determined by the utilised experimental apparatus.
To avoid the kinematic limitations, the proposed experiment intends to exploit information stored inside the radiative tail, which offers new insight into GEp at Q**2 as low as 0.0001 (GeV/c)**2. Due to many different processes that contribute to radiative tail, measurements need to be studied in conjunction with the results of precise Monte-Carlo simulation, in order reach GEp. First comparison of the data to the simulation demonstrated that with this technique agreement to a percent level can be achieved in the region of the radiative tail that reaches more than 200 MeV away from the elastic peak. This is the precision required for a detailed study of the form-factor at very small Q**2 and motivates us to perform the full experimental campaign.
The experiment will be performed at the Mainz Microtron Accelerator Facility using continuous-wave electron beam, solid-state plastic target and the three high-resolution spectrometer setup of A1 collaboration. The measurements are proposed for 42 kinematic settings at three different beam energies (195, 330 and 495 MeV), for which four weeks
of beam time are required. The remaining available time will be used for development of the full simulation, preparation of the experiment and data analysis.
Significance for science
The first goal of the project was to validate the theoretical description of the radiative corrections to the elastic cross-section measurements. With the performed experiments we demonstrated for the first time that the radiative corrections are indeed understood to a percent level even in momentum regions that reach more than 200 MeV away from the elastic peak. This is important for many electron-induced experiments, like virtual Compton scattering, which is dedicated to precise extraction of the proton polarizabilities. It is also crucial for upcoming precise experiments with excited states of light nuclei (He, Li), which aim to get information about nuclear structure and dynamics. Precise understanding of the background generated by the Bethe-Heitler processes is important also for the experiments committed to searches of the dark photon, where the Bethe-Heitler processes represent the main source of the experimental background. However, the main contribution of the project to the scientific community are the new precise measurements of the proton charge form factor at extremely small momentum transfers. Project offers 25 new data points in the region between Q^2=0.001 GeV^2 and Q^2=0.017 GeV^2. Especially interesting are the points below Q^2(0.004 GeV^2 where no previous measurements existed. With the reached precision the new data allow for better determination of the normalisation parameters of the form factor fit functions and together with the previous measurements offer more robust and accurate extraction of the proton charge radius.
Significance for the country
The problem of the proton charge radius is one of the key open questions of today’s atomic and nuclear physics. It is being investigated by scientists from all main research institutions in the world (e.g. MIT, College of William and Mary, NIST, PSI, Argonne National Lab, Thomas Jefferson National Accelerator Facility, TUM Garching). The measurement of the proton charge form factors that was performed within the scope of this project by the Slovenian scientists together with the German colleagues from Mainz, represents the first nuclear measurement after the observation of the discrepancy in 2010, thus offers valuable new data that allow a more precise determination of the proton charge radius. This puts Slovenian researchers at the very center of the scientific map and results in receiving continuous invitations to various international conferences, where they proficiently present latest results, discuss advances in the theory and offer possibilities for future experiments. By doing this, they promote both Slovenian science and country. The discussed measurements were performed at the accelerator MAMI of the Johannes Gutenberg University. The project involved not only research scientists but also Slovenian students, who participated in all parts of an experiment (preparation, execution and analysis). This offered them a hands-on approach to world-class experimental nuclear physics, and allowed them to experience the work-flow of a true physics experiment. Educating new generations of nuclear scientists and motivating them to continue research in this field is essential for the successful further developments in this field of research in Slovenia. However, the project's main contribution to the Slovenian society would be a new knowledge on a fundamental property of the proton, its radius. The proton is a basic constituent of matter and detailed understanding of its properties is relevant for many fields of science and education.
Most important scientific results
Interim report,
final report
Most important socioeconomically and culturally relevant results
Interim report,
final report
