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Register a new userThe proton radius puzzle
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Added by
Maurizio Bonesini M.
Publication date
2016
fields
Physics
and research's language is
English
Authors
M. Bonesini
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The FAMU (Fisica degli Atomi Muonici) experiment has the goal to measure precisely the proton Zemach radius, thus contributing to the solution of the so-called proton radius puzzle. To this aim, it makes use of a high-intensity pulsed muon beam at RIKEN-RAL impinging on a cryogenic hydrogen target with an high-Z gas admixture and a tunable mid-IR high power laser, to measure the hyperfine (HFS) splitting of the 1S state of the muonic hydrogen. From the value of the exciting laser frequency, the energy of the HFS transition may be derived with high precision and thus, via QED calculations, the Zemach radius of the proton. The experimental apparatus includes a precise fiber-SiPMT beam hodoscope and a crown of eight LaBr3 crystals and a few HPGe detectors for detection of the emitted characteristic X-rays. Preliminary runs to optimize the gas target filling and its operating conditions have been taken in 2014 and 2015-2016. The final run, with the pump laser to drive the HFS transition, is expected in 2018.
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In 2010 the proton charge radius was extracted for the first time from muonic hydrogen, a bound state of a muon and a proton. The value obtained was five standard deviations away from the regular hydrogen extraction. Taken at face value, this might be an indication of a new force in nature coupling to muons, but not to electrons. It also forces us to reexamine our understanding of the structure of the proton. Here I describe an ongoing theoretical research effort that seeks to address this proton radius puzzle. In particular, I will present the development of new effective field theoretical tools that seek to directly connect muonic hydrogen and muon-proton scattering.
The proton size, specifically its charge radius, was thought known to about 1% accuracy. Now a new method probing the proton with muons instead of electrons finds a radius about 4% smaller, and to boot gives an uncertainty limit of about 0.1%. We review the different measurements, some of the calculations that underlie them, some of the suggestions that have been made to resolve the conflict, and give a brief overview new related experimental initiatives. At present, however, the resolution to the problem remains unknown.
High-precision measurements of the proton radius from laser spectroscopy of muonic hydrogen demonstrated up to six standard deviations smaller values than obtained from electron-proton scattering and hydrogen spectroscopy. The status of this discrepancy, which is known as the proton radius puzzle will be discussed in this paper, complemented with the new insights obtained from spectroscopy of muonic deuterium.
The difference in proton radii measured with $mu p$ atoms and with $ep$ atoms and scattering remains an unexplained puzzle. The PSI MUSE proposal is to measure $mu p$ and $e p$ scattering in the same experiment at the same time. The experiment will determine cross sections, two-photon effects, form factors, and radii independently for the two reactions, and will allow $mu p$ and $ep$ results to be compared with reduced systematic uncertainties. These data should provide the best test of lepton universality in a scattering experiment to date, about an order of magnitude improvement over previous tests. Measuring scattering with both particle polarities will allow a test of two-photon exchange at the sub-percent level, about a factor of four improvement on uncertainties and over an order of magnitude more data points than previous low momentum transfer determinations, and similar to the current generation of higher momentum transfer electron experiments. The experiment has the potential to demonstrate whether the $mu p$ and $ep$ interactions are consistent or different, and whether any difference results from novel physics or two-photon exchange. The uncertainties are such that if the discrepancy is real it should be confirmed with $approx$5$sigma$ significance, similar to that already established between the regular and muonic hydrogen Lamb shift.
We review the status of the proton charge radius puzzle. Emphasis is given to the various experiments initiated to resolve the conflict between the muonic hydrogen results and the results from scattering and regular hydrogen spectroscopy.
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