The smaller r p we have now measured supports the value found by two previous muonic hydrogen experiments 1,7. Our result, r p = 0.831 ± 0.007 stat ± 0.012 syst femtometres, is smaller than the most recent high-precision e-p measurement 5 and 2.7 standard deviations smaller than the average of all e-p experimental results 6. We used a magnetic-spectrometer-free method along with a windowless hydrogen gas target, which overcame several limitations of previous e-p experiments and enabled measurements at very small forward-scattering angles. Here we report on the proton charge radius experiment at Jefferson Laboratory (PRad), a high-precision e-p experiment that was established after the discrepancy was identified. In fact, there is a discrepancy between the two most recent spectroscopic measurements conducted on ordinary hydrogen 3,4. Despite experimental and theoretical efforts, the puzzle remains unresolved. In 2010, a new method using muonic hydrogen atoms 1 found a substantial discrepancy compared with previous results 2, which became known as the 'proton radius puzzle'. 19 Yerevan Physics Institute, Yerevan, Armenia.Įlastic electron-proton scattering (e-p) and the spectroscopy of hydrogen atoms are the two methods traditionally used to determine the proton charge radius, r p.18 Norfolk State University, Norfolk, VA, USA.17 College of William and Mary, Williamsburg, VA, USA.16 Hampton University, Hampton, VA, USA.15 University of Massachusetts, Amherst, MA, USA.14 Alikhanov Institute for Theoretical and Experimental Physics NRC "Kurchatov Institute", Moscow, Russia.13 Old Dominion University, Norfolk, VA, USA.12 Massachusetts Institute of Technology, Cambridge, MA, USA.11 Kharkov Institute of Physics and Technology, Kharkov, Ukraine.10 University of North Carolina, Wilmington, NC, USA.9 Argonne National Laboratory, Lemont, IL, USA.8 North Carolina A&T State University, Greensboro, NC, USA.7 Mississippi State University, Mississippi State, MS, USA.6 Thomas Jefferson National Accelerator Facility, Newport News, VA, USA.5 University of Virginia, Charlottesville, VA, USA.4 Idaho State University, Pocatello, ID, USA. 3 Mississippi State University, Mississippi State, MS, USA. 2 North Carolina A&T State University, Greensboro, NC, USA.1 Duke University and Triangle Universities Nuclear Laboratory, Durham, NC, USA.
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