In the category of a big deal, if true, a new theoretical approach set forth in a four page paper for converting electron scattering data to a proton charge radius value calculates a proton charge radius in ordinary hydrogen (i.e. with an electron) that is the same as the value for muonic hydrogen (a proton-muon system).
This would solve one of the major unsolved problems of physics (I have previously put this problem in the top twelve experimental data points needed in physics), called the "proton radius problem" or "muonic hydrogen problem" in which the radius of a proton in a proton-muon system appeared to be 4% smaller than measurements of the radius of a proton in ordinary hydrogen, contrary to the Standard Model. See previous substantive posts on the topic at this blog which can be found on November 5, 2013, April 1, 2013, January 25, 2013, September 6, 2011, and August 2, 2011.
The measurements of muonic hydrogen traditions imply a proton charge radius of 0.84087(39) fm. The CODATA value obtained from electronic hydrogen transitions and some information from electron scattering data was 0.8751(61) fm. Their new analysis is that the proton charge radius based upon electron scattering data is 0.844(7) fm, which is consistent with the muonic hydrogen measurement. Any discrepancy is a problem, because the internal structure of a proton shouldn't be impacted materially by the kind of charged lepton that is "orbiting" it.
It has long been clear that the problem was probably in the old CODATA number, which used comparative old data, because measurements using muonic hydrogen transitions are inherently much more precise than measurements using electron hydrogen transitions (the muonic hydrogen measurement has error bars about 16 times smaller than the CODATA value).
But why?
There was really no fault to be found in the determination of the statistical or systemic errors in the electron scattering measurements.
It has long been clear that the problem was probably in the old CODATA number, which used comparative old data, because measurements using muonic hydrogen transitions are inherently much more precise than measurements using electron hydrogen transitions (the muonic hydrogen measurement has error bars about 16 times smaller than the CODATA value).
But why?
There was really no fault to be found in the determination of the statistical or systemic errors in the electron scattering measurements.
In the view of these authors, the proton radius problem is simply a case of inferior analysis of the available experimental electron scattering data to their analysis, and of a failure to include theoretical errors involved in fitting the data to their proton charge radius conclusion in their margin of error.
Essentially, these authors found a way to more meaningfully incorporate electron scattering data in a larger swath of the momentum transfer energy scale into their calculation of the proton charge radius from the data. This brings in more data and the data that it brings in is further from the extremes of what can be measured experimentally where systemic measurement errors tend to be greatest. Their fitting method makes the result less error prone because it is less sensitive to flukes in the smaller data sets used previously which were largely confined to very low energy data points.
Put another way, in the view of these authors, previous estimates of the proton charge radius based upon electron scattering data approximately correctly state the combined statistical and systemic errors in their experimental observations of about 0.7%, but omit a theoretical uncertainty arising from the method used to fit their data into a proton charge radius, which was on the order of 1.9% (a number that, it turns out, is itself hard to calculate or evaluate properly). Alternately, there may have been an underestimate of the systemic experimental error since their fitting function overweighted the extreme low points of the distribution where systemic errors are greater, in addition to failing take into account at all theoretical uncertainty associated with an imperfect function for fitting the data to their conclusion.
The abstract and the paper are as follows:
Put another way, in the view of these authors, previous estimates of the proton charge radius based upon electron scattering data approximately correctly state the combined statistical and systemic errors in their experimental observations of about 0.7%, but omit a theoretical uncertainty arising from the method used to fit their data into a proton charge radius, which was on the order of 1.9% (a number that, it turns out, is itself hard to calculate or evaluate properly). Alternately, there may have been an underestimate of the systemic experimental error since their fitting function overweighted the extreme low points of the distribution where systemic errors are greater, in addition to failing take into account at all theoretical uncertainty associated with an imperfect function for fitting the data to their conclusion.
The abstract and the paper are as follows:
We extract the proton charge radius from the elastic form factor data using a theoretical framework combining chiral effective field theory and dispersion analysis. Complex analyticity in the momentum transfer correlates the behavior of the spacelike form factor in different Q2 regions and permits the use of data up to Q2 ∼ 0.5 GeV2 in constraining the radius. The predictive theory describes the data with the same accuracy as current descriptive models (global fits). We obtain a radius of 0.844(7) fm, consistent with the high-precision muonic hydrogen results.
J. M. Alarcón, D. Higinbotham, C. Weiss, Z. Ye, "Proton charge radius extraction from electron scattering data using dispersively improved chiral effective field theory" (September 17, 2018).
The discussion in the body text explains (citations omitted) that:
The proton charge radius is a fundamental quantity of nuclear physics and attests to the hadron’s finite spatial extent and composite internal structure. It is defined as the derivative of the proton electric form factor (FF) at zero momentum transfer,
(rpE)2 ≡ −6dGpE/dQ2 (Q2 = 0),
and describes the leading finite-size effect in the interaction with long-wavelength electric fields. The electric and magnetic FFs at Q2 > 0 are measured in elastic electron-proton scattering experiments. The radius is also extracted from nuclear corrections to atomic energy levels measured in precision spectroscopy experiments. . . .
Determining the charge radius from electron scattering data amounts to inferring the derivative of the FF at Q2 = 0 from the data at finite Q2 . From an empirical point of view, the problem presents itself as one of “extrapolation” of the measured FF to Q2 → 0. Two approaches have been taken in most studies so far. Descriptive fits (e.g. higher-order polynomial fits) provide excellent descriptions of the data over a wide range of Q2, but the functions are generally not well-behaved outside the fitted region. Predictive models (e.g. fits with low-order polynomials or other smoothly varying functions) permit stable extrapolation but are constrained by either the selected functional form or tightly bounded parameters. In both approaches the question arises over what Q2 range the extrapolation should optimally be performed, and what uncertainties are associated with this choice. . . .
In our analyticity-based framework the main impact on the proton radius comes from FF data at moderate Q2 (∼0.1–0.5 GeV2 ) rather than at the lowest available Q2. . . .
The proton radius was extracted previously from dispersive FF fits in which the two-pion spectral functions were constructed using empirical πN amplitudes. Our approach is different in that the two-pion spectral functions are computed in DIχEFT and contain low energy constants, which can vary (consistently with the nucleon radii) and adjust the strength of the spectral functions in the ρ meson peak and above. This increases the flexibility of the FF description and represents a major advantage of our approach. We note that the empirical dispersive fits have consistently obtained radii ∼0.84 fm, in agreement with our result.
Other Recent Physics News
The Weak Radius Of The Proton
Another new study calculates the analogous "weak" radius of the proton (i.e. its effective size for purpose of weak force interactions as opposed to electromagnetic ones).
The Weak Radius Of The Proton
Another new study calculates the analogous "weak" radius of the proton (i.e. its effective size for purpose of weak force interactions as opposed to electromagnetic ones).
The weak charge of the proton determines its coupling to theC. J. Horowitz, "Weak radius of the proton" (September 17, 2018).Z0 boson. The distribution of weak charge is found to be dramatically different from the distribution of electric charge. The proton's weak radiusRW≈1.580±0.033 fm is over 80% larger than its charge radiusRch≈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.
BSM Lepton and Lepton Family Number Violations Constrained
Further afield, the Standard Model physics constraints of possible violations of lepton number and lepton family number have been considerably tightened as reported in a short LHCb paper.
Theorists would really like for their to be low energy sources of baryon number violation and lepton number violation as this could explain the baryon asymmetry of the universe in cosmology, but the hard reality is that both of these numbers of been perfectly conserved in every experiment to date and those experiments are among the most precise experiments ever conducted. There is simply no experimental data on any of the multiple fronts (including searches for proton decay, neutrinoless double beta decay, the lepton number violating decays searched for in three different kinds of decays reported on in the LHCb paper linked above, and flavor changing neutral current searches), to believe that baryon number or lepton number are not conserved, and the only theoretical hint that there might be violations of these numbers involve temperatures only found in the very early moments of the Big Bang that could not plausibly ever be reproduced in an experiment or a "natural experiment" in Nature at any observable point of the history of the universe.
Cross-Sections Of Interaction Of High Energy Neutrinos
Theorists would really like for their to be low energy sources of baryon number violation and lepton number violation as this could explain the baryon asymmetry of the universe in cosmology, but the hard reality is that both of these numbers of been perfectly conserved in every experiment to date and those experiments are among the most precise experiments ever conducted. There is simply no experimental data on any of the multiple fronts (including searches for proton decay, neutrinoless double beta decay, the lepton number violating decays searched for in three different kinds of decays reported on in the LHCb paper linked above, and flavor changing neutral current searches), to believe that baryon number or lepton number are not conserved, and the only theoretical hint that there might be violations of these numbers involve temperatures only found in the very early moments of the Big Bang that could not plausibly ever be reproduced in an experiment or a "natural experiment" in Nature at any observable point of the history of the universe.
Cross-Sections Of Interaction Of High Energy Neutrinos
Measurements of the cross-section of high energy neutrinos and nucleons have been improved using IceCube data from highly energetic cosmic ray neutrinos.
Invisible Higgs Boson Decays
And, constraints on invisible decays of the Higgs boson (which would imply BSM physics) have tightened in light of new data that continues to confirm the Standard Model, although the constraints aren't yet particularly tight (a limit on branching fractions not in excess of 37%).
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