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Title Extraction and Parametrization of Isobaric Trinucleon Elastic Cross Sections and Form Factors
Authors Scott Barcus
JLAB number JLAB-PHY-19-3076
LANL number (None)
Other number DOE/OR/23177-4819
Document Type(s) (Thesis) 
Supported by U.S. Naval Research: No
Supported by Jefferson Lab LDRD Funding: No
Funding Source: Nuclear Physics (NP)
  Thesis
A PHD thesis
Advisor(s) :
   Todd Averett (W&M)
Publication Abstract: By mining data from Je?erson Lab Hall A experiment E08-014 a new 3He elastic cross section was extracted from a large quasielastic background. This measurement was taken with an initial beam energy of 3.356 GeV and an angle of 20.51?. The cross section was found to be 1.345 × 10?6 µb/sr ± 0.086 × 10?6 µb/sr at Q2 = 34.19 fm?2. This new data point falls approximately halfway between the ?rst and second di?ractive minima of the 3He form factors. When combined with recent high Q2 3He elastic cross section measurements from Je?erson Lab and MIT-Bates this new data point improves our knowledge of the cross section and form factors at large momentum transfers. The new high Q2 data motivate a reanalysis of the 3He elastic cross section world data and provide an improved understanding of the magnetic form factor in particular. For this analysis the elastic cross section world data for 3He, and its mirror nuclei 3H, were collected. The world data spans a time frame from 1965 to 2016. The dataset contains electron energy ranges from tens of MeV to above 12 GeV for measurements performed at many di?erent facilities. The world data were then ?t using a sum of Gaussians parametrization which allowed for the extraction of both targets’ magnetic and electric form factors which were then used to calculate charge densities and radii. The new charge and magnetic form factors for 3H and the charge form factor for 3He are in good agreement with the 1994 ?ts from Amroun et al. However, the addition of the new high Q2 data has caused the 3He magnetic form factor’s ?rst di?ractive minimum to shift up in Q2 by 1-3 fm?2 while also decreasing the magnitude of the magnetic form factor above Q2 ? 20 fm?2. The ?rst di?ractive minima for 3H are located at Q2 ? 13 fm?2 and Q2 ? 23-24 fm?2 for the charge and magnetic form factors respectively. The ?rst di?ractive minima for 3He are located at Q2 ? 11 fm?2 and Q2 ? 17-20 fm?2 for the charge and magnetic form factors respectively. The charge radius for 3He was found to be 1.90 fm ± 0.00144 fm in reasonable agreement with past measurements, and the charge radius for 3H was found to be 2.02 fm ± 0.0133 fm which is much larger than past measurements. However, each of these charge radii has an additional uncertainty that must be applied to them due to allowing all parameters to ?oat freely during the sum of Gaussians ?tting procedure. This additional uncertainty should be small for 3He, but it is likely quite signi?cant for 3H and would help bring this charge radius closer to agreement with past measurements that made di?erent ?tting choices. Unfortunately, this analysis was unable to quantify this additional uncertainty. The new form factor ?ts were compared to modern theoretical predictions from the 2016 paper of Marcucci et al. The ‘conventional’ theoretical approach applied in this paper modelled two and three-body nucleon interactions with relativistic corrections and was reasonably successful at predicting the charge form factors of 3H and 3He. ?EFT predictions were also often successful. However, while the ‘conventional’ approach still performed best, theory failed to accurately predict the magnetic form factors of either 3H or 3He. The ?rst di?ractive minimum of the new 3He magnetic form factor ?ts actually moved further away from theory. This disagreement between theory and experiment provides motivation for new asymmetry measurements using polarized 3He and a polarized electron beam. When the beam is scanned in Q2 on the target the sign of the asymmetry will ?ip at the form factor minima pinning down their true location.
Experiment Numbers: E08-014
Group: Hall A
Document: pdf
DOI:
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