Author: Andrew Puckett

New paper published on future positron-proton polarization transfer experiment

The European Physical Journal A has now published a paper led by Professor Puckett together with Professors Axel Schmidt of George Washington University and Jan Bernauer of Stony Brook University describing a future experiment to measure polarization transfer in positron-proton elastic scattering at large momentum transfers. These measurements would provide new information to constrain the effects of multi-photon-exchange in elastic lepton-proton scattering, thought to explain the discrepancy between cross section and polarization measurements in extractions of the proton electromagnetic form factor ratio at large momentum transfers.

https://inspirehep.net/literature/1860752

https://doi.org/10.1140/epja/s10050-021-00509-5

FF ratio vs Q2
Future positron-proton polarization data (green “x”-es) compared to existing cross section and polarization data as a function of the momentum transfer Q^2
Form factor ratio vs. epsilon
Proton form factor ratio at Q^2 = 2.5 GeV^2 versus the virtual photon polarization epsilon, comparing projected future positron-proton measurements (blue triangles and red square) to existing electron-proton scattering data.

 

New precision measurements of proton form factors at high energy

The Hall A Collaboration at Jefferson Lab is reporting new precision measurements of the cross section for elastic electron-proton scattering at large values of the momentum transfer Q2 and low values of the virtual photon polarization ε. These new data significantly improve the precision of our knowledge of the proton’s magnetic form factor at large Q2 values. When combined with existing high-ε data at similar Q2, the new Hall A data allow us to nearly double the Q2 range for which it is possible to directly separate the contributions of longitudinally (L) and transversely (T) polarized virtual photons to the cross section, in a procedure known as Rosenbluth separation or L/T separation. In the one-photon-exchange approximation, this procedure allows us to separate the electric (GE) and magnetic (GM) contributions to the scattering. The new data are consistent with approximate form factor scaling; i.e., μp GE/GM = 1 (here μp = 2.79284734462(82) (according to PDG), is the proton’s magnetic dipole moment in units of the nuclear magneton). This result contradicts the expectations for this ratio based on precise measurements of the proton’s electric/magnetic form factor ratio using the polarization transfer technique. The new data significantly increase the range of Q2 for which significant two-photon-exchange contributions to the elastic electron-proton scattering cross section are  conclusively established.

A manuscript reporting the new results has been submitted to Physical Review Letters. The preprint of the manuscript can be found here.

proton magnetic form factor from GMp12
Proton magnetic form factor (in the one-photon exchange approximation) from the Hall A GMp12 experiment.
FFR GMp12
Proton form factor ratio (in the one-photon-exchange approximation) from the new Hall A GMp12 experiment.

New precision measurement of the neutron skin thickness of Lead-208

The PREX/PREX-II collaboration is reporting new measurements of the parity-violating asymmetry in elastic electron scattering from the Lead-208 nucleus. This asymmetry, which is generated by the interference between the electromagnetic and weak neutral current interactions between the electron and the nucleus, is highly sensitive to the difference in size between the proton and neutron distributions in the Lead-208 nucleus.

The new measurement improves on the statistical precision of the PREX-I measurement by more than a factor of 3, and has significant implications for the size and composition of neutron stars.

  • Preprint of the manuscript reporting the PREX-II result, prepared for submission to Physical Review Letters
  • The PREX-I result, published in Physical Review Letters in 2012

New Nature publication: More anti-down quarks than anti-up quarks in the proton

The SeaQuest/E906 Collaboration at Fermilab has published new data on measurements of the ratio of anti-down over anti-up quarks in the proton, obtained by studying the Drell-Yan process in collisions of a 120 GeV proton beam from the Fermilab main injector with protons and neutrons in liquid hydrogen and deuterium targets.

In the Drell-Yan process, quark-antiquark annihilation events are clearly identified by detecting high-energy muon-antimuon pairs resulting from the annihilation of a quark (antiquark) in the beam proton with an antiquark (quark) in the target proton or neutron.

By studying Drell-Yan events in kinematics dominated by beam quarks and target antiquarks with large momentum, and comparing measurements on hydrogen and deuterium under the assumption of charge symmetry (same number of down/anti-down quarks in the neutron as up/anti-up quarks in the proton), the collaboration was able to establish an excess of anti-down quarks over anti-up quarks in the proton. This important and highly anticipated result has no clearly agreed-upon theoretical explanation, and contradicts naive expectations of quark “flavor” symmetry in the antiquark distributions.

The paper can be read here.

 

Fig. 2

 

Professor Puckett awarded new three-year grant from the Department of Energy

Professor Puckett was recently awarded a new three-year grant from the US Department of Energy, Office of Science, Office of Nuclear Physics (topic area: Medium-Energy Nuclear Physics) to support our group’s efforts in the Super BigBite Spectrometer (SBS) Collaboration in Jefferson Lab’s Experimental Hall A. The research supported by the grant, titled “Three-dimensional structure of the nucleon”, includes measurements of nucleon electromagnetic form factors at large momentum transfers and single-spin asymmetries on a transversely polarized Helium-3 target. The figure below shows the projected results of the form factor part of the program.

Projected results from SBS form factor program
Projected results on nucleon electromagnetic form factors at large momentum transfers (Q^2) from the upcoming SBS program in Hall A at Jefferson Lab. These measurements are a major component of the research supported by the new DOE grant

Preprint of ECT* Diquark Workshop Proceedings submitted to PPNP

The manuscript of the proceedings paper from the September 2019 ECT* “Diquark Correlations” workshop in Trento, Italy has recently been submitted for publication to Progress in Particle and Nuclear Physics. The preprint of the paper can be found at the following link. Professor Puckett gave an invited talk at the workshop and wrote the section of the paper titled “Super BigBite Spectrometer Programme on High-Q2 Space-like Nucleon Form Factors”.

 

Two new experiments approved by Jefferson Lab Program Advisory Committee

Two new experiments using the Super BigBite Spectrometer (SBS) have been approved by the Jefferson Lab Program Advisory Committee (PAC) at its 48th meeting.

The goal of the first experiment (E12-20-010), with UConn postdoc Eric Fuchey as co-spokesperson and contact person, is to perform the first precise measurement of the so-called “Rosenbluth slope” in the elastic electron-neutron scattering cross section at a large momentum transfer, to determine whether there are significant contributions to this process beyond the dominant single-photon-exchange contribution, as is already well established for elastic electron-proton scattering. The PAC approved this experiment for two days of new beam time with a scientific rating of “A-“.

The slides from Dr. Fuchey’s PAC presentation can be found here

The goal of the second experiment (E12-20-008), with Professor Puckett as co-spokesperson and contact person, is to perform a first measurement of the polarization transfer observables in the photoproduction of charged pions in the reaction γ+n → π+p at high energies and wide scattering angles in the center-of-mass frame. These measurements can unambiguously test predictions based on the framework of Generalized Parton Distributions (GPDs). The PAC approved this experiment for two days of new beam time with a scientific rating of “B+”.

The slides from Prof. Puckett’s PAC presentation can be found here