Home page of the Nab and pNab experiments at the SNS

This is the home page for the Nab and pNab collaborations. Nab is an approved and funded experiment; the Nab apparatus is currently being installed and commissioned at the Fundamental Neutron Physics Beamline (FnPB) at the Spallation Neutron Source (SNS), Oak Ridge, Tennessee. The pNab experiment is the planned follow-up with polarized neutrons, and is currently at the Letter of Intent stage.


The Nab collaboration has undertaken to perform a precise measurement of a, the electron-neutrino correlation parameter, and b, the Fierz interference term in neutron beta decay, at the Fundamental Neutron Physics Beamline at the SNS, using a novel electric/magnetic field spectrometer and detector design. The experiment is aiming at the 10-3 accuracy level in a, and will provide an independent measurement of λ = GA/GV, the ratio of axial-vector to vector coupling constants of the nucleon. We will also perform a measurement of b in neutron decay, which will provide an independent limit on the tensor weak coupling.

Following the completion of Nab measurements, pNab proposes to take over and modify the apparatus. The goal of pNab will be to measure the electron and proton asymmetries with respect to the spin of the decaying neutron, which will lead to an even more precise determination of  λ.

Active collaborating institutions on Nab and pNab

  • Arizona State University
  • Technische Hochschule Karlsruhe
  • Eastern Kentucky University
  • University of Kentucky
  • Western Kentucky University
  • Los Alamos National Laboratory
  • University of Manitoba
  • Universidad Autonoma de Mexico
  • University of Michigan
  • North Carolina State University
  • Oak Ridge National Laboratory
  • University of South Carolina
  • University of Tennessee, Chattanooga
  • University of Tennessee, Knoxville
  • University of Virginia
  • University of Winnipeg

Documentation and Publications

  • Transparencies of presentations relating to Nab at FnPB PRAC meetings, DoE reviews, as well as at workshops and conferences. Some photos of equipment in development are also given.

Links to early, now superseded documents

  • The abBA experiment proposal (9 Nov 2007); superseded by the pNab LoI .
  • The PANDA experiment proposal (9 Nov 2007); superseded by the pNab LoI.
  • The abBA Letter of Intent (24 Aug 2005); superseded by the abBA proposal and the pNab LoI.
  • The Nab Letter of Intent, submitted to the SNS Program Advisory Committee on 29 July 2005; superseded in 2007 by the Nab proposal.

Useful internal links for Nab collaborators

  • Nab Wiki: The Nab collaboration Wiki pages.
  • Nab CMS: content management system for ongoing work.
  • Repository of internal documents for use by collaborators (through 2016).


Dinko Počanić, 29 Jul 2005.
Last updated: 8 Jan 2021.

This work has been supported by the US National Science Foundation, the US Department of Energy, University of Virginia, Arizona State University, and from other sources.