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Neutron Beta Decay Asymmetry Nab

General Description

Nab uses a large, novel magneto-electrostatic spectrometer, highly optimized for precision measurements of key parameters describing neutron beta decay: "a", the neutrino-electron correlation, and "b", the Fierz interference term. The spectrometer will be initially mounted at the Fundamental Neutron Physics Beamline (FnPB) of the Spallation Neutron Source (SNS) within the Oak Ridge National Laboratory (ORNL)

The so-called standard model (SM) of elementary particles and interactions, developed and refined during the last forty years, has described and predicted a vast array of observations and measurement results with unprecedented breadth and precision, in effect producing a new "periodic table of elementary particles" accurately describing nature at a million-times smaller length scale than the Mendelyeyev's original "periodic table of elements". However, notwithstanding its legendary success, the SM is known to be incomplete. Search for its extensions is arguably the most importan goal of modern physics in the subatomic domain. Basic weak-interaction processes offer the cleanest framework for testing of SM boundaries. Beta decay, the longest known and best studied such process, still remains at the forefront of these efforts.

Detailed Description

The Nab spectrometer will be a premier instrument for precision study (at the level of a part in ~1000) of neutron beta decay in the US and in the world. The Nab experiment is expected to provide critical results toward resolving longstanding discrepancies in the neutron decay world data set. The proposed device, coupled with state of the art neutron beam, will enable the collaboration to set stringent limits on certain processes that would extend the SM, with strong implications in astrophysics. Then Nab spectrometer is the first step in a longer experimental program. In this first step, the spectrometer analyzes decays of unpolarized neutrinos. From the propoerties of the decay particles, the neutrino electron correlation coefficient "a" and the Fierz interference term "b" are to be infered. Further experiments with the spectrometer that use a polarized neutron beam are planned, which aim to determine the beta asymmetry "A", the neutrino asymmetry "B", and the proton asymmetry "C". A more technical description of the spectrometer can be found at the Nab website at University of Virginia

Technical Description

The Nab spectrometer will be set up at the Fundamental Neutron Physics Beamline (FnPB) of the Spallation Neutron Source (SNS). A sketch of the setup is shown to the right. Neutrons enter the spectrometer magnet (in red) from the left, and exit to the right. Electrons and protons from neutron decays in the fiducial volume inside the spectrometer magnet are caught by the magnetic field, which guides them to one of the two detectors at the top and the bottom of the spectrometer magnet. Details can be found here [[2]].


"Nab: measurement principles, apparatus and uncertainties", paper presented at the International Workshop for Particle Physics with Slow Neutrons, ILL, Grenoble, 29 - 31 May 2008; Nucl. Instrum. Meth. A611, 211-215 (2009) [Note: includes a spectrometer concept that is out of date]

Nab funding request to DOE, Jan 2010

Funding request for neutron beta decay spectrometer magnet to NSF, Jan 2011

For More Information

The following links will let you learn more about this topic:

• [[1]] Fundamental Neutron Physics Beamline at the SNS

• [[2]] Nab website at University of Virginia

• [[3]] Spallation Neutron Source

• [[4]] Oak Ridge National Laboratory


• Arizona State University (Ricardo Alarcon, ricardo.alarcon@asu.edu)

• University of Kentucky (Christopher Crawford, c.crawford@uky.edu)

• Los Alamos National Laboratory (W. Scott Wilburn, wilburn@lanl.gov)

• University of Manitoba (Michael Gericke, mgericke@physics.umanitoba.ca, and Shelley Page, spage@cc.umanitoba.ca)

• University of Michigan (Tim Chupp, chupp@umich.edu)

• University of New Hampshire (John Calarco, jrc@einstein.unh.edu, and Bill Hersman, hersman@unh.edu)

• Universidad Nacional Autonoma de Mexico (Libertad Barron Palos, libertad@fisica.unam.mx)

• North Carolina State University (Albert Young, albert_young@ncsu.edu)

• Oak Ridge National Laboratory (David Bowman*, bowmanjd@ornl.gov, Vince Cianciolo, cianciolotv@ornl.gov, Robert Grzywacz, grzywaczrk@ornl.gov, and Seppo Penttila‡ , penttilasi@ornl.gov)

• RMKI Budapest (Ferenc Gluck, ferenc.glueck@kit.edu)

• University of South Carolina (Vladimir Gudkov, gudkov@asg.sc.edu)

• University of Sussex (James Byrne, james.byrne11@btinternet.com)

• University of Tennessee (Geoff Greene, ggreene@utk.edu)

• University of Virginia (Dinko Počanić*, pocanic@virginia.edu, and Stefan Baessler†, baessler@virginia.edu)

• University of Winnipeg (Jeff Martin, j.martin@uwinnipeg.ca)

*Co-Spokespersons, † Project manager, ‡ Local (to ORNL) project manager