PARITY-VIOLATING SPIN-ROTATION OF POLARIZED NEUTRONS PASSING THROUGH LIQUID HELIUM
C. D. Bass 1,2, T. D. Bass 2, B. E. Crawford 3, J. M. Dawkins 2, K. Gan 4, D. G. Haase 6, B. R. Heckel 5, J. C. Horton 2, C. R. Huffer 2, P. R. Huffman 6, D. Luo 2, D. M. Markoff 7, A. M. Micherdzinska 2, H. P. Mumm 1, J. S. Nico 1, A. K. Opper 4, M. G. Sarsour 2, E. Sharapov 8, W. M. Snow 2, H. E. Swanson 5, S. C. Walbridge 2, V. Zhumabekova 9
1 National Institute of Standards and Technology, Gaithersburg, MD 20899, USA
2 Indiana University / IUCF, Bloomington, IN 47408, USA
3 Gettysburg College, Gettysburg, PA 17325, USA
4 The George Washington University, Washington D.C. 20052, USA
5 University of Washington / CENPA, Seattle, WA 98195, USA
6 North Carolina State University / TUNL, Raleigh, NC 27695, USA
7 North Carolina Central University, Durham, NC 27707, USA
8 Joint Institute for Nuclear Research, Dubna, Russia
9 Al-Farabi Kazakh National University, Almaty, Kazakhstan
In the forward elastic scattering of transversely-polarized low-energy neutrons propagating through a medium, the most prominent parity-violating observable is the rotation of the neutron spin vector about its momentum vector: parity-violating neutron spin-rotation, which is due to nucleon-nucleon (NN) weak interaction. Parity-violating neutron spin rotation can be described in terms of the meson exchange model nucleon-meson weak coupling amplitudes  as well as the pionless chiral-EFT coupling parameters [2,3,4,5]. The values of these couplings are not well-constrained by theory, so a measurement of the parity-violating neutron spin rotation through liquid helium can constrain the poorly-understood properties of the NN weak interaction. In addition, parity-violating neutron spin rotation is important to understand because NN weak interactions are in principle a new probe of strong QCD and knowledge of NN weak couplings are needed to interpret measurements of parity violation in many nuclear and atomic observations.
For a liquid helium target, the expected parity-violating neutron spin rotation is of order 10-6 rad/m. An apparatus was designed and built to perform a precision measurement of the parity-violating neutron spin rotation in helium with a sensitivity goal of 3×10-7 rad/m. The Neutron Spin Rotation experiment conducted at the NCNR in 2007-2008 employed a crossed neutron polarizer/analyzer pair surrounding a liquid helium target system. The tiny size of the parity-violating observable placed severe constraints on the target design. In particular, isolation of the parity-odd component of the neutron spin rotation from a much larger (~106) background rotation due by magnetic fields required a nonmagnetic cryostat and target system supported within magnetic shielding, which allowed nonmagnetic motion of liquid helium between separated target chambers. This poster provides a description of the design, function, and performance of the liquid helium target system and neutron polarimeter apparatus.
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Christopher D. Bass
Mentors Name: Jeff S. Nico
Ionizing Radiation Division (846), Physics Laboratory
Room B181, Bldg 235, MS 8641
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