Abstract
The neutron spin rotation (NSR) collaboration used parity-violating spin rotation of transversely polarized neutrons transmitted through a 0.5 m liquid helium target to constrain weak coupling constants between nucleons. While consistent with theoretical expectation, the upper limit set by this measurement on the rotation angle is limited by statistical uncertainties. The NSR collaboration is preparing a new measurement to improve this statistically-limited result by about an order of magnitude. In addition to using the new high-flux NG-C beam at the NIST Center for Neutron Research, the apparatus was upgraded to take advantage of the larger-area and more divergent NG-C beam. Significant improvements are also being made to the cryogenic design. Details of these improvements and readiness of the upgraded apparatus are presented. We also comment on how recent theoretical work combining effective field theory techniques with the 1/Nc expansion of QCD along with previous NN weak measurements can be used to make a prediction for dϕ/dz in 4He. An experiment using the same apparatus with a room-temperature target was carried out at LANSCE to place limits on parity-conserving rotations from possible fifth-force interactions to complement previous studies. We sought this interaction using a slow neutron polarimeter that passed transversely polarized slow neutrons by unpolarized slabs of material arranged so that this interaction would tilt the plane of polarization and develop a component along the neutron momentum. The results of this measurement and its impact on the neutron-matter coupling gA2 from such an interaction are presented. The NSR collaboration is also preparing a new measurement that uses an upgraded version of the room-temperature target to be run on the NG-C beamline; and it is expected to constrain gA2 by at least two additional orders of magnitude for λc between 1 cm and 1 μm.
Highlights
The DDH models the hadronic weak interaction (HWI) between nucleons as an exchange of a light meson (π, ρ, ω) where the meson couples to the nucleons via strong coupling at one vertex and weak coupling at the other
In Ref. [11], Gardner, Holstein and Haxton have for the first time reorganized the theory analysis to take into account the recent theory results from the Quantum Chromodynamics (QCD) 1/Nc expansion, which gives a natural scale for the relative size of the weak couplings
The apparatus consists of an upgraded version of that used in the previous neutron spin rotation (NSR) measurement [40,41,42]
Summary
The uncertainty in details of the hadronic weak interaction (HWI) stems largely from the very short range of the quark-quark weak interaction (∼0.01 fm) compared. [11], Gardner, Holstein and Haxton have for the first time reorganized the theory analysis to take into account the recent theory results from the QCD 1/Nc expansion, which gives a natural scale for the relative size of the weak couplings It leads to a completely different picture of the NN weak interaction. Including the N2LO LECs and NPDGamma results [29] in the EFT+leading-Nc expansion gives a neutron spin rotation angle in 4He of (9.0 ± 1.5) × 10−7 rad/m [11, 36]. In n+4He, the NSR collaboration used parity violating spin rotation in liquid 4He to constrain hadronic weak coupling constants and measured a neutron spin rotation angle per unit length (dφ/dz) of [+ 1.7 ± 9.1 (stat.) ± 1.4 (sys.)] × 10−7 rad/m [4]. Because f PV is proportional to the parity-odd correlation σ n · kn with σ n the neutron spin vector and kn the neutron momentum, it tends to a constant for low energy neutrons in the absence of resonances [38]
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