The n3He ExperimentProbing the Hadronic Weak Interaction

Abstract: Although QCD has had tremendous success in describing the strong interaction at high energy, the structure of nuclear matter remains elusive due to the difficulty of QCD calculations in the low energy frontier. Thus nuclear structure has typically been explored through electromagnetic interactions, like electron scattering. The hadronic weak interaction (HWI) is an attractive alternative because it involves only nucleons, but the weak component is short-range and precisely calculable at low energies. While the HWI is dominated by the strong force by a factor of 107, it can be isolated due to its unique property of parity violation (PV). N3He is a precision experiment designed to measure the proton asymmetry through the reaction n + 3He p + t.

G.L. Greene, S. KucukerUniversity of Tennessee

V. Gudkov, Y. SongUnviersity of South Carolina

A. Barzilov, I. NovikovWestern Kentucky University

S. BaesslerUniversity of Virginia

M. VivianiIstituto Nazionale di Fisica Nucleare,

Sezione di Pisa

CalarcoUniversity of New Hampshire

M.A. Brown, C.B. Crawford, E. MartinUniversity of Kentucky

J.D. Bowman, S. PenttilaOak Ridge National Laboratory

P.N. SeoTriangle Universities Nuclear Laboratory

L. BaronUniversidad Nacional Autónoma de

México

M.T. Gericke, S. Page, WTH. Van Oers,R. Mahurin, V. Tvaskis, M. McCrea, D. HarrisonUniversity of Manitoba

Parity ViolationThe weak interaction is 107 times smaller than its strong counterpart. However, experiments can probe this small component of the hadronic interaction by observing a unique property of it, parity violation (PV). Weak interactions look different under spatial inversion (looking at them in a mirror.)

The goal of the n3He is to determine the single-spin proton asymmetry in the reaction .The asymmetry is evident in the direction of the proton emission with respect to the polarity of the incoming neutron. Studying such a PV circumstance will shed light on the Hadronic Weak Interaction.

Hadronic Weak CouplingsIn the DDH meson exchange model, the strength of the HWI is specified by coupling constants at the vertex where (when) an exchange meson is emitted or absorbed. The fundamental weak interaction occurs at the vertex. There are six unique couplings characterized by the type of meson exchanged and details of the vertex. By investigating the many different hadronic nuclear reactions with varying sensitivities to these couplings, experimental values can be obtained to test the DDH theory and eventually the EFT theories once the calculations have been completed in that context.

The n3He experiment is one of the experiments which will allow for values of the coupling constants to be obtained. Once a number of the HWI experiments have been completed, you can develop a system of equations involving the asymmetries along with the coupling constants and their theoretically calculated counterparts (Ex. Below).

Spallation Neutron SourceThe SNS, located at Oak Ridge National Laboratory, is an intense neutron beam produced by pulsing a high energy proton beam on a mercury target. The velocity, energy, and wavelength can all be extrapolated from the TOF of the neutrons in the 60 Hz pulse structure. The energy spectrum of the neutrons is nearly thermal, slightly higher than the temperature of the LH2 moderator, located prior to the guide for the FnPB where the n3He experiment will be conducted.

Experimental Setup

Supermirror PolarizerThe supermirror polarizer is the same polarizer used in NPDGamma. The SM polarizer has a polarization efficiency of 95%. Neutrons striking the mirror are reflected if their energy is less then the nuclear potential of the neutrons within the Fe/Si coating. This is due to the repulsive potential of many nuclear cores felt simultaneously by the spread out neutron wave packet. If the SM coating is magnetic, the nuclear potential is modified by the magnetic dipole interaction which repels one spin state and attracts the other into the coating. The transmitted neutrons are absorbed by boron in the glass, producing a soft 0.5 MeV gamma. The mirror uses an Fe/Si magnetic coating to reflect the neutrons rather than FeCoV/TiN coating in order to prevent activation of Co-60. The polarizer channels are curved slightly more than the optimal angle in order to prevent any direct line of sight between the moderator and the spin rotator, which prevents any neutrons from distorting the beam profile.

RF Spin Rotator

N3He is using a spin flipper with transverse windings which allows for both longitudinal and transverse spin rotation. It is being developed at UK based on calculations with the magnetic scalar potential. For this experiment, longitudinal polarized neutrons are required which called for the change from the NPDGamma RFSF. The spin rotator is based on NMR, where the neutron spin precesses at the Larmor frequency around a magnetic field. The spin rotator creates an RF field BRF which rotates in resonance with the Larmor frequency making it appear static, thus causing the neutron spin to also precess around this field, rotating the spin 180o. The RFSF is ramped inversely proportional to the time of flight of the neutrons within the flipper to ensure that all the neutrons within a pulse are rotated efficiently.

3He Target / Ion Chamber

The target/ion chamber will serve both as an unpolarized 3He target and an in situ detector of the proton current as a function of emission direction. As neutrons capture on the 3He they create an excited 4He nucleus which then decays into a proton and triton. Both particles ionize the gas as they travel. The negative ions collect on the sense wires at ground, while the positive ions travel to the high voltage field wire. The asymmetry is detected in current mode by looking at the where the ion current is greater. If the current is higher downstream, then the proton was emitted in the forward direction.

Uncertainties

Systematics• Beam Fluctuations• RFSF Efficiency• Polarization• Alignment (beam,

field, chamber)

PV was discovered by C.S. Wu in 1957, by observing a correlation between the polarization of Co nuclei and the direction of beta emission.

The difference in R+ and R- leads to the high neutron polarization.

𝑨𝒑≈𝝈𝒏 ∙𝒌𝒑

TOF Spectrum

Ionization distribution for a single capture event due to the proton carrying 3 times as much energy as the triton and depositing its energy at the end of its track

StatisticalThe statistical uncertainty is dependent on the detector efficiency, the neutron flux, and the polarization.

N = 2.2x1010 n/s x 107 sP = 96.2%σd = 6

𝛿 𝐴=𝜎 𝑑

𝑃 √𝑁≈1.3×10−8

10 Gausssolenoid

RF spinrotator

3He target /ion chamber

supermirrorbender polarizer

(transverse)

FnPB coldneutron guide

3He BeamMonitor transition field

(not shown)

shim coils(not shown)