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Atomic Nuclei: Nuclear Relaxation Processes01:23

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In the absence of an external magnetic field, nuclear spin states are degenerate and randomly oriented. When a magnetic field is applied, the spins begin to precess and orient themselves along (lower energy) or against (higher energy) the direction of the field. At equilibrium, a slight excess population of spins exists in the lower energy state. Because the direction of the magnetic field is fixed as the z-axis,  the precessing magnetic moments are randomly oriented around the z-axis.
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Atomic Nuclei: Nuclear Spin State Population Distribution01:14

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Near absolute zero temperatures, in the presence of a magnetic field, the majority of nuclei prefer the lower energy spin-up state to the higher energy spin-down state. As temperatures increase, the energy from thermal collisions distributes the spins more equally between the two states. The Boltzmann distribution equation gives the ratio of the number of spins predicted in the spin −½ (N−) and spin +½ (N+) states.
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Nuclear relaxation restores the equilibrium population imbalance and can occur via spin–lattice or spin–spin mechanisms, which are first-order exponential decay processes.
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The number of nuclear spins aligned in the lower energy state is slightly greater than those in the higher energy state. In the presence of an external magnetic field, as the spins precess at the Larmor frequency, the excess population results in a net magnetization oriented along the z axis. When a pulse or a short burst of radio waves at the Larmor frequency is applied along the x axis, the coupling of frequencies causes resonance and flips the nuclear spins of the excess population from the...
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Area of Science:

  • Nuclear Physics
  • Particle Physics
  • Atomic Physics

Background:

  • The Standard Model of particle physics describes fundamental forces, but searches for physics beyond it are ongoing.
  • Exotic tensor contributions to the weak interaction could indicate new physics, such as right-handed neutrinos.

Purpose of the Study:

  • To perform the first precise measurement of beta-recoil correlation in radioactive 6He.
  • To search for evidence of exotic tensor-type contributions to the charged weak current.
  • To set limits on the ratio of tensor to axial-vector coupling constants (CT/CA).

Main Methods:

  • Radioactive 6He atoms were confined using a magneto-optical trap.
  • Beta-recoil correlations were precisely measured.
  • Experimental uncertainties in time response and detector distance were analyzed.

Main Results:

  • The measurement yielded |CT/CA|^2 ≤ 0.022 (90% C.L.) for right-handed neutrinos.
  • For left-handed neutrinos, the limits found were 0.007 < CT/CA < 0.111 (90% C.L.).
  • Experimental uncertainties were identified as the main limitation to measurement sensitivity.

Conclusions:

  • The study provides stringent limits on tensor currents in beta decay.
  • The results constrain theories with exotic tensor interactions and right-handed neutrinos.
  • Improved experimental techniques are needed to further enhance sensitivity.