Search for spin-dependent short range interaction of the bound neutron in He-3/Xe-129 clock comparison experiments
In the quantum chromodynamics (QCD), which describes the strong interaction, a CP violating term appears in the Lagrangian that arise from the non-trivial QCD vacuum structure. Since a CP violation in strong interaction never has been observed this problem is referred as the “strong CP problem”. To solve this problem a new light, weak interacting particle, the so called axion (Fig.1), was introduced in the Peccei-Quinn model .
Fig.1: Short range interaction between a fermion N and the spin of another fermion Ň mediated by the axion
This hypothetical particle can mediate interaction between a fermion and the spin of another fermion, respectively between polarized and unpolarized matter. This interaction can be described by a Yukawa-type potential with monopole-dipole coupling:
Thereby λ is the range of the potential which is indirect proportional to the mass of the axion mɑ, gs and gp are the coupling constants between the axion and the fermions, mn is the mass of the fermion with the spin σ and n is the unit distance vector which points from the polarized to the unpolarized matter. This potential produces a frequency shift Δω = Vave/ħ whereby Vave is the average of the potential V(r) over the volume of the polarized and unpolarized matter.
This experiment is also performed in the magnetically shielded room (BMSR-2) of the Physikalisch-Technische Bundesanstalt (PTB) in Berlin . We use the 3He/129Xe co-magnetometer, described in , to get rid of magnetic field dependence and therefore to get sensitive to smaller frequency shifts. In this experiment we want to measure the change in spin precession frequency Δω due to the interaction of the spin sample with an unpolarized test mass. Therefore we measure the spin precession frequency when the unpolarized test mass is “close” and when it is “far away” from the spin sample (Fig.2).
Fig.2: Principle of our measurements. The short range interaction between the polarized helium atoms and the unpolarized atoms of the test mass causes a frequency shift Δω in the precession frequency of helium. We measure the helium precession frequency when the unpolarized test mass is “close” and when it is “far away” from the spin samples.
The frequency shift Δω due to the interaction between the spin sample and the unpolarized test mass is then given by the difference of the two measured frequencies: Δω = Vave/ħ = ωClose - ωFar. For our spin samples we used a cylindrical cell which was made out of aluminosilicate glass (GE-180) with a diameter of 60 mm and a length of 60 mm. The cell was filled outside the BMSR-2 with a mixture of polarized 3He and 129Xe and nitrogen which was used as buffer gas in order to suppress the Xenon relaxation due to van der Waals molecular coupling. After transportation into the BMSR-2, the cell was installed directly beneath the Dewar which contains SQUIDs for the detection of the spin precession signals. To the left, respectively, to the right side, of the cylindrical sample cell a cylindrical glass tube with a length of 1 m and an inner diameter of 60 mm is placed on a separate support with its axis along the axis of the cylindrical sample cell (Fig.3).
Fig.3: Experimental setup
At its open end towards the polarized sample cell, a test mass (Pb-glass or BGO (Bismuth Germanate Bi4Ge3O12)) was installed. The tube and with it the test mass could be moved horizontally from “close” position to “far away” position during a measurement. In March 2009 we carried out feasibility studies which clearly demonstrate the high sensitivity and thus the high predictive power of investigating short-range interactions by using the free spin precession co-located 3He/129Xe spin samples. In September 2010, a new measurement run was performed at PTB Berlin. The precession frequency shift in the presence of an unpolarized mass was measured to determine the coupling of pseudoscalar particles to the spin of the bound neutron. For boson masses between 2 and 500 μeV (force ranges between 3×10-4 m and 10-1 m) we improved the laboratory upper bounds by up to 4 orders of magnitude .
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