Test of Lorentz symmetry by using a He-3/Xe-129 comagnetometer
In the framework of the so-called Standard Model Extension (SME) of Kostelecký et al. , Lorentz- and CPT violating terms should cause measurable effects in high-precision experiments at low energies. Our experiment looks for an anomalous spin coupling to an anisotropy in space (Fig.1).
Fig.1: The Earth and with it the magnetic field B0 in the laboratory rotate around the axis Ωs with respect to a hypothetical Lorentz-violating background field ε.
This coupling causes a periodic variation of the relative Larmor frequencies Δω, respectively phases, of the co-located 3He and 129Xe spin samples σ, during a sidereal day as the laboratory reference frame rotates with Ωs with respect to a hypothetical background field with magnitude <b> and pointing in direction ε :
To determine the leading-order effects of a Lorentz violating potential V, it suffices to use a non-relativistic description for the particles involved given by :
with: J = X, Y, Z ; w = e, p, n. Since in the 3He and 129Xe atoms the nuclear spin is carried by a neutron only we are sensitive to the parameter bJ of the bound neutron.
This experiment is performed in the magnetically shielded room (BMSR-2) of the Physikalisch-Technische Bundesanstalt (PTB) in Berlin . Inside the room two quadratic Helmholtz coils produce a magnetic field of about 400nT. Directly below the Dewar which contains about 300 individual SQUIDs that can be used for the detection of the spin precession signals we place the 3He/129Xe co-magnetometer. The design and operation of the two-species 3He/129Xe co-magnetometer has been shown recently . By composing the weighted frequency difference of 3He and 129Xe we get rid of the magnetic field dependence, so that we are sensitive to the periodic Lorentz violating frequency. With our measurements in March 2009 we have set an upper limit on neutron spin coupling to possible Lorentz and CPT violation background tensor fields of < 3.7·10-32 GeV (95% CL) .
In March 2012, a new measurement run was performed at PTB Berlin. The essential improvement there was the increase of the transverse relaxation time of Xenon, now reaching more than 8 h. Together with improved 129Xe polarization-levels coherent spin precession could be recorded at least for 24 h and a gain of 2-3 in the SNR was achieved. As a result we obtain an upper limit on the equatorial component of the background field interacting with thespin of the bound neutron of < 8.4·10-34 GeV (68% C.L.) . Our result improves our previous limit  by a factor of 30 and the world’s best limit by a factor of 4.
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