Searching for new spin-dependent interactions withSmCo5spin sources and a spin-exchange-relaxation-free comagnetometer

Abstract

We propose a novel method to search for possible new macro-scale spin-and/or velocity-dependent forces (SVDFs) based on specially designed SmCo5 spin sources and a spin exchange relaxation-free (SERF) comagnetometer. A simulation shows that, by covering a SmCo5 permanent magnet with a layer of pure iron, a high net spin density source of about 1 × 10 22 /cm 3 could be obtained. Taking advantages of the high spin density of this iron-shielded SmCo5 and the high sensitivity of the SERF, the proposed method could set up new limits of greater than 10 orders of magnitude more sensitive than those from previous experiments or proposals in exploring SVDFs in force ranges larger than 1 cm.Searches for anomalous spin-and/or velocity-dependent forces (SVDFs) have drawn considerable attentions in the past few decades. Various theories beyond the Standard Model have predicted weakly coupled scalar, pseudoscalar, vector, or axial-vector bosons with light masses [1][2][3]. It is believed that these light bosons may be the answers to many fundamental questions related to, for examples, the CP or CPT violation [4,5], Lorentz violation [6], and the dark matter [7] etc. Obviously, how to experimentally set limits on coupling constants of such bosons or even find them is an important step for human beings to further understand the mother nature.The light bosons, if exist, can mediate long-range SVDFs between macroscopic objects[1]. Many highly sensitive experimental techniques have been employed to search for these new SVDFs, for examples, the torsion balance [8,9], the resonance spring [10,11], the spin exchange relaxation free (SERF) comagnetometer [12], and other nuclear magnetic resonance (NMR) based methods [13], etc.In all these experimental techniques, the spin density of the source is one of the most critical factors. The force strength mediated by a boson having non-zero mass drops exponentially, for example [14],whereσ 1 ,σ 2 are the spins of the two particles respectively, λ is the interaction range, and r is the distance between the two particles. An effective magnetic field B ef f = f 2 cσ 2 e −r/λ /(4πr) can be employed to detect the boson, and increasing the spin density of the source in the interaction range λ can significantly improve the detection sensitivity. Therefore, various methods have been employed to improve the spin densities [15][16][17].In this letter we propose a new specially designed high spin density material, an iron-shielded SmCo 5 permanent magnet (ISSC) together with a SERF comagnetometer to constrain the coupling strengths of the various terms in SVDFs [14]. In the following of this letter, we will give an overview of the proposed setup first, and then a short introduction of the SERF comagnetometer. The structure of the ISSC and its finite element analysis (FEA) simulation are provided. Then the comparisons between the sensitivities of this proposal and others will be presented. The limits on the coupling strengths set by this proposal could be improved by as large as more than 10 orders of m...

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