Abstract
Abstract Resonance states of axion-like particles were searched for via four-wave mixing by focusing two-color pulsed lasers into a quasi-vacuum. A quasi-parallel collision system that allows probing of the sub-eV mass range was realized by focusing the combined laser fields with an off-axis parabolic mirror. A 0.10 mJ/34 fs Ti:sapphire laser pulse and a 0.14 mJ/9 ns Nd:YAG laser pulse were spatiotemporally synchronized by sharing a common optical axis and focused into the vacuum system. No significant four-wave mixing signal was observed at the vacuum pressure of $3.7 \times 10^{-5}$ Pa, thereby providing upper bounds on the coupling-mass relation by assuming exchanges of scalar and pseudoscalar fields at a 95% confidence level in the mass range below 0.21 eV. For this search, the experimental setup was substantially upgraded so that the optical components were compatible with the requirements of the high-quality vacuum system, hence enabling the pulse power to be increased. With the increased pulse power, a new kind of pressure-dependent background photon emerged in addition to the known atomic four-wave mixing process. This paper shows the pressure dependence of these background photons and how to handle them in the search.
Highlights
The spontaneous breaking of a global symmetry accompanies a massless Nambu– Goldstone boson (NGB) [1]
Because it is difficult to evaluate the physical masses of pNGBs theoretically, model-independent laboratory experiments are indispensable for determining the physical masses as comprehensively as possible at relatively low masses compared to those of high-energy charged-particle colliders
We have reported on the first search for scalar-type pNGBs with laser beams in a quasi-parallel collision system (QPS) [6], and the second search for sub-eV scalar and pseudoscalar fields in the QPS [7]
Summary
The spontaneous breaking of a global symmetry accompanies a massless Nambu– Goldstone boson (NGB) [1]. As a scalar-field example, dilaton is predicted as a source of dark energy [5] These weakly coupling pNGBs are known generically as axion-like particles (ALPs). The scattering probability increases in proportion to (i) the number of photons in the inducing laser and (ii) the square of the number of photons in the creation laser [8,9,10,11] This process is kinematically similar to four-wave mixing in the context of atomic physics [12]. The main purpose of the present search is to show the extensibility of this searching method in such a high-quality vacuum system, even if new types of background sources emerge as a result of increased laser intensities
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