Single-Point Search for eV-scale Axion-like particles with Variable-Angle Three-Beam Stimulated Resonant Photon Collider

TL;DR

This work presents a laboratory, model-independent search for axion-like particles (ALPs) in the eV mass range using a variable-angle three-beam stimulated resonant photon collider (SRPC). By varying the incidence angle, the center-of-mass energy $E_{\rm cms}=2\omega_c\sin\theta_c$ is scanned to probe ALP resonances around $m_a$ in the eV regime, while maintaining spatiotemporal overlap of three femtosecond/nanosecond laser pulses in vacuum. The measurement yields no excess, enabling a 95% confidence limit on the two-photon coupling $g/M$ with a minimum sensitivity of $g/M \simeq 4.2\times10^{-10}~\mathrm{GeV}^{-1}$ at $m_a=2.27~\mathrm{eV}$, reaching the KSVZ benchmark in this mass range. This single-point result demonstrates the feasibility of smooth, wide-range mass scans with tabletop SRPC techniques and paves the way for future comprehensive eV-scale ALP searches.

Abstract

We report a laboratory search for axion-like particles (ALPs) in the eV-mass range using a variable-angle three-beam stimulated resonant photon collider. The scheme independently focuses and collides three laser beams, providing a cosmology- and astrophysics-independent test. By varying the angles of incidence, the center-of-mass energy can be scanned continuously across the eV range. In this work, we operated the collider in a vacuum chamber at a large-angle configuration, verified the spacetime overlap of the three short pulses, and performed a first search centered at $m_a\simeq 2.27~\mathrm{eV}$. No excess was observed. We thus set a $95\%$ C.L.\ upper limit on the pseudoscalar two-photon coupling, with a minimum sensitivity of $g/M\simeq 4.2\times 10^{-10}~\mathrm{GeV}^{-1}$ at $m_a=2.27~\mathrm{eV}$. This provides the first model-independent upper limit on the coupling that reaches the KSVZ benchmark in the eV regime and demonstrates the feasibility of eV-scale mass scans in the near future.

Figures (12)

• Figure 1: Concept of the three-beam stimulated resonant photon collider (${}^{\mathrm{t}}$SRPC). Three short-pulsed laser beams with photon number densities $\rho_k (k=c_1, c_2, i)$ are independently focused at the collision point. The two creation beams (green) cross at an incidence angle $\theta_c$ and produce an ALP resonance, while the inducing beam (red) stimulates its decay. The creation photons have energies $\omega_1$ and $\omega_2$ and incidence angles $\theta_1$ and $\theta_2$ around the central values $\omega_c$ and $\theta_c$, respectively. Similarly, the inducing beam with central energy $\omega_i$ contains a frequency component $\omega_4$ that enhances emission of signal photons with energy $\omega_3$ (blue) via energy-momentum conservation.
• Figure 2: Schematic of the search setup. A Ti:Sapphire laser (green, $\lambda_c=811\,\mathrm{nm}$) enters from the upper left and serves as the creation laser, while an Nd:YAG laser (red, $\lambda_i=1064\,\mathrm{nm}$) enters from the left and serves as the inducing laser. Both are brought to intersect at the collision point (CP) inside a vacuum vessel, where axion-like particles (ALPs) can be resonantly produced and induced to decay. Creation path (green): Upstream, the beam is clipped by a aperture plete (AP) into two creation beams, c1 and c2. Each is then cut to a diameter of $5\,\mathrm{mm}$ by apertures $A_{c1}$ and $A_{c2}$. A delay line (DL) on the c2 arm tunes the optical path length. In each optical path, a 1.81-mm-thick half-wave plate ($\lambda/2$) is inserted to make the polarization states orthogonal at the CP. The beams are directed to the variable-angle three-beam stimulated resonant photon collider on the right side of the vacuum chamber and focused to the CP by parabolic mirrors ($PM_{c1},PM_{c2}$) at incidence angles $\theta_{c1,c2}=\pm 47.9^\circ$ to produce an ALP resonance. And after collimation by a lens, c1 line is measured by a photodetector $PD_c$, and the laser profile c1, c2 at CP is monitored by a collision-point monitor (CPM2). Inducing path (red): Upstream, a mechanical shutter (MS) generates a special trigger pattern. The beam is propagated in a circular polarization state by quarter plate $\lambda/4$ and focused onto the CP by a parabolic mirror ($PM_i$) at an incidence angle $\theta_i=-66.4^\circ$ to induce the decay of the resonant state. After collimation by a lens, inducing laser is recorded by $PD_i$, and the CP profile is verified with CPM2. Variable-angle three-beam stimulated resonant photon collider: The collider is a four-layer assembly of rotation stages and aluminum plates. To independently vary collision angles and focus for each laser beam, parabolic mirrors are mounted on an aluminum plate. The lowest (black) signal layer collimates the ALP-mediated signal photons (blue: $\sim 655\,\mathrm{nm}$) emerging from the CP with a parabolic mirror and guides them to photomultiplier-tube (PMT1). The middle layers (cyan for c1, orange for c2) focus the creation beams. The top (magenta) layer hosts a collision-point monitor (CPM1) for alignment of the laser profiles and the incidence angles; this camera is removed during data taking. At the collider center, a target system is set to ensure spatiotemporal overlap of all three beams and to enable automated vertical-axis switching among a cross-shaped wire, a nonlinear crystal (NLC), and vacuum. Calibration and signal path (blue): To measure the acceptance of PMT1 for photons originating at the CP, a He-Ne laser $633\,\mathrm{nm}$ is injected by temporarily inserting a calibration mirror (CM) and routing the beam along the c1 line from the left. During the acceptance measurement, the c1 layer (cyan) is temporarily aligned to match the signal layer's (black) divergence $\sim 34.0^\circ$ from the CP, reproducing the signal-photon path to PMT1.
• Figure 3: Left, middle: Beam-profile images of c1, c2, and i recorded with the collision-point monitor (CPM2). In left three panels, a $10\,\mu\mathrm{m}$-diameter cross-shaped wire is placed at the collision point (CP) as a common target. The spatial overlap of the three laser beams is ensured by imaging onto the wire at the focal point and aligned to coincide. Along the focusing direction ($z$-axis), the image is formed so that the vertical wire is in focus. For each beam, single camera pixel corresponding to the center of the cross-shaped wire is defined. During the search, the cross-wire is removed and each beam's profile centroid is aligned to the defined camera pixel at the search-mode waist as shown in middle three pannels. Right: Oscilloscope trace of four-wave mixing (FWM). A nonlinear crystal is installed at the CP under vacuum to ensure three-beam spatio-temporal overlap. Scanning the delay line shows that second-harmonic generation (SHG, yellow) occurs only when the two creation beams (c1, c2, green) overlap at the CP, whereas FWM (cyan) appears only when all three beams-including the inducing beam i overlap.
• Figure 4: Trigger pattern used during the search. A four-state trigger sequence is introduced by combining the emission states of the three beams to form S, I, C, and P patterns. The classifications are: S for both laser pulses, C for only the creation laser pulses, I for only the inducing laser pulses, and P for pedestals without laser pulses.
• Figure 5: Arrival time distribution of FWM photons for the S pattern (500 shots) via the atomic process when the NLC was placed at CP. The red lines thus provide the expected time window for FWM photons via ALP-exchange to arrive.