Any Light Particle Searches with ALPS II: Description of the first science campaign

Abstract

From February to May of 2024 the Any Light Particle Search II (ALPS II) conducted its first science campaign using the `light-shining-through-a-wall' technique to search for pseudo-Goldstone bosons that lie beyond the Standard Model of particle physics and which are inaccessible by accelerator-based experiments. The experimental setup consists of two strings of superconducting dipole magnets, each more than 100 m long, that are separated by a wall. Laser light is directed through the first magnet string and a heterodyne detection system is used to measure the electromagnetic power that traverses a wall via the conversion to and then from a bosonic field. After the wall, a high-finesse optical cavity resonantly enhances the signal power. Two searches were carried out, one with the laser polarized perpendicular to the magnetic field direction and another with its polarization state aligned parallel to the magnetic field. No evidence for the existence of new bosons was found. In its first science campaign, ALPS II reached photon-boson conversion probability sensitivities of a few $10^{-13}$. The ongoing upgrade of the optical system aims to increase this sensitivity by about four orders of magnitude.

Figures (24)

• Figure 1: Side view of the design of the full ALPS II experimental system. The HPL beam and PC spatial eigenmode is shown in red, while the BSM field traveling to the right is shown in blue. The dashed purple lines show the projection of the RC spatial eigenmode. A HWP before the PC can be used to configure the polarization state of the HPL with respect to the direction of the magnetic field (shown as the black arrow).
• Figure 2: Top down view of the optical system used during the ALPS II first science campaign.
• Figure 3: Frequencies of the lasers and PLLs.
• Figure 4: Flow chart of the analysis for the science PD data. The data first undergo the process of double demodulation by the heterodyne detection system. The calibration procedure is then applied to the resulting data series to calculate the final result at the heterodyne signal frequency. The background is then estimated by performing the same analysis at alternative frequencies and a detection threshold is calculated based on the statistics of the background at these other frequencies. A discovery can be claimed if the signal at the expected frequency is above the detection threshold. If the signal is below the detection threshold, as was the case in the case in both runs of the first science campaign, exclusion limits can then be calculated.
• Figure 5: (a) $H[n]$ represented on the complex plane. (b) $Z$-function represented on the complex plane after summing over 100 coherent points of $H[n]$.