We report on the measurement of the Casimir force between conducting surfaces in a parallel configuration. The force is exerted between a silicon cantilever coated with chromium and a similar rigid surface and is detected by looking at the shifts induced in the cantilever frequency when the latter is approached. The scaling of the force with the distance between the surfaces was tested in the $0.5–3.0\mu \mathrm{m}$ range, and the related force coefficient was determined at the $15\%$ precision level.

The Physics Beyond Colliders initiative is an exploratory study aimed at exploiting the full scientific potential of the CERN’s accelerator complex and scientific infrastructures through projects complementary to the LHC and other possible future colliders. These projects will target fundamental physics questions in modern particle physics. This document presents the status of the proposals presented in the framework of the Beyond Standard Model physics working group, and explore their physics reach and the impact that CERN could have in the next 10–20 years on the international landscape.

We have searched for light scalar and/or pseudoscalar particles that couple to two photons by studying the propagation of a laser beam ($\lambda =514\mathrm{}$ nm) through a transverse magnetic field. A limit of 3.5×${10}^{-10\mathrm{}}$ rad was set on a possible optical rotation of the beam polarization for an effective path length of 2.2 km in a 3.25 T magnetic field. We find that the coupling $g_{\alpha \gamma \gamma }<3.6\mathrm{}\times {10}^{-7\mathrm{}}$ Ge${\mathrm{V}}^{-1\mathrm{}}$ at the 95% confidence level, provided $m_a<{10}^{-3\mathrm{}}$ eV. Similar limits can be set from the absence of ellipticity in the transmitted beam. We also searched for photon regeneration in a magnetic field and found the limit $g_{\alpha \gamma \gamma }<6.7\mathrm{}\times {10}^{-7\mathrm{}}$ Ge${\mathrm{V}}^{-1\mathrm{}}$ for the same range of particle mass.

We report the experimental observation of a light polarization rotation in vacuum in the presence of a transverse magnetic field. Assuming that data distribution is Gaussian, the average measured rotation is $(3.9\pm 0.5)\times {10}^{-12}\mathrm{rad}/\mathrm{\text{pass}}$, at 5 T with 44 000 passes through a 1 m long magnet, with $\lambda =1064\mathrm{nm}$. The relevance of this result in terms of the existence of a light, neutral, spin-zero particle is discussed.

To account for the dark-matter content in our Universe, postinflationary scenarios predict for the QCD axion a mass in the range $(10–{10}^3)\mu \mathrm{eV}$. Searches with haloscope experiments in this mass range require the monitoring of resonant cavity modes with frequency above 5 GHz, where several experimental limitations occur due to linear amplifiers, small volumes, and low quality factors of copper resonant cavities. In this paper, we deal with the last issue, presenting the result of a search for galactic axions using a haloscope based on a $36{\mathrm{cm}}^3$ NbTi superconducting cavity. The cavity worked at $T=4\mathrm{K}$ in a 2 T magnetic field and exhibited a quality factor $Q_0=4.5\times {10}^5$ for the TM010 mode at 9 GHz. With such values of $Q$, the axion signal is significantly increased with respect to copper cavity haloscopes. Operating this setup, we set the limit $g_{a\gamma \gamma }<1.03\times {10}^{-12}{\mathrm{GeV}}^{-1}$ on the axion photon coupling for a mass of about $37\mu \mathrm{eV}$. A comprehensive study of the NbTi cavity at different magnetic fields, temperatures, and frequencies is also presented.