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The shielded pair resonator method is a useful tool in the measurement of accelerator components, such as the beam screens used in the Large Hadron Collider (LHC), the High-Luminosity (HL) LHC, or future accelerators. It can measure the resistive losses at several frequency points by separating the resistive losses on the sample from other sources of losses. We built a new resonator to be inserted into a superconducting dipole magnet (peak magnetic field of 9.5 T) and to measure the surface resistance of beam screens, such as LHC beam screens coated with amorphous carbon (a-C). The device can measure surface resistance at any temperature between 4.2 K and 300 K, in the frequency range of 400 MHz to 1600 MHz. We conducted the first surface resistance measurements of two a-C coated beam screens at 4.2 K and showed that the 200 nm to 400 nm titanium underlayer plus 50 nm a-C only has a limited effect on the surface resistance. This first result supports the choice of this coating as baseline for the HL-LHC triplets magnets upgrade. The resonator will have an important role in the characterization of next-generation beam screens, such as a beam screen with laser-engineered surface structure (LESS). Further measurements of the LHC beam screen in the presence of magnetic fields up to 9.5 T and throughout the full temperature range are going to be reported separately. [ABSTRACT FROM AUTHOR]

We review the results of recent laser spectroscopy experiments on metastable pionic helium atoms at the Paul Scherrer Institute’s 590 MeV cyclotron facility that was carried out by the PiHe collaboration. Some future perspectives are briefly discussed.

When atoms are placed into liquids, their optical spectral lines corresponding to the electronic transitions are greatly broadened compared to those of single, isolated atoms. This linewidth increase can often reach a factor of more than a million, obscuring spectroscopic structures and preventing high-resolution spectroscopy, even when superfluid helium, which is the most transparent, cold and chemically inert liquid, is used as the host material1–6. Here we show that when an exotic helium atom with a constituent antiproton7–9 is embedded into superfluid helium, its visible-wavelength spectral line retains a sub-gigahertz linewidth. An abrupt reduction in the linewidth of the antiprotonic laser resonance was observed when the liquid surrounding the atom transitioned into the superfluid phase. This resolved the hyperfine structure arising from the spin–spin interaction between the electron and antiproton with a relative spectral resolution of two parts in 106, even though the antiprotonic helium resided in a dense matrix of normal matter atoms. The electron shell of the antiprotonic atom retains a small radius of approximately 40 picometres during the laser excitation7. This implies that other helium atoms containing antinuclei, as well as negatively charged mesons and hyperons that include strange quarks formed in superfluid helium, may be studied by laser spectroscopy with a high spectral resolution, enabling the determination of the particle masses9. The sharp spectral lines may enable the detection of cosmic-ray antiprotons10,11 or searches for antideuterons12 that come to rest in liquid helium targets.The spectral lines of antiprotonic helium atoms are shown to retain their sub-gigahertz linewidth upon submersion in a bath of superfluid helium, enabling the hyperfine structure to be resolved. [ABSTRACT FROM AUTHOR]

The mass of the antiproton (the antiproton-electron mass ratio $m_{\overline{p}}/m_e$) can be deduced from laser spectroscopy measurements of antiprotonic helium, compared to 3-body QED calculations. The most precise spectroscopy measurements have so far been limited by Doppler-broadening at finite temperatures. The status of our ongoing precision measurements is presented, within the framework of the Asacusa experiment at CERN. [ABSTRACT FROM AUTHOR]

The spin magnetic moment $\mu^{\overline{p}}_{s}$ of the antiproton can be determined by comparing the measured transition frequencies in $\overline{p}^4$He with three-body QED calculations. A comparison between the proton and antiproton can then be used as a test of CPT invariance. The highest measurement precision of the difference between the proton and the antiproton spin magnetic moments to date is 0.3%. A new experimental value of the spin magnetic moment of the antiproton was obtained as $\mu^{\overline{p}}_{s} = -2.7862(83)\mu_{N}$, slightly better than the previously best measurement. This agrees with $\mu^{p}_{s}$ within 0.24%. In 2009, a new measurement with antiprotonic He has been started. A comparison between the theoretical calculations and experimental results would lead to a stronger test of the theory and address systematic errors therein. A measurement of this state will be the first HF measurement on $\overline{p}^3$He. We report here on the new experimental setup and the first tests. [ABSTRACT FROM AUTHOR]