Pion electronic decay and lepton universality

In common with a number of simple processes involving elementary particles, charged pion decays are profoundly shaped by applicable Standard Model (SM) symmetries and properties. Given the highly precise SM theoretical description, pion decays are used as selective probes of SM parameters, and of possible SM extensions. The PEN experiment at PSI is studying the \pi^+ \to e^+\nu_e(\gamma)π+→e+νe(γ), or \pi_{e2(\gamma)}πe2(γ) decay. The primary goal is to reach the relative precision of 5 \times 10^{-4}5×10−4 in R_{e/\mu}^\piRe/μπ, the branching ratio for \pi_{e2(\gamma)}πe2(γ) decay. We review the PEN research program, its present status, and prospects.

The mass of the $\pi^-$

The most precise values of the mass of the negatively charged pion have been determined from several measurements of X-ray wavelengths for transitions in pionic atoms at PSI. The Particle Data Group gives the average m_{\pi^-}mπ− = (139.570 61 \pm± 0.000 24) MeV/c^22.

The $\pi^0$ mass and the first experimental verification of Coulomb de-excitation in pionic hydrogen

The most precise value for the \pi^0π0 mass was obtained from the measurement of the mass difference m_{\pi^-}-m_{\pi^0} = 4.593\,64(48)mπ−−mπ0=4.59364(48),MeV/c^22 in the charge exchange reaction \pi^-π−p \rightarrow \pi^0→π0n at PSI. With the most precise charged pion mass value, m_{\pi^+} = 139.570\,21(14)mπ+=139.57021(14),MeV/c^22 and the validity of the CPT theorem (m_{\pi^-} = m_{\pi^+}mπ−=mπ+), a value m_{\pi^0} = 134.976\,57(50)mπ0=134.97657(50),MeV/c^22 is obtained. The measurements also revealed, for the first time, evidence of an unexpectedly large contribution from Coulomb de-excitation states during the pionic atom cascade.

Meson bunches formed by pulsed laser-induced processes in ultra-dense hydrogen H(0)

Ultra-dense hydrogen H(0) (reviewed in Holmlid and Zeiner-Gundersen, Physica Scripta 2019 ) consists of small strongly bound molecules with interatomic distance of 0.56 pm in spin state s = 1. It is a useful nuclear fuel for energy generation, giving heat above break-even (Holmlid, AIP Advances 2015) in laser-induced processes (Holmlid, Int. J. Hydr. Energy 2021). Nuclear processes in H(0) emit particles in typical meson decay chains with kinetic energy up to 100 MeV. These mesons decay and generate fast muons at up to 500 MeV energy at current densities of several mA cm-2 at 1–2 m distances, which corresponds to 1013 -1014 muons formed per laser pulse. It is shown that the mesons decay in chain processes with well-defined meson time constants in the range 10–60 ns. The time varying signals from H(0) agree well with mesons M in decay chains as A ◊ M ◊ N where N is a signal muon. M may be a charged kaon K± (decay time constant at rest 12.4 ns) or a charged pion π± (decay time constant at rest 26 ns) or a long-lived neutral kaon \({\text{K}}_{L}^{0}\) (decay time constant at rest 51 ns). Ultra-dense protium p(0) gives the same time constants as D(0) but slightly different decay-chains. The meson bunches observed are similar to the meson bunches from nucleon + antinucleon annihilation. The energy gain in the nuclear process is at least 8000, strongly indicating baryon annihilation for which process further evidence is given in other recent publications.

Laser Spectroscopy Measurements of Metastable Pionic Helium Atoms at Paul Scherrer Institute

We review recent experiments carried out by the PiHe collaboration of the Paul Scherrer Institute (PSI) that observed an infrared transition of three-body pionic helium atoms by laser spectroscopy. These measurements may lead to a precise determination of the charged pion mass, and complement experiments of antiprotonic helium atoms carried out at the new ELENA facility of CERN.

Charged rho superconductor in the presence of magnetic field and rotation

In this work, we mainly explore the possibility of charged rho ($$\rho ^\pm $$ ρ ± ) superconductor in the presence of parallel magnetic field and rotation within three-flavor Nambu–Jona-Lasino model. By following similar schemes as in the previous studies of charged pion ($$\pi ^\pm $$ π ± ) superfluid, the $$\rho ^\pm $$ ρ ± superconductor is found to be favored for both choices of Schwinger phase in Minkowski and curved spaces. Due to the stability of the internal spin structure, charged rho begins to condensate at a smaller threshold of angular velocity than charged pion for the given large magnetic fields. Even the axial vector meson condensation is checked – the conclusion is that $$\rho ^\pm $$ ρ ± superconductor is the robust ground state at strong magnetic field and fast rotation, which actually sustains to very large angular velocity.