Laser spectroscopy of anti-protonic and mesonic helium atoms
Dr. Masaki Hori (MPI für Quantenoptik, Garching/Institut für Physik, Uni Mainz)

A Metastable antiprotonic helium is a Rydberg exotic atom composed of a
helium nucleus, electron, and an antiproton. It is among the hadron-anti-
hadron bound systems with the longest known lifetimes. Intense beams
of laser light can be used to excite atomic transitions involving the anti-
proton orbital. By utilizing sub-Doppler two-photon laser spectroscopy
or buffer gas cooling, its atomic transition frequencies were measured to
ppb-scale precision. Comparisons with the results of QED calculations
allowed the antiproton-to-electron mass ratio to be determined as
1836.1526734(15). The results were used to set upper limits on fifth
forces between antiprotons and nucleons at atomic length scales, and
on forces that may arise between an electron and antiproton mediated
by hypothetical bosons by Mainz theoretical groups. Efforts are
currently underway to improve the experimental precision using
CERN’s ELENA facility.

We also observed narrow spectral lines of these atoms formed in super
fluid helium with asurprisingly high spectral resolution of 2 parts per million.
This revealed the hyperfinestructure arising from the spin-spin interaction
between the antiproton and electron,despite the fact that the atom was
surrounded by a dense matrix of normal atoms. Thisphenomenon may
imply future possibilities in condensed matter or astrophysical fields.

Metastable pionic helium (πHe+) contains a negative pion occupying a
state of n≈l-1≈17, and retains a 7 ns average lifetime. We recently used
the 590 MeV ringcyclotron facility of Paul Scherrer Institute near Zurich
to synthesize the atoms, and irradiated them with resonant infrared laser
pulses. This induced a pionic transition within the atom and triggered
an electromagnetic cascade that resulted in the π- being absorbed into
the helium nucleus. This constitutes the first laser excitation and
spectroscopy of an atom containing a meson. By improving the
experimental precision, the pion mass may bedetermined to a high
precision as in the antiproton case. We wish to extend these studies
to other atoms containing kaons or hyperons that includes the strange
quark.

Bio: Masaki Hori obtained his PhD in 2000 at the University of Tokyo
in the field ofnuclear physics. After CERN and JSPS fellowships in Geneva
involving antiprotonexperiments and building LHC injector parts, he
became group leader at the Max Planck Institute of Quantum Optics
in 2008. He obtained a Habilitation and became Privatdozent in 2020
at the Ludwig Maximillians University, while working in a commercial
company that develops optical frequency combs. He joined the Institute
of Physics of Mainz today as a Heisenberg position. He is spokesperson
of the laser spectroscopy experiments of exotic helium atoms at CERN and PSI.