Observation
of Charge Symmetry Breaking in the Reaction d-d → 4He-π0
Charge
symmetry breaking (CSB) in the strong interaction arises from the difference in
the masses of the up and down quarks (md > mu) and
from electromagnetic interactions.
These effects cause the neutron to be heavier than the proton and
nuclear mass differences within isospin multiplets. Other independent experiments in light systems are needed to
separate the hadronic and electromagnetic terms in the effective chiral
Lagrangian. The long-sought d-d → 4He-π0 reaction is forbidden by charge symmetry;
its observation would be a measurement of the square of a CSB matrix element
with a different combination of hadronic and electromagnetic terms. Our group at the Indiana University
Cyclotron Facility recently made the first unambiguous observations of the d-d
→ 4He-π0
reaction at two energies just above the π0
production threshold (225.5 MeV) using the IUCF electron-cooled storage
ring. The forward-going cone of 4He
nuclei was separated from the circulating deuteron beam in a 6° bending magnet
and captured and identified in a magnetic channel consisting of a septum magnet
followed by three magnetic quadrupoles.
The two photons from π0 decay
were observed in two arrays of Pb-glass Čerenkov detectors that surrounded
the cold D2 gas jet target.
While the cross section was very low ( ~ 10 pb), a three-fold
coincidence removed essentially all background. Separation of the CSB 4He-π0 events
from the allowed continnum of double radiative capture 4He-γ-γ events
depended on a reconstruction of the pion mass from channel position and time of
flight. The results show a π0 production cross section that is small but
rising with energy in a manner consistent with s-wave production.
* For the Cooler-CSB Collaboration