This thesis is part of the feasibility studies for a search for an Electric
Dipole Moment (EDM) of charged particles in a storage ring. The evidence
for a non-vanishing EDM at the sensitivity of present or planned experiments
would clearly prove the existence of new CP violating meachanisms
beyond the Standard Model. The proposed solution to measure the EDM
of charged particles is the use of a storage ring where the polarized charged
particle beam can be kept circulating while interacting with a radial electric
field. Starting with a longitudinally polarized beam, the EDM signal
would be detected as a polarization precession starting from the horizontal
plane and rotating toward the vertical direction. A long horizontal polarization
lifetime, called spin coherence time, is required since it represents
the time available to observe the EDM signal. In order to have a sensitivity
about 10−29 e·cm to the deuteron EDM, the spin coherence time should
reach 1000 s while the measurement of a vertical polarization change should
detect angles as small as micro-radians.
The aim of this work is the analysis of the mechanisms which control
the spin coherence time in a storage ring. The measurements presented
here were made at the COSY (COoler SYnchrotron) ring located at the
Forschungszentrum-J¨ulich GmbH (Germany).
There are two set of measurements presented in this thesis: the first is a
study of a spin resonance induced by a radio-frequency (rf) solenoid and the
second shows the results from the first direct measurement of the horizontal
polarization as a function of time.
The first experiment sought to estimate the spin coherence time by measuring
the width of a deuteron spin resonance induced by an rf-solenoid.
Since the width of the resonance depends on the spin tune spread and thus
on particle momentum distribution, each mechanism that can change the
particle velocity in the beam could contribute to the spin tune spread. In
particular, these mechanisms are betatron oscillations related to the beam
emittance and synchrotron oscillations that are present only in a bunched
beam.
The experiment consisted in the measurement of the vertical polarization
measurements with the rf-solenoid running at fixed frequency on and
off resonance, for both uncooled and cooled bunched beam. In order to
interpret the data, a simple “no-lattice” model was developed based on two
rotation matrices for the spin precession about the vertical axis and the
solenoid kick about the longitudinal axis; synchrotron oscillations were included
as simple harmonic motion. The model demonstrated that the effect
of synchrotron oscillations on the induced spin resonance were large enough
to hide any dependence on emittance.
The second experiment was the direct measurement of the horizontal
polarization as a function of time. This task was accomplished through
the development of a dedicated data acquisition system synchronized with
the revolution frequency of the beam. By changing the horizontal beam
emittance with a white noise electric field, the measurements gave the first
experimental evidence of a dependence of the spin coherence time on the
horizontal beam size. The dependence is due to the path lengthening introduced
by betatron oscillations which forces the particles to go faster in order
to respect the isochronous condition in a bunched beam. A possible method
to correct for emittance effects is to use sextupole magnets. In fact the field
varies as the square of the radius from the center and provides an adjustment
to the particle orbit to remove the term driving the spin tune change.
It has been demonstrated that for a particular value of sextupole strength
the contribution from the horizontal emittance was canceled, reaching a
spin coherence time of a hundred seconds