A physics potential adding positron polarization has recently been demonstrated in a
precise measurement of 
on Z-pole. [28]
With 
% and 
%,
the systematic error( 
)can be 5-times smaller than that of single polarization(%), that is
0.00005. This accuracy would significantly constrain the mass
of heavy Higgs boson if the first-phase collider can not find a Higgs boson.
In addition, a new physics signal can be ambiguously measured in the initial states of
and 
for hardly interacting in the standard model. For
instance, the process of 
can
be observed with no SM background. [29]
T.Hirose described the R&D program and the present status of polarized
positrons. [30]
The two methods have been considered as follows.
The first is to use the 
decay of an unstable nucleus, that is 
, where 
is made in the process of 
by protons bombarding an aluminum plate.
The present setup is based on a commercial proton cyclotron, CYPRIS, of Sumitomo Heavy
Industries Ltd., which accelerates protons up to 18MeV with a beam intensity of 1
A.
The positrons from the decay must be monochromized energetically in a moderator
having a 25m
tungsten plate, and are then accelerated. The present intensity
of positrons is expected to be 
/sec for a polarization of more than 80%
with a 10
efficiency(
) of the moderator.
The first R&D milestone is to measure the actual intensity and polarization of
positrons.
In order to fulfill the requirement of 10
/sec for the JLC-I, 40MeV proton
beams of 100mA are needed in addition to a dramatic improvement in the efficiency of the
moderator (
).
The second method is to use 
pair creations by polarized photons which are
back-scattered from Compton scattering events between circularly polarized laser light and
high-energy electrons. A test experiment is being conducted using 1.5 GeV 
beams of the ATF-linac at KEK. A beam of 2
10
/bunch collides with a
2.33eV YAG-laser beam of 0.55J/pulse to produce polarized photons(<80MeV) of
4.410
/bunch. Polarized positrons (>80%) are expected to be created
with an intensity of 1.410
/bunch from a 3mm tungsten target hit by the
photons. [31] For JLC-I, a high-intensity 5-GeV linac of more than 10
/bunch is needed together with a multi-converter system comprizing 
100(number of
bunches) CO
lasers of 5J/500ps which are operational at 150Hz and are driven by a
Nd:YAG laser so as to precisely control their timings.
Different methods have recently been proposed to produce high-energy polarized
photons emitted in undulators at 100200 GeV linac and at a relativistic
ion-storage ring for a more efficient source of polarized positrons. [32]
Although many R&D and breakthroughs are necessary to realize a polarized positron beam, it is worth pursuing.