next up previous contents
Next: Ground Motions Up: EXPERIMENTATION AT LINEAR COLLIDERS Previous: Calorimetry

Positron Polarization

A physics potential adding positron polarization has recently been demonstrated in a precise measurement of tex2html_wrap_inline1873 on Z-pole. [28] With tex2html_wrap_inline1875% and tex2html_wrap_inline1877%, the systematic error( tex2html_wrap_inline1879 )can be 5-times smaller than that of single polarization(tex2html_wrap_inline1875%), that is tex2html_wrap_inline18830.00005. 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 tex2html_wrap_inline1885 and tex2html_wrap_inline1887 for hardly interacting in the standard model. For instance, the process of tex2html_wrap_inline1889 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 tex2html_wrap_inline1891 decay of an unstable nucleus, that is  tex2html_wrap_inline1893, where tex2html_wrap_inline1895 is made in the process of tex2html_wrap_inline1897 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 1tex2html_wrap_inline1589A. The positrons from the tex2html_wrap_inline1891 decay must be monochromized energetically in a moderator having a 25tex2html_wrap_inline1589mtex2html_wrap_inline1905 tungsten plate, and are then accelerated. The present intensity of positrons is expected to be tex2html_wrap_inline1907/sec for a polarization of more than 80% with a 10tex2html_wrap_inline1909 efficiency(tex2html_wrap_inline1911) 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 10tex2html_wrap_inline1913/sec for the JLC-I, 40MeV proton beams of 100mA are needed in addition to a dramatic improvement in the efficiency of the moderator (tex2html_wrap_inline1915).

The second method is to use tex2html_wrap_inline1399 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 tex2html_wrap_inline1829 beams of the ATF-linac at KEK. A beam of 2tex2html_wrap_inline179910tex2html_wrap_inline1923/bunch collides with a 2.33eV YAG-laser beam of 0.55J/pulse to produce polarized photons(<80MeV) of 4.4tex2html_wrap_inline179910tex2html_wrap_inline1929/bunch. Polarized positrons (>80%) are expected to be created with an intensity of 1.4tex2html_wrap_inline179910tex2html_wrap_inline1935/bunch from a 3mmtex2html_wrap_inline1905 tungsten target hit by the photons. [31] For JLC-I, a high-intensity 5-GeV linac of more than 10tex2html_wrap_inline1939/bunch is needed together with a multi-converter system comprizing tex2html_wrap_inline1421100(number of bunches) COtex2html_wrap_inline1943 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 100tex2html_wrap_inline1421200 GeV tex2html_wrap_inline1829 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.


next up previous contents
Next: Ground Motions Up: EXPERIMENTATION AT LINEAR COLLIDERS Previous: Calorimetry

Toshiaki Tauchi
Fri, Dec 20, 1996 02:24:05 PM