Heavy Meson Production at a Low-Energy Photon Collider Page: 4 of 16
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are produced directly. In a way that is complementary to e+e- interactions,
'y'y interactions produce particles having final states with C of +1, where C is
the charge conjugation number.
Participating photons can be either virtual or real. Virtual photons are an
irreducible description of charged particle beams: all e+e- colliders generate
virtual -y-y luminosity - in the case of PEP-II or KeK-II the virtual luminos-
ity can rival the e+e- luminosity. An alternate mechanism for producing 'yy
luminosity is with real photons. Typically, low energy photons are made to
Compton-backscatter off of an energetic charged particle beam to produce
highly boosted photons. Opposing photon beams are kinematically focused
and interact to produce real 'y'y luminosity. Although this production mech-
anism requires the added complication of introducing a laser and associated
optics into the interaction region, the substantial benefits include control over
the 'y'y luminosity profile, polarization and magnitude. Technology advances
in optics and lasers have progressed to the point that one could construct a
low energy 'yy testbed as a proof-of-principle for the NLC, where it would
be possible to singly produce the Higgs boson (whereas it must be produced
in pairs at e+e- colliders). The potential this offers for doing precision Higgs
studies is a major driving force behind the impetus for a 'y'y option at the NLC.
For the purposes of this study, however, we restrict our attention to photons
with a center of mass energy of 60 GeV, which is sufficient for an engineer-
ing demonstration. One possible site for the testbed is the Stanford Linear
Collider, where polarized electron beams meet opposing positrons beams at
center-of-mass energies up to the mass of the Zo. In this proceeding we demon-
strate that with suitable choice of laser and optics parameters the resulting ryy
luminosity can be made to exceed the virtual 'y'y luminosity from e+e- collid-
ers. This luminosity can thus serve as the basis for a physics program at the
testbed facility, e.g., heavy quarkonia studies. In subsequent sections we com-
pare and contrast real and virtual -y-y luminosities and discuss the implications
for heavy quark meson spectroscopy.
2 Real photon luminosity
Depending on the energy transfer scale the structure of an otherwise feature-
less photon can be resolved, resulting in an irreducible hadron and leptonic
background that would complicate precision Higgs measurements. Hence, a
primary goal of a 'y'y testbed would be to measure the 'y'y luminosity and com-
pare the result with simulation. In this study Compton-backscattered lumi-
nosity was simulated with CAIN 2.le (1) using the laser and beam parameters
listed in Table 1, where the electron and laser polarizations refer to circular
polarization. Beam parameters in this table reflect NLC-like expectations
about achievable beam sizes, while the laser parameters were chosen to maxi-2
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Asztalos, S. Heavy Meson Production at a Low-Energy Photon Collider, article, April 15, 2004; Livermore, California. (https://digital.library.unt.edu/ark:/67531/metadc1406998/m1/4/: accessed March 29, 2024), University of North Texas Libraries, UNT Digital Library, https://digital.library.unt.edu; crediting UNT Libraries Government Documents Department.