Abstract
for Gamma-Gamma Section of the LC Documment
Many physicists have recognized the unique potential
that a high
energy gamma-gamma collider offers, and its complementarity
to the
e+e- physics programs. The main reasons are due
to the fact that:
(1) the effective cross sections for producing Higgs
and
Supersymmetric particles are as large or even larger
than the
corresponding ones for e+e-, (2) the capability of this
machine to
produce linearly polarized beams not only can provide
conclusive
information about the charge-parity (CP) nature of these
particles, but
also will allow us to detect any CP admixture.
To quantify the physics capabilities of the machine we
have been
considering two machine scenarios with e-e- beams and
one with e+e-.
In the first e-e- scenario, we focus on a `Higgs-factory'
based on
high energy photon beams that are optimized for s-channel
production
of a Standard Model-like Higgs boson in the mass range
between 100-130
GeV (REF1). The properties of this Higgs boson would
be explored in a
dedicated fashion. Among these is a precision measurement
of the
h-->gamma-gamma partial width, which is sensitive to
new charged
particles such as the top squark, and also the top Yukawa
coupling.
In addition, certain rare decays that are highly sensitive
to MSSM
parameters, like h-->gamma-gamma, h-->gamma-Z, and h-->ZZ^*
can be
measured due to the low background level. In the
second e-e-
scenario, a gamma-gamma collider runs at the maximum
available e-e-
energy and study the heavier neutral Higgs bosons, H
and A (REF2), and the
charged Higgs bosons, H+ and H- (REF3). This is
probably one of the
strongest cases for the gamma-gamma collider because
the machine is
capable of discovering these Higgses, even in tan_beta
and M_A regions
that are not accessible to the e+e- machine or the LHC.
The last
scenario starts from e+e- because it is devoted to the
study of the CP
nature of the Higgs. These studies will be done with
linearly
polarization photons, therefore we can use positrons
because we don't
need the initial lepton to be polarized, but we do need
them to have
very high energies even to study the light Higgs, in
order to end up
with a system of well defined CP.
In all scenarios a gamma-gamma collider will require a
second
interaction region (IR), with a different crossing angle
and the optics and
laser system needed to produce the photon beams from
Compton
backscattering of lasers off the incoming electron beams.
In the
first two scenarios we need high degree of polarization,
therefore the
LC should be capable of running both in e+e- and e-e-
mode. The need
for a second IR could be avoided if the crossing angle
is made large enough.
At a gamma-gamma collider the high energy Compton photons
provide an
additional and dominant source of soft hadron backgrounds
coming from
the hadronic interactions of the photons. As a consequence
the hit
multiplicities at the Silicon detector are higher for
the e+e-
detector. The assumption that the e+e- and gamma-gamma
detectors will
be based on the same technology or have comparable performance
may not
be valid, if we are required to integrate many beam crossings.
Studies
on this topic are in progress (REF3).
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REF1 M. M. Velasco {\it et al.},
``Photon photon and electron photon colliders with energies
below a
TeV,'' in {\it Proc. of the APS/DPF/DPB Summer Study
on the Future of
Particle Physics (Snowmass 2001) } ed. R.~Davidson and
C.~Quigg,
arXiv:hep-ex/0111055.
REF2 D. Asner, J. Gronberg and J.F.Gunion,
``Detecting and studying Higgs bosons in two-photon collisions
at a
linear collider,''
arXiv:hep-ph/0110320.
REF3 D. Asner (LLNL), B. Grzadkowski (Warsaw), J.F. Gunion
(U.C. Davis),
H.E. Logan (FNAL), V. Martin (NWU), M. Schmitt (NWU),
M.M. Velasco (NWU),
``New results for a photon-photon collider,''
arXiv:hep-ph/0208219.
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