Dear Physics Group Conveners,
I appreciate very much all your hard work that is
helping to produce compelling reports
on the high energy colliding beam approach to exploring
high energy physics.
I do want to re-state more clearly my remark
from yesterday at the
end of Mark Palmer's capabilities talk on
lepton colliders.
I think it is a mistake and very
misleading to lump all "lepton colliders" together in the
physics reports - and more generally in discussion of our
field.
I urge you to say e+e- when you mean e+e- and say mu+mu- when
you mean mu+mu-.
This is essentially the same remark as I made
at the Seattle workshop in response
to elements of the Higgs group report. It was also heavily
triggered by the repeated use
of the word "lepton colliders" in the new physics summary
talk when in fact the relevant
conclusions and inputs to the working group were only
applicable to the proposed
high energy e+e- colliders ILC and CLIC.
The e+e- and mu+mu- approaches are
fundamentally different.
Much of the rationale for a future high
energy lepton collider is to explore
the Higgs and explore new physics
possibilities in a way that is complementary to LHC.
The e+e- approach has shown that it is very
well suited to measuring final states with
missing energy and such states are at the
heart of the envisaged ILC and CLIC Higgs programs.
e+e- is a well established "stable lepton
collider" accelerator technology
with well understood and
comprehensive detector capabilities with high longitudinal
polarization capabilities for linear
colliders. It is a real option that has been
under development for decades and is on the
table now for ILC with realistic detector
designs and an understanding of the machine
backgrounds. The detector hermeticity
capability is impeccable. Precision absolute
normalization is possible using Bhabhas at the 0.1% level.
The mu+mu- collider is a highly speculative
"decaying lepton collider" with
much R&D to do to establish the
accelerator technology and luminosity performance
with a potential niche application to things
like a Higgs resonance scan, heavy Higgs
and direct production of Z'. It can in
principle do very well on beam energy determination.
It features a "novel" (according to Mark),
insane according to others, background regime
from muon decays in the detector. This makes
instrumentation of close to 4pi steradians
extremely difficult at a muon collider and
will severely limit the ability to detect final states with
missing transverse momentum.
Instrumentation below something like 150
mrad is not known to
be feasible at a muon collider. Assuming no
instrumentation below 150 mrad, it has been shown
for an e+e- collider from simple kinematic
considerations that this would limit the clean region
of detection of missing energy to transverse
momenta of about 30% of the beam energy.
Given the actual minimum detection angle for
e+e- (15 mrad), the reach is extended by a
factor of 10 to about 3% of the beam
energy.
As an example, the direct production of WIMP
pairs in association
with a soft initial state photon and missing
energy is something that can be
done very well in e+e-. Exploration of
"compressed" SUSY spectra is also one of the main issues
of complementarity to LHC - a potentially
natural explanation of current LHC results
(SUSY particles are being produced - but with not enough missing
ET to be detectable).
This will be a much greater strength of
e+e- compared to mu+mu- at the same center-of-mass energy
for the same reasons.
In conclusion, please be careful to avoid
implicitly assuming that what is feasible and
documented in e+e- is also obviously
feasible in mu+mu-.
It has been demonstrated that an e+e-
machine is very well suited to measurements with missing energy
such as nu-nu-H and supersymmetry.
regards
Graham Wilson