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 remarkas 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
--
Graham W. Wilson
Associate Professor
Dept. of Physics and Astronomy
University of Kansas
Lawrence, KS 66045
Office Tel. 785-864-5231
Web: http://heplx3.phsx.ku.edu/~graham/
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