Graham,

While it might indeed be a mistake to lump electron and muon
colliders into the same category, it is equally a mistake to lump
CLIC together with the ILC or TLEP.  

No one would argue that the ILC is a well studied
and feasible machine.  However, if the LHC results or
indirect measurements point to a mass scale out
of the range of the ILC, then it might well be
that the muon collider is the better option to reach
those scales in a "lepton collider".  

I've said many times that accelerators have historically
been evolutionary rather than revolutionary and that
"No accelerator has ever done two new things at
once".  The Tevatron did one new thing (superconducting
magnets in a synchrotron).  The LHC did nothing new,
and that turned out to not be without risks.

In contrast, both CLIC and the muon collider require
*lots* of new things and are both "impossible"
to first order. Which one is declared more feasible is
largely a function of one's on experience and prejudice.  
Certainly, no one can axiomatically declare one
more reasonable than the other.

As with the ILC, the Higgs factory would be a step
along the way, rather than a "niche application".

The Muon Acceleration Program has something of an advantage
(IMHO) in that it supports a lot of interesting physics
along the way, even if a collider is never built, whereas
CLIC is totally uninteresting until it succeeds (if
it succeeds).

How things are categorized is largely book keeping, but
it's important that they be represented and compared
correctly.

-Eric


On Jul 31, 2013, at 3:23 PM, Graham W. Wilson wrote:

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
--
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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