Dear WWSOC,

WWS is asked by CCB to respond to the CCR recently submitted
by BDS AG. And MDI panel was asked by WWS to make input.

Attached below is the report from the MDI panel. It has a form of
the minutes of the meeting held on August 15. It went through
one round of comments/suggestions, but please consider it
preliminary. It covers folowing topics:

1. 14mrad crossing

2. Single experimental hall covering two IRs

3. Surface assembly of detectors

4. Muon spoiler

The items directly related to the CCB already submitted by BDS AG
are 1 and 2. The item 4 can save considerable money and will
be used in the cost estimation. Item 3 will be submitted soon
to CCB and included here since it is closely related to item 2.

Best

- Hitoshi (as MDI panel chair)

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MDI panel meeting minutes


1. 14mrad crossing angle

For physics, the mode that is most affected by the
crossing angle is the slepton pair production where the
slepton-LSP mass difference is small (so-called
co-annihilation region). Here, the main
background is 2-photon processes and an efficient
low-angle electron tag by BEAMCAL is needed to veto them.
There are several studies and the difference in the
amount of expected background is now understood as
the different levels of simulation for the veto
efficiency. The study using full simulation shows
that the signal to noise will be about 4 to 1 for
the nominal machine parameter case with 2mrad
crossing angle.

For a large crossing angle (14 or 20mrad),
anti-DID is needed to collimate the pair background
along the outgoing beam. For 14mrad crossing with
anti-DID, the amount of background is expected to
be comparable to the 2mrad case while the signal
efficiency reduces by about 30% to 40%. This is mainly
due to the 2nd hole of BEAMCAL that is needed for the
large crossing angle which will force additional cuts
to remove the 2-photon and other backgrounds.
This is not based on a complete analysis but on a
study of the pair background distribution on the BEAMCAL:
that for 20mrad crossing with anti-DID was found to be
essentially the same as the 2mrad case. A complete
analysis is needed for 14mrad with anti-DID, also
covering different values of the mass difference
(namely, for different SUSY parameter space). Backgrounds
considered here is mainly the pair background and a lesser
extent Bhabha events. More studies are sorely needed in
this area.

With this limited information, the MDI panel thinks that
the 14mrad is acceptable as the baseline at this time.
However, we would like to stress that the 2mrad crossing
angle is clearly desirable than larger crossing angles for
the slepton search, and R&Ds related to 2mrad should be
encouraged.

nb: The luminosity loss is less than a few % when anti-DID
is used regardless of the size and strength of the
detector solenoid. And also the rotation of polarization
vector within the detector solenoid should be manageable.

2. Single experimental hall

When the crossing angles of the two IR are both 14mrad,
the transverse separation between two IRs will be 28.4m.
With 4m-thick wall between two IRs, this will give 12.7m from
the beamline to the separating wall (with 3m-thick
separation wall) Even though this is tight and seems
to constrain the design of detector access,
it seems to be manageable. There is a concern about the
mechanical coupling of the two IRs such as vibration;
the problem, however, seems to be non-critical. Also, there
os a possibility of doing without the separating wall
using self-shielding detectors. Thus, MDI panel believes
that the single experimental hall containing two IRs is
acceptable as the baseline.

3. Surface assembly of detector

It is generally agreed within the MDI panel that one cannot
meet the requirement that beam commissioning is t0+7yrs and
physics run start t0+8yrs if the detector is assembled
completely underground. The surface detector assembly
can save 2 years or more of time since the detector can
be assembled while the experimental hall is being
excavated and outfitted. Thus, the surface assembly
seems to be necessary for the scheduling reason.

Also, each of three concepts thinks that
surface detector assembly can be accommodated into each
detector assembly design at least with some modifications
to the exact CMS style assembly. One example was
suggested for GLD where medium-weight (~400tons)
movable crane is used both on surface and underground.
There, the detector is lowered underground in
pieces larger than the CMS case (~400 tons instead of
~1500 tons). The detail of assembly procedure is still to be
worked out, and the cost estimation should be done for both
surface assembly and the complete underground assembly
cases. Even though the time saving seems to be certain,
The cost benefit for the surface assembly is not clear
at this time. With the surface assembly. the underground
hall may be smaller than otherwise, but there is a need
for a large assembly hall on surface and some procedures/hardware
may have to be duplicated on surface and underground. The
detail will also depend on the detector concepts.

Thus, the MDI panel thinks that the surface detector assembly
is needed for scheduling reason and it seems possible that the
actual assembly design can be worked out for each of the three
detector concepts. However, it should be stressed that the design
of the assembly as well the geometry of the experimental hall
and shafts are in brainstorming stage now, and much work is
still needed (and on-going).

4. Muon spoiler

With one 5m-thick spoiler per beam, the expected rate of muon in TPC is
384 muons hitting TPC per 200 bunch crossing (500 GeV cm).
This corresponds to roughly 0.15% occupancy and, according to experts, should be manageable. The beam loss rate assumed is considered
quite conservative but it has a large uncertainty.
With the provision that there is a possiblity
of reinforcing the 5m-thick spoiler with more of it, the MDI panel
believes that 5m-thick muon spoiler is acceptable as the baseline.
The other purposes of the muon spoiler is radiation safety and
protection against accidents. Even though these are important
issues, the MDI panel will not deal with them since they are
purely machine-related.

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