Sent to CCB on Aug 19, 06

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 given SUSY parameters).

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 coannihilation region slepton search, and R&Ds related
to 2mrad should be encouraged. We also note that the slepton
signal above is a representative of physics signals with
two slow particles with large missing energy and is not restricted
to SUSY scenarios.

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 3m-thick wall between two IRs, this will give 12.7m from
the beamline to the separating 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.