This memorandum is a short progress report regarding plans for FELIX, a possible full acceptance detector at the LHC.
As you will recall, the possibility of pursuing full acceptance physics at the LHC was raised in oral presentations by J.D. Bjorken and K. Eggert at the November 1994 LHCC workshop on new physics for the LHC. The physics topics were then outlined in a memorandum to the LHCC (LHCC 95/01) by the ALICE collaboration together with other interested parties. Following the encouragement of the LHCC (LHCC 95/03 and LHCC 96/28) to further investigate the subject of diffractive physics, together with developments on the machine side which potentially open I4 as an experimental area optimized for forward and full acceptance physics, a workshop was held at CERN to consider the possibility of a dedicated full acceptance detector. The workshop concluded that such a detector placed in I4 represents the best environment for pursuing this kind of physics (LHCC 95/51).
Significant work has been done since that time, leading to a magnetic architecture for the collision region and the experiment. Since the detector extends into the far forward region, the interplay between the machine and the experiment is vital. Together with requirements for free space for detectors, and the desire for relatively inexpensive magnets for momentum analysis in the central region, the design of the magnetic architecture of a genuinely full acceptance experiment is non trivial. Nevertheless, we have now settled on a candidate architecture which satisfies the machine needs as well as the experimental needs.
We have proceeded on the assumption that the cost of FELIX will be no more than 50 MSF, not including about 30 MSF for the intersection region and machine magnets. Our working design minimizes costs by reusing the ALEPH magnet (already located in I4) and the UA1 magnet (split into two pieces with new coils) together with forward dipoles which serve as both spectrometer and machine magnets. The infrastructure of I4 is well suited to FELIX, and will need only minor modifications. Note that while FELIX will have high precision detectors in the forward directions, we require only modest instrumentation in the central region for much of the FELIX program of physics. Since the forward detectors will be quite compact, this means that the cost of FELIX can be small compared to CMS and ATLAS. Should FELIX need to be staged, we would first establish the magneitc architecture, fully instrument the less expensive forward regions, and build the inner tracking system. On the other hand, if sufficient money were available, the ALEPH magnet could accommodate a more substantial central detector than we envision. A brief description of the physics ideas and the experimental layout can be found in the paper we attach as an appendix to this letter.
The experiment will run at luminosities of 
. We expect the first
several years of operation to be completely devoted to forward and full acceptance p-p
physics. We are also interested in heavy ion runs and especially p-A runs which are
made possible by the split RF cavities. It is conceivable that the insertion could be later
modified
by placing the focussing quadrupoles closer to the experiment leading to higher
luminosities at the cost of compromising the detection of far-forward particles.
We are proceeding with our studies which we expect to lead to a formal LoI early next year. To this end, we have formed a number of working groups, including both experimentalists and theorists. The various items on the potential physics agenda are being addressed with an eye towards defining the physics reach of the detector, and better understanding the implications for the detector. We are beginning to identify candidate technologies for detector components. With a shorter time scale for design and construction, we expect to benefit tremendously from the R&D carried out by the other collider experiments. We have begun detailed GEANT simulations, and expect soon to carry out serious studies of benchmark physics processes in order to determine well the physics reach of the proposed detector. And, perhaps most importantly, we are continuing to work to identify and expand the potential community of users.
FELIX will substantially broaden the range of physics accessible at the LHC for a comparatively modest cost. While we recognize the many difficulties in realizing this goal, we ask that the LHCC (1) agrees that the I4 intersection region be kept open to accommodate an experimental program such as FELIX; and (2) agrees that the machine studies and optimizations should include the option of a collision region at I4. Such encouragement will also make the task of assembling the experimental collaboration much easier.
11 October 1996 G.K.