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felix will be the first full acceptance detector at a hadron collider . it will be optimized for studying the structure of individual events over all of phase space ( see figure [ prodplot ] ) . felix will observe and measure all charged particles , from the central region all the way out to diffractive protons which have lost only @xmath0 of their initial energy . it will even see elastic protons which have a momentum transfer of at least @xmath1 gev@xmath2 . this comprehensive , precision tracking is accompanied by equally superb electromagnetic and hadronic calorimetry . felix will observe and measure photons and neutrons down , literally , to zero degrees , giving it an unparalleled ability to track the energy flow . in contrast , the other lhc detectors are sensitive over only a fraction of phase space and see less than 10% of the typical energy flow . felix is thus uniquely able to pursue physics complementary to that of the other detectors planned for the lhc . the felix design involves the coordinated arrangement of three distinct systems : the magnetic architecture responsible for getting the beams through the i4 straight section , the tracking system , and the calorimetry . each system must be complete in its own right , without compromising the characteristics of the other systems . the magnetic apertures must not be limiting apertures of either the tracking or calorimeter systems . there must be sufficient physical space for both tracking and calorimetry . the calorimeters must be physically large enough to have good resolution , and must not interfere with either the tracking or the magnetic systems . all of this requires a lot of space , and the detector must be carefully integrated into the design of the machine . full acceptance can not be achieved by  adding on " to central detectors optimized for high @xmath3 physics . here felix is fortunate . the decision to split the rf cavities at i4 , moving them to @xmath4 140 m from the interaction point ( ip ) , combined with the fact that felix s  low " luminosity permits the focusing quadrupoles to be moved more than 120 m from the ip , provides the necessary longitudinal space . i4 is also ideal from the point of view of transverse space . the beams are separated by 42 cm at the location of the rf cavities , providing room for zero degree calorimetry . since the existing infrastructure , including the aleph solenoid , can be re - used with minimal modifications , i4 is clearly a superb location for a full acceptance detector . ( the central part of felix , which nicely fits into the existing cavern , and the extensions upstream into the forward regions , are shown in figure  [ figi ] . )    nevertheless , the task of integrating a detector with genuinely full acceptance into the available space at i4 is not trivial . the felix letter of intent @xcite outlines how it can be done , using well - understood magnets and compact detectors , for a comparatively modest price : we estimate a cost of about 25 mchf for the machine magnets and the infrastructure , and about 50 mchf for the detector outlined here and presented in more detail in the felix loi . the heart of the felix physics agenda is qcd : felix will be the ultimate qcd detector at the lhc .    surprisingly , the need for such a detector is not obvious to many members of the high energy community . in part , this may be because of the success of the interplay between theory and experiment in the case of electron - positron collisions . the cleanliness of the process , together with the low event rate and full - acceptance capability of the detectors , has led to an especially fruitful interaction between the qcd aspects of that experimental program with the remainder . the case of hadron - hadron collider physics is quite different . the high-@xmath3 , low cross section physics is accessed by highly selective triggers . the phase - space acceptance of the detectors is largely limited to the central rapidity region . full acceptance has not been attained since the bubble - chamber era of fixed - target physics . therefore the basic data base is much more limited . this situation is all the more serious because of the great variety in event classes for hadron - hadron collisions . there are soft collisions with large impact parameters ; angular momenta of tens of thousands instead of the unique @xmath5 of the @xmath6 world . central collisions produce much higher multiplicities than are seen in @xmath6 annihilation . there are the diffraction classes of events , with and without jet activity , that comprise several to tens of percent of typical subsamples ( if seen in full acceptance ) and which present a major challenge to theory . there are poorly understood strong bose - einstein - like correlations seen at very low @xmath3 and low relative @xmath3 in hadron - hadron collisions which do not occur in @xmath6 collisions . but at collider energies this is only based on one sample of low-@xmath3 data from ua1 , because until now no other detector has had the measurement capability . finally , there is little if any data in the forward fragmentation regions , where cosmic ray experiments insistently claim that anomalies exist . given this richness of phenomena , and given the importance of qcd to the interpretation of the new - physics data expected to emerge from the lhc , it is clearly very important to improve the data - base with an lhc detector and experimental group fully dedicated to the observation and interpretation of as broad a range of qcd phenomena as possible . this is of course the mission of the felix initiative .    many of these new opportunities in qcd physics at the lhc are not well known , and the felix collaboration has accordingly placed high priority in in providing a description of them in the felix loi . we briefly summarize a few of the main themes here . the parton densities at small @xmath7 are themselves a very important thing to measure . up to now hera has provided data down to @xmath7 values of order @xmath9 for @xmath10 in the perturbative domain of several gev@xmath2 . felix will have the capability to extend these measurements to @xmath7 values below @xmath8 via observation of dileptons , low - mass dijets , and low - mass jet - photon systems carrying large longitudinal momenta . in this regime one expects ( especially for proton - ion collisions ) the breakdown of the usual dglap / bfkl evolution - equation formalism and significant nonlinear effects to be observed . the need for a vastly improved qcd data - base for hadron - hadron collisions is made even more urgent by the fact that qualitative changes are expected even in the structure of generic events because of the rapid increase with energy of gluon parton densities in the primary protons . thanks to the measurements at hera , this is not only the theoretical expectation but also a data - driven one . the parton densities at a @xmath11 gev scale become so large that minijet production in central collisions may become common place , with minijet @xmath3 large enough for reasonably clean observability . these very high parton densities create , at a perturbative short distance scale , `` hot spots '' in the spacetime evolution of the collision process within which there may be thermalization or other nonperturbative phenomena not easy to anticipate in advance of the data . particle spectra themselves may evolve to something quite distinct from what has been so far observed , with strangeness , heavy flavors , and/or baryon and antibaryon production enhanced . especially in central proton - ion collisions , where the total gluon - gluon luminosity per collision is maximized , and where the evolution of a single proton fragment is followed , one can expect this class of phenomena to be most prominent and surprises most probable . diffractive final states will comprise almost @xmath12 of all final states at the lhc . the soft diffraction at very large impact parameter , which perhaps sheds light on pion - cloud or glueball physics , is at one extreme , and hard diffraction , where rapidity gaps coexist with jets , is at the other . there are a large variety of hard diffraction processes , including some with two and three rapidity gaps , which are of basic interest to study . in this class there are expected to be , for example , an extraordinary class of events where the complete event consists of a coplanar dijet accompanied by the two unfragmented beam protons detected in roman pots , and absolutely nothing else in the detector . certainly atlas and cms can also detect such events , provided they sacrifice a luminosity factor of about 30 relative to their hard - earned peak luminosity . however , to really understand this event class , one will need , at the very least , to examine the @xmath13-distribution of the roman - pot protons , as well as to study the generalizations of this process to the cases where one or both of the protons undergoes soft diffraction dissociation to a low mass resonance or a high mass continuum , or to a high-@xmath3 system containing a tagging jet . only felix would have such a capability . in addition to this class of hard diffraction and very soft diffraction processes , there is another very interesting class of semihard diffractive phenomena associated with the conjectured fluctuation of the initial - state projectile into a transversely compact configuration , which therefore interacts with an unusually small cross section . evidence for this is seen in vector - meson photoproduction at hera , especially @xmath14 production , which exhibits the expected rapid increase of cross section with energy . also at fermilab , diffraction dissociation of a high energy pion into dijets , with all the initial pion energy going into the dijet system , is being studied by experiment e791 . exactly the same process is available at the lhc with felix , as well as a similar process where one beam proton dissociates diffractively into three jets , one for each quark . the @xmath15 dependence of these processes is remarkable , roughly @xmath16 , because this diffractive process should occur even in central collisions , thanks to the small size of the initial configuration . the existence of events with a very high final - state multiplicity of minijets and their associated hadrons has other implications . the products of such interactions for the most part can be expected to explode from the initially compact collision volume in all directions at the speed of light . because of the high multiplicity density , the time of hadronization of all these degrees of freedom will be lengthened from the usual low - energy value of 1 - 2 fm to several fm . up to this time of hadronization , the expanding `` fireball '' containing most of the partonic collision products is arguably a rather thin spherical shell , of thickness of order a fm . so even before hadronization there is a large interior volume of hundreds of @xmath17 , isolated from the exterior vacuum , which may evolve toward a chirally disordered vacuum . consequently in such events there might be a large pulse of semiclassical , coherent pions of relatively low @xmath3 emitted when this false vacuum eventually decays : disoriented chiral condensate . this is at present only a speculative possibility , although experimental searches , especially in the context of ion - ion collisions , are underway . more generally , one may ask : if disoriented vacuum is not what is in the interior of this quasi - macroscopic fireball , what is ? if the interior  vacuum " is broken into domains of various chiral orientations , then topological obstructions might lead to production of ( skyrmionic ) baryons and antibaryons of unusually low @xmath3 . and if there is activity deep inside the light cone , no matter what it is , then this activity has eventually to be turned into emission of particles ; hence a new particle production mechanism which deserves to be studied . it would seem that the only alternative available for the @xmath18 of new phenomena emergent from the deep interior of the light cone under these circumstances is that that region relaxes back to the true vacuum , despite its being isolated from the true vacuum by a fireball shell and despite there not being enough elapsed time for chiral orientation to be distinguished energetically from chiral disorientation . in general , the chiral vacuum condensate is distorted in the neighborhood of impurities such as an isolated proton . this is just the long - range pion cloud surrounding it . the pion - cloud structure can be probed especially well in high energy @xmath19 collisions at very large impact parameters , say 2 to 3 fm . these interactions are , because of the larger radii of interaction at the lhc , a bigger component of the cross section , and can lead to larger final - state multiplicities than found at lower energies . perhaps here too there may be coherence in the structure of the pion emission , and this class of events may turn out to be of special interest . again a detection capability at very low @xmath3 , 100 mev and less , as possessed by felix , is important for such studies .      together with these many novel phenomena , there will be new methods for experimentally tagging different kinds of events . the impact parameter of the collision is obviously of importance to be determined event - by - event . this is done routinely in ion - ion collisions via zero degree measurements of nuclear fragments and by the amount of transverse energy produced . at the lhc , the felix instrumentation in the forward direction allows a data - driven approach for attacking the problem by the former method . the large yield of minijets , strongly dependent upon impact parameter , may allow the latter method , based upon transverse energy production , to be used more effectively at the lhc ( by all detector groups ) because of the stronger correlation of multiplicity with impact parameter than at lower energy . a combination of both methods , unique to felix , is likely to be the best of all . a second important tag available to felix is the choice of beam . by tagging on a leading neutron or @xmath20 at very low @xmath13 , one can reasonably cleanly isolate the one - pion - exchange contribution , and thereby replace the lhc @xmath19 collider with a somewhat lower energy , lower luminosity @xmath21 collider . in a similar spirit , and including @xmath22 tags , one can study collisions of any combination of @xmath23 , @xmath24 , or @xmath25 with each other . the beam - dependence of phenomena has historically been of considerable importance , and it may find important applicability , especially with respect to questions of valence - parton structure , at the lhc energy scale . a special case of these tags is that of a photon tag in ion - ion collisions , via forward detection of the undissociated ions . the luminosity for @xmath26 collisions is very high , and the capability of felix to exploit this luminosity is also very high . another class of tags which has been underutilized is the diffractive tag , where leading protons are detected via roman - pots . as discussed above , this leads to a very rich stratum of up - to - now poorly - measured , poorly understood , but potentially important physics . finally , there may be pattern tags . the event structure in final states containing jets is dependent upon the color flow . typically , neighboring jets in phase space are connected by a partonic color line ( antenna ) . for quarks , one antenna line emerges from the jet , for gluons two . along these antenna lines in phase space , hadronization and minijet production is enhanced . recently the tevatron collider experiments have observed these effects . in principle this technique might allow one in the future to identify in an individual multijet event quarks versus gluons , and even fully classify the event structure according to the color flow . clearly such a pattern - analysis technique is very difficult , and needs to be data - driven . felix , with full acceptance , will be optimal for making the attempt . we now introduce the major features of the felix design . more details can be found in the felix loi . @xcite . a full acceptance detector must be able to analyze the global structure event - by - event . this means that it should run at a luminosity no greater than @xmath27 @xmath28 s@xmath29 ; that is , with less than about one interaction per crossing . this luminosity can be achieved at i4 by means of an insertion which can be tuned from @xmath30 m to @xmath31 m without changing the magnetic elements . there are two significant features of this insertion . first , the final - focus quadrupoles can be placed more than 120 m from the ip , providing the space needed to accommodate the felix dipoles . second , it is economical . the necessary quadrupoles are already in the lhc baseline design . the ability to tune the insertion also has several nice features . at @xmath31 m , felix is optimized for the study of low - t elastic scattering . at @xmath32 m , where felix s luminosity is about 4 x 10@xmath33 @xmath28 s@xmath29 when the lhc is at design luminosity , the beam size in the heart of felix detector ( @xmath4 120 m ) is minimized , permitting the roman pot detectors in these locations to come as close as 3 mm to the beam . finally , @xmath34 23 m permits felix to reach luminosities as high as 2 x 10@xmath35@xmath36 . felix will implement a  kissing scheme " in which the two beams are brought together at @xmath37 in the horizontal plane and then returned to the same inner or outer arc ( see figure [ setupvfnewi ] ) . to accomplish this , we need some 67 t - m to first bring the beams together ( d2 magnets ) , and then another 67 t - m ( d1 magnets ) to make them parallel . this has to be accomplished within the 120 m available . both sets of magnets must be superconducting machine dipoles . the d1 magnets must also have large bores , to accomodate both beams and to provide acceptable tracking and calorimetry apertures . felix is fortunate that brookhaven national laboratory ( bnl ) has designed large aperture superconducting dipole magnets for use at rhic . with a coil aperture of 18 cm and a design field of 4.28 t ( felix will use them at 3.62 t ) , these magnets are suitable for use as d1 magnets . bnl is committed to producing these magnets for rhic and thus will be able to supply well - understood magnets on the felix time scale . the constraints on the d2 magnets are somewhat less severe , and several options are available . of these , felix proposes existing superconducting dipoles constructed as prototypes for unk . while these are single aperture magnets , the 42 cm beam separation permits two unk cold masses to be assembled in a common cryostat for use as d2 magnets .    in order to avoid parasitic beam - beam interactions and long - range tune shift effects , the beams will collide with a vertical crossing angle of @xmath38 mrad . to do this while optimizing the match of the magnetic architecture to tracking and calorimetry , we propose to re - use the existing ua1 magnet , split longitudinally into two halves and equipped with new coils . we will also build two 5 meter long , 2 t warm dipole ( d0 ) magnets .    the magnetic architecture is completed by the re - use of the existing aleph solenoid , which is well - matched with the use of the ua1 magnet . an important feature of this overall design is that the strengths of the magnetic fields increase in the forward direction , always well - matched to the typical momenta of the particles , resulting in momentum resolution which is reasonably uniform over all of phase space . finally , we note that all magnets can be accommodated in the existing aleph collision hall and adjacent tunnels without any significant civil construction . some 50 tracking stations , located as far as 430 m from the ip , are needed to ensure full acceptance and uniform resolution . the positions of most of the stations ( vertical lines ) are indicated in figure [ setupvfnewi ] . felix will instrument radially outward , emphasizing compact , near - beam tracking . how close we will approach the beams depends on the location . in general , we will use roman pot detectors to aggressively approach the beams wherever the location is accessible and the pot mechanical structure does not interfere with other tracking or calorimetry . elsewhere , we propose to use fixed - radius tracking , approaching to within 2.5 cm of the beams . the acceptance for charged particles as a function of pseudorapidity ( a ) and their momenta ( b ) ( see figure  [ acc - fract ] ) is almost 100% over the entire phase space . an important consideration is the occupancy within the tracking detectors . high particle densities close to the beam pose a significant pattern recognition problem . each tracking station should thus have sufficient resolution and redundancy to be able to locally reconstruct track segments . track segments are then matched , station - to - station , resulting in a very powerful spectrometer . these considerations lead to a common conceptual design for most felix tracking stations , based on two technologies : si pixel detectors out to radii of about 8 cm , supplemented by gas electron multiplier ( gem ) chambers at larger radii . we are also exploring the possibility of using gem as the basis for very compact micro - tpc s . a conceptual design for a  standard " fixed - radius tracking station is shown in figure [ beam_cyl_009i ] . the same technologies will be used for a compact microvertex detector . felix proposes four calorimeters on each side of the ip to provide complete electromagnetic and hadronic calorimetry for angles @xmath39 radian , that is , for @xmath40 . the coverage of the calorimeters is illustrated in figure [ felixcalsi ] . the interplay with the magnets and tracking systems is illustrated in figures [ figi ] . the calorimeters must have superb energy and spatial resolution , and must provide the information needed to identify neutrons , electrons and gammas . this must be done in limited space , and in a high - radiation environment . these considerations determine the structure of the calorimeters , the choice of sampling materials and the kinds of photodetectors and front end electronics which can be used for the readout . the ua1 endwall calorimeter , which is expected to have a radiation dose of less than 5 mrad for 10 years running , is a sampling calorimeter based on plastic scintillators and wavelength shifting fibers . the very forward ( d0 , d1 and zero degree calorimeters ) see much higher radiation levels , and will thus be  spaghetti "- type calorimeters , based on either thin capillaries filled with liquid scintillator or on quartz fibers . all three very forward calorimeters are similar in construction , differing only in their overall dimensions . each consists of a preshower detector , an em calorimeter , and two hadron calorimeter sections . the basic structure of the trigger is a multi - level triggering scheme which must reduce the trigger rate from 40 mhz to a rate acceptable for data recording . a schematic view of the trigger scheme together with the rates and the latencies of the three trigger levels is shown in figure  [ trig ] . the initial level 1 triggering must be done deadtimeless for each bunch crossing . it must establish clean evidence of the presence of an event from the triggering constituents and produce a constituent - based trigger decision . this is achieved with simple algorithms based upon the topology of the elements of the detectors which fired , or upon special trigger counters and energy deposited therein . these local trigger decisions are produced near the detector front - end electronics by special circuitry . during the level 1 latency , the sampled data of all channels of all detectors are stored in data pipelines . after a positive level 1 trigger decision , the output data are associated with specific bunch crossings , formatted into event fragments , and stored for subsequent filtering by the level 2 trigger . the output rate from level i is estimated to be around 100 khz .    the level 2 system performs a fast decision on the event quality , based upon data from separate detectors or portions thereof , using more sophisticated algorithms . these may be realized by a set of dedicated hardware or software processors . the output rate from level 2 is estimated to be roughly 10 khz . the level 2 filtered data are combined into complete events by the event builder , and are filtered by the level 3 computer farm . level 3 produces the complete event - based decision , and its task is to reduce the trigger rate from 10 khz to one acceptable for data recording ( around 100 mbytes / sec ) . we estimate the mean felix event size to be around 0.5 mbytes . therefore the level 3 rate might be around 200 hz .      the first major data set containing inelastic events used for serious physics analyses is assumed to be of order @xmath41 @xmath19 collisions , taken with essentially a minimum - bias trigger . the only selectivity provided at trigger level would address the rejection of bunch crossings with no interactions ( or more than one interaction ) , and rejection of candidate events seriously contaminated with beam - gas background . after such a minimum - bias run , an intermediate - luminosity run could be naturally implemented using only the level 2 trigger strategy and a choice of interaction rate compatible with the lack of rejection that level 1 would provide . such a strategy would seem to maximize the flexibility of choice of detailed triggers , and leave to a minimum the amount which is hard - wired into the front end of the daq system . we assume here that this intermediate - luminosity run might yield of order @xmath41 recorded events for @xmath42 @xmath19 interactions in the felix detector , i.e. have a rejection of order @xmath43 . finally , the design run , fully implementing the level 1 and level 2 triggers , should be expected to yield @xmath41 recorded events per @xmath44 interactions in the detector . the detailed choice of the trigger algorithms should probably be deferred for as long as possible , for at least two reasons . one is technological ; the rate of change is so large that one should opt for as modern hardware ( and software ) as possible . the other is the physics itself , which is evolving and will continue to evolve rather rapidly with time . it is not easy to anticipate in detail what will be the highest - priority qcd physics to address at the time of lhc commissioning . after the presentations of j.d . bjorken and k. eggert about possible forward physics in pp and p - a collisions at the lhcc `` workshop on further physics topics '' ( nov . 1994 ) the lhcc committee recommended this kind of physics by noting : _ `` the lhcc noted the interest in diffraction , and expects that such studies may also form part of the lhc experimental programme . the committee encourages interested parties to work together on an integrated approach towards this physics , whilst bearing in mind the lhc physics priorities already established '' _ when the possibility for a new interaction region in i4 became reality ( summer 1995 ) several workshops took place to discuss the layout of a full acceptance detector . in may 1996 the lhcc defined new rules for coming activities : _  the lhcc urges that any new experimental initiative should be consistent with the restricted resources likely to be available , and combined as far as possible with one of the foreseen experiment . " _    in an oct . 1996 memorandum@xcite to the lhcc , felix responded to these new guidelines by describing , in detail , the felix set - up , strategy and financial assumptions . the group received general encouragement from the cern management to go ahead with the letter of intent .    during the spring and summer of 1997 , the felix collaboration mobilized for the preparation of the loi , which was submitted to the lhcc in august 1997 .    in november 1997 , the lhcc chose to address the felix letter of intent , finding    _ ... that the felix loi is not responsive to these guidelines . while the physics topics addressed by the programme proposed in the loi are of interest ( particularly the complete reconstruction of diffractive events ) , the likely costs of constructing the proposed dedicated detector and of the modifications to the lhc collider are very high in comparison with the probable physics output . finally , the composition and strength of the collaboration seem inadequate for carrying out a strong programme addressing these physics topics . @xcite _    the cern research board has since endorsed the decision of the lhcc . the felix collaboration believes that these decisions were reached in a precipitate manner , with gross violation of due process . in particular , there has been no thorough scientific review of the felix proposal . indeed , a primary grievance is that the lhcc referees never contacted the proponents before arriving at its negative conclusion , nor were the proponents permitted to directly present the initiative in person to the committee . important issues , including possible staging scenarios to reduce cost , and ongoing efforts to build collaboration strength , were thus never presented to the committees . the justification of the decision which has been presented by the lhcc , the research board and by the director general clearly has to do with costs : cern is under great financial stress , and the issue of affordability is of course a very real one , an issue not unnoticed by the collaboration . it is clear that the felix collaboration as presently constituted is far from being able to provide the resources , a point which was reinforced in private discussions by the cern director general , who has indicated that he might have considered the felix loi more seriously if the collaboration would have been stronger , and with more collaborators from cern member states . felix has formally protested both the conclusions of the lhcc and the procedure by which the felix loi has been considered by the lhcc . the lhcc and research board have , however , raised several critical points . felix had originally expected to address such issues via direct interaction with the referees and the lhcc through the usual procedures . in the present situation , we believe that the best way of proceeding is to present an addendum to the felix loi to the lhcc which will contain a thorough discussion of the following points :    * the complementarity of the capabilities of felix with those of the already foreseen experiments ; * staging scenarios for the felix detector ; including * the possibility to construct a preliminary version of the felix experiment at fnal , hera or rhic with a stronger collaboration to demonstrate both the technical feasibility as well as to obtain a first glimpse of the physics . felix welcomes all additional collaborators , and will continue to expand the collaboration , with particular emphasis on cern member states . felix will also work to identify funding sources . finally , the entire felix collaboration will continue to work on substantive issues as outlined above , and in the loi . in particular , we are proceeding with the design and construction of a prototype forward tracking station , as sketched in figure  [ beam_cyl_009i ] , to be tested at one of the current generation of colliders . prototypes of the various forward calorimeters are also under construction . felix looks forward to a more positive response from the committees . it is clear , however , that more people must soon join the effort if felix is to succeed . many people have made invaluable contributions to felix without formally joining the collaboration . we would particularly like to acknowledge the contributions of the approximately 100 people in this category who directly contributed to the preparation of the felix letter of intent , and who are thanked in that document by name .      the felix collaboration . `` felix , a full acceptance detector at the lhc , letter of intent '' . cern / lhcc 97 - 45 , lhcc / i10 , 1 august 1997 . the full text of the loi is available on the web at the felix webpage http://www.cern.ch/felix j.d . bjorken , k. eggert and c. taylor , progress report about felix , lhcc 96 - 37 , http://www.cern.ch/felix/news/memor.html lhcc minutes , cern / lhcc 97 - 61 lhcc31
the felix collaboration has proposed the construction of a full acceptance detector for the lhc , to be located at intersection region 4 , and to be commissioned concurrently with the lhc . 
 the primary mission of felix is qcd : to provide comprehensive and definitive observations of a very broad range of strong - interaction processes . 
 this paper reviews the detector concept and performance characteristics , the physics menu , and plans for integration of felix into the collider lattice and physical environment . 
 the current status of the felix letter of intent is discussed .