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A full acceptance detector will maximize the ability to search for new physics in unexplored regions of phase space while pursuing a substantial agenda of more conventional physics. Many of these topics are not well understood, either theoretically or experimentally, and we believe that the prospect of acquiring data will serve as a powerful stimulus to the further development of these topics.
The physics agenda for FELIX will thus include the complete range of non-perturbative and semi-perturbative QCD processes which are of considerable intrinsic interest, and which appear as backgrounds to other interesting and potential new physics.
At the lowest luminosity we will concentrate on soft physics such as inclusive distributions of forward and very forward reaction products and leading particle effects. The data will enable us to search for disoriented chiral condensates and other related phenomena by studying multiple particle correlations.
Particle beams can be defined via leading particle tags and
meson-nucleon interactions at energies of several TeV can be observed. FELIX
will have the unique capability to see leading particles,
with 
.
If a leading nucleon or delta is detected, the remainder of the event represents
a collision of a beam nucleon with the exchanged nonstrange meson; hence the LHC
becomes an effective meson-nucleon collider, and most pp processes studied,
hard or soft, can be compared with the corresponding meson-proton process to
determine the beam-composition dependence. A kaon ``beam" can be made in a like
manner with a leading lambda tag.
The physics agenda continues with the study of soft single and double diffraction (Pomeron exchange) and of Pomeron-Pomeron interactions via triple and higher order diffraction. FELIX will measure the ultimate diffractive process-elastic scattering-over a wide range of momentum transfers: small enough to determine the total cross-section and large enough to see the dip structure and the distribution beyond the dip.
Increasing the luminosity by an order-of-magnitude to some
will lead us to semi-hard and hard diffractive physics.
There is much current interest in the nature of the
``hard Pomeron" mediating these processes, which, it is hoped, can
be quantitatively understood in terms of a theoretical
construct arising from multi-peripheral gluon exchange. We will
continue the studies already begun at HERA, Fermilab and the CERN SPS collider.
More generally, the structure of the Pomeron will be elucidated through
the measurement of the rapidity distributions of jets, heavy flavors and gauge
bosons. Related studies, probing the fractal nature of QCD phase space, and the
onset of color-coherence effects, include the analysis of multi-jet global
patterns and of jets-within-jets.
Rapidity gap tags may provide a useful tool in the search for new physics. For example, Pomeron-Pomeron interactions may be an effective way of producing new particles in a rather clean environment, particularly if the Pomeron has a hard gluon structure function. Further, while rapidity gaps are usually thought of as a consequence of a strongly interacting color singlet (the pomeron or hard pomeron), hard collisions mediated by the exchange of gauge bosons can also create rapidity gaps. Together with the expectation that hard diffractive processes may be enhanced by orders-of-magnitude compared to the most naive two-gluon exchange mechanism if the parton-parton center of mass momentum is sufficiently high, we are led to the study of two gauge-boson physics, new physics and diffractive production of heavy flavors via rapidity-gap tags.
The measurement of forward reaction products and leading particle effects will be of utmost importance in interpreting the highest energy cosmic ray air showers. Not much is known about the energy flow in the far-forward region and the interpretation of high energy cosmic ray shower is mainly based on some forward production models in the Monte-Carlo calculations of extensive air showers. Furthermore, a variety of cosmic ray exotica, such as the knee in the cosmic ray spectrum at a centre-of-mass energy of about 5 TeV, Centauro and Anti-Centauro events, forward multi-muon bundles, aligned clusters and possible changes in the interaction length may find a natural explanation or may be confirmed if the energy flow in the far-forward region is better understood.
We want to emphasize the importance of studying proton-ion collisions which could become available due to the new design in which the RF cavities are split into two, each for one ring. The present extensive program of A-dependence physics, centered on shadowing and color-transparency phenomena, could be extended to these extreme collider energies. In addition, it may be that any kind of exotic physics which is associated with beam fragmentation into a very high mass system (e.g. Centauro, anti-Centauro, disoriented chiral condensate) occurs with higher frequency in proton-ion collisions than in proton-proton collisions.
Finally, the detector we envision should also be well-suited to two photon physics in pp and heavy ion collisions, opening a new field of photoproduction physics (pioneered by HERA), and searches for new massive particles and multi-quark states. Intense heavy-ion beams represent a prolific source of quasireal photons and hence reasonable statistics can be obtained at two-photon masses well above the reach of LEP 200.
