Proposed material for the summary of the working ================================================= group on lepton-nucleon colliders at Snowmass. ================================================= (asci file) 1. Context: ========= Future ep collider and its relation to the present ee and future pp colliders The basic issue which needs to be addressed before considering the future ep (eA) collider options is to identify the research domains where the merits of the ep (eA) colliders are unique and to identify those where such a collider would be redundant with respect to existing (future) ee and pp colliders. 2. Statement - 1: ================ The ep (eA) collider can not compete with the ee and pp colliders in the domain of large energy (short distance) frontier of particle physics. The impressive precision tests of the standard model and resulting constraints on its possible extensions constitute the highlights of the research program at at the LEP and Tevatron (ee and pp) colliders. The HERA (ep) collider results, unique in several other research domains, turned out not to be competitive with LEP and Tevatron results in this field. The future high energy frontier ep colliders,being discussed during this workshop, are bound to follow similar path. THERA and/or eLHC can not match the precision and can hardly enhance the discovery potential of their parent TESLA and LHC colliders at the short distance frontier of particle physics. There are several reasons for that, the most important being the constrains of achievable luminosity increase for collisions of two different particle species with increasing CM energy. 3. Statement - 2: =============== The design of a future ep (eA) collider has to be optimized from a perspective of long term goal of creating a facility for a generic QCD studies and the design criteria (parameters) have to reflect such a choice of research domain. The domain in which ep (eA) machines have unmatched merits is the domain of strong interactions. Contrary to existing folklore I have first to stress that exploring the high CM- energy frontier of ep collisions is of secondary importance. Since the theory of strong interaction, QCD, is asymptotically free the interactions of quark and gluon at short distances are precisely controlled by the perturbative methods of QCD calculations. At this frontier, if one considers it interesting, the pp collisions provide already adequate environment to study high energy interaction of quarks and gluons. The physics of low x partons can be experimentally accessed by using nuclear, rather than proton, beams at intermediate energies. Using nuclei instead of nucleons allows to experimentally resolve the partonic density and the coherence length effects. The ep (eA) collisions are, on the other hand, indispensable to investigate quark and gluon interactions at distances comparable to the size of hadrons, where collective partonic degrees of motion are of importance and QCD has a limited predictive power. It is worthwhile to mention in this context that, as far as the collider energy is concerned, the collider design should be optimized to allow for the large range of collision energies and for the experimentally most suitable ratio of lepton and proton (ion) energies rather then for the highest collision energy. The other important parameters which define the quality factors for a future ep (eA) collider include: - luminosity and quality of particle beams - achievable polarization of the electron proton and light ion beams and quality of experimental control of the polarization over short and long time intervals - available ion species - achievable beam emmittances - a smart, QCD-dedicated, interaction point optics simplifying the design of a full-event-detector capable of measuring all particles produced in the interactions - provision of extending the ep (eA) collision program to the pp and pA collision program using a dedicated detector for precision studies of medium effects (three-beam-collider scenario) The above quality factors have to be optimized if a ep (eA) collider is meant to become a dedicated QCD facility providing unique means to study QCD dynamics by means of a femtotechnology of manipulating quarks and gluons in various environment (vacuum, hadronic matter, nuclear matter), and by observing quark and gluons at variable distance, time and quark-gluon density scales. 4. My proposed set of the design parameters (criteria) =================================================== for the future ep (eA) collider. ================================== a) Center-of-mass energy and its tun-ability range The optimal CM energy range which allows both for measurements in the perturbative QCD region and for extending the measurement towards and beyond the following distance scales: R_p, R_A, 1/Lambda_QCD, 1/m_Pi, 1/m_rho, which are pertinent for strongly interacting objects is ~10-100 GeV. Note, that the above energy range allows for the optimal dynamical resolution range of the electron probe both in the transverse and in the collision (light-cone) direction. The low energy limit can be approached by parasitic fixed target running with electron beam and as such can be moved upwards for the beam colliding mode. b) Ratio of electron to proton (ion) beam energies At the highest CM energies, where the low x domain can be covered in the DIS regime, choosing the ratio of the proton (ion) to electron beam energies in the range 10-20 facilitates various aspects of the full event detector design and preserves a good reconstruction quality and an efficient triggering of deep inelastic scattering events over the whole x_Bj range. In addition large value of the beam energies ratio allows for early decoupling the electron and proton (ion) beam optics thus facilitating the design electron beam insertion. Last but not least, it allows for the high electron currents with controllable level of synchrotron radiation at the interaction point. c) Charge and polarization of the electron While highest achievable and precisely controllable polarization of the electron and the proton (light ion) beam is indispensable, a possibility of switching from electron to positron beam is of secondary importance. A majority of physics processes leading to the charge asymmetries can be investigated by looking at spin asymmetries. d) Choice of ion species and tun-ability of ion atomic number It is indispensable to collide electron with heavy ions for maximizing the nuclear medium effects and with lightest ions (e.g. deuteron) for studying proton/neutron asymmetries. But, in addition, it is highly desirable to cover uniformly the A_range on the A**1/3 scale. A strong emphasis is here on storing isoscalar ions, in particular, if the collider compaction factor allows for simultaneous storing of of the two, isoscalar ion bunch trains. e) Luminosity While highest achievable luminosity L*A ~ 10**33 is necessary for high precision measurements of spin asymmetries, for studies of exclusive and semi-exclusive processes, rare fluctuations of nuclear densities, for the x>1 physics, etc..., there exist a vast physics program which can be start already at luminosities three orders of magnitude smaller than that quoted above. This provides an opportunity to consider the collider/detector project as evolutive with several initially open scenarios. In addition it allows for running at lower luminosities at comfortable beta* for precise studies of the electron, proton and ion fragmentation. f) Beam parameters at fixed luminosity In order that the future ep (eA) collider opens a domain of a femtotechnology of manipulating quarks and gluons in experimentally controllable nuclear environment, minimizing beam emmittances at the Laslet and the beam-beam tune shift luminosity limit rather then minimizing beta* is of virtue. As a thumb rule the beam divergences at the interaction point should be kept largely below the level of P_Fermi/P_nucleon ~ 5*10**(-4). A particular emphasis, in particular at high beam collision frequency, has be put on cleanness of the beam, i.e. smallest possible cloud of ''out-of-bunch'' particles to allow for efficient triggering scheme over all the x-range. g) Interaction Point geometry One of the merits of the collider configuration discussed above, related to the electron beam energy range and to a large ratio to the ion (proton) to electron beam energies, boils down to a drastic reduction of interference of the the electron insertion magnetic lattice with respect to magnetic lattice of the proton (ion) beam. In such a scenario the electron insertion magnet is placed in vicinity of the beam crossing zone and used in the design of a spectrometer for particles produced at small angles in the proton (ion) and electron direction. Owing to small electron beam energies the synchrotron radiation in the detector can be kept at tolerable level . Such a configuration is of particular interest for the linac-ring scenario in which the spin of the beam-electrons need not to be rotated. In addition, late electron beam insertion facilitates the design of the detection system of nuclear (proton) fragments. h) ''Three-beam-collider'' The Brookhaven National Lab with its nuclear and polarized proton beams in an optimal energy range (lower energy ions (protons) can be stored at RHIC if cooled by the electrons) and with its available IP12 interaction point provides a cost effective site to conduct the future ep (eA)research program in close synergy with the present RHIC pp, AA and pA programs. The interplay of these programs could open new vistas for understanding QCD in its full complexity. In this context, an ambitious but extremely interesting scenario is to design the IP12 collision point as a collision point of 3 beams. If one of the ion (proton) beam and the electron beam could be guided in and out of the beam_beam collision zone, not only the ep (eA) physics could be done without and interference with the ongoing BNL program, but more importantly, one can perform precise relative studies of medium-dependence of basic QCD processes. Such an idea seems to realizable for the collider parameters discussed above (in the limit of electron beam energy being lower than the proton (ion) energy) The collider parameters discussed above, in particular the particular the choice of the range of electron energies with respect to the energies of RHIC beams may facilitate such a design.