From: Paul Grannis and Charlie Baltay As you may have heard, the two of us (Paul Grannis and Charlie Baltay) have agreed to work together to coordinate the US Study of Physics and Detectors of Future e+ e- Linear Colliders. The motivation for this is a desire to bring a wider community of US physicists into these studies. If a linear collider is to be in our future, it will require a strong and broad consensus that it is the optimal facility for extending our understanding of basic physics issues. The directors designate of Fermilab and SLAC have blessed this arrangement. We have been discussing future directions for the study following the successful international e+e- meeting at Sitges, and have the following suggestions: 1. Evolution of Detector Designs The US studies presented at Sitges concentrated mostly on common designs for a small and large detector, which were labelled S1 and L1. How will these designs evolve, and how frequently should we update or modify the designs? In discussion with various people we arrived at the suggestion that twice a year is about right for the next few years. We therefore propose that we do the next round of modifications by August 1999. We urge everyone involved to start thinking now about what modifications they would want to see. You can find the current designs on the worldwide web at the following address: http://hepwww.physics.yale.edu/lc/graphics.html We are not prejudging at this time whether these modifications should be minor or major. We also ask the Detector Working Groups to have discussions with their membership about what changes make sense at this time. There is a meeting scheduled at SLAC on August 4, 1999 to discuss the proposed changes and arrive at a consensus on what designs S2 and L2 should look like. We imagine that the next set of design iterations after August would be at the "Sequel to KEYSTONE" meeting that's planned for early 2000. 2. Evolution of the Physics Studies Up to the Sitges meeting the focus of the physics studies have been to evaluate the capabilities of the e+ e- collider and how the physics affects the detector designs. This kind of study clearly should continue. In addition, however, we have to keep in mind that a project of this magnitude requires that the case for a superb scientific pay-off be nearly foolproof, and that there be a clear and strong consensus within the community on the appropriateness of this facility as our next step. This has to be done in the context of the LHC, which is likely to start its physics program years before the next e+e- linear collider. Thus the physics goals and capabilities of the e+e- collider must be evaluated as a comparison with the capabilities of the LHC. Our suggestion on how to get started on this is to make up several scenarios of that the LHC is likely to find and ask how the e+e- collider will contribute to further our understanding beyond what we learned from the LHC. For instance: 2.1 Let us assume that SUSY exists at the expected mass scale (many states below 1 Tev) * What are the realistic estimates of the LHC experiments' ability to unravel this spectroscopy? Recent LHC physics studies make a case that the mass spectrum for gauginos, squarks/gluinos and sometimes sleptons will be established to reasonable precision; that some branching ratios will also be determined. In this scenario, what are the essential advances that will be made at the Linear Collider beyond LHC? (To what extent are better mass measurements, detailed branching ratios and couplings, quantum numbers critical to our understanding of Nature's choice of the low mass Susy phenomenology?) * Are there significant regions of Susy parameter space in which one could expect LHC to fail to disentangle the basic particle content, but for which LC could do so? How dependent is this on LC energy? * Most LHC studies have been done in relatively constricting Susy models: minimal SUGRA or gauge mediated Susy. To what extent can the Linear Collider experiments understand more complex supersymmetry sectors? For example, for what model assumptions could one determine that there are more than two Higgs doublets? or more than five Susy unification parameters? or failure to achieve gauge coupling unification? * What information can be gained from NLC experiments on the supersymmetry hidden sector scale and its symmetry properties? * An ambitious program is outlined for LC experiments to do precision Susy and Susy Higgs measurements, together with studies of salient Standard Model processes. Given a reasonable assumption of LC luminosity growth to design value, what is the length of running time to accomplish this program? * With what precision can LC determine the J CP composition of a Higgs boson candidate at 130 GeV? at 500 GeV? at 1000 GeV? 2.2 Let us assume that the LEP/Tevatron/LHC uncover no evidence for TeV scale supersymmetry or a Susy Higgs sector. * If there is a SM Higgs, what is the outline of the linear collider program to extend our knowledge of it beyond the LHC? * If there is no SM Higgs up to the LHC reach, what is the program of the NLC? Can one make a convincing case that the linear collider extends the search for strong coupling beyond that possible at LHC? 2.3 Should a 500 GeV limited energy linear collider become available somewhere in the world, what is the physics case for construction of a second facility at 1 TeV? 1.5 TeV? 3 TeV? 2.4 What are the physics cases beyond the issues of Electroweak Symmetry breaking? * For TeV scale compactification and millimeter scale gravity, or other indications of physics of more than 4 dimensions. * New insights into hot dark matter. * What if the CKM triangle fails to describe Nature; is there any role for a linear collider to play? * What insights into Yukawa coupling unification, or the origin of flavors? We realize that the answer to some of these questions are implicit in the studies that have been carried out so far and these answers just have to be pulled together in the context of an LHC comparison. Some of the other questions need a fresh approach. We would like to get everyone's suggestions about how to do all this and how to get the Physics Groups started so that by the "Sequel to KEYSTONE" meeting we could start serious discussions along these lines. 3. Future Meetings August 4-5, 1999 at SLAC to discuss evolution of the detector designs (August 4) followed by a simulation meeting (August 5). Sequel to KEYSTONE - probably at Berkeley, February-March 2000 or so. The next international meeting, Sequel to Sitges, will be at Fermilab, September-October 2000. Please let us know your comments and suggestions on all of this. With best regards, Paul and Charlie