The B-factory Review Committee met at the Stanford Linear Accelerator Center during the week of June 21, 1993, and at the Cornell Laboratory of Nuclear Studies during the week of June 28, 1993.
The charge to the Committee was developed jointly by the agencies and communicated to the laboratory directors. The principal objective was to thoroughly review the technical aspects of the proposed colliders and the proposed detectors and to assess their potential for achieving design objectives and physics goals. Evaluation criteria included ability to achieve physics goals, technical feasibility and flexibility, credibility of cost estimates and schedules, and adequacy of R&D and infrastructure. The Committee evaluated each proposal separately. The Committee was explicitly instructed not to undertake a comparative review and not to rank proposals.
The CP symmetry violation will manifest itself in the difference in decay times between a B and a Bbar meson that are produced simultaneously. At existing electron positron colliders, where the incident beams are of equal energy and the BBbar pair is produced almost at rest, it is impossible to measure this difference. However, producing the BBbar pair in motion with respect to the laboratory frame would mean that their decay points are separated by measurable distances along their flight paths. Accurately measuring the decay asymmetry requires (of order) 100 million BB events. At an asymmetric energy B-factory with an integrated luminosity of 30 fb-l per year, it would be possible to measure the CP-violating asymmetries over the entire range expected within the Standard Model.
High collider luminosity is achieved by operating with large beam currents and many bunches. Limiting effects include the maximum allowable beam-beam tune shift and the minimum machine beta function at the collision point. Beam currents may be limited by single- and multi-bunch instabilities and average power dissipation considerations. Beam lifetime and detector backgrounds are influenced by the quality of the vacuum. Since beams in such colliders typically last only a few hours before becoming depleted, injection must be rapid to maintain high average luminosity.
There is no unique design for constructing a B-factory. Technical approaches are influenced by site considerations, experience, and costs. One significant design choice is that of collision geometry. Two possibilities are either: standard head-on collisions or an approach involving crossing beams at a small angle with slightly tilted bunches to provide for effective head-on collisions. This is known as crab-crossing. The design of the interaction region, the ability to minimize parasitic collisions, and the ability to absorb large amounts of synchrotron radiation and limit detector backgrounds are all strongly influenced by this choice. In addition, the colliders require high radiofrequency power to maintain the mean energy and short bunch length. Either room-temperature or superconducting radiofrequency cavities powered with klystron amplifiers may be used. This choice influences the ability to achieve the required accelerating gradients, to minimize beam impedance, and to extract and absorb large amounts of higher-order-mode power. Other important engineering choices include the design of the vacuum system and the interaction region components.
The collider is designed to operate near the maximum allowable beam-beam tune shift and the minimum machine beta functions at the collision point. The existing linear accelerator would be used as the injector for the storage rings. The collider satisfies the physics requirements for studying CP symmetry violation in the B meson system. The machine design is fairly conservative, and the engineering approach promises high integrated luminosity. The detailed engineering design also aims to achieve high operational efficiency, ease of maintenance and repair, and overall reliability of components and basic systems. Single-bunch parameters are consistent with those in many storage rings. PEP-II will use the standard head-on collision scheme. This conservative approach has been used in every high luminosity electron-positron collider that has ever been built, but it offers less flexibility in the interaction region design and layout; because of the relatively short bunch spacing, great care must be taken to avoid parasitic bunch collisions. In addition, the synchrotron radiation load close to the interaction region is very high. The design team has paid careful attention to component placement and masking in the region where the two beams enter and leave the interaction point, so as to limit the detector backgrounds to acceptable levels. The existing linac is an excellent injector for the B-factory.
Constructing the PEP-II collider poses many engineering challenges. As a consequence of the unprecedentedly high synchrotron radiation load in the interaction region, high demands will be placed on the vacuum system. Although the choice of room temperature technology for the radiofrequency cavities is a natural one for the PEP-II rings, the required maximum power density dissipated in the cavities exceeds that which has been routinely achieved anywhere so far. However, the physics goals would not be greatly compromised if the cavities had to be operated at levels that have already been achieved. Multi-bunch instabilities will be controlled by suppressing higher-order-modes in the accelerating cavities and by minimizing the number of cavities. Impedance of the higher order-modes is reduced by a large factor. Nevertheless, an active damping system is absolutely essential. The radiofrequency power handling capabilities of existing input windows may not be adequate, but other design approaches are available.
Many of the engineering issues have reasonable technical alternatives that will assure acceptable integrated luminosity. In addition, the ongoing R&D program should provide answers to many of the open questions. Extensive R&D studies have demonstrated the feasibility of the conceptual design and allowed it to be refined and optimized for both performance and cost.
Engineering challenges, such as noted above, are not unusual in large accelerator projects whose goal is a significant advance beyond what has been achieved so far. The Committee believes that in time they would all be met and overcome.
The PEP-II design, because of the large ring size and large number of long straight sections, offers operational flexibility and several possibilities for future upgrades. These include an increase in luminosity to 10^34 cm-2s-l and/or an increase in the beam energy asymmetry from 9 GeV on 3.1 GeV to 12 GeV on 2.5 GeV, which might allow a more careful study of B_s mixing at the resonance energy of the Upsilon(5S).
The collaborating Laboratories have prepared a detailed and credible baseline cost estimate, in FY 1993$, for constructing PEP-II. The Committee believes that the collider can be constructed for a total estimated cost of $162.7M, including $30.6M of contingency but excluding the detector. An additional $7.7M would be needed for R&D and pre operations costs during construction. The Committee supports the Laboratories' estimate of $26.8M as the total annual cost for operating the collider; provided that 80% of the linac cost can be absorbed by the other Laboratory programs.
The Committee also reviewed the preliminary design and cost for a new detector, BaBar, for experiments at the B-factory. The detector design is well conceived for studying CP violation. There are no major technical challenges associated with the construction of the proposed detector, although some subsystems still need extensive development. At this early stage in the design, the Committee estimated the total detector cost, including contingency, to be approximately $68M. The Laboratories hope to obtain a significant fraction of this cost from foreign sources.
It is anticipated that approximately 300 physicists, including foreign participants, will build and use the BaBar detector. The Stanford Linear Accelerator Center can provide adequate infrastructure support for these users.
The Laboratories have developed a detailed and realistic schedule for constructing PEP-II. It identifies project milestones and a critical path. The planned construction period is four years. The Laboratories have a detailed plan to obligate the first $36M of construction funds in FY 1994.
The detector construction schedule extends over five years, and the detector would not be available for experiments until FY 1999, just as the collider is ready to begin physics operation.
The combination of the three collaborating institutions-Stanford Linear Accelerator Center, Lawrence Berkeley Laboratory, and Lawrence Livermore National Laboratory constitutes a very powerful team for constructing the B-factory. A large pool of talented people is available for the project, which requires a total of approximately 759 person-years of effort. This number is a relatively small fraction of the total staff that is available.
The collider is designed to operate near the maximum allowable beam-beam tune-shift and the minimum machine beta functions at the collision point. The combination of the existing linear accelerator and the (relocated) synchrotron would be used as the injector. The collider satisfies the physics requirements for studying CP symmetry violation in the B meson system. The basic ring lattice designs are conservative and flexible. Single bunch parameters are based on direct operating experience with CESR and appear to be a conservative choice. Cornell has taken a new approach to the collision geometry to avoid the problem of parasitic collisions by using a crab-crossing scheme. The resulting layout is compact and flexible, and synchrotron radiation in the interaction region is reduced. The Committee believes that the crab-crossing scheme should work, but it has not been tested in an accelerator. The injector is adequate.
Constructing the CESR-B collider poses many engineering challenges. As a consequence of the unprecedentedly high synchrotron radiation load, very high demands will be placed on the vacuum system. The crab-crossing technique adds flexibility in the interaction region design, and Cornell has paid careful attention to component placement and masking to control and limit detector backgrounds. The superconducting radiofrequency accelerating and crab cavities may provide high accelerating gradients, but their application here is a significant extrapolation of parameters achieved in existing machines. However, the physics goals would not be greatly compromised if the accelerating cavities had to be operated at levels that have already been achieved. Cornell has an optimized cavity design with low narrow-band impedances, which allows for ring operation with high current and many bunches; as a result, no longitudinal feedback and only a modest amount of transverse feedback are needed. Ferrite absorbers, which act as higher-order mode dampers, must function properly under very demanding conditions. The crab cavity design is not yet mature, and only a small reduction in the cavity design field strength is tolerable for proper operation. Even if the nominal parameters can be demonstrated, reliable operation of superconducting cavities under very high beam loading at high gradients may pose new and serious problems. However, the choice of superconducting cavities is a natural one for Cornell given the machine parameters and the staff's extensive experience with this technology.
Many of the potentially difficult engineering problems have reasonable technical alternatives that pose only minimal risks to luminosity. In addition, the ongoing R&D program is expected to provide answers to many of the open questions. The fact that CESR is available for machine studies that are relevant to CESR-B is a great asset. The ongoing program for increasing the peak luminosity of CESR to (of order) 5x10~32 cm-2s-l serves as an excellent engineering testbed for many of the components needed for the B-factory, including an injector upgrade.
Engineering challenges, such as noted above, are not unusual in large accelerator projects whose goal is a significant advance beyond what has been achieved so far. The Committee believes that in time they would all be met and overcome.
The CESR-B design provides good operational flexibility. The interaction region geometry permits easy tuning of the energy ratio, the number of bunches can be easily increased, and no systems are shared with other major Laboratory programs.
Upgrade possibilities include a luminosity increase to 10^34 cm-2s-l and an increase in the center-of-mass energy to 13 GeV, which would be above all b-hadron thresholds.
Cornell has prepared a reasonable cost estimate, in FY 1993$, for constructing CESR-B. The Committee believes that the collider can be constructed for a total estimated cost of $99.5M, including $17.5M of contingency but excluding the detector. An additional $5.1M would be needed for R&D and pre-operations costs during construction. (The Committee has also budgeted an additional $7M to cover costs to Cornell if CESR-B were constructed under Department of Energy rules and oversight.) The Committee estimates the annual costs for operating the collider to be $17.6M.
The Committee also reviewed the preliminary design and cost for upgrading the existing CLEO-II detector for use with the B-factory. The detector upgrade is well conceived for studying CP violation. No major technical challenges are associated with the upgrade of the detector, although some subsystems still need extensive development. At this early stage in the design, the Committee estimated the total cost, including contingency, of the detector upgrade to be approximately $18.2M. The Laboratory hopes to obtain a significant fraction of this cost from foreign sources.
It is anticipated that approximately 300 physicists, including foreign participants, will build and use the CLEO-III detector. Available office space will be approximately doubled as part of the B-factory construction efforts. As a result, Cornell University will be able to provide adequate infrastructure support for these users.
Cornell has developed an achievable schedule for constructing CESR-B. Project milestones and a critical path have been identified. The planned construction period is four years. The Laboratory has a plan to obligate the first $12.1M of construction funding in FY 1994. The schedule for the detector upgrade also extends over four years. The detector will be available for physics at the same time the collider is being commissioned.
The CESR staff constitute a very strong and experienced team. Most participated in the original construction and commissioning of CESR and currently operate it as the highest luminosity electron-positron collider in the world. However, the project requires a total of 394 person-years of effort, a large fraction of the available total. Operating CESR, carrying out the present luminosity upgrade program, and starting CESR-B construction simultaneously will heavily tax the Laboratory staff.