Tau Physics Prospects at the Tau-charm Factory and at Other Machines

The prospects for tau physics at future high-luminosity facilities are briefly discussed. Although important (and often complementary) contributions will be made from other machines, the unique experimental environment near threshold makes the Tau-Charm Factory the best experimental tool for τ physics. Talk given at the Marbella Workshop on the Tau-Charm Factory Marbella, Spain, June 1993 ∗ On leave of absence from Departament de F́ısica Teòrica, Universitat de València, and IFIC, Centre Mixte Universitat de València–CSIC, E-46100 Burjassot, València, Spain. † Work supported in part by CICYT (Spain), under grant No. AEN93-0234. CERN-TH.7065/93 November 1993 TAU PHYSICS PROSPECTS AT THE TAU-CHARM FACTORY AND AT OTHER MACHINES A. Pich Theory Division, CERN, CH-1211 Geneva 23 Abstract The prospects for tau physics at future high-luminosity facilities are briefly discussed. Although important (and often complementary) contributions will be made from other machines, the unique experimental environment near threshold makes the Tau-Charm Factory the best experimental tool for τ physics.The prospects for tau physics at future high-luminosity facilities are briefly discussed. Although important (and often complementary) contributions will be made from other machines, the unique experimental environment near threshold makes the Tau-Charm Factory the best experimental tool for τ physics. The physics programme of the Tau-Charm Factory (τcF) was established in the 1989 SLAC workshop [1]. At that time, the experimental knowledge on the τ was quite poor [2], and the main part of the physics community was not giving much attention to that (still exotic) lepton. The situation has completely changed since then. ARGUS and CLEO have substantially increased their ττ data sample, and the four LEP collaborations have demonstrated the potential of this machine for making clean τ physics. Moreover, a small, but very important, contribution has been accomplished at the Beijing collider. The growing interest on the τ particle has been reflected in several specialized reviews [3, 4], which have emphasized the unique properties of this heavy lepton for testing the Standard Model, and in the first two workshops [5, 6] devoted entirely to the τ . The qualitative change of τ physics can be appreciated in Table 1, which compares the status of several τ measurements in the 1990 compilation of the Particle Data Group [7] with the more recent world averages [6,8]; the main experimental sources of the improvements are also indicated. Obviously, our knowledge on the τ lepton properties is going to be further improved in the next few years. To get a proper feeling on the quantitative impact of the τcF in this field, one should analyze the precisions that can be reached with present facilities. Moreover, the possible contributions of other future machines should also be considered. Figure 1 shows the energy and luminosity of present and future ee colliders in the energy range 1 GeV ≤ Ec.m. ≤ 100 GeV. All machines above √ s = 2mτ produce τ ’s and can make significant contributions to τ physics. Figure 1: Luminosity/Energy plot of present and future ee colliders, in the energy range 1 GeV ≤ Ec.m. ≤ 100 GeV. The different running energies have their own advantages and problems; thus, a given collider can be very good for measuring some parameter and totaly insensitive to other properties of the τ . The luminosity is clearly an important ingredient, but not always the decisive one. There are three energy regions worth while to be considered: the ττ threshold (BEPC, τcF), the Υ region (DORIS, CESR, BF) and the Z peak (LEP, ZF). LEP has the great advantage of producing τ ’s with a sizeable boost and low backgrounds. It is obviously the best machine for lifetime measurements. Adding the 1990–1992 data sample of the four LEP experiments (2× 10 τ pairs) an accuracy of about 3 fs has been obtained [9]: ττ |LEP = 293.5± 2.8 fs. In spite of having accumulated a much larger statistics (2 × 10 τ pairs), the CLEO result [8], ττ |CLEO = 296 ± 10 fs, is less precise; a smaller average flight path and larger hadronic backgrounds make difficult to achieve a better sensitivity [10]. At threshold, to measure the lifetime is clearly not possible. The τ -mass measurement requires completely different experimental conditions. With only 7 (ττ → eμ + 4ν) events, taken at threshold, BES [11] has been able to achieve an unbelievable On leave of absence from Departament de F́ısica Teòrica, Universitat de València, and IFIC, Centre Mixte Universitat de València–CSIC, E-46100 Burjassot, València, Spain. Work supported in part by CICYT (Spain), under grant No. AEN93-0234.